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APPROVED
DATE: 08/22/22
FILE NUMBER: FP- 02aoo22
STORM DRAINAGE REPORT AND CALCULATIONS
FOR
SKYBREAK SUBDIVISION NO. 2
�y
Q
16970
6-2-22
c)o��y� OF
�NE M .
MERIDIAN, IDAHO
PREPARED BY:
CORINNE M GRAHAM, PE
.June 2, 2022
csw
CIVIL SIT E
w o rz K s
PO BOX 6059
BOISE, ID 83707
(208)946-3874
cgraham@cswengineering.com
CUEIDR IAy
APPROVED IUCTION
DATE 08/22/22
ng 11.12-acre site for the proposed Skybreak Subdivision No. 2 is currently agricultural row
FILE NUMBER: FP-zozz-oozz
Ind. The proposed development includes 62 single family residential lots, 10 landscape
common lots, and associated public streets, landscaping, and utilities.
HISTORIC DRAINAGE
The existing project site sits southeast of the intersection of E. Lake Hazel Road and S. Eagle Road in
Meridian. The eastern portion of the site that was developed in Phase 1 lies along Meridian's southern
rim.
Surficial soils are comprised of sandy silts and silty sands. At depths of 3-6 feet, free draining poorly
graded gravel with sand was encountered during geotechnical exploration. Groundwater was not
encountered at depths up to 15.2 feet and is not anticipated to reach levels higher than 20 feet below
existing ground.
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 40' 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.
A high -flow bypass will be installed on Basin #1 for larger storm events.
APPENDICES
• Drainage Basin Map
• Storm Drainage Calculations
• Project Geotechnical Report
APPROVED
0 25 50 100 150
SCALE: 1" = 50'
SKYBREAK SUBDIVISION NO. 2
C.SW
C I V I L S I T E
w O R K B
DRAINAGE BASIN MAP
PO BOX 6059
BOISE, ID 83707
Ph: (208) 946-3874
cgraham@cswengineering.com
Prepared By: Corinne Graham, PE
June 2, 2022
CUE IDI y r ST E
APPROVED Skybreak Subdivision No. 2
DATE 08/22/22
FILE NUMBER:FP-2o22-.22 Storm Drainage Calculations
Drainaee Basin Characteristics
C Values
Impervious = 0.90
Landscape = 0.20
ACHD Basin
#1
ACHD
Basin #2
ACHD
Basin #3
Total Area (SF)
157388
86334
83303
Roadway Area (SF)
44883
1 23842
21608
Sidewalk Area (SF)
14448
7521
7699
House/Driveway Area (SF)
52787
26579
25237
Landscape Area (SF)
45270
28392
28759
C Value:
0.70
0.67
0.66
Catch Basin Calculations
CB #1A
CB #113
CB #2A
CB #213
CB #2C
CB #3A
CB #313
CB #3C
Total Area (SF)
74453
82935
31153
40582
14599
45191
25645
12467
Roadway Area (SF)
17670
27213
8603
10698
4541
10003
6880
4725
Sidewalk Area (SF)
5772
8676
2604
3558
1359
3718
2399
1582
House/Driveway Area (SF)
30549
22238
9080
12501
4998
17722
7515
0
Landscape Area (SF)
20462
24808
10866
13825
3701
13748
8851
6160
C Value:
0.71
0.69
0.66
0.66
0.72
0.69
0.66
0.55
i,�j(E IDS IAN-
�J ACHD Calculation Sheet for Finding Peak Discharge/Volume -Rational Method
APPROVED is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology. These
08/22/22
DATE blish a minimum requirement. The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
ME NUMBER: PP-2o22-cD22
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 Is area drainage basin map provided?
(map must be included with stormwater calculations)
3 Enter Design Storm (100-Year or 25-Year With 100-Year Flood Route)
4 Enter number of storage facilities (25 max)
5 Area of Drainage Subbasin (SF or Acres) SF
Acre
6 Determine the Weighted Runoff Coefficient (C)
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg
7 Calculate Overland Flow Time of Concentration in Minutes (Tc) or use default 10 user calculate
min
0 Min.
8 Determine the average rainfall intensity (i) from IDF Curve based on Tc
9 Calculate the Post -Development peak discharge (QPeak)
i
Qpcak
2.58
6.51
2.58 in hr
6.51 cfs
10 Calculate total runoff vol (V) (for sizing primary storage)
V
8,724
8,724 ft
V = Ci (Tc=60)Ax3600
11 Calculate Volume of Runoff Reduction Vrr
Enter Percentile Storm I (95th percentile = 0.60 in)
80th
0.34 in
Enter Runoff Reduction Vol (95th Percentile=0.60-in x Area x C)
Vrr
3,090
3,090 ft'
12 Detention: Approved Discharge Rate to Surface Waters (if applicable)
0.00 cfs
13 Volume Summary
Surface Storage: Basin
Basin Forebay
V
872
ft'
Primary Treatment/Storage Basin
V
7,852
ft'
Subsurface Storage
Volume Without Sediment Factor (See BMP 20 Tab)
V
8,724
8,724 ft'
YES
flirk M Chn , Mnra 1i hha<inc f1
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
1
Subbasin 2
3
4
5
Subbasin 6
7
8
9
10
44,883
14,448
52,787
45,270
3.61
0.90
0.90
0.90
0.20
0.70
Estimated Runoff Coefficients for Various Surface
Type of Surface
Runoff Coefficients "I
Business
Downtown areas
0.70-0.95
Urban neighborhoods
0.50-0.70
Residential
Single Family
0.35-0.50
Multi -family
0.60-0.75
Residential (rural)
0.25-0.40
Apartment Dwelling Areas
0.70
Industrial and Commercial
Light areas
0.80
Heavy areas
0.90
Parks, Cemeteries
0.10-0.25
Playgrounds
0.20-0.35
Railroad yard areas
0.20-0.40
Unimproved areas
0.10-0.30
Streets
Asphalt
0.95
Concrete
0.95
Brick
0.95
Roofs
0.95
Gravel
0.75
Fields: Sandy soil
Soil Type
Slope
A
B
C D
Flat: 0-2%
0.04
0.07
O.il 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\22001Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_SD_CALCS_022322
Version 10.5, November 2018
6/2/2022, 6:23 PM
CUEIDR IAN,--
�� HD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
t is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
APPROVED
n methodology. These calculations shall establish a minimum requirement. The Engineer's methodology must result in
08/22/22 r exceed these calculations in order to be accepted.
DATE t pulls information from the "Peak Q,V" tab
FILE NUMBER: Fa D22 D22
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)
3 Design Storm
4 Weighted Runoff Coefficient C
5 Area A (Acres)
6 Approved discharge rate (if applicable)
7 Is Seepage Bed in Common Lot? Yes
1
100
0.70
3.61 acres
0.00 cfs
Link to: o'v
QV TR55
V 8,724 8,724 ft3
8 Set Total Design Width of All Drain Rock
W
20.0 ft
9 Set Total Design Depth of All Drain Rock
D
10.0 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
8.00 in/hr
12 Size of WQ Perf Pipe (Perf 180°)
Dia pipe
18 in
13 Size of Overflow Perf Pipe (Perfs 360°), REQD if Q100>3.3 cfs
12 in
14 Calculate Total Storage per Foot
Spf
94.0 ft,/ft
15 Calculate Design Length L
Override Value Required for Chambers
16 Variable Infiltration Window L SWL
17 Variable Infiltration Window W SWW
18 Time to Drain
90%volume in 48-hours minimum
19 Length of WQ & Overflow Perf Pipes
20 Perf Pipe Checks. Qperf>= Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers
This assumes chambers are organized in a rectangular layout.
1 Type of Chambers
2 Volume to Store
V
3 Installed Chamber Width
Cw
Installed Chamber Depth
Cd
Installed Chamber Height
Ch
4 Chamber Void Factor
5 Chamber Storage Volume, Without Rock, Per Manuf
6 Chamber Storage Volume, With Rock, Per Manuf
7 Total Number of Units Required
8 Area of Infiltration
Aperc
9 Volume Infiltration
Vperc
10 Time to Drain
90%volume in 48-hours minimum
93
93 ft
93
93.0 ft
20.0
207 ft
6.3 hours
93
93 ft
ftz
0 ft3/hr
hours
µ3
0% Sediment
C:\Users\cgrah\Dropbox\Projects\22001 Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_SD_CALCS_022322 6/2/2022, 6:24 PM
Version 10.0, May 2018
i,�j(E IDS IAN-
�J ACHD Calculation Sheet for Finding Peak Discharge/Volume -Rational Method
APPROVED is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology. These
08/22/22
DATE blish a minimum requirement. The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
ME NUMBER: PP-2o22-cD22
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?
(map must be included with stormwater calculations)
3 Enter Design Storm (100-Year or 25-Year With 100-Year Flood Route)
4 Enter number of storage facilities (25 max)
5 Area of Drainage Subbasin (SF or Acres) SF
Acre
6 Determine the Weighted Runoff Coefficient (C)
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg
7 Calculate Overland Flow Time of Concentration in Minutes (Tc) or use default 10 user calculate
min
0 Min.
8 Determine the average rainfall intensity (i) from IDF Curve based on Tc
9 Calculate the Post -Development peak discharge (QPeak)
i
Qpcak
2.58
3.42
2.58 in hr
3.42 cfs
10 Calculate total runoff vol (V) (for sizing primary storage)
V
4,588
4,588 ft
V = Ci (Tc=60)Ax3600
11 Calculate Volume of Runoff Reduction Vrr
Enter Percentile Storm I (95th percentile = 0.60 in)
80th
0.34 in
Enter Runoff Reduction Vol (95th Percentile=0.60-in x Area x C)
Vrr
1,625
1,625 ft'
12 Detention: Approved Discharge Rate to Surface Waters (if applicable)
0.00 cfs
13 Volume Summary
Surface Storage: Basin
Basin Forebay
V
459
ft'
Primary Treatment/Storage Basin
V
4,129
ft'
Subsurface Storage
Volume Without Sediment Factor (See BMP 20 Tab)
V
4,588
4,588 ft'
YES
flirk M Chn , Mnra 1i hha<inc f1
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
1
Subbasin 2
3
4
5
Subbasin 6
7
8
9
10
23,842
7,521
26,579
28,392
1.98
0.90
0.90
0.90
0.20
0.67
Estimated Runoff Coefficients for Various Surface
Type of Surface
Runoff Coefficients "I
Business
Downtown areas
0.70-0.95
Urban neighborhoods
0.50-0.70
Residential
Single Family
0.35-0.50
Multi -family
0.60-0.75
Residential (rural)
0.25-0.40
Apartment Dwelling Areas
0.70
Industrial and Commercial
Light areas
0.80
Heavy areas
0.90
Parks, Cemeteries
0.10-0.25
Playgrounds
0.20-0.35
Railroad yard areas
0.20-0.40
Unimproved areas
0.10-0.30
Streets
Asphalt
0.95
Concrete
0.95
Brick
0.95
Roofs
0.95
Gravel
0.75
Fields: Sandy soil
Soil Type
Slope
A
B
C D
Flat: 0-2%
0.04
0.07
O.il 0.
Average: 2-6%
0.09
0.12
1 15 0.
Steep:>6%
0.13
0.18
0.23 0.
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\22001Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_SD_CALCS_022322
Version 10.5, November 2018
6/2/2022, 6:23 PM
(E IDS IAN�
HD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
APPROVED t is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
In methodology. These calculations shall establish a minimum requirement. The Engineer's methodology must result in
DAM 08/22/22 exceed these calculations in order to be accepted.
pulls information from the "Peak QV" tab
FILE NUMBERost-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)
3 Design Storm
4 Weighted Runoff Coefficient C
5 Area A (Acres)
6 Approved discharge rate (if applicable)
7 Is Seepage Bed in Common Lot? Yes
10U
0.67
1.98 acres
0.00 cfs
V 4,588
Unk to: O v
QV TR55
4,588 ft3 O%Sediment
8 Set Total Design Width of All Drain Rock
W
15.0 ft
9 Set Total Design Depth of All Drain Rock
D
10.0 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
8.00 in/hr
12 Size of WQ Perf Pipe (Pert 180°)
Dia pipe
18 in
13 Size of Overflow Perf Pipe (Perfs 360% READ if Q100>3.3 cis
in
14 Calculate Total Storage per Foot
Spf
70.2
70.2 ft3/ft
15 Calculate Design Length
L
65
65 ft
Override Value Required for Chambers
16 Variable Infiltration Window L
SWL
65
65.0 ft
17 Variable Infiltration Window W
SWW
15.0
15.0 ft
18 Time to Drain
6.4 hours
90%volume in 48-hours minimum
19 Length of WQ & Overflow Perf Pipes
65
65 ft
20 Perf Pipe Checks. Qperf >= Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers 9r
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
6 Chamber Storage Volume, With Rock, Per Manuf
74.90 ft3
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\22001 Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_SD_CALCS_022322 6/2/2022, 6:24 PM
Version 10.0, May 2018
(E IDS IAN,_-
�J ACHD Calculation Sheet for Finding Peak Discharge/Volume -Rational Method
APPROVED is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the Engineer's calculation methodology. These
08/22/22
DATE blish a minimum requirement. The Engineer's methodology must result in facilities that meet or exceed these calculations in order to be accepted.
FILE NUMBER:Fehod calculat111111111 e
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?
(map must be included with stormwater calculations)
3 Enter Design Storm (100-Year or 25-Year With 100-Year Flood Route)
4 Enter number of storage facilities (25 max)
5 Area of Drainage Subbasin (SF or Acres) SF
Acre
6 Determine the Weighted Runoff Coefficient (C)
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avg
7 Calculate Overland Flow Time of Concentration in Minutes (Tc) or use default 10 User Calculate
min
0 Min.
8 Determine the average rainfall intensity (i) from IDF Curve based on Tc
y Calculate the Post -Development peak discharge (QPeak)
i
QPeak
2.58
3.25
2.58 in hr
3.25 cfs
10 Calculate total runoff vol (V) (for sizing primary storage)
V
4,351
4,351 ft
V = Ci (Tc=60)Ax3600
11 Calculate Volume of Runoff Reduction Vrr
Enter Percentile Storm I (95th percentile = 0.60 in)
80th
0.34 in
Enter Runoff Reduction Vol (95th Percentile=0.60-in x Area x C)
Vrr
1,541
1,541 ft'
12 Detention: Approved Discharge Rate to Surface Waters (if applicable)
0.00 cfS
13 Volume Summary
Surface Storage: Basin
Basin Forebay
V
435
ft'
Primary Treatment/StorageBasin
V
3,916
ft'
Subsurface Storage
Volume Without Sediment Factor (See BMP 20 Tab)
V
4,351
4,351 ft'
YES
flirk to Chnw Mnre 4ihhacinc f1
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
Subbasin
1
Subbasin 2
3
4
5
Subbasin 6
7
8
9
10
21,608
7,699
25,237
28,759
1.91
0.90
0.90
0.90
0.20
0.66
Estimated Runoff Coefficients for Various Surface
Type of Surface
Runoff Coefficients "I
Business
Downtown areas
0.70-0.95
Urban neighborhoods
0.50-0.70
Residential
Single Family
0.35-0.50
Multi -family
0.60-0.75
Residential (rural)
0.25-0.40
Apartment Dwelling Areas
0.70
Industrial and Commercial
Light areas
0.80
Heavy areas
0.90
Parks, Cemeteries
0.10-0.25
Playgrounds
0.20-0.35
Railroad yard areas
0.20-0.40
Unimproved areas
0.10-0.30
Streets
Asphalt
0.95
Concrete
0.95
Brick
0.95
Roofs
0.95
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\22001 Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_ SD_CALCS_022322
Version 10.5, November 2018
6/2/2022, 6:23 PM
(E IDS IAN�
HD Calculation Sheet for Sizing Seepage Bed With Optional Chambers
APPROVED t is intended to be a guideline to standardize ACHD checking of drainage calculations and shall not replace the
In methodology. These calculations shall establish a minimum requirement. The Engineer's methodology must result in
DAM p8/22/22 exceed these calculations in order to be accepted.
pulls information from the "Peak QV" tab
FILE NUMBERost-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)
3 Design Storm
4 Weighted Runoff Coefficient C
5 Area A (Acres)
6 Approved discharge rate (if applicable)
7 Is Seepage Bed in Common Lot? Yes
100
0.66
1.91 acres
0.00 cfs
V 4,351
Linkto: Ov
QV TR55
4,351 ft3 O%Sediment
8 Set Total Design Width of All Drain Rock
W
14.0 ft
9 Set Total Design Depth of All Drain Rock
D
11.0 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
8.00 in/hr
12 Size of WQ Perf Pipe (Pert 180°)
Dia pipe
18 in
13 Size of Overflow Perf Pipe (Perfs 360% READ if Q100>3.3 cis
in
14 Calculate Total Storage per Foot
Spf
71.1
71.1 ft3/ft
15 Calculate Design Length
L
61
62 ft
Override Value Required for Chambers
16 Variable Infiltration Window L
SWL
62
62.0 ft
17 Variable Infiltration Window W
SWW
14.0
147 ft
18 Time to Drain
6.8 hours
90%volume in 48-hours minimum
19 Length of WQ & Overflow Perf Pipes
62
62 ft
20 Perf Pipe Checks. Qperf >= Qpeak;
where Qperf=CdxAxV(2xgxH)
Optional Storage Chambers 9r
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
6 Chamber Storage Volume, With Rock, Per Manuf
74.90 ft3
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\22001 Skybreak Sub Ph2\Reports\Storm Drainage\ACHD_SD_CALCS_022322 6/2/2022, 6:24 PM
Version 10.0, May 2018