CC - Storm Drainage Calcs STORM DRAINAGE REPORT AND CALCULATIONS
FOR
IMPRESSIVE EAST RIDGE SUBDIVISION NO. 2
MERIDIAN, IDAHO
ENS FO PREPARED BY:
Q `W1
16970 CORINNE M GRAHAM, PE
July 8, 2020
CIVIL SIT E
W O R K S
335 E. 36TH STREET
GARDEN CITY, ID 83714
(208) 946-3874
cgraham@cswengineering.com
INTRODUCTION
The existing 7.91-acre site for the proposed Impressive East Ridge Subdivision No. 2 is currently vacant
ground that is bordered by Phase 1 on the north and east.The proposed development includes 38 single
family residential lots, 9 common lots, and a private road, landscaping, parking, and utilities.
HISTORIC DRAINAGE
The existing project site is located north of Lake Hazel Road between Eagle and Locust Grove Roads.
Historically the majority of the site drains from southwest to northeast to the Grimmett Lateral. Slopes
on site vary from relatively flat to moderately steep.
Surficial soils are comprised of lean clays and sandy silt.At depths of approx. 7 feet,free draining poorly
graded gravels were encountered during geotechnical exploration. Groundwater was not encountered
in any test pits on site nor has groundwater and groundwater is expected to be well below storm design
elevations.
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 the City of Meridian and ACHD storm water policies. 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 801h percentile storm event. Storm water runoff from 40' behind the sidewalk, including
residential driveways and part of the building rooftops, will be conveyed to the private stormwater
system to be owned and maintained by the HOA. Remaining lot drainage will be contained within
individual lots.
Storm water is conveyed via curb and gutter to drain inlets within the roadway then piped to
underground seepage beds where it is treated and finally infiltrated. Storm drain manholes with a 2 foot
sump will be installed to provide pre-treatment for the water quality event. Overflow runoff will be
directed straight to the seepage bed by means of an overflow manhole.
APPENDICES
• Drainage Basin Map
• Storm Drainage Calculations
• Project Geotechnical Report
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IMPRESSIVE EAST RIDGE SUB NO. 2
DRAINAGE BASIN MAP SCALE: 1' = 120'
Prepared By:Corinne Graham,PE
Date:July 8,2020
CIVIL SIT E
Impressive East Ridge Subdivision No. 2
Storm Drainage Calculations
Boise Area IN Data
Intensity(inches/hour)
Design Storm 25 100
Tc
10 min 1.85 2.58
60 min 0.69 0.96
Basin Characteristics
Basin#1 Basin#2
Total Area(SF) 200061 134531
Roadway Area(SF) 54147 34362
Building/Drive Area(SF) 69255 49700
Landscape Area(SF) 76659 50469
C Value: 0.63 0.64
Peak Flow Calculations
25-year 100-year
Intensity(Tc=10 min) 1.85 2.58
Basin#1 Peak Flow(CFS) 5.37 7.49
Basin#2 Peak Flow(CFS) 3.64 5.08
Storage Volume Calculations
1-hr 100-year rainfall 0.96 inch
Basin#1 Basin#2
Water Quality Volume(CF) 3552 2409 80th Percentile Storm(0.34 inch)
Water Quality Flow(CFS) 0.99 0.67 Water Quality Volume/3600 sec
100-year storage volume(CF) 1 10028 1 6803 V=C*A*Total 1 hr Rainfall
Prepared By:Corinne Graham,PE
csW Date:July 8,2020
CIVIL SIT E
Catch Basin Characteristics
CB#1A CB#1B CB#1C CB#1D CB#2A CB#2B
Total Area(SF) 42692 15907 96984 44478 75771 58760
C Value: 0.63 0.63 0.63 0.63 0.64 0.64
Peak Flow Calculations
25-year 100-year
Intensity(Tc=10 min) 1.85 2.58
CB#1A Peak Flow(CFS) 1.15 1.60
CB#1B Peak Flow(CFS) 0.43 0.60
CB#1C Peak Flow(CFS) 2.60 3.63
CB#11)Peak Flow(CFS) 1.19 1.66
CB#2A Peak Flow(CFS) 2.05 2.86
CB#2B Peak Flow(CFS) 1.59 2.22
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.
�Fps for Peak Dis
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 Private Basin#1
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 Subbasin
1 Subbasin 2 3 4 5 6 7 8 9 10
5 Area of Drainage Subbasin(SF or Acres) SF 54,147 69,255 76,659
Acres 4.59
6 Determine the Weighted Runoff Coefficient(C) 0.90 0.90 0.20
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avgl 0.63
7 Calculate Overland Flow Time of Concentration in Minutes(Tc)or use default User calculate
10 min 10 Min. Estimated Runoff Coefficients for Various Surface
Type of Surface Runoff Coefficients"I
8 Determine the average rainfall intensity(i)from IDF Curve based on i 2.58 2.58 in hr Business
Downtown areas 0.70-0.95
9 Calculate the Post-Development peak discharge(QPeak) Qp-k 7.49 7,49 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 10,028 10,028 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 3,552 3,552 ff' 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
Basin Forebay V 1,003 f Asphalt 0.95
t'
Concrete 0.95
Primary Treatme nt/Sto rage Basin V 9,025 ft' Brick 0.95
Subsurface Storage Roofs 0.95
Private Facility-No Sediment Factor Req'd V 10,028 10,028 ft' Gravel 0.75
Fields:Sandy soil Soil Type
Slope A B C
Flat:0-2% 0.04 0.07 0.11
Average:2-6% 0.09 0.12 0.15]
Steep:>6% 0.13 0.18 0.23
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\19013 Eastridge Sub Phase 2\Reports\Storm Drainage\ACHD_SD_CALCS_12-15-16 7/8/2020,3:03 PM
Version 9.0,December 2016
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 Private Basin#1
2 Enter number of Seepage Beds(25 max) 1
3 Design Storm 100
4 Weighted Runoff Coefficient C 0.63 Link to: LQV
QV TR55
5 Area A(Acres) 4.59 acres
6 Approved discharge rate(if applicable) 0.00 cfs
7 Design Volume V 10,028 8,588 ft3
1st Hour of Drainage 1440
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 1800) Dia pipe lb
13 Size of Overflow Perf Pipe(Perfs 3600),READ if Q100>3.3 cfs
14 Calculate Total Storage per Foot Spf ft3/f
Spf=Apf=WxD-APerf_PipexVOids+1/2 Perf—Area
15 Calculate Design Length L
Override Value Required for Chambers
16 Variable Infiltration Window L SWL 108
17 Variable Infiltration Window W SWW
18 Time to Drain
90%volume in 48-hours minimum OK
19 Length of WQ&Overflow Perf Pipes 108
20 Perf Pipe Checks.Qperf>=Qpeak; OK
where Qperf=CdxAxV(2xgxH)
18 in 15.8
in 0.0
C:\Users\cgrah\Dropbox\Projects\19013 Eastridge Sub Phase 2\Reports\Storm Drainage\ACHD_SD_CALCS_12-15-16 7/8/2020,3:03 PM
Version 9.0,December 2016
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.
�Fps for Peak Dis
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 Private 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 Subbasin
1 Subbasin 2 3 4 5 6 7 8 9 10
5 Area of Drainage Subbasin(SF or Acres) SF 34,362 49,700 50,469
Acres 3.09
6 Determine the Weighted Runoff Coefficient(C) 0.90 0.90 0.20
C=[(C1xA1)+(C2xA2)+(CnxAn)]/A Weighted Avgl 0.64
7 Calculate Overland Flow Time of Concentration in Minutes(Tc)or use default User calculate
10 min 10 Min. Estimated Runoff Coefficients for Various Surface
Type of Surface Runoff Coefficients"I
8 Determine the average rainfall intensity(i)from IDF Curve based on i 2.58 2.58 in hr Business
Downtown areas 0.70-0.95
9 Calculate the Post-Development peak discharge(QPeak) Qp-k 5.08 5.08 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 6,803 6,803 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 2,409 2,409 ff' 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
Basin Forebay V 680 { Asphalt 0.95
t'
Concrete 0.95
Primary Treatme nt/Sto rage Basin V 6,123 ft' Brick 0.95
Subsurface Storage Roofs 0.95
Private Facility-No Sediment Factor Req'd V 6,803 6,803 ft' Gravel 0.75
Fields:Sandy soil Soil Type
Slope A B C
Flat:0-2% 0.04 0.07 0.11
Average:2-6% 0.09 0.12 0.15]
Steep:>6% 0.13 0.18 0.23
Adapted from ASCE
C:\Users\cgrah\Dropbox\Projects\19013 Eastridge Sub Phase 2\Reports\Storm Drainage\ACHD_SD_CALCS_12-15-16 7/8/2020,3:03 PM
Version 9.0,December 2016
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 Private Basin#2
2 Enter number of Seepage Beds(25 max) 1
3 Design Storm 100
4 Weighted Runoff Coefficient C 0.64 Link to: LQV
QV TR55
5 Area A(Acres) 3.09 acres
6 Approved discharge rate(if applicable) 0.00 cfs
7 Design Volume V 6,803 5,833 ft3
1st Hour of Drainage 970
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(Perf 1800) Dia pipe 18 in
13 Size of Overflow Perf Pipe(Perfs 3600),READ if Q100>3.3 cfs in
14 Calculate Total Storage per Foot Spf 60.2 ft3/ft
Spf=Apf=WxD-APerf_PipexVOids+1/2 Perf—Area
15 Calculate Design Length L 97 97 ft
Override Value Required for Chambers
16 Variable Infiltration Window L SWL 97 ft
17 Variable Infiltration Window W SWW 15.0 ft
18 Time to Drain 5.4 hours
90%volume in 48-hours minimum
19 Length of WQ&Overflow Perf Pipes 97 228 ft
20 Perf Pipe Checks.Qperf>=Qpeak;
where Qperf=CdxAxV(2xgxH)
C:\Users\cgrah\Dropbox\Projects\19013 Eastridge Sub Phase 2\Reports\Storm Drainage\ACHD_SD_CALCS_12-15-16 7/8/2020,3:03 PM
Version 9.0,December 2016