27694 DRAINAGE TECHNICAL REPORT
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DRAINAGE TECHNICAL REPORT
for the
TURTLE CREEK
SECOND ADDITION PLAT
OCTOBER 1998 QpR~VE4 ~
SPO E COUNTY EI~tG.
W.O. No. 98389 ^ BY , ~
O • ~~J. 7$
Prepared for : ^IS 7•R u CT
DAHM DEVELOPMENT
12810 East Nora Avenue PERMIT REQUIRE.D !
Spokane, WA 99216 NOTIFY THE PERMIT ENGINEER
a56-3soo
24 HRS. PRIOR TO CONSTRUCt10N.
Prepared by:
Inland Pacific Engineering Company
707 West 7`h Avenue Suite 200
Spokane, WA 99204
(509) 458-6840
DRAINAGE REPORT
for the
Turtle Creek Second Addition Plat
Spokane County, Washington
OCTOBER 1998
e'~'e ts s o 't~h's se olans a~n ca uiationcorm to
a ic i~ 'a o t - S o •a - o n S~tan ar s ~or ' oad an e r ct'oa d
a 'pos e oum Gdftg@es & to m" r aa gem" u d e* p do~~k~ D ave
be ao ed y t~ Spo •ane ou~nty :gie-r. a~ p o~ t~ es -,a s~$Icula~tio s} or
cns ~ u~ n.
This report has been prepared by Ann L. Winkler and Susan K. Murphy of Inland Pacific
Engineering Company under the direction of the undersigned professional engineer whose seal and
signature appear hereon.
A. SC
Q ~oF wASy,
229, 6 Pjj4
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F~/sTEa~°
SS~ONAL ENG
EXPIRES: V7.tljq
Timothy A. Schwab, P.E.
: . Turtle Creek Second Addition
- Drainage Narrative
GENERAL
Turtle Creek Second Addition is a 9.73 acre development of 28 single family residential lots
located west and south of Barker Road and EigHth Avenue in the Spokane Va11ey, within
Spokane County, Washington. The site is currently vacant with alfalfa as the predominant land
cover and vegetation. Single family residential development lies to the north, east and west. A
vicinity and location map have been included within the technical section of this document.
PURPOSE
The purpose of this report is to determine the extent of storm drainage facilities which will be
required to dispose of the increase in stormwater runoff created by the development of the Turtle
Creek Phase 2 plat. The storm drainage facilities on this project will be designed to dispose of
runoff from a ten year design storm, as required by the Spokane County Guidelines for
Stormwater Management for onsite basins and the 50 yeaz storm event for basins which include
' offiste areas. Calculations are included to check the adequacy of all storm drainage facilities for
the 50-year storm event as a measure of safety. This development is within the Aquifer Sensitive
. Area of Spokane County and is subject to '208' requirements. For this project, the 10- and 50-
year values from the Spokane, Medical Lake, Reardon, Cheney and Rockford intensity curves
were used, as well as the SCS iso-pluvial rainfall curves.
ANALYSIS METHODOLOGY
Per the Spokane County Guidelines for Stormwater Management, the Rational Method, which is
recommended for basins less than ten acres in size, will be used to determine the peak d.ischarges
and runoff volumes for all basins.
PROJECT DESCRIPTION
The Turtle Creek Second Addition plat comprises 281ots of the approved 101 lots in the
approved preliminary plat. Twenty-one lots were platted previously in the Turtle Creek First
Addition, for a total of 49 developed lots within the approved preliminary plat. The Turtle Creek
subdivision is located in the SE 1/4 of Section 19, T. 25 N., R. 45 E., W.M. within Spokane
County, Washington.
TOPOGRAPHY
The existing site naturally drains generally to the southwest, to Saltese Creek. The existing ditch
line will allow the natural drainage route to remain in place from that point west. The general
Inland Pacific Engineering Company D 1 Turtle Creek Second Addition Drainage Repori
slopes within the Turtle Creek Second plat would be characterized as rolling, from 1 to 13%. For
development pu.rposes, the natural slope was used for design and is maintained as much as
.
possible throughout the project.
SOILS
As can be seen from the accompanying soils map from the Spokane County Soils Survey as
performed by the SCS, the site consists of three types of soils, two within the Class B type and
one within the Class A type. These soil types are described as follows.
GgA - Garrison gravelty loam, 0 to 5 percent slopes: Soils within this soil type are some what
excessively drained soils formed in gravelly glacial outwash material from a variety of igneous
rock. Surface runoff is slow, and the hazard of erosion is slight. Spokane County Guidelines for
Stormwater Manaizement indicate this to be a Soil Group Type B and pre-approved for drywell
installation.
Hydrologic Soil Classication - B
GmB - Garrison very gravelly loam, 0 to 8 percent slopes: Soils within this soil type are
some what excessively drained soils formed in gravelly glacial outwash material from a variety
of igneous rock. Surface runoff is slow, and the hazard of erosion is slight. GmB soils have a
thinner, more gravelly surface layer than GgA soils. Spokane County Guidelines for Stormwater
• Manaeement indicate this to be a Soil Group Type B and pre-approved for drywell installation.
Hydrologic Soil Classification - B
MaC - Marble loamy sand, 0 to 30 percent slopes: Soils within this soil type are very deep,
excessively drained soils formed from sandy glaciofluvial material on dunelike, nearly level to
steep terraces and terrace breaks. Surface runoff is slow, and the hazard of erosion is slight.
Spokane County Guidelines for Stormwater Mana2ement indicate this to be a Soil GrQup Type A
and pre-approved for drywell installation.
Hydrologic Soil Classification - A
DRAINAGE NARRATIVE
Offsite
Per Spokane County Stormwater Guidelines, a 50 year storm analysis has been done for Basins
A, B, C, E, F and G as required for basins with include offsite areas.
Inlaqd Pacific Engineering Company DZ Turtle Creek Second Addition Drainage Report
. " Onsite
.
As this plat is within the aquifer sensitive area of Spokane County, all onsite and future drainage
will be collected and treated using the "208" runoff inethod as described in the Spokane County
Guidelines for Stormwater Management. To facilitate this analysis, Turtle Creek Second
Addition was divided into 6 permanent sub-basins (A through F). Table 1 lists the basins and
pond for easy reference. . ,
TABLE 1
. ~ .
.
~ a's o al Area ~ well~in,g ~ (GA ' 'eigffld
A 873,44612.01 6 2.98 0.25
B 865396\1.98 5.5 2.78 0.25
_ C 383,232\0.88 3 3.5 0.35
D 251,769\0.59 2 3.4 0.30
' E 166,013\3.81 17.5 * 4.59 0.40
F 284,027\6.52 19.5 2.99 0.25
G 81552\0.20 .5 2.50 0.25
* Includes 8 proposed units and 9.5 future units
Includes 5 proposed units and 14.5 future units
"208" Calculations
Within each basin the streets are divided into sub-basins by high and low roadway profiles.
These facilities capture upstream stormwater runoff from the impervious street surface and direct
the runoff into grassy '208' ponds. As shown on the '208' calculation worksheets included, the
'208' storage volume provided is adequate to perform '208' treatment for the runoff created by the
first half inch of rainfall.
DRAINAGE CALCULATIONS SUMMARY
Calculations have been included for Drainage Basin considerations such as Peak Flow, Time of
Concentration, "208" sizing, drywell requirements, inlet sizing and curb inlets, as well as culverts
and pipe flows, some of which are summarized below with the remaining calculations included
' lnland Pacific Engineering Company D3 Turtle Creek Second Addition Drainage Report
. ' in the appendix.
' Within the calculation section and where it has been determined necessary for clarity, several
sketches, as well as details from the plans have been incorporated to demonstrate assumptions
and conclusions.
BASIN DESCRIPTIONS
Basins A through G
Within this plat, and due to the general geographic features, a basic assumption was followed and
can be used to describe each basin; namely that ponds will be of the lot pond, or tract
configuration. Table No. 2- Pond and 208 Basin Summary
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a~ ~n ee ~08OS ea '0$ Area ~ ~e i s~
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a
A A 71223 301 602 608 1-
B B 111,714 488 976 1,012 1-"B"
C C 6,116 255 510 515 1-"B"
D D 69151 256 512 544 1-"A"
E E 28,627 13193 2,386 2,500 2-"B"
F F 47,781 1,991 3,982 4,240 2-"B"
G G 23436 102 204 216 1-"B"2
1- Type A is a Spokane Standard Drywell - Single Depth, Capacity 0.3 cfs
1- Type B is a Spokane Standard Drywell - Double Depth, Capacity 1.0 cfs
2- Installed as a part of Turtle Creek Phase 1
" Inland Pacific Engineering Company D4 Turtle Creek Second Addition Drainage Report
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- - - - - - - _ 1_-~'i'- - _ - - - _
50-year STORM BOWSTRING METHOD , PROJECT: Turtle Creek 2nd . ,
PEAK FLOW CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: A `
BASIN A DESIGNER: ALW ' Tot. Area 2.01 Acres DATE: 10114
Imp. Area 7223 SF '
C = 0.25
Time Increment (min) 5
CASE 1 Time of Conc. (min) 5.00
Outflow (cfs) 1
50 ft. Overland Flow Design Year Flow 50
Area (acres) 2.0075
Ct = 0,15 Impervious Area (sq ft) 7223
L= 50 ft. 'C' Factor 0.25
n= 0.40 '208' Volume Provided 304
S= 0,020 Area' C 0.502
Tc = 2.93 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.ln Vol.Out Storage
160 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1= 50.0 5.00 300 4.58 2.30 924 300 624
Z2 = 3,5
n= 0.016 5 300 4.58 2.30 924 300 624
S= 0.011 10 600 3.20 1,61 1127 600 527
15 900 2.45 1,23 1232 900 332
d= 0.202 ft. 20 1200 1.95 0,98 1274 1200 74
25 1500 1.62 0.81 1302 1500 -198
A R Q Tc Tc total I Qc 30 1800 1.56 0.78 1489 1800 -311
35 2100 1.28 0.64 1415 2100 -685
1.09 0.10 2.30 1.26 5.00 4.58 2.30 40 2400 1.19 0.59 1488 2400 -912
45 2700 1.08 0,54 1519 2700 -1181
Qpeak for Case 1= 2.30 cfs 50 3000 0.99 0.50 1541 3000 -1459
55 3300 0.92 0.46 1571 3300 -1729
60 3600 0.87 0.44 1616 3600 -1984
CASE 2 65 3900 0.82 0.41 1647 3900 -2253
70 4200 0,80 0.40 1727 4200 -2473
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 0.39 1778 4500 -2722
peak flow =.90(4.58)(Imp. Area) = 0.68 cfs 80 4800 0.75 0.38 1845 4800 -2955
85 5100 0.72 0.36 1880 5100 -3220
90 5400 0.71 0.36 1961 5400 -3439
So, the Peak flow for the Basin is the greater of the lwo flows, 95 5700 0.69 0.35 2009 5700 -3691
2.30 cfs 100 6000 0.67 0.34 2052 6000 -3948
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Volume
= Impervious Area x.5 in I 12 inlft 301 cu ft
50 year Volume Provided 304 cu ft
S'CO ~M lf UUL l~l-~l.l..~
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM
Maximum Storage Required by Bowstring 624 cu ft/
N (~08( q~ ~5(1,215~ ~
Number and Type of Drywells Required 0 Single (a ca2 CF,
1 Double
(O(P2-7 6z4
INLAND PACIFIC ENGINEERING '~j'• ~ ` Cq (•S z
_ 1 -7 1 C <G
50-year STORM BOWSTRING METHOD - PROJECT: Turtle Creek 2nd ~
PEAK FLOW CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: B . `
BASIN B DESIGNER: ALW ` Tot. Area 1.98 Acres DATE: 10l14
Imp. Area 11714 SF `
C = 0.25
Time Increment (min) 5
CASE 1 Time of Conc. (min) 5.00
Outflow (cfs) 1
50 ft. Overland Flow Design Year Flow 50
Area (acres) 1.9834
Ct = 0.15 Impervious Area (sq ft) 11714
L= 50 ft. 'C' Factor 025
n= 0.40 '208' Volume Provided 506
S= 0.020 Area' C 0.496
Tc = 2.93 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.ln Vol,Out Storage
350 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1= 50.0 5.00 300 4.58 2.27 913 300 613
Z2 = 3.5
n= 0.016 5 300 4.58 2.27 913 300 613
S= 0.025 10 600 3.20 1.59 1114 600 514
15 900 2,45 1.21 1217 900 317
d= 0.172 ft. 20 1200 1.95 0.97 1259 1200 59
25 1500 1.62 0.80 1287 1500 -213
A R Q Tc Tc total I Qc 30 1800 1.56 0.77 1471 1800 -329
35 2100 1.28 0.63 1398 2100' -702
0.79 0.09 2.27 2.04 5.00 4.58 2.27 40 2400 1.19 0.59 1470 2400 -930
45 2700 1.08 0.54 1501 2700 -1199
Qpeak for Case 1= 2.27 cfs 50 3000 0.99 0.49 1523 3000 -1477
55 3300 0.92 0.46 1552 3300 -1148
60 3600 0.87 0.43 1597 3600 -2003
CASE 2 65 3900 0.82 0.41 1627 3900 -2273
70 4200 0.80 0.40 1707 4200 -2493
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 0.38 1757 4500 -2743
peak flow =,90(4.58)(Imp. Area) = 1.11 cfs 80 4800 0.75 0.37 1823 4800 -2977
85 5100 0.72 0.36 1857 5100 -3243
90 5400 0.71 0.35 1931 5400 -3463
So, the Peak flow for the Basin is the greater of the iwo flows, 95 5700 0.69 0.34 1985 5700 -3715
2.27 cfs 100 6000 0.67 0.33 2027 6000 -3973
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Valume
= Imperoious Area x,5 in ! 12 in/ft 488 cu ft
50 year Volume Provided 506 cu ft~~M
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM
Maximum Storage Required by Bowstring 613 cu ft~
10~2(,q~+ I2Z(I,ZIS~=
Number and Type of Drywells Required 0 Single 10-59 CF
1 Double
los~~ l013.'.ofZ..
INLAND PACIFIC ENGINEERING
50-year STORM BOWSTRING METHOD • PROJECT: Turtle Creek 2nd ~ PEAK FLOW CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: C
BASIN C DESIGNER: ALW ~ Tot. Area 0.88 Acres DATE: 10114
Imp. Area 6116 SF C = 0.35
Time Increment (min) 5
CASE 1 Time of Conc. (min) 5.00
OutAow (cfs) 0,3
50 ft. Overland Flow Design Year Flow 50
Area (acres) 0.8777
Ct = 0.15 Impervious Area (sq ft) 6116
L= 50 ft. 'C' Factor 0.35
n= 0.40 '208' Volume Provided 257
S= 0.020 Area * C 0.307
Tc = 2.93 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.ln Vol.Out Storage
180 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1= 50.0 5.00 300 4.58 1.41 566 90 476
Z2 = 3.5
n= 0.016 5 300 4.58 1.41 566 90 476
S= 0.015 10 600 3.20 0.98 690 180 510
15 900 2.45 0.75 754 270 484
d= 0.159 ft. 20 1200 1.95 0.60 780 360 420
25 1500 1.62 0.50 797 450 347
A R Q Tc Tc total I Qc 30 1800 1.56 0.48 911 540 371
35 2100 1.28 0.39 866 630• 236
0,67 0.08 1.41 1,43 5.00 4.58 1,41 40 2400 1.19 0,36 911 720 191
45 2700 1.08 0,33 930 810 120
Qpeak for Case 1= 1.41 cfs 50 3000 0.99 0,30 943 900 43
55 3300 0.92 0.28 961 990 -29
60 3600 0.87 027 989 1080 -91
CASE 2 65 3900 0.82 0.25 1008 1170 -162
70 4200 0.80 0.25 1057 1260 -203
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 0.24 1089 1350 -261
peak flow =.90(4.58)(Imp. Area) = 0.58 cfs 80 4800 0.75 0.23 1129 1440 -311
85 5100 0.72 022 1151 1530 -379
90 5400 0.71 022 1200 1620 -420
So, the Peak flow for the Basin is the greater of the iwo flows, 95 5700 0.69 0.21 1230 1710 -480
1.41 cfs 100 6000 0.67 0.21 1256 1800 -544
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Valume
= Impervious Area x.5 in 112 in/ft 255 cu ft
50 year Volume Provided 257 cu ft'~;Ttq~~ U01- u- ry\;;~7
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM -gfZ~3V IOC-~) =
Maximum Storage Required by Bowstring 510 cu ft
SI~C.ei) + 850
Number and Type of Drywells Required 1 Single S~~ LF
0 Double
5c~c~ ) 5~~ a
INLAND PACIFIC ENGINEERING
BOWSTRfNG METHOD PROJECT; Turtle Creek 2nd "PEAK FLOW CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: D
BASIN: D 10-Year Design Starm DESIGNER: ALW '
Tot, Area 0.59 Acres DATE: 10113 Imp. Area 6151 SF
C = 0.30
Time Increment (min) 5
CASE 1 Time of Conc. (min) 5.00
Outflow (cfs) 1
60 ft. Overland Ftow Design Year Flow 10
Area (acres) 0.5916
Ct = 0.15 ImpeNious Area (sq ft) 6151
L= 60 ft. 'C' Factor 0.3
n= 0,40 '208' Volume Provided 272
S= 0.020 Area * C 0.177
Tc = 3,27 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.in Vol.Out Storage
150 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1 = 50,0 For Z2 5.00 300 3.18 0.56 227 300 -73
Z2 = 3.5 Type B =1.0
n= 0.016 Rolled = 3.5 5 300 3.18 0.56 227 300 -73
S= 0.011 10 600 2.24 0.40 219 600 -321
15 900 1,77 0.31 315 900 -585
d= 0.119 ft. 20 1200 1.45 0.26 335 1200 -865
25 1500 1,21 0.21 344 1500 -1156
A R Q Tc Tc total I Qc 30 1800 1.04 - 0.18 351 .1800 -1449 .
- - - 35 2100 0.91 0.16 356 2100 -1744
0.38 0.06 0.56 1.68 5.00 3,18 0.56 40 2400 0.82 0.15 364 2400 -2036
45 2700 0.74 0.13 368 2700 -2332
Qpeak for Case 1= 0.56 cfs 50 3000 0.68 0.12 374 3000 -2626
55 3300 0.64 0.11 386 3300 -2914
60 3600 0,61 0.11 401 3600 -3199
CASE 2 65 3900 0.60 0.11 426 3900 -3474
70 4200 0.58 0.10 443 4200 -3757
Case 2 assumes a Time of Concentration less than 5 minutes so that the • 75 4500 0.56 0.10 457 4500 4043
peak flow =.90(3.18)(Imp. Area) = 0.40 cfs 80 4800 0.53 0.09 461 4800 4339
85 5100 0.52 0.09 480 5100 -0620
90 5400 0.50 0.09 488 5400 -0912
So, the Peak flow for the Basin is the greater of the two flows, 95 5700 0.49 0.09 505 5700 -5195
0.56 cfs 100 6000 0.48 0.09 520 6000 -5480
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Volume
= Impervious Area x.5 in ! 12 inlft 256 cu ft
208' Storage Volume Provided 272 cu ft
DRYWELL REQUIREMENTS -10 YEAR DESIGN STORM
Maximum Storage Required by Bowstring -73 cu ft
Number and Type of Drywells Required 0 Single
1 Double
50-year STORM BOWSTRING METHOD PROJECT: Turtle Creek 2nd
PEAK FLOW CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: D 4~
BASIN D DESIGNER: ALW
Tot. Area 0.59 Acres DATE: 10114 ,
Imp. Area 6151 SF
C = 0.30
Time Increment (min) 5
CASE 1 Time of Conc. (min) 5.00
Outflow (cfs) 0.3
60 ft. Overland Flow Design Year Flow 50
Area (acres) 0.5916
Ct = 0.15 Impervious Area (sq ft) 6151
L= 60 ft. 'C' Factor 0.3
n= 0.40 '208' Volume Provided 272
S= 0.020 Area' C 0.177
Tc = 3.27 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.ln Vol.Out Storage
150 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1 = 50.0 5.00 300 4.58 0.81 327 90 237
Z2 = 3.5
n= 0.016 5 300 4.58 0.81 327 90 237
S= 0.011 10 600 3.20 0.57 399 180 219
15 900 2.45 0,43 436 270 166
d= 0.137 ft. 20 1200 1.95 0.35 451 360 91
25 1500 1.62 0.29 461 450 11
A R Q Tc Tc total I Qc 30 1600 1.56 0.28 527 540 -13
35 2100 1.28 023 500 630' -130
0,50 0.07 0.81 1.54 5.00 4.58 0.81 40 2400 1.19 0,21 526 720 -194
45 2700 1.08 0,19 537 810 -273
Qpeak for Case 1= 0,81 cfs 50 3000 0.99 0.18 545 900 -355
55 3300 0.92 0.16 555 990 -435
60 3600 0.87 0.15 572 1080 -508
CASE 2 65 3900 0.82 0.15 582 1170 -588
70 4200 0.80 0.14 611 1260 -649
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 0.14 629 1350 -721
peak flow =.90(4.58)(Imp. Area) = 0.58 cfs 80 4800 0.75 0.13 653 1440 -787
85 5100 0.72 0.13 665 1530 -865
90 5400 0,71 0.13 693 1620 -927
So, the Peak flow for the Basin is the greater of the iwo flows, 95 5700 0.69 0.12 711 1710 -999
0.81 cfs 100 6000 0.67 0.12 726 1800 -1074
208' DRAINAGE POND CALCULATIONS'
Required'208' Storage Volume
= Imperoious Area x.5 in I 12 in/ft 256 cu ft
50 year Volume Provided 272 cu ft
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM
Maximum Storage Required by Bowstring 237 cu ft
Number and Type of Drywells Required 1 Single
0 Double
INLAND PACIFIC ENGINEERING
50-year STORM BOWSTRING METHOD PROJECT: Turtle Creek 2nd
PEAK FLOUV CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: E f:
BASIN E DESIGNER: ALW
Tot. Area 3.81 Acres DATE: 10l13 ~
Imp. Area 29731 SF
C = 0.40
Time Increment (min) 5
CASE 1 Time of Conc. (min) 13.52
Outflow (cfs) 2
50 ft. Overland Flow Design Year Flow 50
Area (acres) 3.81 .
Ct = 0.15 Impervious Area (sq ft) 29731
L= 50 ft. 'C Factor 0.4
n= 0.40 '208' Volume Provided 1250
S= 0.020 Area' C 1.524
Tc = 2.93 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens. Q Devel. Vol.ln Vol.Out Storage
1150 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1= 50.0 13.52 811 2.65 4.04 4390 1622 2768
Z2 = 3.5
n= 0.016 5 300 4,58 6.98 2806 600 2206
S= 0.005 10 600 3.20 4,88 3921 1200 2721
15 900 2.45 3.73 4390 1800 2590
d= 0.289 ft. 20 1200 1.95 2.97 4386 2400 1986
25 1500 1.62 2.47 4384 3000 1384
A R Q Tc Tc total I Qc 30 1800 1.56 2.38 4935 3600 1335
35 2100 1.28 1.95 4634 4200 . 434
2.23 0.14 4.04 10.59 13.52 2.65 4.04 40 2400 1.19 1,81 4832 4800 32
45 2700 1.08 1.65 4898 5400 -502
Qpeak for Case 1= 4.04 cfs 50 3000 0.99 1,51 4942 6000 -1058
55 3300 0.92 1.40 5013 6600 -1587
60 3600 0.87 1.33 5139 7200 -2061
CASE 2 65 3900 0.82 1.25 5218 7800 -2582
70 4200 0.80 1.22 5457 8400 -2943
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 1.17 5604 9000 -3396
peak flow =.90(4.58)(Imp. Area) = 2.81 cfs 80 4800 0.75 1.14 5802 9600 -3798
85 5100 0.72 1.10 5899 10200 -4301
90 5400 0.71 1,08 6141 10800 -4659
So, the Peak flow for the Basin is the greater of the two flows, 95 5700 0.69 1,05 6284 11400 -5116
4.04 cfs 100 6000 0.67 1.02 6408 12000 -5592
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Volume
= Impervious Area x.5 in I 12 inlft 1239 cu ft
50 year Volume Provided 1250 cu ft
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM S-MIZM UoL(,l,Yl'lG'
Maximum Storage Required by Bowstring 2768 cu ft -prz-tVI DED'.
Number and Type of Drywells Required 0 Single
2 Double
t° f~3 $D CF
/ ~Z~c~~ ~3so ~ 2~~a
INLAND PACIFIC ENGINEERING 2 , ~
50-year STORM BOWSTRING METHOD• PROJECT: Turtle Creek 2nd
PEAK FL01N CALCULATION PROJECT Turtle Creek 2nd DETENTION BASIN DESIGN BASIN: F
BASIN F DESIGNER; ALW
Tot. Area 6.61 Acres DATE: 10114 ~
Imp. Area 47781 SF
C = 0.25
Time Increment (min) 5
CASE 1 Time of Conc. (min) 10.65
Outflow (cfs) 2
50 ft. Overland Flow Design Year Flow 50
Area (acres) 6.6098
Ct = 0.15 Impervious Area (sq ft) 47781
L= 50 ft. 'C' Factor 0.25
n= 0.40 '208' Volume Provided 2120
S= 0.020 Area' C 1.652
Tc = 2.93 min., by Equation 3-2 of Guidelines
Time Time Inc. Intens, Q Devel, Vol.ln Vol.Out Storage
1310 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu ft) (cu ft) (cu ft)
Z1= 50.0 10.65 639 3.10 5.12 4385 1279 3106
Z2 = 3.5
n= 0.016 5 340 4.58 7.57 3042 600 2442
S= 0.014 10 600 3.20 5.29 4251 1200 3051
15 900 2.45 4.05 4524 1800 2724
d= 0.260 ft. 20 1200 1.95 3.22 4567 2400 2167
25 1500 1,62 2.68 4597 3000 1597
A R Q Tc Tc total I Qc 30 1800 1,56 2.58 5200 3600 1600
35 2100 1.28 2.12 4902 420Cr 702
1.81 0.13 5.12 7.73 10.65 3,10 5.12, 40 2400 1.19 1.96 5125 4800 325
45 2700 1.08 1.78 5206 5400 -194
Glpeak for Case 1= 5.12 cfs 50 3000 0.99 1.64 5263 6000 -737
55 3300 0.92 1.52 5347 6600 -1253
60 3600 0.87 1.44 5488 7200 -1712
CASE 2 65 3900 0.82 1,36 5579 7800 -2221
70 4200 0.80 1.32 5640 8400 -2560
Case 2 assumes a Time of Concentration less than 5 minutes so that the 75 4500 0.77 1,27 6002 9000 -2998
peak flow =.90(4.58)(Imp. Area) = 4.52 cfs 80 4800 0.75 1.24 6218 9600 -3382
85 5100 0.72 1.19 6326 10200 -3674
90 5400 0.71 1.17 6591 10600 -4209
So, the Peak flow for the Basin is the greater of the two flows, 95 5700 0.69 1.14 6747 11400 -0653
5.12 cfs 100 6000 0.67 1.11 6883 12000 -5117
208' DRAINAGE POND CALCULATIONS
Required'208' Storage Volume
= Impervious Area x.5 in 112 inlft 1991 cu ft
50 year Volume Provided 2120 cu ft
DRYWELL REQUIREMENTS - 50 YEAR DESIGN STORM
Maximum Storage Required by Bowstnng 3106 cu ft~~'~N1 UO'L.LIMe
Number and Type of Orywells Required 0 Single 0
~ 2bq~1,215~-
2 Double 4240(.9)
Q137 ~F
INLAND PACIFIC ENGINEERING
q13q }31 040.'0~'~-
50-yeat STORM BOWSTRING METHQD. PROJECT; Turile Creek 2nd
P6AK FLD}JV CALCUtATION PROJECT Turtle Creek 2nd DETENTION 6ASIN DESEGh! BASIN: C
BRSIN G DESIGfVER: ALW
Tat. Area 0.20 Rcres DATE: 10l13
Imp. Area 2436 SF '
C = Q.25 Time fncrement (min) 5
CASE 1 Time pf Cpnc. (min) 5,00
Outflow (cfs) 1
Q f#. Overland Flow Design Year f low 50
Area (acres) 0.1963
Ct = D.9 5 ]mpervious Area (sq fi) 2438
L= Q ft. `C' Factor 0.25
n = 0.40 '208' Volume Provided 108
S= 0.020 Area ~ C O.049
Tc = 0.00 min., by Equatian 3-2 af Guidelines
Time rme Inc. Inlens. Q Devel. Vvl,ln VaI.OWt Storage
150 ft. Gutter flow (min) (sec) (inlhr) (cfs) (cu tt) (cu ft) (cu ft)
ZI = 50.0 ========5.00 = ===300 4.58 0.22 90 =====300 -21 D
Z2 = 3.5
n= 0.016 5 300 4.58 022 90 300 -21 0
S= 0,030 10 600 3.20 0,16 190 600 -A90
15 soa 2.45 0.12 120 soa -780
d= 0.069 ft. za 1200 1.95 0.10 125 1200 -1075
25 1504 1.62 0.08 127 1500 -1373
A R Q Tc Tctotal I 30 1800 1,56 0,08 146 1800 -1654
= ~~w 35 2100 1.28 0.06 138 2100 -1962
0.13 &03 012 1 A7 5.00 4.58 6.22 40 2404 1.19 0.06 146 2400 -2254
45 2700 1.08 0.05 149 2700 -2551
Qpeak for Case 1= 4.22 cfs 50 3000 0.99 0,05 151 3000 -2849
55 3304 0.92 0.05 154 3300 -3146
60 3600 0.87 0.04 158 3800 -3442
CASE 2 65 3900 0.82 0,04 161 3900 -3739
70 4204 4.80 0.44 169 4200 -4031 Case 2 assumes a 1'ime of Concentratian less than 5 minutes so that the 75 4500 0.77 0,04 174 4500 -4326
pealc flow =,90(4.58)(Img. Area) = 0.23 & 80 4800 0.75 0,04 180 4800 -4620
85 5100 ' 0,12 0.04 184 5100 -4916
90 5400 0.71 0.03 1e2 5400 -5208
Sa, the Peak flow for #he Basin is #he greater of the two flows, 95 5700 0.69 0.03 196 51G0 -5504
0.23 cfs 100 6000 0,67 0.03 201 sooa -5799
208'DRAINAGE PONo CALCIJLATIONS
Required "206° Storage Volume
= Imperviaus Area x.5 in ! 12 infft 142 cu ft
54 year Volume Pravided 108 cu ft
DRYVIlELL REQ4lIRENkENTS - 50 YEAR DESIGN STQRM
Maximum Stnrage Required by Bouus#ring -210 cu {i
Nurnber and Type af Qrywells Required 0 Single
1 Double
INLANQ PACIFfC ENGINEERING
1
~ v►
i i
S P O K A N E C O U N T Y
DIVISION OF ENGINEERING AND ROADS • A DIVISION OF THE PUBLIC WORKS DEPARTMENT
William A. Johns, P.E., County Engineer Dennis M. Scott, P.E., Director
September 30, 1998 .
Susan Murphy, E.I.T.
Inland Pacific Engineering
707 W. 7th Avenue, Suite 200
Spokane, WA 99204
Re: Road & Drainage Comments 1 for Turtle Creek 2"d Addition, P-1753A
Susan,
The following items must be addressed prior to approval of the plans:
1. Please submit a typical lot plan for the "no drainage structure" swales.
2. The maintenance responsibilities for Tracts A& B need to be documented. Describe
^ who will be maintaining them, the specific duties, the approximate annual cost of such
maintenance, and submit a copy of the operation and maintenance manual.
3. The temporary cul de sac on 10`h Avenue needs to be surfaced with compacted asphalt
grindings.
4. Street signs need to be shown at the road intersections.
5. It appears from the drainage calculations that the only impervious areas included for
the sizing of the drywells were the roadway surfaces. The front half of the houses and
the driveways need to be included in these calculations.
6. The drywell rim in Pond F is shown 1.55' above the bottom of the swale. Is this what
was intended?
7. The construction bond estimate for this project is $169,100.00, which I have enclosed
a copy of with this letter. This bond needs to be submitted prior to finalization of the
plat. As noted in your transmittal this project falls under the old road standards;
therefore, the County will perform the inspection of the road and drainage
construction.
1026 W. Broadway Ave. • Spokane, WA 99260-0170 •(509) 456-3640 FAX: (509) 324-3478 TDD: (509) 324-3166
Please call if you have any questions.
- Sincerely,
Spokane County Engineers
;
Doug Busko, P.E.
Spokane County Engineers
c: Rich Dahm, Sponsor
P-1753A Project File encl. Road Bond Estimate