HomeMy WebLinkAbout009a_201104 - Sewer Demand Report
INTRODUCTION
The proposed Thriftway Site Plan is located on a 5.036-acre parcel located on the northeast corner
of Davis Lane and Baxter Lane in the City of Bozeman where the current zoning designation is B-
2 (community business district). The lot is legally described as the S. 1/2, SW ¼, SW ¼, SW ¼ of
Section 35, Certificate of Survey No. 2970, T. 1 S., R. 5 E. of P.M.M.. The site plan consists of a
proposed retail space and office building, gas pumps, and associated parking.
Water and sanitary sewer mains are located within Davis Lane from which water and fire services
for the retail space and office building will be provided services. A cut-in manhole will be installed
along the existing sanitary sewer main along Davis Lane to extend an 8-inch main into the
property, servicing the proposed building as well as continuing through the property to provide
conveyance for future development to the adjacent properties. The existing water main located
within Baxter Lane to the south will be extended into the property, providing service and fire
protection for the adjacent properties. The proposed water service, 4-inch fire service, and sanitary
sewer service will connect to the proposed building as shown on civil sheet C3.0.
Water Distribution System Demands
The design parameters used herein are according to the City of Bozeman Design Standards and
Specifications Policy (DSSP) dated May 2017. Values used for assessing the peak hour demand
for the facility were taken from Table 3.1-1 in the Montana Department of Environmental Quality
(DEQ) Circular Number 4 – Montana Standards for Subsurface Wastewater Treatment Systems.
Average Daily Water Usage (store) = 3 gpd customers, 10 gpd employee
Average Daily Users = 192 persons (180 customers & 12 employees)
Peak Hour Demand (Peaking Factor = 3.0)
Water Demands
Average Day Demand =
(180 customers x 3gpd/person) + (12 employees x 10gpd/person) = 660 gpd = 0.458 gpm
Peak Hour Demand = 0.458 gpm x 3.0 = 1.374 gpm
Sanitary Sewer Design Load
The flow rates used herein are designed using the anticipated generation rates found in DEQ
Circular 4, Section 3.1, Table 3.1-1, typical wastewater flows from commercial, industrial, and
other nonresidential sources. Based on Table 3.1-1, the sewer discharge rate for a store is 12
gallons/day per employee and 3 gallons/day per customer. As such, the proposed Thriftway store
would have a sanitary sewer discharge rate of 684 gallons per day. The peaking factor for the
design area is determined by figuring the equivalent and inserting the population into the Harmon
Formula. An estimated total of 12 employees (assuming 3 shifts of 4 employees) and 180
customers (assuming 10 customer facility users 12hrs/day and 5 users 12hrs/day).
Average Faily Flow Rate:
(180 customers x 3gpd) + (12 employees x 12gpd) = 684gpd
Harmon Formula: Peaking Factor = (18 + P0.5)/(4 + P0.5)
where: P = Population in thousands
Peaking Factor = (18 + 0.1920.5)/(4 + 0.1920.5)
Peaking Factor = 4.15
Assumed infiltration rate = 150 gallons/acre/day = 150 (5.036 acres) = 755.40 gal/day
The peak flow rate is calculated by multiplying the anticipated average daily flow rate by the
peaking factor, and adding the infiltration rate:
Peak Flow Rate = (684) (4.15) + 755.40 gpd = 3,594 gpd
= 2.496 gpm
= 0.006 cfs
The capacity of a 8-inch main is checked using Manning’s Equation:
Qfull = (1.486/0.013)AR2/3S1/2
For an 8-inch main:
Manning's n = 0.013 for PVC Pipe
Minimum Slope = 0.004 ft/ft
A = area = (3.14/4)d 2 = (3.14/4)(8/12)2 = 0.3491 ft2
P = perimeter = 2(3.14)r = 2(3.14)(4/12) = 2.0944 ft
R = hydraulic radius = A/P = 0.3491/2.0944 = 0.1667 ft
R2/3 = 0.3029 ft
S = 0.004 ft/ft
S1/2 = 0.0632 ft/ft
Qfull = (1.486/0.013)( 0.3491)( 0.3029)(0.0632) = 0.7643 cfs
Q75% = 0.75*Qfull = 0.75*0.7643 cfs = 0.5732 cfs
Connection:
Q/Qfull = 0.006/0.7643 = 0.0079 or 0.79%
Based on these calculations, an 8-inch sewer main has adequate capacity to carry the design flows
for the project an anticipated future development. The City of Bozeman will confirm that their
wastewater system has the capacity to handle this peak flow downgradient of the project.