HomeMy WebLinkAboutmem_th erosion study_Memo 5_20150225
CITY OF RENTON
Technical Memorandum No. 5
Preliminary Erosion Hazard Evaluation
Thunder Hills Sewer Interceptor
February 13, 2015
Prepared By:
Phil A. Haberman, P.G., P.E.G.
Senior Engineering Geologist
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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Contents
1.0 PROJECT DESCRIPTION ................................................................................................. 1
2.0 EXISTING CONDITIONS ................................................................................................. 1
3.0 REACH LOCATIONS ....................................................................................................... 2
4.0 SITE GEOLOGY .............................................................................................................. 2
5.0 AREA HYDROLOGY ....................................................................................................... 3
6.0 SITE HYDROLOGY OBSERVATIONS ............................................................................... 4
7.0 EROSION POTENTIAL ..................................................................................................... 4
8.0 BANK STABILITY ............................................................................................................. 5
9.0 EROSION HAZARD ESTIMATION .................................................................................... 6
10.0 EROSION POTENTIAL AT CRITICAL CROSS SECTION LOCATIONS ............................... 7
11.0 GENERALIZED EROSION POTENTIAL AT REACH LOCATIONS ....................................... 8
12.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................. 9
13.0 ROCK BUTTRESS/ROCKERY............................................................................................ 9
14.0 FURTHER STUDY .............................................................................................................. 9
15.0 CLOSURE ...................................................................................................................... 10
Appendices
APPENDIX A – VICINITY MAP
APPENDIX B – SITE PLANS (FIGURES 2-7)
APPENDIX C – CROSS SECTIONS (FIGURES 8-11)
APPENDIX D – BOOTH & HENSHAW:
RATES OF CHANNEL EROSION IN SMALL URBAN STREAMS
TECHNICAL MEMORANDUM NO. 5
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TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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1.0 Project Description
Proposed utility work in this area could include replacement or rehabilitation of the
existing sewer interceptor that extends along Thunder Hills Creek from approximately
Grant Avenue South to Interstate 405 (I-405) and/or diverting flow from this basin into
sewer infrastructure west of Talbot Road South.
For the alternatives located in the Thunder Hills Creek basin, construction may include
adding sanitary sewer manholes where none currently exist, creating/rehabilitating an
access roadway to the area near I-405 for maintenance equipment; and potentially,
constructing retaining structures to support the access roadway and sewer main.
2.0 Existing Conditions
The Thunder Hills Creek project area is located between I-405 and Grant Avenue South,
just east of the Berkshire Apartment Home development (Figure 1). For the purposes of
this evaluation, the site area includes the sewer alignment, access roadways along the
alignment, Thunder Hills Creek, and adjacent areas potentially affected by stream
erosion.
The site consists of the existing sewer line alignment through the valley along with a
gravel improved access roadway located adjacent to the stream for much of the
alignment. The access roadway has been damaged significantly by soil movement
and erosion near Station 5+00. North of this location, the roadway is improved
(partially) with quarry rock north to a flat area near I-405. There is a gap where there is
no access roadway where the sewer line crosses beneath Thunder Hills Creek (Station
11+80).
Thunder Hills Creek appears to flow continuously throughout the year and there are
numerous intermittent, seasonal streams that join the creek. The site plans show the
locations of various streams that join Thunder Hills Creek from the east, as well as
wetland areas located in and around the creek. Minor springs and seeps seasonally
add volume to the creek throughout the valley. There are numerous residential yard
drains that add stormwater to the stream. Near Station 5+30, a relatively large creek
joins Thunder Hills Creek from the southeast, approximately doubling the volume of
runoff.
South of Station 5+50, variable amounts of sediment are present in the stream channel
indicating some level of deposition. There is also evidence of periodic flooding and
heavy erosion locally in the northern portion of the alignment. North of Station 5+50, the
stream has incised into the underlying sandstone (Renton Formation).
Four to 12-inch sized quarry rock is present in the stream bed and banks in many areas
along the entire alignment. Larger quarry rock, generally ½ to 4 man sized basalt, has
been used to stabilize the stream banks and/or to prevent ongoing stream erosion.
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Specifically in the vicinity of Station 5+00, large quarry rock has been used to fill the
stream channel. Smaller rockeries, gabion walls, and areas of smaller ballast rock are
present locally along portions of the stream banks from Grant Avenue to I-405.
The site is bordered to the north by I-405, to the west by the Berkshire Apartment Homes,
to the east by undeveloped land (easements) and single family residences, and to the
south by Grant Avenue South, easements, and residential developments.
3.0 Reach Locations
For the purposes of this report, we have separated the overall alignment into eight
reaches of varying length and gradient. The bank and stream characteristics are very
similar south of Station 12+50; however, further study, including setting up survey
monitoring points for a period of years, would be necessary to adequately determine
channel erosion and deposition over time.
Reach Location Approximate Gradient (%)
0+00 to 1+00 Not Applicable (Culvert)
1+00 to 5+50 15.1
5+50 to 8+30 4.3
8+30 to 11+00 8.2
11+00 to 12+30 10.4
12+30 to 18+90 5.2
18+90 to 26+75 7.1
26+75 to 28+00 14.4
4.0 Site Geology
The Geologic Map of King County, indicates that the site is located near the contacts
between Vashon Glacial Till and Tertiary Bedrock.
Based on our site explorations and observations, the Thunder Hills Creek valley is
underlain by the following materials:
Colluvium
Alluvium
Fill
Landslide Debris
Glacial Till
Renton Formation (primarily sandstone)
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Colluvium includes highly weathered glacial till and sandstone that overlies more dense
materials along sloped areas above the valley. In general, these materials consist of
loose mixtures of silt, sand, and clay and are up to 5 feet in thickness.
Alluvium includes soil materials deposited by stream action within the base level of the
valley. These materials consist of loose mixtures of silt, sand, and gravel, and are up to 4
feet in thickness (where present).
In general, fill is present along the existing access roadways within the valley. Deeper
areas of fill are present below the Berkshire Apartment buildings extending to Thunder
Hills Creek. Our borings indicate that the fill consists of silty-sand with variable amounts
of gravel to sandy silt. Local areas of organic debris and construction materials were
observed within the fill. The fill is loose to medium dense and up to 15 feet in thickness
locally.
Landslide debris is present near the toe of steep slopes within the north portion of the
valley. Landslide debris generally consists of very loose to loose mixtures of silt, sand,
and clay, and are up to several feet in thickness, where observed. In general, landslide
debris consists of colluvium, or highly weathered native soils.
Vashon Glacial Till is typically characterized by an unsorted, nonstratified mixture of
clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are
typically dense and relatively impermeable. The poor sorting reflects the mixing of the
materials as these sediments were overridden and incorporated by the glacial ice. In
general, weathered glacial till is yellowish brown and medium dense to dense.
Unweathered glacial till is dense to very dense and typically blue-gray or olive gray in
color. Vashon Glacial Till is present throughout the site area south of Station 5+50.
Tertiary Bedrock in this area consists of the Renton Formation. The Renton Formation
includes feldspathic fine to medium grained sandstone with beds of coal,
carbonaceous siltstone, and claystone. Tertiary Bedrock locally outcrops south of I-90
and the Seattle Fault Zone due to uplift associated with seismic activity. Variable
thicknesses of highly weathered bedrock (soil) are present locally within the valley and
consist of medium stiff to stiff/medium dense mixtures of sandy silt and silty sand.
5.0 Area Hydrology
Groundwater in the Thunder Hills Creek valley is primarily influenced by area streams
and surface water runoff/infiltrating surface waters. The overall area that contributes
surface water to Thunder Hills Creek extends from the site area to the south toward
Puget Drive Southeast, to the east to Beacon Way South, and to the west to the ridge
slightly east of Benson Road South.
The approximate area of this basin is 230 acres and it is about 50 percent developed. A
majority of the development consists of single family residential plats constructed
between the 1950’s and 1980’s. Newer construction, including apartment building
developments, is located directly adjacent to Thunder Hills Creek in the site vicinity.
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We expect that surface water runoff flows through available stormwater infrastructure
and/or ditches, where present. We expect that stormwater that infiltrates into the
subsurface materials becomes perched between the fill and weathered glacial
deposits and the underlying dense to very dense glacial till and/or sandstone. The very
dense till and sandstone are nearly impermeable and shallow perched groundwater
will slowly infiltrate vertically through fractures and joints, and/or migrate laterally.
Perched groundwater that migrates laterally will either daylight along a slope as a seep
or spring, or continue below grade toward area rivers.
6.0 Site Hydrology Observations
There are areas of the site near I-405 where surface water and groundwater is at the
same level (ground surface) and areas where groundwater is not encountered below
stream depths due to stream channel confinement within the Renton Formation
sandstone (Stations 5+50 to 6+50).
We observed numerous 4-inch diameter (or larger) drain pipes extending into or
towards Thunder Hills Creek north of Grant Avenue South.
We anticipate low to moderate seasonal seepage from the valley/channel sidewalls
contributing to the overall volume in Thunder Hills Creek. A majority of volume in
Thunder Hills Creek originates south and upland from the site. Surface waters appear to
contribute a majority of the stream volume.
Some streams that join Thunder Hills Creek are identified on the site plans. During our
site reconnaissance work, we observed minor flows at Stream B (near Station 0+90) and
Stream D (near Station 2+60), both of which are located on the east side of Thunder Hills
Creek. A significant stream joins Thunder Hills Creek from the east at approximately
Station 5+25. We did not observe significant surface water contributions at any other
stream or pipe locations along the alignment during our site visits, nor did we observe
evidence of significant erosion from areas below these pipes, indicating minimal flows
occur at these locations.
7.0 Erosion Potential
The Natural Resources Conservation Services (NRCS) maps for King County indicate that
the project area and directly adjacent side slopes are underlain by Alderwood and
Kitsap soils (very steep), Alderwood gravelly sandy loam (8 to 30 percent slopes) and
Beausite gravelly sandy loam (15 to 30 percent slopes).
Based on our observations and explorations, it is our opinion that the valley area is
underlain by Beausite gravelly sandy loam from Station 0+00 to 5+50 and Alderwood
gravelly sandy loam from Station 5+50 to Grant Avenue South. Kitsap soils are present in
the north portion of site along steep slopes.
The parent material for Beausite soils includes residuum of sandstone (Renton
Formation). The parent material for Alderwood soils include glacial drift (till).
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Since the project is located within an actively incising stream environmental adjacent
to very steep slope areas, all soils should be considered to have “Severe” to “Very
Severe” erosion potential with regard to grading activities and earthwork (soil
exposure). In an undisturbed state, unweathered glacial till and slightly weathered
Renton Formation (sandstone) has very low to low rates of erosion.
8.0 Bank Stability
A bank stability evaluation system, originally proposed by Henshaw & Booth (2000) for
evaluation of urban watersheds in the Northwest, was adapted for the purposes of
identifying areas of increased erosion during the site assessment. The degree (or class)
of bank stability was used as a basis for determining mitigation/stabilization options.
For example, a site with a higher degree of instability may require an engineered bank
stability structure, while a stable reach may not require any treatment for erosion. Table
1 provides a summary of the physical features typically used to classify the banks at the
site. This protocol was adapted to separately describe portions of the bank where the
banks are naturally-occurring or where armoring or manmade structures influence bank
stability.
Table 1
Class
IV Stable
Vegetation (other than grasses) to ordinary high water line
No raw or undercut banks (some erosion on outside of meander bends OK with deposition on
inside bank)
No recently‐exposed roots
No recent tree falls
If bank armoring is present it does not appear to contribute to overall stability
IV.A Stable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring appears stable and/or was observed in conjunction with the above features
III Slightly Unstable
Vegetation to ordinary high water line in most places
Some scalloping of banks
Minor erosion and/or bank undercutting
Recently exposed tree roots rare but present
If bank armoring is present it does not appear to contribute to overall stability
III.A Slightly Unstable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring appears to be degrading slightly or is located such that it only partially stabilizes
the bank II Moderately Unstable
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Vegetation to waterline sparse (mainly scoured or stripped by lateral
erosion) Bank held mainly by hard points (trees, boulders) and eroded
back elsewhere Extensive erosion and bank undercutting
Recently exposed tree roots and fine root hairs common
If bank armoring is present it does not appear to contribute to overall stability
II.A Moderately Unstable with Bank Armoring
Bank armoring is present and appears to be the primary source of bank stability
Bank armoring is degrading extensively or is located such that it only stabilizes the bank somewhat
I Completely Unstable
No vegetation (other than grasses) at ordinary high water line
Banks held only by hard points
Severe erosion of both banks (straight runs) or outside bank with no deposition on inside bank
(bends) Recently exposed tree roots common
Tree falls and/or severely undercut trees common
If bank armoring is present it does not appear to contribute to overall stability
Note: Adapted from Henshaw and Booth (2000).
Preliminarily, we classify the overall site as having classes III, IV, and IV.A , showing signs
of active erosion and loss of bank materials. Minimizing excessive erosion with treatment
actions would include implementing one of the stabilization options described in this
report and the geotechnical report, such as toe protection or slope re-grading (if
feasible).
9.0 Erosion Hazard Estimation
The erosion factor (whole soil) is 0.15 for Alderwood soils and 0.10 for Beausite soils. The
erosion factor (K) indicates the susceptibility of a soil to sheet and rill erosion by water
and is one of six factors used in the Revised Universal Soil Loss Equation (RUSLE) to
predict the annual rate of soil loss by water erosion. Values of K range from 0.02 to 0.69
with the lower values indicating soils more resistant to erosion.
It is our opinion that estimating the rate of erosion using this equation for this site area is
not practical or effective. Determining several of the other factors used in the equation
is difficult if not impossible due to the variable slope conditions within the valley,
variable vegetation patterns, and variations in soil types with depth. Also, this equation
and method is used primarily to determine soil loss over a large uniform area, such as
grazing or farm land.
A scientific study conducted in 2001 on small urban streams in King County provides
estimated rates of channel erosion more applicable to the site area. This study can be
found as an attachment to this report.
This study found that the rates of channel change did not correlate very closely with
development intensity, especially in areas where developments have been established
for a longer period of time.
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February 24, 2015
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The study found that the type of geologic material that underlies the stream strongly
influenced the rate of erosion. The study concluded that granular soil deposits eroded
more readily than the more cohesive silt-clay deposits, which showed low or very low
rates of erosion.
The Thunder Hills Creek area is underlain by glacial till and sandstone. While sandstone
is generally granular, it contains silt particles and is also well indurated and locally
cemented. Both soil units that underlie the site area are cohesive and consistent with
those indicated in the study to have a low to very low rate of erosion. Their respective K
values confirm a low erosion potential when undisturbed.
From this study, our site observations and drilled borings, we anticipate that the rate of
vertical channel erosion is approximately 0.5 inches per year (on average). The rate of
bank erosion varies widely with channel shape and soil characteristics.
Other factors will influence the erosion rate in any given stream. These include gradient
changes, additional stormwater volumes, large storm events, ballast placement or
other mitigation work, and local variations in soil composition/density. Furthermore,
some areas of any given stream will experience soil deposition and not incision. In the
site area, we observed the following general aspects of the Thunder Hills Creek
channel.
Location Observation
Station 1+00 to 5+40 Incision
Station 5+40 to 8+30 Deposition
Station 8+30 to 27+50 Primarily Incision, Minor Local Deposition
10.0 Erosion Potential at Critical Cross Section Locations
We created eight cross sections approximately perpendicular to the stream and sewer
line direction to analyze the potential effect of stream erosion on bank stability (Figures
8-11). Site plans showing the location of the cross sections can be found in Figures 2
through 7.
For this evaluation, we chose the most critical situations based on proximity of the sewer
line to the stream, sewer line depth relative to stream elevation, soil conditions, and
topography between the sewer line and stream.
We utilized these cross sections, along with estimated soil characteristics and
groundwater levels, to assess the potential long term effect of stream erosion on the
existing sewer line. We estimated soil erosion at a rate of 0.5 inches per year while
considering stream level fluctuations and stream bank erosion (channel widening). We
then analyzed the slope stability to determine where potential failure planes may occur
with regard to the location of the sewer line (as necessary). We used an 80 year
effective lifespan for the sewer line and assumed a current stable condition for the
sewer line at all locations.
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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The overall stability of the soils supporting the sewer line at the eight cross section
locations, following 20, 40, and 80 years of stream erosion at approximately 0.5
inches/year is as follows:
Location 20 Years 40 Years 80 Years
9+03 Stable Stable Marginal
9+70 Stable Stable Marginal
11+85 Stable Marginal Unstable
17+47 Stable Stable Stable
20+70 Stable Stable Stable
24+00 Stable Stable Stable
25+85 Stable Stable Stable
27+50 Marginal Unstable Unstable
Additionally, six cross sections were created as part of our Geotechnical Study for the
City of Renton dated February 13, 2015. This study was primarily focused on the portion
of the site north of Station 6+00 and considered both landslide and erosion hazards
within that area. Erosion and landslide hazard mitigation recommendations can be
found in that report.
It should be noted that these results are estimates only based on assumed soil
conditions. Also, the schematic cross sections indicate possible levels of erosion
assuming no maintenance or mitigation of the access roadway and stream banks over
time. We expect, and recommend, periodic observation and documentation of
erosion (at least annually) and implementation of rock armoring and other slope
stabilization as needed.
11.0 Generalized Erosion Potential at Reach Locations
In terms of overall sections of the sewer line alignment, the stability of the soils
supporting the sewer line following 20, 40, and 80 years of stream erosion at
approximately 0.5 inches/year are as follows:
Section/Reach 20 Years 40 Years 80 Years
0+00 to 1+00 Stable Stable Stable
1+00 to 5+50 Marginal Unstable Unstable
5+50 to 8+30 Stable Stable Stable
8+30 to 11+00 Stable Stable Marginal
11+00 to 12+30 Stable Marginal Unstable
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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12+30 to 18+90 Stable Stable Stable
18+90 to 26+75 Stable Stable Stable
26+75 to 28+00 Marginal Unstable Unstable
12.0 Conclusions and Recommendations
Our preliminary erosion analyses indicate that some mitigation work will be required to
achieve the desired 80 year life span for the sewer line extending along Thunder Hills
Creek.
Specifically, the areas between Station 1+00 and 5+50 and near Station 27+50 require
bank stabilization in the near future. The section of stream between Station 11+00 and
12+30 may require mitigation within approximately 40 years. Prior to the 80 year lifespan
of the sewer line, approximately 975 feet of the alignment will likely require some level
of bank stabilization and erosion protection (Station 1+00 to 5+50, 8+30 to 12+30, and
26+75 to 28+00).
It should be noted that our estimation assumes only stream incision and that there will
be no periods of cyclical deposition and erosion or mitigation efforts/maintenance of
existing slopes/banks. Other than near Station 27+50, erosion prevention and
maintenance of existing slopes and stream banks may reduce the effect of stream
erosion during the useful life span of the sewer line and access roadways.
The sewer line near Station 27+50 should be stabilized from the effects of soil erosion
during project construction. We anticipate that the most cost effective means of sewer
line protection and erosion prevention is slope armoring/rockery construction in this
area. Note: gravity wall recommendations as part of bank stabilization are included in
the Stantec Geotechnical Report dated February 13, 2015.
13.0 Rock Buttress/Rockery
We recommend using 1 to 4 man sized rocks and embedding the base rocks into the
dense to very dense glacial till at least 2 feet. This may mean excavation through a
variable thickness of weathered glacial till and/or other soil materials before achieving
the dense till. We can investigate the area using hand borings prior to construction to
estimate the excavation depths required. Alternative recommendations may be made
at that time if soft soils are present to greater depths.
14.0 Further Study
Additional geomorphologic analyses, including further bank stability analyses, reach
delineation, and a surveyed study of channel migration, erosion, and deposition should
be conducted over a period of 5 to 10 years. This type of geomorphologic study would
more adequately determine the extent, limits, and effects of stream erosion within the
TECHNICAL MEMORANDUM NO. 5
February 24, 2015
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site area and on sewer line stability, as well as provide suitable bank stabilization options
as needed.
15.0 Closure
This report was prepared for the exclusive use of the City of Renton and their appointed
consultants. Any use of this report or the material contained herein by third parties, or
for other than the intended purpose, should first be approved in writing by Stantec.
The general recommendations contained in this report are based on site observations
and boring explorations, along with the preliminarily proposed construction.
TECHNICAL MEMORANDUM NO. 4
February 24, 2015
APPENDIX A
Vicinity Map
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
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SITE
N
VICINITY MAP
FIGURE 1
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Project
Location
Renton
WASHINGTON
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
TECHNICAL MEMORANDUM NO. 4
February 24, 2015
APPENDIX B
Site Plans (Figures 2-7)
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
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11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 2
Cross Section
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
9+03 Section
9+70 Section
N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 3
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
11+85 Section
Cross Section
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Cross Section
17+47 Section
SITE PLAN
FIGURE 4
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 5
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 6
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
24+00 Section
Cross Section
Potential Location of
Gravity Wall or Rockery
N
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
27+50 Section
25+85 Section
Cross Section SITE PLAN
FIGURE 7
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Potential Location of
Gravity Wall or Rockery
TECHNICAL MEMORANDUM No. 4
February 24, 2015
APPENDIX C
Cross Sections (Figures 8-11)
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TECHNICAL MEMORANDUM No. 4
February 24, 2015
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Cross Sections
Figure 8
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
9+03
155’
165’
.
9+70
157’
167’
.
A
B
A
B
T
T
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
Elevation
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 9
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
11+85
170
180
.
17+47
204’
214’
.
A
A
B
B
T
T
Building Rockery
A
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
.
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 10
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
20+70
227’
237’
.
.251’
261’
24+00
A
A
B
B
T
T
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
.
.
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
Cross Sections
Figure 11
11130 NE 33rd Place, Suite 200Bellevue, WA 98004(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
256’.
266’
25+85
.
27+50
280’
290’
A
A
B
B
T
T
A Generally loose to medium dense soils (SM-ML)
B Generally medium dense to very dense soils (SM)
T Thunder Hills Creek.Approximate Sewer Location
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Note: All contacts and elevations are approximate
Schematic of possible erosion only
Elevation
Elevation
Potential Area Eroded in 20 year span
.
.
Potential Area Eroded in 40 year span
Potential Area Eroded in 80 year span
..
TECHNICAL MEMORANDUM No. 4
February 24, 2015
APPENDIX D
Booth & Henshaw: Rates of Channel
Erosion in Small Urban Streams
v:\2002\active\2002003607\civil\design\report\mem_th erosion study_20150224.docx
TECHNICAL MEMORANDUM No. 4
February 24, 2015
v:\2002\active\2002003607\civil\design\report\mem_th erosion study_20150224.docx
1
Rates of Channel Erosion in Small Urban Streams1
By Derek B. Booth and Patricia C. Henshaw
Center for Urban Water Resources Management
Department of Civil Engineering
University of Washington
ABSTRACT
We address four objectives, focused on urban and urbanizing watersheds that drain forested (or once-forested)
landscapes in humid regions: to document rates of channel change, to evaluate the relationship between development
intensity and the rate of channel change, to evaluate geologic and topographic controls on channel change, and to
determine if predevelopment conditions can be used to predict erosion-susceptible reaches. We used an 11-year data set
covering 21 urban and suburban channels in western Washington, draining watersheds from 0.1 to 20 km2, a range that
covers both seasonal and perennial channels that generally respond readily and rapidly to watershed disturbance. The
results indicate:
1. Rates of vertical channel change vary from below the range of measurement error (<20 mm vertical change
between visits) to about 1 m (width-averaged) per year. The median rate for this sample population was 60 mm per
year.
2. Within these lightly to moderately urbanized watersheds, rates of channel change did not correlate with
development intensity.
3. The nature of the geologic substrate strongly influenced whether or not significant channel change occurred, but no
unconsolidated substrate appears immune to change given sufficiently severe watershed disturbance. Erosion rates
are independent of channel gradient within the range investigated (0.013-0.52).
4. Channels with the greatest susceptibility to rapid vertical change share the following characteristics, which can be
used to predict and so reduce the consequences of future urban development on natural stream systems:
Erosion-susceptible geologic substrate;
Moderate to high gradient;
Absence of natural or artificial grade controls;
Predevelopment runoff inputs predominantly via subsurface discharge, likely to be converted to surface (point)
discharge in the post-development condition.
INTRODUCTION
Changes to channel morphology are among the most common and readily visible effects of urban development on
natural stream systems in humid environments. The actions of deforestation, channelization, and paving of the uplands
can produce tremendous changes in the delivery of water and sediment into the channel network. In channel reaches that
are alluvial, subsequent responses can be rapid, dramatic, and readily documented. Channels widen, deepen, and in
extreme cases may incise many meters below the original level of their beds. Alternatively, they may fill with sediment
derived from farther upstream and braid into multiple rivulets threading between gravel bars. In either case, they become
aesthetic eyesores and biological invalids; natural populations of benthic invertebrates and fish are decimated, to be
replaced by limited numbers and taxa of disturbance-tolerant species.
This chapter reviews the current understanding of the behavior and physical changes reported from stream channels in
urban and urbanizing watersheds. Our focus is on those draining forested, or once-forested, landscapes in humid regions
because the subsequent channel changes there appear to be most dramatic and detrimental to physical, biological, and
aesthetic attributes. The historic data on channel changes is supplemented by our own 11-year data set on urban and
suburban channels in western Washington, draining watersheds from 0.1 to 20 km2, a range that covers both seasonal and
perennial channels that generally respond readily and rapidly to watershed disturbance.
1 D.B. Booth and P.C. Henshaw, 2001, Rates of channel erosion in small urban streams: chapter in M. Wigmosta and S. Burges,
eds., Land Use and Watersheds: Human Influence on Hydrology and Geomorphology in Urban and Forest Areas: AGU
Monograph Series, Water Science and Application Volume 2, pp. 17–38.
2
Previous Studies
Although urban-induced channel changes are widely recognized, their magnitudes, rates, and controls are largely
matters of sparse data and anecdotal information. As a result, we have only limited understanding of the physical
determinants of channel change and even less predictive ability of the likely consequences of urban development on
downstream channels. Although a variety of papers have explored the phenomenon of channel change in urban
environments, they do not establish a consistent framework because they have analyzed the process from multiple
perspectives:
Watershed hydrology, where the most detailed analysis is conducted on the hydrologic changes brought by urbanization,
and any channel response is shown or presumed to follow those changes directly;
Sediment loading, where the delivery of sediment into the channel from an urban or urbanizing watershed is analyzed
most completely, and the observed channel response is explained in whole or part through the trends of that sediment
delivery over time; or
Conceptual models of channel erosion and sedimentation, where a sequence of channel responses, initiated by
watershed change, follows a predictable path somewhat independent of the details of the initiating upstream activity.
Watershed Hydrology. Changes in the hydrologic response of an urban watershed, notably the increase in stream-flow
discharges, are demonstrably the clearest single determinant of urban channel change. Even where the channel is
physically isolated from both physical disturbance and new inputs of coarse sediment, the occurrence and magnitude of
increased discharges generally are mirrored by observed increases in channel dimensions. Previous studies that present
such relationships include Hammer [1972], Hollis and Luckett [1976], Morisawa and LaFlure [1982], Neller [1988],
Whitlow and Gregory [1989], Moscrip and Montgomery [1997], and Booth and Jackson [1997]. Yet this relationship,
although common and intuitive, is not universal. A few studies note a reduction in channel width or depth with increases
in watershed urbanization and, presumably, the discharge that accompanies it [e.g., Leopold, 1973; Nanson and Young,
1981; Ebisemiju, 1989a; Odemerho, 1992].
Sediment Loading. Delivery of sediment into the channel network is a common consequence of urban development with
potentially significant expression in the channel form. The broad relationship between stages of watershed development
and resulting sediment loads have been presented in studies such as Wolman [1967], Graf [1975], and Douglas [1985;
Table 1, below]. Increased sediment loads, generated at particular stages in the forest-agriculture-urban sequence of North
American land development, exert an opposing tendency on the channel to that of increasing discharge and probably
explain much of the channel narrowing or shallowing that is sometimes measured.
TABLE 1. Conceptual relationship between stages of development, sediment yield, and channel stability [from Douglas, 1985].
“Stage” Land Use Lag time Sediment Yield Channel Stability
A Natural forest or
grassland 100 years low Relatively stable with some
bank erosion
B Heavily grazed 80 low to moderate Somewhat less stable
C Cropping 75 moderate to heavy Some aggradation and
increased bank erosion
D
Abandonment of
cropping;
permanent grass
85 low to moderate Increased stability
E Urban construction 40 very heavy Rapid aggradation and some
bank erosion (can braid)
F Stabilization 25 moderate Degradation and severe
bank erosion
G Stable urban 15 low to moderate Relatively stable
Conceptual Models of Channel Erosion and Sedimentation. Previous efforts to integrate the generally similar, but
locally disparate, observations of channel change [see Park 1997] into a unified model generally articulate a sequence of
anticipated changes over time. For example, Douglas [1985] suggested a specific pattern of watershed development and
channel response (Table 1).
3
Simon [1989] evaluated the consequences of channelization and described a predictable evolutionary sequence of
undercutting, bank failure, channel widening, and restabilization that closely resembles that of urbanization. Arnold et al.
[1982] also recognized the interplay of spatial factors, notably upstream stream erosion and downstream deposition, that
can result in multiple “responses” along the same channel, a theme of complex spatial and temporal response echoed by
Gregory et al. [1992] and Park [1997].
Lessons from Previous Work
In any given locality, observed correlations between channel size, rate of channel change, and watershed characteristics
are likely to be fortuitous or at least non-universal. In general, the dimensions of channels in an urban stream network will
tend to follow the overall pattern of discharge across that network—larger flows beget larger channels. Consequently, a
naïve prediction of channel change based on the magnitude of anticipated hydrologic change [Booth, 1991] is also
probably justified as a first-order estimate (e.g., Figure 1 as an example from the Pacific Northwest). Yet details of the
channel, the watershed, and the timing and location of the measurement itself may overwhelm this presumption of
channel-discharge equilibrium for the following reasons:
Location of the measurement station in the channel network: Is the measurement located in a “transport” reach,
where water and sediment are passed downstream with little channel adjustment, or a “response” reach, where
channel form readily adjusts to changing conditions? Not every channel responds to increasing sediment load or
water discharge in the same way [e.g., Montgomery and Buffington, 1997]. Local channel gradient and the pattern of
gradient changes across a channel network are particularly important factors, but they are rarely reported or
incorporated into case-study analyses.
Location of urban development relative to the channel network: This includes the obvious factors that headwater
development will affect more of the channel network than one that drains into the stream farther downstream, and that
the influence of a particular area of disturbance will be proportionally greater on progressively smaller catchments.
Similarly, developments that concentrate urban effects in only a few areas tend to have less impact on the channel
network as a whole than equivalent development spread across the watershed [Ebisemiju, 1989b]. In addition, flow
increases introduced at one point in the channel network may be far more effective at eroding sediment than at
another, because of the spatial variability of watershed soils and the distribution of alluvial and bedrock (or other non-
alluvial) reaches.
Interplay of the timing of watershed development, large storms, and stream-channel observations: Many of the
“relationships” advocated in the literature between channel form and the magnitude and age of the watershed
development are probably artifacts of a particular combination of unique temporal or geomorphic factors [Henshaw,
1999]. “Stable” stream channels may simply reflect a lack of recent rainfall [e.g., Bird, 1982]. They are expected in
mature systems where fluvial equilibrium has truly been reestablished (as anticipated for example by Hammer 1972,
Neller 1988, Ebisemiju 1989b]; but alternatively, they may simply be the product of flushing all mobile sediment
from the system to produce a relatively static, non-alluvial channel, where change may still occur [e.g. Tinkler and
Parish, 1998] but at rates generally slower than reported elsewhere. If equilibrium can be achieved in a disturbed
fluvial system, it will depend not only on the at-a-station fluvial processes but also on factors outside of, and perhaps
wholly unaccounted by, fluvial conditions in the immediate channel reach. These factors include adjacent hillslope
stability, which may have a dramatically longer time scale for stabilization than the fluvial system; and channel
stability farther upstream, particularly the absence of large upstream sediment sources.
Most previous studies have reported examples of particularly dramatic channel changes. These sites are commonly
erosional, because this response is generally more rapid and more localized than deposition, and because the occurrence of
channel erosion (particularly downcutting, commonly the first such fluvial response) can initiate adjacent hillslope failures
that mobilize substantially more sediment over a wider area than the original fluvial process. Thus they call attention to
themselves from researchers and the public alike. That attention is entirely appropriate—such changes are among the most
serious environmental disruptions for both human and biological use of streams in the urban environment. However, such
a level of attention introduces a bias into our assessment of what constitutes “urban channel changes.”
4
Figure 1. Bankfull channel widths, segregated by percent effective impervious area (EIA; see Dinicola, 1990)
contributing to the measurement point. A discrimination at 6 percent EIA was chosen because it approximates
the rural-to-suburban transitional land use in this region. From Booth and Jackson [1997].
Channel Types and Classification
Principles and Limitations. Geomorphologists and biologists have been organizing and categorizing the myriad array of
stream channels for about a century. The purpose of such an organization is fundamental: if a channel of interest can be
placed in a group, and the properties of that group are already known, then the properties of the new channel will also be
known with little additional work [Kondolf, 1995]. Those “properties” depend on the organizational scheme, but they
include such attributes as the channel’s response to environmental change (such as increased sediment load or placement
of an artificial habitat-enhancement structure) or its importance in supporting stream biota [Mosley, 1987]. Intrinsic
differences between channels will strongly influence channel response to urban development.
Yet the influence of a classification scheme can be detrimental, by suggesting an overly simplistic range of channel
conditions that obscures critical differences between channels that are ostensibly “the same.” It may also impart a false
understanding if the classification method is taken outside of where it was developed to where the dominant landscape
processes, or range of landscape conditions, are significantly different: channels may be “classified” but the predictive
power of that classification will be low or misleading.
Two examples, both relevant to urban stream channels of the Pacific Northwest, illustrate this problem. The
classification method of Rosgen (e.g., 1994 and prior informal publications), applied widely throughout the United States,
does not include the influence of large logs and other woody debris on channel processes, reflecting the non-forested
environment in which this method was first developed. A forested stream may be “classified” by this method but the
nature of its response to human disturbance may be poorly predicted. In contrast, the classification of Montgomery and
Buffington [1997] was established explicitly to address the channels found in forested watersheds of the Pacific
Northwest, where such large woody debris (LWD) is ubiquitous and its influence can be dominant. Yet this method was
developed in mountain drainage basins sharing a typical downstream progression from steep headwater catchments
underlain by bedrock to gentler, larger watershed areas in broad alluvial valleys. This orderly sequence may not be
matched in a lowland setting—the smallest watersheds of urbanizing Puget Sound can be quite flat, with steeper reaches
located some distance farther downstream. Sediment-delivery processes and sources of channel roughness are very
different in lowland urban channels than in nearby mountainous channels, and so this classification system also may not
fully predict the response of a particular urban stream.
Criteria. Despite these caveats, different channel “types” display different intrinsic channel behaviors and have
different responses to watershed disturbance. No framework has been fully developed for our environment of interest, but
we are using the conceptual approach of Montgomery and Buffington [1997] because of its orientation on channel-forming
processes, its development in the same climatic region as the present study, and its explicit recognition of the influence of
LWD and other such obstructions on channel morphology. By their terminology, most of our channels are either “plane-
bed” or “forced pool-riffle” channels—relatively flat-bottomed channels lacking well-defined bedforms and instead
displaying long, and commonly channel-wide, reaches of uniform riffles or glides which can aggrade or degrade rapidly in
response to changing water and sediment fluxes. Development of a more heterogeneous morphology depends on the
presence of immobile material, most commonly LWD. By restricting our evaluation to such channels, we may be limiting
the potential utility of our work. Yet the vast majority of the small, responsive, urban streams in our region fit these
categories, and this selectivity helps avoid the risk of transferring results inappropriately.
5
A Conceptual Framework to Assess Channel Change in Urban Watersheds
Past studies and repeated observation suggest a “typical” scenario for channel change in an urbanizing lowland
watershed. Recognizing that this scenario does not encompass the full range of potential watershed conditions or stream-
channel responses, it nonetheless characterizes the most common “problems” of urban channel change and highlights those
settings where an unexpected response suggests the presence of atypical channel or watershed conditions.
Consider a watershed of some tens of hectares up to several square kilometers, where development has blanketed the
upper watershed and so the first-order channel(s) are the most fully affected of any in the channel network. In most cases,
channel expansion of at least several times the original cross sectional area accompanies the progression from rural to
suburban to urban land uses. Whether or not the response of the channel to these flow increases is “orderly” (i.e. channel-
size increases in approximate proportion to discharge increases in the sense of Booth, 1990) or “catastrophic” (i.e. rapid
incision) is largely independent of the magnitude of the watershed disturbance (see below). Even low levels of land-cover
changes, if accompanied by an efficient collection system (e.g., road ditches) can produce significant increases in
headwater channel discharges, which in turn will initiate increased in-channel erosion and sediment transport.
Because such land-use changes typically occur over a period of many years or decades, they tend to produce continuous
changes in the downstream channel subject only to the variability of seasonal runoff. Any tendency towards “equilibrium,”
either dynamic or static, is completely obscured during this period. Sparse long-term data suggest that true equilibrium
may be possible in watersheds with constant land use, over a years-to-decades time lag [Henshaw, 1999], but actually
observing such a condition depends on achieving stable hillslope conditions as well, which may take many times longer.
With these complications, it is not surprising that “reequilibration” may be more useful as a theoretical construct than as a
widely observed condition.
The sediment released by this scenario of headwater flow increases may or may not accumulate as it passes through the
downstream channel network. The potential input of additional urban-flow-induced sediment from other lateral tributaries
will combine to influence whether sediment, eroded from upstream reaches, can remain in active transport or will
accumulate in noteworthy volumes. Curiously, the vagaries of human infrastructure, particularly small roadway culverts
that were sized and installed during an earlier pre-headwater-development era when only lower discharges of water (and
tremendously lower discharges of sediment) occurred, appear to be one of the strongest single determinants of whether the
urban channel change is perceived to be mainly a problem of “erosion” or one of “deposition” (Figure 2).
Figure 2. Deposition of stream-channel sediment, eroded from upslope reaches of tributary 0143G (station C8).
6
FIELD INVESTIGATIONS
No single study can cover all settings in which urban-induced channel change is observed. Yet even a geographically
limited set of new data can increase our understanding and prediction of this threat to aquatic-system integrity. This study
was initiated to provide some of that new data, focused on a part of western Washington state where (and beginning at a
time when) the rate of new urban development was accelerating to historically unprecedented rates. It also began when the
social and political desire to alleviate the worst environmental consequences of that development far exceeded the concrete
knowledge necessary to achieve that goal.
Starting in 1986, 35 stations along an equal number of independent streams were established to monitor long-term
channel changes in urbanizing watersheds. The purpose of this effort was four-fold:
1. To document erosion and deposition rates in a variety of physiographic settings;
2. To test the hypothesis that urban development consistently increases the rate of channel change, and that higher
levels of urban development are correlated with faster rates of channel change;
3. To test the hypothesis that certain geologic and/or topographic settings are particularly susceptible to urban-
induced channel changes; and
4. To improve identification of the most susceptible sites before development, and thus before degradation, has
begun.
Methods
Study sites. The choice of channel reaches for monitoring began in early 1986, following a particularly large storm in
January that resulted in many instances of channel modification and property damage from high discharges. These first
sites were chosen because of known stream-channel erosion, reported downstream problems, or knowledge of impending
development that might prove problematic. Over the next several years, a number of additional sites were identified and
some unsuitable sites were relocated or abandoned, mainly due to unrepresentative channel conditions but also because of
subsequent obliteration by development activity. A range of channel conditions, particularly slope, degree of upstream
development, and geographic location, were covered by the final set of 21 selected sites (see Figures 3 and 4 and Table 2).
Previous observations had suggested that channel changes were particularly rapid downstream of recent urban
development in small headwater catchments and in channels traversing hillslope deposits of a specific regionally common
geologic deposit, so these characteristics were emphasized in the initial site selection.
7
TABLE 2. Stream erosion station characteristics.
Station
Station Name
Drainage
Area
(km2)
Local
Slope
%
%
EIA1
Channel
Width
(m)
Substrate
Type
Length
of
Record
(yrs)
C1 McAleer Creek 18 2 22 9.8 silt-clay 7
C2 McAleer
tributary 0.6 2 20 4.1 silt-clay 11
C3 Holmes Point
trib. 0.5 5 16 2.4 sand 4
C4 Juanita Point
trib. 0.4 4 12 1.7 sand 11
C6 Skookum
tributary 1.7 2 5 2.5 sand 9
C7 Timberline trib.
0143F 0.2 14 1 2.3 sand 7
C8 Timberline trib.
0143G 0.1 48 15 2.3 sand 11
C9 upper 0164A 0.5 3 16 4.9 sand 7
C11 Ginger Creek 1.8 5 22 7.9 sand 5
C12 mid-Madsen
Creek 6.5 3.5 10 6.6 sand 10
C13 Hollyw'd Hills
trib. 4 2 7 4.6 sand 7
G2 Garrison Creek 2.6 5 10 1.8 sand 2
G3 Mill Creek 4 2.8 19 4.3 sand 3
G5 Cobble Creek 0.7 5.5 11 4.8 silt-clay 11
PS3 Easter Lake
outlet 0.6 1.3 40 3.7 sand 11
PS4 Olympic View
Park 1.5 2.1 17 2.4 sand 11
PS7 Boeing Creek 5 2 20 8 sand 2
S2 Chasm Creek
mainstem 0.2 5 3 2.4 sand 4
S3 Pepper Creek 1 5 5 6.6 sand 7
S4 Lk. Alice Estates
trib. 0.1 52 10 2.6 sand 11
S5 Joule short plat
trib. 0.1 5 5 2.4 sand 4
1EIA = Effective impervious area
Most of the sites were, broadly speaking, alluvial channels [Leopold et al., 1964]: carved by running water into the very
sediment carried by that flow in the past, and that presumably could be carried by that flow in the future. These "self-
formed" channels are free to adjust their shape in response to subsequent changes in flow and thus were anticipated to
respond most sensitively to future development. However, as the channel changes in response to increased flows (and
particularly if it begins to incise) the underlying hillslope deposit becomes more dominant as the channel-bounding
sediment and the alluvial “character” of the channel can be reduced. In contrast, a channel formed in alluvial sediment but
also choked with immovable roughness elements, such as logs, is not strictly “alluvial.” Yet if those logs are removed, or
if progressive bed erosion strands those logs above the elevation of the flow, the channel will become more
characteristically alluvial over time.
The sample population was chosen to explore the influence of the underlying geology by emphasizing sites located on a
particularly erodible substrate. Most of the stations have as their underlying substrate a thick and widespread sandy
deposit with local concentrations of pebble to cobble gravel, laid down by glacial outwash streams during the last advance
of the continental ice sheet (regionally named the “Vashon” by Armstrong and others [1965] and spanning an interval of
8
about 17,000-13,000 years ago). This emphasis was established to quantify rates in what previously had been observed
locally to be the most erosion-susceptible deposit. A moderate number of sites with other substrates were also included to
test this hypothesis more precisely.
Figure 3. Location of stream measurement stations listed in Table 2.
Seattle
C2
C1
C3
C4
C6
C13
C8
C7
C9
C12
C11
S3
S2
S5S4
PS3
PS4
PS2
PS1
G2
G3
G5
10 mi
20 km
122 15’
o 122 00’
o
P
u
g
e
t So
und
Ta co m a
Green Ri v er
C
e
d
ar River
Sno
q
u
a
l
m
i
e
River
47 45’
o
47 30’
o
47 15’
o
1
10
100
0.1 1 10 100
Drainage Area (square km)
Lo
c
a
l
C
h
a
n
n
e
l
G
r
a
d
i
e
n
t
(
p
e
r
c
e
n
t
)
.
High Urban (EIA > 20%)
Medium Urban (20% > EIA > 6%)
Low Urban (6% > EIA)
Figure 4. Summary attributes of measurement stations listed in Table 2.
9
The work was initially sponsored by the jurisdiction of King County, Washington, so all sites were located within its
boundaries. By virtue of its location and size, however, this 5000-square-kilometer area cuts a remarkably diverse and
representative swath across the Puget Lowland and Cascade Range of western Washington; it also spans a range of land
uses from forested wilderness through agriculture, suburbia, and intensely urban. In the rapidly developing suburban
fringe targeted by this study, annual precipitation averages about 1000 mm, falling primarily from October to April as
large frontal storms of several days’ duration. Unlike much of the rest of the continent, short but intense storms are rare.
The 100-year 6-hour rainfall intensity in this region is only about 5 mm/hr [Miller et al., 1973], whereas the largest stream
discharges are generally associated with moderate-intensity rainfall following a period of extended wintertime
precipitation or snowmelt.
Measurement Techniques. Cross sections were measured using two procedures, modified from that suggested in Dunne
and Leopold [1978]. A specific location along a relatively straight and uniform part of the channel, qualitatively judged to
be “representative” of the reach in question, was selected. Two endpoints were established to define a line approximately
perpendicular to the channel. Where available, streamside trees were used and the precise endpoint was marked with a
12d galvanized nail driven nearly flush with the trunk. Where trees were unavailable, 0.6-meter-long pieces of reinforcing
bar (“1/2-inch rebar”) were driven into the ground to provide a suitable monument. We rarely had much difficulty in
relocating such markers, even after four years’ absence, with sufficiently detailed notes.
In the early years of this study, a 50-m steel tape was stretched and held at constant tension between endpoints to
provide both horizontal and vertical reference. The vertical distance between the tape and the ground surface was recorded
at 0.3-m intervals, together with additional intermediate measurements at marked breaks in slope. At each station, the
vertical angle and the parabolic sag of the tape were measured and used to correct the raw data. This method had the
advantage of speed and minimal field equipment but had some inherent inaccuracies. In the last year of the study we used
an automatic level and surveyor’s rod to determine channel depths, relying on the stretched tape only to specify horizontal
distance. By direct comparison of these two measurement methods, we could determine the precision of the early method;
its error was consistently less than 0.02 m, at or below the degree of inescapable measurement imprecision imposed by
ground irregularities and sediment clasts on the channel bed.
In the early years of the project, measurements at most sites were made annually in the summer, the season when
qualitative observations suggest that little or no channel changes occur from about June until October. After collecting
data in the summer of 1990 from changes during the large storms of January 1990, the primary objectives of the study had
been achieved and measurement frequency was reduced, with visits to most sites only in 1993 and 1997.
Results
Overview. Rates of erosion and deposition vary by over two orders of magnitude (see Figure 5). In this population, the
minimum amount of the annual width-averaged vertical channel change was below the level of measurement error (about
20 mm); the maximum was about 1 m per year. Over the 11-year period, 80 percent of all measurements show an annual
width-averaged vertical change (erosion or deposition) of less than 0.2 meters, with the median of all measurements at 60
mm/year.
-3
-2
-1
0
1
2
3
4
0246810
CHANNEL WIDTH (m)
Ch
a
n
n
e
l
E
r
o
s
i
o
n
(
s
q
u
a
r
e
m
)
.
0.06 m incision
0.06 m aggradation
Figure 5. Results of all measurements, expressed as the average change in cross-section area per year between visits. Dotted lines plot
the median vertical change of 0.06 m.
10
The most consistent pattern is the correlation of rainfall with channel change. This outcome is qualitatively intuitive,
although the nature of this relationship is more complex than might be first anticipated. For example, 1990 channel
changes (i.e. occurring between the 1989 and 1990 measurements) are the largest, by a significant degree, at nearly all
sites (Figure 6). Although the 1990 rainfall intensities are also the largest in the period as well (Figure 7), they do not
exceed other “large” years (1991 and 1996) by nearly as much as the erosion/deposition measurements would suggest.
0
10
20
30
40
50
60
70
%
o
f
St
a
t
i
o
n
s
.
1987-
1988
1988-
1989
1989-
1990
1990-
1993
1993-
1997
0-0.06 m
0.06-0.25 m
> 0.25 m
Figure 6. Distribution of stations below, above, and greatly above the median vertical change of 0.06 m/year. Channel changes are
calculated across the full period between measurements and are not annual averages. Note that the distribution of stations in the 1989-
1990 interval is nearly identical to that of 1993-1997, even thought the earlier period is only one-quarter as long.
0
10
20
30
40
50
60
70
80
90
1988 1989 1990 1991 1992 1993 1994 1995 1996
Water Year
Ma
x
i
m
u
m
2
4
-
H
o
u
r
R
a
i
n
f
a
l
l
(
m
m
)
.
Figure 7. Maximum 24-hour rainfall for each year of the interval 1988-1996. The chosen gauge (“Maplewood”) was centrally located
for the study sites (near station C11 on Figure 3).
11
Most noteworthy of this data set, however, is the overall absence of general relationships between measured channel
changes and simple, physical parameters of the stream or of the watershed, such as slope (Figure 8) or imperviousness
(Figure 9). This condition bodes poorly for the kinds of simple predictive methods favored by local governmental
jurisdictions in the prediction and avoidance of environmental impacts. Only the role of geologic materials shows any
consistency, with cohesive silt-clay substrates generally permitting only low rates of channel adjustment.
The poor correlation between effective impervious area (EIA) and channel change is quite robust. It is displayed by
both the station averages for the period of record (Figure 9) and the single-year (1989-1990) data (Figure 10). We
therefore reject the first of our initial hypotheses, that urban development consistently increases the rate of channel change,
and that higher levels of urban development are correlated with faster rates of channel change.
Evidence against this hypothesis is particularly clear at the following stations, discussed below in greater detail:
Moderate to high development, moderate to minimal changes:
Easter Lake outlet (40% EIA, moderate change)
Olympic View Park (17% EIA, very little change)
McAleer Tributary (20% EIA, very little change)
Little development, large changes:
Pepper Creek (3% EIA, very large change)
Although hydrologic processes may impose a general tendency for increased urbanization to yield greater channel change,
the expression of that change is completely swamped by the vagaries of local conditions.
Our other initial hypothesis, the association of particular topographic or geologic conditions with rate of change, finds
much more consistent support from the data. Granular hillslope deposits, normally mantled by alluvium but accessible to
streamflow in an incising environment, were anticipated to display the greatest changes for a given degree of upstream
urbanization; indeed, a majority of sites were chosen on the basis of this very attribute. The type of deposit does appear to
exert a significant influence on channel-change rates; the most common alternative, cohesive silt-clay deposits,
consistently showed low or very low rates of change.
0.01
0.10
1.00
110100
Channel Slope (%)
In
c
i
s
i
o
n
o
r
A
g
g
r
a
d
a
t
i
o
n
,
1
9
8
9
-
1
9
9
0
(
m
)
.
Sandy Deposits
Silt-clay Deposits
Figure 8. Demonstration of the poor correlation between local channel slope and the magnitude of one year’s width-averaged vertical
change. A more consistent pattern is suggested by the relatively low change shown at each of the three stations underlain by cohesive
(silt and clay) hillslope deposits.
12
0.0
0.1
0.2
0.3
0.4
0.5
0 1020304050
Effective Impervious Area Percentage in Watershed
Av
e
r
a
g
e
A
n
n
u
a
l
I
n
c
i
s
i
o
n
o
r
D
e
p
o
s
i
t
i
o
n
.
Sandy Deposits
Silt-clay Deposits
(m
e
t
e
r
s
)
Figure 9. Demonstration of the poor correlation between contributing impervious area and the magnitude of the annual width-averaged
vertical change, averaged over the full duration of each station’s measurement history.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1020304050
Effective Impervious Area Percentage in Watershed
A
v
e
r
a
g
e
I
n
c
i
s
i
o
n
o
r
D
e
p
o
s
i
t
i
o
n
.
(
m
e
t
e
r
s
)
Sandy deposits
Silt-clay deposits
Pepper Ck.
Easter Lk. Outlet
McAleer trib.
Olympic View Pk.
(0143G)
(0143F)
Figure 10. The equivalent parameters as for Figure 9 but considering only the measurement interval with the greatest change, 1989-1990.
There are no appreciable differences between the pattern expressed by either the single-year or the time-averaged results. Labeled
points are discussed in the following section.
13
Specific Site Conditions (see Figure 10)
Olympic View Park. This channel is located in a lightly developed parkland (Figure 11), established around the long-
protected riparian corridor of the stream. The surrounding watershed has been almost fully developed for several decades,
primarily with single-family residences. The ravine that contains the channel and associated park is excavated into
deposits of the sandy Vashon advance outwash. Incision has clearly been part of the channel’s past history; several
hundred meters downstream of the measured cross section, large gabion baskets stabilize what must have been a major
knickpoint in the 1970’s. Yet the current decade of measurements is noteworthy in its near-negligible change from one
year to the next (Figure 12), although the channel morphology is distinctly unappealing from either a biological or an
aesthetic standpoint. It is relatively uniform, slightly sinuous, with virtually no heterogeneity or variability in size, shape,
or roughness. Much of this uniformity is surely the result of close human contact—foot traffic up and down the channel
(commonly dry in the summertime) is frequent, and any sticks or twigs would be promptly “cleaned up.” Yet even where
encroaching riparian shrubs limit the immediate access of people, conditions are essentially unchanged.
Figure 11. View of the channel of Olympic View Park (Station PS4) in 1986, looking upstream. Line across the channel (below the scale
bar) is the tape measure used to determine horizontal distance at the cross section location.
0
0.5
1
1.5
2
2.5
3
3.5
4
1234567
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 12. All cross section measurement of Station PS4, showing only minor changes from 1987 to 1993 (the total width-averaged
erosion during this interval is 8 cm) and from 1993 to 1997 (total average erosion 7 cm).
14
Easter Lake Outlet. This channel is also in a well-established part of King County, with a high level of urban
development in the contributing watershed but with most of it predating 1970. About 80 percent of the runoff from that
watershed drains through Easter Lake, which occupies about 10 percent of the contributing surface area of the watershed
and which provides significant hydraulic control of discharges. Continuous hydrologic modeling of this lake [King
County, 1990] displays a marked reduction in the unit-area discharges here relative to other catchments without lakes in
the immediate region.
The channel-measurement station lies within a reasonably well-protected riparian buffer, located entirely on private
property and generally not accessed by nearby residents because of topography (Figure 13). The channel is incised about
1.5 m into a narrow upland terrace, probably the old floodplain, set within a broader valley. A complex of logs and a large
tree root, about 20 m downstream of the section, have formed the lip of a 1-m knickpoint and probably have inhibited
more dramatic degradation.
3 m
Figure 13. View of the channel of the Easter Lake outlet (Station PS3) in 1986, looking upstream.
15
This site displays the interplay of fluvial and hillslope processes and it also demonstrates why cross-sectional
measurements can be a very incomplete characterization of stream-channel conditions. Net change at this site has not been
dramatic, but bed scour has clearly oscillated with bank failure on a multi-year scale (Figure 14). For example, the steep
high right bank remaining after the 1986 storms degraded over the next several years, with progressive bank collapse and
channel widening contributing sediment to an aggrading channel bed. Renewed scour during the large flows of January
1990 lowered the bed by nearly 0.5 meters in the active part of the channel, and additional sediment was flushed out
through at least 1993. The 1993-1997 interval was a period of substantial channel widening; but in contrast to many other
stations during this time, channel deepening did not occur, and in fact aggradation was substantial.
Changes in channel cross section were accompanied by a progressive simplification of in-stream morphological features.
Over the 11 years of observations this stream has become more like a drainage ditch, with a marked lack of heterogeneity
in either sediment or bedforms. This has not required any direct human intervention; simplification of the channel, with
attendant loss of aesthetic and biological benefits, has occurred only through the indirect effects of watershed disturbance.
0
0.5
1
1.5
2
2.5
3
3.5
4
2345678
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1988
1987
1986
Figure 14. All cross section measurement of Station PS3, showing episodic deepening in 1990 and 1993 but aggradation both before and
after. Widening has been progressive but particularly during the interval 1986-1988.
16
McAleer Tributary. The third example of a relatively highly impervious, minimally changing channel is a small stream
which, for a 200-m-reach, is surrounded by a surprisingly intact riparian buffer and wetland system. It is isolated from
most human traffic by private property and distance from the adjacent county road. This part of the valley has been eroded
into resistant silt and clay deposits, which are nowhere visible in the stream bed itself but do lie within a few decimeters of
the ground surface and which contribute to the generally silty cohesive banks.
Systematic change at this station over the past decade has been minimal (Figure 15). The channel has aggraded slightly,
and the channel banks have become more rounded and have retreated a few tenths of a meter. The clearest interval of
change was between 1990 and 1993, when two threads of the flow evolved into a more broadly flowing single channel.
Some textural changes in the bed sediment have been noticed over the years, with areas of gravel riffle in one visit
becoming patches of silty sand in the next, but no long-term trends in these changes are evident.
Pepper Creek. In contrast to the modest changes observed at the previous stations, this site has shown tremendously
variable conditions (Figure 16). It collects runoff from a watershed in the very earliest stages of urbanization; the major
hydrologic changes have been related to channelization and road-ditch interception of shallow subsurface flow [Burges et
al., 1989], whereas the total fraction of contributing imperviousness is still quite low. The channel is extremely well
protected from direct human intrusion, lying several hundred (very brushy) meters from the nearest structure or public road
and entirely on private property. It is eroded into sandy valley-bottom deposits, delivered by episodic landslides from the
surrounding hillsides and locally reworked by past fluvial action.
The likely magnitude of channel changes was first suggested by extensive deposition on the downstream alluvial fan of
the stream, beginning in about 1980 and coincident with the first extensive road construction and forest removal in recent
history. Following first measurements in 1986, two episodes of significant erosion were evident: 1986-1987 and 1989-
1990. Channel readjustment, but little net erosion/deposition, occurred in 1987-1988. Near-static conditions persisted
during the low-rainfall year of 1989. Substantial erosion continued following 1993, but landsliding off the hillside above
the right bank, probably in 1996, completely obliterated the measurement station.
0
0.5
1
1.5
2
2.5
3
3.5
4
0.5 1.5 2.5 3.5 4.5 5.5 6.5
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 15. All cross section measurement of Station C2, one of the three sites underlain by cohesive deposits and showing almost no
change 1987-1997.
0
0.5
1
1.5
2
2.5
3
3.5
4
02468101214
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1993
1990
1989
1988
1987
1986
Figure 16. All cross section measurement of Station S3, showing active change in every measurement interval. Vertical exaggeration 2:1
(note expanded horizontal scale).
17
Timberline Tributaries. In a topographic and geologic setting remarkably similar to that of Pepper Creek, these smaller
tributaries drain adjacent areas generally subjected to intensive urban development. A primary difference between them,
however, is that the upland storm-drain system bypasses one (0143F) and discharges into the other (0143G). The stability
of 0143F (Figure 17) throughout the measurement period demonstrates that there is nothing inherently unstable about these
channels, even where gradients are steep and deposits are erodible. Curiously, the discharge into 0143G was constructed
with some advance awareness of the susceptibility of the downstream channel to erosion: rather than simply releasing the
runoff from the end of a pipe, the discharges are first attenuated in a detention pond and then released at the head of a
swale through a 20-m-long level dispersion pipe. Despite these efforts, first documented in Booth [1989], channel incision
of more than one meter occurred in 1989-1990 (Figure 18). In contrast, the station without such flows remained virtually
unchanged throughout the period of its measurement.
The respective (in)stabilities of these two channels emphasize a fundamental point about streams in urbanizing
environments. Prior to watershed disturbance, “stability” (whether static or dynamic) is the norm. Although gradients are
steep and substrate is easily erodible, woody debris and other in-channel roughness elements maintain an overall balance
with the tractive stress of the steeply flowing water and the delivery of sediment from farther up in the watershed. If urban
development alters any element in this balance, however, the relative stability of the entire system can be lost with rapid
and sometimes catastrophic results.
0
0.5
1
1.5
2
2.5
3
3.5
4
-1012345
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1993
1990
1989
1987
Figure 17. All cross section measurement of Station C9, the Timberline tributary that does not receive any appreciable runoff from the
stormwater system of the upslope development.
0
0.5
1
1.5
2
2.5
3
3.5
4
0123456
Distance from Left Bank Datum (m)
El
e
v
a
t
i
o
n
(
m
)
.
1997
1993
1990
1989
1987
Figure 18. All cross section measurement of Station C8, the Timberline tributary that does receive significant stormwater runoff from the
upslope development and which responded with abrupt downcutting in 1989-1990, and further incision by 1997,.
18
Rates of Channel Restabilization
In the final days of 1996, a major storm in the Seattle metropolitan area resulted in the failures of a road embankment
and adjacent berm of a regional stormwater detention facility. Massive quantities of water and sediment were flushed
down the North Fork and main stem of Boeing Creek, a stream draining several hundred hectares of primarily residential
land use, and filled the valley of that channel as deep as 2 m with deposited sediment. Following this deposition, the
channel of Boeing Creek began to reincise immediately, presenting an unexpected opportunity to document the
establishment of a channel where watershed land cover was essentially stable (and highly developed) and the sediment to
be eroded was very easily transportable by even modest stream discharges. Seven cross sections were established in the
300-m reach between the failed detention pond and Hidden Lake, an artificial pond that marks the downstream end of the
alluvial channel on the main stem of Boeing Creek. The normal protocol of annual measurements was replaced by
monthly, and in some instances weekly, field visits.
The recovery of the channel of Boeing Creek was anticipated to take a period of some years and to be controlled in part
by the reestablishment of floodplain vegetation to help bind and stabilize the very sandy fill that was deposited. Instead,
reestablishment of an apparently stable, “equilibrium” channel was very rapid and occurred at most cross sections in a
matter of a few months (Figures 19 and 20). The subsequent year’s measurements demonstrated that this stability was not
an artifact of the termination of rainfall in spring 1997; the 1998 channel, after another winter’s high discharges, remained
largely unchanged.
0 5 10 15 20
Distance from Left Bank Datum (m)
Ele
v
a
t
i
o
n
(
m
)
.
9/98
11/97
4/22/97
4/4/97
3/97
2/23/97
2/2/97
1/97
0
1
2
3
4
Figure 19. All measurements at Cross Section 4 of Boeing Creek, spanning 20 months since shortly after the site was obliterated by
failure of the upstream detention pond embankment. Within weeks the channel achieved a “metastable” form that persisted for about
two months (2/97-3/97); following several rainstorms in late March and early April 1997, channel form has remained nearly stable.
Vertical exaggeration 2.5:1.
10 m
Figure 20. View of the channel of Boeing Creek (Station PS7) at Cross Section 4 on September 9, 1998 (the last measurement plotted in
Figure 19), looking downstream.
19
Discussion
The factors anticipated to influence the annual rate of channel change are generally well represented by the results.
They include:
Abundant rainfall,
Easily erodible substrate, and Presence or absence of watershed urbanization.
These factors resist simple quantification, however, because of the tremendous variability imposed by the multiplicity of
local geologic conditions, channel type, downstream grade controls (natural or artificial), the location of a chosen site in
the context of the upstream channel network, and the variety of development ages and styles (e.g. residential density, or
sewered vs. unsewered). Hammer [1972] recognized the last of these complications, and he developed a complex
regression equation to express the observed relationship between different development types and channel dimensions.
Yet these results are not readily transferred anywhere else, and the ever-changing patterns and styles of development
render only the most general conclusions of lasting value.
The population of sample sites was neither varied enough nor large enough to allow a systematic evaluation of every
relevant condition. However, several useful observations can be drawn:
1. The average annual rate of change can increase in a single channel by as much as 2 orders of magnitude
between dry and wet years (e.g., the winter of 1989-1990, in this sample population); more typically, the greatest
interannual change is about 5-fold. Because the study years include some of the largest lowland storms in recent
memory as well as several quite unexceptional years, this variability is probably representative of most long-term
conditions. In virtually all cases, the rate of channel change returns to nearly equivalent pre-event levels within
one measurement interval (typically one or two years).
2. The previously recognized characteristics of erosion-susceptible channels are broadly correct—moderate to
steep slopes, susceptible geologic materials, and significant (and recent) upstream development. The unique
factors of any given catchment, however, can greatly influence these predictions. No unconsolidated substrate
appears immune from change, given sufficiently severe watershed disturbance. The streams draining large basins
are more resistant than those draining small ones. Steep slopes in and of themselves are not critical, but they may
increase the magnitude of the response to disturbance.
3. The age of the upstream development appears to be quite significant (as first recognized by Hammer, 1972)
but the reason for this influence is enigmatic [see Henshaw, 1999]. In general, channels draining established
neighborhoods [C1, C2, PS1, and PS4] show low rates of change. Possible explanations include (1)
reequilibration of channel dimensions and sediment size with the increased (but now stable) flow regime; (2)
removal of all erodible sediment from the channel perimeter, leaving non-erosive bed and banks; (3) cementation
of channel sediments, a ubiquitous condition at these sites; or (4) reestablishment of bank vegetation following
initial disruption of the channel by increased flows. Each of these explanations applies to certain sites, although
(1) and (2) appear to be the most significant in a majority of cases. The reestablishment of equilibrium, however,
does not necessarily coincide with a reestablishment of overall stream function or habitat quality: the channel
capable of resisting the frequent, flashy discharges that roll out of an urban catchment is generally inhospitable to
most aquatic organisms.
4. Results are most unpredictable in the smallest basins (those of a few tens of hectares). In these basins, even a
relatively small amount of development can have significant downstream effects if flow concentration occurs as a
result of ditches or road crossings [S4, S5]. These effects, however, are not well represented by traditional
methods of characterizing urbanization, such as impervious-area or disturbed-area percentages.
5. Any potential influence of channel slope is not well displayed by this data set (see Figure 8). All channels
here have slopes of at least 1.3 percent, so true low-slope channels (also correlating, typically, with larger
channels) are not represented. Within this data set, the only apparent relationship is that the very largest changes
(>0.3 m/yr) appear to require a steep slope (4 percent or greater). The experiences from this study also suggest several cautionary notes for future long-term channel monitoring. Most
importantly, single-site measurements do not reliably characterize the overall status of the channel. Conditions both
upstream and downstream of the selected site can be very different from those at the measured station. For example,
delivery of sediment from an unstable upstream source can completely obscure the local behavior of the measured section.
The headward migration of a downstream knickpoint, one obvious (and common) manifestation of an unstable stream
reach [Booth, 1990], can cause dramatic changes in the bed elevation after a long period of apparent channel stability as
measured at a single monitoring site.
Furthermore, the “stability” of a channel, as measured by the absence of change at single cross sections, does not
necessarily equate with other desirable conditions, such as high-quality aquatic habitat. The converse statement, however,
is generally correct: instability does correlate well with low habitat quality. Therefore, evaluating only channel stability
20
does not provide unequivocal information on habitat conditions; if that information is needed, additional measurements are
required.
SUMMARY
This study was begun with four objectives: to document rates of channel change, to evaluate the relationship between
development intensity and the rate of channel change, to evaluate geologic and topographic controls on channel change,
and to determine if predevelopment conditions could be used to predict susceptible reaches. Our results indicate the
following conclusions:
1. Rates of vertical channel change vary from below the range of measurement error (<20 mm vertical change
between visits) to about 1 m (width-averaged) per year. The median rate for this sample population was 60 mm per
year.
2. Within these lightly to moderately urbanized watersheds, rates of channel change did not correlate with
development intensity.
3. The nature of the geologic substrate strongly influenced whether or not significant channel change occurred.
Other likely influences included local downstream grade control, riparian vegetation, and the age of the upstream
development. Gradient was not a significant factor across the range of channels measured (0.013-0.52), but the
importance of local grade controls suggest that low-gradient channels may show lower rates of change for a given
level of disturbance and geologic susceptibility.
4. Channels with the greatest susceptibility share the following characteristics, which could be used to reduce the
consequences of future urban development on natural stream systems:
Erosion-susceptible geologic substrate
Moderate to high gradient
Absence of natural or artificial grade controls
Water inputs via predominantly subsurface discharge, likely to be converted to surface (point) discharge in
the post-development condition.
MANAGEMENT IMPLICATIONS
These results imply several consequences for watershed management. First, urban development is an obvious force in
channel change, yet not all channels respond equivalently. The locations of potential susceptibility can be determined well
in advance, at least in the Puget Lowlands of Washington, based on geologic conditions. Finally, channel changes, if and
when they do occur, can happen so rapidly after development begins that remediation, to be effective, must occur prior to
development.
The results of this study also suggest that channel changes are very responsive to varying rainfall. This source of
variability is obviously beyond the ability of surface-water or land-development agencies to control, yet its effects can be
as significant as those of urban development. The most extreme effects of high rainfall are felt in the urbanized channels,
and so one result of large storms is to amplify the differential response of developed and undeveloped watersheds. This
imposes a challenging task for watershed managers: during low-rainfall years, any “warning” of impending channel-
erosion disaster is muted, along with the public's concern for such issues. When a large storm arrives, however, the
magnitude of channel change in urbanizing watersheds can cause significant damage, and its consequences invariably
surprise almost everyone.
Acknowledgements. We are indebted to the associates and assistants who have helped with project design, site selection,
and field assistance over the years, particularly Laura Casey, Kelin Whipple, Adelaide Johnson, John Buffington, Fred
Bentler, Arny Stonkus, Karen Comings, and Marit Larson. Joan Blainey was instrumental in establishing the initial sites
and making the first-year measurements on Boeing Creek. Partial support for the successful conclusion of this project was
provided by the U. S. Environmental Protection Agency’s Waters and Watersheds Program Grant no. R825284-01-0, and
the Center for Urban Water Resources Management at the University of Washington. The manuscript was greatly
improved from careful reviews by Ellen Wohl, Jim Pizzutto, and Kelin Whipple.
21
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