HomeMy WebLinkAbout010 Stormwater Design Report
DESIGN REPORT
STORMWATER MANAGEMENT
JENNIFER PLACE CONDOMINIUMS
LOT 1, BLOCK 1
LAUREL GLEN SUBDIVISION PHASE 1
Prepared for:
Great Western Investment, LLC
15525 Horse Creek Road
Bozeman, MT 59715
Prepared by:
Project Number: 190065
March 2019
INTRODUCTION
The Jennifer Place Condominium Master Site Plan proposes to develop Lot 1, Block 1 Laurel Glen
Subdivision. The proposed development consists of 10 2-unit condominiums with a proposed
access road and associated utilities. A combination of site grading, curb and gutter, drainage
swales and retention ponds will be used to manage stormwater runoff on the site. The proposed
stormwater design retains limits the runoff from the development to below the allocated amount
for the lot.
EXISTING STORMWATER DESIGN
Existing City stormwater infrastructure was installed with the subdivision’s infrastructure
improvements. Using the Bozeman Design Standards and Specifications Policy (DSSP) runoff
coefficient estimation for dense residential (C=0.50) the existing site generates 3,014 cf of runoff.
The proposed layout has a weighted C value of 0.68 which generates 4,082 cf of runoff. Therefore,
a minimum of 1,069 cf of runoff from the development will need to be retained and infiltrated on
site. Supporting calculations and drainage area maps are included in Appendices A & B,
respectively.
PROPOSED STORMWATER DESIGN
The site was divided into 8 drainage areas as shown on the Proposed Drainage Area Map included
in Appendix B. The runoff coefficient for Drainage Area 1 is 0.75 over an area of 24,115 sf which
translates to 1,227 cf of runoff. Drainage Area 1 will be retained in Retention Pond 1 located to the
northwest corner of the site. Retention Pond 1 has a storage volume of 1,346 cf, a top of pond
elevation of 4832.02’, a bottom elevation of 4830.52’, and 4:1 (H:V) side slopes.
Drainage Area 2 consists of the roofs, landscape and hardscape areas on the east and south sides of
the access road. Drainage Area 2 has a runoff coefficient of 0.71 over an area of 41,038 sf which
translates to 1,961 cf of runoff. Drainage Area 2 will runoff into Loxley Drive and Annie Street
which drains to existing subdivision stormwater infrastructure. Drainage Area 3 consists of a
portion of the access road and landscape areas on the east side of the lot. These areas will drain to
Laurel Parkway and deposit into the existing city stormwater infrastructure. Drainage Area 3 has a
composite C value of 0.57. Over an area of 22,910 sf, Drainage Area 3 generates 886 cf of runoff.
Combined these drainage areas will contribute 2,847 cf of runoff to the subdivision infrastructure
which is less than the 3,014 cf that the R-2 lot was estimated to generate.
APPENDIX A
STORMWATER CALCULATIONS
OVERALL SITE
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 20691 19656
Roofs 0.95 35910 34115
Landscape 0.2 32343 6469
Total 88944 60240
C=Weighted C Factor 0.68
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.68
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 2.04
Q = runoff (cfs) 0.57
V = RUNOFF VOL (ft3)4082
REQUIRED RETENTION VOLUME (ft3)1069
DRAINAGE AREA 1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 17716 16831
Landscape 0.2 6399 1280
Total 24115 18110
C=Weighted C Factor 0.75
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.75
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.55
Q = runoff (cfs) 0.17
V = REQUIRED STORAGE VOL. (ft3)1227
DRAINAGE AREA 2
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 27641 26259
Landscape 0.2 13398 2680
Total 41038 28938
C=Weighted C Factor 0.71
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.71
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.94
Q = runoff (cfs) 0.27
V = RUNOFF VOL TO SUBDIVISION (ft3)1961
DRAINAGE AREA 3
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 11324 10758
Landscape 0.2 11586 2317
Total 22910 13075
C=Weighted C Factor 0.57
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.57
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.53
Q = runoff (cfs) 0.12
V = REQUIRED STORAGE VOL. (ft3)886
APPENDIX B
DRAINAGE AREA MAP