HomeMy WebLinkAboutSWP272327(2) PRELIMINARY DRAINAGE ANALYSIS
FOR THE
GURUDWARA SINGH SABA CHURCH
Renton, Washington
April 19, 1.996
RECEIVED
MAY 2 8 1996
DEVELOPMENT PLANNING
CITY OF RENTON
a I `c�F
Ot WAS,,
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Job No.: 96-046
SEGX Engineers/
Struct�ivil Consulting Engineers
22928 SE 3120 ST (360)886-1017 .
KLmt,WA 1181142
Cop%ru.ii 1976_:i E(;A Kngine�n
I
PRELIMINARY DRAINAGE AN ALYSIS
FOR THE
GURUDWARA SINGH SABA CHURCH
SECTION 31 , TOWNSHIP 23 NORTH, RANGE 5 EAST, W.M.
i
RENTON, WASHINGTON
April 5, 1995
Introduction
This portion of the analysis is included to give a brief overview of the project and a description
of the existing storm drainage system on and off the site. For this analysis two site visit(s) were
made to note the condition of the existing drainage system. Field observations were completed
on April 8, 1996.
The proposed project involves the construction of a new temple building, parking facilities, and
associated improvements. The project also involves demolition of a two existing buildings.
Existing site topographic maps, U.S.G.S. maps, City of Renton Storm Drainage maps, and a
field investigation were used to determine the-existing drainage configuration.
Proposed Site
The proposed Sikh Temple is located on Talbot Road approximately 1/4 mile north of the
intersection of S 55th Street and Talbot Road, in Renton, Washington. The site is approximately
4 acres and is currently developed with two existing temple buildings, a private residence,
landscaping, a paved parking lot and a grass area used for parking. The new temple building
will be located at the west central portion of the site. An existing residence on the north side of
the site and the eastern most temple building will be demolished and removed.
Approximately 64,000 sf of new paved parking lots will be constructed on the north, south'and
west side of the new Temple building. The existing church parking lot (approximately 20,000
sf) will be re-graded to match new grades.
Existing Drainage Conditions
Surface runoff from the existing site is conveyed to a storm drain system located on the temple
grounds. Surface runoff from the parking lot is collected in catch basins and conveyed to the
existing 12-inch storm sewer at the southwest comer of the existing parking lot. Flows are then
conveyed into an underground detention system located at the southwest corner of the site.
Runoff from the detention pipe is conveyed into a swale that flows southerly along Talbot
Road. Approximately 120 feet south of the site,.runoff enters a culvert that crosses Talbot Road
from east to west and discharges into a wetland area.
Two well defined drainage channels convey storm water off-site from the area to the east of the
site. The southern most channel conveys storm water from uphill areas and a well house that
i
i
Gurudwara Singh Saba Church.
Preliminary Drainage Analysis.
is located at the south east corner of the site. Flows in this channel are conveyed into an 8-inch
ductile iron pipe that connects to the existing storm water collection system on the site. The
northern swale conveys flows from the uphill areas to the east of the site. The tributary area
for both of these swales is approximately 7.35 acres. This area is wooded and slopes to the
west at about 7%.
An existing wetland is located at the southeast end of the site. This wetland has been
delineated by wetland consultants and is approximately 0.28 acres. The wetland is located on a
hillside with slopes to 7°e. A portion of the wetland (approximately 0.10 acres) will be
removed as a result of this project. To mitigate this a new wetland area (approximately 0.12
acres) will be constructed on the east side of the site.
Method of Analysis
Drainage calculations were performed using methods described in Chapter 3 -- Hydrologic
Analysis, of the King County Surface Water Drainage Manual. Peak runoff rates for the pre-
developed and post developed conditions were calculated using the Santa Barbara Urban
Hydrograph Method.The Water Works design program by Engenious Systems was used to
develop peak runoff rates for the various hydrographs.
King County Drainage Manual -- Core Requirements
Core Requirement # 1. Discharge at the Natural Location:
Runoff in the area of the proposed building addition currently discharges into a series of
ditches and storm drain pipes that convey Flows southerly along the east side of Talbot Road.
Approximately 120 feet south of the site, runoff enters an 18-inch storm drain pipe that crosses
Talbot Road from east to west and discharges into a wetland area.
The proposed development will in no way divert or change the location of the existing surface
water discharge point. Runoff from the proposed development will be tight-tined into the
existing on-site conveyance system and will be routed into a new two cell combination
detention/wet pond. Storm water will be conveyed from this facility into a new storm drain
system that will be constructed along the east side of Talbot Road. Runoff will be discharged
into an existing ditch just south of the site. These flows will continue to drain south in the
existing ditch to the storm drain pipe that crosses Talbot Road.
The existing drainage swale that carries flows from the area to the east of the site will be routed
into the new conveyance system in Talbot Road and conveyed into the ditch to the west of the
site.
The combination detention/wet pond facility will be designed to discharge peak flows from the
developed site from the 2-yr., 10-Vr. and 100-vr. 24-hr. storms at the pre-developed now rates
(refer to Core Requirement #3). Runofffrvm the entire area of development will be routed into
the detention facility. Runoff from the areas to the east of the development will be diverted
aroun3 the site and routed into the new off-site convevance system along Talbot Road. There
should no significant adverse impacts to the downstream properties.
2
Gurudwara Singh Saba Church.
Preliminary Drainage Analysis.
y
Core Requirement#2. Off Site Analysis:
There is approximately 7.35 acres of upstream area tributary to the site. Runoff from this site
currently discharges into two swales located on the site. Runoff from the entire site is currently
conveyed in these swales to the series of ditches and culverts that are presently located along
Talbot Road. From these ditches flows are conveyed to the storm drain pipe that crosses Talbot
Road to the west and discharges into a wetland area on the west side of the roadway.
The proposed conceptual drainage system will intercept the off-site flow and route it into the
existing channel to the north of the development. The runoff will be conveyed westerly
through the channel intb the new storm drain system at Talbot Road. This storm drain system
will convey flows to the existing ditch west of the site.
Currently there is no evidence of any drainage problems that may exist in the project area, nor
are any drainage problems predicted after development. The level I downstream analysis
indicated that downstream conveyance system appeared adequate. At the time of our field
investigation there was no evidence of flooding in the storm drainage system
Core Requirement#3: Runoff Control:
The existing site contains approximately 28,300 square feet(0.65 ac) of impervious area. After
development, the new impervious area will be about 95,800 square feet (2.20 ac), resultuig in a
net increase of 67,500 square feet of impervious area. The proposed developed site will contain
60% impervious area and 40% pervious area. The drainage analysis indicates that the pre-
developed condition peak flow for the 100-vr., 24-hr storm is 1.83 cfs, and the post-developeu
condition peak flow for the 100-yr., 24 hour storm is 2.52 cfs. Since the increase in the peak
runoff for the 100-yr. storm event exceeds the 0.5 cfs threshold per King Countv SWDNI, on-
site storm water detention will be provided for this project.
The detention pond will be constructed at the west end of the site near the entrance driveway.
The maximum storage elevation will be 149.0 feet (100-vr., 24-hour storm event). Discharge
From the detention system will be controlled by an outlet device with three orifices that will be
sized for the controlled discharge rate requirements of the 2, 10 and 100 year storm events. The
bottom orifice will be 5.58 inches the second orifice will be 5.13 inches and the third orifice will
be 5.25 inches. The detention pond will provide 2-feet of live storage. The wetpond portion of .
the basin will have a minimum3epth of three feet for water quality treatment. The proposed
pond will provide an additional qntfaQt of dead storage for sedimentation. A 10-foot
emergency spillway will be constructed at elevation 149.5.
The King County SWM requires a 30 percent safety factor for the detention pond volume.
Detention for the site will be provided by a two cell detention pond that will provide
approximately 6,680 cf of detention capacity. The required volume for the site is 4,050 cubic
feet. The proposed pond will have a safety factor of 1.65.
Computer generated hydrographs for the developed site were routed through the proposed
detention pond and control manhole to determine the capacity of the proposed system. The
Gurudwara Singh Saba Church.
Preliminary Drainage Analysis.
results of the routing analysis indicate that the proposed system meets current requirements.
See results of hydrograph routing analysis below:
Table 1 H dro ra h Routing
Storm Event Existing Peak Developed Peak Storage Peak Outflow
Inflow Inflow Required (cf)
cfs cfs cf
2-vr. Storm 0.61 1.09 1,720 0.61
10 r. Storm 1.16 1.76 2,927 1.16
100- r. Storm 1.83 2.52 4,063 1.83
As shown in the table above, the proposed pond and outlet control manhole with orifice riser
will provide sufficient detention capacity.
A 3-feet high (maximum) retaining wall will be constructed along two of the sides of the
detention pond. A guard rail will be placed along the top of the wall. The other two sides of
the pond will be sloped at 3:1 maximum. The pond will be constructed in a driveway island
and will be accessible from all sides. The maximum length from the driveway to the center of
either cell is 30 feet. Since access for maintenance can be from the adjacent road, a standard
maintenance access road into the pond will not be provided.
Water Quality Control:
Approximately 1.55 acres of new impervious vehicular parking and driving surface will be
created as a result of this project. Therefore, water quality treatment will be provided for the
new impervious surfaces. Physical constraints on the site make construction of a biofiltration
swale very difficult. A combined detention/wet pond will be provided for water quality
control. The King County SWM requires that the area provided for water quality be at least 1
percent of the new impervious surface area and that the volume provided for water quality be
1/3 of the runoff volume for the 2-yr.-24 hour storm event for the new impervious surface area.
The required water quality treatment surface area is 700 square feet and the water quality
storage volume was determined to be 3,300 cubic feet. The wet pond will provide
approximately 4,100 cubic feet of water quality storage and will have a surface area of
approximately 3,000 square feet. A 1-foot deep sediment storage collection area will be
provided at the wet ponds.
Core Requirement#4: Conveyance System:
The proposed on-site conveyance system will consist of a roof collection system, parking lot
catch basins, and an underground storm sewer system that will carry flows to the detention
basin.
The proposed conveyance system will be sized to carry flow from the 25 year storm event.
Since the design drainage basin is less than 25 acres, and the time of concentration is less than
100 minutes, the capacity of the conveyance elements will be determined using the Modified
Rational Method as described in SWDM Section 4.3.3.
4
Gurudwara Singh Saba Church.
Preliminary Drainage Analysis.
The existing channel on the north side of the site was sized to determine the required cross
section for carrying flows from the uphill areas. The results of the sizing show that a 1.5 foot
deep channel with 3:1 side slopes will have sufficient capacity to carry flows for the 25-yr
storm, and will not crest during the 100-yr. storm event.
Core Requirement #5: Erosion & Sedimentation Control Plan:
A temporary erosion and sediment control plan will be prepared for the site to reduce the risk
of erosion or sediment laden runoff leaving the site. We recommend the following measures
be taken:
• A silt fence shall located along the west side of the new parking along the frontage with
Talbot Road. The fence should be extended approximately 1.20 feet east along either side of
the development area.
• Temporary drainage interceptor ditches with rock check dams placed at 50 foot intervals
should be constructed to convey runoff from the uphill areas of the site into a temporary
sedimentation pond to be installed in the location of the proposed detention/wet pond
facility.
• The temporary sedimentation pond should be equipped with a cone riser and gravel filter
and will be connected to the new storm drainage conveyance system. When construction is
completed the pond will be cleaned of silt and debris and converted into the
detention/wetpond facility. '
• Existing catch basins will be fitted with filter fabric to prevent sediment from entering the
storm drain system.
• New catch basins will be fitted with a filter BMP as pavement is installed.
• The site should be monitored and additional$MP's implemented, if necessary, as
construction continues.
• The project construction drawings will include the City of Renton Standard Construction
and Erosion Control Notes. These notes address the additional T.E.S.C. BNIP's that may be
necessary.
Core Requirement#6:Maintenance and Operation: i
A maintenance and operation requirement for private'tacilities will be provided per the King
County SWM.
Core Requirement #7: Bonds and Liability:
Bonding will be provided per the King County SWM.
Special Requirement #5: Special Water Quality Controls:
i'1 1
The proposed drainage system will drain into an existing wetland area. Special water quality
controls will be provided for the project. The combined detention/wet pond will be sized to
meet the King County SW1vl requirements for special water quality controls. See Core
Requirement #3: Runoff Control: above.
3
Gurudwara Singh Saba Church.
Preliminary Drainage Analysis.
Summary
Our investigation indicated that there is no record of any drainage problem within the 1/4 mile
downstream conveyance system. The proposed development will not divert runoff from the
natural existing location. A detention/wet pond facility will be constructed to control release
rates from the site for the 2, 10, and, 100 year storm events. The proposed expansion is in
compliance with the City of Renton drainage codes and the King Countv Surface Water Design
Manual. There is no evidence of any drainage problems that may exist in the project area, nor
are any drainage problems predicted after development. The Level I downstream analysis
indicates the conveyance system appears adequate and there was no evidence of any bank
sloughing or erosion.
t
6
Hydrograph Input Worksheet
J
Project: SAL
k L nn C
Drainage Basin Identification (5 char): 2- r �d - r< u r• r t,
nr ,
Description (30 char): F Y-o I�n o c0 2 S F
T �
Total Area: 3 b-S
Precipitation:
(500/.)2yr_ 2yr_0 5yr ioy�r z .q zsyr Soy r iooyr-2-,.i—
Hydrograph Numbers:
(50%)2yr_ 23,r Syr ioyr 25yr 50yr 100y1
Time of Concentration:
length slope surface type 'In"value time
�•�a s a'/3 /6•S/
�O O 6.6 � Gra 4 a /
o 6 .6 Gross / GrU / 13
/ 7 7/
I
Total Travel Time: 1 7. Lfi I
Pervious Area: J'•0 3 Impervious Area:
Curve Number. (pervious)^ (impervious)
Land Use: G rc,s s
J c� Kir� � �G ve G� �Gac-s ,j ra<5 /iJr•�:ti
Soil Type: AI I2rc 1�r SL e-
Hydrologic Soil Group: L
CPL Job No. 7l - 0066' CI 1
Date:
'I I PIwE STREET.W.E_`:J SEAIILE.wA 9E101 P!1)6/54-0c60 -F206CK}5651
Hydrograph Input Worksheet
Project
Drainage Basin Identification(5 char): 2
DesQiption(30 char): L) r/ S' ��
F
Total Area: R
Precipitation:
(50%)2yr_ 2yr 2 . O 5}r,_10}T 2. 25}rt-- 50yr__ 100}'r 3 , _
Hydrograph Numbers:
(50%)2yr-__ 2yr Syr 10yr 25yr 50yr 100yr
Time of Concentration:
lengthslope surface 'In" value time
/0 /7;,n
I
Total Travel Time: /U m'tl
Pervious Area: /' y�6 A G Impervious Area: 2, 2 0 f
Curve Number. (pervious) 'F6 (impervious) 2 F
Land Use- /A✓K,n /�G v l��r l /n^ L^ 1/5 = aaln
Soil Type: f)I r1
Hydrologic Soil Group:
CPLjobNo-
y6 'CGd�' C/
Date: 411117Z76
v
^11 Pu+E STREET SURE-D SEhIILE�%A 95101 P:206134?-0 60 ,F:816,U335651
4/16/96 5 : 53 : 34 pm _ v page 1
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BASIN SUMMARY
BASIN ID: 100yr-dv NAME : 100-YR DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE . . . . : TYPEIA PERV IMP
PRECIPITATION. . . . : 3 . 90 inches AREA. . : 1 . 48 Acres 2 . 20 Acres
TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 6 . 00 98 . 00
TC. . . . : 10-. 00 min 10 . 00 min
ABSTRACTION COEFF : 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1500 p2yr: 2 . 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE: 2 . 52 cfs VOL: 0 . 97 Ac-ft TIME : 480 min
BASIN ID: 100yr-pr NAME : 100-YR PRE DEVELOPED SITE
SEUH METHODOLOGY
TOTAL AREA. . . . .+ . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE . . . : TYPElA PERV IMP
PRECIPITATION. . . . : 3 . 90 inches AREA. . : 3 . 03 Acres 0 . 65 Acres
TIME INTERVAL... . . : 10 . 00 min CN. . . . : 85 . 00 98 . 00
TC. . . . : 17 . 91 min 17 . 91 min
ABSTRACTION COEFF : 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1300 p2yr : 2 . 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 13 . 00 s : 0 . 0660
PEAK RATE : 1 . 83 cfs VOL: 0 . 80 A_c-ft TIME : 480 min
BASIN ID : 10yr-dev NAME : 10-YR DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE . . . . : TYPElA PERV IMP
PRECIPITATION. . . . : 2 . 90 inches AREA. . : 1 . 48 Acres 20 I:cre=
TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00
TC . . . . 10 . 00 min 10 . 00 min
ABSTRACTION COEFF : 0 . 20
TcReach -Sheet L: 300 . 00 ns : 0 . 1500 D2yr : 2. 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE : 1 . 76 cfs VOL: 0 . 68 Ac-ft TIME : 480 min
BASIN ID : 10yr-pre NAME : 10-YR PRE DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE. . . . : TYPElA PERV IMP
PRECIPITATION. . . . : 2 . 90 inches AREA. . : 3 . 03 Acres 0 . 65 Acres
TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 8 . 00 9700
_ ,TC. . . . : 17 . 91 min 17 . 91 min
ABSTRACTION COEFF : 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1500 p2yr : 2 . 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE : 1 . 16 cfs VOL: 0 . 52 Ac-ft TIME : 480 min
4/16/96 5 : 53 :34 pm page 2
____________-_______
BASIN SUMMARY
BASIN ID: , 2Vr-dev NAME : 2-YR DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE. .. . : TYPEIA PERV IMP
PRECIPITATION. . . . : 2 . 00 inches AREA. . : 1 . 48 Acres 2 . 20 Acres
TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 9S"U-
TC. . . . : 10 . 00 min 10 . 00 min
ABSTRACTION COEFF: 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1500 p2yr: 2 . 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE: 1 . 09 cfs VOL: 0 . 43 Ac-ft TIME : 480 min
BASIN ID: 2yr-pre NAME: 2-YR PRE DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 3 . 68 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE. . . . : TYPElA PERV IMP
PRECIPITATION.. . . : 2 . 00 inches AREA. . : 3 . 03 Acres 0 . 65 Acres
TIME INTERVAL. . . .: 10 . 00 min' CN. . . . : 85 . 00 98 . 00
TC. . . . : 17 . 91 min 17 . 91 min
ABSTRACTION COEFF: 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1500 p2yr: 2 . 00 s : 0 . 0660
TcReach - `Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE: 0 . 61 cfs VOL: 0 . 30 Ac-ft TIME: 480 min
i
4/18/96 4 : 15 :36 pm page 1
------------------------
STAGE STORAGE TABLE
CUSTOM STORAGE ID No. D-POND1
Description: DETENTION POND 1
STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE---->
(ft) ---cf--- --Ac-Ft- (ft) - -cf--- --AC-Ft- (ft) ---cf--- --AC-Ft- ' (ft) ---cf--- --Ac-F�_
................................................................................................7-3-97..
147.00 0.0000 0.0000 147.60 2004 � 0.0460 148.20 4008 0.0920 148.80 6012 0.1380
147.10 334.00 0.0077 147.70 2338 0.0537 148.30 4342 0.0997 148.90 6346 0.1457
147.20 668.00 0.0153 147.80 2672 0.0613 148.40 4676 0.1073 149.00 'R�6680 0.1534
147.30 1002 0.0230 147.90 3006 0.0690 148.50 5010 0.1150
147,40 1336 0.0307 148.00 3340 0.0767 148.60 5344 0.1227
147.50 1670 0.0383 148.10 3674 0.0843 -148.70 5678 0.1303
# 6 6 8U C.F. 570na6a /D,ZovioED
y056 C.F 57-o2AcE IZSQ�,RLD
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X'n) Co. SufirJ
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4/18/96 4 : 15 : 36 pm _ page 2
------------------------------------------
DISCHARGE STRUCTURE LIST
COMBINATION DISCHARGE ID No . NEW-COMB
Description: NEW COMBINATION ORIFIC/RISER
Structure: NEW-ORIF Structure : .
Structure: NEW-RISR Structure :
Structure:
MULTIPLE ORIFICE ID No. NEW-ORIF
Description: NEW MULTIPLE ORIFICE
Outlet Elev: 147 . 00
Elev: 147 . 00 ft Orifice Diameter : 5 . 5840 in.
Elev: 147 . 60 ft Orifice 2 Diameter : 5 . 1328 in.
Elev: 148 . 00 ft Orifice 3 Diameter : 5 . 2500 in'.
RISER DISCHARGE ID No . NEW-RISR
Description: NEW RISER
Riser Diameter' (in) : 12 . 00 elev: 150 . 00 ft
Weir Coefficient . . . : 9 . 739 height : 150 . 50 ft
Orif Coefficient . . . : 3 . 782 increm : 0 . 10 ft
4/18/96 4 : 15 : 37 pm page 3
--------------------------
LEVEL POOL TABLE SUMMARY
MATCH INFLOW -STO- -DIS- <•PEAK, STORAGE Pr-A
<--------DESCRIPTION---------> (cfs) (cfs) --id- --id- <•STAGE> id VOL (cf)
cF5
2 YEAR STORM .0 61 1.09 D-POND1 NEW-COMB 147.51 1 1715.01 cf 0. 41
10 YEAR STORM ................ 1.16 1.76 D-POND1 NEW-COMB 147.87 2 2909.53 cf 1. 14
100 YEAR STORM ............... 1.83 2.52 D-POND1 NEW-COMB 148.21 3 4045.09 cf /, s3
I
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SEGA Engineers PROJECT
If Structural & Civil Consulting Engineers �1K� TIcn�P�E
JOB NO. g�_ RGURED BY r
22928 SE 312th ST • Kent, Washington 98042 ---- �J�/
(360) 886-1017 • Fax (360) 886-1016 CHECKED BY DATE'/ jjjt, SHLLT OF
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Copyright 096, SEGA Ftwmccrx
4/17/96 10 : 9 : 16 am page 1
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BASIN SUMMARY
BASIN ID: 2yr-wq NAME: 2-YR DEVELOPED SITE
SBUH METHODOLOGY
TOTAL AREA. . . . . . . : 1 . 55 Acres BASEFLOWS : 0 . 00 cfs
RAINFALL TYPE. . . . : TYPEIA PERV IMP
PRECIPITATION. . . . : 2 . 00 inches i .;AREA. . : 0 . 00 Acres 1 . 55 Acres
TIME INTERVAL. . . . '. 10 . 00 min , CN. . . . : 86 . 00 98 . 00
TC. . . . . 10 . 00 min 10 . 00 min
ABSTRACTION COEFF: 0 . 20
TcReach - Sheet L: 300 . 00 ns : 0 . 1500 p2yr: 2 . 00 s : 0 . 0660
TcReach - Shallow L: 280 . 00 ks : 11 . 00 s : 0 . 0660
PEAK RATE: 0 . 61 cfs VOL: 0 . 23 Ac-ft TIME: 480 min
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.213 SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS
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 I 66 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 older second growth 42 64 76 81
Wood or forest land: young second growth or brush 55 72 81 86
Orchard: with cover crop 81 88 _ 92 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area 68 8o 86 90
fair condition: grass cover on 50%
to 75% of the area 77 85 90 92
Gravel roads and parking lots I 46 85 89 91
Dirt roads and parking lots 72 82 87 89
Impervious surfaces, pavement, roofs, etc. 98 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 too
Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3)
1.0 DU/GA 15 i 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.
3.5.2-3 11192
{
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2B SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS
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 Iand(1j: 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 older second growth 42 54 76 81
Wood or forest land: young second growth or brush 55 72 81 86
Orchard: with cover crop 81 88 92 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area 68 so as 90
fair condition: grass cover on 50% 77 85 90 92
to 75% of the area
Gravel roads and parking lots ( 76 85 89 91
Dirt roads and parking lots 72 82 87 89
Impervious surfaces, pavement, roofs, etc. I 98 98 98 98
Open water bodies: lakes, wetlands, ponds; etc. 100 100 100 100
Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3) Separate curve number
1.0 DU/GA 15
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 o4
7A 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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