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Black Olive Apartment Building
Water, Sewer, Storm Water Engineering Report
September 28, 2016
Prepared for: Black & Olive, LLC
Prepared by: Stahly Engineering and Associates Cordell D. Pool, PE – Project Engineer
Introduction The Black Olive building is an infill redevelopment of the property located at 202 South Black
Avenue. The site is currently occupied by a 2-story (with basement) office/retail building and parking lot. The existing building is currently served by a 1” water service from South Black Avenue and two
4” sanitary services to South Black Avenue.
The existing building will be demolished and replaced with a new apartment building. The new
building will not have a basement. The proposed building and site improvements are shown on the
Civil plans provided with the Site Plan submittal. This Engineering Report supports the design of the water and fire building service lines, sewer service, and storm water mitigation.
Civil Specifications and Design Standards
The civil specifications for the project will be the Montana Public Works Standard Specifications (MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans will be developed in accordance with the City of Bozeman Design Standards.
Water
City water mains exist in South Black Avenue and Olive Street. The water main in South Black Avenue is 14” and the water main in Olive Street is 12”. The existing water service on South Black
Avenue will be capped at the main, and two new services (fire and water) will be provided from the
water main in Olive Street.
The new water service size is based on the plumbing fixture counts in the building and the Uniform
Plumbing Code flow rate for this number of fixture counts. The total fixture counts are 700.5 water fixture units, equating to a maximum flow rate of 160.9 gpm. Based on a maximum velocity of 10
ft/sec in the water meter, this flow rate will require a 3” water meter. The service to the building will be a 4” line reducing to a 3” meter in the building. The fire service line size has not been determined yet, but a 6” service is anticipated. The water and fire service lines will be designed by a PE and
submitted for review as required by the building permit. Irrigation water supply will be primarily from detained building roof stormwater. There may be some
make up water required from the potable system, when stored rainwater is used up. It is estimated that only 20% of the total irrigation needs will need to come from potable water supply.
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A fire flow test of the existing fire hydrant at the corner of Black and Olive has been conducted by the City and is included with this report. The static water pressure is approximately 128 psi. The
hydrant provided a flow of 1802 gpm from one of the 2.5” nozzles. This flow resulted in a 2 psi drop in the hydrant located 1 block to the east. This fire flow test shows that the minimum required fire
flow of 1500 gpm at 20 psi residual is met by this hydrant. Also, this test indicates that the water
system network in this area can provide the anticipated peak domestic demand of the Black Olive building with very little impact on the system pressures.
Water use for the Black Olive building has been estimated based on City of Bozeman engineering standards. The 2015 Wastewater Facility plan measured a wastewater flow rate of 64.4
gallons/day/person for residential uses. This value is an average for all city residential users, and is higher than some new apartment buildings which utilize low-flow fixtures. The estimated water use is provided in Table 1.
Table 1. Black Olive Estimated Water Use
Domestic Water Use
Unit Type (Use) # Units Occupants Gpd/Occupant Gallons/ Day
Studio Apartment 16 1 64.4 1030
1-Bedroom Apartment 24 1.25 64.4 1932
2-Bedroom Apartment 15 2 64.4 1932
Leasing Office 1 2 15 30
Retail Space 1 1 500 500
Total Domestic Use 5,424
Total Annual Use (ac ft) 6.08
Irrigation Water Use
Irrigation Area (sf) Irrigation Rate (in./year) Gallons conversion Factor Total Annual Water (Gallons)
Sod/spray 2,235 29 0.623 40,380
Landscape/drip 7,938
Total Irrigation (gallons) 48,318
Total Irrigation (ac ft) 0.15
% Potable Make-up 20%
Potable Irrigation (ac ft) 0.03
Existing Water Use
(From last 1-year of billing)
Average Flow (gallons/day) 181.7
Total Annual Use (gallons) 66,310
Total Annual Use (ac ft) 0.20
The total new annual water use from the Black Olive building is 6.08+0.03-0.20 = 5.91 ac ft.
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Sewer
A 6” clay sewer exists in South Black Avenue. The existing sewer services are not adequate to serve the new building and will be abandoned. The new sewer service is sized based on drainage
fixture units and sizing criteria from the Uniform Plumbing Code. The Black Olive building drainage
fixture units are 699. For this number of fixture units, the plumbing code requires a 6” service at a 2% slope. Daily wastewater generation is anticipated to be equal to the domestic use of 5425
gallons per day. Using a peaking factor of 12 (the entire day’s use in 2 hours), the peak hour flow is
45.2 gallons per minute.
Storm Water The site is currently developed and has a substantial impervious area. Currently site storm water is
directed by surface drainage towards the adjacent streets. Runoff is then collected in existing curb inlets and transported by a storm drain in South Black Avenue. Though antiquated, the existing storm drainage system appears to be functioning adequately. The existing curbs are badly
deteriorated and will be replaced with this project. New curb inlets and storm drain laterals will be installed with the curb replacement.
Storm water mitigation is based on the requirement in the City Design Standards to “infiltrate, evapotranspire, or capture for reuse the runoff generated from the first 0.5” of rainfall”. Additionally,
peak flows were analyzed to make sure that runoff generated by the Black Olive building is less than
existing flows, ensuring that existing pipe capacity is not exceeded.
Due to the existing impervious areas, the most limiting criteria is the capture of the first 0.5” of
rainfall. Since the building covers 80% of the lot, roof-top runoff will be captured for reuse. This runoff is relatively clean and can be easily reused. Runoff will be captured by a detention tank
located beneath the building, and used to supply the landscape irrigation system. Flows from storms exceeding 0.5” will flow through the tank and discharge to one of the new curb inlets in Olive Street.
Storm water calculations are provided in Table 2. This table provides the existing and post development site statistics and resulting runoff coefficients. Design storm information is provided from the 0.5” regulatory storm to the 100-year 24-hour storm. Existing and post development runoff
is calculated for each storm event using City of Bozeman standards. The 100-yr post development runoff is used to determine the size of the building storm sewer line. A 10” pipe will convey the 100-
year storm, preventing building damage. The existing and post development runoff volumes are
compared to determine mitigation volumes for each storm event. Lastly, a detention calculation for the 100-year storm event is determined using City of Bozeman standards. This calculation provides
the detention volume required to keep post development flow rates equivalent to existing conditions.
Comparing the mitigation volumes and detention volume (highlighted) shows that the 0.5” storm
capture requires the largest volume of 638.59 cubic feet, or 4777 gallons. Capture and reuse of this
volume will reduce post development runoff flow rates and volumes to less than existing. In order to provide additional storage volume for irrigation reuse, a slightly larger detention tank is proposed.
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Table 2. Black Olive Storm Water Calculations
Site Statisitcs
Land Classification Runoff
Coefficients Existing Area (sf) Post Dev Area (sf)
Rooftops 0.9 3,546 15,238
Pavement 0.9 8,412 0
Sidewalk 0.9 1,419 1,171
Landscape 0.9 5,822 2,790
Total 0.2 19,199 19,199
Weighted Coeff. 0.69 0.80
Design Storm Information
Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres) 0.441 0.441 0.441 0.441 0.441
Drainage Area (sf) 19199 19199 19199 19199 19199
Slope (%) 2 2 2 2 2
Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0
24 Hour Precipitation
Volumes (in) 0.50 1.84 2.16 2.42 2.67
Existing Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg.
29) (in/hr) 0.00 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) 0.00 0.98 1.16 1.44 1.62
Runoff Volume (cf) 0.00 2024.57 2376.67 2662.75 2937.82
Post Dev Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
New Development (0.5inch) 0.5 0.5 0.5 0.5 0.5
Intensity at Tc (Figure I-2 pg.
29) (in/hr) 3.22 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) 1.13 1.13 1.35 1.67 1.88
Runoff Volume (cf) 638.59 2350.00 2758.70 3090.76 3410.06
Mitigaton Requirements
Runoff Increase (cfs) 1.13 0.16 0.19 0.23 0.26
Runoff Increase (cf) 638.59 325.43 382.03 428.02 472.23
Runoff Increase (gallons) 4777 2434 2858 3202 3532
Peak Flow Detention
To reduce runoff flows to preconstruction flow rates
100-yr Storm (only largest value shown)
Storm Duration Intensity Future
Runoff Rate
Runoff
Volume
Release
Volume
Required
Storage
(minutes) (in/hr) (cfs) (cf) (cf) (cf)
1440 0.12 0.04 3651.21 3145.58 505.63