HomeMy WebLinkAboutCC - Storm drainage calcs STORM DRAINAGE REPORT AND CALCULATIONS
FOR
WINDROW SUBDIVISION NO. 3
MERIDIAN, IDAHO
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16970 CORINNE M GRAHAM, PE
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C I Nor I L S ITE
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PO BOX 6059
BOISE, ID 83707
(208) 946-3874
cgraham@cswengineering.com
INTRODUCTION
The existing 7.92-acre site for the proposed Windrow Subdivision No. 3 is currently vacant ground.The
proposed development includes 37 single family residential lots, six landscape common lots,two
common drives, and associated public streets, storm drainage, landscaping, and utilities.
HISTORIC DRAINAGE
The existing project site sits northeast of the intersection of W.Amity Road and S. Linder Road in
Meridian.The site is bisected from southwest to northeast by the Calkins Lateral, which is also the high
point along the property. The Calkins lateral was recently piped by the Boise Project Board of Control.
Surficial soils are comprised of silt and silty sands. At depths of 2-6 feet, free draining coarse sand was
encountered during geotechnical exploration. Groundwater was not encountered at depths up to 11
feet.
Because of the presence of free draining materials and the depth to groundwater,the site is suitable for
the use of infiltration facilities.
PROPOSED DRAINAGE
The storm drainage calculations and design for the proposed improvements are intended to meet the
requirements of ACHD's storm water policy. Storm water facilities are designed to store and treat runoff
from a 100-year storm event and to pretreat the water quality volume defined as the 80t" percentile
storm event. Storm water runoff from 50' behind the sidewalk, including residential driveways,will be
conveyed to the public stormwater system to be owned and maintained by ACHD. Remaining lot
drainage will be contained within individual lots.
Stormwater runoff from the public streets is conveyed via curb and gutter to drain inlets within the
roadway then piped to one of three underground seepage beds where it is treated, stored, and finally
infiltrated. A sand and grease trap will be installed to provide pre-treatment for the water quality event.
Temporary storm drain facilities were constructed in previous phases at the Selawick/Routeburn
intersection and the northern stub of Saltwood Ave.The street design within this phase will route runoff
from the temporary basins to permanent facilities.
APPENDICES
• Drainage Basin Map
• Storm Drainage Calculations
• Project Geotechnical Report
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SCALE: 1 - 50' DRAINAGE BASIN MAP PO BOX 6059
BOISE, ID 83707
Ph: (208) 946-3874
cgraham@cswengineering.com
'r� Prepared By: Corinne Graham, PE
May 11, 2026
CIVIL SITE
W O R K S
Windrow Subdivision No. 3
Storm Drainage Calculations
C Values
Impervious=0.90
Landscape=0.20
ACHD Basin ACHD Basin ACHD Basin
#1 #2 #3
Total Area (SF) 34915 112616 63647
Roadway Area (SF) 9846 27158 14956
Sidewalk Area (SF) 3503 8599 5873
House/Driveway Area (SF) 6885 36884 17878
Landscape Area (SF) 14681 39975 24940
C Value: 0.61 0.65 0.63
Catch Basin Calculations
CB#1A CB#1B CB#2A CB#213 CB#3A CB#3B CB#3C
Total Area (SF) 29410 5505 73821 38795 21666 27295 14686
Roadway Area (SF) 8146 1700 18042 9116 4042 7536 3378
Sidewalk Area (SF) 2901 602 5346 3253 1559 2996 1318
House/Driveway Area (SF) 6885 0 24722 12162 3802 6040 8036
Landscape Area (SF) 11478 3203 25711 14264 12263 10723 1954
C Value: 0.63 0.49 F-o.66 0.64 0.50 0.63 0.81
ACHD Calculation Sheet for Finding Peak Discharge/Volume-Rational Method
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology.These
calculations shall establish a minimum requirement.The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
for Peak Discharge Rate using th r post-developm
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin k1
2 Is area drainage basin map provided? YES
(map must be included with stormwater calculations)
3 Enter Design Storm(100-Year or 25-Year With 100-Year Flood Route) 100
4 Enter number of storage facilities(25 max)
Click to Show More Subbasins ❑
Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin
1 Subbasin 2 3 4 5 Subbasin 6 7 8 9 10
5 Area of Drainage Subbasin(SF or Acres) SF 9,846 3,503 6,885 14,646
Acres 0.80
6 Determine the Weighted Runoff Coefficient(C) 0.90 0.90 0.90 0.20
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg 0.61
7 Calculate Overland Flow Time of Concentration in Minutes(Tc)or use default 10 User Calculate
min
10 Min. I Estimated Runoff Coefficients for Various Surface
Type of Surface Runoff Coefficients"I
8 Determine the average rainfall intensity(i)from IDF Curve based on Tc i 2.58 2.58 in hr Business
Downtown areas 0.70-0.95
9 Calculate the Post-Development peak discharge(ClPeak) Cip'A 1.25 1.25 cfs Urban neighborhoods 0.50-0.70
Residential
Single Family 0.35-0.50
10 Calculate total runoff vol(V)(for sizing primary storage) V 1,677 1,677 ft Multi-family 0.60-0.75
V=Ci(Tc=60)Ax3600 Residential(rural) 0.25-0.40
11 Calculate Volume of Runoff Reduction Vrr Apartment Dwelling Areas 0.70
Industrial and Commercial
Enter Percentile Storm I(95th percentile=0.60 in) 80th 0.34 in Light areas 0.80
Enter Runoff Reduction Vol(95th Percentile=0.60-in x Area x C) Vrr, 594 594 ff' Heavy areas 0.90
12 Detention:Approved Discharge Rate to Surface Waters(if applicable) 0.00 cfs Parks,cemeteries 0.
Playgrounds 0.20-0.0-0.35
5
Railroad yard areas 0.20-0.40
13 Volume Summary Unimproved areas 0.10-0.30
Surface Storage:Basin Streets
Asphalt 095
Basin Foreba V 168 ft' .
Y Concrete 0.95
Primary Treatment/StorageBasin V 1,509 ft' Brick 0.95
Subsurface Storage Roofs 0.95
Volume Without Sediment Factor(See BMP 20 Tab) V 1,677 1,677 ft' Gravel 0.75
Fields:Sandy soil Soil Type
Slope A B C D
Flat:0-2% 0.04 0.07 0.11 0.
Average:2-6% 0.09 0.12 0.15 0.
Steep:>6% 0.13 0.18 0.23 0.
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:37 AM
Version 10.5,November 2018
ACHD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
Engineer's calculation methodology. These calculations shall establish a minimum requirement.The Engineer's methodology must result in
facilities that meet or exceed these calculations in order to be accepted.
Note this spreadsheet pulls information from the"Peak QV"tab
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin#1
2 Enter number of Seepage Beds(25 max) 1
3 Design Storm 100
4 Weighted Runoff Coefficient C 0.61 Unkto: —
QV TR55
5 Area A(Acres) 0.80 acres
6 Approved discharge rate(if applicable) 0.00 cfs
7 Is Seepage Bed in Common Lot? Yes V 1,677 1,677 ft' 0%Sediment
8 Set Total Design Width of All Drain Rock W 12.0 ft
9 Set Total Design Depth of All Drain Rock D 6.0 ft
Rock Only,Do Not Include Filter Sand Depth or Cover
10 Void Ratio of Drain Rock Voids 0.4
0.4 for 1.5"-2"drain rock and 3/4"Chips
11 Design Infiltration Rate(8 in/hr max) Perc 2.00 in/hr
12 Size of WQ Perf Pipe(Perf 1800) Dia pipe 18 in
13 Size of Overflow Perf Pipe(Perfs 3601),READ if Q100>3.3 cfs in
14 Calculate Total Storage per Foot Spf 31.0 31.0 ft3/ft
15 Calculate Design Length L 54 54 ft
Override Value Required for Chambers
16 Variable Infiltration Window L SWL 54 54.0 ft
17 Variable Infiltration Window W SWW 12.0 12.0 ft
18 Time to Drain 14.0 hours
90%volume in 48-hours minimum
19 Length of WQ&Overflow Perf Pipes 54 54 ft
20 Perf Pipe Checks.Qperf>=Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers
Note:This assumes chambers are organized in a rectangular layout.
1-StormTech,
1 Type of Chambers SC740
2 Volume to Store V 0 ft3
3 Installed Chamber Width Cw 4.25 ft
Installed Chamber Depth Cd 2.50 ft
Installed Chamber Height Ch 7.12 ft
4 Chamber Void Factor
5 Chamber Storage Volume,Without Rock,Per Manuf 45.90 ft3/Unit
6 Chamber Storage Volume,With Rock,Per Manuf 74.90 ft3/Unit
7 Total Number of Units Required 0 ea
8 Area of Infiltration Aperc ftz
9 Volume Infiltration Vperc 0 ft3/hr
10 Time to Drain hours
90%volume in 48-hours minimum
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:38 AM
Version 10.0,May 2018
ACHD Calculation Sheet for Finding Peak Discharge/Volume-Rational Method
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology.These
calculations shall establish a minimum requirement.The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
for Peak Discharge Rate using th r post-developm
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin#2
2 Is area drainage basin map provided? YES
(map must be included with stormwater calculations)
3 Enter Design Storm(100-Year or 25-Year With 100-Year Flood Route) 100
4 Enter number of storage facilities(25 max)
Click to Show More Subbasins ❑
Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin
1 Subbasin 2 3 4 5 Subbasin 6 7 8 9 10
5 Area of Drainage Subbasin(SF or Acres) SF 27,158 8,599 36,884 40,010
Acres 2.59
6 Determine the Weighted Runoff Coefficient(C) 0.90 0.90 0.90 0.20
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg 0.65
7 Calculate Overland Flow Time of Concentration in Minutes(Tc)or use default 10 User Calculate
min
10 Min. I Estimated Runoff Coefficients for Various Surface
Type of Surface Runoff Coefficients"I
8 Determine the average rainfall intensity(i)from IDF Curve based on Tc 2.58 2.58 in hr Business
Downtown areas 0.70-0.95
9 Calculate the Post-Development peak discharge(ClPeak) peak 4.35 4.35 cfs Urban neighborhoods 0.50-0.70
Residential
50
10 Calculate total runoff vol V for sizing rims storage) V 5,822 5,822 ft Single Family 0.60-0.75
( )( g primary g ) Multi-family 0.60-0.75
V=Ci(Tc=60)Ax3600 Residential(rural) 0.25-0.40
11 Calculate Volume of Runoff Reduction Vrr Apartment Dwelling Areas 0.70
Industrial and Commercial
Enter Percentile Storm I(95th percentile=0.60 in) 80th 0.34 in Light areas 0.80
Enter Runoff Reduction Vol(95th Percentile=0.60-in x Area x C) Vrr, 2,062 7062 ft' Heavy areas 0.90
12 Detention:Approved Discharge Rate to Surface Waters(if applicable) 0.00 cfs Parks,Cemeteries 0.10-0.25
Playgrounds 0.20-0.35
Railroad yard areas 0.20-0.40
13 Volume Summary Unimproved areas 0.10-0.30
Surface Storage:Basin Streets
Asphalt 0.95
Basin Foreba V 582 ft'
Y concrete 0.95
Primary Treatment/StorageBasin V 5,240 ft' Brick 0.95
Subsurface Storage Roofs 0.95
Volume Without Sediment Factor See BMP 20 Tab V 5,822 5,822 ft' Gravel 0.75
( ) Fields:Sandy soil Soil Type
Slope A B C D
Flat:0-2% 0.04 0.07 0.11 0.
Average:2-6% 0.09 0.12 0.15 0.
Steep:>6% 0.13 0.18 0.23 0.
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:37 AM
Version 10.5,November 2018
ACHD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
Engineer's calculation methodology. These calculations shall establish a minimum requirement.The Engineer's methodology must result in
facilities that meet or exceed these calculations in order to be accepted.
Note this spreadsheet pulls information from the"Peak QV"tab
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin#2
2 Enter number of Seepage Beds(25 max) 1
3 Design Storm 100
4 Weighted Runoff Coefficient C 0.65 Unkto: —
QV TR55
5 Area A(Acres) 2.59 acres
6 Approved discharge rate(if applicable) 0.00 cfs
7 Is Seepage Bed in Common Lot? Yes V 5,822 5,822 ft3 0%Sediment
8 Set Total Design Width of All Drain Rock W 15.0 ft
9 Set Total Design Depth of All Drain Rock D 7.0 ft
Rock Only,Do Not Include Filter Sand Depth or Cover
10 Void Ratio of Drain Rock Voids 0.4
0.4 for 1.5"-2"drain rock and 3/4"Chips
11 Design Infiltration Rate(8 in/hr max) Perc 2.00 in/hr
12 Size of WQ Perf Pipe(Perf 1800) Dia pipe 18 in
13 Size of Overflow Perf Pipe(Perfs 3601),READ if Q100>3.3 cfs in
14 Calculate Total Storage per Foot Spf 44.7 44.7 ft3/ft
15 Calculate Design Length L 130 130 ft
Override Value Required for Chambers
16 Variable Infiltration Window L SWL 130 130.0 ft
17 Variable Infiltration Window W SWW 15.0 15.0 ft
18 Time to Drain 167 hours
90%volume in 48-hours minimum
19 Length of WQ&Overflow Perf Pipes 130 130 ft
20 Perf Pipe Checks.Qperf>=Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers
Note:This assumes chambers are organized in a rectangular layout.
1-StormTech,
1 Type of Chambers SC740
2 Volume to Store V 0 ft3
3 Installed Chamber Width Cw 4.25 ft
Installed Chamber Depth Cd 2.50 ft
Installed Chamber Height Ch 7.12 ft
4 Chamber Void Factor
5 Chamber Storage Volume,Without Rock,Per Manuf 45.90 ft3/Unit
6 Chamber Storage Volume,With Rock,Per Manuf 74.90 ft3/Unit
7 Total Number of Units Required 0 ea
8 Area of Infiltration Aperc ftz
9 Volume Infiltration Vperc 0 ft3/hr
10 Time to Drain hours
90%volume in 48-hours minimum
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:38 AM
Version 10.0,May 2018
ACHD Calculation Sheet for Finding Peak Discharge/Volume-Rational Method
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology.These
calculations shall establish a minimum requirement.The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
for Peak Discharge Rate using th r post-developm
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin#3
2 Is area drainage basin map provided? YES
(map must be included with stormwater calculations)
3 Enter Design Storm(100-Year or 25-Year With 100-Year Flood Route) 100
4 Enter number of storage facilities(25 max)
Click to Show More Subbasins ❑
Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin Subbasin
1 Subbasin 2 3 4 5 Subbasin 6 7 8 9 10
5 Area of Drainage Subbasin(SF or Acres) SF 14,956 5,873 17,878 24,940
Acres 1.46
6 Determine the Weighted Runoff Coefficient(C) 0.90 0.90 0.90 0.20
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg 0.63
7 Calculate Overland Flow Time of Concentration in Minutes(Tc)or use default 10 User Calculate
min
10 Min. I Estimated Runoff Coefficients for Various Surface
Type of Surface Runoff Coefficients"I
8 Determine the average rainfall intensity(i)from IDF Curve based on Tc 2.58 2.58 in hr Business
Downtown areas 0.70-0.95
9 Calculate the Post-Development peak discharge(ClPeak) peak 2.36 2.36 cfs Urban neighborhoods 0.50-0.70
Residential
50
10 Calculate total runoff vol V for sizing rims storage) V 3,160 3,160 ft Single Family 0.60-0.75
( )( g primary g ) Multi-family 0.60-0.75
V=Ci(Tc=60)Ax3600 Residential(rural) 0.25-0.40
11 Calculate Volume of Runoff Reduction Vrr Apartment Dwelling Areas 0.70
Industrial and Commercial
Enter Percentile Storm I(95th percentile=0.60 in) 80th 0.34 in Light areas 0.80
Enter Runoff Reduction Vol(95th Percentile=0.60-in x Area x C) Vrr, 1,119 1,119 ff' Heavy areas 0.90
12 Detention:Approved Discharge Rate to Surface Waters(if applicable) 0.00 cfs Parks,cemeteries 0.
Playgrounds 0.20-0.0-0.35
5
Railroad yard areas 0.20-0.40
13 Volume Summary Unimproved areas 0.10-0.30
Surface Storage:Basin Streets
Asphalt 095
Basin Foreba V 316 ft' .
Y Concrete 0.95
Primary Treatment/StorageBasin V 2,844 ft' Brick 0.95
Subsurface Storage Roofs 0.95
Volume Without Sediment Factor(See BMP 20 Tab) V 3,160 3,160 ft' Gravel 0.75
Fields:Sandy soil Soil Type
Slope A B C D
Flat:0-2% 0.04 0.07 0.11 0.
Average:2-6% 0.09 0.12 0.15 0.
Steep:>6% 0.13 0.18 0.23 0.
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:37 AM
Version 10.5,November 2018
ACHD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
NOTE:This worksheet is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
Engineer's calculation methodology. These calculations shall establish a minimum requirement.The Engineer's methodology must result in
facilities that meet or exceed these calculations in order to be accepted.
Note this spreadsheet pulls information from the"Peak QV"tab
Calculate Post-Development Flows(for pre-development flows,increase number of storage facilities to create new tab)
User input in yellow cells.
1 Project Name ACHD Basin#3
2 Enter number of Seepage Beds(25 max) 1
3 Design Storm 100
4 Weighted Runoff Coefficient C 0.63 Unkto: —
QV TR55
5 Area A(Acres) 1.46 acres
6 Approved discharge rate(if applicable) 0.00 cfs
7 Is Seepage Bed in Common Lot? Yes V 3,160 3,160 ft' 0%Sediment
8 Set Total Design Width of All Drain Rock W 12.0 ft
9 Set Total Design Depth of All Drain Rock D 8.0 ft
Rock Only,Do Not Include Filter Sand Depth or Cover
10 Void Ratio of Drain Rock Voids 0.4
0.4 for 1.5"-2"drain rock and 3/4"Chips
11 Design Infiltration Rate(8 in/hr max) Perc 2.00 in/hr
12 Size of WQ Perf Pipe(Perf 1800) Dia pipe 18 in
13 Size of Overflow Perf Pipe(Perfs 3601),READ if Q100>3.3 cfs 12 in
14 Calculate Total Storage per Foot Spf 41.0 ft3/ft
15 Calculate Design Length L 77 77 ft
Override Value Required for Chambers
16 Variable Infiltration Window L SWL 77 77.0 ft
17 Variable Infiltration Window W SWW 12.0 12.0 ft
18 Time to Drain 18.5 hours
90%volume in 48-hours minimum
19 Length of WQ&Overflow Perf Pipes 77 77 ft
20 Perf Pipe Checks.Qperf>=Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers
Note:This assumes chambers are organized in a rectangular layout.
1-StormTech,
1 Type of Chambers SC740
2 Volume to Store V 0 ft3
3 Installed Chamber Width Cw 4.25 ft
Installed Chamber Depth Cd 2.50 ft
Installed Chamber Height Ch 7.12 ft
4 Chamber Void Factor
5 Chamber Storage Volume,Without Rock,Per Manuf 45.90 ft3/Unit
6 Chamber Storage Volume,With Rock,Per Manuf 74.90 ft3/Unit
7 Total Number of Units Required 0 ea
8 Area of Infiltration Aperc ftz
9 Volume Infiltration Vperc 0 ft3/hr
10 Time to Drain hours
90%volume in 48-hours minimum
C:\Users\cgrah\Dropbox\Projects\26003 Windrow Sub Ph3+Ph4\Reports\Storm Drainage\ACHD_SD_CALCS_Ph3 5/11/2026,10:38 AM
Version 10.0,May 2018