HomeMy WebLinkAboutStorm Water Design Report 061022 Storm Water Design Report
Lots 4 & 5, Block 18, Baxter Meadows Subdivision, Phase IIA
Located in the S 1/2 of Section 34, T. 1S., R. 5E., P.M.M.
City of Bozeman, Gallatin County, Montana
Prepared By:
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June, 2022
STORM WATER OVERVIEW
Storm water runoff from the full development of Lots 4 & 5 was accounted for in the sizing
of the Baxter Meadows Subdivision storm water infrastructure. The storm water
infrastructure installed with the subdivision includes roadway and adjacent parking
asphalt, curb and gutter, underground storm sewers, storm water retention/detention
facilities.
LOT 4 & 5 STORM WATER MANAGEMENT
Attached in Appendix A are relevant sections of the Storm Water Design Report prepared
for Baxter Meadows Subdivision, Phase 2A. The project site is located within Drainage
Basin #3 and highlighted on the "Basin 3 Drainage Area Map". The original Detention
Basin #3 calculations are also attached highlighting the post-development runoff
coefficient assumed for the site (C=0.60-Commercial Neighborhood).
Lots 4 & 5 combined contain a gross area of 11,814 square feet. The proposed
development of Lots 4 & 5 will include:
Roof Area (C=0.85) = 6,105 sq ft
Concrete/Asphalt Area (C=0.90) = 1,827 sq ft
Landscaping (C=0.30) = 3,882 sq ft
The Composite C coefficient for the proposed developed area is:
C = (0.85(6,105) + 0.90(1,827) + 0.30(3,882)) / 11,814
C = 0.68
The proposed site development includes a slightly higher impervious area than that which
was assumed with the original Baxter Meadows Phase 2A infrastructure design report (C
of 0.68 vs. C of 0.60). To avoid unplanned impacts to the existing storm water
infrastructure we are proposing to construct a shallow swale retention area to retain the
surface runoff from the portion of the site between the proposed buildings. This area is
identified as Drainage Area #1 on the "Drainage Area Exhibit" attached within Appendix
A.
Attached in Appendix A is a "Storm Water Design Calculations" spreadsheet comparing
the 10-year storm water volumes for the assumed site runoff (C=0.60) and the proposed
site runoff (C=0.68). The calculations show that a minimum on-site retention volume of
68 cubic feet is required to limit the off-site flows to the original projection. The proposed
shallow storm water retention swale will provide a volume of 70 cubic feet which is
adequate to handle the minimum of 68 cubic feet required for the project and the
calculated 57 cubic feet required to handle all of Drainage Area #1.
Although this site is less than 1-acre and this standard is not required to be met, we will
also check that the pond is sized to capture the runoff generated from the fire 0.50 inches
of rainfall from a 24-hour storm. A half inch of rain falling on Drainage Area #1 (2,037
feet) with a runoff coefficient of 0.41 equates to a total rain volume of 35 cubic feet of
water. The 70 cubic feet of storage available within the proposed ponds is more than
adequate to capture this volume.
The proposed on-site storm water facilities are to be maintained by the Lot 4 & 5 owners.
Attached in Appendix A is a copy of the "Storm Water Maintenance Plan" outlining the
owners maintenance responsibilities for the on-site retention swale. The existing off-site
storm water conveyance and detention facilities are to continue to be maintained by the
Baxter Meadows Subdivision property owner's association. The owner's association
maintenance responsibilities are spelled out in the subdivision covenants and
homeowners association documents.
Appendix. A
Baxter Meadows Ph2A Drainage Basin 3 Map
Baxter Meadows Ph2A Detention Basin 3 Calcs.
Lot 4 Storm Water Exhibit
Lot 4 Storm Water Design Calculations
Lot 4 Storm Water Maintenance Plan
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Baxter Meadows-Phase II
Detention Basin 3
Design Stom1 Frequency 10 yr
Basin Area(Ac) 12.16
Pre0evelopment CoafBdant C Poet Devolopment Coefficient C
Area(Ac) C Area(Ac) C
Open Space 12.15 0.20 Open Space 0.20
Mod.Rea, 0.35 Mod.Res. 0.35
Dense Res. 0.50 Dense Res. 3.18 0.50
Comm.Neigh. 0.60 Comm.Neigh. 0 0.60
Comm Down 0.80 Comm.Down o.80
Industrial 0.80 Industrial 0.80
Total Area: 12.15 Compoape C: 0.2 Total Area: 12.16 Composite C: 0.57
Pre-Development Posl43evelopment
Runoff Distance 1300 Q Runoff Dletance 1900 it
Average Slope 0.80 % Average Slops 0.50 %
Time of Concentration 67 min Time of Concentration 38 min
Average Rainfall intensity 0.60 idhr Averepe Rainfall Intensity 0.68 mr
Maximum Runoff 1.45 Cie Maximum Runoff 8.01 ds
Storm Intensity Developed Developed Undeveloped Required
Duration 10 Yr Flower Runoff Runoff Storage
(min) (k tx) (c1s) (d) (on (CO
5 3.22 22.46 8735 434 63W
10 2.05 14:31 8684 869 7715
15 1.58 10.99 9892 1303 8690
Z0 1.31 9.12 109.40 1737 9203
26 1.13 7.89 11829 2171 9868
;. 30 1.00 7.00 12809 2800 10003
35 0.91 6.34 13308 3040 10258
40 0.83 6.81 13844 3474 10470
45 0.77 ' 8.38 14531 3908 10023
50 0.72 5.03 14077 4343 10734
65 0.68 4.72 155" 4777 10611
80 0.84 4.46 10070 5211 10656
O5 0,$1,, AW, a + �rp mot; 14 i+ib t
70 0.58 4.04 1e981 0080 10582
r 76 0.66 3.86 17376 6614 10662
80 0.53 Wo 17773 69" 10524
86 0.51 3.68 18164 7383.; 10771
90 0.40 3.43 1$521 7817 10704
96 0.47 3.31 18875 8261 10023
100 DA8 3.20 10216 am 10531
106 0.44 3.10 19647 9120 10428
110 0.43 3.01 tons 9654 10314
115 0.42 2.92 20180 9988 10192
x 120 0.41 2.84 20483 10422 10000
r 126 0.40 2.77 20M 10867 9921
130 0.39 2.70 21005 11201 9774
136 0.38 2.64 21346 11725 9620
140 0.37 2.67 211618 12159 9469
145 0.36 2.62 21885 12694 9292
150 0.36 2.46 22147 13028 0119
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Minimum Basin Area
Minimum Volume¢ 10882 d
Minimum Area(le"basin depth) 7256 of Actual Area(shaped to drain) 146M of
i
Minimum Area for Sediment Control->146 of per 1 cis
Peak Developed Flow > 4.24 cis
Minimum Area= 615 of
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STORM WATER EXHIBIT
LOTS 4 &5, BLOCK 18, BAXTER MEADOWS SUBDIVISION, PHASE IIA
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SEWER SEF'JICE 4'SEWER SERVICE� 1
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ELEV•4890.60 I I INSTALL SHALLOW BIDBWAL
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#22001
Storm Water Design Calculations
Project: Lots 4&5 of Block 18, Baxter Meadows Phase 2A Date: 6/10/2022
From City of Bozeman Design Standards: Q=CIA
V=7200Q
Q=runoff(cfs)
C=Weighted C factor
I=Rainfall Intesity(in/hr)
A=Area(acres)
V=Volume(ft)
Rainfall Intensities(from figures 1-2&1-3)
110=0.4079 in/hr(10 yr,2 hr event)
125=0.5005 in/hr(25 yr,2 hr event)
150=0.5822 in/hr(50 yr,2 hr event)
1100=0.6348 in/hr(100 yr,2 hr event)
Full Site Composite Runoff Coefficient Calculation
Area ft2 Area(acres) C factor
Hardscape= 1,827 0.0419 0.90
Roof= 6,105 0.1402 0.85
Landscaping= 3,882 0.0891 0.30
Total Area= 11,814 0,2712
Weighted C= 0.68
Full Site Retention(C=0.68)
For a 10-year storm event:
Area= 11,814 ft2
Q10= 0.0749 cfs
Vrqd= 539 ft3
Retention Provided by Underlyina Subdivision(C=0.60)
For a 10-year storm event:
Area= 11,814 ft2
Q10= 0.0664 cfs
Vrqd= 478 ft3
Minimum On Site Retention Required=539 ft3-478 ft3=68 ft3
Drainage Area#1 Calculations
Drainage Area#1 Composite Runoff Coefficient Calculation
Area ft2 Area(acres) C factor
Hardscape= 380 0.0087 0.90
Landscaping= 1,657 0.0380 0.30
Total Area= 2,037 0.0468
Weighted C= 0.41
Area= 2,037 ft2
Q10= 0.0079 cfs
Vrqd= 57 ft3
-> On Site Retention Provided=57 ft3
4 Drainage Area#1,and its associated storm water retention swale is large enough
to ensure the subdivision design runoff from this lot is not exceeded. The on-site retention
provided is also sufficiently sized to handle the runoff from Drainage Area#1.
STORM WATER MAINTENANCE PLAN
Lot 4, Block 18, Baxter Meadows Subdivision, Phase IIA
City of Bozeman, Montana
PROPERTY OWNER'S RESPONSIBILITIES
FOR ROUTINE INSPECTION AND MAINTENANCE:
1. Routine Maintenance Activities (1-3 month interval)
• Designate no cut zones in the bottom of basins
• Remove trash, leaves, grass clippings and debris
• Establish a chemical free zone in and around the basin
• Inspect for uniform ponding, and that water disappears within three days
of rain events
2. Annual Maintenance Activities (Annually)
• Cut vegetation to 6" and remove clippings
• Re-establish vegetation on eroded and barren areas
• Remove excess sediment build-up
• Update maintenance plan and inspection log
3. Long Term Maintenance Activities (5-10 year interval)
• Consult a qualified professional to inspect and return storm water basin
back to initial design found on the subdivision and/or site plan engineering
plans.
• Dredge basin if sediment buildup is greater than 6"
• Re-establish vegetation
22001\Storm Water Maintenance PIan.Doc