CC - Drainage Calcs PROJECT NO . 10 = 17 =007
DRAINAGE CALCULATIONS
FOR
The Oaks Subidivions No . 6
City of Meridian
Ada County , Idaho
February 2017
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J • U • B ENGINEERS , INC .
250 S . Beechwood Drive , #201
Boise , Idaho 83709
208 =376 -7330
PROJECT NO. 10-17-007
The Oaks Subidivions No.6
SITE OVERVIEW
The Oaks Subdivision No. 6 is located at the southwest corner of W. McMillan Road and N. Rustic
Oak Way in Meridian, Idaho.The proposed development consists of 20 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 north and east,and agricultural land to the
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.
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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- 17-007
The Oaks Subidivions No. 6
Contributing Catchment Areas : A
Required Water Quality Volume : 1 ,720 ft'
Required Water Quality Flow Rate : 0 .88 cfs
Required Storage Volume : 4, 856 ft'
Seepage Bed
ThE Volume Calculations Storage Duration
Depth = 4 ft
Length = 203 ft Infiltration Area = 812 ft2
Width = 15 ft Infiltration Rate = 8 in ./hr
Void Ratio = 40 % Trench Infiltration = 541 ft3/hr
Trench Volume = 41872 ft' Time for 90% Percolation = 2 .86 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWo = 0 . 88 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0 . 13 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ft2 Throat Velocity must be < 0 . 50 fUsec
F:\Projects\JUB\10-17-007-Oaks No 6\Drainage\Oaks 6 Drainage-Copy.xlsx Page 4 of 7
Basin B PROJECT NO. 10- 17-007
The Oaks Subidivions No. 6
Contributing Catchment Areas : B
Required Water Quality Volume : 1 , 236 ft'
Required Water Quality Flow Rate : 0 . 60 cfs
Required Storage Volume : 39491 ft3
Seepage Bed
Volume Calculations Storage Duration
Depth = 4 ft
Length = 182 ft Infiltration Area = 728 ft2
Width = 12 ft Infiltration Rate = 8 in ./hr
Void Ratio = 40 % Trench Infiltration = 485 ft3/hr
Trench Volume = 3 ,494 ft' Time for 90% Percolation = 2 . 29 hr
Sand and Grease Trap Calculations
Box Volume = 1000 gallon
Interior Box Width = 48 in QWQ = 0 . 60 ft3/sec
Baffle Separation = 20 in Throat Velocity = 0 . 09 ft/sec
Number of Boxes = 1
Throat Area = 6 . 67 ft' Throat Velocity must be < 0 . 50 ft/sec
F:\Projects\JUB\10-17-007-Oaks No 6\Drainage\Oaks 6 Drainage-Copy.xlsx Page 5 of 7
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3DO by Autodesk,Inc. Thursday,Jan 26 2017
PIPE BASIN Al-A2
Circular Highlighted
Diameter(ft) = 1.00 Depth (ft) = 0.38
Q (cfs) = 2.340
Area (sqft) = 0.27
Invert Elev (ft) = 100.00 Velocity (ft/s) = 8.52
Slope (%) = 2.74 Wetted Perim (ft) = 1.33
N-Value = 0.010 Crit Depth, Yc (ft) = 0.66
Top Width (ft) = 0.97
Calculations EGA (ft) = 1.51
Compute by: Known Q
Known Q (cfs) = 2.34
Elev (ft) Depth
Section
102.00 2.00
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101.50 1.50
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101.00 1.00
100.50 0.50
100.00 0.00
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99.50 -0.50
0 1 2 3
Reach (ft)
�Chalnnel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk,Inc. Thursday,Jan 26 2017
PIPE BASIN A2
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.50
Q (cfs) = 1.610
Area (sqft) = 0.39
Invert Elev (ft) = 100.00 Velocity (ft/s) = 4.08
Slope (%) = 0.50 Wetted Perim (ft) = 1.57
N-Value = 0.010 Crit Depth, Yc (ft) = 0.54
Top Width (ft) = 1.00
Calculations EGL (ft) = 0.76
Compute by: Known Q
Known Q (cfs) = 1.61
( ) Depth
Elev ft De Section p (
102.00 2.00
101.50 1.50
i
101.00 1.00
100.50 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk,Inc. Thursday,Jan 26 2017
PIPE BASIN 131-132
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.31
Q (cfs) = 1.590
Area (sqft) = 0.21
Invert Elev (ft) = 100.00 Velocity (ft/s) = 7.60
Slope (%) = 2.65 Wetted Perim (ft) = 1.18
N-Value = 0.010 Crit Depth, Yc (ft) = 0.54
Top Width (ft) = 0.93
Calculations EGL (ft) = 1.21
Compute by: Known Q
Known Q (cfs) = 1.59
Elev (ft) Depth
j Section
I
102.00 2.00
101.50 1.50
i
101.00 1.00
I
c
100.50 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for AutodeskO AutoCADO Civil 3D0 by Autodesk,Inc. Thursday,Jan 26 2017
PIPE BASIN B2
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.43
Q (cfs) = 1.230
Area (sqft) = 0.33
Invert Elev (ft) = 100.00 Velocity (ft/s) = 3.78
Slope (%) = 0.50 Wetted Perim (ft) = 1.43
N-Value = 0.010 Crit Depth, Yc (ft) = 0.47
Top Width (ft) = 0.99
Calculations EGL (ft) = 0.65
Compute by: Known Q
Known Q (cfs) = 1.23
Elev (ft) Depth
Section
102.00 2.00
101.50 1.50
101.00 1.00
100.50 0.50
100.00 0.00
j
99.50 -0.50
0 1 2 3
Reach (ft)
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