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CITY OF RENTOW
FILE NO. 739-005-991 Johl
PREPARED BY
TOUMA ENGINEERS
6632 SOUTH 191ST PLACE, SUITE E-102
KENT, WA. 98032
(425) 251-0665
May 9, 1999
. 7
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1. PROJECT OVERVIEW
The subject property involved in this plat covers an area of approximately 3.81 acres. The
property is situated in the north end of the City. Bounded on the west by Union Avenue
NE (132"d Avenue SE—King County Street designation). It is bounded on north, east and
south by fairly large single parcels. Access to the property is made from Union Avenue
NE via a cul-de-sac that would serve the proposed single lots.
The project area is hilly with slopes ranging from 12% to 28%, except for a small area
with steep slopes exceeding 40%. These areas exist along the northeast portion of the site.
The development is split into two small basins. There exist a knoll about the east third of
the site which directs surface water generated from the most easterly portion of the site to
the east into another shallow swale that eventually meets with the surface water generated
from the westerly portion of the site approximately 800 feet downstream of the site. The
easterly basin covers the areas within the proposed Lots 10 through 14. The westerly
basin covers the remaining lots. The easterly basin will be designed to control flow
release utilizing single family retention facility.
The soil type of the site is Alderwood AgD, glacial till. Existing groundcover consists
mainly of forest.
The proposed plat will have 21lots with lot sizes ranging from 4500 square feet to overt
13,000 square feet. The plat improvements will be designed to City of Renton Standards
and requirements for streets, storm, sewer, water and streetlights.
See the figures on following pages:
Il. RELIMINARY CONDITIONS SUMMARY
CORE REQUIREMENT #l: DISCHARGE AT NATURAL LOCATION
The allowable outflow from the site will be discharged to its natural location. The lowest
elevation for the easterly basin is approximately 373.00; and the lowest elevation for the
westerly basin is approximately 366. Refer to topography map prepared for the site.
CORE REQUIREMENTS #2: OFF-SITE ANALYSIS
Refer to off-site analysis on subsequent pages.
CORE REQUIREMENTS #3: RUNOFF CONTROL
Preliminary calculations are provided on subsequent pages addressing peak runoff. The
sizing for peak rate runoff control facility and its routing calculations are included.
Water quality control will be provided most likely in a form of wet-vaults.
CORE REQUIREMENTS #4: CONVEYANCE SYSTEM
The conveyance system will be designed using the rational method as presented in the
King County 1990 SWM Manual.
CORE REQUIREMENTS #5: EROSION/SEDIMENTATION CONTROL PLAN
The Erosion/Sedimentation control facilities will consist of filter fabric fences to be
placed along the northeast and northwest boundary of the site. Wet vaults will be
considered for water quality control for this plat.
SPECIAL REQUIREMENTS—THOSE APLLICABLE TO THIS PROJECT
1. Critical Drainage Area - The drainage basin is not part of any critical drainage
area.
2. Compliance with existing Master Drainage Plan—N/A
3. Conditions Requiring Master Drainage Plan—N/A
4. Adopted Basin or Community Plans—N/A, However, the site is located in the
May Creek Basin. Refer to attached Figures.
5. Special Water Quality Controls - The proposal will create 0.58 acres of new road.
These special water quality controls will likely be in the form of wet-vaults.
Refer to the last pages of this level 1 analysis for preliminary sizing.
6. Coalescing Plate Oil-Water Separators—N/A
7. Closed Depression—N/A
8. Use of Lakes, Wetlands, or Depressions for Detention — N/A
9. Delineation of 100-year Flood Plain—N/A
10. Flood Protection for Type 1 and 2 Steam — N/A
11. Geotechnical Analysis and Report—Geotechnical study is being prepared in
conjunction with this application.
12. Soil Analysis and Report—Geotechnical report is prepared for this site and
addresses the soil classifications and stability.
13. A Wetland Mitigation Report —N/A.
OFF-SITE ANALYSIS
1. UPSTREAM
The site is the situated within a small drainage basin. The entire basin area is calculated at
approximately 5.7 acres. This includes off-site area of 1.9 acres and 3.8 acres of site area.
The off-site runoff will be intercepted by adequate conveyance system and will be by-
pass the site drainage. The storm detention facilities will be designed to account for off-
site runoff to by-pass the site.
2. DOWNSTREAM
Surface runoff from the site and tributary area is directed to the northwest of the site. The
westerly basin is directed to open ditches along the easterly side of Union Avenue NE.
That portion of the easterly basin in directed to a swale which conveys the surface water
to north and west. The runoff from the two basins join again at approximately the
intersection of 132"d Avenue SE (Union Avenue NE) and SE 95 Way(Also known as
Boss Blvd). The runoff from this point directed toward the existing 18-inch culvert under
SE 95`h Way. Thence it discharged into a defined drainage swale north of SE 95t' Way
and directs the flow into Coal Creek approximately300 to 350 feet.
No downstream erosion has been noted.
d 13Z tii p2Er- pl-4-T mrty „ �999
L; SCS Nll IY_5 DHV
PERFORMANCE : INFLOW TARGET-OUTFLOW ACTUAL-OUTFLOW PK-STAGE STORAGE
DESIGN HYD: 1 . 90 -y"e. . 74 . 62 4. 34 7572
TEST HYD 1 : 1 . 21 2-Y2 . 31 . 25 3 . 32 5780
TEST HYD 2 : 2 . 76 ioo-y0 1 . 34 2 . 16 5 . 23 9120
STRUCTURE DATA: R/D-VAULT
RISER-HEAD VAULT-BOTTOM-AREA STOR-DEPTH STORAGE-VOLUME
5 . 00 FT 1744 . 4 SQ-FT 5 . 00 FT 8722 CU-FT
DOUBLE ORIFICE RESTRICTOR: DIA( INCHES) HT(FEET) Q-MAX(CFS)
BOTTOM ORIFICE : 2 . 26 . 00 . 310
TOP ORIFICE : 3 . 51 3 . 35 .430
ROUTING DATA:
STAGE(FT) DISCHARGE(CFS) STORAGE(CU-FT) PERM-AREA(SQ-FT)
. 00 . 00 . 0 . 0
. 50 . 10 872 . 2 . 0
1 . 00 . 14 1744. 4 . 0
1. 50 . 17 2616 . 6 . 0
2 . 00 . 20 3488. 8 . 0
2 . 50 . 22 4361 . 0 . 0
3 . 00 . 24 5233 . 2 . 0
3 . 35 . 25 5843 . 8 . 0
3 . 50 . 39 6105 . 4 . 0
4 . 00 . 55 6977 . 6 . 0
4. 50 . 65 7849 . 8 . 0
5 . 00 . 74 8722 . 0 . 0
5 . 10 1. 14 8896. 5 . 0
5 . 20 1 . 86 9070 . 9 . 0
5 . 30 2 . 79 9245 . 3 . 0
5 . 40 3 . 88 9419 . 8 .0
5 . 50 4. 99 9594. 2 .0
5 . 60 5 .41 9768 . 7 . 0
5 . 70 5 . 79 9943 . 1 . 0
5 . 80 6. 14 10117 . 5 . 0
AVERAGE VERTICAL PERMEABILITY: . 0 MINUTES/INCH
SPECIFY: F - FILE, N - NEWJOB, P - PRINT IF/OF, R - REVISE, S - STOP
DETENTIOMRETENTION CALCULATIONS (3.81 acres)
Existing site conditions
Time of concentration- s a a s
T, _ .42(NgL) /((P2) (So)' ) _ .42(.4X300) /(2.1) (.16).4T, =27.8 min.
T2 = UK(So)s(60) = 130/3(.16)s(60) = T== 1.8 min.
Tc=29.6 min.
CN ?
CN; 3.81 acres @ cn = 81
Developed conditions
Time of concentration
T, _ .42(N,L)8/((P2)-s(So)a) _ .42(.15X1 30)a/(2.1)s(.024)4 T, = 13.87 min
T2 = UK(So)s(60) = 150/42(.11)"(60) T2=0.18 min
T3 = UK(So)s(60) = 310/42(.129)5(60) T3=0.34 min
Ta = L/K(So),5(60) = 120/42(.08)5(60) T4= 0.17 min
Tc= 14.56 min.
CN ?
21 lots on 3.81 acres DU/GA 21/3.81 = 5.1 DU/GA= 50% impervious
1.91 acres pervious, landscaping cn = 86
1.90 acres impervious, road, and roof cn = 98
P2= 2.1", P,o= 2.95", P100= 3.95"
Computer Input
1EX =3.81,81,0,98,29.6
1DE = 1.91,86,1.90,98,14.6
Orifice restrictor to release 2 year and 10 year at existing condition rate.
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SBUH/SCS METHOD FOR COMPUTING RUNOFF HYDROGRAPH
STORM OPTIONS :
1 - S .C . S. TYPE-1A
2 - 7-DAY DESIGN STORM
3 - STORM DATA FILE
SPECIFY STORM OPTION:
1
S .C . S . TYPE-1A RAINFALL DISTRIBUTION
ENTER: FREQ(YEAR) , DURATION(HOUR) , PRECIP( INCHES)
2 , 24, 2 . 1
----------------------------------------------------------------------
******************** S .C . S . TYPE-1A DISTRIBUTION ********************
********* 2-YEAR 24-HOUR STORM **** 2 . 10" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV) , CN(PERV) , A( IMPERV) , CN( IMPERV) , TC FOR BASIN NO. 1
3 . 81 ,81,0, 98, 29 . 6
DATA PRINT-OUT :
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
3 . 8 3 . 8 81 . 0 . 0 98 . 0 29 . 6
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
. 31 8. 00 9209
ENTER [d: ] [path]filename[ .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
lex2yr
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
C
----------------------------------------------------------------------
ENTER: A(PERV) , CN(PERV) , A( IMPERV) , CN( IMPERV) , TC FOR BASIN NO. 2
1 . 91 , 86, 1 . 90 ,98 , 14 . 6
DATA PRINT-OUT :
AREA(ACRES ) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
3 . 8 1 . 9 86 . 0 1 . 9 98 . 0 14 . 6
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
1 . 21 7 . 83 19301
ENTER [d: ] [pathlfilename[ .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
1de2yr
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT , S - STOP
21
S .C. S . TYPE-lA RAINFALL DISTRIBUTION
ENTER: FREQ(YEAR) , DURATION(HOUR) , PRECIP( INCHES )
10 ,24, 2 . 95
----------------------------------------------------------------------
******************** S .C. S . TYPE-1A DISTRIBUTION ********************
********* 10-YEAR 24-HOUR STORM **** 2 . 95" TOTAL PRECIP. *********
----------------------------------------------------------------------
ENTER: A(PERV) , CN(PERV) , A( IMPERV) , CN( IMPERV) , TC FOR BASIN NO. 1
3 . 81 ,81 , 0 ,98 , 29 . 6
DATA PRINT-OUT :
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES )
A CN A CN
3 . 8 3 . 8 81 . 0 . 0 98 . 0 29 . 6
PEAK-Q(CFS ) T-PEAK(HRS) VOL(CU-FT)
. 74 7 . 83 17542
ENTER [d: ] [path]filename[ .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
lexlOyr
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
c
----------------------------------------------------------------------
ENTER: A(PERV) , CN(PERV) , A( IMPERV) , CN( IMPERV) , TC FOR BASIN NO. 2
1 . 91,86, 1. 90,98 , 14. 6
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
3 . 8 1. 9 86 . 0 1 . 9 98 . 0 14 . 6
PEAK-Q(CFS ) T-PEAK(HRS) VOL(CU-FT)
1 . 90 7 . 83 29926
ENTER [d: ] [path]filename[ . ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
lde10yr
SPECIFY: C - CONTINUE , N - NEWSTORM, P - PRINT, S - STOP
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KING COUNTY, WASIIIN (; TON, SURFACE WATER DES ► GN MANUAL
TABLE 3.5.2111 SCS WESTERN WASIIINGTON RUNOF`CURVE NUMBERS
SC
S 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 42 64 76 81 ,
Wood or forest land: young second growth or brush 55 72 d 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 80 (j�) 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 87 89
Impervious surfaces, pavement, roofs, etc. 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 100 100
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 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 2/89
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2C "n" AND "k" VALUES USED IN TIME CALCULATIONS FOR HYDROGRAPHS
'n;Sheet Flow Equation Manning's Values(For the Initial 300 If of travel) n.'
Smooth surfaces(concrete,asphalt,gravel,or bare hard packed soY) 0.011
Fallow fields or loose soh surface(no residue) 0.05
Cultivated soil with residue cover(s <-0.20 h/h) 0.06
Cultivated soli with residue cover(S>0.20 It/h) 0.17
Short prairie grass and lawns 0.15
0.24
Dense grasses
Bermuda grass 0.41
Range(natural) 0.13
Woods or forest with light underbrush 0.40
Woods or forest with dense underbrush 0.80
*Manning values for sheet flow only,from Overton and Meadows 1976(See TA-55, 1986)
'k'Values Used in Travel Time/Time of Concentration Calculations
Shallow Concentrated Flow (After the Initial 300 ft.of sheet flow.R -0.1) k.
1. Forest with heavy ground liner and meadows(n=0.10) 3
2. Brushy ground with some trees(n-0.060) 5
3. Fallow or minimum ti lage cultivation(n.0.040) 6
4. High grass(n =0.035) 9
5. Short grass,pasture and lawns(n-0.030) 1i
6 Nearly bare ground(n=0.025) 13
7. Paved and gravel areas(n-0.012) 27
Channel Flow(InlermittenQ(At the beginning of visible channels:R=0.2) k.
1. Forested Swale whit heavy ground thief In = 0.10) 5
2. Forested drainage course/ravine with defined channel bed(n=0.050) t0
3. Rock-lined waterway(n=0.035) 15
4. Grassed waterway(n-0.030) 17
5. Earth-lined waterway(n=0.025) 20
6. CMP pipe(n=0.024) 21
7. Concrete pipe(0.012) 42
8. Other waterways and pipes - 0.508/n
Channel Flow(Continuous stream,R =0.4) k.
9. Meandering stream with some pods(n =0.040) 20
10. Rock-lined stream(n=0.035) 23
11. Grass-lined stream(n-0.030) 27
12. Other streams,man-made channels and pipe 0.807/n"
**See Chapter 5,Table 5.3.6C for additional Manningy'n'values for open channels
3.5.2-7 I/90