CC - Drainage Calcs PROJECT NO . 10 = 17 =014
DRAINAGE CALCULATIONS
FOR
Fall Creek No . 4
City of Meridian
Ada County , Idaho
March 20 , 2017
106
OF
CimumB
10U0B ENGINEERS , INC .
250 S . Beechwood Drive , #201
Boise , Idaho 83709
208 =376 - 7330
PROJECT NO. 10-17-014
Fall Creek No.4
SITE OVERVIEW
The Fall Creek Subdivision No.4 is located south of W.Overland Road between S.Stoddard Road and
S. Linder Road in the Fall Creek Subdivision Meridian, Idaho.The proposed development consists of
46 single family residential lots located on approximately 18.2 acres.
EXISTING DRAINAGE
The existing project site presently consists of a vacant agricultural field. The site is bounded by
developed single family residential properties to the south, east and west,and agricultural land to
the north.The ground slopes gently from southeast to northwest at grades generally less than 2%.
DRAINAGE DESIGN CONCEPT
The proposed stormwater improvements include the installation of roadways, pipe networks,
stormwater treatment systems,and storage facilities.The surface flow will be directed to and
contained within the road sections to designed low points where inlets will capture the stormwater
and convey it through a pipe network to a discharge location. A treatment system will be installed
for each pipe network prior to allowing the stormwater to be be discharged to the storage facility.
The site was divided into catchment areas based on the inlet locations. Each catchement area was
analyzed for the 25-year flow rate for conveyance,the water quality event for treatment and the
100-year runoff for storage.
Pre-development discharge has been piped to discharge to the Hardin Drain.The required onsite
storage was adjusted to allow for the pre-development discharge. The full volume of Basin D and
the excess volume of Basin B minus the water quality volume is being discharged as pre-
development.
CALCULATION METHODS
Calcualtions were prepared in accordance with the current edition of the Ada County Highway
District Stormwater Policy Manual using the rational method (Q=CIA).The catchment areas are
shown on the Drainage Catchment Plan. The average rainfall intensity is estimated from the
"Rainfall Intensity-Duration-Frequency Relationship"figure for each storm event noted. The time of
concentration is determined using the TR-55 method and combines the longest overland grass flow
time to gutter flow time. Calculations and spreadsheets are included with this report for all
catchment areas.
SOIL CONDITIONS
Based upon the preliminary soils and subsurface investigation provided in the soils report for this
subdivision,the existing soils will accept the runoff. However, in the event that during construction
the contractor encounters either groundwater or rock,a revised design will be done based upon
field conditions.
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Basin A PROJECT NO. 10- 17-014
Fall Creek No, 4
Contributing Catchment Areas : A
Required Water Quality Volume : 1 ,534 ft3
Required Water Quality Flow Rate : 0.70 cfs
Required Storage Volume : 3 ,767 ft3
Seepage Bed
Volume Calculations Storage Duration
Depth = 8 ft
Length = 118 ft Infiltration Area = 11180 ft
Width = 10 ft Infiltration Rate = 0 . 93 in ./hr
Void Ratio = 40 % Trench Infiltration = 91 ft3/hr
Trench Volume = 3 ,776 ft3 Time for 90% Percolation = 15. 10 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWQ = 0 . 70 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0. 1 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ftz Throat Velocity must be < 0 . 50 ft/sec
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Basin B PROJECT NO. 10-17-014
Fall Creek No, 4
Contributing Catchment Areas : B
Required Water Quality Volume : 21548 ft3
Required Water Quality Flow Rate: 1 . 17 cfs
Pre-Development discharge Volume 3,708 ft3
Required Storage Volume : 2,547 ft3
Pre-development discharge volume to be discharged to the Hardin Drain .
Seepage Bed
Volume Calculations Storage Duration
Depth = 8 ft
Length = 68 ft Infiltration Area = 816 ft
Width = 12 ft Infiltration Rate = 0 . 93 in ./hr
Void Ratio = 40 % Trench Infiltration = 63 ft3/hr
Trench Volume = 21611 ft3 Time for 90% Percolation = 36. 26 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWa = 1 . 17 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0. 17 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ft' Throat Velocity must be < 0 . 50 ft/sec
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Basin C PROJECT NO. 10-17-014
Fali Creek No. 4
Contributing Catchment Areas : C
Required Water Quality Volume : 1 , 154 ft'
Required Water Quality Flow Rate : 0.57 cfs
Required Storage Volume : 2,834 ft'
Seepage Bed
Volume Calculations Storage Duration
Depth = 8 ft
Length = 89 ft Infiltration Area = 890 ft
Width = 10 ft Infiltration Rate = 0 . 93 in ./hr
Void Ratio = 40 % Trench Infiltration = 69 ft3/hr
Trench Volume = 21848 ft3 Time for 90% Percolation = 15.06 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWa = 0 . 57 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0.08 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ftz Throat Velocity must be < 0 . 50 ft/sec
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Basin D PROJECT NO. 10-17-014
Fall Creek No. 4
Contributing Catchment Areas : D
Required Water Quality Volume : 338 ft3
Required Water Quality Flow Rate: 0. 57 cfs
Pre-Development discharge Volume 829 ft3
Required Storage Volume : 0 ft3
Pre-development discharge volume to be discharged to the Hardin Drain .
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWa = 0 . 57 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0.08 ft/sec
Number of Boxes = 1
Throat Area = 6 .67 fe Throat Velocity must be < 0 . 50 ft/sec
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Basin E PROJECT NO. 10- 17-014
Fall Creek No. 4
Contributing Catchment Areas : E
Required Water Quality Volume: 746 ft3
Required Water Quality Flow Rate : 0. 18 cfs
Required Storage Volume : 19832 ft3
Seepage Bed
Volume Calculations Storage Duration
Depth = 8 ft
Length = 48 ft Infiltration Area = 576 ft2
Width = 12 ft Infiltration Rate = 0 . 93 in ./hr
Void Ratio = 40 % Trench Infiltration = 45 ft3/hr
Trench Volume = 19843 ft3 Time for 90% Percolation = 15 .04 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWo = 0 . 18 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0.03 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ft' Throat Velocity must be < 0 . 50 ft/sec
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Basin F PROJECT NO. 10-17-014
Fall Creek No. 4
Contributing Catchment Areas : F
Required Water Quality Volume : 493 ft3
Required Water Quality Flow Rate : 0. 12 cfs
Required Storage Volume : 1 , 210 ft3
Seepage Bed
ThE Volume Calculations Storage Duration
Depth = 8 ft
Length = 30 ft Infiltration Area = 360 ftz
Width = 12 ft Infiltration Rate = 0 . 93 in ./hr
## Void Ratio = 40 % Trench Infiltration = 28 ft3/hr
Trench Volume = 11152 ft3 Time for 90% Percolation = 15.89 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in Qwo = 0 . 12 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0.02 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ft2 Throat Velocity must be < 0 . 50 ft/sec
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Boise Area Intensity- Duration- Frequency
Intensity ( inches per hour)
Design Storm 2 5 10 25 50 100
Tc
0 . 17 10 min 0 . 69 1 . 15 1 . 45 1 . 85 2020 2 . 58
0 . 25 15 min 0 . 59 0 . 97 1 . 22 1 . 56 1986 2 . 18
0 . 33 20 min 0 .49 0 . 81 1 . 01 1 . 30 1954 1 . 81
0 . 42 25 min 0 . 43 0 . 71 0 . 89 1 . 14 1 . 35 1 . 58
0 . 50 30 min 0 . 41 0 . 67 0 . 85 1 . 08 1 . 29 1 . 51
0 . 58 35 min 0 . 34 0 . 56 0 . 70 0 . 90 1907 1 . 25
0 . 67 40 min 0 . 31 0 . 51 0 . 64 0 . 82 0 . 98 1 . 15
0 . 75 45 min 0 . 29 0 . 48 0 . 60 0 . 77 0 . 91 1 . 07
0 . 83 50 min 0 . 27 0 . 45 0 . 56 0 . 72 0 . 85 1 . 00
0 . 92 55 min 0 . 26 0 . 43 0 . 54 0 . 69 0 . 82 0 . 96
1 . 00 1 hour 0 . 26 0 . 43 0 . 54 0 . 69 0 . 82 0 . 96
2 . 00 2 hours 0 . 16 0 . 25 0 . 31 0 . 39 0 . 46 0 . 54
3 . 00 3 hours 0 . 13 0 . 19 0 . 23 0 . 29 0 . 34 0 . 40
6 . 00 6 hours 0 . 09 0 . 12 0 . 14 0 . 18 0 . 21 0 . 25
12 . 00 12 hours 0 . 06 0 . 08 0 . 10 0 . 12 0 . 14 0 . 16
24 . 00 24 hours 0 . 04 0 . 06 0 . 06 0 . 08 0 . 09 0 . 10
NOAA Atlas 2
Intensity Duration Frequency
3 . 5
-- - -- -- 2 year
3 -
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2 . 5 -x- 10 year
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V \ - 1 50 year
1 . 5 * - -
?4 \ � 100 year
C
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I Ads WIN
10 min 15 min 30 min 1 hour 2 hours 3 hours 6 hours 12 hours 24 hours
Duration in minutes and hours
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FALL CREEK SUBDIVISION NO. 4
l— Meridian, Idaho
DRAINAGE CATCHMENT PLAN
a o 1-U-8 ENGINEERS,INC.