CC - Drainage Calcs PROJECT NO . 10 = 16� 060
DRAINAGE CALCULATIONS
FOR
Silverwater South Subdivision
City of Meridian
Ada County , Idaho
Februrary 2017
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J • U • B ENGINEERS , INC .
250 S . Beechwood Drive , #201
Boise , Idaho 83709
208 = 376-7330
PROJECT NO. 10-16-060
Silverwater South Subdivision
SITE OVERVIEW
Silverwater South Subdivision is located on the south side of Victory Road and approximately 2,000-
ft east of Meridian Road in Meridian, Idaho.The subdivision will include the development of 48
single family residential lots on 12.1 acres.
EXISTING DRAINAGE
The project site presently consists of empty agriculture fields with several irrigation ditches located
throughout the property. The ground slopes gently from southeast to northwest at grades generally
less than 1%.The project is surrounded on all sides by existing single family residential
developments.
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.
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The site was divided into catchment areas based on the inlet locations and high points within the
roadway. 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.
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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 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: A
Required Water Quality Volume: 1,401 ft'
Required Water Quality Flow Rate: 0.73 cfs
Required Storage Volume: 3,957 ft'
100-Year Flow Rate: 2.69 cfs
Seepage Bed
Volume Calculations Storage Duration
Depth= 6 ft
Length= 165 ft Infiltration Area= 1,650 fiz
Width= 10 ft Infiltration Rate= 8 in./hr
Void Ratio= 40 % Trench Infiltration= 1,100 ft3/hr
Trench Volume= 3,960 ft3 1 Time for 90%Percolation= 1.15 hr
Sand and Grease Trap Calculations
Box Volume= 1000 gallon
3
Interior Box Width= 48 in QWQ= 0.73 ft/sec
Baffle Separation= 20 in Throat Velocity= 0.11 ft/sec
Number of Boxes= 1
Throat Area= 6.67 ft' Throat Velocity must be<0.50 ft/sec
Overflow Bypass Calculations
Max Flow Through Sand&Grease Trap= 3.33 cfs (ACHD Policy Manual Section 8202.11)
100-Yr Flow Rate= 2.69 cfs
100 year flow is less than maximum flow rate allowed through the sand and grease trap.Therefore,an overflow bypass is not required.
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F:\Projects\JUB\10-16-060-Silverwater 4\Drainage\10-16-060_Silverwater 4 Drainage.xlsx
Basin B
Contributing Catchment Areas: B
Required Water Quality Volume: 1,830 ft'
Required Water Quality Flow Rate: 0.88 cfs
Required Storage Volume: 5,167 ft'
100-Year Flow Rate: 3.27 cfs
Seepage Bed
Volume Calculations Storage Duration
Depth= 8.5 ft
Length= 152 ft Infiltration Area= 1,520 ft2
Width= 10 ft Infiltration Rate= 8 in./hr
Void Ratio= 40 % Trench Infiltration= 1,013 ft3/hr
Trench Volume= 5,168 ft3 Time for 90%Percolation= 1.63 hr
Sand and Grease Trap Calculations
Box Volume= 1000 gallon
Interior Box Width= 48 in Qwa= 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 ft/sec
overflow Bypass Calculations
Max Flow Through Sand&Grease Trap= 3.33 cfs (ACHD Policy Manual Section 8202.11)
100-Yr Flow Rate= 3.27 cfs
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100-year flow is greater than maximum flow rate allowed through the sand and grease trap.Therefore,an overflow bypass is required.
Bypass Flow Rate= -0.06 cfs
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F:\ProjectsWUB\10-16.060-Silverwater 4\Drainage\10-16-060_Silverwater 4 Drainage.xlsx
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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 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
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Duration in minutes and hours
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SILVERWATER SOUTH SUBDIVISION
�- MERIDIAN, IDAHO
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o `JOB I DRAINAGE CATCHMENT PLAN
s 1-U-8 ENGINEERS,INC.
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