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HomeMy WebLinkAbout19_Water and Sewer Report Memo PLAN 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 L:\213611\Engineering Reports\Water and Sewer Report Memo PLAN.doc Page 1 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. Stormwater storage and design is also included in this report. 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) Peak Hour Flow = 26.56 gallons per minute L:\213611\Engineering Reports\Water and Sewer Report Memo PLAN.doc Page 2 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) (0.25 cfs) X (60 sec./min) X (7.48 gal./cf) = 112.2 gpm Using City of Bozeman estimated average day sewer flows and estimated retail average day sewer flows and multiplying by a peaking factor of 4.24, determined from equation 10-1 in DEQ 2, the estimated peak hour sanitary sewer demand for the proposed development is 26.56 gallons per minute. The proposed development will utilize a 4” sanitary sewer service which has been calculated to have a capacity for 112.2 gallons per minute 75% full providing adequate capacity for the developments demands. The proposed service will connect to the 18” sewer main in West Babcock Street where sewage will flow west. The existing 18” sewer main terminates into a manhole in the intersection of 9th and West Babcock where sewer flows west into a 20” sewer main. The existing 10” sewer main that runs out of this manhole north down 9th Avenue has an invert out elevation higher than the 20” invert out to the west, meaning that sewer flows west and does not go north through the main in 9th Avenue. Further more the channel bottom of the manhole has the troughs built so that sewer flow heads west. Between the lower invert to the west and the trough channel design, it is highly unlikely that sewage from the proposed development would flow north down 9th Avenue. In the event that flow rates were so high as to overtop the manhole’s trough very little flow would actually flow north, the large majority would still be flowing west down West Babcock in the exiting 20” main. 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 Peak Demand Using a ration of 3:1 ratio for peak hour demand to average day from the City of Bozeman Design Standards: (17,329.4 gpd) X 3 = 51,988.2 gallons per day (51,988.2 gpd) / (900 min/day) = 57.76 gallons per minute peak hour Hydrant curves for nearby fire hydrants were supplied by the city. Due to similar conditions fire hydrant #129 was analyzed to determine adequate fire flows are available. Hydrant #129 is at the intersection of 9th Avenue and West Olive Street. Hydrant #129 provides approximately 4,000 gallons per minute with a residual pressure of 20 psi. Using a fire flow demand of 1,000 gallons per minute and a peak domestic demand of 57.76 gallons per L:\213611\Engineering Reports\Water and Sewer Report Memo PLAN.doc Page 3 minute total fire flow is 1,057.76 gallons per minute. See attached for the City’s fire flow documentation. 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. The post 10-yr 2-hr storm retention volume is 1084 cubic feet. The proposed underground retention system’s storage volume is 1557 cubic feet satisfying the required retention design storm volume. The 100-yr 2-hr storm was analyzed as a short duration high intensity storm as well as the 100-yr 24-hr as a long duration low intensity storm. When analyzing the 100-yr events infiltration was accounted for. Infiltration rates were set at 2.6 in/hr per DEQ 4 estimates for gravels. The proposed retention system has been over sized for the 10-yr 2-hr design storm to accommodate for larger storm events. The proposed system has adequate storage for the 100-yr events. The underground retention system will infiltrate the runoff volume in 48 hours or less. 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. trench drains will also be provided to capture runoff flowing back towards portions of the building. Runoff collected by the trench drains will also be conveyed to the inlet structure or infiltration chamber directly. The parking lifts 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 manufacture’s recommendations and design standard. Runoff from portions of the post-developed site will not be captured by the infiltration chamber. The pre-development runoff volumes were compared against the post development runoff volumes to verify that runoff rates and volumes will not be increased. Exhibits A and C show the pre-development and post-development runoff basins. Currently there is no onsite stormwater storage and all runoff is discharged into the surrounding streets where it is collected by the curb and gutter and ultimately the storm sewer system. The post development conditions will also have areas discharging into the existing storm sewer system. These areas include all site sidewalk, landscaping, as well as the paved alley. The entirety of the proposed structure will not discharge runoff into city storm sewer, only the surrounding areas. The pre-development runoff for a 10-year 2-hour storm is 801 cubic feet, the post-development 10-year 2-hour runoff volume is 400 cubic feet. The proposed development will not increase runoff rates or volumes.