HomeMy WebLinkAboutDesign Report Phase 1 Sanitary Sewer Demand Calculations
INTRODUCTION
This project includes the proposed re-development of a portion of the existing Gallatin Valley Mall
property. The re-development includes the demolition and re-development of a portion of the
existing Gallatin Valley Mall and construction of four (4) new commercial buildings. A large
portion of the existing parking lot will also be re-developed to add landscape and sidewalk islands
and re-align the parking stalls. The property is located within the Bozeman city limits and is
currently zoned B-2 commercial. This project is located in the City of Bozemanβs South Pressure
Zone (HGL 5125).
Previously, a Master Site Plan was submitted and conditionally approved by the City of Bozeman.
We received a letter attesting to adequate capacity for the sewer and water distribution systems on
May 2nd, 2023. This report is indented to outline the sanitary sewer demands for the purpose of the
Phase 1 and 2 infrastructure site plan submittals. The proposed Phase 1 and 2 sanitary sewer main
extension is shown on the civil plan set and intends to serve the Phase 1 area and Phase 2 area.
As part of the Phase 1 and 2 projects, a proposed 8β sanitary sewer mains will be extended in the
Phase 1 area to serve the existing re-developed mall located in the Phase 1 area and the four (4)
new commercial buildings located within the Phase 2 area. An existing manhole is located north
of the existing Gallatin Valley Mall building and a proposed 8β sanitary sewer main will be
extended southwestward into Phase 2 from there. The proposed 8β sanitary sewer line will utilize
a total of 4 proposed manholes located as each bend of the proposed sewer main extension.
Sanitary Sewer Design Load
The design parameters used herein are based on the DEQ Circular 4, specifically section 3.1, table
3.1-1. An average daily flow rate of 10 gpd per employee, 3 gpd per customer and 3 gpd per meal
was used. The existing mall is occupied by approximately 120 employees while the new
businesses associated with the redevelopment are estimated to produce approximately 80 new
employees. As such, a total of 200 employee is used for the demand analysis.
The number of proposed customers is based on the foot traffic analytics survey conducted by
Placer Labs, Inc. at the Gallatin Valley Mall for the 2021 calendar year. In summary, there were
1.5 million visits that year, which is approximately 4,110 visits per day. The existing mall is
about 300,000 square feet in size and results in a rate of 0.0137 visits per square foot.
Approximately 50,000 square feet of retail space will be added as part of the re-development, but
25,500 square feet will be demolished from the existing mall, resulting in a total square footage of
324,500 square feet. Therefore, a total of 4,450 visitors per day is assumed. The restaurant area
is anticipated to serve 500 meals per day. An infiltration rate of 150-gallons/acre/day is added to
the total sanitary sewer flow calculations.
Projected Sewer Flows:
(200 employees)(10 gal/day/person) = 2,000 gpd
(4,450 visitors per day)(3 gal/day/person) = 13,350 gpd
(500 meals per day)(3 gal/day/meal) = 1,500 gpd
Infiltration (37.42 acres)(150 gal/day/acre) = 5613 gpd
Total QAvg. = 22,463 gpd = 15.60 gpm = 0.035 cfs
Peaking Factor:
ππππ₯
ππ΄π£π
=18+π.5
4+π.5 ,(P =Pop./thousands),=18+(4.45).5
4+(4.45).5 =3.29
Peak Flow = (3.29) x 0.035 cfs = 0.115 cfs
Sanitary Sewer Hydraulic Analysis:
The capacity of an 8-inch main is checked using Manningβs Equation:
Q = (1.486/n)AR2/3S1/2
For an 8-inch PVC sewer main:
Manning's n = 0.013 for PVC
Minimum Slope = 0.004 ft/ft (~0.01 ft/ft is proposed)
A = area = (3.14/4)d 2 = (3.14/4)(8/12)2 = 0.34907 ft2
P = perimeter = 2(3.14)r = 2(3.14)(4/12) = 2.0944 ft
R = hydraulic radius = A/P = 0.34907/2.0944 = 0.16667 ft
R2/3 = 0.30105 ft
S = 0.004 ft/ft
S1/2 = 0.0632 ft/ft
Qfull = (1.486/0.013)(0.34907)(0.30105)(0.0632) = 0.7592 cfs
Q0.75 = (0.75)(0.7592) = 0.5694 cfs > Qpeak = 0.121 cfs β΄ adequate
Conclusion
In conclusion, the 8-inch sanitary sewer main provides adequate capacity to service the proposed
development.
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Design Report.Docx