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STORM DRAINAGE REPORT DEVELOPMENTPLgNNING
CITY OF RENTON
Ifor
SUMMIT PARK
CITY OF RENTON, WASHINGTON
1
' December 13, 1993
Triad Job No. 93-064
1
1
b Prepared : Reviewed By:
p Y
JENNIFER A. STEIG MICHAEL L. MATHESON, P.E.
1
1
1
1
1
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1 - - CIATES
' STORM DRAINAGE REPORT
' for
SUMMIT PARK
' CITY OF RENTON, WASHINGTON
December 13, 1993
Triad Job No. 93-064
'
Prepared r d b Reviewed By:
y
JENNIFER A. STEIG MICHAEL L. MATHESON, P.E.
WAS
% NAL
o ECG �31a3
12
' EXPIRES: 3/19/y
CIATES
DRAINAGE REPORT
' FOR
SUMMIT PARK
' INTRODUCTION:
' The proposed project is to subdivide approximately 12.25 acres into 24 single
family townhouse dwelling units and four 6-plex multifamily dwelling units (48 single
units), a park, open space and future development area. The rectangular shaped parcel lies
on the east side of Talbot Road, near the City of Renton/King County Line (see Vicinity
' Map). More generally, the site lies within Section 31, Township 23 North, Range 5 East,
in King County, Washington.
' Vegetation on the east half of the site consists of scattered second and third-
growth coniferous and broad-leafed trees native to the Pacific Northwest. The understory
' consists of northwest native species including salal, sword fern, berry vines and grasses.
The western half of the site is covered with tall pasture grasses and clumps of berry vines.
' The site generally slopes from east to west at approximately 10-13 percent. The S.C.S.
Soils Map, located in the appendix, indicates the site is underlain by Alderwood soils.
1
EXISTING & PROPOSED DRAINAGE FEATURES:
' There are no existing man-made drainage features on the site, but a large wetland is
located in the center of the site. This wetland will remain undisturbed and will be
' surrounded by a 25 foot buffer area.
' Proposed drainage features for the site include a a tightline conveyance system to a
detention pond and biofiltration swale. The sizing of the detention pond and biofiltration
' swale are located within this report.
1
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2 14
CORE & SPECIAL REQUIREMENTS:
Listed below is each of the Core and Special Requirements and how each one was addressed.
CORE RE UIREMENTS:
1. Discharge at the Natural Location:
Discharge from the site will occur at the natural location, Talbot Road.
2. Off-Site Analysis:
A Level 1 Analysis has been prepared and is located in the appendix.
3. Runoff Control:
A detention pond is proposed for this site to limit peak runoff to the pre-development peak
' rates.
4. Conveyance System:
' A tightline conveyance system will be provided to convey developed on-site runoff to the
proposed detention system. The off-site tributary area will be routed around the proposed
' developed area.
' 5. Erosion/Sedimentation Control Plan:
The final construction plans for this site will include a Temporary Erosion/Sedimentation
' Control Plan.
6. Maintenance and Operation:
' The proposed drainage facilities will be part of the public system and will be maintained by
the City of Renton.
7. Bonds and Liability:
' The drainage facilities will be constructed in conformance with the bond and liability
requirements of King County Code 9.04.100.
��jTRR��ppp�
=ASSOCIATES
3/14
SPECIAL REQUIREMENTS:
1. Critical Drainage Areas:
' The site does not lie within a Critical Drainage Area.
2. Compliance with an Existing Master Drainage Plan:
1 The site does not lie within an Existing Master Drainage Plan Area.
' 3. Conditions Requiring a Master Drainage Plan:
' The project does not meet the conditions requiring a Master Drainage Plan.
4. Adopted Basin or Community Plans:
The site does not lie within an Adopted Basin or Community Plan area.
' 5. Special Water Quality Controls:
The proposed project does not meet the requirements for Special Water Quality Controls.
6. Coalescing Plate Oil/Water Separators:
' The proposed project does not meet the requirements for Coalescing Plate Oil/Water
Separators.
' 7. Closed Depressions:
There are no closed depressions located on the site.
8. Use of Lakes Wetlands or Closed Depressions for Peak Rate Runoff Control:
' There will be no use of lakes, wetlands or closed depressions for peak rate runoff control.
9. Delineation of 100-Year Flood plain:
A wetland is located in the center of the site, on a hillside ranging in slope from 10 to 20
' percent. Due the steep nature of the wetland there will not be an associated flood plain.
A 25-foot NGPE buffer will be placed around the wetland.
10. Flood Protection Facilities for Type 1 and 2 Streams:
The site does not contain or abut a Type 1 or 2 stream.
�1T
=—MIATES
' 4/14
11. Geotechnical Analysis and Report:
' The proposed detention pond does not meet the requirements for a geotechnical analysis.
12. Soils Analysis and Report:
The soils underlying the site have been mapped by the Soils Conservation Service.
��T
M&MTES
PRELIMINARY DETENTION ANALYSIS AND DESIGN:
SITE HYDROLOGY : (see Existing and Developed Conditions Map in
' appendix)
' The impervious and pervious area of lots 1-3 will not drain to the proposed detention
system. To compensate for the bypass area, the detention system release rate will be
restricted to ensure that the flow leaving the entire site during a 2-year, 10-year and 100-
' year developed storm is equivalent to one hundred percent of the flow from the 2-year,
10-year and 100-year existing storm, respectively. The area of the detention system will
then be increased by 30 percent per the KCSWDM. The orifices were sized for the pond
before the 30 percent area increase. Calculations are provided below for the release rate
' and detention volume.
The soils are classified as "Alderwood series", hydrologic group "C", per the King County
Soils Survey maps in the appendix.
' Approximately 7 acres of offsite area is tributary to the plat of Summit Park. The flow
from the offsite area will be conveyed around the developed area.
1
The precipitation for a 24-hour period, in inches, for each storm is listed below. Isopluvial
' maps are provided in the appendix.
' P2 = 2.00 inches
P 10 = 2.90 inches
P 100 = 3.90 inches
1
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6/14
EXISTING CONDITIONS
Area= 3.65 acres (area to be developed)
' 3.65 acres Pervious (pasture) @ CN = 85
Time of Concentration= 21.45 minutes
' TcReach - Sheet L: 300 FT ns: 0.24 s: 0.13 70 2yr: 2.00
TcReach - Shallow L: 245 FT ks: 9.00 s: 0.1184
' EXISTING CONDITIONS BASIN SUMMARY
BASIN ID: EX2 NAME: 2 YEAR EXISTING
SBUH METHODOLOGY
TOTAL AREA.......: 3.65 Acres BASEFLOWS: 0.00 cfs
' RAINFALL TYPE...... USERI PERVIOUS AREA
PRECIPITATION....: 2.00 inches AREA..: 3.65 Acres
TIME INTERVAL....: 10.00 min CN....: 85.00
' TIME OF CONC.....: 21.45 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 0.00 Acres
CN....: 98.00
' PEAK RATE: 0.48 cfs VOL: 0.24 Ac-ft TIME: 480 min
' BASIN ID: EX10 NAME: 10 YEAR EXISTING
SBUH METHODOLOGY
TOTAL AREA.......: 3.65 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
PRECIPITATION....: 2.90 inches AREA..: 3.65 Acres
TIME INTERVAL....: 10.00 min CN....: 85.00
' TIME OF CONC.....: 21.45 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 0.00 Acres
CN....: 98.00
PEAK RATE: 1.06 cfs VOL: 0.46 Ac-ft TIME: 480 min
r
��T
=��IATES
7/14
BASIN SUMMARY CONT.
BASIN ID: EX100 NAME: 100 YEAR EXISTING
' SBUH METHODOLOGY
TOTAL AREA.......: 3.65 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
' PRECIPITATION....: 3.90 inches AREA..: 3.65 Acres
TIME INTERVAL....: 10.00 min CN....: 85.00
TIME OF CONC.....: 21.45 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 0.00 Acres
CN....: 98.00
PEAK RATE: 1.77 cfs VOL: 0.72 Ac-ft TIME: 480 min
DEVELOPED CONDITIONS
Area= 3.40 acres (entire site plus offsite less bypass area)
2.72 acres Impervious (Roofs, Roads, etc.) @ CN = 98
0.68 acres Pervious (lawn) @ CN = 86
' Time of Concentration = 56.40 minutes
TcReach - Sheet L: 100 FT ns: 0.15 s: 0.1100 2yr: 2.00
TcReach - Sheet L: 200 FT ns: 0.0110 s: 0.1000 2yr: 2.00
TcReach - Shallow L: 120 FT ks: 27.00 s: 0.1000
TcReach - Channel L: 180 FT kc: 42.00 s: 0.0500
i
DEVELOPED CONDITIONS BASIN SUMMARY
' BASIN ID: DEV2 NAME: 2 YEAR DEVELOPED
SBUH METHODOLOGY
' TOTAL AREA.......: 3.40 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
PRECIPITATION....: 2.00 inches AREA..: 0.68 Acres
' TIME INTERVAL....: 10.00 min CN....: 86.00
TIME OF CONC.....: 8.22 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 2.72 Acres
CN....: 98.00
PEAK RATE: 1.39 cfs VOL: 0.45 Ac-ft TIME: 480 min
' TNCIATES
8/14
BASIN SUMMARY CONT.
' BASIN ID: DEV 10 NAME: 10 YEAR DEVELOPED
SBUH METHODOLOGY
TOTAL AREA.......: 3.40 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
PRECIPITATION....: 2.90 inches AREA..: 0.68 Acres
TIME INTERVAL....: 10.00 min CN....: 86.00
TIME OF CONC.....: 8.22 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 2.72 Acres
CN....: 98.00
PEAK RATE: 2.14 cfs VOL: 0.69 Ac-ft TIME: 480 min
BASIN ID: DE100 NAME: 100 YEAR DEVELOPED
SBUH METHODOLOGY
TOTAL AREA.......: 3.40 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
PRECIPITATION....: 3.90 inches AREA..: 0.68 Acres'
TIME INTERVAL....: 10.00 min CN....: 86.00
TIME OF CONC.....: 8.22 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 2.72 Acres
CN....: 98.00
' PEAK RATE: 2.97 cfs VOL: 0.97 Ac-ft TIME: 480 min
OwmaATES
9/14
BYPASS CONDITIONS
' Area= 0.25 acres
0.05 acres Pervious (lawn) @ CN = 86
0.20 acres Pervious (lawn) @ CN= 86
Time of Concentration= 6.05 minutes
TcReach - Sheet L: 100 FT ns: 0.15 s: 0.1200 2yr: 2.00
BYPASS CONDITIONS BASIN SUMMARY
BASIN ID: BY2 NAME: 2 YEAR BYPASS
' SBUH METHODOLOGY
TOTAL AREA........ 0.25 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
' PRECIPITATION....: 2.00 inches AREA..: 0.05 Acres
TIME INTERVAL..... 10.00 min CN....: 86.00
TIME OF CONC...... 6.05 min IMPERVIOUS AREA
' ABSTRACTION COEFF: 0.20 AREA..: 0.20 Acres
CN....: 98.00
PEAK RATE: 0.11 cfs VOL: 0.03 Ac-ft TIME: 470 min
BASIN ID: BY10 NAME: 10 YEAR BYPASS
' SBUH METHODOLOGY
TOTAL AREA.......: 0.25 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE....: USER1 PERVIOUS AREA
' PRECIPITATION....: 2.90 inches AREA..: 0.05 Acres
TIME INTERVAL....: 10.00 min CN....: 86.00
TIME OF CONC.....: 6.05 min IMPERVIOUS AREA
' ABSTRACTION COEFF: 0.20 AREA..: 0.20 Acres
CN....: 98.00
PEAK RATE: 0.16 cfs VOL: 0.05 Ac-ft TIME: 470 min
BASIN ID: BY100 NAME: 100 YEAR BYPASS
SBUH METHODOLOGY
' TOTAL AREA.......: 0.25 Acres BASEFLOWS: 0.00 cfs
RAINFALL TYPE..... USER1 PERVIO+US AREA
PRECIPITATION....: 3.90 inches AREA..: 0.05 Acres
TIME INTERVAL....: 10.00 min CN....: 86.00
TIME OF CONC.....: 6.05 min IMPERVIOUS AREA
ABSTRACTION COEFF: 0.20 AREA..: 0.20 Acres
' CN....: 98.00
PEAK RATE: 0.23 cfs VOL: 0.07 Ac-ft TIME: 470 min
ti
s��TES
' 10/14
RELEASE RATE:
The allowable release rate from the entire site for the developed 2-year, 10- ear y and 100-year
' storms is 100% of the 2-year, 10-year and 100-year existing peak flows, respectively.
2-year allowable release rate= 0.48 cfs
10-year allowable release rate = 1.06 cfs
100-year allowable release rate = 1.77 cfs
The outflow hydrograph from the detention pond will be added to the developed condition
bypass hydrograph to check that the release rate from the site is less than or equal to the
allowable release rate.
1 HYDROLOGIC ANALYSIS:
The detention system will consist of 40' x 40' x 4' deep pond with 3:1 side slopes. The system
has been designed to meet the detention requirements of the King County Surface Water
Design Manual, with a 30 percent increase in surface area (the theoretical design pond for
orifice sizing was 40' x 28' x 4'). Flows released from the detention system will be tightlined to
' a biofiltration swale for water quality treatment.
The following pages show the output from the WaterWorks program for the stage-storage, the
stage-discharge and the level pool summary. Each of the existing and developed basins were
copied to a hydrograph. A stage-storage relationship was input into the program and used,
along with the stage-discharge and the basin design feature, to determine through level-pool
routing the size of the orifices and that the detention pond design was adequate to meet the
detention requirements.
The assumed invert elevation for the detention pond is 145.50 and the overflow elevation is
' 149.50.
1 d'=T
&IATES
1 IUl4
1
Detention Pond Stage-Storage Table
iStorage ID: STOI
Size: 40' x 28' x 4' deep with 3:1 side slopes
STAGE STORAGE STORAGE VOLUME WITH
[FT] [CF] [AC-FT] 30% LENGTH
iINCREASE [CF]
145.50 0.0000 0.0000 0.0000
' 146.00 613 0.0141 862
146.50 1336 0.0307 1852
147.00 2179 0.0500 2980
i147.50 3152 0.0724 4256
148.00 4262 0.0979 5687
i 148.50 5520 0.1267 7284
149.00 6933 0.1592 9054
IL 149.50 8512 0.1954 11008
i
Detention Pond Stage-Discharge Table
Discharge ID No. DIS I Description: FLOW CONTROL STRUCTURE FOR POND
iOutlet Elev:
Elev: 143.50 Orifice 1 Diameter: 3.0625 in.*
Elev: 148.00 Orifice 1 Diameter: 4.8120 in.*
Elev: 148.80 Orifice 1 Diameter: 4.5620 in.*
1
STAGE DISCHARGE
i [EL-_ CFS
145.50 0.0000
i 146.00 0.1797
146.50 0.2541
147.00 0.3112
i 147.50 0.3594
148.00 0.4018
i 148.50 0.8841
149.00 1 1.3556
149.50 1.7493
1
i ��TRR��SOpppp
=6ASCIATES
12/14
* Orifice sizing was performed using the 40' x 28' x 4' deep pond.
' LEVEL POOL TABLE SUMMARY
Description Inflow Storag Discharge Peak Stage2 Outflow Outflow + Bypass
(CFS) e ID (ft) (CFS) (CFS)
' ID
2 - 2 1.39 STO1 DIS1 147.45 0.35 0.39
10 - 10 2.14 STO1 DIS 1 148.30 0.77 0.82
100 - 100 2.97 STOI DIS1 149.02 1.38 1.45
1 Outflow+Bypass is the addition of the outflow hydrograph to the developed bypass hydrograph to
insure that the flow leaving the site is less than or equal to the allowable site release rate for a given
storm event.
2 Peak Stage is measured in the 40' x 40'x 4'deep pond.
BIOFILTRATION SWALE CALCULATIONS:
1
The biofiltration swale will be sized using the following variables:
Trapizodal Channel:
n = 0.35
Slope = 2.0%
Water depth= 3" (total depth= 1.00')
Side slope= 3:1
The swale will be sized for the 2-year existing flow and checked for the 100-year developed
flow from the site.
�I��SOCIATES
' 13/14
BIOFILTRATION SWALE SIZING
The base width for the swale at the outlet of the detention pond is sized below:
' CHANNEL INPUT INFORMATION
---------------------------------------------------
NORMAL DEPTH(FEET) = 0.33
' CHANNEL Z(HORIZONTAL/VERTICAL) = 3.00
CONSTANT CHANNEL SLOPE(FEET/FEET) = 0.02
UNIFORM FLOW(CFS) = 0.40
' MANNINGS FRICTION FACTOR= 0.35
NORMAL-DEPTH FLOW INFORMATION:
----------------------------------------_--------
»»> BASEWIDTH(FEET) 3.85
FLOW TOP- WIDTH(FEET) = 5.83
' FLOW AREA(SQUARE FEET) = 1.60
HYDRAULIC DEPTH(FEET) = 0.27
FLOW AVERAGE VELOCITY(FEET/SEC.) = 0.25
' UNIFORM FROUDE NUMBER= 0.084
PRESSURE + MOMENTUM(POUNDS) = 15.53
tSPECIFIC ENERGY(FEET) = 0.331
' The swale should have a bottom width of 4.0 feet for 200 feet. The slope of the biofiltration
swale will be 0.5 percent. Check the swale at 4.0 feet wide for the 100-year capacity.
' aM&IATES
14/14
CHECK 100-YEAR CAPACITY
' CHANNEL INPUT INFORMATION
---------------------------------------------------
CHANNEL Z(HORIZONTAL/VERTICAL) = 3.00
BASEWIDTH(FEET) = 4.00
CONSTANT CHANNEL SLOPE(FEET/FEET) = 0.005
' UNIFORM FLOW(CFS) = 3.19
MANNINGS FRICTION FACTOR= 0.03
' - NORMAL-DEPTH FLOW INFORMATION:
- --------------------------------------------
»»> NORMAL DEPTH(FEET) = 0.39
' FLOW TOP- WIDTH(FEET) = 6.32
FLOW AREA(SQUARE FEET) = 2.00
' HYDRAULIC DEPTH(FEET) = 0.32
FLOW AVERAGE VELOCITY(FEET/SEC.) = 1.60
UNIFORM FROUDE NUMBER= 0.50
' PRESSURE + MOMENTUM(POUNDS) = 32.20
AVERAGED VELOCITY HEAD(FEET) = 0.40
SPECIFIC ENERGY(FEET) = 0.427
At the 100-year flow the swale will have approximately 5" of water depth. Therefore, the
swale has adequate capacity for the 100-year developed storm.
LEVEL 1 DOWNSTREAM ANALYSIS:
A Level 1 Downstream Analysis has been prepared for this site and is attached in the appendix.
' 42O CIATES
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LEVEL 1 DRAINAGE ANALYSIS
' FOR
' THE PLAT OF SUMMIT PARK
' CITY OF RENTON, WASHINGTON
i
1
December 13, 1993
' Triad Job No. 93-064
' PREPARED BY: REVIEWED BY:
JENNIFER A. STEIG MICHAEL L. MATHESON, P.E.
W��'c��P
sss
O
WPM 3119/qq
CIATES
' SUMMIT PARK
INTRODUCTION
' The proposed project is to subdivide approximately 12.25 acres into 24 single
family townhouse swelling units and four 6-plex multifamily dwelling units (48 single
' units), a park, open space and future development area. The rectangular shaped parcel lies
' on the east side of Talbot Road, near the City of Renton/King County Line (see Vicinity
Map). More generally, the site lies within Section 31, Township 23 North, Range 5 East,
' in King County, Washington.
A site visit was made on November 9, 1993, an overcast day. Vegetation on the
' east half of the site consists of scattered second and third-growth coniferous and broad-
leafed trees native to the Pacific Northwest. The understory consists of northwest native
' species including salal, sword fern, berry vines and grasses. The western half of the site is
covered with tall pasture grasses and clumps of berry vines. The site generally slopes from
' east to west at approximately 10-13 percent. The S.C.S. Soils Map, located in the
' appendix, indicates the site is underlain by Alderwood soils.
' Runoff from the site sheet flows in a westerly direction and enters the existing
ditch, flowing southerly along Talbot Road. Flow in the ditch continues southerly and
' then enters Springbrook Creek, which is tributary to the Green River.
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UPSTREAM DRAINAGE ANALYSIS FOR THE BASIN
The area upstream of the site is approximately 7 acres in size. The area is partially
developed with approximately one dwelling unit per acre (please refer the Upstream/
' Downstream Map in the appendix).
' UPSTREAM ANALYSIS FOR POINT QUARTER MILE DOWNSTREAM OF BASIN
' The point that lies one quarter mile downstream of the site is located within the existing
ditch along Talbot Road. The area tributary to this point is approximately 480 acres. Please
refer to the Upstream/Downstream Map located in the appendix.
DOWNSTREAM DRAINAGE ANALYSIS
Please refer to the Upstream/Downstream Drainage Map and accompanying
' spreadsheet in the appendix for identification of specific reaches in the downstream drainage
system. As indicated in the spreadsheet, there were no visual problems with flooding, capacity,
' or overtopping.
A&MMM-TES
RESOURCES USED FOR ANALYSIS
' Adopted Basin Plan
The site is located in the Green River Basin.
Basin Reconnaissance Summary Report
rThe site lies within the Black River Drainage Basin as defined in the King County
Basin Reconnaissance Program Summary Volume I. Runoff from the site enters
Springbrook Creek. The Reconnaissance Summary indicates that Springbrook Creek has
' erosion problems upstream of Talbot Road, but no problems have been reported
downstream of Talbot Road where the flow from Summit Park will enter the creek.
Critical Drainage Area Maps
' The site is not located in any of the critical drainage areas discussed in the
' reference section of the 1990 King County Surface Water Manual.
' Flood Plain/Floodway Maps
The FEMA Flood Insurance Rate Map indicates the site lies outside the 500-year
floodplain of the Green River.
t
Sensitive Areas Folio
The King County Sensitive Areas Maps did not reveal the site to be in a problem area
in regards to wetlands, erosion hazard, landslide hazard, flood plain or seismic hazards (please
refer to the appendix for the appropriate maps).
SWM Division Drainage Investigation Section Problem Map
The SWM Division Drainage Investigation Section Problem Map was researched,and
' there have been no complaints in this particular area.
' Wetland Inventory Notebook
The Wetland Inventory Notebook indicated there were no wetlands located on the site
or in the surrounding area, but a wetland has been flagged on the site by Terra Associates, Inc.
dM&IATES
DOWNSTREAM REACH CHART
FOR
SUMMIT PARK
BASIN A
REACH REACH APPROX. APPROX. REACH OBSERVED POTENTIAL
��
DESCRIPTION SLOPE LENGTH(FT) CHARACTERISTICS PROBLEMS PROBLEMS
oR
6-1
YA'deep ditch,6'wide at the top,
A DITCH FLOW 1-2% 1320 feet approx.6"-1'flow depth NONE NONE
6.1
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V \�, ��• Basin Boundary
Subcatchment Boundary -
o ;
9 Collection Point ; �I � :: 3 °°o � , I Q
Stream
0006 Tributary Number Tukwila,_
00301 Proposed Project
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July,1987
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Trib Collect Existing Anticipated
Item River Mile Point Categiory Prop. Proi. Conditions and Problems Conditions and Problems Recommendations
30 0020 21 habitat Excess sediment from Further sedimentation from Several options: construct sediment
RM .60 upstream has destroyed a upstream sediment sources. pond above Talbot Road. Reduce
private trout farm. flow rates and volumes in Trib.
Thousands of fish were 0021. Increase R/D upstream of
killed during Jan. '86 Talbot Rd. Down-zone land to
storm. reduce surface water impacts of
future development.
31 0021 22, Geology Bank erosion, landsliding Problems will continue. - Control storm flows: R/D above
RM .00 in ravine due to outfalls RM .40 in 0021; reroute or
0.50 at end of SE 196th St. control flows from vicinity of
and from R/D pond at SE SE 200th (tightline flows west
200th plus natural on 200th).
sensitivity (landslides). - Provide energy dissipation at R/D
Heavy damage in Jan. '86 outfall (RM .40).
storm. Sedimentation above - Restrict development on north side
old road and in trout farm of 0021 (runoff to be tightlined
below Talbot Rd. (0020), as or routed around canyon).
well as erosion in the
canyon.
32 0021 22 Hydrology 0302 The upper reaches of Development will continue - Construct a regional R/D faci-
RM .40 Springbrook Creek lie and impervious area will lily in the natural drainage
within the city of Renton continue to grow. Increased Swale at the upper reaches of
City Watershed. The creek ninoff will further erode this system.
originates in highly cro- the unstable soils carrying - Constrict a dam across the swale
sive and steeply sloping sediment and debris with an outlet control structure
soils. Alterations of the downstream, where they will and an overflow spillway.
natural drainage patterns reduce the efficiency of or - Tightline existing drainage into
by development, roadway even destroy culverts, pipe- area to further reduce erosion and
construction, and poor lines, and streambeds. raise existing roadway to prevent
practice in handling Flooding could result from flooding.
runoff have increased ero- the inefficierft facilities
sion and sediment coupled with the greater
transport to the lower runoff.
gradient downstream
reaches.
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PR E L IMI N A R Y 93-064
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