United States
Environmental Protection
Agency
Office of Water
(4504F)
EPA 842-F-95-001E
September 1995
&EPA
Shell Creek Stormwater
Diversion Project
Demonstrating Practical Tools For Watershed
Management Through The National Estuary Program
Puget Sound, Washington
Characteristics:
• The Puget Sound drainage basin covers about 16,000 square
miles, of which 80 percent is land surface.
• Approximately 3.4 million people live in the Sound area.
• Land use in the Puget Sound area is 9 percent intense urban
development, 9 percent agricultural and rural, and 82 percent
forest.
• The Shell Creek watershed, which discharges into Puget Sound,
is an 80 percent developed urban watershed.
The Problem: Stormwater runoff and other nonpoint sources from
the Shell Creek watershed have deposited sediment and pollutants into
Puget Sound, decreasing water quality and impacting marine life.
• Over half of Puget Sound's original wetlands have been lost.
• Over 22 square miles of commercial shellfishing beds are under harvesting restrictions
largely due to nonpoint source pollution.
• Nonpoint source pollution will likely worsen, as urban development is expected to
advance by 62 percent over the next 15 years in the Sound area.
The Project: The Shell Creek Stormwater Diversion Demonstration Project involved
constructing a bypass system in two creeks and providing stream bank restoration upstream of
the bypass system, cutting the amount of sediment and pollutants from runoff.
The National Estuary Program
tLstuaries and other coastal and marine waters are national
resources that are increasingly threatened by pollution, habitat
loss, coastal development, and resource conflicts. Congress
established the National Estuary Program (NEP) in 1987 to
provide a greater focus for coastal protection and to demon-
strate practical, innovative approaches for protecting estuaries
and their living resources.
As part of this demonstration role, the NEP offers funding
for member estuaries to design and implement Action Plan
Demonstration Projects that demonstrate innovative approaches
to address priority problem areas, show improvements that can
be achieved on a small scale, and help determine the time and
resources needed to apply similar approaches basinwide.
The NEP is managed by the U.S. Environmental Protection
Agency (EPA). It currently includes 28 estuaries: Albemarle-
Pamlico Sounds, NC; Barataria-Terrebonne Estuarine Complex,
LA; Barnegat Bay, NJ; Buzzards Bay, MA; Casco Bay, ME;
Charlotte Harbor, FL; Columbia River, OR and WA; Corpus
Christi Bay, TX; Delaware Estuary, DE, NJ, and PA; Delaware
Inland Bays, DE; Galveston Bay, TX; Indian River Lagoon, FL;
Long Island Sound, CT and NY; Maryland Coastal Bays, MD;
Massachusetts Bays, MA; Mobile Bay, AL; Morro Bay, CA;
Narragansett Bay, RI;New Hampshire Estuaries, NH; New York-
New Jersey Harbor, NY and NJ; Peconic Bay, NY; Puget Sound,
WA; San Francisco Bay-Delta Estuary, CA; San Juan Bay, PR;
Santa Monica Bay, CA; Sarasota Bay, FL; Tampa Bay, FL; and
Tillamook Bay, OR.
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Introduction To Puget Sound
1 uget Sound is located in northwest Washington State.
Over 900 feet at its deepest point, the Sound is a highly produc-
tive waterway, supporting numerous plant and animal species,
including salmon, crab, shellfish, and shrimp. Wetland ecosys-
tems, teeming with grasses, also thrive in the Sound area.
In recent times, however, Puget Sound and its resources have
been threatened by nonpoint source pollution. Nonpoint source
pollution has contributed to declines in the Sound's water quality
and has resulted in numerous shellfishing area closings.
Additionally, large amounts of fine sediment deposited into
the Sound from Shell Creek could cause problems in the Sound
if left unchecked. Part of the potential sediment problem lies in
the geology of the Sound itself. Puget Sound is shaped much
like a bathtub, with shallow ridges separating the four deep
basins. Sediment deposited into the Sound by surrounding
watersheds could be kept in motion by strong tidal currents
which cause the water-suspended sediments to slosh back and
forth. Although large particles settle out, excessive fine sedi-
ments that stay in solution could cloud the water, blocking out
the sunlight needed by seagrasses for growth. In addition, a lack
of penetrating sunlight in the Sound could result in a lower
amount of phytoplankton, a necessary food source for marine
life.
Overview Of Shell Creek
Groundwater
(Aquifer)
Stormwater carries nonpoint source pollutants and
sediments from the Shell Creek Watershed to Puget
Sound.
tJhell Creek is located in southwest Snohomish County in
the City of Edmonds, Washington. The creek, which discharges
into Puget Sound, receives stormwater runoff from 2 square
miles of suburban neighborhoods. The neighborhoods, much
like other watersheds in the Sound area, were almost completely
developed before on-site stormwater quality control was required
starting in the late 1970s. Because rapid development occurred
without regard for stormwater control, the Shell Creek ecosys-
tem, and consequently the water quality and resources of Puget
Sound, have been steadily degrading over the years.
Lack of stormwater control in a highly developed watershed
can set into play a devastating chain of events. In the Shell
Creek watershed, additional development added to runoff
flowing into the creek, causing stream bed erosion and loss of
the vegetation that was essential to filter pollutants and stabilize
sediments. Without vegetation and stable stream beds to help
control water flow, the water velocity and volume increased,
which led to area flooding. In addition, the rapid water flow
swept up the loose sediment and bottom gravel of Shell Creek
and discharged the load, along with other pollutants from
nonpoint sources, into Puget Sound.
The result of this continuous process was damaging to both
Shell Creek and Puget Sound. The increased volume and
velocity of water flow in the creek cut away at the stream bed
until only clay remained at the bottom of the bed. Clay bottoms
are slick and provide no resistance to water flow, increasing the
water flow velocity. As a result, there was no pooling in the
creek, which made it impossible for the creek's fish, such as
cutthroat trout and coho and chum salmon, to successfully
spawn in the channel. Populations of trout and salmon in the
creek dwindled. In effect, without stormwater management
techniques in place, the habitat viability of Shell Creek was
diminished, while huge volumes of sediments and pollutants
were discharged into Puget Sound.
Realizing the crisis facing Shell Creek and the Sound, the
City of Edmonds and Snohomish County prepared the Shell
Creek Basin Plan in 1987. This plan recommended comprehen-
sive approaches to slow the resource degradation that was
occurring in Shell Creek and impacting Puget Sound. The plan
addressed problems such as flooding, severe erosion of the
stream bed, very heavy sedimentation, and increased pollutant
loading. Secondary problems included reduced capacity in
culverts and loss of fish habitats.
Based on the recommendations in the 1987 Basin Plan, the
Shell Creek Stormwater Diversion Demonstration Project was
initiated in 1988 by the City of Edmonds with support from the
Puget Sound Estuary Program. Shell Creek was chosen as a
demonstration project because many communities in Puget
Sound share the same topology as the Shell Creek area, making
the problems of Shell Creek common to many regions in the
Sound. EPA joined forces with the City of Edmonds,
Snohomish County, and two volunteer groups, Quest Northwest
and a Citizen's Surface Water Advisory Committee, to plan and
implement the demonstration project.
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Project Objectives
L he primary objectives of the Shell Creek project were to
manage stormwater flows and reduce sediment and pollutant
loadings into Puget Sound. This would be achieved by stream
bed restoration and construction of a stormwater diversion and
sediment entrapment system in Shell Creek and its tributary,
Hindley Creek.
Diversion And Entrapment System
Construction
L redesign analysis began in June 1988 and included review
of six alternative routes for a diversion system. Construction of
a system, which would divert peak flows from Shell Creek and
Hindley Creek to a new storm sewer system and outfall, was
recommended by the Shell Creek Basin Plan. The Edmonds
City Council approved a recommended route which was de-
signed to divert approximately 100 cubic feet per second (cfs)
from Shell Creek and 50 cfs from Hindley Creek. Nearly 90
percent of the runoff from the drainage basin could drain
through this diversion system.
Almost a mile of pipeline was installed during the project.
The Shell Creek diversion structure has a vertical slot entrance
that restricts flow and causes water to crest over two weirs. This
entrance includes a fish ladder to help fish migrate. Screens to
prevent trout and salmon fingerlings from entering the diversion
lines were installed along with trash racks to stop floating debris.
The pipeline route was
designed to divert stormwater
from both Shell Creek and
Hindley Creek.
The diversion at
Hindley Creek is a man-
hole which was added to
the existing culvert. The
manhole has a 12-inch
outlet pipe which carries
the stream's base flow
and a 36-inch outlet pipe
which carries the diver-
sion flow. Manholes
throughout the system are
typically designed to cap-
ture large amounts of
sediment and debris be-
fore they get to Puget
Sound. Construction of
the diversion system was
completed in fall 1990.
Stream Bed Restoration
1 o re-establish trout and salmon populations and to restore
stream bed and bank stability, the demonstration project
included a restoration component focusing on a mile of Shell
Creek upstream from the diversion structure. To encourage
salmon and trout populations to return to the stream channel,
water flow had to be slowed down and desirable stream bed
conditions had to be created. The clay bottom was replaced with
gravel, which helped pool the water, enabling fish to enter the
channel. The gravel also created an adequate spawning ground
for salmon by providing protective niches for the eggs.
Prior to the demonstration project, Shell Creek was very
poorly vegetated. Brush, which does not stabilize soil
adequately, was the main plant life along the stream banks.
Revegetation to provide bank and sediment stabilization was
essential to the long-term success of the demonstration project.
With the help of a local Boy Scout Troop, the stream banks were
planted with willows, snowberry, and serviceberry plants. In
addition, bank log armoring and log check dams were
constructed to protect the re-established vegetation and reduce
erosion.
Willow
Serviceberry
The shallow,
expansive root
systems of
willow and
serviceberry
trees provide
superb stream
bank
stabilization.
The Shell Creek Success Story
L he project reduced sediment entering Puget Sound by 5.7
tons in the first year and is estimated to have reduced stream bed
erosion by 65 percent. As a result of the diversion system and
stream bank restoration, citizens report that clear water now runs
through the creek where muddy water used to be prevalent.
There is evidence that the stream bed and banks are stabilizing
most of the sediment and that the reduced water flow now allows
for the settling of loose particles.
In addition, the restoration of the stream bed and the
employment of the diversion system have eliminated flooding
and erosion, which in turn has reduced pollutant loadings
downstream. Restoration has also re-established the fish
spawning habitat. According to area biologists, both trout and
salmon have returned to Shell Creek. In addition, the reduction
in stream bed erosion has helped the willow, snowberry, and
serviceberry plants to flourish on the stream banks, which has
further reduced erosion and created more opportunities for
trapping pollutants.
The project was successful in its public involvement efforts
as well. The public participated in the decision making process
through the Citizen's Surface Water Advisory Committee. The
Committee provided input to the Shell Creek Basin Plan and
helped the Edmonds City Council choose among the diversion
route alternatives. Citizen groups are also involved in the
ongoing monitoring of the conditions of the stream bed and
vegetation.
Several groups have been involved in monitoring the project.
The City of Edmonds Public Works Division monitors and
estimates amounts of sediment collected and attends to needed
maintenance, such as system repairs and sediment trap cleaning.
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The snowberry
shrub, with white,
fleshy fruit,
encourages wildlife
habitation along the
restored stream
bed.
The advanced biology class of a local
high school has surveyed the stream's
biological community, while a volun-
teer group has monitored the stream's
water quality using EPA's stream walk
guidelines. Although local monitoring
has shown a marked improvement in
Shell Creek's stream conditions, plant
and fish vitality, and water flow, such
improvements in Puget Sound are
difficult to attain through one water-
shed improvement effort. Because of
the Sound's vastness, sediment and
nonpoint source pollution controls will
have to be widespread to ensure the
long-term preservation of Puget
Sound's resources.
Lessons Learned
L he Shell Creek Stormwater Diversion Demonstration
Project was successful in reducing erosion and sedimentation
from storm flows that exceeded Shell Creek's channel capacities.
Prior to the implementation of the project, sediment transport
was estimated at about 10 tons per year. In the first year, 5.7
tons of sediment were trapped, indicating that the structure can
trap about half the sediment transported in the creek, as
estimated prior to the project. Among the other lessons learned:
Printed on recycled paper
Stream bed restoration and erosion control were accom-
plished upstream of the diversion at a much lower cost than
the diversion system. However, methods that worked
above the diversion were not practical as a long-term
solution in the lower reaches of Shell Creek because the
lower portion of the creek is located in residential back-
yards.
The diversion system operates infrequently by design.
Because it operates during periods when water flow
exceeds channel capacities, the system came on-line only
six times in the first 18 months after it was completed.
This infrequency of operation makes formal evaluation
difficult.
Sedimentation downstream of the diversion was reduced
because of the reduced erosion upstream. Reducing
velocity of flow by diversion and revegetating stream beds
are efficient ways to control stormwater runoff.
For more information contact:
Don Fiene
City of Edmonds
Community Services Dept.
250 5th Avenue North
Edmonds, WA 98020
(206)771-0220
(206) 771-0221 FAX
John Armstrong
Office of Coastal Programs
U.S. EPA Region 10
1200 Sixth Avenue
Seattle, WA 98101
(206) 553-1368
(206)553-0165 FAX
?/EPA
United States
Environmental Protection
Agency
(4504F)
Washington, DC 20460
Official Business
Penalty for Private Use
$300
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