HomeMy WebLinkAbout029 Stormwater Management PackageGran Cielo I Block 4 Minor Subdivision
Preliminary Plat Application
29 – Stormwater Management
City of Bozeman Staff has approved a partial waiver for this section during the pre-application process.
Attached is a supplementary Memo.
895 Technology Blvd Suite 203 Bozeman, MT 59718 (406) 586-0262
895 Technology Boulevard, Suite 203, Bozeman, MT 59718 | 406.586.0262
March 18th, 2025
City of Bozeman Planning Department
20 East Olive
Bozeman, MT 59715
We have prepared this memo as part of our Preliminary Plat application for Gran Cielo Block 4
Minor Subdivision. This memo hopes to address the comment provided on Gran Cielo Block 4
Minor Subdivision Pre-Application. The comment is provided below:
” - A.10 – Stormwater management. Provide a supplemental memo from a professional engineer
registered in the State of Montana demonstrating that adequate stormwater capacity exists to serve
the proposed subdivision.”
We have provided the previously approved Stormwater report that shows the previously accepted
stormwater capacities. We have also provided the following information with supplementary
calculations showing the increase in runoff with the addition of 2 lots. By comparing the
previously approved reports to the proposed subdivision, this shows that we have adequate
stormwater capacity.
Stormwater
With the addition of two lots, there will be two additional single family detached households. Our
calculations were based off the conservative assumptions that Block 4 would be 60% impervious
with 5 lots and 80% Impervious 7 lots. The increase of impervious area would come with the
addition of two new buildings, driveways, and walkways. A 20% increase a conservative estimate,
but was used to demonstrate there would still be adequate stormwater runoff capacity.
Refer to Appendix A for existing stormwater plan and layout for Gran Cielo I. Table 1 below
shows the previously accepted required volume for the stormwater Pond 1. Block 4 is a part of
Basin 13, and all runoff contributes to Pond 1. Page 2 in that report identifies that the required
volume for Pond 1 is 12,105 cubic feet. The Basins approximate size is 14,427 cubic feet,
meaning Pond 1 could support an additional 2,322 cubic feet of stormwater runoff.
Project: Gran Cielo Block 4 Subdivision
Page 2 of 4
895 Technology Blvd Suite 203 Bozeman, MT 59718 (406) 586-0262
Our calculations estimated that the addition of two lots would create an increase of 286 cubic feet.
Block 4 would contribute 1,267 cubic feet of stormwater runoff with 5 lots and 1,553 cubic feet
with the addition of 2 lots. This is a negligible increase compared to the Pond 1’s potential
capacity.
Attached: Appendix A: Gran Cielo Subdivision Drainage Basin Calculations
Appendix B: Previously Accepted Gran Cielo I Stormwater Report
WWC/Madison Engineering
______________________
Erik Ringsak, PE Department Manager
Project: Gran Cielo Block 4 Subdivision
Page 3 of 4
895 Technology Blvd Suite 203 Bozeman, MT 59718 (406) 586-0262
APPENDIX A
Gran Cielo I Subdivision Drainage Basin Calculations
Gran Cielo I
Block 4 Minor Sub
Stormwater Detention/Retention Calculations
Detention Pond 1
Calculation of Required Volume for Storm Water Retention & Infitration System
Design Rainfall Freq.10 10 Year - 2 Hour event
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
5 Lot Calculations:
Total Basin Area:30,152 sf
C
Areas (ft2):
Impervious 18,091 0.90
Landscaped 12,061 0.20
Total:30,152 0.62
Retention Basin Calculation:
Q = CIA
C =0.62 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.69 acres
Qpost =0.18 cfs
Required retention storage (ft3) =1,267 ft3 (10-yr, 2-hr storm)
7 Lot Calculations:
Total Basin Area:30,152 sf
C
Areas (ft2):
Impervious 24,122 0.90
Landscaped 6,030 0.20
Total:30,152 0.76
Retention Basin Calculation:
Q = CIA
C =0.76 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.69 acres
Qpost =0.22 cfs
Required retention storage (ft3) =1,553 ft3
Additional Stroage required with the addition of 2 Lots:286 ft3
Additional Storage available for Pond I:2,322 ft3
* See Gran Cielo I
Subdivision Stormwater
design report page 2
Detention Pond Requirements.xlsx
Project: Gran Cielo Block 4 Subdivision
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895 Technology Blvd Suite 203 Bozeman, MT 59718 (406) 586-0262
APPENDIX B
Previously Accepted Gran Cielo I Stormwater Report