CC - Drainage CalculationsDRAINAGE CALCULATIONS
FOR
The Oaksubo ivion
City of Meridian
Ada County, Idaho
3 -Nov -2016
fJmUmB
J -U -B ENGINEERS, INC.
250 S. Beechwood Drive, #201
Boise, Idaho 83709
208-376-7330
PROJECT NO. 10-16-114
The Oaks Subidivions No. 5
SITE OVERVIEW
The Oaks Subdivision No. 5 is located at the southwest corner of W. McMillan Road and N. Rustic
Oak Way in Meridian, Idaho. The proposed development consists of 51 single family residential lots
located on approximately 13.2 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 east, and agricultural land to the north, west,
and south. The ground slopes gently from northeast to southwest 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.
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.
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
Contributing Catchment Areas:
Required Water Quality Volume:
Required Water Quality Flow Rate:
Required Storage Volume:
Seepage Bed
3,019 fte
1.39 cfs
7,413 its
PROJECT NO, 10-16-114
The Oaks Subidivions No. 5
Volume Calculations
Box Volume =
Storage Duration
Depth =
4 ft
46 in
()Wo = 1.39 ft'/sec
Length =
185 ft
Infiltration Area =
740 fe
Width =
26 ft
Infiltration Rate =
0.93 in./hr
Void Ratio =
40 %
Trench Infiltration =
57 ft3/hr
Trench Volume =
7,696 ft3
Time for 90% Percolation =
47.38 hr
Sand and Grease Trao Calculations
\\boisefiles\public\Projeef5\fUB\10-16-114 Oaks Phase 5\Drainage\Oaks 5 Drainage.xlsx Page 4 of 6
Box Volume =
1000 gallon
Interior Box Width =
46 in
()Wo = 1.39 ft'/sec
Baffle Separation =
20 in
Throat Velocity = 0.21 ft/sec
Number of Boxes =
1
Throat Area =
6.67 ftz
Throat Velocity must be < 0.50 ft/sec
\\boisefiles\public\Projeef5\fUB\10-16-114 Oaks Phase 5\Drainage\Oaks 5 Drainage.xlsx Page 4 of 6
Temp Basins
Contributing Catchment Areas:
Required Water Quality Volume:
Required Water Quality Flow Rate:
Required Storage Volume:
Storm Pond
Temp
1,475 ft'
0.41 cfs
4,164 ft'
PROJECT NO. 10-16-114
The Oaks Subidivions No. 5
Volume Calculations
Storage Duration
Forebay Bottom Area =
357 ft
Infiltration Area =
1,462 ftp
Primary Basin Bottom Area =
1105 ft
Infiltration Rate =
8 in./hr
Trench Infiltration =
975 ft3/hr
Time for 90% Percolation =
3.85 hr
Contributing Catchment Areas:
Required Water Quality Volume:
Required Water Quality Flow Rate:
Required Storage Volume:
Storm Pond
Temp
783 ft'
0.22 cfs
2,212 ft'
Volume Calculations
Storage Duration
Forebay Bottom Area =
166 ft
Infiltration Area =
480 fe
Primary Basin Bottom Area =
314 ft
Infiltration Rate =
8 in./hr
Trench Infiltration =
320 ft3/hr
Time for 90% Percolation =
6.22 hr
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Boise Area Intensity -Duration -Frequency
NOAA Atlas 2
Intensity Duration Frequency
3.5 T
0.5 --------
0
10 min 15 min 30 min 1 hour 2 hours 3 hours 6 hours 12 hours 24 hours
Duration in minutes and hours
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
2.20
2.58
0.25
15 min
0.59
0.97
1.22
1.56
1.86
2.18
0.33
20 min
0.49
0.81
1.01
1.30
1.54
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
1.07
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 T
0.5 --------
0
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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THE OAKS SUBDIVISION NC
Meridian, Idaho
DRAINAGE CATCHMENT PLAN
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1 -U -B ENGINEERS, INC.
THE OAKS SUBDIVISION NC
Meridian, Idaho
DRAINAGE CATCHMENT PLAN