SECTION 3
CHARACTERISTICS OF THE STUDY AREA
3.1
STUDY AREA
The study area for the Comprehensive Stormwater Management Plan, as shown
in Figure 3-1, includes the incorporated area of the City of North Bend and the
adjoining unincorporated areas of King County within the City’s urban growth
area. The unincorporated area includes the Maloney Grove, Tanner, and Wood
River communities. Most of the study area lies within a broad, relatively flat
alluvial floodplain between the South and Middle Forks of the Snoqualmie River.
While the unincorporated area is included for the overall stormwater program
evaluation, the capital improvement plan (CIP) study area is limited to areas
within the City’s corporate limits. A total of six study areas were addressed by
the CIP (see Figure 5-1). Those CIP study areas include the Central Business
District (North and South), Silver Creek Northeast, Si View Southeast, Forster
Woods Southwest, and Ribary/Gardiner Creek West (see Section 5 and Figure
5-1). The hydrology for drainage basins within those study areas was evaluated
for the cumulative tributary area within and beyond the corporate limits.
However, drainage problems and solutions assessment, including drainage
system mapping and hydraulic analysis for CIP development, was limited to
areas within existing City of North Bend corporate limits.
3.2
CLIMATE
The climate in the Upper Snoqualmie River basin varies from moist alpine
conditions in the Cascade mountain range headwaters, to moist temperate
conditions in the lower valley in and around the study area. The Cascade
mountain range forms a barrier to the easterly movement of moist air from the
Pacific Ocean, resulting in significant precipitation during the late fall and winter
months. Winter snowfall and subsequent snowmelt also result in elevated
spring runoff conditions in the rivers, which can result in flooding.
Annual precipitation on the west side of the Cascades is typically around 100
inches. Most precipitation in the headwaters falls as snow from October through
January and as rain for the remainder of the year. Rainfall dominates the lower
elevation portions of the upper river basin (EES/KCM, 1999). In the North Bend
area, the average annual precipitation is 60 inches per year (URS Greiner, 2000).
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3.3
TOPOGRAPHY, SOILS, AND GEOLOGY
The study area is within a broad, relatively flat alluvial floodplain between the
South and Middle Forks of the Snoqualmie River at the foothills of the Cascade
mountain range. To the west, east, and south of the study area, the topography
is moderate to steep. Mount Si is located to the east of the City along the east
side of the Middle Fork of the river. The river floodplain continues to the north
and northwest toward the city of Snoqualmie. Figure 3-1 shows the study area
and topography based upon USGS topographic information.
Soils in the study area are largely the result of sediment load deposits, termed
alluvium, from the rivers on the flat valley floor. The alluvium within the Three
Forks area primarily consists of unconsolidated gravel, sand, and silt that have
been laid down and reworked by surface water flows in the active floodplain.
3.4
LAND USE
Land use within the study area consists of a mixture of single family, multifamily, commercial/industrial, agricultural/pasture, and open spaces. The
historic development pattern of the City is along its northwest-southeast axis,
parallel to the old Highway 10 corridor. Much of the more intense uses,
including commercial, community business, and multi-family, are along this
corridor. Following the completion of Interstate 90 south of the City, newer
commercial and manufacturing park activities developed along North Bend
Boulevard, the major access to downtown North Bend from I-90. Residential
development is spread throughout the City.
TABLE 3-1
EXISTING LAND USE
CITY OF NORTH BEND
Percent of City
Parks/Open Space
General Undeveloped
Single Family Residential
Multi-Family Residential
Manufacturing
Commercial
Retail/Community Business
Total
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26%
5%
34%
7%
20%
6%
2%
100%
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SOURCE: 1995 CITY OF NORTH BEND COMPREHENSIVE PLAN
3.5
DRAINAGE SYSTEM
Much of the City is located in the 100-year floodplain of the Middle and South
Forks of the Snoqualmie Rivers. Upstream of North Bend, the river system has a
collective basin area of approximately 347 square miles.1 A combination of high
annual precipitation and melting snow in the Upper Snoqualmie Basin
contribute to the potential for significant winter flooding from November
through February. Flooding of the lowland from South and Middle Fork river
overflows combined with local drainage systems is one of North Bend’s most
costly natural hazards. Significant river flooding events occurred in 1932, 1933,
1943, 1947, 1951, 1959, 1964, 1975, 1977, 1986, 1990, 1995, and 1996. According to
published records, the largest flood flow on the South and Middle Fork occurred
in 1959, and in 1990 on the mainstem (below North Bend). In the 1990 event,
both the South Fork and Middle Fork Snoqualmie Rivers overflowed and these
river flood flows were delivered through the Ribary and Gardiner creek systems
(from the South Fork) and along the Silver Creek system (from the Middle Fork).
Damages in the 1990 flood were more significant than earlier floods, which is
likely a result of increased development in the floodplain in recent years. The
Corps of Engineers estimated the average annual cost of flood damage in the
Snoqualmie Valley to be over $3 million (1995 Comprehensive Plan). The City’s
Comprehensive Flood Hazard Management Plan is evaluating options to reduce
flood hazards from river flooding and provides more information about the
nature of river flooding.
The South Fork Snoqualmie River, designated as water resource inventory area
(WRIA) #07-0467 (Williams et al. 1975), is classified by King County (1990) as a
Class 1 stream. According to the North Bend Municipal Code (NBMC) Ch.
14.10, Sensitive Areas, this system is classified as a Category 1 stream. Levees
along the banks of the South Fork Snoqualmie River upstream from the
Snoqualmie Valley Trail confine river flows (except for periodic overflows during
major floods, typically greater than approximately 10-year recurrence interval)
so that its width averages 80 feet throughout most of the urban growth area.
However, within the northernmost segment of the urban growth area, the river
floodplain widens and the river channel is less confined.
The Middle Fork Snoqualmie River (designated as WRIA #07-0219) is located
outside the urban growth area, bordering its northeast edge. The undeveloped
1
More specifically, the upstream drainage for the three forks are: South Fork, 82 sq. mi.,
Middle Fork, 169 sq. mi., North Fork, 96 sq. mi. at North Bend gauges, for a total of 347 sq.
mi.
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eastern bank of the Middle Fork Snoqualmie River is confined by steep slopes
rising up to Mount Si, while the west bank is relatively flat and is bordered by
residential development and agricultural land uses (USGS 1968 and 1993). The
Middle Fork Snoqualmie River channel averages 150 feet in width and exhibits a
more braided channel across the floodplain compared to the South Fork. The
segment of the river east of the City’s east corporate limits is subject to periodic
overflows over its southwest bank (upstream and downstream from the Mount
Si Bridge) during major flood events (typically greater than approximately
10-year recurrence interval) as discussed below.
Within the North Bend study area, there are several tributaries that flow through
the corporate limits and into the South Fork Snoqualmie River. These include
Gardiner Creek, Ribary Creek, and the Silver Creek system, including additional
unnamed tributaries (see Figure 3-1). Each stream system is described in more
detail below.
Gardiner Creek (within WRIA #7), with a tributary drainage area of
approximately 1.3 square miles, originates on steep slopes of Rattlesnake
Mountain southwest of North Bend and flows through the Forster Woods
Southwest study area, under I-90 (by culvert), and through the Ribary/Gardiner
Creek study area with multiple road/rail-line crossings within the northwest
corporate limits. According to NBMC Ch. 14.10, Gardiner Creek is classified as a
Category 2 stream. Gardiner Creek is lined by a narrow band of riparian
vegetation along the majority of its length, and joins the South Fork Snoqualmie
River just north of the north corporate limits.
Ribary Creek (designated as WRIA #07-470) has a tributary drainage area of
approximately 1.3 square miles, and also originates on the steep slopes of
Rattlesnake Mountain southwest of North Bend. As with Gardiner Creek, it
flows through the Forster Woods Southwest study area, under 1-90, and north
through the Ribary/Gardiner Creek study area (with multiple road/rail-line
crossings within the corporate limits) as it parallels the South Fork Snoqualmie
River for several miles. Ribary Creek discharges to the South Fork channel
downstream of the Snoqualmie Valley Trail. According to the NBMC Ch. 14.10,
Ribary Creek is classified as a Category 2 stream, and King County (1990)
classifies it as a Class 2 stream.
Silver Creek (within WRIA #7), with a tributary drainage area of approximately
0.5 square miles (drainage areas of other tributaries in Silver Creek Study area
total approximately 0.2 square miles), has been fragmented by residential
development and numerous road/driveway crossings, resulting in restrictions to
flood flows and loss of riparian vegetation. This study area is actually a large
alluvial fan that spans between the Middle and South Forks of the Snoqualmie
River (the apex of the fan is located near the Mount Si Bridge crossing of the
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CHARACTERISTICS OF THE STUDY AREA
Middle Fork). The Silver Creek tributaries generally flow from the Maloney
Grove residential area within the northern portion of the urban growth area to
the South Fork Snoqualmie River with outfall below the Snoqualmie Valley Trail.
Portions of Silver Creek outside developed areas have not yet been
specifically classified. However, based on field investigations and the
categories of streams defined in the NBMC Ch. 14.10.230, the reaches of Silver
Creek that lie between the South Fork Snoqualmie River and Ballarat Avenue
would likely be classified as Category 2 streams. Using the King County
classification system (1990) they would be classified as Class 2 streams. The
Silver Creek stream reaches that lie east of Ballarat Avenue are culverted under
numerous roads and are conveyed in maintained ditches through sometimes
highly manicured, residential lawns. NBMC Ch. 14.10 classifies the reaches of
Silver Creek east of Ballarat Avenue as Category 4 streams.
In addition to the rivers and smaller stream, the study area includes other local
drainage systems that collect and convey surface water runoff. These consist of
open channels and roadside ditches, storm sewer pipelines, wetlands, infiltration
systems, and retention/detention systems.
As a part of this planning study, the City’s existing drainage system was mapped
using inventory data and supplemental inventory data collected for this plan.
The system maps were developed using an ArcView Geographical Information
System (GIS). The drainage inventory includes several large maps at a scale of
1”=200’ which show drainage system features within the various CIP study
areas. These maps are on computer files with the City’s Public Works
Department. These inventory maps show element numbers that reference
attribute information for each drainage system facility (i.e., catch basin, manhole,
or culvert). Accompanying these figures are tables that list the attribute
information for each facility identified and indexed by element numbers on the
figures. For example, the information associated with an element number for a
structure describes the structure itself as well as the downstream pipe connected
to the structure. When available, the attribute information lists the structure
type, elevations (including rim/grate/invert), downstream pipe diameter,
material of construction, and length.
Information on individual
structures/culverts was gathered from a large number of existing sources,
including existing drainage studies, roadway improvement design drawings,
developer plans, prior City drainage system survey data, and field survey
pickup data conducted for this plan within the various CIP study areas.
Because the data was compiled from many sources and is not, in many areas,
“as-built” information, the drainage system inventory should be used for general
planning guidance. It was used for preliminary evaluation of CIP solutions.
Specific analysis of the City’s drainage system is discussed in Section 5.
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3.5.1 RELATIONSHIP OF RIVER AND LOCAL DRAINAGE SYSTEM FLOODING
Flooding conditions within the City of North Bend are influenced by a
combination of river and local drainage system effects. These conditions are
interrelated given the City’s location on an alluvial plain adjacent to two major
forks of the Snoqualmie River and the hydraulic connectivity that exists between
the river and local drainage systems. Direct river flooding results from periodic
overflows of the South and Middle Fork river channels during major flood
events (typically greater than 10-year recurrence interval). However, local
drainage system flooding (backwater flooding) can result from the effects of high
river system flood stages (tailwater) on local drainage systems (i.e., creeks and
storm drainage systems). High groundwater conditions are also present that can
affect local drainage system performance.
High groundwater is more
pronounced under winter month conditions, as affected by high river stages and
the permeable nature of underlying soils in the alluvial plain between the
Middle and South Fork channels.
Direct river system flooding is most prominent where overflows enter the
Ribary/Gardiner Creek system (from the South Fork) and the Silver Creek
system (from the Middle Fork). On the South Fork, the southwest levee system
has a lower top elevation than the northeast levee (which protects the North
Bend downtown commercial area), and was designed to overtop during major
flooding events. Flooding analyses conducted for the Tollgate EIS (URS Greiner,
2000) suggest that the 100-year event South Fork overflows upstream from
Bendigo Avenue South (SR-202) would be approximately 2,700 cfs. By
comparison, Ribary and Gardiner Creek 100-year discharge estimates from these
drainage basins alone are 250 and 300 cfs, respectively. South Fork overflows
during the November 1990, 25-year recurrence interval event were estimated at
approximately 700 cfs. After overtopping of SR-202 and combining with Ribary
Creek flows, South Fork overflow floodwaters can also flow to Gardiner Creek
(and subsequently to Kimball Slough to the west). Since the stream channels and
crossings of those local drainages are not large enough to convey the larger flood
flows associated with river system overflows, extensive flooding within the
Ribary/Gardiner Creek study area and adjacent urban growth areas are
predicted to occur under those conditions.
On the Middle Fork Snoqualmie River, infrequent high river stage overflows
from above and below the Mount Si Bridge can have a significant effect on the
flooding conditions along Silver Creek and its adjacent tributaries. The most
recent FEMA estimate (1999 Draft) of 100-year Middle Fork overflow through the
Silver Creek study area channels, combined with local flows, is approximately
6,500 cfs. This compares with 100-year Silver Creek and tributaries combined
peak flow estimated from this drainage basin alone to be approximately 600 cfs.
During the November 1990 event where Silver Creek flooding did occur, Middle
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CHARACTERISTICS OF THE STUDY AREA
Fork overflows were estimated to be less than 1,000 cfs. In the FEMA study it
was reported that three of the four overflow channels that carry this flow travel
through the Silver Creek study area directly to the South Fork channel; a fourth
drains to Reid Slough and back to the Middle Fork floodplain. As a result, the
majority of the Silver Creek study area lies within the 100-year Middle Fork
floodplain. FEMA has also designated one-foot rise floodways along these Silver
Creek overflow corridors that the City will be required to regulate in accordance
with its flood hazard ordinance.
Non-structural and structural solutions to those river-induced flooding problems
are being addressed independently by the City’s Comprehensive Flood Hazard
Management Plan. Stormwater CIP improvements listed in Section 5 do not
include analysis of these river overflow effects since it is not practicable to design
improvements to these local creek systems to carry the larger river-induced
flows. However, stormwater plan CIP improvements in the Silver Creek and
Ribary/Gardiner Creek study areas will assist in lowering flood levels and
evacuating flooding volumes behind restrictive crossings more rapidly after
large flooding events when river overflows occur. Therefore, non-structural
solutions such as overflow floodway corridor preservation, in combination with
local drainage system improvements, may be the appropriate comprehensive
flood reduction management solution to areas potentially affected by river
overflows (i.e., where alternative river system structural flood reduction
measures are not feasible).
Local drainage system flooding that is affected by high river stages and elevated
groundwater tables was considered in the drainage system solutions evaluation
process. This was done by assuming 10-year coincident river flood elevations at
outfalls combined with drainage system peak flows for hydraulic analysis of
drainage system conveyance facilities. Groundwater inflow effects were
considered by assuming a 10 percent added impervious area cover factor in
hydrologic analysis. In addition, evaluation of solutions considered the potential
for high groundwater tables to be encountered with their installation and
operation. For example, this eliminated consideration of any infiltration
solutions to local drainage problem areas where high groundwater elevations
were likely to exist.
The City should integrate the solution recommendations contained in both this
Comprehensive Stormwater Management Plan and the Comprehensive Flood
Hazard Management Plan such that project improvements and development
policies/regulation resulting from each plan are consistent.
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3.6
ENVIRONMENTAL RESOURCES
Environmental resources, including fisheries/riparian habitats, were studied on a
qualitative level supplemented with minor field investigations as a part of this
plan. The investigations are summarized below.
3.6.1 EXISTING AQUATIC ENVIRONMENT
The portions of both the South Fork and Middle Fork Snoqualmie River within
the urban growth area lie three miles upstream of Snoqualmie Falls. Although
this natural barrier prevents upstream migration of anadromous salmon, these
systems provide excellent habitat for resident fish populations, including
cutthroat and rainbow trout, white fish, and sculpin.
Gardiner Creek supports a healthy population of cutthroat trout. Ribary Creek
primarily supports cutthroat trout, with some presence of rainbow trout as well
(Raedeke Associates 1996).
3.6.2 THREATENED AND ENDANGERED FISH
The City of North Bend is situated upstream of Snoqualmie Falls, which is an
impassible barrier to the upstream migration of anadromous fish in the
Snoqualmie River. No anadromous fish are found in the South Fork and Middle
Fork Snoqualmie River or its tributaries within the urban growth area (KCM
1999). As a result, the urban growth area is not directly affected by the recent
listing of chinook salmon under the Endangered Species Act. However, water
quality- and quantity-related impacts to the Snoqualmie River resulting from
activities in the City of North Bend could contribute to cumulative impacts on
chinook downstream of the falls.
Information on the historical and current distribution of bull trout throughout
the Puget Sound area is poorly understood, because comprehensive data are
lacking. An extensive two-year survey on the North Fork Snoqualmie River and
its tributaries has not found any bull trout in this drainage basin. No native char
have been observed by Washington Department of Fish and Wildlife biologists
above Snoqualmie Falls within the last 10 years, nor by the U.S. Army Corps of
Engineers during surveys conducted in the early and mid-1990s (R2 Consultants
2000). Although the USFWS may indicate (via their species database letters) that
bull trout may occur in the vicinity of the project, they are not currently known to
occur in the North Bend area (Chan 2000 personal communication). Because
there is a general lack of data on bull trout presence, this assessment may change
pending more extensive sampling. There is also a possibility that if bull trout are
not present, they may return to the North Bend area in response to restoration
efforts in the Snoqualmie River system.
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CHARACTERISTICS OF THE STUDY AREA
3.6.3 THREATENED AND ENDANGERED WILDLIFE
The South and Middle Fork Snoqualmie River and associated riparian areas
provide habitat for federally threatened bald eagles, state endangered peregrine
falcons, and several species of concern and state priority species. There are no
known bald eagle nest sites within or near the urban growth area. However,
bald eagles are observed throughout the year foraging within the Snoqualmie
River corridor (URS Greiner 2000). A pair of peregrine falcons reportedly nests
approximately one mile outside the urban growth area on the west face of
Mount Si. No data are available on the home range or foraging territory for this
pair. However, peregrine falcons forage over wide areas, and their territory can
extend up to eight miles from a nest site (URS Greiner 2000).
3.6.4 WETLANDS
Within the urban growth area there are several palustrine and riverine wetlands
that are associated with the South Fork Snoqualmie River and its tributaries
(Raedeke Associates 1996, North Bend 1995). These wetland systems include
forested, scrub/shrub, and emergent vegetation classes. Some of these systems
have been severely fragmented by development and farming, particularly in the
southeast portion of the urban growth area, where wetlands have been disturbed
and tributaries to the South Fork Snoqualmie River have been channelized and
fragmented by roads.
Figure 3-2 illustrates wetlands within the study area and is based upon the City’s
GIS mapping. The primary source of this information is National Wetland
Inventory. Note that these sources indicate only the potential for the presence of
wetlands. Jurisdictional wetland determinations must be based upon data
obtained during detailed field investigations in accordance with the Corps of
Engineers Wetlands Delineation Manual (Environmental Laboratory 1987). Some
areas depicted on Figure 3-2 may, therefore, not be jurisdictional wetlands;
furthermore, some areas not mapped as wetlands by the resources cited may, in
fact, be jurisdictional wetlands. The City’s GIS mapping contains more
information about the wetlands within the city limits.
3.6.5 WETLAND FUNCTIONS AND VALUES
Wetlands play important roles that provide valuable benefits to the environment
and society. Detailed scientific knowledge of wetland functions is limited, so
that evaluations of the functions of individual wetlands are often necessarily
qualitative and dependent upon professional judgment.
Several wetland functional evaluation methods have been developed. The most
common methods applicable in the Pacific Northwest were developed by the
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SECTION 3
U.S. Army Corps of Engineers. These methods were modified for use in wetland
evaluations of the following wetland functions: (1) water quality improvement,
(2) storm and flood flow attenuation and storage, (3) hydrologic support, and
(4) natural biological support.
Water quality improvement functions of wetlands include the ability of wetlands
to remove sediments from surface waters passing through the wetlands. This
helps prevent the siltation of fish spawning gravels, particularly for
economically important salmonid species.
Because many pollutants are
associated with particulates, sediment removal results in better water chemistry
in receiving waters. Further, many wetland plants and microbial communities
associated with plants have the ability to directly remove pollutants or to
transform them into less harmful chemical compounds.
Storm and flood flow attenuation and storage results in smoother (less “flashy”)
hydrographs for streams and other surface waters. This helps prevent flooding
conditions on private and public lands, reduces streambank erosion, and
maintains the hydrology necessary to support wetland plants.
Because stormwater is detained in wetlands, water is released to surface and
occasionally to groundwater receiving waters at a slower rate. Such hydrologic
support helps maintain proper flow rates in streams and may help recharge
aquifers. The hydrologic support function of some wetlands may also assist in
providing readily available irrigation water for agricultural uses.
Natural biological support functions of wetlands include providing the
necessary hydrologic regime for aquatic organisms and providing the habitat
resources (for example, food, cover, and nesting materials) for wildlife.
Wetlands may be particularly important for biological support because many
organisms are partially or completely dependent on wetlands for their survival.
In Washington, a large percentage of sensitive, threatened, and endangered
species are associated with wetlands.
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