HomeMy WebLinkAboutStormwater Design Report 06-28-2021INTRODUCTION
The proposed Fallon Condominium project includes the construction of 22 condominium units on
Lots 1 and 2, Block 14 of Norton East Ranch Subdivision, Phase 4. Lots 1 and 2 are intended to
be aggregated into a single lot totaling 2.099 acres in size.A combination of site grading, curb
and gutter,and drywells will be used to manage stormwater runoff on the site.Supporting
stormwater calculations are attached to this report.A Drainage Area Map is included in Appendix
A.Calculations for each individual drainage area (total area, weighted C factor, and time of
concentration) are included in Appendix B.
DRYWELL DESIGN
All drywells have been sized according to City of Bozeman Design Standards. Retention facilities
are sized to capture the entire volume of the 10-year 2-hour storm event.Calculations used for
sizing the drywells can be found in Appendix C.
EXISTING STORMWATER INFRASTURCTURE –NORTON RANCH SUBD. PH. 4
Existing Detention Ponds #1 and #2.
There are existing Detention Pond Nos.1 and 2, which treat stormwater from the underlying
Norton East Ranch Subdivision and are located in the Norton Ranch Park, just north of May Fly
Street.As shown in the Norton East Ranch Ph. 4 stormwater report,Lots 1 and 2, Block 14,are
both in DA#8 and drain into existing Detention Pond #2 via surface flow and gutters within May
Fly Street.With the Norton East Ranch Phase 4 development, these ponds were expanded to
account for the proposed development within Phase 4.The Norton East Ranch Subd. Ph. 4
Stormwater Design Report can be found in Appendix D
In the Phase 4 Design Report, a C-factor of 0.5 (dense residential) was applied to Lots 1 and 2,
Block 14. Using 2.099 acres and a C coefficient of 0.50, the runoff for the 10-year 2-hour storm
was calculated to be 3,098 cubic feet. Therefore, it would be allowable for 3,098 cubic feet of
stormwater runoff to discharge off site.As shown later in the report, only 1,505 cubic feet of runoff
is being discharged off site into the adjacent street right of ways.All of the post-development site
runoff will drain to Detention Pond #2,with no runoff going to Detention Pond #1.
Drainage Area 1 -Proposed Drywell #1
The proposed Drywell #1 is located on the northwest portion of the property within the drive aisle.
It receives roof runoff from Drainage Area 1,totaling 0.66 acres. Runoff from Drainage Area 1 is
conveyed by sheet flow and curb and gutter flow to proposed Drywell #1.The required drywell
volume was calculated to be 1,527 cubic feet. The proposed drywell volume is 850 cubic feet with
a gravel depth below the structure of 0’ and a gravel offset of 7’. Excavation for the drywell will
go down to native gravels and will be backfilled with a well-draining pit-run.The excess runoff
from Drainage Area 1 is 677 cubic feet,which will flow northward into Fallon Street as an
allowable discharge to the underlying Norton East Ranch Subdivision Phase 4.
Drainage Area 2 -Proposed Drywell #2
The proposed Drywell #2 is located on the northeast portion of the property within the drive aisle.
It receives roof runoff from Drainage Area 2, totaling 0.80 acres. Runoff from Drainage Area 2 is
conveyed by sheet flow and curb and gutter flow to proposed Drywell #2. The required drywell
volume was calculated to be 1,772 cubic feet.The proposed drywell volume is 850 cubic feet
with a gravel depth below the structure of 0’ and a gravel offset of 7’. Excavation for the drywell
will go down to native gravels and will be backfilled with a well-draining pit-run. The excess
runoff from Drainage Area 2 is 922 cubic feet, which will flow northward into Fallon Street as an
allowable discharge to the underlying Norton East Ranch Subdivision Phase 4.
Drainage Area 3
Drainage Area 3 is located along the western, northern, and eastern perimeter of the property and
will discharge a total of 606 cubic feet of stormwater off site via sheet flow, which is an allowable
discharge to the underlying Norton East Ranch Subdivision Phase 4.
Drainage Area 4 –Storm Swale #2
Drainage Area 4 is located within the south-central portion of the property and the 10-year 2-hour
storm event results in a total of 522 cubic feet of stormwater.Runoff from Drainage Area 4 is
conveyed by sheet flow to proposed Storm Swale #2.The proposed storm swale will function as
a retention facility and was designed with a volume of 560 cubic feet. To be conservative,
infiltration was not accounted for and only the volume of the swale was calculated for retention.
Therefore, the proposed Storm Swale #2 will fully retain the stormwater generated within Drainage
Area 4 and off-site discharge will be mitigated.
Drainage Area 5 –Storm Swale #3
Drainage Area 5 is located at the southeast corner of the property and the 10-year 2-hour storm
event results in a total of 61 cubic feet of stormwater.Runoff from Drainage Area 5 is conveyed
by sheet flow to proposed Storm Swale #3.The proposed storm swale will function as a retention
facility and was designed with a volume of 80 cubic feet. To be conservative, infiltration was not
accounted for and only the volume of the swale was calculated for retention. Therefore, the
proposed storm swale #3 will fully retain the stormwater generated within Drainage Area 5 and
off-site discharge will be mitigated.
Drainage Area 6 –Storm Swale #1
Drainage Area 6 is located at the southwest corner of the property and the 10-year 2-hour storm
event results in a total of 75 cubic feet of stormwater.Runoff from Drainage Area 6 is conveyed
by sheet flow to proposed Storm Swale #1.The proposed storm swale will function as a retention
facility and was designed with a volume of 80 cubic feet. To be conservative, infiltration was not
accounted for and only the volume of the swale was calculated for retention. Therefore, the
proposed Storm Swale #3 will fully retain the stormwater generated within Drainage Area 6 and
off-site discharge will be mitigated.
DEPTH TO GROUNDWATER
Groundwater monitoring has been conducted by C&H Engineering during the 2018 calendar year
and the results are included in Appendix D. The monitoring well nearest to Lots 1 and 2,Block
14,were used to verify that the proposed stormwater infrastructure is above the high groundwater
table. The property is located at Monitoring Well (MW)#2.MW#2 had a measured high
groundwater recording of approximately 5.1 feet below ground surface.
The bottom of the proposed underground drywells are approximately 5.0 feet below the proposed
grade.It should be noted that the proposed grade is about one foot higher than existing grade, in
which the previous groundwater depths were measured by.Therefore, the anticipated seasonal-
high groundwater depth is not expected to affect the proposed drywells.
OFF-SITE RUN-ON CONSIDERATIONS
Stormwater run-on from adjacent properties is not expected to adversely impact this development.
The surrounding properties have a slight downgradient slope to the north and, as a result, the only
run-on would be from the adjacent property to the south, which is currently an undeveloped
vegetated pasture. As such, the current conditions would not result is a measurable amount of
run-on to this property. Any future development of the adjacent property to the south would be
required to contain stormwater runoff within their property,and would not be allowed to discharge
stormwater onto this property per City code. Given the existing and potential future conditions
at adjacent properties, off-site run-on would have a negligible effect on this development.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
Lots 1 and 2, Block 14 - Norton Ranch Subdivision Phase 4
Stormwater Design Calculations
Calculate required volume Hardscape 52831
Q = CiA Landscape 38611
V=7200Q
Drainage Area Area (acre)C FACTOR
Hardscape 1.21 0.95
Landscape 0.89 0.20
Total 2.10
Weighted C=0.63
ALL STORMWATER GENERATED ON SITETotal Area =2.10 acres
Weighted C =0.63
Rainfall intensity =0.41 in./hr.(C.O.B Design Standards 10-yr, 2-hr storm)
Runoff, Q =0.55 cfs
Volume, V =3925 ft3
Proportion of allowable C directed towards sub-division0.5 C Factor from TD&H Stormwater Repor (December 2002)
VOLUME OF STORMWATER ALLOWED TO BE DISCARGED OFF SITE INTO NORTON RANCH SUBD. PH. 4
Total Area =2.10 acres
Weighted C =0.50
Rainfall intensity =0.41 in./hr.(C.O.B Design Standards 10-yr, 2-hr storm)
Runoff, Q =0.43 cfs
Volume, V =3098 ft3
TOTAL VOLUME TO BE RETAINED ON SITE 826 ft3
DRAINAGE AREA # 1
DRYWELL 1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 22537 21410
Landscape 0.20 6214 1243
Total 28751 22653
A = Area (acres)0.6600
C = Weighted C Factor 0.79
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.79
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.66
Q = Runoff (cfs)0.21
V = REQUIRED VOL (ft3)1527
3. Calculate Drywell Volume
Existing Soil Condition Gravel
Percolation Rate (min/in)6 (see Circular DEQ 4,
Percolation Rate (ft/hr)0.83 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft)7
Infiltration
Drywell Gravel Area (ft2)278.5
Infilitration Volume (ft3)464.13
Gravel Void Volume
Gravel Bed Depth (below MH)0.00
Gravel Volume (ft3)1040.62
Gravel Storage Volume (ft3)312.19
Manhole Volume Manhole Depth (ft)4.00
Manhole Volume (ft3)73.29
Provided Volume Inc. Perc. (ft3)850
DRAINAGE AREA # 2
DRYWELL 2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 26303 24988
Landscape 0.20 6517 1303
Total 32820 26291
A = Area (acres)0.7534
C = Weighted C Factor 0.80
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.80
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.75
Q = Runoff (cfs)0.25
V = REQUIRED VOL (ft3)1772
3. Calculate Drywell Volume
Existing Soil Condition Gravel
Percolation Rate (min/in)6 (see Circular DEQ 4,
Percolation Rate (ft/hr)0.83 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft)7
Infiltration
Drywell Gravel Area (ft2)278.5
Infilitration Volume (ft3)464.13
Gravel Void Volume
Gravel Bed Depth (below MH)0.00
Gravel Volume (ft3)1040.62
Gravel Storage Volume (ft3)312.19
Manhole Volume Manhole Depth (ft)4.00
Manhole Volume (ft3)73.29
Provided Volume Inc. Perc. (ft3)850
DRAINAGE AREA #3
OFFSITE RUNOFF
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 7055 6702
Landscape 0.20 11462 2292
Total 18517 8995
A = Area (acres)0.4251
C = Weighted C Factor 0.49
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.49
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.43
Q = Runoff (cfs)0.08
V = REQUIRED VOL (ft3)606
DRAINAGE AREA #4
STORM SWALE
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 6822 6481
Landscape 0.20 6274 1255
Total 13096 7736
A = Area (acres)0.3006
C = Weighted C Factor 0.59
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.59
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.30
Q = Runoff (cfs)0.07
V = REQUIRED VOL (ft3)522
3. Calculate Storm Swale Volume
Storm Swale Width (ft)4
Storm Swale Length (ft)140
Storm Swale Depth (ft)0.5
Cross Sectional Area (ft2)4.00
Storm Swale Volume
Storm Swale Volume (ft3)560.00
Provided Volume Inc. Perc. (ft3)560
DRAINAGE AREA #5
STORM SWALE
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 607 577
Landscape 0.20 1660 332
Total 2267 909
A = Area (acres)0.0520
C = Weighted C Factor 0.40
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.40
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.05
Q = Runoff (cfs)0.01
V = REQUIRED VOL (ft3)61
3. Calculate Storm Swale Volume
Storm Swale Width (ft)4
Storm Swale Length (ft)20
Storm Swale Depth (ft)0.5
Cross Sectional Area (ft2)4.00
Storm Swale Volume
Storm Swale Volume (ft3)80.00
Provided Volume Inc. Perc. (ft3)80
DRAINAGE AREA #6
STORM SWALE
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 808 768
Landscape 0.20 1692 338
Total 2500 1106
A = Area (acres)0.0574
C = Weighted C Factor 0.44
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.44
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.06
Q = Runoff (cfs)0.01
V = REQUIRED VOL (ft3)75
3. Calculate Storm Swale Volume
Storm Swale Width (ft)4
Storm Swale Length (ft)20
Storm Swale Depth (ft)0.5
Cross Sectional Area (ft2)4.00
Storm Swale Volume
Storm Swale Volume (ft3)80.00
Provided Volume Inc. Perc. (ft3)80
APPENDIX C
GROUNDWATER MONITORING DATA
Project Engineer:Matt H
Project:
Well Information:bgs = below ground surface ags = above ground surface
MW-1 MW-2 MW-3 MW-4
10'10'10'10'
0.58 1.71 0.71 0.92
Groundwater Information:
MW-1 MW-2 MW-3 MW-4
02/14/18 6.81 6.29 4.20 4.38
03/13/18 6.94 6.39 4.20 4.34
04/13/18 6.10 5.10 3.12 3.30
05/11/18 6.87 6.36 4.20 4.35
05/25/18 6.78 6.18 4.14 4.31
06/08/18 6.99 6.62 4.37 4.54
06/22/18 6.30 6.51 4.48 4.60
07/06/18 8.64 8.21 5.91 6.10
07/20/18 7.49 7.32 4.85 5.10
08/03/18 7.47 7.32 4.85 5.21
08/17/18 7.57 7.10 5.00 5.21
08/30/18 7.15 6.82 4.50 4.70
09/14/18 7.25 6.85 4.65 4.80
Monitor Well Data
Project Number:161140
Norton East Ranch Subdivision, Phase 4
Project Location:Bozeman
Well ID
Well Depth (feet-bgs)
Top of Well (feet-ags)
Date Depth to Ground Water (feet-bgs)
BB+z z2:1 Δσ EtBqH DCompressible soil layerRigid soil layer
APPENDIX D
NORTON PHASE 4 STORMWATER
DESIGN REPORT
DESIGN REPORT
STORMWATER MANAGEMENT
NORTON RANCH SUBDIVISION, PHASE 4
Prepared for:
Norton Properties, LLC
63020 NE Lower Meadow Road, Suite A, Bend, OR 97702
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
Project Number: 161140
February 2018
INTRODUCTION
The proposed Norton Ranch Subdivision, Phase 4 is a 21-lot subdivision located on a 15.58-acre
parcel in the East Half and the West Half of Section 9, Township 2 South, Range 5 East of P.M.M.,
Gallatin County, City of Bozeman. A combination of site grading, curb and gutter, storm inlets,
and piping will be used to manage stormwater runoff on the site. The stormwater infrastructure
used in this phase is all existing and was installed with Norton Ranch Subdivision, Phase 1.
Supporting stormwater calculations are attached to this report. A Drainage Area Map is included
in Appendix A. Calculations for each individual drainage area (total area, weighted C factor, and
time of concentration) are included in Appendix B.
RETENTION/DETENTION POND DESIGN
All ponds have been sized according to City of Bozeman Design Standards. Retention ponds are
sized to capture the entire volume of the 10-year 2-hour storm event. They are designed with An
effective depth of 1.5 feet, and maximum side slope of 4:1. Existing Detention Pond 1 and Existing
Detention Pond 2 are sized to limit discharge to pre-development rates for the 10-year storm event.
Both ponds have been sized to retain the first 0.5 inches of rain before the outlet structure begins
to discharge. Calculations used for sizing each pond can be found in Appendix C. Design pond
capacities were calculated using volume surfaces in AutoCAD Civil3D.
Existing Detention Pond #1
Existing Detention Pond #1 is located north of May Fly Street, between Bull Frog Drive and Laurel
Parkway. It receives runoff from Drainage Areas 1, 2, 3, and 4, totaling 8.66 acres. The pond
currently receives runoff from Norton Ranch Subdivision, Phase 1. The existing pond volume is
2,904 cubic feet at an effective water depth of 1.5’. For Norton Ranch Subdivision, Phase 4, the
pre-development time to concentration was calculated to be 40 minutes and the pre-development
runoff rate for the 10-yr storm was calculated to be 1.43 cfs. The existing outlet structure
discharges into the existing wetland area in the park to the north. The outlet structure will need to
be raised to retain the first 0.5 inches of stormwater runoff. The new flow out elevation will be set
to 4804.25’ and the slot will be expanded to a width of 2.8”. The new required pond volume was
calculated to be 8,964 cubic feet. The new provided pond volume is 9,146 cubic feet at an effective
water depth of 1.5’ above the seasonal high groundwater level (see Appendix D and the
Geotechnical Investigation Report for Phase 4). See construction plans for the expansion of the
pond and modifications to the outlet structure. In the case of a storm exceeding the 10-yr design
storm, runoff will overflow the outlet structure top grate into the outlet pipe and flow into the
existing wetland area. Supporting calculations for the pond sizing can be found in Appendix C.
Existing Detention Pond #2
Existing Detention Pond #2 is located north of May Fly Street, between Bull Frog Drive and Laurel
Parkway. It receives runoff from Drainage Areas 5, 6, 7, and 8, totaling 14.58 acres. The pond
currently receives runoff from Norton Ranch Subdivision, Phase 1. The existing pond volume is
6,072 cubic feet at an effective water depth of 1.5’. For Norton Ranch Subdivision, Phase 4, the
pre-development time to concentration was calculated to be 58 minutes and the pre-development
runoff rate for the 10-yr storm was calculated to be 1.91 cfs. The existing outlet structure
discharges into the existing wetland area in the park to the north. The outlet structure will need to
be raised to retain the first 0.5 inches of stormwater runoff. The new flow out elevation will be set
to 4803.50’ and the slot will be expanded to a width of 3.7”. The new required pond volume was
calculated to be 14,321 cubic feet. The new provided pond volume is 14,915 cubic feet at an
effective water depth of 1.5’ above the seasonal high groundwater level (see Appendix D and the
Geotechnical Investigation Report for Phase 4). See construction plans for the expansion of the
pond and modifications to the outlet structure. In the case of a storm exceeding the 10-yr design
storm, runoff will overflow the outlet structure top grate into the outlet pipe and flow into the
existing wetland area. Supporting calculations for the pond sizing can be found in Appendix C.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
DRAINAGE AREA #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 12158 8967
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 72612 36306
Existing Development 0.5 0 0
Total 84771 45273
A = Area (acres) 1.95
C = Weighted C Factor 0.53
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.46 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 249 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)18.57
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.52%
L = length of gutter (ft) 214
V = mean velocity (ft/s) 2.17
Tc Gutter Flow (minutes) =1.64
Tc Total = 20.21
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.53 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.56 (25-yr storm)
A = area (acres) 1.95 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 1.63 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.52%
Q = PROVIDED GUTTER CAPACITY (cfs) 2.69
DRAINAGE AREA #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 11868 8752
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 0 0
Existing Development 0.5 0 0
Total 11868 8752
A = Area (acres) 0.27
C = Weighted C Factor 0.74
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 23 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)5.11
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.52%
L = length of gutter (ft) 285
V = mean velocity (ft/s) 2.17
Tc Gutter Flow (minutes) =2.20
Tc Total = 7.31
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.74 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.00 (25-yr storm)
A = area (acres) 0.27 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 0.60 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.52%
Q = PROVIDED GUTTER CAPACITY (cfs) 2.69
DRAINAGE AREA #3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 60231 44420
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 189509 94755
Existing Development 0.5 0 0
Total 249740 139175
A = Area (acres) 5.73
C = Weighted C Factor 0.56
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 0.86 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 350 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)26.30
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.07%
L = length of gutter (ft) 784
V = mean velocity (ft/s) 3.10
Tc Gutter Flow (minutes) =4.21
Tc Total = 30.51
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.56 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.20 (25-yr storm)
A = area (acres) 5.73 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 3.84 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.07%
Q = PROVIDED GUTTER CAPACITY (cfs) 3.85
DRAINAGE AREA #4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 30818 22728
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 0 0
Existing Development 0.5 0 0
Total 30818 22728
A = Area (acres) 0.71
C = Weighted C Factor 0.74
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.18 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 25 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)6.31
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.27%
L = length of gutter (ft) 663
V = mean velocity (ft/s) 3.38
Tc Gutter Flow (minutes) =3.27
Tc Total = 9.59
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.74 (calculated above)
I = 0.78 Tc-0.64 (in/hr)2.52 (25-yr storm)
A = area (acres) 0.71 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 1.32 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.27%
Q = PROVIDED GUTTER CAPACITY (cfs) 4.19
DRAINAGE AREA #5
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 6870 5067
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 18920 9460
Existing Development 0.5 0 0
Total 25790 14527
A = Area (acres) 0.59
C = Weighted C Factor 0.56
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.73 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 206 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)16.00
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.50%
L = length of gutter (ft) 82
V = mean velocity (ft/s) 2.12
Tc Gutter Flow (minutes) =0.64
Tc Total = 16.65
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.56 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.77 (25-yr storm)
A = area (acres) 0.59 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 0.59 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.50%
Q = PROVIDED GUTTER CAPACITY (cfs) 2.63
DRAINAGE AREA #6
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 56228 41468
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 120620 60310
Existing Development 0.5 0 0
Total 176848 101778
A = Area (acres) 4.06
C = Weighted C Factor 0.58
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 0.90 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 170 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)18.05
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.56%
L = length of gutter (ft) 569
V = mean velocity (ft/s) 3.74
Tc Gutter Flow (minutes) =2.53
Tc Total = 20.58
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.58 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.55 (25-yr storm)
A = area (acres) 4.06 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 3.61 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.56%
Q = PROVIDED GUTTER CAPACITY (cfs) 4.64
DRAINAGE AREA #7
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 9297 6856
Park 0.2 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 0 0
Existing Development 0.5 0 0
Total 9297 6856
A = Area (acres) 0.21
C = Weighted C Factor 0.74
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 26 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)5.42
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.31%
L = length of gutter (ft) 165
V = mean velocity (ft/s) 1.66
Tc Gutter Flow (minutes) =1.66
Tc Total = 7.08
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.74 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.06 (25-yr storm)
A = area (acres) 0.21 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 0.48 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.31%
Q = PROVIDED GUTTER CAPACITY (cfs) 2.06
DRAINAGE AREA #8
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 54523 40211
Park 0.2 35811 7162
Low-Med Density Residential 0.35 0 0
Dense Residential 0.5 91442 45721
Existing Development 0.5 241552 120776 *See Norton Ranch Phase
Total 423328 213870 1 Stormwater Design Report
for C Factor
A = Area (acres) 9.72
C = Weighted C Factor 0.51
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.17 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 160 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)16.05
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.91%
L = length of gutter (ft) 845
V = mean velocity (ft/s) 2.86
Tc Gutter Flow (minutes) =4.93
Tc Total = 20.98
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.51 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.53 (25-yr storm)
A = area (acres) 4.17 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 3.27 (assuming no carry flow)
*Note: The existing development was removed for gutter capacity calculations due to a different
time of concentration.
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.91%
Q = PROVIDED GUTTER CAPACITY (cfs) 3.55
APPENDIX C
POND SIZING CALCULATIONS
EXISTING DETENTION POND # 1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.74 115074 84867
OS/Park 0.20 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.50 262121 131061
Existing Development 0.50 0 0
Total 377196 215928
A = Area (acres) 8.66 Storm
C = Weighted C Factor 0.57 Return (yrs)Cf
2 to 10 1
2. Calculate T c (Pre-Development)11 to 25 1.1
Tc Overland Flow 26 to 50 1.2
Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25
S = Slope of Basin (%) 1.47
C = Rational Method Runoff Coefficient 0.2
Cf = Frequency Adjustment Factor 1.1
D = Length of Basin (ft) 780
Tc (Pre-Development) (minutes) 40
3. Calculate Rainfall Intensity (Duration = Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.67 (Tc Pre-Development)
i = rainfall intensity (in./hr.) 0.83
4. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.) 0.83 (calculated above)
A = Area (acres) 8.66 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs) 1.43
5. Calculate Required Pond Volume
Total Area (acres) = 8.66 acres
Weighted C = 0.57
Discharge Rate (cfs) = 1.43 cfs (Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage (ft3)
35 0.58 0.91 4.50 9457 516 8942
36 0.60 0.89 4.42 9551 602 8949
37 0.62 0.88 4.34 9643 688 8955
38 0.63 0.86 4.27 9734 774 8960
39 0.65 0.85 4.20 9823 860 8963
40 0.67 0.83 4.13 9910 946 8964
41 0.68 0.82 4.06 9996 1032 8964
42 0.70 0.81 4.00 10081 1118 8963
43 0.72 0.79 3.94 10164 1204 8960
44 0.73 0.78 3.88 10246 1290 8956
*Note: The outlet structure will not release any water until the initial 8,923 CF of runoff from the first
0.5"of rain has been retained. This occurs within the first 29 minutes of the storm event.
PROVIDED VOLUME (ft3)9,146
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs) 1.43
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft) 1.5
L = Horizontal Length (ft) 0.23
L = Slot Width (inches) 2.8
DETENTION POND #1
REQUIRED STORAGE FOR FIRST 0.5 INCHES OF RAIN
FROM 24-HOUR STORM EVENT
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.74 115074 84867
Park 0.20 0 0
Low-Med Density Residential 0.35 0 0
Dense Residential 0.50 262121 131061
Existing Development 0.50 0 0
Total 377196 215928
C=Weighted C Factor 0.57
2. Calculate Required Volume
Q = CIA
V=22194Q
C = Weighted C Factor 0.57
I = intensity (in/hr) 0.08 (10 yr, 24 hr storm)
A = Area (acres) 8.66
Q = runoff (cfs) 0.40
V = REQUIRED VOL (ft3)8923
*A 10-year, 24-hour storm event produces 0.5" of rain after 6.165 hours.
(I=0.0811in/hr --> 0.5in / 0.0811in/hr = 6.165 hours)
**The required volume was calculated by using the 6.165 hours it takes
to produce the 0.5" of rain. (6.165hr * 60min/hr * 60sec/min * Q = 22194Q)
EXISTING DETENTION POND # 2
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.74 126918 93602
OS/Park 0.20 35811 7162
Low-Med Density Residential 0.35 0 0
Dense Residential 0.50 230982 115491
Existing Development 0.50 241552 120776 *See Norton Ranch Phase
Total 635263 337031 1 Stormwater Design Report
for C Factor
A = Area (acres) 14.58 Storm
C = Weighted C Factor 0.53 Return (yrs)Cf
2 to 10 1
2. Calculate T c (Pre-Development)11 to 25 1.1
Tc Overland Flow 26 to 50 1.2
Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25
S = Slope of Basin (%) 1.02
C = Rational Method Runoff Coefficient 0.2
Cf = Frequency Adjustment Factor 1.1
D = Length of Basin (ft) 1261
Tc (Pre-Development) (minutes) 58
3. Calculate Rainfall Intensity (Duration = Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.97 (Tc Pre-Development)
i = rainfall intensity (in./hr.) 0.65
4. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.) 0.65 (calculated above)
A = Area (acres) 14.58 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs) 1.91
5. Calculate Required Pond Volume
Total Area (acres) = 14.58 acres
Weighted C = 0.53
Discharge Rate (cfs) = 1.91 cfs (Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage (ft3)
45 0.75 0.77 5.97 16119 1833 14286
46 0.77 0.76 5.89 16243 1947 14296
47 0.78 0.75 5.80 16366 2062 14305
48 0.80 0.74 5.72 16487 2176 14311
49 0.82 0.73 5.65 16607 2291 14316
50 0.83 0.72 5.57 16724 2405 14319
51 0.85 0.71 5.50 16841 2520 14321
52 0.87 0.70 5.43 16956 2634 14321
53 0.88 0.69 5.37 17069 2749 14320
54 0.90 0.69 5.30 17181 2863 14318
*Note: The outlet structure will not release any water until the initial 13,927 CF of runoff from the first
0.5"of rain has been retained. This occurs within the first 29 minutes of the storm event.
PROVIDED VOLUME (ft3)14,915
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs) 1.91
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft) 1.5
L = Horizontal Length (ft) 0.31
L = Slot Width (inches) 3.7
DETENTION POND #2
REQUIRED STORAGE FOR FIRST 0.5 INCHES OF RAIN
FROM 24-HOUR STORM EVENT
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Composite ROW 0.74 126918 93602
OS/Park 0.20 35811 7162
Low-Med Density Residential 0.35 0 0
Dense Residential 0.50 230982 115491
Existing Development 0.50 241552 120776
Total 635263 337031
C=Weighted C Factor 0.53
2. Calculate Required Volume
Q = CIA
V=22194Q
C = Weighted C Factor 0.53
I = intensity (in/hr) 0.08 (10 yr, 24 hr storm)
A = Area (acres) 14.58
Q = runoff (cfs) 0.63
V = REQUIRED VOL (ft3)13927
*A 10-year, 24-hour storm event produces 0.5" of rain after 6.165 hours.
(I=0.0811in/hr --> 0.5in / 0.0811in/hr = 6.165 hours)
**The required volume was calculated by using the 6.165 hours it takes
to produce the 0.5" of rain. (6.165hr * 60min/hr * 60sec/min * Q = 22194Q)
APPENDIX D
POND PROFILES