HomeMy WebLinkAboutSWP272253(3) 1
' PRELIMINARY DRAINAGE REPORT
' FOR
' SPRINGBROOK
' CITY OF RENTON, WASHINGTON
�
_ 22� S3
January 5, 1996
Triad Job No. 95-202
' PREPARED BY:
Shawn G. Moore, E.I. T.
REVIEWED BY:
Roy E. Lewis, Jr., P.E.
' BAN 89 1996
' DEVELOPNt'1*4 f PLANNING
CITY OF RENTON
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1
TRIAD ASSOCIATES
1
1
1 PRELIMINARY DRAINAGE REPORT
1 FOR
1 SPRINGBROOK
1 CITY OF RENTON, WASHINGTON
1
1
1 January 5, 1996
1 Triad Job No. 95-202
1 .
1 PREPARED BY: LE y�; w41,1
Shawn G. Moore, E.I. T.
REVIEWED BY:
Roy E. Lewis, Jr., P.E. •. 9 l AB
1 �SS�oNAL
1
1
1
TRIAD ASSOCIATES
' TABLE OF CONTENTS
INTRODUCTION.........................................................................................................1
DRAINAGECONCEPT...............................................................................................2
DOWNSTREAM ANALYSIS......................................................................................3
DETENTION CALCULATIONS.................................................................................4
' EXISTING CONDITIONS.....................................................................................................4
DEVELOPEDCONDITIONS................................................................................................. 7
Stage - Storage/Discharge Table.............................................................................. S
LevelPool Routing Table ........................................................................................ 9
LiveStorage............................................................................................................ 9
' Dead Storage........................................................................................................... 9
ProposedVault........................................................................................................ 9
APPENDIX
' Soils Map
Curve Number Table
' 2, 10, & 100-Yr Isopluvials
' PRELIMINARY STORM DRAINAGE REPORT-PAGE-i
INTRODUCTION
' The proposed project includes 90 condominiums with a looped access road and utilities on
approximately seven acres of land. The site is located along the east side of Talbot Road
between Southwest 43rd Street and South 55th Street in the City of Renton. Please refer
to the Vicinity Map located below. More generally, the site is located in Section 32,
Township 23 North, Range 5 East, W.M., King County, Washington.
Two existingsingle-family
s gle fanuly residences are currently located on the site. Associated with
' these residences is approximately 0.33 acres of impervious surfaces and approximately
6.64 acres of pervious surfaces. The pervious area is predominantly a combination of
pasture and young second growth forest. The site slopes from east to west at
approximately 10 to 15 percent. Runoff is collected in the roadside ditch along the east
' side of Talbot Road and conveyed to a tightlined storm drainage system to the north.
According to the"Soil Survey- King County Area", the site is underlain with Alderwood,
' type `C' soils.
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VICINITY MAP
PRELIMINARY STORM DRAINAGE REPORT-PAGE-1
1 -
' DRAINAGE CONCEPT
The storm drainage system for the proposed project is designed with reference to
standards found within the 1990 King County Surface Water Design Manual
(KCSWDM). In general, runoff from the site will be conveyed within a tightline drainage
system to an underground detention vault to be located near the western property line.
The detention vault will be designed as a wet vault, incorporating live and dead storage,
and providing water quality enhancements. The detained and treated storm drainage will
connect to a tightline conveyance system in Talbot Road (which will be constructed in
association with the frontage improvements for this project). The storm drainage will then
' discharge into the existing drainage ditch along Talbot Road, at the northwestern corner
of the site. Runoff from off-site tributary areas to the east of the project will be collected
' and conveyed along the southern property line of the site in a separate conveyance system.
' The wet vault will be designed to provide peak rate runoff control for the 2, 10, and 100-
' year, 24-hour storm events. The vault will be designed such that the post-developed peak
runoff rates are attenuated to the pre-developed peak runoff rates for each of the 2, 10,
' and 100-year, 24-hour storm events. A 30-percent volume correction factor (factor of
safety) will also be applied to the live storage volume. Water quality enhancement will be
provided within the vault through the use of "dead storage". Dead storage will be
provided for the volume of the `water quality" (6-month) storm event. The precipitation
' to be used for the water quality storm event will be one-third of the 2-year, 24-hour
precipitation.
' PRELI
MINARY STORM DRAINAGE REPORT-PAGE-2
DOWNSTREAM ANALYSIS
A downstream drainage analysis was performed on December 7, 1995, a cloudy and cold
day. There was a small amount of runoff being conveyed within the existing drainage
' ditches and tightline storm drainage systems. Runoff from the site currently sheet flows in
a westerly direction and is collected within the existing drainage ditch on the east side of
' Talbot Road South. The drainage ditch is approximately three feet deep and has side
slopes of 2:1, with a one foot base width. The ditch flows northerly along Talbot Road
' South at a slope of approximately one-half to one percent. Immediately north of the
property, flows are conveyed beneath an existing driveway via a 12-inch diameter HDPE
' culvert. The ditch then continues northerly for approximately 200 feet. Runoff is then
collected into a City of Renton storm drainage system via a 12-inch diameter CMP
culvert, across from South 45th Place. Runoff is conveyed within this tightlined
conveyance system to beyond a point a quarter-mile downstream.
' The downstream conveyance system was in good shape and showed no evidence of
drainage related problems. The system appears to have more than adequate capacity to
' handle the existing flows. The King County Sensitive Areas Folio was researched and the
site was not found, nor was it tributary to (within atleast a quarter mile), any sensitive
areas (wetlands, erosion hazard areas, etc). As such, it is our opinion that a detention
standard more restrictive than matching the pre-existing flows for the 2, 10, and 100-year,
' 24-hour storms is not warranted.
PRELIMINARY STORM DRAINAGE REPORT-PAGE-3
' DETENTION CALCULATIONS
' The following detention calculations were performed using Engenious Systems, Inc.'s
hydrology program Water Works. The hydrographs were developed using the S.B.U.H.
' methodology with a King County Type I-A, 24-hour (User 1) rainfall distribution.
Separate S.C.S. curve numbers were applied for the impervious and pervious portions of
' the site. The developed hydrographs were routed through a "theoretical" vault to
determine the theoretical volume of storage required. The theoretical volume of live
storage was then increased by the 30-percent volume correction factor (factor of safety) to
determine the actual volume of live storage required. Please refer to the Existing and
Developed Condition Maps and the Curve Numbers Table in the Appendix of this report.
The total precipitation for the water quality, 2, 10, and 100-year, 24-hour storm events are
' 0.67", 2.0", 2.9", and 3.9", respectively.
EXISTING CONDITIONS
Total Area= 6.97 Ac —
Impervious Area= 0.33 Ac @ CN-98 (2-residences& associated parking/patios)
' Pervious Area=6.64 Ac @ CN-83.9
Gravel Roads= 0.20 Ac @ CN-89
Lawn/Landscaping= 0.68 Ac @ CN-86
Pasture= 3.56 Ac @ CN-85
2°d Growth Forest/Brush= 2.20 Ac @ CN-81
Time of Concentration=25.31 min.
Reach 1: 70' Sheet Flow @ 30%; `n' = 0.8 (Forest, heavy ground litter)
Reach 2: 230' Sheet Flow @ 9.56%; `n' = 0.13 (Tall Grass/Pasture)
Reach 3: 375' Shallow Concentrated Flow @ 10.4%; `k' = 11 (Short Grass/Lawn)
' PRELIMINARY STORM DRAINAGE REPORT-PAGE-4
' Existing Flows
BASIN ID: EX2 NAME: EX 2 YEAR HYDRO
SBUH METHODOLOGY
' TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE..: USERI PERVIOUS AREA
PRECIPITATION..: 2.00 inches AREA..: 6.64 Acres
' TIME INTERVAL..: 10.00 min CN..: 83.90
TIME OF CONC..: 25.31 min IMPERVIOUS AREA
ABSTRACTION COEFF . 0.20 AREA... 0.33 Acres
CN..: 98.00
PEAK RATE: 0.84 cfs VOL: 0.46 Ac-ft TIME: 480 min
BASIN ID: EX10
NAME: EX 10 YR HYDRO
' SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE... USERI PERVIOUS AREA
PRECIPITATION..: 2.90 inches AREA..: 6.64 Acres
TIME INTERVAL..: 10.00 min CN..: 83.90
TIME OF CONC..: 25.31 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 0.33 Acres
' CN..: 98.00
PEAK RATE: 1.84 cfs VOL: 0.86 Ac-ft TIME: 480 min
BASIN ID: EX00 NAME: EX 100 YR HYDRO
' SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE..: USERI PERVIOUS AREA
PRECIPITATION-: 3.90 inches AREA..: 6.64 Acres
TIME INTERVAL..: 10.00 min CN..: 83.90
TIME OF CONC... 25.31 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 0.33 Acres
' CN..: 98.00
PEAK RATE: 3.10 cfs VOL: 1.36 Ac-ft TIME: 480 min
' PRELIMINARY STORM DRAINAGE REPORT-PAGE-5
Allowable Release Rates
' 2-yr Allowable Release Rate =Peak Rate -Existing 2-yr Hydro
2-yr Allowable Release Rate = 0.84 cfs
10-yr Allowable Release Rate=Peak Rate - Existing 10-yr Hydro
' 10-yr Allowable Release Rate= 1.84 cfs
' 100-yr Allowable Release Rate=Peak Rate - Existing 100-yr Hydro
100-yr Allowable Release Rate=3.10 cfs
' PRELIMINARY STORM DRAINAGE REPORT-PAGE-6
DEVELOPED CONDITIONS
Total Area= 6.97 Ac
Impervious Area = 3.22 Ac @ CN-98
' Pervious Area= 3.75 ac @ CN-85.17
Lawn/Landscaping= 3.13 Ac @ CN-86
2r-d Growth Forest/Brush= 0.62 Ac @ CN-81
Time of Concentration= 15.33 min.
Reach 1: 90' Sheet Flow @ 33%; `n' = 0.8 (Forest, Heavy Ground Litter)
Reach 2: 75' Shallow Concentrated Flow @ 5%; `k' = 11 (Lawn)
Reach 3: 500' Pipe Flow @ 9.0%; `k' =42
BASIN ID: DVWQ NAME: DEV WATER QUALITY STORM
SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE... USERI PERVIOUS AREA
PRECIPITATION..: 0.67 inches AREA..: 3.75 Acres
TIME INTERVAL..: 10.00 min CN..: 85.17
TIME OF CONC..: 15.33 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 3.22 Acres
' CN..: 98.00
PEAK RATE: 0.36 cfs VOL: 0.14 Ac-ft TIME: 480 min
' BASIN ID: DV2 NAME: DEV 2 YR HYDRO
SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE..: USERI PERVIOUS AREA
PRECIPITATION-: 2.00 inches AREA..: 3.75 Acres
TIME INTERVAL..: 10.00 min CN..: 85.17
TIME OF CONC..: 15.33 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 3.22 Acres
CN..: 98.00
PEAK RATE: 1.92 cfs VOL: 0.73 Ac-ft TIME: 480 min
BASIN ID: DV10 NAME: DEV 10 YR HYDRO
SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE..: USERI PERVIOUS AREA
PRECIPITATION..: 2.90 inches AREA..: 3.75 Acres
' TIME INTERVAL..: 10.00 min CN..: 85.17
TIME OF CONC..: 15.33 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 3.22 Acres
CN..: 98.00
PEAK RATE: 3.23 cfs VOL: 1.19 Ac-h TIME: 480 min
PRELIMINARY STORM DRAINAGE REPORT-PAGE-7
1
BASIN ID: DV00 NAME: DEV 100 YR HYDRO
SBUH METHODOLOGY
TOTAL AREA..: 6.97 Acres BASEFLOWS 0.00 cfs
RAINFALL TYPE..: USER1 PERVIOUS AREA
PRECIPITATION-: 3.90 inches AREA..: 3.75 Acres
TIME INTERVAL-: 10.00 min CN..: 85.17
TIME OF CONC..: 15.33 min IMPERVIOUS AREA
ABSTRACTION COEFF : 0.20 AREA..: 3.22 Acres
CN..: 98.00
PEAK RATE: 4.75 cfs VOL: 1.73 Ac-ft TIME: 480 min
Stage- Storage/DiScharee Table
' VAULT ID No. DET 1
DESCRIPTION: Theoretical Detention Vault
Length: 100.00 ft. Width: 18.00 ft.
DISCHARGE STRUCTURE ID No. DIS 1
DESCRIPTION: Multiple Orifice
Outlet Elev: 100.00(Assumed)
Orifice#1 - Elev: 98.00 ft, Diameter: 4.6348 in.
Orifice#2 -Elev: 102.20 ft, Diameter: 5.0391 in.
Orifice#3 -Elev: 103.50 ft, Diameter: 4.8750 in.
STAGE STORAGE DISCHARGE
VI CF CFS
100.00 0 0.0000
100.50 900 0.4122
101.00 1800 0.5829
101.50 2700 0.7139
102.00 3600 0.8244
102.50 4500 1.2991
103.00 5400 1.6260
' 103.50 6300 1.8762
104.00 7200 2.5464
104.50 8100 2.9265
105.00 9000 3.2464
Note: Elevations are relative and are for the purposes of these calculations only. Actual elevations will
vary.
r
' PRELIMINARY STORM DRAINAGE REPORT-PAGE-8
Level Pool Routing Table
The following table represents the results of routing the developed hydrographs through
the "theoretical" detention vault. This determines the theoretical volume of live storage-,
the minimum volume of storage required to meet the allowable release rates.
Description Inflow Storage Discharge P. Stage Volume Outflow P. Time
[CFSJ ID ID C CFS [Min
Theoretical 2-yr 1.92 DET1 DIS1 102.08 3744 0.841 540
Storm
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Theoretical 10-yr 3.23 DETI DIS1 103.43 6166 1.841 520
Storm
Theoretical 100-yr 4.75 DETI DIS1 104.76 8565 3.097 510
' Storm
Live Storage
' The theoretical volume of live storage required is 8,565 cubic-feet. Therefore, the actual
volume of live storage required is 11,135 cubic feet(1.30 x 8,565 ft3). The vault for this
project will provide at least 11,135 cubic-feet of live storage.
1 Dead Storage
The vault will be designed to provide dead storage in the amount equal to the volume of
the water quality storm event. The volume of the water quality hydrograph was
' previously determined to be 6,238 cubic-feet (approx. 0.14 Ac-ft). The actual vault for
this project will provide at least 6,238 cubic-feet of dead storage.
' Proposed Vault
The vault for this project could be approximately 115' long, 20' wide and 9.0' deep. The
' vault would provide 5' of live storage and 2.75' of dead storage with 1.25' of freeboard.
PRELIMINARY STORM DRAINAGE REPORT-PAGE-9
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' CURVE NUMBERS TABLE
' SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS (Published by SCS in 1982)
Runoff curve numbers for selected agricultural, suburban and urban land use for Type 1A
rainfall distribution, 24-hour storm duration.
CURVE NUMBERS BY
HYDROLOGIC SOIL GROUP
' LAND USE DESCRIPTION A B C D
Cultivated land(1): winter condition 86 91 94 95
' Mountain open areas: low growing brush and grasslands 74 82 89 92
Meadow or pasture: 65 78 85 89
' Wood or forest land: undisturbed or oldeFor
growth 42 64 81
Wood or forest land: young second growsh 55 72 81 86
Orchard: with cover crop 81 88 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area 68 80 O 90
fair condition: grass cover on 50%
to 75% of the area 77 85 90 92
Gravel roads and parking lots 76 85 89 91
' Dirt roads and parking lots 72 82 89
Impervious surfaces, pavement, roofs, etc. 98 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 0 100
' Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3)
' 1.0 DU/GA 15 Separate curve number
1.5 DU/GA 20 shall be selected
2.0 DU/GA 25 for pervious and
2.5 DU/GA 30 impervious portion
' 3.0 DU/GA 34 of the site or basin
3.5 DU/GA 38
4.0 ,DU/GA 42
4.5 DU/GA 46
' 5.0 DU/GA 48
5.5 DU/GA 50
6.0 DU/GA 52
6.5 DU/GA 54
' 7.0 DU/GA 56
Planned unit developments, % impervious
condominiums, apartments, must be computed
' commercial business and
industrial areas.
(1) For a more detailed description of agricultural land use curve numbers refer to National Engineering
' Handbook, Section 4, Hydrology, Chapter 9, August 1972.
(2) Assumes roof and driveway runoff is directed into street/storm system.
(3) The remaining pervious areas (lawn) are considered to be in good condition for these curve numbers.
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ILI
ATom
r
NO
ILI WNW
ArAr11l
, . . . r ►
1
1
1
1
1
1
1
1
1
1
1