HomeMy WebLinkAboutSWP273170(27)Congestion Relief & Bus Rapid Transit Projects
September 1, 2005
Subject: Springbrook Wetland and Habitat Mitigation Bank:
Springbrook Creek Wetland Restoration Memorandum
Executive Summary
The Washington State Department of Transportation (WSDOT) and the City of Renton are
working together to establish the Springbrook Wetland and Habitat Mitigation Bank (the Bank).
The Bank is comprised of five wetlands units: A, B, C, D, and E. This memorandum addresses
the rehabilitation of Units A and B and the reestablishment of wetland at Unit E.
Rehabilitation of Units A and B will be achieved by providing hydrologic connections with
Springbrook Creek. Reestablishment of Unit E wetlands will depend on providing sufficient
water to the site to establish vegetation typically found in saturated soil conditions. Unit E will
depend solely on flows entering the site from Springbrook Creek for its source of water. Units C
and D are addressed in a separate memorandum because these sites do not depend on
Springbrook Creek for their source of hydrology. Instead,', the hydrology of Units C and D is
characterized by using a water balance that includes stormwater runon volumes, piezometric
data, infiltration, and evapotranspiration calculations.
Springbrook Creek hydrology was investigated using the Hydrologic Simulation Program —
FORTRAN (HSPF) program. A continuous hourly precipitation record from Seattle -Tacoma
International Airport for water years 1949 to 2001 was used to generate 53 years of continuous
flow in Springbrook Creek. The model was calibrated to streamflow data from water years 1995
and 1996. Analysis of the model was used to determine the inundation and duration at Units A,
B, and E.
The rehabilitation of Units A and B will depend on creating breaches to provide more efficient
connection of flows with Springbrook Creek through the existing levee. The breach elevations
were set at the elevations of the existing delineated wetlands. It is anticipated that the 12-foot
elevation of the breach will provide 77 hours (3.2 days) of inundation annually, and 23 hours (1
day) of inundation during the mesic growing season defined from March 1 through October 31.
It is anticipated that the depth of flows on the site will range from 0 to 2 feet as a result of
inundation from Springbrook Creek.
The success of the reestablishment of Unit E depends on inundation by Springbrook Creek,
duration of the flows, and on the saturation of soils. Breaches have been created to facilitate
Springbrook Creek flows into and out of the site without causing erosion to the site, flooding to
surrounding properties, or stranding of fish species. It is anticipated that the depth of flows at
Unit E will range from 0 to 7 feet. The deepest inundation will occur at the connection with
Springbrook Creek. It is anticipated that Springbrook Creek flows will last approximately 1 hour
during the growing season. It is estimated that Elevation 10 will be inundated with water
approximately 2 feet deep for 15 days during the growing season.
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Introduction/Purpose
The Washington State Department of Transportation (WSDOT) and the City of Renton are
working together to establish the Springbrook Wetland and Habitat Mitigation Bank (the Bank).
The Bank is comprised of five wetlands units: A, B, C, D, and E. This memorandum addresses
the rehabilitation of Units A and B and reestablishment of wetlands at Unit E.
Rehabilitation of Units A and B will be achieved by providing hydrologic connections with
Springbrook Creek. Reestablishment of Unit E will depend on providing sufficient water to the
site to establish vegetation typically found in saturated soil conditions. Unit E will depend solely
on flows entering the site from Springbrook Creek for its source of water. Units C and D are
addressed in a separate memorandum because these sites do not depend on Springbrook
Creek for their source of hydrology. Instead, the hydrology of Units C and D is characterized by
using a water balance that includes stormwater runon volumes, piezometric data, infiltration,
and evapotranspiration calculations.
This memorandum describes the hydrologic modeling and analyses conducted for evaluating
Springbrook Creek flows and for determining inundation of the units. This memorandum
includes three parts:
• Part I identifies the criteria and methodology whichneed to be met in order to receive
mitigation credit.
• Part II provides analysis and discussion` of hydrology as it pertains to supporting the
rehabilitation of Units A and B and the re-establishment of Unit E.
• Part III provides a summary and conclusion.
1. Criteria and Project Methodology
To receive mitigation credit, Units A, B, and E must meet multiple functions and natural
processes criteria. Rehabilitating the existing wetlands will allow Units A and B to meet these
criteria. Unit E will meet the criteria when wetlands are re-established onsite.
Units A and B
Wetland rehabilitation is the dominant mitigation treatment within Units A and B. Wetland
rehabilitation will be achieved by providing hydrologic connections between the existing
wetlands and Springbrook Creek. It is anticipated that these connections will augment wetland
hydrology in Units A and B, increase stream interaction with the floodplain, and potentially
reduce the elevation and duration of peak flows in Springbrook Creek. It should be noted that it
is not necessary to establish wetland hydrology at Units A and B because it already exists.
Unit E
Wetland hydrology currently does not exist at Unit E. Mitigation treatments for Unit E include
wetland re-establishment, riparian enhancement, upland habitat enhancement, and buffer
creation. To re-establish wetlands in Unit E, the definition of wetland hydrology must be met or
exceeded, assuming that the soil and vegetation parameters are achieved. The Washington
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State Department of Ecology's 1997 Washington State Wetlands Identification and Delineation
Manual defines wetlands as:
Those areas that are inundated or saturated by surface or ground water
at a frequency and duration sufficient to support, and that under normal
circumstances do support, a prevalence of vegetation typically adapted
for life in saturated soil conditions.
In order to be considered a wetland, the following hydrologic criterion must be met:
Areas which are seasonally inundated and/or saturated to the surface for
a consecutive number of days greater than 12.5 percent of the growing
season are wetlands provided the soil and vegetation parameters are
met. Areas inundated or saturated between 5 percent and 12.5 percent
of the growing season in most years may be wetlands. Areas inundated
or saturated to the surface for less than 5 percent of the growing season
are non -wetlands.
Because individuals from different disciplines will review this document, terms in the definition
above are clarified below:
Inundation —A condition in which water from any source temporarily or
permanently covers a land surface.
Saturation — A condition in which all easily drained voids (pores between
soil particles) in the root zone are temporarily or permanently filled with
water to the soil surface at pressures, greater than atmospheric.
Growing Season - The growing season is the portion of the year when
soil temperatures at 19.7 inches below the soil surface are higher than
biologic zero (51 C). For western' Washington the mesic growing season
is often considered to be approximately March 1 to October 31 (245
days).
Duration — The length of time during which water stands at or above the
soil surface (inundation), or during which time the soil is saturated.
Frequency — The periodicity of coverage of an area by surface water or
soil saturation.
To determine the extent of the inundation and the duration of Springbrook Creek flows, the
Hydrologic Simulation Program — FORTRAN (HSPF) program was used. HSPF is a continuous
time series hydrologic program that can be used to investigate a wide range of hydrologic
conditions such as low flow conditions, flow durations, and length of inundation. HSPF is used
extensively in western Washington by King County Water and Land Resources, the United
States Geological Society (USGS), and engineering consultants to assess effects of changes in
a drainage basin over time.
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The Springbrook Creek model was used to support the City of Renton's floodplain re -mapping
study, a part of the recent Federal Emergency Management Agency (FEMA) floodplain mapping
revison (Hartley and Stewart, 2004). A continuous hourly precipitation record from Seattle -
Tacoma International Airport (NWS gage 7473 at Sea-Tac) for water years 1949 to 2001 was
used to generate 53 years of continuous flow in Springbrook Creek. The model was calibrated
to streamflow data from water years 1995 and 1996 using land use, channel, and basin storage
conditions (Hartley and Stuart, 2004). Storms were typically large and numerous during the
calibration period, making this a very good period for calibrating (Hartley and Stuart, 2004).
For the Springbrook Wetland and Habitat Mitigation Bank project, the model was updated with
recently surveyed stream cross sections. The updated model was used to evaluate current and
future flow conditions in Springbrook Creek. The current flow conditions were based on: (a) the
1999 land use designations determined from aerial photography of the drainage basin, and
(b) the anticipated near -term conveyance improvements, including replacing the culvert at SW
34th Street and connecting the proposed Unit E wetland to, Springbrook Creek.
The future flow condition consists of current condition near term conveyance improvements,
removing the bridge north of SW 27th Street, and replacing the culvert at the intersection of SW
41 st Street and Oakesdale Avenue SW. The future land use scenario represents the maximum
development allowed under the City of Renton zoning: ordinances in place at the time of the
2001floodplain mapping study.
Unit E improvements were included in all model runs. > The two modeled scenarios provide
anticipated conditions at the sites when the project is completed and the anticipated buildout
conditions for channel improvements along Springbrook Creek. Note that analysis of current
and future conditions does not provide an evaluation of the impacts or benefits of the proposed
project.
II. Analysis and Discussion
The extent of the inundation and the duration of Springbrook Creek flows were evaluated at
three locations for the current and future conditions. The three locations (Figure 1) are:
1. Upstream of SW 27th Street (adjacent to Units A and B)
2. Upstream of SW 34th Street (located just north and downstream of Unit E)
3. Downstream of Oakesdale Avenue SW (located just south and upstream of Unit E)
The analysis for each location included counting the number of times (events) that the water
surface exceeded a certain elevation (inundation), and for how long it remained at that elevation
(duration). For example, the analysis could tell the investigator that Elevation 11 was exceeded
36 times for 134 hours during the growing season at Location 1. This type of analysis is often
referred to as an event -stage (elevation) -duration analysis and can be achieved only by using a
continuous time series program. The analysis cannot predict how long an area will be
continuously inundated or saturated. To address the continuous inundation question, a second
analysis is necessary and is discussed in the second part of this section.
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11.1 Event - Stage - Duration Analysis
Units A and B
Event — stage — duration analyses for Units A and B is based on the 53-year continuous record
of Springbrook Creek flows upstream of SW 27th Street. Table 1 provides current condition
information about the hours and days of inundation in half -foot increments up to Elevation 12,
and in 1-foot increments from Elevations 13 to 16, for four time periods. The table also lists the
corresponding plant zones proposed for each elevation range. Plants will not be installed at a
given elevation if the plant community is not identified in the plant zone column. For Units A and
B, no plantings are anticipated below Elevation 12 along Springbrook Creek. The proposed
plantings for Units A and B are shown in the attached drawings WM1, WM2, WM3, and WM4.
The time periods listed in Tables 1 through 8 are: (a) annual, (b) the growing season (March —
October), (c) the early growing season (March — May), and (d) from June through August.
Subdividing the information into different time periods helpsillustrate changes in Springbrook
Creek hydrology over time.
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Table 1 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites under Current
Conditions - Upstream of SW 27th Street
Hours (days) Inundated
Growing
March - May
Plant Zone
Elevation
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
7.5
7,964
4,274
2,193
1,853
(331.8)
(178.1)
(91.4)
(77.2)
8
2,485
908
541
114
(103.5)
(37.8)
(22.5)
(4.8)
8.5
1,454
488
265
53
(60.6)
(20.3)
(11.0)
(2.2)
9
976
322
163
32
(40.7)
(13.4)
(6.8)
(1.3)
9.5
674
216
101
20
(28.1)
(9.0)
(4.2)
(0.8)
10
473
149
66
13
(19.7)
(62)
(2.8)
(0.5)
10.5
323
101
41
8
(13.5)
(4.2)
(1.7)
(0.3)
11
212
65
23
5
(8.8)
2.7)
(1.0)
(0.2)
11.5
137
41
14
3
(5.7)
(1.7)
(0.6)
(0.1)
Wetland Shrub #2
12
77
(3.2)
14
(0.6)
8
(0.3)
2
(0.1)
Wetland Shrub #2
13
21
(0.9)
4
(0.2)
3
(0.1)
0
(0.0)
Wetland Shrub #2
14
3
(0.1)
1
(0.0)
0
(0.0)
0
(0.0)
Riparian
15
0
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
Riparian
16
0
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
Riparian
20
0
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
Table 2 provides the future conditions hours and days of inundation by elevation for specific
seasons and proposed plant communities.
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Table 2 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites
under Future Conditions - Downstream of SW 27th Street
Hours (days) Inundated
Growing
March - May
Elevation
Plant Zone
(feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
7.5
7,085
4,274
2,163
1,309
(295.2)
(178.1)
(90.1)
(54.5)
8
2,467
908
529
121
(102.8)
(37.8)
(22.0)
(5.0)
8.5
1,511
488
277
60
(63.0)
(20.3)
(11.5)
(2.5)
9
1,067
322
180
40
(44.5)
(13.4)
(7.5)
(1.7)
9.5
766
216
118
26
(31.9)
(9.0)
(4.9)
(1.1)
10
559
149
81
18
(23.3)
(6.2)
(3.4)
(0.8)
10.5
401
101
54
13
(16.7)
(4.2)
(2.3)
(0.5)
11
276
65
33
8
(11.5)
(2.7)
(1.4)
(0.3)
11.5
188
41
20
5
(7.8)
(1.7)
(0.8)
(0.2)
Wetland Shrub #2
12
114
(4.8)
23
(1.0)
11
(0.5)
3
(0.1)
Wetland Shrub #2
13
35
(1.5)
7
(0.3)
4
(0.2)
1
(0.0)
Wetland Shrub #2
14
7
(0.3)
2
(0.1)
1
(0.0)
0
(0.0)
Riparian
15
1
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
Riparian
16
0
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
Riparian
20
0
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and
August represent 2,208 hours (92 days).
Tables 3 and 4 present the expected hours of inundation, expressed as a percent of the time
periods upstream of SW 27th Street under current and future conditions.
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Table 3 — Predicted Percent Inundation of Wetland Mitigation Sites
under Current Conditions — Downstream of SW 27th Street
Plant Zone
Elevation
(feet)
Percent Inundation
Annual
Growing
Season
(March -October)
March - May
(Early
growing
season)
June -
August
7.5
91 %
73%
99%
84%
8
28%
15%
25%
5%
8.5
17%
8%
12%
2%
9
11%
5%
7%
1%
9.5
8%
4%
5%
1%
10
5%
3%
3%
1%
10.5
4%
2%
2%
0%
11
2%
1%
1%
0%
11.5
2%
1%
1 %
0%
Wetland Shrub #2
12
1%
0%
0%
0%
Wetland Shrub #2
13
0%
0%
0%
0%
Wetland Shrub #2
14
0%
0%
0%
0%
Riparian
15
0%
0%
0%
0%
Riparian
16
0%
0%
0%
0%
Riparian
20
0%
0%
0%
0%
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
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Table 4- Predicted Percent Inundation of Wetland Mitigation Sites
under Future Conditions — Downstream of SW 27th Street
Percent Inundation
Plant Zone
Elevation
(Feet)
Annual
Growing
Season
(March-
October)
March - May
(Early
growing
season)
June —
August
7.5
81 %
73%
98%
59%
8
28%
15%
24%
5%
8.5
17%
8%
13%
3%
9
12%
5%
8%
2%
9.5
9%
4%
5%
1 %
10
6%
3%
4%
1%
10.5
5%
2%
2%
1 %
11
3%
1%
1%
0%
11.5
2%
1 %
1 %
0%
Wetland Shrub #2
12
1 %
0%
0%
0%
Wetland Shrub #2
13
0%
0%
0%
0%
Wetland Shrub #2
14
0%
0%
0%
0%
Riparian
15
0%
0%
0%
0%
Riparian
16
0%
0%
0%
0%
Riparian
20
0%
0%
0%
0%
1 vear is aooroximately 8.760 hours (365 days). Growina season is aonroximately 5.580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and
August represent 2,208 hours (92 days).
Tables 1 through 4 show that the hours and days of inundation are reduced as the elevation
increases. The hours of inundation are also reduced as the time period is reduced from the
annual period to a smaller June — August time period. It is also noted that the change between
the current condition and the future condition is insignificant. This is most easily observed when
comparing the current (Table 3) and future (Table 4) percent inundation. Except for elevation
range 7.0 to 7.5, which represents the Springbrook Creek bed, the change in percent inundation
between the two conditions is 1 percent or less.
The proposed levee breaches at Units A and B are set at 12 feet to provide the necessary
hydrologic connections with Springbrook Creek. This proposed elevation is based on the
elevations of existing delineated wetlands located behind the levee. It is anticipated that these
connections will augment wetland hydrology in Units A and B, increase stream interaction with
the floodplain, and reduce the elevation and duration of Springbrook Creek's peak flows. Per
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Table 1, it is anticipated that areas at Elevation 12 will be inundated for 77 hours (3.2 days)
annually, for approximately 14 hours during the growing season, and for 8 hours during the
early growing season (March through May). The depth of flows at Elevation 12 will range from 0
to 5 feet. From Table 2, areas at Elevation 12 will be inundated approximately 114 hours (4.8
days) during the year, and to 23 hours (1 day) during the growing season.
Unit E
Event — stage — duration analyses for Unit E are based on the 53-year continuous record of
Springbrook Creek flows upstream of SW 34th Street. Similar to Tables 1 and 2, the
information in Table 5 and 6 provides the current and future condition hours and days of
inundation in half -foot increments up to Elevation 12, and in 1-foot increments from Elevations
13 to 16, for four time periods respectively. The time periods are identical to those provided for
Units A and B. The table also provides the corresponding plant zones proposed for each
elevation. The proposed plantings for Unit E are shown in the attached drawings WM11 and
WM12.
Similar to the analysis for Units A and B, there is little change in the hours of inundation
between the current and future conditions. The most significant change between the two
conditions occurs in area less than 8.5 feet in elevation which occur at the connection with
Springbrook Creek. Changes are higher elevations are on the order of 1 percent of the period
or less.
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Table 5 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites
under Current Conditions - Upstream of SW 34th Street
Hours (days) Inundated
Growing
March - May
Plant Zone
Elevation
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
8
8,764
5,866
2,208
2,208
(365.2)
(244.4)
(92.0)
(92.0)
Wetland Shrub #2
8.5
3,908
1,711
1,034
258
(162.8)
(71.3)
(43.1)
(10.8)
Wetland Shrub #2
g
2,250
798
474
100
(93.8)
(33.3)
(19.8)
(4.2)
Wetland Shrub #1
9.5
1,604
516
300
62
(66.8)
(21.5)
(12.5)
(2.6)
Wetland Shrub #1
10
1,105
326
189
37
(46.0)
(13.6)
(7.9)
(1.5)
Wetland Shrub #1
10.5
759
206
117
24
(31.6)
(8.6)
(4.9)
(1.0)
Wetland Shrub #1
11
508
128
72
15
(21.2)
(5.3)
(3.0)
(0.6)
Wetland Shrub #1
11.5
337
77
43
9
(14.0)
(3.2)
(1.8)
(0.4)
Wetland Shrub #1
12
214
44
23
5
(8.9)
(1.8)
(1.0)
(0.2)
Upland (Forest or
13
66
12
7
1
Riparian)
(2.8)
(0.5)
(0.3)
(0.0)
Upland (Forest or
14
14
3
2
0
Riparian)
(0.6)
(0.1)
(0.1)
(0.0)
Upland (Forest or
15
2
0
0
0
Riparian)
(0.1)
(0.0)
(0.0)
(0.0)
Upland (Forest or
16
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
Upland (Forest or
20
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
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Table 6 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites
under Future Conditions - Upstream of SW 34th Street
Hours (days) Inundated
Growing
March - May
Elevation
Plant Zone
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
8
8,763
5,866
2,208
2,208
(365.1)
(244.4)
(92.0)
(92.0)
Wetland Shrub #2
8.5
3,619
155
900
242
(150.8)
(164.6)
(37.5)
(10.1)
Wetland Shrub #2
g
2,241
807
466
106
(93.4)
(33.6)
(19.4)
(4.4)
Wetland Shrub #1
9.5
1,645
547
310
69
(68.5)
(22.8)
(12.9)
(2.9)
Wetland Shrub #1
10
1,178
364
203
45
(49.1)
(15.2)
(8.5)
(1.9)
Wetland Shrub #1
10.5
845
(35.2)
244
(10.2)
133
(5.5)
30
(1.3)
Wetland Shrub #1
11
592
160
86
20
(24.7)
(6.7)
(3.6)
(0.8)
Wetland Shrub #1
11.5
413
105
55
13
(17.2)
(4.4)
(2.3)
(0.5)
Upland (Forest or
12
274
64
33
8
Riparian)
(11.4)
(2.7)
(1.4)
(0.3)
Upland (Forest or
13
96
20
9
2
Riparian)
(4.0)
(0.8)
(0.4)
(0.1)
Upland (Forest or
14
24
5
3
0
Riparian)
(1.0)
(0.2)
(0.1)
(0.0)
Upland (Forest or
15
4
1
0
0
Riparian)
(0.2)
(0.0)
(0.0)
(0.0)
Upland (Forest or
16
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
Upland (Forest or
20
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
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Congestion Relief & Bus Rapid Transit Projects
Table 7 — Predicted Percent Inundation of Wetland Mitigation Sites
under Current Conditions — Upstream of SW 34th Street
Percent Inundation
Growing
March - May
Elevation
Plant Zone
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
8
100%
100%
100%
100%
Wetland Shrub #2
8.5
45%
29%
47%
12%
Wetland Shrub#2
9
26%
14%
21%
5%
Wetland Shrub #1
9.5
18%
9%
14%
3%
Wetland Shrub #1
10
13%
6%
9%
2%
Wetland Shrub #1
10.5
9%
4%
5%
1 %
Wetland Shrub #1
11
6%
2%
3%
1 %
Wetland Shrub #1
11.5
4%
1 %
2%
0%
Upland (Forest or
12
2%
1%
1%
0%
Riparian)
Upland (Forest or
13
1%
0%
0%
0%
Riparian)
Upland (Forest or
14
0%
0%
0%
°
0%
Riparian)
Upland (Forest or
15
0%
0%
0 /°
°
/° 0
Riparian)
Upland (Forest or
16
0%
0%
0%
0 /° °
Riparian)
Upland (Forest or
20
0%
0%
0 /0
°
/° 0
Riparian)
1 year is approximately 8,760-',,hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
20050829 Springbrook Wwetland Restoration Memo editted.doc 13
Congestion Relief & Bus Rapid Transit Projects
Table 8 — Predicted Percent Inundation of Wetland Mitigation Sites
under Future Conditions — Upstream of SW 34th Street
Elevation
Percent` Inundation
GrowingMarch
Season
—May
Plant Zone
(Feet)
Annual
(March -
(Early growing
June - August
October)
season)
8
100%
100%
100%
100%
Wetland Shrub #2
8.5
41 %
26%
41 %
11 %
Wetland Shrub #2
9
26%
14%
21 %
5%
Wetland Shrub #1
9.5
19%
9%
14%
3%
Wetland Shrub #1
10
13%
6%
9%
2%
Wetland Shrub #1
10.5
10%
4%
6%
1 %
Wetland Shrub #1
11
7%
3%
4%
1 %
Wetland Shrub #1
11.5
5%
2%
2%
1 %
Upland (Forest or
12
3%
1%
1%
0%
Riparian)
Upland (Forest or
13
1%
0%
0%
0%
Riparian)
Upland (Forest or
14
0%
0%
0%
0%
Riparian)
Upland (Forest or
15
0%
0%
0% °
°
0 /o
Riparian)
Upland (Forest or
16
0%
0%
0%
0%
Riparian)
Upland (Forest or ''
20
0%
0%
0%
0%
Riparian)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
The event -stage -duration analyses of Springbrook Creek at Units A and B indicate that:
• Areas below 8 feet in elevation will be permanently inundated. The areas at this elevation
are associated with the Springbrook Creek stream channel at Unit E.
• Areas between 8 and 8.5 feet in elevation will be inundated approximately 26 to 29 percent
(approximately 71 to 165 days) of the growing season and almost half of the early growing
season. The depth of water at the site will range from 0 to 0.5 feet.
• Areas from 8.5 to 9 feet in elevation will be inundated 14 percent of the growing season
(approximately 33 days) and approximately a fifth of the early growing season. The depth of
water will range from 0 to 1 foot.
20050829 Springbrook Wwetland Restoration Memo editted.doc 14
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• Areas from 9 to 10.5 feet in elevation will be inundated 4% of the growing season
(approximately 8.5 to 10 days). Depth of water will range from 0 to 2.5 feet.
• Elevations above 10.5 feet in elevation will be inundated less than 4 percent (8.5 to 10 days)
of the growing season and about a tenth of the early growing season. Depth of water will
range from 0 to 7 feet at its deepest point. This point occurs for approximately 1 hour during
the growing season.
Saturation
The event -stage -duration analysis described above is capable of predicting water surface
elevations associated with Springbrook Creek hydrology. The analysis is not able to account for
temporary or permanent saturation of the soils at Unit E.
Soil saturation is based on capillary rise, which is a measure of the soil's ability to draw water
above the water surface elevation. The amount of rise depends on the soil properties and the
space (pores) between the individual particles of soil. Contiguous, small pores fill to a higher
elevation above the water table than larger pores. Soils richer in organic matter draw water to a
higher elevation than sandy, gravelly soils. Sandy soils with effective pore diameters of 0.01
centimeters (cm) can draw water 6 inches above the water table. A loam with effective pore
space half that size (0.005 cm) draws water 12 inches above the water table. Some organic
soils draw water 18 inches or more above the water table.
Soil saturation information can typically be obtained from soil surveys or other similar sources.
In this case, much of the material at the site is fill from previous construction activities. Recent
geotechnical investigations at the site suggest that much of the material at the proposed grading
elevations is expected to be silts with peat, silts to sandy silt, and brown peat with organic silts.
The fill material at the site is generally characterized as silty sand and sand. It is assumed from
this analysis that the materials found over the majority of the site after construction will be
mostly silts capable of drawing water up 12 inches or more. Silty sand and sand fill materials on
the side slopes of the wetland will be amended with organic material to establish the necessary
planting and will be capable of drawing water 12 inches above the water table. This assumption
is consistent with the Washington State Wetland Identification and Delineation Manual (Ecology,
1997) which states that for soil saturation to affect vegetation it must occur within the major
rooting zone of the prevalent vegetation (usually within 12 inches of the surface).
Hydrologic Zones
With the event -stage -duration analysis and the saturation component established, the expected
hydrologic zones can be identified. Table 9 shows the expected hydrologic zones adapted by
the Corps of Engineers Wetland Delineation Manual (1987) from Clark and Benforado (1981).
The information presented in Table 9 was based on the percent inundation information and
adding 12 inches to the elevation to account for the influence of saturation.
The following example is provided for clarity: From Table 9, the elevation corresponding to
6 percent inundation during the growing season is 10 feet. Adding 12 inches to the elevation
provides saturated soils to Elevation 11 for approximately 6 percent of the growing season. The
following table was constructed following this procedure.
20050829 Springbrook Wwetland Restoration Memo editted.doc 15
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Table 9 — Unit E Hydrologic Zones
Zone* Name Duration"* Elevation Range***
It Permanently inundated 100% Below 8.0 feet
II Semi -permanently to nearly >75 % - <100%
permanently inundated
III Regularly inundated >25% - 75% Areas up to elevation 9.5
IV Seasonally inundated >12.5% - 25% Areas up to elevation 10.0
V Irregularly inundated 5% - 12.5% Areas between 11 and 11.5
feet in elevation.
VI Intermittently or never <5% Areas above 11.5 feet in
inundated elevation
" Zones adapted from Clark and Benforado (1981).
** Refers to duration of inundation and/or soil saturation during the growing season.
*** Assumes 12 inches of capillary rise
t This defines an aquatic habitat zone.
Based on the hydrologic zones for Unit E (Table 9) and plant zones listed in Tables 5 though 8,
wetland hydrology can be provided up to Elevation 11.5 feet. The lower elevations of Wetland
Shrub Community #2 will be regularly inundated to seasonally inundated. Wetland Shrub
Community #1 will be regularly inundated to irregularly inundated. Upper elevations of the
wetland shrub community are shown to be intermittently inundated, while the lower elevations of
uplands are inundated less than intermittently or never inundated.
11.2 Continuous Duration Analysis
A second analysis was performed to address how long will Unit E will be continuously saturated.
A continuous duration analysis is not required for Units A and B since wetland hydrology already
exists on site.
The analysis examined four individual storm events at Unit E to determine the duration of typical
Springbrook Creek flows at specific elevations. The storms represent conditions that can be
expected during the early growing season (March through May and during the remaining portion
of the growing season (June through October). Selection of these events was based on
identifying the 2-year return period and the annual return period (1.01 percent exceedance).
Figures 2 through 5 provide information about the continuous inundation that can be expected
for an individual storm.
20050829 Springbrook Wwetland Restoration Memo editted.doc 16
Congestion Relief & Bus Rapid Transit Projects
Table 10 — Peak Flows for Selected Typical Storm Events
SW 34th Street
Return Period
Early Growing Season
Growing Season
(March — May)
(March - October)
146 cfs
231 cfs
1.01 years
(March 1996)
(June 2001)
274 cfs
385 cfs
2 years
(May 2000)
(August 2001)
cfs= cubic feet per second
Upstream SW 34th Street and Oakesdale Avenue SW, March - May
• March 11, 1996 with a peak simulated flood flow of 146 cfs, representing a moderate, more
frequent event that is expected to be equaled or exceeded approximately 99 percent of the
time.
• May 10, 2000 with a peak simulated flood flow of 274 cfs, representing a 2-year return
period expected to be equaled or exceeded once every 2 years on average.
Upstream SW 34th Street and Oakesdale Avenue SW,_ June — October
• June 11, 2001 with a peak simulated flood flow of 231 cfs, representing a moderate, more
frequent event that is expected to be equaled or exceeded approximately 99 percent of the
time.
• August 22, 2001 with a peak simulated flood flow of 380 cfs, representing a 2-year return
period expected to be equaled or exceeded once every 2 years on average.
Figures 2 through 5 show that frequent events will inundate areas up to an elevation of 8.5 feet
(corresponding to a saturated elevation of 9.5 feet) for durations that range from 93 hours (3.9
days) to 157 hours (6.5 days). Surface flows will typically take 2 to 3 days to recede. The
cumulative time corresponding to the inundation of the site to the recession of water from the
site is expected to last from 5.9 days to 9.5 days.
It is anticipated that in areas up to 9 feet in elevation (corresponding to a saturated elevation of
10.0 feet), frequent events will last from 23 hours (0.9 days) to 65 hours (2.7 days). Flows will
typically take 2.5 to 3.5 days recede. The cumulative time would range from 3.4 days to 6.2
days.
Areas up to Elevation 10.5 (corresponding to a saturated elevation of 11.5 feet) could typically
be inundated from 10 hours (0.4 days) to 36 hours (1.5 days) in duration. Flows would typically
take 3.5 to 4.5 days to recede. The cumulative time corresponding to the inundation of the site
to the recession of water from the site is expected to last from 3.9 days to 6.0 days.
20050829 Springbrook Wwetland Restoration Memo editted.doc 17
Congestion Relief & Bus Rapid Transit Projects
While none of these storms last for 12.5 percent of the growing season (roughly 30 days), it is
expected that the wetland areas would be frequently inundated or saturated to provide sufficient
water to establish wetland hydrology between 5 percent and 12.5 percent of the growing season
It should be noted that the slow recession of water from Unit E should allow any fish seeking
refuge during higher flows ample time to respond and remain in deep water, provided that the
sites are graded to drain toward Springbrook Creek and avoid any isolated water that is not
hydrologically connected to Springbrook Creek.
III. Summary and Conclusion
To receive mitigation credit, Units A and B were designed to provide a more effective hydrologic
connection to Springbrook Creek by providing a number of breaches through the levee. The
design and success of Unit E depends on inundation and saturation by Springbrook Creek
flows. Success is defined as the ability to provide the inundation' and duration sufficient to
establish vegetation typically adapted for life in saturated soil conditions.
To address inundation and duration during the growing season as well as other seasonal
periods, an HSPF program was used to model Springbrook Creek current and future hydrology
conditions at locations adjacent to Units A, B, and E. Continuous inundation is addressed by
the inspection of typical storm events and the duration of inundation at specific elevations.
Analysis of Units A and B demonstrates that modifying the existing levee with breaches will
provide hydrologic connections between the existing wetlands and Springbrook Creek, augment
the wetland hydrology in Units A and B, increase stream interaction with the floodplain, and
potentially reduce the elevation and duration of Springbrook Creek peak flows.
Analysis of Unit E provides the '"hours "``of inundation for specific time periods, their associated
elevations, and the percentage of period inundated for the current and future flows upstream of
SW 34th Street. Proposed elevations of Unit E are based upon inundation and saturation
during the growing season. The analysis indicates that areas up to 10.5 feet in elevation are
seasonally inundated and areas up to 11.5 feet in elevation are irregularly inundated. Areas
higher than 11.5 feet in elevation' are intermittently or never inundated and therefore would likely
be considered uplands.
Four individual simulated events were selected to represent typical storms that can be expected
during the early growing season (March through May) and during the remaining portion of the
growing season (June through October). Inspection of the storm events shows that areas up to
9.5 feet in elevation (0 to 1.5 feet deep) are inundated or saturated for approximately 4 to 6.5
days. Areas up to 10.5 feet in elevation (0 to 2.5 feet) are inundated or saturated for
approximately 1 to 2.5 days. Areas up to Elevation 11.5 (0 to 3.5 feet deep) are inundated or
saturated for approximately 0.5 to 1.5 days. Flows will typically take 2.5 to 4.5 days to recede.
20050829 Springbrook Wwetland Restoration Memo editted.doc 18
Congestion Relief & Bus Rapid Transit Projects
References
Clark, J. R., and Benforado, J., eds. 1981. Wetlands of Bottomland Hardwood Forests,
Proceedings of a Workshop on Bottomland Hardwood Forest Wetlands of the
Southeastern United States. Elsevier Scientific Publishing Company, New York.
Environmental Laboratory. (1987). "Corps of Engineers Wetlands Delineation Manual,"
Technical Report Y-87-1, U.S. Army Engineer Waterways Experiment Station,
Vicksburg, MS.
Washington State Department of Ecology, 1997. Washington State Wetlands Identification and
Delineation Manual. Publication #96-94.
Hartley, David and Derek Stuart, 2004, Hydrologic Analysis for Floodplain Mapping Study of
Springbrook Creek, King County, Washington. n^h"or�hwest Northwest hydFaU'*G Hydraulic
Consultants. Memorandum. Tukwila, Washington.
20050829 Springbrook Wwetland Restoration Memo editted.doc 19
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DURI.DAT
1
Duration analysis operation no. 1
DURANL OF FLOW D1
Start date: 1948/ 9/30 24: 0 End date: 1998/ 9/30 24: 0
Analysis season starts: 12/31 24: 0 Ends: 12/31 24: 0
1
FRACTION OF TIME EACH LEVEL EQUALED OR EXCEEDED WITH DURATION >= THE SPECIFIED
DURATIONS
FRACTION IS RELATIVE TO TOTAL TIME SPAN
EVENTS GREATER THAN
DURATIONS
1
LEVELS
5.824
0.1937E-02
5.960
0.1814E-02
6.096
0.1666E-02
6.233
0.1545E-02
6.369
0.1419E-02
6.505
0.1321E-02
6.641
0.1228E-02
6.778
0.1145E-02
6.914
0.1043E-02
7.050
0.9674E-03
7.186
0.9149E-03
7.323
0.8282E-03
7.459
0.7689E-03
7.595
0.7164E-03
7.731
0.6662E-03
7.868
0.5978E-03
8.004
0.5636E-03
8.140
0.5065E-03
8.277
0.4814E-03
8.413
0.4472E-03
EVENTS LESS THAN
DURATIONS
1
LEVELS
5.824
0.9981
5.960
0.9982
6.096
0.9983
6.233
0.9985
6.369
0.9986
6.505
0.9987
6.641
0.9988
6.778
0.9989
6.914
0.9990
7.050
0.9990
7.186
0.9991
Page 1
1
DURI.DAT
7.323
0.9992
7.459
0.9992
7.595
0.9993
7.731
0.9993
7.868
0.9994
8.004
0.9994
8.140
0.9995
8.277
0.9995
8.413
0.9996
UNDEFINED EVENTS (NO WATER)
DURATIONS
1
0.000
SUMMARY
TOTAL LENGTH OF DEFINED EVENTS: 438288. INTERVALS
TOTAL LENGTH OF UNDEFINED EVENTS: 0. INTERVALS
TOTAL LENGTH OF ANALYSIS: 18262.00 DAYS
SAMPLE SIZE: 438288
SAMPLE MAXIMUM: 17.86
SAMPLE MINIMUM: 0.000
SAMPLE MEAN: 0.1586
SAMPLE STANDARD DEVIATION: 0.6195
Page 2
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Congestion Relief & Bus Rapid Transit Projects
August 31, 2005
TO: Bank Oversight Committee (BOC)
FROM: Springbrook Bank Project Team
- Beth Peterson
- Hans Ehlert
REm�E®
SEP " 2005
CITY OF RENTON
UTILITY SYSTEMS
Subject: Springbrook Creek Review Materials for BOC Meeting September 20, 2005
Included in this packet are the additional materials, as indicated in our previous packet, we are
submitting for the BOC to review in advance of the September 20, 2005 meeting. The meeting
will be held at the USACE's office. Included in this packet are:
• Unit C Water Balance Memorandum
• Springbrook Creek Wetland Restoration Memorandum (for Units A, B and E)
• Tables of wetland function attributes for each unit
We look forward to a productive meeting on September 20, 2005.
AM'
GWashington State
, Department of Transportation
Congestion Relief & Bus Rapid Transit Projects
August 31, 2005
Subject: Springbrook Wetland and Habitat Mitigation Bank:
Unit C Water Balance Memorandum
Purpose and Findings
This memorandum describes water balance calculations for the proposed wetland re-
establishment area at Unit C as part of the Springbrook Wetland and Habitat Mitigation
Bank. The purpose of the water balance calculations is to identify whether sufficient
water is available to establish wetlands. The findings from this analysis are:
• Water pumped from the 180th Street grade separation site will extend the period of
available wetland hydrology within the wetland re-establishment area at Unit C.
• Preliminary results of the geotechnical investigation at Unit C indicate that infiltration
rates could range from 0.0012 to 0.06 inches per day.
If the infiltration rate is 0.06 inches per day, water pumped from the 180th Street
grade separation pump station could extend the period of available wetland
hydrology by approximately 1 week in the late spring and late summer.
If the infiltration rate is 0.0012 inches per day, water pumped from the 180th
Street grade separation pump station could extend the period of available
wetland hydrology by approximately 1 month at late spring and late summer.
• To extend the period of available wetland hydrology, the design will incorporate
some type of amendment to the bottom of the wetland re-establishment area to
decrease the infiltration rate to be more consistently 0.0012 inches per day or less.
Methodology
Water balance calculations are performed by estimating all inflows to the wetland and all
outflows from the wetland. The total inflows are compared to the total outflows. When
the inflows exceed the outflows, water would be available to support wetland
establishment. When the outflows exceed the inflows, some type of wetlands may still
be supported.
The calculations were conducted based on monthly averages. Components of the
inflows and outflows and data sources used in the analysis are presented below.
Inflows
The Unit C inflow components considered in this analysis consist of: (a) pumped flow
from the 180th Street grade separation pump station, (b) direct precipitation falling on
Springbrook Wetland Habitat and Mitigation Bank:
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
the wetland, (c) stormwater runon from areas upstream of Unit C, and (d) soil moisture
uptake when the groundwater is relatively high.
• Pumped Flow — The 180th Street grade separation pump station began operating
during the summer of 2003. Hours of operation for each pump are recorded monthly
by Tukwila operations staff. Pump hour data were obtained from June 2003 through
May 2005. However, data were not collected each month. For months in which no
data were obtained, the values were interpolated using a straight-line graph. Figure
1 shows a graph of calculated pump flows based on the pump hours.
Tukwila Pump Station Data
90000
70000
60000
50000
Pump 1
Pump 2
40000
>
30000-
20000
10000„
r
x
0
M
O
M
O
M
O
O O O O O O
Lr)
O O
Lr)
O
O
N
O
N
O
N
O O O O O O
N N N N N N
O O
N N
O
N
Q0
0000
O
N N tD 00 O
N N
Dates with Data
Figure 1 — Tukwila Pump Station Data
Volume of water pumped was computed by multiplying the recorded pump hours by
the design flow rate of the pumps.
Through October 2003, the flow from each pump was approximately 10,000
cubic feet per month.
Throughout 2004, the flow from each pump increased to between 40,000 and
50,000 cubic feet per month.
Springbrook Wetland Habitat and Mitigation Bank: 2
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
° In 2005, the pumped flows were almost 80,000 cubic feet per month in February,
but dropped to around 50,000 cubic feet per month in April. In May, the flows
increased to over 60,000 cubic feet per month.
For the water balance analysis, pumped flow was assumed to be the same as for
2004 because the 2003 data appear to be low and not enough data were collected in
2005 to justify using the slightly higher values from 2005. The calculated pumped
flow from each pump was added together, and the total was converted to a depth of
water over the proposed Unit C wetland creation area (13.8 acres).
• Precipitation — The precipitation used in the analysis is average monthly
precipitation for a 53-year period recorded at Sea-Tac International Airport.
Runon — Approximately 80 acres of land directly south of Unit C drain to Unit C.
Two-thirds of the area is undeveloped and one-third is developed and impermeable.
Evapotranspiration and infiltration in the upstream area is expected to be higher
during the summer months; therefore, it was assumed that 20 percent of the
precipitation would flow to Unit C from October through April and 10 percent of the
precipitation would flow to the unit from May through September. The precipitation
used is the average monthly precipitation from the 53-year period described above.
Soil Moisture Uptake — Soil moisture uptake is groundwater that is available to the
wetland. The City of Renton has three piezometers installed in Unit C. Data from
the two piezometers nearest the wetland re-establishment area were evaluated. Data
were collected by the City between June 30, 1997 and May 11, 2001. Data were
collected on 20 dates. Ten data points occurred in 1997, five in 1998, one in 2000,
and 4 in 2001. The data from the groundwater monitoring at the two piezometers are
shown in Figure 2. Most of the recorded points (12) occurred between November
and February, outside the typical growing season. Water levels recorded at the two
piezometers were averaged monthly. The soil moisture uptake in inches was
calculated to be the monthly average water level in inches above Elevation 15.
Elevation 15 was selected for comparison because it is 1 foot lower than the
proposed wetland bottom. For monthly average water levels below Elevation 15, no
inflow contribution was calculated from soil moisture uptake.
Springbrook Wetland Habitat and Mitigation Bank:
Unit C Water Balance Memorandum
Congestion Relief b Bus Rapid Transit Projects
1s
18
17
12
10
18.48
18.17
17.37 .; 17.38
16.88
16.63 16.43
16.23
1ss
15 53
15 JB 15.48 15.53 ,y. 15.33 4.2
15.08
u.1a14 �
13.78 13.62 .7
13.43 13.46
ze
12.83 13.02 12. 12.78
12.49
j 12.31
12.03 '
1 J'.62
G W-08
10.33
rn m `8i `Si �i `� rn w rn m rn m rn m w m rn rn m rn rn rn rn E o 0 0 0 0
rn m m m rn w w m rn m rn m rn m rn m m rn m rn rn rn rn � o 0 0 0 0
n m •- e n o � � � m rn o � � � � a A C `� a cO can c � � in a �n
m
Date Water Levels Were Read
Figure 2 - Groundwater Monitoring Data
Outflows
Outflow components considered in the analysis consist of evapotranspiration and
infiltration. Surface water discharges were not evaluated. The basis of each component
is discussed below.
• Evapotranspiration - Wetland evapotranspiration is well represented by
approximately 0.70 to 0.80 percent of the Class A Pan Evaporation from a nearby
open site. Monthly Class A Pan Evaporation values from the Seattle Maple Leaf
Reservoir were obtained from the U.S. Department of Agriculture (USDA) Natural
Resources Conservation Service. The monthly pan evaporation values were
converted to evapotranspiration by multiplying by 0.80.
• Infiltration - A Geotechnical Investigation of Unit C is being conducted by Hart
Crowser. Preliminary results of the investigation indicate that infiltration rates at the
site could range from 0.0012 to 0.06 inches per day. Monthly infiltration rates were
calculated by multiplying the days per month by the infiltration rates, using both the
low and high estimated rates.
Springbrook Wetland Habitat and Mitigation Bank: 4
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
Results
Results of the inflow/outflow calculations are presented in Table 1 and are shown
graphically in Figures 3 and 4. Figure 3 depicts outflow conditions at the higher
infiltration rate (0.06 inches per day) and Figure 4 depicts outflow conditions at the lower
infiltration rate (0.0012 inches per day). Total inflow with and without pump contribution
is plotted on both figures to assess whether the pump contribution is significant.
Table 1 - Estimated Monthly Inflows and Outflows of Unit C (in inches)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Pump
1.80
1.65
1.75
1.86
1.97
2.07
1.95
1.83
1.71
1.89
1.86
2.10
Precipitation
5.62
4.16
3.73
2.52
1.64
1.46
0.76
1.09
1.71
3.46
5.98
5.83
Runon
6.52
4.82
4.32
2.92
0.95
0.85
0.44
0.63
0.99
4.01
6.93
6.76
Soil Moisture
1
21
6
5
1
0
0
0
0
0
0
8
Total Inflow 14.93 31.64 15.80 12.30 5.56 4.37 3.15 3.56 4.41 9.36 14.77 ZZ.69
Infl. w/out pump 13.14 29.98 14.05 10.44 3.59 2.31 1.20 1.72 2.70 7.47 12.91 20.59
Eva otrans iration
1 0.46
1 0.70
1 1.39
1 2.51
3.717
4.04
1 5.34
1 4.22
1 2.78
1 1.32
1 0.56
0.42
Infiltration
6.2
5.8
6.2
6
12.4
12
12.4
12.4
12
6.2
6
6
Total Outflow 6.66 6.50 7.59 8.51 16.11 16.04 17.74 16.62 14.78 7.52 6.56 6.4Z
Springbrook Wetland Habitat and Mitigation Bank: 5
Unit C Water Balance Memorandum
r
Congestion Relief & Bus Rapid Transit Projects
35.00
30.00
25.00
20.00
.0
V
15.00
10.00
5.00
0.00
s Inflow with pump
k:e
__Outflow (high
------------- ---------------------
infiltration rate)
Inflow without pump
__prdxlmately 1: Approximately 1
k;extension In week extension inetland hydrology wetland hydrology- --------- -------------------------- -- -- r
Jan Feb Mar Apr May Jun Jul Aug Sepi Oct Nov Dec
Month
Figure 3 — Comparison of Inflows and Outflows
(high infiltration rate = 0.06 in/day)
Springbrook Wetland Habitat and Mitigation Bank:
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
35.00
30.00
25.00
20.00
t
= 15.00
10.00
5.00
0.00
—s Inflow with pump
Outflow (low infiltration rate)
Inflow without pump
,
Approximately 5 Approximately 3
week extension of; weep extension of
wetland hydrology, wetland hydrology
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 4 — Comparison of Inflows and Outflows
(low infiltration rate = 0.0012 in/day)
Springbrook Wetland Habitat and Mitigation Bank: 7
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
Hydrologic Regime
With the water balance established, the expected hydrologic regime can be identified.
The percent duration of inundation was calculated based on the percent of the growing
season where inflow was greater than outflow, and shown in Table 2 for both the low
and high infiltration rates, and with and without the flow from the 180th Street grade
separation pump.
Table 2 — Percent Duration of Inundation of Unit C Wetland Re-establishment Area
The introduction of flow from the 180th Street grade separation pump will extend the
duration of inundation in the wetland re-establishment area. The hydrologic regime
identified is adapted by the Corps of Engineers Wetland Delineation Manual (1987) from
Clark and Benforado (1981). The hydrologic regime in the wetland re-establishment
area will primarily be a regularly inundated wetland based on a duration of inundation or
saturation between 25% and 75% of the growing season.
Conclusions and Recommendations
When inflow exceeds outflow, water is considered to be available to support wetland
hydrology. When outflow exceeds inflow, water may be unavailable to support wetland
hydrology. The analysis indicates that outflow generally exceeds inflow from May
through September without pumped water from the 180th Street grade separation pump
station. The addition of water to Unit C from the pump is available to support wetland
hydrology at the site, and the re-establishment will be classified as a regularly inundated
wetland. Figure 3 shows that at the higher estimated rate of soil infiltration (0.06 inches
per day), the inflow almost matches the outflow during May and September when the
pumped flow is added. At this higher infiltration rate, the period of available wetland
hydrology is extended by approximately a week in both May and September. Figure 4
shows that at the lower estimated rate of soil infiltration (0.0012 inches per day), outflow
exceeds inflow only during July and August. At this lower infiltration rate, the pumped
flow extends the period of available wetland hydrology by about 5 weeks in the late
spring and 3 weeks in late summer.
The following actions are recommended to further confirm the results of this analysis:
• Identify engineering measures required to limit infiltration at Unit C to approximately
0.0012 inches per day.
Springbrook Wetland Habitat and Mitigation Bank:
Unit C Water Balance Memorandum
Congestion Relief & Bus Rapid Transit Projects
• Continue to obtain pump hour measurements from the 180th Street grade separation
pump station. New data may confirm that more water is available from the pump
station than is included in the analysis.
• Existing on -site piezometers are being monitored to validate the groundwater levels.
Springbrook Wetland Habitat and Mitigation Bank:
Unit C Water Balance Memorandum
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E VALLF_Y RO
Wetland Functions Will Substantially Increase at the Springbrook Creek Mitigation Bank
Water quality improvement, hydrologic, and habitat functions are expected to increase substantially following wetland re-
establishment, rehabilitation, and enhancement at the Springbrook Creek Mitigation Bank. The design team compared pre -and post -
construction wetland functions to demonstrate the expected functional "lift". The attributes examined are based on Method for
Assessing Wetland Functions (Hruby et al., 1999) and the Wetland Rating System for Western Washington (Hruby, 2004). These
methods include comprehensive lists of attributes that contribute to wetland function.
In general, work performed at the Springbrook Bank will improve hydrologic regimes, diversify vegetation communities, add more
habitat structures, and increase wetland size. The following four tables summarize the existing and proposed wetland function
attributes at Springbrook Bank. Existing and proposed conditions are rated qualitatively using three categories (Low, Moderate, and
High).
Table 1. Existing and Proposed Function Attributes for Springbrook Creek Mitigation Bank —Units A and B (Riverine)
Function attribute
I Existing Condition
I Mitigation Work
Proposed Condition
WATER QUALITY IMPROVEMENT FUNCTION ATTRIBUTES
Vegetation classes
Low to Moderate — 2 to 3 vegetation
Mow, apply herbicide, create
Moderate to High — Forested and scrub -shrub
classes present (Unit A: forested,
micro -topography, add mulch,
vegetation classes will replace large reed
scrub -shrub, emergent; Unit B:
and plant trees and shrubs in
canarygrass emergent wetlands. Establishing
forested, emergent).
large areas dominated by reed
tree and shrub classes in the riparian areas
canarygrass, including riparian
may help improve water quality in
areas.
Springbrook Creek.
Understory vegetation
Low to Moderate — Limited
Plant native trees and shrubs.
Moderate — Area and complexity of
understory development
understory vegetation will increase.
Width ratio of wetland to
Low — Wetland is 30 to 70 times the
Breach 20-foot berm sections
Moderate to High — Reconnecting the
stream
stream width, but is hydrologically
next to Springbrook Creek:
wetland floodplain to Springbrook Creek
disconnected from Springbrook
- 3 breaches in Unit A;
substantially increases the effective width
Creek.
- 4 breaches in Unit B.
ratio of wetland to stream.
HYDROLOGIC FUNCTIONS ATTRIBUTES
Storage capacity
Low to Moderate — Wetland has
Breach berm sections next to
High — Breached berm will allow floodwater
potential to store large volumes of
Springbrook Creek.
from Springbrook Creek to more easily enter
stormwater. Low opportunity to store
the wetland, increasing available flood storage
flood water because berms prevent
capacity.
connection.
Size ratio of wetland to
Low — Wetlands represents a small
Breach berm sections next to
Low — The effective floodplain area will be
basin
portion of total basin area.
Springbrook Creek.
increased, but that increase is relatively small
compared to the basin drainage area.
Ratio of wetland to stream
High — Stream extends total length of
Breach berm sections next to
High — No change in ratio, but connectivity
wetland, but connectivity is low.
S rin brook Creek.
will be substantially increased.
Cover by woody vegetation
Low to Moderate — Woody
Plant native trees and shrubs.
High — Overall cover by woody vegetation
vegetation covers approximately 33
will increase in wetlands dominated by reed
percent of Unit A and 60 percent of
canarygrass, including riparian areas.
Unit B.
HABITAT FUNCTIONS ATTRIBUTES
Buffer condition
Low — Buffers are relatively narrow
Within a 40-foot buffer screen
Moderate — Weed removal and plantings will
and disturbed.
around the perimeter of Units A
improve buffer condition.
and B, remove weeds and plant
native trees and shrubs. Plant
native trees and shrubs in
riparian enhancement area.
Canopy closure over
Varies from Low to Moderate —
Mow, apply herbicide, create
High — Overall canopy closure by woody
wetlands
Woody vegetation covers
micro -topography, add mulch,
vegetation will increase in reed canarygrass
approximately 33 percent of Unit A
and plant trees and shrubs in
removal areas.
and 60 percent of Unit B.
large areas dominated by reed
canarygrass.
Canopy closure over stream
Low — Very little woody vegetation
Mow, apply herbicide, add
High — Canopy closure over stream will
present along stream
mulch and jute matting; plant
increase. Woody vegetation will replace reed
trees and shrubs along riparian
canarygrass.
corridor currently dominated by
reed canarygrass.
Number of vegetation strata
Moderate — 3 strata present (tree,
Plant native trees and shrubs to
Moderate to High — Tree and shrub strata
shrub, herb).
increase number of strata in
will replace the herb layer in large areas
large areas dominated by reed
dominated by reed canarygrass.
canarygrass.
Number of snags
Low — Few or no snags.
Install vertical snags throughout.
High — Number of snags will substantially
increase.
Number of LWD
Low — Little or no LWD.
Install large woody debris and
High — Amount of LWD and number of brush
brush piles throu hout.
piles will substantially increase.
Vegetation interspersion
Moderate — Most of the areas have a
Install native trees and shrubs
Moderate — Increase vegetation interspersion
moderate degree of interspersion.
and create micro -topography in
with structurally complex boundaries by re -
large areas dominated by reed
habilitating forested and scrub -shrub wetlands
canarygrass.
and enhancing riparian u .lands.
Number of hydrologic
Moderate — 3 hydrologic regimes
Excavate small areas to create
Moderate — No change to number of
regimes
(seasonally saturated, occasionally
micro -topography in reed
hydrologic regimes, but the micro -topography
inundated, seasonally inundated).
canarygrass removal areas.
will increase the wetland area with the various
hydrologic regimes.
Number of water depth
High — 2 depth classes (0-8", 8-40").
Excavate small areas to create
High — No change to number of depth classes,
classes
micro -topography in reed
though complexity will increase by creating
canarygrass removal areas.
micro -topography.
Species richness
Low — Between 4 to 8 species
Plant up to 5 additional species
Moderate — Native species richness will
present, depending on area. No
in reed canarygrass removal
increase.
conifers are present.
areas.
Mature woody vegetation
Moderate — Areas of mature woody
Retain existing mature woody
High — Plantings will provide more mature
vegetation are present.
vegetation. Plant native trees and
woody vegetation as the site becomes
shrubs in large areas dominated
established.
by reed canarygrass.
SummaryFunctions by Unit—rev8-31-05.doc August2005
Table 2. Existine and Proposed Function Attributes for Springbrook Creek Mitigation Bank —Unit C (Devressionab
Function attribute
I Existing Condition
I Mitigation Work
Proposed Condition
WATER QUALITY IMPROVEMENT FUNCTION ATTRIBUTES
Vegetation classes
Moderate — 3 vegetation classes
Create forest/scrub-shrub in
Moderate — Same number of vegetation
present (deciduous forest, scrub-
wetland re-establishment area;
classes, but improved composition (deciduous
shrub, emergent).
Create forest/scrub-shrub
forest, mixed forest, scrub -shrub). Reed
wetlands in reed canarygrass
canarygrass-dominated emergent wetlands
removal areas; Create mixed
will be replaced by forest/scrub-shrub. Mixed
forest by underplanting
forest will be created by underplanting
coniferous trees in deciduous
conifers. Area of forest/scrub-shrub will be
forest.
enlarged in wetland re-establishment area.
Understory vegetation
Low to Moderate — Limited
Plant native trees and shrubs,
Moderate — Area and complexity of
understory development. Forested
and conifers in the understory of
understory vegetation will increase.
areas have shrub understory that is
existing deciduous forest.
largely Himalayan blackberry.
Area seasonally inundated
Moderate — Wetland C-1 has some
No action planned to increase
Moderate to High — Additional wetlands
areas of seasonal inundation
area seasonally inundated.
onsite will provide substantial new area of
seasonal inundation.
HYDROLOGIC FUNCTIONS ATTRIBUTES
Storage capacity
Moderate — Wetland has capacity to
Excavate fill to re-establish
Moderate — Newly created wetlands will
store additional water.
forest/scrub-shrub wetlands.
increase storage capacity.
Size ratio of wetland to
Moderate to High — Wetland
Excavate fill to re-establish
Moderate to High — The wetland area will be
basin
represents approximately 15 percent
forest/scrub-shrub wetlands.
increased, but that increase is relatively small
of sub -basin drainage area.
compared to the sub -basin drainage area.
Cover by woody vegetation
Varies from Low to High — Woody
Plant native trees and shrubs
High — Overall cover by woody vegetation
vegetation covers approximately 83
will increase.
percent of the wetlands in Unit C.
HABITAT FUNCTIONS ATTRIBUTES
Buffer condition
Moderate —Buffers are greater than
Plant native trees and shrubs
Moderate — Plantings will improve buffer
100 meters for 50% of wetland.
within a 40-foot buffer screen
condition.
along Oakesdale Ave. SW and
portion of BNSF property to the
south.
Canopy closure
Varies from Low to High — Woody
Plant native trees and shrubs.
High — Overall canopy closure by woody
vegetation covers approximately 83
vegetation will increase.
percent of the wetlands in Unit C.
Number of vegetation strata
Moderate — Three strata present
Plant native trees and shrubs.
Moderate — Tree and shrub strata will
(tree, shrub, herb).
Underplant native coniferous
develop within the wetland re-establishment
trees in deciduous forest.
and reed canarygrass removal areas. Conifers
underplanted in deciduous forest will
contribute to near -term development of the
sub -canopy stratum.
Number of snags
Low to Moderate — Few or no snags
Install vertical snags throughout.
High — Number of snags will substantially
in Wetlands C-2/C-3; up to 4 classes
increase.
of snags in Wetland C-1.
Number of LWD
Varies from Low to High — Little or
Install large woody debris and
High — Amount of LWD and number of brush
no LWD in Wetlands C-2/C-3, 6
brush piles throughout.
piles will substantially increase.
classes of snags present in Wetland
C-1.
Vegetation interspersion
Low to Moderate — Most areas have
Excavate fill and plant
High — Increase vegetation interspersion with
low to moderate degree of
forest/scrub-shrub in wetland re-
structurally complex boundaries by re -
interspersion
establishment area;
establishing, rehabilitating, and enhancing
Establish forest/scrub-shrub
forested and scrub -shrub wetlands.
wetlands in reed canarygrass
removal areas; Create mixed
forest by underplanting conifers
in deciduous forest.
Number of hydrologic
Moderate — 3 hydrologic regimes
Re-establish new wetland area.
Moderate — No change to number of
regimes
(seasonally saturated, occasionally
Excavate micro -topography in
hydrologic regimes, but the wetland area with
inundated, seasonally inundated).
the wetland re-establishment and
the various hydrologic regimes will be
reed canarygrass removal areas.
increased.
Number of water depth
Moderate — 2 depth classes (0-8", 8-
Re-establish new wetland area.
Moderate — No change to number of water
classes
40").
Excavate micro -topography in
depth classes, but the wetland area with the
the wetland re-establishment and
depth classes will be increased.
reed can ass removal areas.
Species richness
Moderate — From 6 to 8 species
Plant 10 additional wetland
High — Native species richness will increase.
present, depending on area. Few
native tree and shrub species.
conifers are present.
Area seasonally inundated
Moderate — Wetland C-1 has areas of
Excavate historic fill expanding
Moderate to High — Additional wetlands
seasonal inundation
wetland area onsite and
onsite will provide substantial new area of
providing additional areas with
seasonal inundation.
seasonal inundation.
Mature woody vegetation
Moderate — Areas of mature woody
Retain existing mature woody
High — Plantings will provide more mature
vegetation are present.
vegetation. Plant native trees and
woody vegetation as the site becomes
shrubs throughout.
established.
Summary Functions by Unit—rev8-31-05.doc August 2005
Table 3. Existine and Proposed Function Attributes for Springbrook Creek Mitieation Bank —Unit D (Denressionall
Function attribute
I Existing Condition
I Mitigation Work
Proposed Condition
WATER QUALITY IMPROVEMENT FUNCTION ATTRIBUTES
Vegetation classes
Moderate — 3 vegetation classes
Create mixed forest by
Moderate to High — Increased number of
present (deciduous forest, scrub-
underplanting coniferous trees in
vegetation classes and improved composition
shrub, emergent).
deciduous forest.
(deciduous forest, mixed forest, scrub -shrub,
emergent). Mixed forest will be created by
underplanting conifers.
Understory vegetation
Low — Limited understory
Plant native conifers in the
Moderate — Area and complexity of
development
understory of existing deciduous
understory vegetation will increase.
forest.
Storage capacity
Moderate — Wetlands that exist
No action planned to increase
Moderate — No change anticipated
onsite store water.
storage capacity
Area seasonally inundated
Moderate- Wetland D-1 large areas
Add additional water from 180th
Moderate- Additional water may extend
of seasonal inundation
Street grade separation project.
length of seasonal inundation
HYDROLOGIC FUNCTIONS ATTRIBUTES
Storage capacity
Moderate — Wetland has capacity to
No action planned to increase
Moderate — No change.
store stormwater.
storage capacity.
Size ratio of wetland to
Moderate — Wetland represents
No action planned to size of
Moderate — No change.
basin
approximately 5 percent of the sub-
wetland to basin.
basin drainage area
Cover by woody vegetation
Moderate — Woody vegetation
Underplant native coniferous
Moderate — No change to percentage of
covers approximately 60 percent of
trees in deciduous forest.
woody vegetation, but composition will
the wetland in Unit D.
improve.
HABITAT FUNCTIONS ATTRIBUTES
Buffer condition
Low — Buffers are narrow and
No action planned to improve
Low — No change.
disturbed.
buffer condition.
Canopy closure
Moderate — Woody vegetation
Underplant native coniferous
Moderate — No change in canopy closure, but
covers approximately 60 percent of
trees in deciduous forest.
composition will improve.
the wetland in Unit D.
Number of vegetation strata
Moderate — 3 strata present (tree,
Underplant native coniferous
Moderate — Conifers underplanted in
shrub, herb).
trees in deciduous forest.
deciduous forest will contribute to near -term
development of the sub -canopy stratum.
Number of snags
Low — Few snags in Unit D.
No action planned to increase
Low — No change.
the number of snags.
Number of LWD
Low — Little LWD in Unit D.
Install brush piles in the wetland
Moderate — Brush piles will increase the
enhancement area.
amount of downed wood.
Vegetation interspersion
Low — Unit D has low degree of
No action planned to increase
Low — No change.
interspersion.
vegetation interspersion.
Number of hydrologic
Moderate — 3 hydrologic regimes
Supplement hydrology with
Moderate — Hydroperiod may be extended.
regimes
(seasonally saturated, occasionally
surface water from 180th Street
inundated, seasonally inundated).
grade separation project.
Number of water depth
Moderate — 2 depth classes (0-8", 8-
Supplement hydrology with
Moderate — No increase to the number of
classes
40").
surface water from 180th Street
depth classes, but the hydroperiod may be
grade separation project.
extended.
Species richness
High — 15 species present in Unit D.
Plant 3 native coniferous tree
High — Native species richness will improve.
No conifers are present.
species to increase native plant
diversity.
Area seasonally inundated
Moderate — Unit D has relatively
Add additional water from 180th
Moderate — Additional water may extend
large areas of seasonal inundation
Street grade separation project.
length of seasonal inundation.
Mature woody vegetation
Moderate — Areas of mature woody
Retain existing mature woody
Moderate — Conifer plantings will provide
vegetation are present.
vegetation. Underplant native
mature woody vegetation.
coniferous trees in deciduous
forest.
SummaryFunctions by Unit_rev8-31-05.doc August 2005
Table 4. Existing and Proposed Function Attributes for Springbrook Creek Mitigation Bank —Unit E (Proposed: Riverine)
Function attribute
I Existing Condition
I Mitigation Work
Proposed Condition
WATER QUALITY IMPROVEMENT FUNCTION ATTRIBUTES
Vegetation classes
Low — No wetlands are present in
Excavate fill and plant native
Moderate — The re-established wetland will
Unit E. The riparian uplands include
trees and shrubs in wetland re-
include two vegetation classes (deciduous
three vegetation classes (deciduous
establishment area;
forest and scrub -shrub). Mixed upland forest
forest, scrub -shrub, emergent).
Plant native trees and shrubs in
will be created by underplanting conifers.
riparian enhancement area;
Create mixed forest by
underplanting coniferous trees in
u land deciduous forest.
Understory vegetation
Low — Limited understory
Plant native trees and shrubs,
Moderate — Area and complexity of
development.
and conifers in the understory of
understory vegetation will increase.
existing upland deciduous forest.
Width ratio of wetland to
Low — No wetlands are present in
Excavate fill and plant native
High — The re-established floodplain wetland
stream
Unit E.
trees and shrubs in wetland re-
will be approximately 45 meters wide
establishment area.
(approximately 11 times the stream width) and
connected to Springbrook Creek.
HYDROLOGIC FUNCTIONS ATTRIBUTES
Storage capacity
Low — No wetlands are present in
Excavate fill to re-establish
High — Re-established wetland will increase
Unit E.
wetland and connect to
storage capacity onsite.
S rin brook Creek.
Size ratio of wetland to
Low — No wetlands are present in
Excavate fill to re-establish
Low — 8 acres of wetland will be re -
basin
Unit E.
wetland and connect to
established. This represents a small portion of
S rin brook Creek.
total basin area.
Ratio of wetland to stream
Low — No wetlands are present in
Excavate fill to re-establish
High — Re-established wetland will extend the
Unit E.
wetland and connect to
length of stream within Unit E.
S rin brook Creek.
Cover by woody vegetation
Low — Uplands in Unit E have
Plant native trees and shrubs.
High — Overall cover by woody vegetation
approximately 40 percent cover of
will increase.
woody vegetation.
HABITAT FUNCTIONS ATTRIBUTES
Buffer condition
Moderate — The riparian buffer is
Plant 40-foot buffer screen along
Moderate to High — Plantings will improve
more than 50 meters wide and is
Oakesdale Ave. SW and
buffer condition.
disturbed by off -road vehicles.
northern edge with native trees
and shrubs. Plant native trees
and shrubs in riparian
enhancement area.
Canopy closure over
Low — Uplands in Unit E have
Plant native trees and shrubs.
Moderate — Overall canopy closure over
wetlands
approximately 40 percent cover of
wetlands by woody vegetation will increase
woody vegetation.
substantially.
Canopy closure over stream
Moderate to High — The majority of
Plant additional native trees and
High — Riparian plantings will increase cover
the stream has mature woody
shrubs in riparian areas.
over Springbrook Creek.
vegetation.
Number of vegetation strata
Low — No wetlands are present in
Plant native trees and shrubs.
High — Tree and shrub strata will develop
Unit E.
within the wetland re-establishment area.
Conifers underplanted in upland deciduous
forest will contribute to near -term
development of the sub -canopy stratum.
Number of snags
Low — No snags in uplands in Unit E.
Install vertical snags throughout
High — Number of snags will substantially
re-established wetlands and
increase.
u land buffers.
Number of LWD
Low — Little LWD in uplands on Unit
Install large woody debris and
High — Amount of LWD and number of brush
E.
brush piles throughout re-
piles will substantially increase.
established wetlands and upland
buffers..
Vegetation interspersion
Low — No wetlands are present in
Excavate fill and plant native
High — Increase vegetation interspersion with
Unit E, therefore it has a no wetland
trees and shrubs in wetland re-
structurally complex boundaries by re -
interspersion. Interspersion of upland
establishment area;
establishing forested and scrub -shrub
vegetation is low.
Plant native trees and shrubs in
wetlands, and enhancing riparian uplands.
riparian enhancement area;
Create mixed forest by
underplanting coniferous trees in
upland deciduous forest.
Number of hydrologic
Low — No wetland hydrology is
Excavate fill to re-establish
High — Wetlands will be re-established with 4
regimes
present in Unit E.
wetland area. Excavate micro-
hydrologic regimes (intermittently flooded,
topography in the wetland re-
temporarily flooded, seasonally flooded, and
establishment area.
semipermanently flooded).
Number of water depth
Low — No inundated areas are present
Excavate fill to re-establish
High — Wetlands will be re-established with 2
classes
in Unit E.
wetland area. Excavate micro-
water depth classes (0-8", 8-40").
topography in the wetland re-
establishment area.
Species richness
Low — No wetlands are present in
Plant 10 wetland native tree and
High - Native species richness will
Unit E. Uplands contain a mixture of
shrub species, and 11 upland
substantially increase.
native and non-native, disturbance
native tree and shrub species.
tolerant species.
Mature woody vegetation
Moderate — Mature woody
Retain existing mature woody
High — Plantings will provide more mature
vegetation present over small area.
vegetation. Plant native trees and
woody vegetation as the site becomes
shrubs throughout.
established.
References
Hruby, T. 2004. Washington State Wetland Rating System for Western Washington — Revised. Washington State Department of Ecology
Publication # 04-06-025. htti)://www.ecy.wa.gov/pubs/0406025.-odf
Hruby, T, T. Granger, K. Brunner, S. Cooke, K. Dublancia, R. Gersib, L. Reinelt, K. Richter, D. Sheldon, E. Teachout, A. Wald, and F.
Weinmann. July 1999. Methods for Assessing Wetland Functions Volume I. Riverine and Depressional Wetlands in the Lowlands of Western
Washington. Washington State Department of Ecology Publication #99-115. http://www.eca.gov/programs/sea/wfai)/index.html
SummaryFunctions by Unit_rev8-31-05.doc August 2005
Congestion Relief & Bus Rapid Transit Projects
600 — 108th Avenue NE, Suite 405
Bellevue, WA 98004
Main 425-456-8500
Fax 425-456-8600
Springbrook Creek Wetland Mitigation and
Habitat Bank
Fisheries Review
November 2005
White Paper
..
Washington State
�AP Department of Transportation
Congestion Relief & Bus Rapid Transit Projects
Springbrook Creek Wetland and Habitat Mitigation Bank, Fish Strading Review
Fish Stranding
Problem Statement: Proposed Units A and B of the Springbrook Creek Wetland Mitigation and
Habitat Bank (Springbrook Bank) have the potential to strand fish more often by connecting the
Units to Springbrook Creek than under the existing Springbrook Creek floodplain conditions.
Resolution Outcome: The levee breaches in Units A and B restore natural floodplain habitat,
which is one of the most biologically important habitat types in the region for fish. Fall Chinook
and other salmon are not prone to stranding. The natural topography of the sites does not
cause nor does it increase the levels of fish stranding. The levee breaches are likely to result in
a decrease in stranding, an increase in juvenile salmon growth rates, and an increase in ocean
survival.
1.0 Introduction
The Washington State Department of Transportation (WSDOT) is developing the Springbrook
Bank in partnership with the City of Renton. Springbrook Bank consists of five units, totaling
129.91 acres, which represent some of the last remaining large tracts of undeveloped land in
the Lower Green River Watershed (WSDOT, 2005).
The goals for the Springbrook Bank include: (a) developing wetland habitat, (b) improving
aquatic habitat characteristics such as hydrology and water quality, and (c) enhancing fish
refuge. While the project's effects on fish life were discussed during development of the
Mitigation Banking Instrument (MBI), fish habitat has not been a primary design consideration
other than for regulatory compliance. The regulatory agencies need assurance that the
Springbrook Bank will not harm Endangered Species Act (ESA) listed, as well as other fish
species populations that inhabit or migrate through the Springbrook Bank area.
This white paper reviews the potential impacts on fish that will result from breaching the
Springbrook Creek levees. The review is limited to mitigation Units A and B. The Springbrook
Bank includes three other Units: C, D, and E. Units C and D are wetland areas without surface
water connections to any fish -bearing streams, and these Units are not located near fish -
bearing streams. Once constructed, Unit E will be adjacent to and have a surface water
connection with Springbrook Creek. Even though Unit E includes side channels and small
amounts of floodplain habitat, it is not included in this review because the existing site will be
completely reconstructed and will have no depressions or habitat that could strand fish. All of
the wetland area slopes into the low flow channel, which will have a year-round surface water
connection to Springbrook Creek.
In the attempt to recover valuable salmon stocks, the most important habitat types (floodplains
and wetlands) are often targeted for restoration because it is believed that they provide the
greatest benefit, both qualitatively and quantitatively. Extreme environmental variation during
flood events, however, limits our knowledge of floodplain relationships to fish. Regardless,
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decision makers rely on the best available science to steer them into restoration projects and
then they must rely on known results from other scientific studies to make calculated guesses
about the potential outcome of a restoration project. Through monitoring efforts, the project
results (effects on fish life) can eventually be tallied and compared either to other studies or the
expected results to determine success. Regulators must utilize similar resources to assess the
potential project impacts and whether or not to permit the project. One intention of this paper is
that it serve as a resource for the regulatory agencies when they are assessing biological
impacts from the Springbrook Bank project.
2.0 Springbrook Creek Watershed
The Springbrook Watershed drains 15,763 acres and is the largest sub -basin in the lower Green
River Basin. It has 12.0 miles of main stem stream, 19.1 miles of tributaries, and 3.8 miles of
drainage ditches with natural flows. Three primary streams make up the watershed:
Springbrook Creek, Mill Creek, and Garrison Creek. Prior to 1994, the amount of accessible
fish habitat was approximately 5.9 miles within Springbrook Creek, 6.6 miles in Mill Creek, and
5.2 miles in Garrison Creek. Mill Creek and Garrison Creek are tributaries to Springbrook Creek.
The Black River Pump Station (BRPS) separates Springbrook Creek and the Black River and
functions as the confluence of Springbrook Creek into the Black River.
The BRPS is located in Renton, Washington, upstream of the Black River and Green River
confluence. The existing pump station was built in 1972 by the U. S. Soil Conservation Service
and is currently operated and maintained by the King County Flood Hazard Reduction Services
Group, Water and Land Resources Division. The pump station completely spans the stream
channel, effectively terminating Springbrook Creek before it enters the Green River, leaving a
1,700-foot remnant channel of the Black River.
The BRPS is strategically located to protect developed areas of Renton on the upstream side of
the structure from flooding. The hydraulic differences at the BRPS between upstream and
downstream water levels often result in a higher downstream than upstream water surface; this
is known as a reverse water level scenario. A unique fish passage facility was constructed to
pass fish around the facility during the reverse water level scenario and during normal flow
levels.
3.0 Units A and B Breaches
Springbrook Bank Units A and B are existing open, natural spaces that encompass 62.8 acres,
of which 55.5 acres are currently wetlands. Unit A has 22.9 acres of wetland and Unit B has
32.6 acres of wetlands. Units A and B lie opposite each other on the banks of Springbrook
Creek. Each site has almost 1,300 feet of stream bank. Springbrook Creek, which flows south
to north, is entirely linear between Units A and B and has been channeled between levees for
flood protection. Currently, each site is drained by a single stream outlet. The outlet stream at
Unit B shows evidence of beaver activity; that is, small beaver dams. The outlet streams are
incised and narrow through the levee areas. Both have a channel width of less than 5 feet and
a wetted perimeter during normal flows of less than 2 feet.
The proposed design for Springbrook Bank includes three breaches for Unit A and four
breaches for Unit B. See Figure 1 for the location of each breach. The three breaches for Unit A
are nearly equidistance apart along the Springbrook levee. The existing outlet channel at the
extreme southern end of the site will remain and continue to function as a natural outlet from the
wetland area. Unit B will have four new breaches in the levee. The breaches are not
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equidistance apart, but they will be constructed at regular intervals along the levee. The
existing outlet channel will remain in place in the northern half of the site.
Each breach will be approximately 20 feet wide. The levee elevation will be reduced to match
that of the existing ground behind the levee, which is the existing elevation of the wetland
boundary. The streambed of each breach will be flat across the levee area and then will slope
waterward at a steady angle. The breach opening will be set back at least 20 feet from the
edge of the existing stream. The stream bank characteristics will represent a mix of upland
habitat (the levee) transitioning into a riparian -stream bank plant community, and wetland
habitat transitioning into a riparian -stream bank plant community habitat. The breaches will
convey flood waters from Springbrook Creek into the wetland mitigation sites and then serve as
outlet drains when the flood waters drain off the wetland. The breaches are not being designed
to serve as natural instream habitat. S�
The breaches will allow a more natural cycle of flood pulsing into the wetlands that also serve as
floodplains for Springbrook Creek. The existing levees are designed for flood protection up to
the 25-year flood event (14.5 feet elevation) and in some locations up to the 100-year event
(15.0 feet in elevation). The breach(s) elevation will be at 12.0 feet. This represents a regular
cycle of floodwater influence every 1 to 2 y a_ �`..' According to the Hydrological Simulation
Program — Fortran (HSPF) model, the 2-year flood event for the mitigation sites is 12.8 feet.
See Table 1 for the hydraulic application of HSPF at Units A and B. For reference purposes,
the Federal Emergency Management Agency (FEMA) 100-year elevation regulatory flood level
for the Springbrook Creek floodplain area is 19.8 feet.
Table 1: Springbrook Wetland Mitigation Bank Units A and B Flood Event Metrics
Inundation Time
Flood Event,
Elevation
Flood Elevation
Units A and B
(feet)
(feet)
(hours)
2-year
12.81 11
12.0
77
(breach elevation)
5-year
13.1
13.0
21
10-year
13.8 ( I� 21)
14.0
3
25-year
14.5 (16. �
100-year
15.0 61
'Using the HSPF Model("LIJU-c7)
4.0 Fisheries Management Background
The King County Flood Hazard Reduction Services Group, Water and Land Resources Division,
collects upstream adult migration data (See Table 2) at the BRPS. Neither the operators nor
the equipment can detect which species moves through the BRPS, only that an adult -size fish
passed by the electronic counter. There is minimal downstream passage data and no species
information for the out -migrants (King County, 2005 and Table 3). Harza (1995) completed a
study that provides information on species use of Springbrook Creek, including that of adult and
juvenile Chinook salmon. Harza (1995) documented other anadromous and resident species,
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including coho salmon, steelhead, and lamprey. The focal species for this white paper is fall
Chinook salmon because of its federal status as threatened under the Endangered Species Act
(ESA).
Table 2: Black River Pump Station Upstream Adult Fish Counts
Year'
Adult Count
Year
Adult Count
1983-84
155
1994-95
268
1984-85
119
1995-96
355
1985-86
47
1996-97
206
1986-87
82
1997-98
265
1987-88
166
1998-99
84
1988-89
95
1999-2000
395
1989-90
77
2000-01
463
1990-91
69
2001-02
594
1991-92
107
2002-03
114
1992-93
291
2003-04
191
1993-94
120
2004-05
307
'No species identified 1983-84 to 2004-05
Table 3: Washington Department of Fish and Wildlife Hatchery Coho Salmon
Supplementation in the Springbrook Creek Basin, 1995 to 2005
Year
Coho fry plants,
(Age 0+)
Juvenile fish out -migration counts;
species and age undetectable
1995
130,000
No data
1996
100,000
No data
1997
96,000
No data
1998
172,000
No data
1999
233,000
1,340
2000
0
1,773
2001
350,000
1,075
2002
297,000 1
2,320
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Year
Coho fry plants,
(Age 0+)
Juvenile fish out -migration counts;
species and age undetectable
2003
280,000
1,751
2004
368,000
1,807
2005'
0
1,689
'Salmon recovery management strategies excluded hatchery supplementation
Historically, the proximity of the Springbrook Bank to the lower Green River system, which is a
year round juvenile salmon staging area used before entering salt water, made the area easily
accessible for fall Chinook and other salmon stocks. Low elevation and low gradient streams
near brackish waters are common refuge habitat for salmon, even if it is not their natal stream.
The primary months for this type of refuge behavior are September through June. Even though
juvenile salmon occupy the lower Green River on a year-round basis, the high water
temperatures and low dissolved oxygen (DO) levels of the lower Springbrook Creek during the
summer low flow periods currently exclude salmon use (Harza, 1995). Currently, when the fish
passage facilities are operating, the BRPS presents a formidable upstream migration barrier for
juveniles. Additionally, King County does not pass fish upstream during the summer months.
The fall Chinook life history cycle includes adults entering lower Springbrook Creek during the
fall (October to December), spawning in the fall (late October to January), fry emergence in
early spring (February to March), and out -migration during the spring months (March to June).
Upon emergence, fall Chinook fry head downstream and enter the estuary or brackish water
areas as age 0+. Their survival depends on reaching the estuary areas before the warm
summer temperatures cause mortality. Chinook juveniles prefer to occupy deep water pools
while resting in the stream.
Other Important Species
Coho life history is somewhat different than that of fall Chinook salmon. Coho adults usually
enter the streams after the Chinook. Peak spawning is often 1 to 2 months later than that of
Chinook, although the timing for juvenile emergence is similar. Once juvenile coho salmon
emerge, they head upstream (Wydoski and Whitney, 2004) to seek cover and refuge in cool
waters for rearing during the warm summer months. Juvenile coho salmon out -migrate from
their natal streams as age 1+ from late March through early June. During flood events and
during normal flow conditions, coho are known to prefer alcove habitat with extensive cover
(Bell et al., 2001 and Cederholm et al., 1978) over backwater areas (Units A and B). (While this
white paper does not focus on Unit E, this information supports the habitat concept for Unit E.)
Coho have been the most abundant salmon species in Springbrook Creek in recent years
(WDFW, 2005), primarily due to an extensive hatchery supplementation program (Table 3) by
the Washington Department of Fish and Wildlife (WDFW). As of 2005, hatchery plants ceased,
so adult hatchery returns are likely to decrease. The fisheries management intent is to manage
the Springbrook Creek basin through natural production. Coho are likely to remain an important
focal species for salmon recovery purposes and possibly for recreational or tribal harvest.
Steelhead exhibit perhaps the most complex life history traits of any species of Pacific salmonid.
They can be anadromous or freshwater residents (rainbow trout). Anadromous steelhead can
spend up to 7 years in fresh water before becoming a smolt, then spend up to 3 years in salt
water prior to first spawning. Steelhead may also exhibit iteroparity, or spawning more than
5
1-405 Transportation Corridor Projects Amok
Springbrook Bank 7 Depaniagton St
artment ofTransportation
Fisheries Review
once in a lifetime. Other than these factors, steelhead follow a generalized life history, which
includes incubation and hatching of embryos; emergence and initial rearing of juveniles in fresh
water; migration to oceanic habitats for extended periods of feeding and growth; and return to
natal waters for completion of maturation and spawning. Steelhead smolts typically migrate to
sea between age 2+ and age 3+, spend 2 years rearing in the ocean, then return to fresh water
as 4- or 5-year-old adults.
Historical steelhead population estimates are not available for Springbrook Creek. During fish
surveys in the Springbrook Creek basin in 1993 and 1994, Harza (1995) documented only a few
steelhead adults. Project biologists found adult rainbow trout and smaller Oncorhynchus mykiss
that could have been juvenile steelhead, but did not locate any steelhead redds or find any
spawned -out carcasses. They speculated that water quality (temperature, DO, and toxic
chemicals) limited steelhead presence. Regardless, it is likely that most of the benefits
identified for fall Chinook salmon apply to juvenile steelhead and rainbow trout.
5.0 Key Findings - Breaching Impacts on Salmon
The potential number of juvenile fall Chinook salmon that are vulnerable to stranding at the
Springbrook Bank site would be very difficult to measure with the current data and the natural
habitat variability. However, an interpretation of the likelihood of the increase or decrease in the
number of fish and frequency of stranding has been estimated. Additionally, the benefit of
restoring the floodplain is provided. This white paper explains how and why floodplains play a
key role in fish productivity and diversity of riverine communities and it identifies a number of
benefits that will help in the salmon recovery effort in Springbrook Creek.
The number of adult fall Chinook that pass upstream through the BRPS is undocumented other
than in the fall of 1994, when Harza (1995) counted a total of 14 adult and jack fall Chinook
salmon that passed upstream through the BRPS. The fish were tagged for tracking purposes
during spawning activities and only one tagged carcass was recovered. No fall Chinook redds
were found. Harza (1995) also reported that no fall Chinook juveniles were found during their
summer low flow (electroshocking) or during the outmigration studies at the BRPS. The lack of
data prevents any valid quantitative estimate of stock production, smolt production, and survival
to adulthood estimates, or habitat use estimates.
Biologists, regulators, and various project proponents often raise legitimate questions about the
value to salmon stocks of restoring floodplains. Up to now there has been a poor scientific
response, because very few large-scale projects in the western United State have been
implemented, monitored, and then report on the impacts to salmon. Several floodplain studies
on spawning and rearing functions have been conducted (Welcomme, 1979; Gutreuter et al.,
2000; Sommer et al.; 1997; Halyk and Balon, 1983; Ross and Baker, 1983; Gehrke, 1992; and
Winemiller and Jepsen, 1998). However, very few studies focus on growth, survival, or
stranding, three important life history variables that are needed to assess long-term impacts
from breaching and floodplain restoration. Two studies (Sommer et al., 2005 and Sommer et
al., 2001) have striking similarities to the Springbrook Bank situation regarding impacts to
salmon species. Both studies focus on fall Chinook salmon and both have similar floodplain
conditions and habitat types that can be compared to the Springbrook Bank. Both studies
clearly demonstrate the benefits of restoring floodplain habitat for fall Chinook salmon. Much of
the following discussion regarding fall Chinook salmon is based on the study described in
Sommer et al., 2005 and Sommer et al., 2001; however, as noted above, several other studies
on floodplain functions have identified numerous benefits not only to salmon, but to other fish
species as well.
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1-405 Transportation Corridor Projects Alll•
Springbrook Bank Ti Washington State
Department of Transportation
Fisheries Review
Based on study results (Sommer et al., 2005 and Sommer et al., 2001) juvenile Chinook salmon
do not appear to be especially prone to stranding mortality and that survival to adulthood may
actually be enhanced by floodplain rearing. During a flood event, juvenile salmon, like many
small fishes, seek out shallow water areas with low velocities (Everest and Chapman, 1972;
Roper et al., 1994; Bradford and Higgins, 2001). Juvenile fall Chinook are vulnerable to
predation and physiological stress because they are only 30 to 70 millimeters (mm) in length
(usually less than a finger length). Remaining in the perennial stream, such as Springbrook
Creek, exposes them to higher predation rates (Ward and Stanford, 1995), fewer forage
opportunities, and a reduced growth rate when compared with moving to the adjacent
floodplain.
The concern about stranding fish in habitat that is lower in elevation than the breaches (Table 1)
was addressed by studying the typical behavior of juvenile Chinook salmon during flood events
(Sommer et al., 2005 and Sommer et al., 2001). Juvenile Chinook were tagged and released
upstream and downstream of the floodplain breaches. Control groups were sampled as they
moved through the contiguous waters adjacent to the floodplain in the estuary. In addition,
control groups were sampled in the following years during ocean fisheries. The latter control
group was used to compare survival to adulthood between Chinook that used the floodplain
during out -migration and those that did not. Ultimately, it was survival data from coded wire
tags that allowed biologists to differentiate significant differences between habitat types.
Enough samples were obtained during the ocean fisheries to determine that, at least for these
two groups, survival of the floodplain users was at least as high for one group, and was
significantly higher in another group.
Sampling was conducted on a variety of habitat types within the floodplain: riparian, wetland,
upland, and isolated ponds. Substrate types were also noted: sand, mud, gravel, pavement,
vegetation, and vegetation clumps. Water velocity was recorded to note the different ranges
preferred by juvenile fall Chinook salmon over each habitat type. Juvenile Chinook were
captured in all types of habitat and in all types of substrate. There was no statistically significant
difference between habitat or substrate type. The fish were distributed throughout the entire
floodplain. Their association to a habitat type was dissimilar to instream habitat preferences.
There were no obvious preferences for pools, heavy vegetation, or deep water habitats (i.e.,
depression zones). Other studies show similar results and that juveniles remain relatively
mobile and avoid stranding during natural high water events on floodplains (Higgins and
Bradford, 1996; Bradford, 1997).
Several pieces of evidence suggest that the majority of fish emigrated successfully from the
floodplain. At Springbrook Units A and B, the amount of potential pond habitat is small relative
to the overall area during flood events. The floodplain that Sommer et al., 2005 sampled had 12
percent potential stranding habitat. The amount of depression zones or isolated pond habitat at
Springbrook Units A and B (Table 4) is comparable to the Sommer et al., 2005 study site. This
is significant for the Springbrook Bank project, because it validates results of the Sommer et al.,
2005 study by comparison. Given the even distribution across all habitat types on the
floodplain, it was not surprising to find no evidence of higher densities in isolated pond areas or
depression zones compared with the densities in contiguous water sources during the flood or
during the drainage. See Table 4 for the amount of potential stranding habitat in Units A and B.
Another key finding was that the floodplain drains efficiently despite the presence of natural
topographical features and levees that could impede drainage and fish emigration.
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1-405 Transportation Corridor Projects AfO► -hing
Springbrook Bank T waDepton State artmartment of Transportation
Fisheries Review
Table 4: Topographic Features; Number of Acres of Potential Stranding Habitat and
Percent of Total Area
Topographic
Elevation'
Unit A
(26.3 acres)
Unit B
(36.5 acres)
(feet)
acres
percent
acres
percent
11.5
6.2
23.6
4.6
12.6
11.0
0.8
3.0
1.5
4.1
10.0
0.02
0.08
0.2
0.5
'Levee breaches are at 12.0 feet elevation. Any elevation less than 12.0 feet may result in
isolated pond habitat.
During a flood event, the density of juvenile fish use appears to be highly variable between a
floodplain and its adjacent contiguous waters, such as a Springbrook Creek. Fish sampling is
often difficult because of the amount of habitat to sample. Biologists in one study found a
significantly higher (P < 0.0001; randomized t-test) level of densities in ponds that are controlled
by managed outlet weirs compared with uncontrolled drainage from the floodplain. This finding
suggests that artificial water control structures can create unusual hydraulics that promote
stranding (Sommer et al., 2005), which supports the concept of breaching to allow the natural
exchange of flood waters with the floodplain behind the levees at Units A and B. Biologists also
found higher densities during the drainage period, which provides evidence of an extended
period of use for rearing, another benefit that increases growth and size of fish.
Studies showed that fish are forced off of the floodplain during drainage, which is natural,
compared with the inability to retreat when there are extensive levees or controlled weirs. This
is a key finding regarding the timing of fish exiting the floodplain. Once the waters started to
recede naturally, the fish were cued to leave. Since juvenile Chinook are distributed equally
across the floodplain area the chance of stranding is reduced. Stranding potential is also
avoided, as the juvenile fish do not prefer deep water habitat over another type (i.e., a
depression zone). Some of the more obvious environmental cues to leaving the floodplain
appear to be velocity related, mean water depth (Sommer et al., 2005), and possibly barometric
pressure.
Two of the most important indirect benefits to salmon when occupying the floodplain appear to
be significant growth and long-term survival. Several studies, including those of Sommer et al.
(2005 and 2001), clearly documented that the amount of forage opportunity is much greater in
the floodplain compared with that in the adjacent perennial stream systems. There is a higher
availability of drift invertebrates in the floodplain (Sommer et al., 2001). Gladden and Smock
(1990) found that invertebrate production was one to two orders of magnitude greater on the
floodplain than in adjacent streams. The density of food items was superior, even in heavily
used agricultural zones. Short and long-term survival for salmon is often associated with size of
the fish. The increased food availability, along with slightly warmer temperatures, result in a
larger fish that will leave the floodplain. Sommer et al. (2005 and 2001) showed that there is a
significant difference in size; the authors speculate that this contributes to higher survival rates
to adulthood.
Water temperature affects juvenile salmon using the floodplain. The fish -use periods during the
year that coincide with high water are usually during the fall, winter, or spring. At those times of
8
1-405 Transportation Corridor Projects AW.
Washington State
Springbrook Bank �� Department of Transportation
Fisheries Review
the year, normal and flood event stream temperatures remain within the tolerance range for
most of the cold water species, including salmon. Some flood events may even lower the water
temperature in the stream and on the floodplain. Additionally, research shows (Sommer et al.,
2001; Wilbur, 1997) that the increases in water temperature on the floodplain generate a higher
metabolic rate for the juvenile fish, which is utilized to seek out more food items, digest them,
and incorporate that energy into higher growth rates. Higher growth rates result in increased
size and subsequently survival.
Bird predation on the floodplain during flood events has also been studied extensively. Usually,
flooding increases the amount of habitat available to wading birds, which in turn significantly
reduces the density of wading birds. Habitat use studies show that fish rear everywhere,
reducing vulnerability to bird predation. Predation by piscivorous fish probably occurs, but at a
reduced rate compared with the adjacent contiguous stream habitat (Brown, 2002). Biologists
found that the amount of refuge habitat increases, thereby reducing the exposure of salmon to
predatory species. Increased turbidity levels in streams have a similar effect. In either case
flooding actually decreases the probability of an encounter with a predator.
Summary of Key Findings
• The project does not create any stranding habitat.
• The project decreases the potential for fish stranding in the floodplain.
• Fall Chinook and the other fish species that utilize floodplain habitat have a higher survival
rate than fish that utilize only perennial streams.
• Fall Chinook are not prone to stranding on a floodplain despite various topographical
features such as isolated ponds.
• Fall Chinook grow faster on floodplain habitat.
• During high water, juvenile salmon distribution is equal across all habitat types on a
floodplain.
• Fall Chinook do not concentrate in isolated ponds.
• Warmer temperature increase metabolic rates that lead to increased growth.
• Bird predation is minimal on the floodplain because salmon rear everywhere.
6.0 Conclusion
A thorough literature review was conducted to address fish stranding impacts for fall Chinook
salmon, but it becomes obvious that the benefits for fish life from the proposed Springbrook
Bank will go beyond Chinook salmon. Study after study demonstrates how reconnecting the
floodplain to an adjacent stream or river has benefits for almost all of the fish species (Brown,
2002). The opportunity to enhance rearing habitat and then indirectly enhance growth is not
exclusive to salmon. In fact, the floodplain provides enhanced spawning, rearing, and foraging
opportunities for resident species such as cyprinids, in addition to a lesser complement of
benefits for rainbow trout, cutthroat trout, and sculpin. All of these species are present in
Springbrook Creek.
There is no evidence that the Springbrook Bank project will increase fall Chinook salmon
stranding or any other species stranding. In fact, it should decrease stranding. Under the
current levee structure, each time Springbrook Creek floods over the top there is likely to be a
number of fish that occupy the floodplain area behind the levees. Sommer et al. (2005) found
that most of these fish will not make it back to the creek because of unnatural drainage timing
and barriers. The proposed breaches will facilitate a natural cycle of flood pulsing which will
reduce stranding when compared to the current habitat conditions.
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1-405 Transportation Corridor Projects TA1111..
Springbrook Bank T WashingtonDepartment of T ansportation
Fisheries Review
As salmon fry disperse during out -migration or during a flood event, their choice of habitat may
ultimately determine their survival. Migratory fish frequently have alternative life histories that
may be influenced by habitat use in early life stages. Salmon are known for adapting to very
dynamic and variable environments. Each habitat has risks, but if that risk is spread out over a
variety of habitats, it often results in a more productive stock of fish. Research validates the
hypothesis that restoration of access to floodplain habitat generates benefits for fish, including a
reduction in stranding, which is the most likely biological outcome for the breaching projects at
the Springbrook Bank.
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1-405 Transportation Corridor Projects �...K-
Springbrook Bank -/' Depa'rtmant oof Traaasportatloa
Fisheries Review
7.0 Bibliography
Bell, E., W. Duffy, and T. Roelofs. 2001. Fidelity and survival of juvenile coho salmon in
response to a flood. Transactions of the American Fisheries Society 130:450-458.
Bradford, M. 1997. An experimental study of stranding of juvenile salmonids on gravel bars
and inside channels during rapid flow decreases. Regulated River 13(5):95-401.
Bradford, M. and P. Higgins. 2001. Habitat-, season-, and size -specific variation in diel activity
patterns of juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout
(Oncorhynchus mykiss). Canadian Journal of Fisheries and Aquatic Sciences 58:365-374.
Brown, T. 2002. Floodplains, flooding, and salmon rearing habitats in British Columbia: a
review. Canadian Science Advisory Secretariat, Research Document 2002/007.
Cederholm, C. J., L. C. Lestelle, B. G. Edie, D. J. Martin, J. V. Tagart, and E. O. Salo. 1978.
The Effects of Landslide Siltation on the Salmon and Trout Resources of Stequaleho Creek and
the Main Clearwater River, Jefferson County, Washington, 1972-1975. University of
Washington, Fisheries Research Institute Final Report FRI-UW-7804, Seattle, WA.
Everest, F., and D. Chapman. 1972. Habitat selection and spatial interaction by juvenile
Chinook salmon and steelhead trout in two Idaho streams. J. of the Fisheries Research Board
of Canada 29(1):91-100.
Gehrke, P. C. 1992. Diel abundance, migration and feeding of fish larvae (Eleotridae) on a
floodplain billabond. J. Fish Biol. 40:695-707.
Gladden, J. and L. Smock. 1990. Macroinvertebrate distribution and production on the
floodplains of two lowland headwater streams. Freshwater Biology 24:533-545.
Gutreuter, S., Bartels, A. D., Irons, K., and Sandheinrich, M. B. 2000. Evaluations of the flood -
pulse concept based on statistical models of growth of selected fishes of the Upper Mississippi
River system. Can. J. Fish. Aquat. Sci. 56: 2282-2291.
Halyk, L. C., and Balon, E. K. 1983. Structure and ecological production of the fish taxocene of
small floodplain system. Can. J. Zool. 61:2446-2464.
Ross, S., and Baker, J. 1983. The response of fishes to periodic spring floods in a
southeastern stream. Am. Midl. Nat. 109:1-14.
Harza. 1995. Comprehensive fisheries assessment of the Springbrook, Mill, and Garrison Creek
watershed. Bellevue, WA. Prepared for City of Kent, Washington.
Higgins, P. and M. Bradford. 1996. Evaluation of a large-scale fish salvage to reduce the
impactsof controlled flow reduction in a regulated river. North American J. of Fish. Management
16:666-673.
King County. 2005. Personal communications with Andy Levesque and Black River Pump
Station operator during July 2005 site review.
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1-405 Transportation Corridor Projects ANI.
Ti Washing State
Springbrook Bank Department
of Transportation
Fisheries Review
Roper, B., D. Scarnecchia, and T. La Marr. 1994. Summer distribution and habitat use by
Chinook salmon and steelhead within a major basin of the South Umpqua River, Oregon.
Transactions of the American Fisheries Society 123(3):298-308.
Ross, S., and J. Barker. 1983. The response of fishes to periodic spring floods in a
ssoutheastern stream. Am Midi. Nat. 109:1-14.
Sommer, T., Baxter, R., and Herbold, B. 1997. The resilience of splittail in the Sacramento —
San Joqaquin Estuary. Trans. Am. Fish. Soc. 126: 961-976.
Sommer, T., M. Nobriga, W. Harrell, W. Batham, and W. Kimmerer. 2001. Floodplain rearing of
juvenile Chinook salmon: evidence of enhanced growth and survival. Canadian Journal of
Fisheries and Aquatic Sciences 58:325-333.
Sommer, T., W. Harrell, and M. Nobriga. 2005. Habitat use and stranding risk of juvenile
chinook salnonb on a seasonal floodplain. North American J. of Fish. Management. 25:1493-
1504.
Ward, J. and J. Stanford. 1995. Ecological connectivity in alluvial river ecosystems and its
disruption by flow regulation. Regulated River: Research and Management 11:105-119.
Washington Department of Fish and Wildlife (VVDFW). 2005. Personal communications with
Tom Cropp, District Fish Biologist in September 2005.
Washington State Department of Transportation. 2005. Springbrook Creek Mitigation Site
Wetland Delineation Report. Washington State Department of Transportation, 1-405 Project
Office, Bellevue, WA.
Welcomme, R. L. 1979. Fisheries Ecology of floodplain rivers. Longman Group Limited,
London.
Wilbur, H. 1997. Experimental ecology of food webs: complex systems in temporary ponds.
Ecology 78(8) 2279-2302.
Winemiller, K. O., and Jepsen, D. B. 1998. Effects of seasonality and fish movement on
tropical food webs. J. Fish Biol. 53 (Suppl. A):267-296.
Wydoski, Richard S. and R. R. Whitney. 2004. Inland Fishes of Washington, Second Edition,
Revised and Expanded. American Fisheries Society in association with University of
Washington Press. 332 pp.
12
1-405 Transportation Corridor Projects ...
Springbrook Bank 7 w c iartmertme t state
Department of Transportation
Fisheries Review
0 Public Notice
Army Corps
of Engineers ofApplication
of
Seattle District RECEIV L
f or Permit MAR 2 3 2006
CITY OF RENTON
-Erratum- vT!Lr, Y SYS , a.,u
Regulatory Branch
Post Office Box 3755 Public Notice Date: March 13, 2006
Seattle, Washington 98124-3755 Erratum Date: March 22, 2006
Telephone (206) 764-6911 Reference: 200600100
ATTN: Sandra Manning, Project Manager Name: Springbrook Mitigation Bank
WSDOT and City of Renton
ERRATUM: This erratum identifies a request that the 404 permit issued to construct the mitigation bank
would expire 7-years from the date of issuance rather than the default 3-year expiration that applies to
Individual 404 permits. The request for the additional time will apply to the portion of the project associated
with the construction of the pedestrian trail. The mitigation banking site will be constructed as soon as the
permit is issued, but the trail construction may be delayed.
All other information shown on Public Notice No. 200600100 remains unchanged including the expiration
date for comments to be received by the Washington State Department of Ecology and the
U.S. Army Corps of Engineers which remains April 12, 2006.
Congestion Relief & Bus Rapid Transit Projects
September 1, 2005
Subject: Springbrook Wetland and Habitat Mitigation Bank:
Springbrook Creek Wetland Restoration Memorandum
Executive Summary
The Washington State Department of Transportation (WSDOT) and the City of Renton are
working together to establish the Springbrook Wetland and Habitat Mitigation Bank (the Bank).
The Bank is comprised of five wetlands units: A, B, C, D, and E. This memorandum addresses
the rehabilitation of Units A and B and the reestablishment of wetland at Unit E.
Rehabilitation of Units A and B will be achieved by providing hydrologic connections with
Springbrook Creek. Reestablishment of Unit E wetlands will depend on providing sufficient
water to the site to establish vegetation typically found in saturated soil conditions. Unit E will
depend solely on flows entering the site from Springbrook Creek for its source of water. Units C
and D are addressed in a separate memorandum because these sites do not depend on
Springbrook Creek for their source of hydrology. Instead, the hydrology of Units C and D is
characterized by using a water balance that includes stormwater runon volumes, piezometric
data, infiltration, and evapotranspiration calculations.
Springbrook Creek hydrology was investigated using the Hydrologic Simulation Program -
FORTRAN (HSPF) program. A continuous hourly precipitation record from Seattle -Tacoma
International Airport for water years 1949 to 2001 was used to generate 53 years of continuous
flow in Springbrook Creek. The model was calibrated to streamflow data from water years 1995
and 1996. Analysis of the model was used to determine the inundation and duration at Units A,
B, and E.
The rehabilitation of Units A and B will depend on creating breaches to provide more efficient
connection of flows with Springbrook Creek through the existing levee. The breach elevations
were set at the elevations of the existing delineated wetlands. It is anticipated that the 12-foot
elevation of the breach will provide 77 hours (3.2 days) of inundation annually, and 23 hours (1
day) of inundation during the mesic growing season defined from March 1 through October 31.
It is anticipated that the depth of flows on the site will range from 0 to 2 feet as a result of
inundation from Springbrook Creek.
The success of the reestablishment of Unit E depends on inundation by Springbrook Creek,
duration of the flows, and on the saturation of soils. Breaches have been created to facilitate
Springbrook Creek flows into and out of the site without causing erosion to the site, flooding to
surrounding properties, or stranding of fish species. It is anticipated that the depth of flows at
Unit E will range from 0 to 7 feet. The detest inundation will occur at the connection_with
Springbrook Creek. Icipated that Springbrook�eek flows will last approximately 1 hour
during the growing season:`lCs estimated that Elevation 10 will be inardated-with wafer L - d4w
approximately 2 feet deep for 15 days during the growing season.
20050829 Springbrook Wwetland Restoration Memo editted.doc
Congestion Relief & Bus Rapid Transit Projects
Introduction/Purpose
The Washington State Department of Transportation (WSDOT) and the City of Renton are
working together to establish the Springbrook Wetland and Habitat Mitigation Bank (the Bank).
The Bank is comprised of five wetlands units: A, B, C, D, and E. This memorandum addresses
the rehabilitation of Units A and B and reestablishment of wetlands at Unit E.
Rehabilitation of Units A and B will be achieved by providing hydrologic connections with
Springbrook Creek. Reestablishment of Unit E will depend on providing sufficient water to the
site to establish vegetation typically found in saturated soil conditions. Unit E will depend solely
on flows entering the site from Springbrook Creek for its source of water. Units C and D are
addressed in a separate memorandum because these sites do not depend on Springbrook
Creek for their source of hydrology. Instead, the hydrology of Units C and D is characterized by
using a water balance that includes stormwater runon volumes, piezometric data, infiltration,
and evapotranspiration calculations.
This memorandum describes the hydrologic modeling and analyses conducted for evaluating
Springbrook Creek flows and for determining inundation of the units. This memorandum
includes three parts:
• Part I identifies the criteria and methodology which need to be met in order to receive
mitigation credit.
• Part II provides analysis and discussion of hydrology as it pertains to supporting the
rehabilitation of Units A and B and the re-establishment`' of Unit E.
• Part III provides a summary and conclusion.
I. Criteria and Project Methodology
To receive mitigation credit, ,Units A, B, and E must meet multiple functions and natural
processes criteria. Rehabilitating the existing wetlands will allow Units A and B to meet these
criteria. Unit E will meet the criteria when wetlands are re-established onsite.
Units A and B
Wetland rehabilitation is the dominant mitigation treatment within Units A and B. Wetland
rehabilitation will be achieved by providing hydrologic connections between the existing
wetlands and Springbrook Creek. It is anticipated that these connections will augment wetland
hydrology in Units A and B, increase stream interaction with the floodplain, and potentially
reduce the elevation and duration of peak flows in Springbrook Creek. It should be noted that it
is not necessary to establish wetland hydrology at Units A and B because it already exists.
Unit E
Wetland hydrology currently does not exist at Unit E. Mitigation treatments for Unit E include
wetland re-establishment, riparian enhancement, upland habitat enhancement, and buffer
creation. To re-establish wetlands in Unit E, the definition of wetland hydrology must be met or
exceeded, assuming that the soil and vegetation parameters are achieved. The Washington
20050829 Springbrook Wwetland Restoration Memo editted.doc 2
Congestion Relief & Bus Rapid Transit Projects
State Department of Ecology's 1997 Washington State Wetlands Identification and Delineation
Manual defines wetlands as:
Those areas that are inundated or saturated by surface or ground water
at a frequency and duration sufficient to support, and that under normal
circumstances do support, a prevalence of vegetation typically adapted
for life in saturated soil conditions.
In order to be considered a wetland, the following hydrologic criterion must be met:
Areas which are seasonally inundated and/or saturated to the surface for
a consecutive number of days greater than 12.5 percent of the growing
season are wetlands provided the soil and vegetation parameters are
met. Areas inundated or saturated between 5 percent and 12.5 percent
of the growing season in most years may be wetlands. Areas inundated
or saturated to the surface for less than 5 percent of the growing season
are non -wetlands.
Because individuals from different disciplines will review this document, terms in the definition
above are clarified below:
Inundation — A condition in which water from any source temporarily or
permanently covers a land surface.
Saturation — A condition in which all easily drained voids (pores between
soil particles) in the root zone are temporarily or permanently filled with
water to the soil surface at pressures, greater than atmospheric.
Growing Season - The growing season is the portion of the year when
soil temperatures at 19.7 inches below the soil surface are higher than
biologic zero(50 C). For western Washington the mesic growing season
is often considered to be approximately March 1 to October 31 (245
days).
Duration — The length of time during which water stands at or above the
soil surface (inundation), or during which time the soil is saturated.
Frequency — The periodicity of coverage of an area by surface water or
soil saturation.
To determine the extent of the inundation and the duration of Springbrook Creek flows, the
Hydrologic Simulation Program — FORTRAN (HSPF) program was used. HSPF is a continuous
time series hydrologic program that can be used to investigate a wide range of hydrologic
conditions such as low flow conditions, flow durations, and length of inundation. HSPF is used
extensively in western Washington by King County Water and Land Resources, the United
States Geological Society (USGS), and engineering consultants to assess effects of changes in
a drainage basin over time.
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The Springbrook Creek model was used to support the City of Renton's floodplain re -mapping
study, a part of the recent Federal Emergency Management Agency (FEMA) floodplain mapping
revison (Hartley and Stewart, 2004). A continuous hourly precipitation record from Seattle -
Tacoma International Airport (NWS gage 7473 at Sea-Tac) for water years 1949 to 2001 was
used to generate 53 years of continuous flow in Springbrook Creek. The model was calibrated
to streamflow data from water years 1995 and 1996 using land use, channel, and basin storage
conditions (Hartley and Stuart, 2004). Storms were typically large and numerous during the
calibration period, making this a very good period for calibrating (Hartley and Stuart, 2004).
For the Springbrook Wetland and Habitat Mitigation Bank project, the model was updated with
recently surveyed stream cross sections. The updated model was used to evaluate current and
future flow conditions in Springbrook Creek. The current flow conditions were based on: (a) the
1999 land use designations determined from aerial photography of the drainage basin, and
(b) the anticipated near -term conveyance improvements, including replacing the culvert at SW
34th Street and connecting the proposed Unit E wetland to Springbrook Creek.
The future flow condition consists of current condition Inear term conveyance improvements,
removing the bridge north of SW 27th Street, and replacing the culvert at the intersection of SW
41 st Street and Oakesdale Avenue SW. The future land use scenario represents the maximum
development allowed under the City of Renton zoningordinances in place at the time of the
2001floodplain mapping study.
Unit E improvements were included in all model runs. The two modeled scenarios provide
anticipated conditions at the sites when the project is completed and the anticipated buildout
conditions for channel improvements along Springbrook Creek. Note that analysis of current
and future conditions does not provide an evaluation of the impacts or benefits of the proposed
project.
II. Analysis and Discussion
The extent of the inundation and the duration of Springbrook Creek flows were evaluated at
three locations for the current and future conditions. The three locations (Figure 1) are:
1. Upstream of SW 27th Street (adjacent to Units A and B)
2. Upstream of SW 34th Street (located just north and downstream of Unit E)
3. Downstream of Oakesdale Avenue SW (located just south and upstream of Unit E)
The analysis for each location included counting the number of times (events) that the water
surface exceeded a certain elevation (inundation), and for how long it remained at that elevation
(duration). F example, the analysis could tell the investigator that Elevation 11 was exceeded
�6_tin�es fo 13 hours during the growing season at Location 1. This type of analysis is often
referred t an event -stage (elevation) -duration analysis and can be achieved only by using a
4f4`v continuous time series program. The analysis cannot predict how long an area will be
5 continuously inundated or saturated. To address the continuous inundation question, a second
analysis Is necessary and Is discussed In the second part of this section.
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11.1 Event - Stage - Duration Analysis
Units A and B
Event — stage — duration analyses for Units A and B is based on the 53-year continuous record
of Springbrook Creek flows upstream of SW 27th Street. Table 1 provides current condition
information about the hours and days of inundation in half -foot increments up to Elevation 12,
and in 1-foot increments from Elevations 13 to 16, for four time periods. The table also lists the
corresponding plant zones proposed for each elevation range. Plants will not be installed at a
given elevation if the plant community is not identified in the plant zone column. For Units A and
B, no plantings are anticipated below Elevation 12 along Springbrook Creek. The proposed
plantings for Units A and B are shown in the attached drawings WM1, WM2, WM3, and WM4.
The time periods listed in Tables 1 through 8 are: (a) annual, (b) the growing season (March —
October), (c) the early growing season (March — May), and;(d) from June through August.
Subdividing the information into different time periods helps illustrate changes in Springbrook
Creek hydrology over time.
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Table 1 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites under Current
Conditions - Upstream of SW 27th Street
Hours (days) Inundated
Growing
March - May
Plant Zone
Elevation `
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
7.5
7,964
4,274
2,193
1,853
331.8)
(178.1)
(91.4)
(77.2)
8
2,485
908
541
114
(103.5)
37.8
(22.5)
(4.8)
8.5
1 454
488
265
53
(60.6)
20.3
11.0)
(2.2)
9
976
322
163
32
(40.7)
13.4
(6.8)
1.3
9.5
674
216
101
20
28.1)
(9.0)
(4.2)
(0.8)
10
473
149
66
13
(19.7)
6.2)
(2.8)
0.5)
10.5
323
101
41
8
(13.5)
(4.2
(1.7)
0.3)
11
212
65
23
5
8.8
2.7
1.0)
0.2)
11.5
137
41
14
3
5.7
1.7)
(0.6)
0.1)
Wetland Shrub #2
12
77
3.2
14
(0.6)
8
(0.3)
2
0.1)
Wetland Shrub #2
13
21
0.9)
4
(0.2)
3
(0.1)
0
(0.0)
Wetland Shrub #2
14
3
(0.1)
1
0.0
0
(0.0)
0
(0.0
Riparian
15
0
0
0
0
(0.0)
0.0)
0.0)
(0.0)
Riparian
16
0
0
0
0
(0.0)
0.0
0.0)
(0.0
Riparian
20
0
(0.0)
0
0.0)
0
(0.0)
0
0.0
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
Table 2 provides the future conditions hours and days of inundation by elevation for specific
seasons and proposed plant communities.
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W1r1V
Table 2 - Predicted Hours (Days) Inundatio i o etland Mitigation Sites
under Future Conditions -Downs arn W 27th Street
Gil
Hours (days) Inundated
Growing
March - May
Plant Zone
Elevation
(feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
7.5
7,085
4,274
2,163
1,309
(295.2)
(178.1)
(90.1)
(54.5)
8
2,467
908
529
121
(102.8)
(37.8)
(22.0)
(5.0)
8.5
11511
488
277
60
(63.0)
(20.3)
(11.5)
(2.5)
9
1,067
322
180
40
(44.5)
(13.4)
(7.5)
(1.7)
9.5
766
216
118
26
(31.9)
(9.0)
(4.9)
(1.1)
10
559
149
81
18
(23.3)
(6.2)
(3.4)
(0.8)
10.5
401
101
54
13
(16.7)
(4.2)
(2.3)
(0.5)
11
276
65
33
8
(11.5)
(2.7)
(1.4)
(0.3)
11.5
188
41
20
5
(7.8)
(1.7)
(0.8)
(0.2)
Wetland Shrub #2
12
114
23
11
3
(4.8)
(1.0)
(0.5)
(0.1)
Wetland Shrub #2
13
35
(1.5)
7
(0.3)
4
(0.2)
1
(0.0)
Wetland Shrub #2
14
7
(0.3)
2
(0.1)
1
(0.0)
0
(0.0)
Riparian
15
1
(0.0)
0
(0.0)
0
(0.0)
0
(0.0)
Riparian
16
0
0
0
0
(0.0)
(0.0)
(0.0)
(0.0)
Riparian
20
0
0
0
0
(0.0)
(0.0)
(0.0)
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and
August represent 2,208 hours (92 days).
Tables 3 and 4 present the expected hours of inundation, expressed as a percent of the time
periods upstream of SW 27th Street under current and future conditions.
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Table 3 — Predicted Percent Inunda ien-oMetla,nd Mitigation Sites
under Current Conditions — Downstream of SW 27th Street ri
Plant Zone
Elevation
(feet)
Percent Inundation
Annual
Growing
Season
(March -October)
March - May
(Early
growing
season)
June -
August
7.5
91 %
73%
99%
84%
8
28%
15%
25%
5%
8.5
17%
8%
12%
2%
9
11%
5%
7%
1%
9.5
8%
4%
5%
1%
10
5%
3%
3%
1%
10.5
4%
2%
2%
0%
11
2%
1%
1%
0%
11.5
2%
1%
1%
0%
Wetland Shrub #2
12
1 %
0%
0%
0%
Wetland Shrub #2
13
0%
0%
0%
0%
Wetland Shrub #2
14
0%
0%
0%
0%
Riparian
15
0%
0%
0%
0%
Riparian
16
0%
0%
0%
0%
Riparian
20 —r-0%
0%
0%
0%
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
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Table 4- Predicted Percent Inundation of Wetland Mitigation Sites
under Future Conditions — Downstream of SW 27th Street
Plant Zone
Elevation
(Feet)
Percent Inundation
Annual
Growing
Season
(March-
October)
March - May
(Early
growing
season)
June —
August
7.5
81 %
73%
98%
59%
8
28%
15%
24%
5%
8.5
17%
8%
13%
3%
9
12%
5%
8%
2%
9.5
9%
4%
5%
1 %
10
6%
3%
4%
1%
10.5
5%
2%
2%
1 %
11
3%
1%
1%
0%
11.5
2%
1 %
1 %
0%
Wetland Shrub #2
12
1 %
0%
0%
0%
Wetland Shrub #2
13
0%
0%
0%
0%
Wetland Shrub #2
14
0%
0%
0%
0%
Riparian
15
0%
0%
0%
0%
Riparian
16
0%°
0%
0%
0%
Riparian
24
07
0%
0%
0%
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and
August represent 2,208 hours (92 days).
Tables 1 through 4 show that the hours and days of inundation are reduced as the elevation
increases. The hours of inundation are also reduced as the time period is reduced from the
annual period to a smaller June — August time period. It is also noted that the change between
the current condition and the future condition is insignificant. This is most easily observed when
comparing the current (Table 3) and future (Table 4) percent inundation. Except for elevation
range 7.0 to 7.5, which represents the Springbrook Creek bed, the change in percent inundation
between the two conditions is 1 percent or less.
The proposed levee breaches at Units A and B are set at 12 feet to provide the necessary
hydrologic connections with Springbrook Creek. This proposed elevation is based on the
elevations of existing delineated wetlands located behind the levee. It is anticipated that these
connections will augment wetland hydrology in Units A and B, increase stream interaction with
the floodplain, and reduce the elevation and duration of Springbrook Creek's peak flows. Per
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Table 1, it is anticipated that areas at Elevation 12 will be inundated for 77 hours (3.2 days)
annually, for approximately 14 hours during the growing season, and for 8 hours during the
early growing season (March through May). The depth of flows at Elevation 12 will range from 0
to 5 feet. From Table 2, areas at Elevation 12 will be inundated approximately 114 hours (4.8
days) during the year, and to 23 hours (1 day) during the growing season.
Unit E
Event — stage — duration analyses for Unit E are based on the 53-year continuous record of
Springbrook Creek flows upstream of SW 34th Street. Similar to Tables 1 and 2, the
information in Table 5 and 6 provides the current and future condition hours and days of
inundation in half -foot increments up to Elevation 12, and in 1-foot increments from Elevations
13 to 16, for four time periods respectively. The time periods are identical to those provided for
Units A and B. The table also provides the corresponding plant zones proposed for each
elevation. The proposed plantings for Unit E are shown in the attached drawings WM11 and
WM12.
Similar to the analysis for Units A and B, there is little change in the hours of inundation
between the current and future conditions. The most significant change between the two
conditions occurs in area less than 8.5 feet in elevation which occur at the connection with
Springbrook Creek. Changes are higher elevations are on the order of 1 percent of the period
or less.
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Table 5 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites
under Current Conditions - Upstream of SW 34th Street
Hours (days) Inundated
Growing
March - May
Elevation
Plant Zone
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
8
8,764
5,866
2,208
2,208
(365.2)
(244.4)
(92.0)
(92.0)
Wetland Shrub #2
8.5
3,908
1,711
1,034
258
(162.8)
(71.3)
(43.1)
(10.8)
Wetland Shrub #2
g
2,250
798
474
100
(93.8)
(33.3)
(19.8)
(4.2)
Wetland Shrub #1
9.5
1,604
516
300
62
(66.8)
(21.5)
(12.5)
(2.6)
Wetland Shrub #1
10
1,105
326
189
37
(46.0)
(13.6)
(7.9)
(1.5)
Wetland Shrub #1
10.5
759
(31.6)
206
(86)
117
(4.9)
24
(1.0)
Wetland Shrub #1
11
508
128
72
15
(21.2)
(5.3)
(3.0)
(0.6)
Wetland Shrub #1
11.5
337
(14.0)
77
(3.2)
43
(1.8)
9
(0.4)
Wetland Shrub #1
12
214
(8.9)
44
(1.8)
23
(1.0)
5
(0.2)
Upland (Forest or
13
66
12
7
1
Riparian)
(2.8)
(0.5)
(0.3)
(0.0)
Upland (Forest or
14
14
3
2
0
Riparian)
(0.6)
(0.1)
(0.1)
(0.0)
Upland (Forest or
15
2
0
0
0
Riparian)
(0.1)
(0.0)
(0.0)
(0.0)
Upland (Forest or
16
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
Upland (Forest or
20
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
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Table 6 - Predicted Hours (Days) Inundation of Wetland Mitigation Sites
under Future Conditions - Upstream of SW 34th Street
Hours (days) Inundated
Growing
March - May
Plant Zone
Elevation
(Feet)
Annual
Season
(Early
June -
(March-
growing
August
October)
season)
8
8,763
5,866
2,208
2,208
(365.1)
(244.4)
(92.0)
(92.0)
Wetland Shrub #2
8.5
3,619
155
900
242
(150.8)
(164.6)
(37.5)
(10.1)
Wetland Shrub #2
g
2,241
807
466
106
(93.4)
(33.6)
(19.4)
(4.4)
Wetland Shrub #1
9.5
1,645
547
310
69
(68.5)
(22.8)
(12.9)
(2.9)
Wetland Shrub #1
10
1,178
364
203
45
(49.1)
(15.2)
(8.5)
(1.9)
Wetland Shrub #1
10.5
845
244
133
30
(35.2)
(10.2)
(5.5)
(1.3)
Wetland Shrub #1
11
592
160
86
20
(24.7)
(6.7)
(3.6)
(0.8)
Wetland Shrub #1
11.5
413
105
55
13
(17.2)
(4.4)
(2.3)
(0.5)
Upland (Forest or
12
274
64
33
8
Riparian)
(11.4)
(2.7)
(1.4)
(0.3)
Upland (Forest or
13
96
20
9
2
Riparian)
(4.0)
(0.8)
(0.4)
(0.1)
Upland (Forest or -
14
24
5
3
0
Riparian)
(1.0)
(0.2)
(0.1)
(0.0)
Upland (Forest or
15
4
1
0
0
Riparian)
(0.2)
(0.0)
(0.0)
(0.0)
Upland (Forest or
16
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
Upland (Forest or
20
0
0
0
0
Riparian)
(0.0)
(0.0)
(0.0)
(0.0)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
20050829 Springbrook Wwetland Restoration Memo editted.doc 12
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Table 7 — Predicted Percent Inundation of Wetland Mitigation Sites
under Current Conditions — Upstream of SW 34th Street
Percent Inundation
Growing
March - May
Elevation
Plant Zone
(Feet)
Season
(Early
June -
Annual
(March-
growing
August
October)
season)
8
100%
100%
100%
100%
Wetland Shrub #2
8.5
45%
29%
47%
12%
Wetland Shrub #2
9
26%
14%
21%
5%
Wetland Shrub #1
9.5
18%
9%
14%
3%
Wetland Shrub #1
10
13%
6%
9%
2%
Wetland Shrub #1
10.5
9%
4%
5%
1%
Wetland Shrub #1
11
6%
2%
3%
1%
Wetland Shrub #1
11.5
4%
1 %
2%
0%
Upland (Forest or
12
2%
1%
1 %
0%
Riparian)
Upland (Forest or
13
1%
0%
0%
0 /° °
Riparian)
Upland (Forest or
14
0%
0%
0%
0%
Riparian)
Upland (Forest or
15
0%
0%
0%
0%
Riparian)
Upland (Forest or
16
0%
0%
0%
0%
Riparian)
Upland (Forest or
20
0%
0%
0%
0 /° °
Riparian)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580
hours (232.5 days). March, April, and May represent 2,208 hours (92 days). June, July,
and August represent 2,208 hours (92 days).
20050829 Springbrook Wwetland Restoration Memo editted.doc 13
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Table 8 — Predicted Percent Inundation of Wetland Mitigation Sites
under Future Conditions — Upstream of SW 34th Street
Elevation
Percent Inundation
GrowingMarch
Season
—May
Plant Zone
(Feet)
Annual
(March -
(Early growing
June -'August
October)
season)
8
100%
100%
100%
100%
Wetland Shrub #2
8.5
41 %
26%
41 %
11 %
Wetland Shrub #2
9
26%
14%
21 %
5%
Wetland Shrub #1
9.5
19%
9%
14%
3%
Wetland Shrub #1
10
13%
6%
9%°
2%
Wetland Shrub #1
10.5
10%
4%
6%
1 %
Wetland Shrub #1
11
7%
:'' 3%
4%
1%
Wetland Shrub #1
11.5
5%
2%
2%
1 %
Upland (Forest or
12
3%
1 %°
1 %
0%
Riparian)
Upland (Forest or
13
1%
0%
0%
0%
Riparian)
Upland (Forest or
14
0%
0%
0%
0%
Riparian)
Upland (Forest or
1,5
0%
0%
0%
°
0 /°
Riparian)
Upland (Forest or
16
0%
0%
0%
0%
Riparian)
Upland (Forest or
20
0%
0%
0%
0 /o °
Riparian)
1 year is approximately 8,760 hours (365 days). Growing season is approximately 5,580 hours
(232.5 days). March, April, and May represent 2,208 hours (92 days). June, July, and August
represent 2,208 hours (92 days).
The event -stage -duration analyses of Springbrook Creek at Units A and B indicate that:
Areas below 8 feet in elevation will be permanently inundated. The areas at this elevation
are associated with the Springbrook Creek stream channel at Unit E.
• Areas between 8 and 8.5 feet in elevation will be inundated approximately 26 to 29 percent
(approximately 71 to 165 days) of the growing season and almost half of the early growing
season. The depth of water at the site will range from 0 to 0.5 feet.
• Areas from 8.5 to 9 feet in elevation will be inundated 14 percent of the growing season
(approximately 33 days) and approximately a fifth of the early growing season. The depth of
water will range from 0 to 1 foot.
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• Areas from 9 to 10.5 feet in elevation will be inundated 4% of the growing season
(approximately 8.5 to 10 days). Depth of water will range from 0 to 2.5 feet.
• Elevations above 10.5 feet in elevation will be inundated less than 4 percent (8.5 to 10 days)
of the growing season and about a tenth of the early growing season. Depth of water will
range from 0 to 7 feet at its deepest point. This point occurs for approximately 1 hour during
the growing season.
Saturation
The event -stage -duration analysis described above is capable of predicting water surface
elevations associated with Springbrook Creek hydrology. The analysis is not able to account for
temporary or permanent saturation of the soils at Unit E.
Soil saturation is based on capillary rise, which is a measure of the soil's ability to draw water
above the water surface elevation. The amount of rise depends on the soil properties and the
space (pores) between the individual particles of soil. Contiguous, small pores fill to a higher
elevation above the water table than larger pores. Soils richer in organic matter draw water to a
higher elevation than sandy, gravelly soils. Sandy soils with effective pore diameters of 0.01
centimeters (cm) can draw water 6 inches above the water table. A loam with effective pore
space half that size (0.005 cm) draws water 12 inches above the water table. Some organic
soils draw water 18 inches or more above the water table.
Soil saturation information can typically be obtained from soil surveys or other similar sources.
In this case, much of the material at the site is fill from previous construction activities. Recent
geotechnical investigations at the site suggest that much of the material at the proposed grading
elevations is expected to be silts with peat, silts to sandy silt, and brown peat with organic silts.
The fill material at the site is generally characterized as silty sand and sand. It is assumed from
this analysis that the materials found over the majority of the site after construction will be
mostly silts capable of drawing water up 12 inches or more. Silty sand and sand fill materials on
the side slopes of the wetland will be amended with organic material to establish the necessary
planting and will be capable of drawing water 12 inches above the water table. This assumption
is consistent with the Washington State Wetland Identification and Delineation Manual (Ecology,
1997) which states that for soil saturation to affect vegetation it must occur within the major
rooting zone of the prevalent vegetation (usually within 12 inches of the surface).
Hydrologic Zones
With the event -stage -duration analysis and the saturation component established, the expected
hydrologic zones can be identified. Table 9 shows the expected hydrologic zones adapted by
the Corps of Engineers Wetland Delineation Manual (1987) from Clark and Benforado (1981).
The information presented in Table 9 was based on the percent inundation information and
adding 12 inches to the elevation to account for the influence of saturation.
The following example is provided for clarity: From Table 9, the elevation corresponding to
6 percent inundation during the growing season is 10 feet. Adding 12 inches to the elevation
provides saturated soils to Elevation 11 for approximately 6 percent of the growing season. The
following table was constructed following this procedure.
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Table 9 — Unit E Hydrologic Zones
Zone*
Name
Duration"*
Elevation Range***
It
Permanently inundated
100%
Below 8.0 feet
II
Semi -permanently to nearly
>75 % - <100%
permanently inundated
III
Regularly inundated
>25% - 75%
Areas up to elevation 9.5
IV
Seasonally inundated
>12.5% - 256/o
Areas up to elevation 10.0
V
Irregularly inundated
5% - 12.5%
Areas between 11 and 11.5 —
feet in elevation.
VI
Intermittently or never
<5%
Areas above 11.5 feet in
inundated
elevation
* Zones adapted from Clark and Benforado (1981).
** Refers to duration of inundation and/or soil saturation during the growing season.
*** Assumes 12 inches of capillary rise
t This defines an aquatic habitat zone.
Based on the hydrologic zones for Unit E (Table 9) and plant zones listed in Tables 5 though 8,
wetland hydrology can be provided up to Elevation 11.5 feet. The lower elevations of Wetland
Shrub Community #2 will be regularly inundated to seasonally inundated. Wetland Shrub
Community #1 will be regularly inundated to irregularly inundated. Upper elevations of the
wetland shrub community are shown to be intermittently inundated, while the lower elevations of
uplands are inundated less than intermittently or never inundated.
11.2 Continuous Duration Analysis
A second analysis was performed to address how long will Unit E will be continuously saturated.
A continuous duration analysis is not required for Units A and B since wetland hydrology already
exists on site.
The analysis examined four individual storm events at Unit E to determine the duration of typical
Springbrook Creek flows at specific elevations. The storms represent conditions that can be
expected during the early growing season (March through May and during the remaining portion
of the growing season (June through October). Selection of these events was based on
identifying the 2-year return period and the annual return period (1.01 percent exceedance).
Figures 2 through 5 provide information about the continuous inundation that can be expected
for an individual storm.
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Table 10 — Peak Flows for Selected Typical Storm Events
SW 34th Street
Return Period
Early Growing Season
Growing Season
(March — May)
(March — October)
146 cfs
231 cfs
1.01 years
(March 1996)
(June 2001)
274 cfs
385 cfs
2 years
(May 2000)
(August 2001)
cfs= cubic feet per second
Upstream SW 34th Street and Oakesdale Avenue SIN, March — May
March 11, 1996 with a peak simulated flood flow of 146 cfs, representing a moderate, more
frequent event that is expected to be equaled or exceeded approximately 99 percent of the
time.
May 10, 2000 with a peak simulated flood flow of 274 cfs, representing a 2-year return
period expected to be equaled or exceeded once every 2 years on average.
Upstream SW 34th Street and Oakesdale Avenue SIN, June — October
• June 11, 2001 with a peak simulated flood flow of 231 cfs, representing a moderate, more
frequent event that is expected to be equaled or exceeded approximately 99 percent of the
time.
• August 22, 2001 with a peak simulated flood flow of 380 cfs, representing a 2-year return
period expected to be equaled or exceeded once every 2 years on average.
Figures 2 through 5 show that frequent events will inundate areas up to an elevation of 8.5 feet
(corresponding to a saturated elevation of 9.5 feet) for durations that range from 93 hours (3.9
days) to 157 hours (6.5 days). Surface flows will typically take 2 to 3 days to recede. The
cumulative time corresponding to the inundation of the site to the recession of water from the
site is expected to last from 5.9 days to 9.5 days.
It is anticipated that in areas up to 9 feet in elevation (corresponding to a saturated elevation of
10.0 feet), frequent events will last from 23 hours (0.9 days) to 65 hours (2.7 days). Flows will
typically take 2.5 to 3.5 days recede. The cumulative time would range from 3.4 days to 6.2
days.
Areas up to Elevation 10.5 (corresponding to a saturated elevation of 11.5 feet) could typically
be inundated from 10 hours (0.4 days) to 36 hours (1.5 days) in duration. Flows would typically
take 3.5 to 4.5 days to recede. The cumulative time corresponding to the inundation of the site
to the recession of water from the site is expected to last from 3.9 days to 6.0 days.
20050829 Springbrook Wwetland Restoration Memo editted.doc 17
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While none of these storms last for 12.5 percent of the growing season (roughly 30 days), it is
expected that the wetland areas would be frequently inundated or saturated to provide sufficient
water to establish wetland hydrology between 5 percent and 12.5 percent of the growing season
It should be noted that the slow recession of water from Unit E should allow any fish seeking
refuge during higher flows ample time to respond and remain in deep water, provided that the
sites are graded to drain toward Springbrook Creek and avoid any isolated water that is not
hydrologically connected to Springbrook Creek.
III. Summary and Conclusion
To receive mitigation credit, Units A and B were designed to provide a more effective hydrologic
connection to Springbrook Creek by providing a number of breaches through the levee. The
design and success of Unit E depends on inundation and saturation by Springbrook Creek
flows. Success is defined as the ability to provide the inundation and duration sufficient to
establish vegetation typically adapted for life in saturated soil conditions.
To address inundation and duration during the growing season as well as other seasonal
periods, an HSPF program was used to model Springbrook Creek current and future hydrology
conditions at locations adjacent to Units A, B, and E. Continuous inundation is addressed by
the inspection of typical storm events and the duration of inundation at specific elevations.
Analysis of Units A and B demonstrates that modifying the existing levee with breaches will
provide hydrologic connections between the existing wetlands and Springbrook Creek, augment
the wetland hydrology in Units A and B, increase stream interaction with the floodplain, and
potentially reduce the elevation and duration of Springbrook Creek peak flows.
Analysis of Unit E provides the k
elevations, and the percentage c
SW 34th Street. Proposed elev
during the growing season. The
seasonally inundated and areas
higher than 11.5 feet in elevatioi
be considered uplands.
s of inundation for specific time periods, their associated
ariod inundated for the current and future flows upstream of
is of Unit E are based upon inundation and saturation
alysis indicates that areas up to 10.5 feet in elevation are
to 11.5 feet in elevation are irregularly inundated. Areas
intermittently or never inundated and therefore would likely
Four individual simulated events were selected to represent typical storms that can be expected
during the early growing season (March through May) and during the remaining portion of the
growing season (June through October). Inspection of the storm events shows that areas up to
9.5 feet in elevation (0 to 1.5 feet deep) are inundated or saturated for approximately 4 to 6.5
days. Areas up to 10.5 feet in elevation (0 to 2.5 feet) are inundated or saturated for
approximately 1 to 2.5 days. Areas up to Elevation 11.5 (0 to 3.5 feet deep) are inundated or
saturated for approximately 0.5 to 1.5 days. Flows will typically take 2.5 to 4.5 days to recede.
20050829 Springbrook Wwetland Restoration Memo editted.doc 18
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References
Clark, J. R., and Benforado, J., eds. 1981. Wetlands of Bottomland Hardwood Forests,
Proceedings of a Workshop on Bottomland Hardwood Forest Wetlands of the
Southeastern United States. Elsevier Scientific Publishing Company, New York.
Environmental Laboratory. (1987). "Corps of Engineers Wetlands Delineation Manual,"
Technical Report Y-87-1, U.S. Army Engineer Waterways Experiment Station,
Vicksburg, MS.
Washington State Department of Ecology, 1997. Washington State Wetlands Identification and
Delineation Manual. Publication #96-94.
Hartley, David and Derek Stuart, 2004, Hydrologic Analysis for Floodplain Mapping Study of
Springbrook Creek, King County, Washington. ^^r*r " rNorthwest hydFawliG Hydraulic
seRssConsultants. Memorandum. Tukwila, Washington.
20050829 Springbrook Wwetland Restoration Memo editted.doc 19
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