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HomeMy WebLinkAbout18 - Design Report - Icon Apartments at Ferguson Farm - Water, Sewer Revised: September 14, 2018 Revised: July 13, 2018 Revised: April 9, 2018 Revised: January 3, 2018 Revised: November 10, 2017 July 26, 2017 Project No. 17007.02 WATER AND SEWER DESIGN REPORT FOR ICON APARTMENT HOMES AT FERGUSON FARM TRACT 3A, MINOR SUBDIVISION NO. 365 SITE NARRATIVE A 348-unit residential apartment development is proposed on Tract 3A of Minor Subdivision No. 365 in Bozeman, Montana. Public water and sewer mains will be constructed in public utility easements within the property. The public water and sewer mains will be owned and operated by the City of Bozeman. This report summarizes the calculations performed for the water and sewer design for the site. WATER Water for the proposed development is provided by a new 8-inch water main looped throughout the development and connecting to an existing 12-inch main in Fallon Street, an existing 8-inch main in Resort Drive and an existing 12-inch main in West Babcock Street. The water main network will have no dead-ends and provide domestic and fire protection water to each individual building by separate service lines branching from the main. The water system within the development is designed for domestic use and fire protection, but not irrigation which will be supplied from on-site wells. Peak hour demand for the residential component of the development was determined to be 251 gallons per minute (gpm) based on 336 units and the following criteria stated in the City of Bozeman Design Standards and Specifications Policy; 2.11 people per dwelling unit, an average daily usage of 170 gallons per day per person, and a maximum hour to average day ratio of 3:1. The clubhouse and pool average daily flow is based on water bills from a similar clubhouse and pool size in Broken Arrow, OK which averages 5,975 gal per month, 196 gal/day or average daily flow of 0.14 gallons per minute. The peaking factor for this type of use is assumed to be very high since there will be times when the pool is being filled. For purposes of this report, the peak hourly flow is assumed to be 27 gpm which is based on a 25 gpm flow determined from filling the pool from a 50 foot, ¾ inch hose with 100 psi pressure plus another 2 gpm for peak hourly general domestic use. The total peak hourly domestic flow summing the clubhouse and residential flow is 278 gpm. A flow test performed August 16, 2018 on the hydrant at the intersection of Palisade Drive and Resort Icon Apartment at Ferguson Farms P:17007.02_Utility_Design_Report 2 (9/14/18) TRP/lew Drive, showed a static pressure of 125 psi, flow of 1680 gpm, residual pressure of 120 psi and a pitot flowing pressure of 100 psi. With an assumed fire demand of 1500 gpm at the hydrant, an assumed fire service demand of 500 gpm at the building with the highest finish floor elevation, and the peak hour demand of 278 gallons for the development, the total peak water demand is 2,278 gpm. At a discharge of 2,278 gpm the extrapolated main residual pressure will drop to 82 psi at the location where the flow test was taken. This is much higher than the minimum residual pressure criteria during fire flow conditions of 20 psi in the distribution system. The water pipe sizes within the development were designed to limit the water velocity below 15 feet per second based upon the total peak water demand of 2,278 gpm as determined above. The network of water mains within the development shall be 8-inch diameter CL 51 ductile iron pipe. Fire hydrant assemblies will be 6-inch diameter CL 51 ductile iron pipe. Without modeling the internal water system, the water main piping within the site is assumed to have adequate capacity since the system is looped with 3 points of connection and the relatively high residual pressure as described above. Using a conservative flow rate of half the fire flow (1,139 gpm), the friction headloss of an 8-inch pipe using Hazen-Williams C value of 130 and a pipe length of 700 ft (distance to centroid of the site), is around 7 psi. With an elevation difference of 8 feet (3.5 psi), the residual pressure would be 71.5 psi (82 psi – 7 psi - 3.5 psi) which is significantly higher than the 20 psi minimum criteria. SANITARY SEWER Sanitary sewer design flow rates have been calculated for each phase to determine the impact to the City of Bozeman municipal sanitary sewer system and for sizing the development’s collection system. The City of Bozeman Design Standards use an average daily flow rate of 89 gpd/person using 2.11 persons per unit with a 150 gallons/day/acre infiltration rate. The apartments in this development will be equipped with low flow fixtures so it is anticipated that the sanitary sewer flows will be significantly lower than the flow criteria stated in the Design Standards. Following numerous discussions with the City Engineering Department, they will allow an estimated wastewater flow of 64.4 gal/capita/day with a 2.17 persons/unit as documented in the City of Bozeman Wastewater Collection Facilities Plan Update, dated May 2015 which is based on actual flow data for the Bozeman collection system including infiltration. In addition, the clubhouse average daily flow of 196 gal/day is added to the flows for phase I which is based on the average monthly water usage for a similar facility in Broken Arrow, Oklahoma with the same pool size. The City of Bozeman has determined that the sanitary sewer average daily flow capacity dedicated to this project is 30,919 gallons per day. Once this capacity is reached, construction of new phases will not be allowed until the developer or combination of area wide developers upgrade a portion of the existing downstream collection system. Upon analysis of the attached flow calculations, it appears that the critical daily flow of 30,919 gal/day would occur at the 11th phase (30,382 gal/day) which allows for the completion of all the apartment buildings south of Palisade Drive and the clubhouse. Sanitary sewer flows from the pool during the filter backwash operation were also evaluated to determine the additional flows to the system. There is a limited daily flow capacity for the pool backwash flow in the amount of 537gal/day based on the difference between the allowable daily Icon Apartment at Ferguson Farms P:17007.02_Utility_Design_Report 3 (9/14/18) TRP/lew flow of 30,919 gal/day and the anticipated flow after construction of the 11th building of 30,382 gal/day. Converting the 537 gal/day to a peak hourly residential flow only allows 1.5 gallons per minute. If a sand filter system was installed with one filter, the pool designer recommends 127 gallons per minute for a duration of 5 minutes to properly clean the filters. In this scenario, a tank would need to be installed to store the backwash. The storage tank would have an orifice to limit the discharge to the municipal system at 1.5 gal/min. The tank capacity would need approximately 635 gallons with is 127 gal/min over 5 minutes and it would take approximately 7 hours to drain at 1.5 gal/min. If multiple filters were installed, the required flow rate and storage volume would be reduced proportionately. For example, if 2 filters were installed, the flow rate would be 63.5 gal/min, requiring a tank capacity of 318 gallons. For multiple filters, each tank backflush would need to be done on different days to allow the storage tank to drain. Upon later evaluating these options, it was decided to not pursue the sand filter system because of limited sewer capacity and space limitations for multiple sand filters and a storage tank. The sand filter system will be replaced with a cartridge filter which can be cleaned with a garden hose on the lawn. The pool will also be partially drained once a year for winterization. To drain the pool, the valves will remain in the same position for circulation but the valve to the waste line will be opened slightly to allow a flow rate less than or equal to 1.5 gallons per minute. During the normal winterization operation, there is approximately 2300 gallons to drain. At 1.5 gallons per minute, it would take around 25 hours to winterize. When the entire pool needs to be drained, a majority of the pool will be drained using the same procedure as winterization but may be expedited by draining with an additional pump that discharges to the grass. Regarding the pipe sizing calculations, the attached Sanitary Sewer Flow Calculation Worksheet shows the capacity of an 8-inch main at 75% full at minimum slope is around 300 gpm. Using the flow calculation method described above, the entire development (127 gpm peak hourly flow) can be served with an 8-inch sewer collection system. The proposed sanitary sewer mains are 8-inch diameter PVC with a minimum slope of 0.4 percent. The mains will be situated within 30-foot-wide public utility easements. The sewer will discharge from the site at one point located at the Babcock and Stafford intersection. The sanitary sewer mains for the development will connect to individual units with 6-inch diameter sanitary sewer services. The main in West Babcock Street flows west to an existing 12-inch main in Cottonwood Road. The City of Bozeman has verified that the mains in Resort Drive and West Babcock have the available capacity to serve the maximum anticipated flow of 84 gpm from the 216 units and clubhouse on the south half of the development. This existing main does not have available capacity to serve the additional 43 gpm demand from the 120 units on the north half of the development. Therefore, they are requiring this development to upgrade the existing 10 sanitary sewer main in Babcock to a 12- inch from the point of connection in Babcock to Cottonwood Road prior to allowing building permits north of Palisades. CONCLUSION The proposed network of new 8 inch looped public water and 8-inch sanitary sewer piping is adequate to serve the 336 residential dwelling units and clubhouse within the Icon Apartment Icon Apartment at Ferguson Farms P:17007.02_Utility_Design_Report 4 (9/14/18) TRP/lew Homes development. The water and sewer mains within the proposed development meet or exceed Montana Department of Environmental Quality performance and dimensional requirements. A portion of the existing city sewer main downstream will need to be upgraded to a larger size once the daily flows from this development exceed 30,919 gallons per day. The upgrade timing is depended upon the actual flows generated from this development. The existing city main within West Babcock Street will need to be upgraded to a 12-inch main prior to commencing with building construction north of Palisades Drive.