HomeMy WebLinkAboutInfrastructure Design Report_08062021Infrastructure Design Report
(Water, Sanitary Sewer & Storm Sewer)
Oak & Cottonwood Apartments
Lots 4 & 5, Block 5
Flanders Creek Subdivision
Located in the SE 1/4 of Section 4, T. 2S., R. 5E., P.M.M.
City of Bozeman, Gallatin County, Montana
Prepared By:
Revised August, 2021
SANITARY SEWER
Sanitary sewer service for the Oak and Cottonwoods Apartments project will be provided
by extension of a new 8-inch main through the primary east-west drive aisle. The sewer
will connect to the existing main found in Twin Lakes Avenue. 4-inch sewer services
will be extended to each building from the new 8-inch sewer main.
The original Flanders Creek Subdivision infrastructure design assumed a total average
daily flow rate of 4.20 gallons per minute (gpm) for Block 5. The downstream sanitary
sewer lift station, located to the west along Oak Street within Laurel Glen Subdivision, is
approaching its ultimate capacity. To alleviate future capacity issues a new sewer
interceptor line is proposed, and currently under construction, with the Northwest
Crossing Subdivision. The interceptor line will connect to the Oak Street sewer main at
the intersection with Rosa Way diverting flows northward through the Northwest Crossing
Subdivision ultimately discharging into the Norton East Ranch outfall sewer.
The Oak & Cottonwood Apartment project proposes a total of 84 2-bedroom apartments.
Estimated Average Daily Sewer Flow:
84-two bedroom living units
2.17 persons/dwelling unit
64.4 gpdpc
(84 units)(2.17 persons/dwelling unit)(64.4 gpdpc)= 11,739 gpd (8.15 gpm)
Peaking Factor =ொ௫
ொ௩ =ଵ଼ାభ మൗ
ସାభ మൗ (P = Population in thousands)
Peaking Factor = ଵ଼ା(.ଵ଼ଶଷ)భ మൗ
ସା(.ଵ଼ଶଷ)భ మൗ = 4.16
Assumed Infiltration = 150 gallons/acre/day x 3.1292 acres = 469.4 gpd
→ Max. Flow, Qmax = (Qavg x Peaking Factor) + Infiltration
= (11,739 gpd x 4.16) + 469.4 gpd = 49,304 gpd (0.0763 cfs)
The capacity of an 8-inch main, at minimum slope, is checked using Manning’s Equation:
Qfull = (1.486/0.013)AR2/3S1/2
Manning's n = 0.013 for PVC Pipe
Minimum Slope = 0.004 ft/ft
A = (π)r 2 = (3.1416)(0.33)2 = 0.3491 ft2
P = 2(π)r = 2(3.1416)(0.33) = 2.0944 ft
R = A/P = 0.3491/2.0944 = 0.1667 ft
R2/3 = 0.3029 ft
S = 0.004 ft/ft
S1/2 = 0.06325 ft/ft
Qfull = (1.486/0.013)(0.3491)(0.3029)(0.06325) = 0.76 cfs
Qmax/Qfull = 0.0763 cfs/0.76 = 0.10 or 10%
The estimated peak sanitary sewer flow rate generated from this project amounts to 10%
of the capacity of the proposed 8” sanitary sewer main. The downstream mains
proposed within the Northwest Crossing Subdivision and the Norton East Ranch outfall
sewer were sized to accommodate the proposed sanitary sewer from this site.
WATER SYSTEM
Water will be provided to the Oak and Cottonwood project with an extension of the existing
8-inch water main line currently stubbed into Lot 5 from the main line in Flanders Way.
The 8-inch main will be extended into the project within a proposed easement within the
main drive aisle, ultimately terminating with a fire hydrant on Lot 4. All structures will be
served with 2” domestic water service lines and 4” fire service lines.
Water service lines were sized using the fixture count method outlined in the Uniform
Plumbing Code. The fixture count totals and corresponding minimum pipe size per UPC
Table 610.3 and Table 610.4 for each 12-unit building are attached in Appendix A:
12-plex = 264 fixtures – Meter & Street Service = 1.5-inch
Building Supply & Branch = 2-inch
Maximum Allowable Service Length = 272 feet
1.5-inch Type K Copper water service lines and meters will be provided to each building
for domestic service. Supply and branch lines shall be upsized to 2-inches after the
meter.
STORM WATER MANAGEMENT
Attached in Appendix B are relevant sections of the Storm Water Design Report prepared
for Flanders Creek Subdivision. 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, for lots zoned R-4). Runoff from Lots 4 & 5 was assumed
to discharge directly into the “Common Area” detention ponds located immediately to the
north of the lots and into adjacent roadways which also convey storm water to the
“Common Area” ponds.
The storm water runoff patterns for the Oak and Cottonwood Apartments project is
proposed to be consistent with that assumed with the subdivision infrastructure design.
The bulk of the runoff from Lots 4 & 5 will run to two retention areas proposed on the north
side of the property. These retention ponds will serve as an initial storage and settling
basin. Overflow structures and pipes will be provided to allow overflow into the existing
detention basins in the adjacent “Common Area”. Peripheral storm water runoff from
Lots 4 and 5 will flow into the adjacent roadways and landscape areas which will also
convey water to the existing “Common Area” detention pond.
Lots 4 & 5 contain a gross area of 136,305 square feet. The proposed development will
include:
Roof Area (C=0.85) = 31,305 sq ft
Concrete/Asphalt Area (C=0.90) = 69,299 sq ft
Landscaping (C=0.30) = 35,701 sq ft
The Composite C coefficient for the proposed developed area is:
C = (0.85(31,305) + 0.90(69,299) + 0.30(35,701)) / 136,305
C = 0.73
The proposed site development includes a slightly higher impervious area than that which
was assumed with the original Flanders Creek Subdivision infrastructure design report (C
of 0.73 vs. C of 0.60). To avoid unplanned impacts to the existing storm water
infrastructure we are proposing to construct two retention areas to retain the difference
on-site.
Attached in Appendix B 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=0.73). The calculations show that an on-site retention volume of 1,207
cubic feet is required to limit the off-site flows to the original projection. The proposed
shallow storm water retention ponds will provide a volume of 1,502 cubic feet which is
more than sufficient to make up for the additional runoff from this project. The ponds are
designed with overflow structures so that when the ponds reach their capacity level the
storm water will enter the overflow structure and be piped to the existing detention pond
found to the north of the site.
Calculations are also provided in the Appendix B spreadsheet for each of the 5 individual
drainage basins. Again, the calculations show that the combination of existing detention
pond plus proposed retention basins provide sufficient storm water capacity to ensure
that the original design capacities are not exceeded.
We will also check that the ponds are 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 136,305 square
foot site with a runoff coefficient of 0.73 equates to a total rain volume of 5,679 cubic feet
of water. The existing “Common Area” detention ponds provide a storage volume of
19,000 cubic feet and the proposed on-site retention areas add 1,327 cubic feet to this
volume for a total storage volume of 20,327 cubic feet. This total volume is more than
adequate to capture this first 0.5-inches of rainfall.
STORM WATER CONVEYANCE STRUCTURE SIZING
Storm water conveyance structures proposed with this project include sidewalk drainage
chases and underground storm sewer pipes. These structures are located within
Drainage Areas #1, #2 and #3. Storm water conveyance structures are sized to convey
the minimum of a 25-year storm event in accordance with City of Bozeman Design
Requirements.
Drainage Area #1 contains a total area of 32,187 square feet (0.7389 acre). For
simplicity we will conservatively assume that all runoff from this drainage area enters the
storm water conveyance structures.
Time of Concentration
Overland flow(35 ft @ 2% on grass, C=0.3) = 6.0 min(see Figure I-1, Appendix B)
Overland flow(155 ft @ 1.6% on asphalt, C=0.9) = 4.0 min(see Figure I-1, App. B)
Total Time of Concentration = 10 minutes(0.167 hours)
For a 25-year storm event I25 = 0.78X-.64 = 0.78(0.167)-.64 = 2.46 in/hr
Q25 = CIA = 0.76(2.46 in/hr)(0.7389 acres) = 1.38 cfs
Attached in Appendix B are results from a channel flow calculator showing the proposed
1’ wide sidewalk chase at 2% slope will flow at a depth of 3.4 inches with a velocity of
4.94 ft/sec during the peak flow from a 25-year storm event.
Also attached in Appendix B are results from a Manning’s pipe calculator showing that
the proposed 10-inch retention pond #1 overflow pipe will flow, with no surcharging, at a
depth of 5.75 inches with of velocity of 4.26 ft/sec during the peak flow from a 25-year
storm event.
Drainage Area #2 contains a total area of 31,284 square feet (0.7182 acre). For
simplicity we will conservatively assume that all runoff from this drainage area enters the
storm water conveyance structures.
Time of Concentration
Overland flow(45 ft @ 2% on grass, C=0.3) = 6.0 min(see Figure I-1, Appendix B)
Overland flow(100 ft @ 2% on asphalt, C=0.9) = 4.0 min(see Figure I-1, App. B)
Total Time of Concentration = 9 minutes(0.15 hours)
For a 25-year storm event I25 = 0.78X-.64 = 0.78(0.15)-.64 = 2.63 in/hr
Q25 = CIA = 0.69(2.63 in/hr)(0.7182 acres) = 1.30 cfs
Attached in Appendix B are results from a channel flow calculator showing the proposed
1’ wide sidewalk chase at 2% slope will flow at a depth of 3.24 inches with a velocity of
4.85 ft/sec during the peak flow from a 25-year storm event.
Also attached in Appendix B are results from a Manning’s pipe calculator showing that
the proposed 10-inch retention pond #2 overflow pipe will flow, with no surcharging, at a
depth of 5.53 inches with of velocity of 4.20 ft/sec during the peak flow from a 25-year
storm event.
Drainage Area #3 contains a total area of 45,019 square feet (1.0335 acre). For
simplicity we will conservatively assume that all runoff from this drainage area enters the
storm water conveyance structures.
Time of Concentration
Overland flow(30 ft @ 2% on grass, C=0.3) = 6.0 min(see Figure I-1, Appendix B)
Overland flow(215 ft @ 2% on asphalt, C=0.9) = 5.0 min(see Figure I-1, App. B)
Total Time of Concentration = 11 minutes(0.183 hours)
For a 25-year storm event I25 = 0.78X-.64 = 0.78(0.183)-.64 = 2.31 in/hr
Q25 = CIA = 0.77(2.31 in/hr)(1.0335 acres) = 1.84 cfs
Attached in Appendix B are results from a Manning’s pipe calculator showing that the
proposed Drainage Area #3 storm sewer pipe will flow, with no surcharging, at a depth of
5.53 inches with of velocity of 4.20 ft/sec during the peak flow from a 25-year storm event.
The proposed on-site storm water facilities are to be maintained by the Oak & Cottonwood
Apartments owners and their property management team. Attached in Appendix C is a
copy of the “Storm Water Maintenance Plan” outlining the owners maintenance
responsibilities for the on-site retention ponds and conveyance structures. The existing
off-site storm water conveyance and detention facilities are to continue to be maintained
by the Flanders Creek Subdivision property owner’s association. The owner’s
association maintenance responsibilities are spelled out in the subdivision covenants and
homeowners association documents.
STORM WATER MAINTENANCE PLAN
Lots 4 & 5, Block 5, Flanders Creek Subdivision
Oak & Cottonwood Apartments
City of Bozeman, Montana
THE FOLLOWING PROPERTY OWNER'S RESPONSIBILITIES FOR ROUTINE
INSPECTION AND MAINTENANCE ARE TO BE PERFORMED BY THE
LANDOWNER (L&S PROPERTIES) OR THEIR DESIGNATED AGENT. SHOULD
THE PROPERTY CHANGE OWNERSHIP THESE RESPONSIBILITIES SHALL
PASS TO THE NEW PROPERTY OWNER(S).
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 basins
• 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
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