HomeMy WebLinkAbout5674ORDINANCE NO. 5 6 7 4
AN ORDINANCE OF THE CITY COUNCIL OF THE CITY OF
AUBURN, WASHINGTON, RELATING TO COMPREHENSIVE
PLANNING, AMENDING THE 1990 COMPREHENSIVE
DRAINAGE PLAN, PURSUANT TO THE PROVISIONS OF R.C.W.
CHAPTERS 36.70A AND 35A.63 OF THE LAWS OF THE STATE
OF WASHINGTON; AND DESIGNATING THESE AMENDMENTS
AS GUIDELINES FOR EXERCISING THE CITY'S AUTHORITY
UNDER THE WASHINGTON STATE ENVIRONMENTAL POLICY
ACT (SEPA).
WHEREAS, the City established a moratorium on the filing of building
and grading permits for an area bounded by SR 18 to the south, 15th Street NW
to the north, Union Pacific Railroad to the east, and SR 167 to the west under
Ordinance No. 5518 on February 20, 2001, to allow for time to determine the
extent of flood potential and to determine alternatives for addressing flooding
within the area; and
WHEREAS, on August 3, 2001, the City by Ordinance No. 5571
extended the moratorium by six months; and
WHEREAS, the City further extended the moratorium by six months
under Ordinance No. 5634 until August 5, 2002, to allow for time to develop
capital improvements and associated mitigation measures required of new
development; and
WHEREAS, the City has now completed the technical studies necessary
to establish the capital improvements and conditions essential with new
Ordinance No. 5674
July 3, 2002
Page 1
development to mitigate storm drainage impacts to other properties within Sub
Basin P and has prepared an amendment to the City of Auburn 1990
Comprehensive Drainage Plan to reflect these studies and improvements; and
WHEREAS, the environmental impacts of the 1990 Comprehensive
Drainage Plan amendment were considered in accordance with procedures of
the State Environmental Policy Act; and
WHEREAS, after proper notice published in the City's official newspaper
at least ten (10) days prior to the date of hearing, the Auburn Planning
Commission on July 2, 2002 conducted a public hearing on the proposed
amendment and thereafter recommended approval by the City Council of said
amendment; and
WHEREAS, the City Council now determines that the amendment to the
Storm Drainage Comprehensive Plan is necessary to allow new development
and redevelopment of properties within Sub Basin P to proceed.
NOW, THEREFORE, THE CITY COUNCIL OF THE CITY OF AUBURN,
WASHINGTON, DO ORDAIN AS FOLLOWS:
Section 1. PURPOSE: The purpose of this ordinance is to amend the
City's 1990 Comprehensive Storm Drainage Plan, an element of the City's
Comprehensive Plan, by providing a revised chapter attached hereto as Exhibit
"A", that more accurately reflects the actual drainage conditions within
Ordinance No. 5674
July 3, 2002
Page 2
Subbasin P by considering the increased level and duration of backwaters from
Mill Creek, and the impacts on the currently adopted recommended projects.
The recommended projects and conditions required of new development are
intended to alleviate localized flooding conditions by providing improved
conveyance, and include water quality facilities to improve the quality of
stormwater runoff;
Section 2. The Comprehensive Plan and amendments is herewith
designated as a basis for the exercise of substantive authority under the
Washington State Environmental Policy Act by the City's responsible
environmental official in accordance with R.C.W. 43.21C.060.
Section 3. The Mayor is hereby authorized to implement such
administrative procedures as may be necessary to carry out the directions of
this legislation.
Section 4. This ordinance shall take effect and be in force five (5) after
its passage, approval, and publication, as provided by law.
INTRODUCED
ML -1-62002
PASSED:
APPROVED- JUL 1 b 2002
PETER B. LEWIS
MAYOR
Ordinance No. 5674
July 3, 2002
Page 3
ATTEST:
J;gt� f �I ///� �.City Clerk
AS -1O FORM:
City Attorney
PUBLISHED:
Ordinance No. 5674
July 3, 2002
Page 4
EXHIBIT "A"
SUBBASIN P
This chapter was updated in June 2002 to address the new floodplain elevations for Mill
Creek and to determine the impacts of improving the conveyance system and not providing
a regional detention facility within Subbasin P.
The 1990 Plan identified flooding and water quality problems in the subbasin and outlined
two alternatives to resolve the problems. The new analysis for the current update also
identifies flooding and water quality problems and outlines four alternative improvements.
The updated improvement alternatives are described in this chapter.
DRAINAGE SUBBASIN AND SYSTEM DESCRIPTION
Subbasin P contributes runoff to. Mill Creek from a drainage area of 155 acres in the west -
central portion of the City. The entire subbasin lies within the city limits. The approximate
boundaries of the subbasin are the Chicago, Milwaukee, St. Paul, and Pacific (CMStP&P)
Railroad on the east; SR 167 on the west; the westward extension of 5th Street NW on the
north; and SR 18 on the south.
For the analysis of the existing drainage system, the subbasin was divided into 13
subcatchments. The subbasin and subcatchment boundaries are shown in Figure P-1.
The corridor between Main Street and SR 18 is a rapidly developing area of the city.
Recent industrial and commercial development of that area has occurred close to Mill
Creek, with additional development pending in areas adjacent to SR 18. Some land east of
Lund Road remains undeveloped. Land use in areas north of Main Street and east of
Western Avenue are a mixture of commercial and residential, with some open space to the
west of Western Avenue. In 1990, approximately 25 percent of Subbasin P was determined
to be impervious, according to the City's Geographic Information System (GIS) database.
The City's preliminary wetlands inventory has identified 36 acres of potential regulated
wetland area within Subbasin P. The majority of that area is along the SR 167 corridor
north and south of Main Street. Additional designated wetlands are in the undeveloped
area south of Main Street and east of Lund Road. Further refinement in regulated wetland
boundaries will result from new development reviews.
The existing drainage system for Subbasin P consists of a network of interconnected storm
drains and open channels. The City's GIS inventory data base for the Subbasin P drainage
system consists of approximately 3,600 feet of storm drainage pipelines ranging in
diameter from 12 to 24 inches, and approximately 2,800 feet of open channel.
The conveyance system conveys runoff primarily to the northwest from the subbasin area
south of Main Street and east of Lund Road; flow is intercepted by storm drainage systems
in those roads. The drainage systems combine at Lund Road and flow along the south side
of Main Street to the east side of SR 167. At this point the flow combines with a secondary
culvert that serves the southwest portion of the subbasin and flows north across Main
Street. From that location, flows are directed to the north by open channel to a 24 -inch -
diameter culvert crossing SR 167 prior to discharge to Mill Creek. Runoff from Subbasin P
is discharged into Mill Creek at outfall identifier MC13, which is located at creek mile 4.98.
Exhibit "A" 2 -Pi
Ord. 5674
LEGEND
0 Subbasin P
--------- Subcatchment boundary & number
Existing drainage pipe,
with diameter
• Existing manhole
Existing calchbasin,_J
Existing ditch or stream
O Mill Creek 100 -year floodplain
S��pMPll OR
Tetra Tech/Figure P-1.
KCM, Inc. City Auburn EXISTING SUBBASIN P
S1917FirstAvenue COMPREHENSIVE DRAINAGE PLAN
Seattle, Washington 98101 DRAINAGE SYSTEM
The trunk drainage system alignment drops 10 feet in a distance of approximately 4,400
feet, for an average gradient of 0.002 feet per foot. Approximately 5,800 feet of the drainage
system was evaluated for the updated Plan.
MODELING APPROACH AND SETUP
Modeling Approach
As part of the 2002 Plan Update, the 1990 SWMM models were converted to XP-SWMM
and existing and future problem areas were again identified. For the update, existing
conditions were the same as for the 1990 plan but new zoning under the City's 1998
Comprehensive Land Use Plan was used to define ultimate buildout for the future
condition. The XP-SWMM model is similar to the original SWMM model, with
enhancements to increase stability and decrease continuity errors. Four storm events were
simulated for both the existing and future land use conditions — the mean annual, 10-, 25-,
and the 100 -year storm. Results from the mean annual storm model were used as guidance
for sizing water quality facilities, and the results from the 25 -year storm were used to size
all new conveyance facilities.
The original SWMM model input data file was spot checked against current City utility
maps to ensure that the conveyance system is accurately represented. The following
discrepancies were noted and changed:
The subbasin boundary was redefined to extend only as far west as State
Route 167. The original subcatchment 1P, which is located on the west side
of SR 167, was assumed to not contribute runoff to the Subbasin P
conveyance system but was instead assumed to contribute runoff directly
to Mill Creek.
A majority of the subcatchment boundaries were modified from the original
1990 delineations to account for the pattern of future development.
In the original 1990 analysis, the pipe located between Junctions P9 and
P11 was assumed to be a 12 -inch RCP. City utility drawings indicate that
this pipe is actually a 24 -inch RCP.
Model Setup
Hydrologic analyses were conducted with XP-SWMM's Runoff block using a 48-hour
simulation with a 10 -minute time step. Inflow hydrographs were then read into the model's
Extran block. A five -second time step was used over the 48-hour hydraulic simulation. The
25 -year SCS Type lA design storm was used and the runoff losses were modeled with the
Horton infiltration function used in the original CH2M Hill model.
A key update to the model used for this analysis was the definition of new tailwater
conditions at Subbasin P outfalls. A new hydraulic analysis of Mill Creek was done in 2002.
The results for future land -use conditions indicate that flows having a 25 -year recurrence
interval cause water to back up through the SR 167 culvert, inundating much of the
northwest portion of Subbasin P and submerging existing drainage channels.
2-P3
For this reason, the downstream boundary conditions in the XP-SWMM model were
defined as fixed water surface elevations at the pipe outfall locations. The fixed water
surface elevations for each outfall were estimated in the 2002 hydraulic analysis and
floodplain mapping of Mill Creek and recent groundwater -monitoring data. The outfall
tailwater elevations used for both the existing and future condition model run's are listed in
Table P-1.
TABLE P-1.
OUTFALL TAILWATER ELEVATIONS USED FOR MODELING
25 -Year Tailwater Elevation
Outfall Location (feet)
North Side of West Main Street Bridge over SR 167 60.5
North Side of West Main Street Near Lund Road 60.5
North End of Clay Street
Western Street (North)
Western Street (South)
57.8
60.0
60.25
The tailwater elevations were assumed to remain constant over the entire hydraulic
simulation period. The 25 -year and 100 -year water surface elevations were found to be
equal in the reach of Mill Creek adjacent to Subbasin P. The 100 -year floodplain map
indicates that open channels would be entirely submerged by Mill Creek backwater; thus,
open channels were not modeled in the updated XP-SWMM model.
Other assumptions used in the current modeling include the following:
New runoff nodes were defined for areas tributary to Clay and Western
Streets. Runoff from subcatchments 2AP and 2DP was input into Western
Street nodes. Three new subcatchments were defined for the Clay Street
drainage (SAP, 8BP, and 8CP), adding about 26 acres to the total area of
Subbasin P. Ground elevations obtained from digital mapping were used to
set rim and outfall invert elevations of new model nodes.
The pipe network in the model was modified to include the existing
Western Street drainage system and possible improvements.
On-site detention for allnew development and re -development was
assumed; based on this assumption, peak flows would not change between
the existing and future land -use conditions.
MODELING RESULTS
Existing Condition
The hydraulic performance of the existing drainage system was evaluated with the new
boundary conditions described above. Results indicate flood volumes of 0.45, 0.27, 0.27,
0.20, and 0.13 acre-feet at nodes P7B, P7C, P7D, P7E, and P12, respectively. Complete
results are presented in Table P-2.
2-P4
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Lund Road and West Main Street Drainage Improvements
Improvements to the drainage network on Lund Road and West Main Street were
simulated using the XP-SWMM model with the new downstream boundary conditions.
Storm hydrographs were input to the model's Extran block, and pipe diameters were
increased incrementally in successive model runs until surface flooding at manhole nodes
was eliminated. The design concept includes the following pipe network changes:
A connection at Node P7 to a 24 -inch trunk line pipe that conveys Lund
Road drainage to the west would be plugged.
A new 36 -inch pipe would be installed to convey flow from Node P7
underneath West Main Street and discharge it to an open channel.
The existing 12 -inch diameter storm pipe along West Main Street between
nodes P11 and P12 would be replaced with an 18 -inch pipe.
All existing 18 -inch pipe segments along Lund Street would be upgraded to
24 -inch pipe.
Model results indicate that no surface flooding would occur with these changes. Model
results are presented in Table P-3.
Western Street Drainage Improvements
An analysis of the Western Street drainage system was conducted. The tributary area
includes approximately 18 acres of Subbasin P. Storm hydrographs from two
subcatchments (2AP and 2DP) were generated in the model's runoff block and then input
to the Extran block at two inflow nodes. Two scenarios were modeled:
Western Street Scenario A—A single pipeline along the entire length of
Western Street, with all runoff discharged at a point just north of the
street's end to a new pipe that conveys flow to the west.
Western Street Scenario B—Two separate systems discharging at two
outfall locations. The existing pipe system that drains the southern half of
Western Street would remain as it is, with runoff discharged into the
wetland area to the west. The second system would be constructed to drain
the northern portion of the street and discharge to a new pipe as in
Scenario A.
Under the expected 25 -year tailwater conditions, both outfalls would be completely
submerged. For Scenario A, pipes were sized by trial -and -error to determine the minimum
size that would eliminate surface flooding. An 18 -inch -diameter pipe at the upstream
(south) end connecting to a 24 -inch pipe about midway along Western Street would prevent
surface flooding, but the maximum hydraulic grade line (HGL) would be within 0.2 feet of
the rim elevation of the upstream node. Using a larger pipe larger in the upstream
segment does not change this condition. The HGL is controlled by tailwater rather than by
pipe size. Hydraulic modeling of Scenario B shows surface flooding of about 0.2 acre-feet at
the furthest upstream node. Model results are presented in Table P-4.
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Clay Street Drainage Improvements
As described earlier, Subbasin P was expanded by about 26 acres to include runoff
tributary to a new Clay Street drainage system. Three new subcatchments were
delineated, and impervious land cover was determined from digital mapping. A small pipe
system exists in the northeast corner of Subbasin P and is the only drainage infrastructure
in the Clay Street subcatchment.
The modeled improvements consist of a single pipeline along the entire length. of Clay
Street, with three outfall scenarios:
Clay Street Scenario A—The new pipe system would discharge freely at a
point just north of the street's end
Clay Street Scenario B—The new pipe system would tie into the existing
smaller system at the end of Clay Street
Clay Street Scenario C—The new pipe system would be tie into the existing
system about 300 feet to the west at an overflow manhole.
For Scenario A, the existing smaller system was neglected, and all runoff was input to the
new Clay Street nodes. Pipe sizes and invert elevations were adjusted in successive model
runs until model results indicated no flooding at the ground surface.' The pipe system
necessary to convey 25 -year flows consists three segments increasing in diameter in the
downstream direction (18- to 24- to 30 -inch).
For Scenario B, the pipe system developed under Scenario A was tied into the existing
smaller system at the north end of Clay Street. Runoff from subcatchment 8CP was re-
distributed to allow inflow into the smaller existing system. Results for the 25 -year event
indicate surface flooding at all nodes. The model was rerun to determine if Scenario B
could convey flow from the 10 -year storm, but the model again showed flooding at all
nodes.
Under Scenario C, a 30 -inch pipe would be installed to convey runoff from the end of Clay
Street to the existing 18 -inch system at a point approximately 300 feet to the west. An
overflow manhole joining the two pipe systems would be allowed to spill water into an
undeveloped area during high -runoff storm events, relieving the back -pressure that caused
surface flooding associated with Scenario B. Invert elevations of the overflow manhole were
computed from a profile plot of the existing system. The overflow rim elevation was set
above the expected 25 -year water surface elevation at 60.0 feet. Model results indicate that
no surface flooding occurs at any of the model nodes, except the intended flooding at the
overflow node.
Model results are presented in Table P-5.
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IMPROVEMENT ALTERNATIVES
No additional alternatives were investigated for Lund Road and West Main Street. For the
Western 'Street improvements, Scenario A was rejected and two alternatives were
developed for Scenario B. For the Clay Street improvements, Scenario B was rejected, and
three alternatives were developed from Scenarios A and C. Design concepts for water
quality facilities, developed following 1998 KCSWDM criteria, are included in the
alternatives. Planning -level cost estimates were produced for each alternative.
Clay Street Alternatives
The Clay Street alternatives (see Figure P-2) all include the installation of a new trunk
system along Clay Street. They differ in water quality treatment and system discharge, as
follows:
Clay Street Alternative 1—The trunk system along Clay Street would
convey stormwater flows to a filtration system (such as a vault) installed in
the Clay Street right-of-way for treatment. Flows exceeding water quality
flow would bypass the facility. A 30 -inch pipe extension from the north end
of Clay Street would convey flow to an overflow manhole 300 feet to the
west, discharging to an existing 18 -inch pipe. The 18 -inch pipe would be
disconnected from an existing 24 -inch pipe used to convey drainage
underneath SR 167, and flow would be directed north through an existing
open channel.
Clay Street Alternative 2—The pipe system would be the same as in
Alternative 1, but a water quality swale would be used to treat runoff
instead of a filtration system. The swale would be constructed along the
west side of Clay Street.
Clay Street Alternative 3—The trunk system along Clay Street would
discharge into a water quality pond or swale just north of Clay Street on
the Gertrude Jones property.
Estimated costs for the Clay Street alternatives are summarized in Table P-6.
2-P12
TABLE P-6.
CLAY STREET ALTERNATIVE DESCRIPTIONS AND ESTIMATED COSTS
Alternative Name
Description
Estimated Cost
Clay Street Alternative 1
New trunk system along Clay Street; water quality
$1,266,000
vault in road right of way; discharge west to new
overflow manhole connected to existing facilities
Clay Street Alternative 2
New trunk system along Clay Street and water quality
$1,333,000
swale along Clay Street discharging west to new
overflow manhole connected to existing facilities
Clay Street Alternative 3
New trunk system along Clay Street discharging to
$1,242,000
water quality pond or Swale just north of Clay Street
2-P12
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Western Street Alternatives
In both Western Street alternatives (see Figure P-3), two separate pipe systems would be
used. The existing pipe system that drains the southern part of the street would remain in
place, and a new 18 -inch trunk would be installed to provide drainage to the northern
section of the road. The alternatives differ in the type of water quality treatment facility to
be used:
Western Street Alternative 1—A filtration system would be installed in the
Western Street right of way and discharge to a new pipe to the west.
Western Street Alternative 2—A water quality pond or swale would be
constructed just north of Western Street. A flow spreader would be used to
discharge diffuse flows to the north, into the Thermod site.
Estimated costs for the Western Street alternatives are summarized in Table P-7.
TABLE P-7.
WESTERN STREET ALTERNATIVE DESCRIPTIONS AND ESTIMATED COSTS
Alternative Name-
Description Estimated Cost
Western Street
New trunk system along Western Street; water quality $374,000
Alternative 1
vault in road right of way; discharge west to a new pipe
Western Street
New trunk system along Western Street discharging to $304,000
Alternative 2
water quality pond north of Western Street that
Total
discharges north to the Thermod site.
Lund Road and West Main Street Improvements
The recommended improvements for Lund Road and West Main Street (see Figure P-4)
redirect runoff through a proposed 36 -inch culvert beneath West Main Street. The
improvements also include improving two segments of open channel, upgrading the West
Main Street trunk to an 18 -in pipe between nodes P11 and P12 and upgrading the Lund
Road trunk system to a 24 -inch pipe over its entire length. Estimated costs for the
individual elements of the improvements for Lund Road and West Main Street are
summarized in Table P-8. Detailed cost estimates are included in Appendix D.
TABLE P-8.
LUND ROAD AND WEST MAIN STREET IMPROVEMENT
DESCRIPTIONS AND ESTIMATED COSTS
SR 167 Open Channel Improvements
$140,000
West Main Street Open Channel Improvements
$77,000
New Culvert Under West Main Street
$191,000
Lund Road Trunk System Upgrade
$420,000
West Main Street Trunk Segment Upgrade
$100,000
Total
$928,000
2-P14
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COMPARISON OF HYDROLOGIC DESIGN METHODOLOGIES
The City of Auburn's current stormwater hydrologic design methodology was compared to
the 2001 Washington Department of Ecology Stormwater Management Manual for
Subbasin P. The City's current design standards require using an event -based design
storm to compute design flows, while Ecology requires continuous hydrologic simulation.
The results of this analysis were used to conduct an additional XP-SWMM hydraulic
analysis of the Clay Street improvements.
Modeling Approach
A representative 10 -acre basin consisting of average land use conditions in Subbasin P was
defined. Runoff time series reflecting average runoff characteristics were generated using
each modeling methodology, avoiding the need to explicitly model each subbasin associated
with the Lund Road, Western Street, and Clay Street drainage systems. Unit peak
discharges. having a 25 -year recurrence interval were calculated using both design
methodologies. The unit peak discharges were then applied to the Lund Road, Western
Street, and Clay Street subbasins to determine whether the conveyance system would need
to be resized to accommodate increased peak flows associated with less stringent
(KCSWDM) runoff control requirements.
Land cover characteristics of the representative basin for existing and buildout conditions
were quantified from AutoCAD mapping of Subbasin P. Four general categories were used
to define land use within each parcel. For each land use category, the proportion of the
total Subbasin P area was calculated:
Category 1—Existing undeveloped area to remain undeveloped
(8.5 percent)
Category 2—Existing undeveloped area likely to be developed (19 percent)
Category 3—Existing developed area with no redevelopment (61 percent)
Category 4—Existing developed area with likely redevelopment (11.5
percent)
The 10 -acre representative basin was defined to consist proportionately of the four land use
categories listed above. The representative basin was further divided into two subbasins:
Subbasin 1—Area not requiring detention (6.95 acres, land use categories 1
and 3)
Subbasin 2—Area requiring detention (3.05 acres, land use categories 2
and 4)
King County Surface Water Design Manual Methodology
The modeling approach following the KCSWDM was as follows:
Compute runoff from each Subbasin using the Santa Barbara Urban
Hydrograph methodology (SBUH)
2-P17
Route Subbasin 2 runoff through a detention facility such that the 25 -year
peak outflow equals the pre -development peak
Combine outflow hydrographs of Subbasins 1 and 2
The following assumptions were made in the SBUH analysis:
• All developed parcels consist of commercial land use, with 83 percent
impervious and 17 percent pervious surfaces
Impervious curve number equals 98
Pervious curve number equals 74
Results of the SBUH analysis are summarized in Table P-9.
TABLE P•9.
RESULTS OF SBUH ANALYSIS
25 -Year Peak (cfs)
Subbasin 1 (no land use change) 3.25
Subbasin-2 (pre -development) 0.88
Subbasin 2 (post -development) 1.21
The KCSWDM requires that runoff control facilities be designed to attenuate 2 -year and
10 -year 24-hour peak flows to the pre -developed magnitude. For flows exceeding the
10 -year flow, the attenuation requirement is based on the potential for downstream
impacts. For this analysis, it was assumed that matching 25 -year Subbasin 2 peak outflow
to the pre -development peak would be a reasonable attenuation requirement.
The King County program HYD was used to size a detention facility to attenuate runoff
from Subbasin 2 to the pre -development peak flow of 0.88 cubic feet per second (cfs). The
King County program RESVOR was used to route the inflow hydrograph through the
detention facility. The outflow hydrographs from Subbasins 1 and 2 were combined,
yielding a peak flow of 3.95 cfs from the representative 10 -acre basin.
Washington Department of Ecology Methodology
Ecology currently requires continuous simulation using the Western Washington
Hydrology Model (WWHM) to determine pre- and post -development runoff. The program
uses a standardized 50 -year rainfall/evaporation time series and runoff parameters unique
to the basin location. Rainfall, evaporation, and runoff parameters in the model database
are used to compute a 50 -year time series of basin outflow from which exceedance
probabilities (the inverse of recurrence intervals) and flow duration statistics are
computed.
The WWHM modeling approach used in this analysis follows closely that described for the
SBUH method:
Compute runoff from Subbasins 1 and 2.
2-F18
Route Subbasin 2 runoff through a detention facility
Combine the peak flows of Subbasins 1 and 2
The runoff control requirements stipulated by Ecology are fundamentally different from
those outlined in the KCSWDM. Instead of matching peak flows, runoff control facilities
are to be designed such that the post -development flow durations are equal to pre -
developed conditions for peak flows ranging from 50 percent of the 2 -year event to the 50 -
year event.
Separate WWHM models were developed for each subbasin. A detention basin to attenuate
runoff from Subbasin 2 was sized using a spreadsheet program developed by Ecology. The
stage -storage -discharge relation for the detention basin was then input into the WWHM
model. In an iterative process, flow durations were matched by trial -and -error to the extent
practicable for the assumed field conditions. Results of the WWHM analysis for are
summarized in Table 6.
TABLE P-10.
RESULTS OF WWHM ANALYSIS
Subbasin 1 (no land use change) 3.16
Subbasin 2 (pre -development) 0.36
Subbasin 2 (post -development 1.42
without detention)
Subbasin 2 (post -development 0.26
with detention)
Unlike the SBUH approach, WWHM generates a 50 -year time series instead of a single
event hydrograph. Since the 25 -year peak outflow from the detention pond is small
compared to the Subbasin 1 peak (0.26 and 3.16 cfs, respectively, or less than 10 percent)
the WWHM time series from Subbasins 1 and 2 were not combined, as this would require
an additional flood frequency analysis to more accurately determine the magnitude of the
25 -year flow. Instead, the 25 -year peak outflows from Subbasins 1 and 2 were assumed to
occur simultaneously and were simply added, yielding an outflow of 3.42 cfs from the 10 -
acre representative basin.
Comparison of 25 -year Peak Flows
The City's design approach yields peak flows about 15 percent higher than those calculated
by Ecology's standards (3.95 cfs vs. 3.42 cfs for the 10 -acre representative basin). These
values are based on average runoff characteristics; the difference in peak flows is likely to
vary with the level of development of each parcel. Table P-11 summarizes the resulting
peak flows from applying the results to the Lund Road,, Western Street, and Clay Street
tributary areas.
2-P19
TABLE P-il.
EXPECTED INCREASES IN DISCHARGE
25 -Year Peak Flow (cfs)
Tributary Area
(acres) SBUH WWHM Difference
Lund Street 40.2 15.8 13.7 2.1
Western Street 17.6 6.9 6.0 0.9
Clay Street 26.2 10.3 9.0 1.3
ADDITIONAL HYDRAULIC ANALYSIS OF CLAY STREET SYSTEM
The hydrologic approach used to evaluate Subbasin P drainage improvements is equivalent
to the Ecology methodology. The higher peak flows using the City's methodology could
affect recommendations for drainage improvements. To assess this effect, the Clay Street
Scenario C improvements were re-evaluated with peak flows increased by 15 percent to .
represent the difference between the two methodologies.
A Horton infiltration function (used in XP-SWMM to compute runoff losses) was assumed
for the Ecology methodology. Rather than remodel runoff from each of the four Clay Street
subbasins using SBUH methodology, the Horton loss function was not changed, and the 15
percent increase in design flows was achieved by systematically increasing the percentage
of impervious area in each subbasin..Successive model runs in XP-SWMM's hydrology
block were conducted until the new design flows were achieved.
New inflow hydrographs were then input to the XP-SWMM Extran block to perform the
hydraulic simulation of the drainage system. Results indicate that the Clay Street pipe
sizes developed for Scenario Care adequate to accommodate the 15 percent increase in
design flows without flooding at any of the Clay Street model nodes. However, flooding was
shown to occur at an existing segment of 12 -inch pipe located along the northern subbasin
boundary. A subsequent model run was conducted and increasing the 12 -inch pipe to an
18 -inch prevented flooding at this node (see Table P-12).
ANALYSIS OF MILL CREEK WATER SURFACE ELEVATION
The existing Hydrologic Simulation Program — Fortran (HSPF) model was modified from
its original form to represent current and anticipated land -use conditions. The runoff time
series produced by HSPF was used as input for Mill Creek hydraulic modeling.
The Full Equations (FEQ) unsteady -flow model was used to evaluate the hydraulic
conditions in Mill Creek. The purpose was to compare the 100 -year .Mill Creek water
surface profile under current land use conditions to a new profile of the future flows
obtained from the HSPF analysis. The results show a change in water surface elevation at
only two locations, both upstream of the 37th Street bridge. The increase at these sections
was 0.01 feet.
2-P20
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