HomeMy WebLinkAbout11_W Babcock Apts_Engineering Report
Water, Sewer, and Storm Drainage
Design Report:
for
West Babcock Apartments
Gallatin County, Montana
February, 2022
Prepared For:
Boundary Development
Prepared By:
Hyalite Engineers, PLLC
Brett Megaard, PE
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West Babcock Apartments
ENGINEER’S WATER, SEWER, AND STORM DESIGN REPORT
Owner: Boundary Development
840 Broderick Street
San Francisco, CA 94115
Introduction
This report provides an estimate of the peak-hour sanitary sewer demand as well as the
average and max day demands of domestic water usage for the proposed site development.
The City of Bozeman Design Standards and Specifications (March 2004) was used as the
primary guideline for this stormwater drainage design.
General Design
The proposed development of the site will contain a five-story apartment building with 46
units as well as a 1,000 square foot retail space. The proposed building will have a new 4”
SDR 26 PVC sewer service, a new 4” domestic water service, and a new 4” fire service.
Final water and fire service sizes are to be verified by mechanical and will be updated to
meet these requirements. The sewer service will be connected to the 18” main that runs
down West Babcock Street. The sewer service will be installed at a minimum slope of ¼-inch
per foot. A sewer cleanout will be provided at the building. A water service will be connected
to the 6” water main in West Babcock Street. The water service will have a curb stop and box
1 foot into the property.
Sewer Demands
The property has three existing sewer services extending from the sewer main in the alley to
the south. These services are to be capped and abandoned at the main per city standards.
46 dwelling units assuming an average of 2.17 people per dwelling.
One retail area assuming 6 employees with 100 customers per day assuming an average
daily flow of 10 gpd per employee and 3 gpd per customer.
(2.17 people per dwelling) X (46 dwellings) = 99.82 people
(99.82 people) X (64.4 gpd) + (6 employees) X (10 gpd) + (100 customers) X (3 gpd) =
6,788.41 gallons per day
Using a peaking factor of 4.24 calculated from equation 10-1 from DEQ Circular 2:
Peaking Factor (DEQ-2) = (18 + 0.099821/2) / (4 + 0.099821/2) = 4.24
(8,998.34 gpd) X 4.25 = 38,242.95 gpd (26.56 gallons per minute)
A 4” diameter PVC sewer pipe is capable of flowing 0.25 cfs at 75% full and a slope of 1/4“
per
foot. (Manning’s equation with n=0.013 and slope=0.02)
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(0.25 cfs) X (60 sec./min/) X (7.48 gal./cf) = 112.2 gpm
Water Demands
The property has two existing water services extending from the water main in West
Babcock Street. These services are to be capped and abandoned at the main per city
standards.
Assuming 2.17 people per dwelling with an average daily usage of 170-gallons per person:
(2.17 people per dwelling) X (46 dwellings) = 99.82 people
(99.82 people) X (170 gallons per day) + (6 employees) X (10 gpd) + (100 customers) X (3
gpd) = 17,329.4 gallons per day
Using a maximum day to average day ratio of 2.3:1 from City of Bozeman Design Standards:
(17,329.4 gpd) X 2.3 = 39,857.62 gallons per day
(38,778.46 gpd) / (900 min/day) = 44.29 gallons per minute
Stormwater
The proposed site development consists of a new residential/commercial building, asphalt
alley, and site concrete. The site will be primary hardscape. Due to the coverage of almost
the entire site, as well as the native gravels and low groundwater levels, an underground
retention structure is proposed. The proposed retention chamber will be constructed from 48”
diameter perforated corrugated metal pipe (CMP). From the geotech investigation performed
on site, it is anticipated that the retention chamber will be directly connected to gravels.
Runoff storage for the proposed building as well as drainage for the proposed parking
structure will be provided by the CMP chamber. Retention storage was sized using a 10-yr 2-
hr design storm and the rational method. Runoff from the site will be retained and then
infiltrate through the CMP chamber into the native gravels. The chamber is over sized to
accommodate the 100-yr storm with infiltration rates considered, see the included CMP
infiltration sizing spreadsheet.
Runoff from the roof will be collected through a series of roof drains that will be directly
connected to the underground chamber. The parking lot is graded in such a way that runoff
generated from this area will sheet flow to a 36” diameter slotted inlet which will then covey
the runoff to the retention chamber through piping. A trench drain will also be provided to
capture runoff flowing back towards a portion of the building. Runoff collected by the trench
drain will also be conveyed to the inlet structure. The parking lift will have a sump pump
installed to convey any runoff generated from the parking lot as well as from vehicles stored
in it. This pump will be directly connected to the storm drain inlet, as per the manufactures
recommendations and design standard.
North 3rd Avenue Duplexes
CMP Infiltration Sizing
Page 1 of 1
2/8/2022
By Miguel Ley
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.46
Post-Dev C (weighted): 0.85
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.43 0.90 0.39
Undisturbed 0.03 0.20 0.01
Totals 0.46 0.39
Cw =0.85
Runoff Rate (Rational Method):
Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr
*Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01
*Storm Coeff. B:0.60 0.65 0.64 0.67 0.67
Composite C: 0.85 0.85 0.85 0.85 0.85
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.09 0.16 0.20 0.25 0.05
Pond Size:
10-Year, 2 Hour Storm 100 Year Storm (Includes Infiltration)
Duration Time: 7200.00 sec
V100YR/2HR : 1796 cf V100YR/24HR : 4078 cf
V10YR/2HR : 1154 cf Infiltration Area: 552 sf Infiltration Area Area:552 sf
Infiltation Rate: 2.6 in/hr Infiltation Rate: 2.6 in/hr
Volume Infltrated : 239 cf Volume Infltrated : 2870 cf
Max CMP Storage: 1557 cf Max CMP Storage: 1557 cf
Max CMP Capacity 1796 cf Max CMP Capacity 4427 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
Notes:
30% Void space used in CMP Storage
Infiltration rate from DEQ 8