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DESIGN REPORT
STORMWATER MANAGEMENT
LOTS 2 - 4, BLOCK 1
GLEN LAKE COMMERCE SUBDIVISION
Prepared for:
Bridger Skyline Commercial Lofts
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
Project Number: 210418
May 2021
INTRODUCTION
The proposed building with seven residential/shop units is located on Lots 2-4, Block 1 of Glen
Lake Commerce Subdivision in Bozeman, Montana. A combination of site grading, curb and
gutter, drywells, and existing subdivision stormwater infrastructure will be used to manage
stormwater runoff for the development of the site. A Drainage Area Map is included in Appendix
A. Calculations for each individual drainage area are included in Appendix B. Photos of the
existing retention pond for the subdivision are included in Appendix E.
DRAINAGE AREAS
Drainage Area 1
Drainage Area 1 consists of the proposed drive approach, a portion of buildings B & D, the
surrounding sidewalks and landscaped areas. Runoff from Drainage Area 1 flows via sheet flow,
downspouts, and curb & gutter into Iron horse Road. The runoff is then carried north into the
existing subdivision retention pond in Lot 9. It was found that the 10-year, 2-hour storm generated
695 cubic feet of runoff.
Drainage Area 2A
Drainage Area 2A consists of a portion of buildings C & D and the northern half of the parking
lot. Runoff from Drainage Area 2A flows via sheet flow, curb & gutter, and a valley gutter into
the proposed Drywell # 1. The drywell was sized according to City of Bozeman Design Standards
to retain the volume from the 10-year, 2-hour storm event. DA 2A will generate 564 cubic feet of
runoff, the proposed Drywell #1 will provide 588 cubic feet of storage. Storage calculations are
included in Appendix C.
Drainage Area 2B
Drainage Area 2B consists of a portion of buildings A & B and the southern half of the parking
lot. Runoff from Drainage Area 2B flows via sheet flow, curb & gutter, and a valley gutter into
the proposed Drywell #2. The drywell was sized according to City of Bozeman Design Standards
to retain the volume from the 10-year, 2-hour storm event. DA 2B will generate 546 cubic feet of
runoff, the proposed Drywell #2 will provide 551 cubic feet of storage. Storage calculations are
included in Appendix C.
Drainage Area 3
Drainage Area 3 consists of a portion of buildings A&C and the surrounding sidewalks and
landscaped areas. Runoff from Drainage Area 3 flows via sheet flow and downspouts into Manley
Road. The runoff is then carried north into the existing subdivision retention pond in Lot 9. It was
found that the 10-year, 2-hour storm generated 700 cubic feet of runoff.
EXISTING RETENTION POND ADEQUACY CHECK
The existing retention pond for the entire subdivision is located on Lot 9, Block 2 and was sized
according to City of Bozeman Design Standards to retain the volume from the 10-year, 2-hour
storm event. The subdivision stormwater design report by Madison Engineering calculated the
required volume for the entire subdivision as 17,364 cubic feet using a C value of 0.65 for all
future developable lots, including Lots 2-4, Block 2. A retention pond with a volume of 19,275
cubic feet was installed providing an excess of 1,911 cubic feet of storage capacity. Using a C
value of 0.65 it was determined that Lots 2-4, Block 2 generated 2,015 cubic feet of runoff prior
to development. The actual C value for the proposed development of Lots 2-4, Block 2 was
calculated as 0.81, which results in an excess of 491 cubic feet of runoff. As described earlier in
the report only DA 1 & 3 contribute runoff to the subdivision retention pond. Combined the site
will send 1,395 cubic feet to the existing retention pond which is approx. 70% of the volume Lots
2-4 contributed prior to development.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
Pre Development Runoff Calculations
1. Calculate Required Volume
Q = CIA
V=7200Q
A = Area (SF)45988 Lots 2-4
C = Weighted C Factor 0.65
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)1.06
Q = RUNOFF (cfs)0.28
V = ALLOWED VOL (ft3)2015
POST DEVELOPMENT RUNOFF VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Landscape 0.2 8687 1737
Hardscape 0.95 37301 35436
Total 45988 37174
A = Area (acres)1.06
C = Weighted C Factor 0.81
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.81
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)1.06
Q = RUNOFF (cfs)0.35
V = REQUIRED VOL (ft3)2506
DRAINAGE AREA #1
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Landscape 0.2 4449 890
Hardscape 0.95 9921 9425
Total 14369 10314
A = Area (acres)0.33
C = Weighted C Factor 0.72
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.72
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.33
Q = RUNOFF (cfs)0.10
V = REQUIRED VOL (ft3)695
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1
Contributing Area Area (ft 2 )
Hardscape 9921
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.67 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.69 decimal
A = Entire drainage area 0.33 acres
RRV = Runoff Reduction Volume 0.0092 acre-ft
RRV = Runoff Reduction Volume 402 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the existing retention pond is sized to handle the larger volume (695 cf).
DRAINAGE AREA #2A
RUNOFF VOLUME FROM DA#2A
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Landscape 0.2 174 35
Hardscape 0.95 8776 8337
Total 8949 8372
A = Area (acres)0.21
C = Weighted C Factor 0.94
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.94
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.21
Q = RUNOFF (cfs)0.08
V = REQUIRED VOL (ft3)564
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2A
Contributing Area Area (ft 2 )
Hardscape 8776
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.93 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.98 decimal
A = Entire drainage area 0.21 acres
RRV = Runoff Reduction Volume 0.0080 acre-ft
RRV = Runoff Reduction Volume 348 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed Drywell # 1 is sized to handle the larger volume (564 cf).
DRAINAGE AREA #2B
RUNOFF VOLUME FROM DA#2B
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Landscape 0.2 167 33
Hardscape 0.95 8497 8073
Total 8665 8106
A = Area (acres)0.20
C = Weighted C Factor 0.94
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.94
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.20
Q = RUNOFF (cfs)0.08
V = REQUIRED VOL (ft3)546
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2B
Contributing Area Area (ft 2 )
Hardscape 8497
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.93 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.98 decimal
A = Entire drainage area 0.20 acres
RRV = Runoff Reduction Volume 0.0077 acre-ft
RRV = Runoff Reduction Volume 337 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed Drywell #2 is sized to handle the larger volume (546 cf).
DRAINAGE AREA #3
RUNOFF VOLUME FROM DA#3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2 )C * Area
Landscape 0.2 3897 779
Hardscape 0.95 10108 9602
Total 14005 10382
A = Area (acres)0.32
C = Weighted C Factor 0.74
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.74
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.32
Q = RUNOFF (cfs)0.10
V = REQUIRED VOL (ft3)700
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft 2 )
Hardscape 10108
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.70 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.72 decimal
A = Entire drainage area 0.32 acres
RRV = Runoff Reduction Volume 0.0094 acre-ft
RRV = Runoff Reduction Volume 408 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the existing retention pond is sized to handle the larger volume (700 cf).
APPENDIX C
DRYWELL SIZING CALCULATIONS
DRAINAGE AREA #2A
DRYWELL #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 173.66 35
Hardscape 0.95 8775.65 8337
Total 8949 8372
A = Area (acres)0.2054
C = Weighted C Factor 0.94
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.94
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.21
Q = Runoff (cfs)0.08
V = REQUIRED VOL (ft3)564
3. Calculate Drywell Volume
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft)6.5 (see Circular DEQ 4,
Table 2.1-1)
Gravel Void Volume
Gravel Bed Depth (below MH)2.00
Gravel Volume (ft3)1656.18
Gravel Storage Volume (ft3)496.85
Manhole Volume
Manhole Depth (ft)5.00
Manhole Volume (ft3)91.61
Provided Volume (ft3)588
DRAINAGE AREA #2B
DRYWELL #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 167.18 33
Hardscape 0.95 8497.39 8073
Total 8665 8106
A = Area (acres)0.1989
C = Weighted C Factor 0.94
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.94
I = Intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.20
Q = Runoff (cfs)0.08
V = REQUIRED VOL (ft3)546
3. Calculate Drywell Volume
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft)6.5 (see Circular DEQ 4,
Table 2.1-1)
Gravel Void Volume
Gravel Bed Depth (below MH)1.50
Gravel Volume (ft3)1531.34
Gravel Storage Volume (ft3)459.40
Manhole Volume
Manhole Depth (ft)5.00
Manhole Volume (ft3)91.61
Provided Volume (ft3)551
APPENDIX D
EXCERPTS FROM GLEN LAKE COMMERCE
SUBDIVISION STORMWATER DESIGN
REPORT, 2018 (MADISON ENGINEERING)
APPENDIX E
PHOTOS OF EXISTING RETENTION POND