Loading...
HomeMy WebLinkAbout9_Engineering Report Engineering Design Report Urban + Farm Phase 1 Infrastructure Improvements Bozeman Gallatin County, Montana August, 2022 Prepared For: Laurel Parkway LLC 11 Lone Peak Dr Big Sky, Montana 59716 Prepared By: Hyalite Engineers, PLLC 2304 N 7th Ave. Suite L Bozeman, MT 59715 Engineering Report – Urban + Farm Phase 1 Table of Contents August 2022 Page ii Version 08/30/2022 Table of Contents Table of Contents ........................................................................................................................ ii 1 Introduction ......................................................................................................................... 3 1.1 Purpose of Report ........................................................................................................ 3 1.2 Scope .......................................................................................................................... 3 2 Location and Site Information ............................................................................................. 3 3 Land Use ............................................................................................................................ 4 3.1 Existing ........................................................................................................................ 4 3.2 Proposed ..................................................................................................................... 4 4 Water System ..................................................................................................................... 4 4.1 Water Use Data ........................................................................................................... 4 4.2 Fire Flow ...................................................................................................................... 4 5 Sewer System ..................................................................................................................... 5 5.1 Design Flow Rates ....................................................................................................... 5 6 Stormwater Network ........................................................................................................... 6 6.1 General Design ............................................................................................................ 6 6.2 Hydrologic Methodology............................................................................................... 6 6.3 Detention Basin ........................................................................................................... 8 6.4 Inlets & Storm Sewer ................................................................................................... 8 6.5 Groundwater ................................................................................................................ 9 6.6 System Maintenance ................................................................................................... 9 6.7 Erosion Sediment Control ............................................................................................ 9 6.8 Flooding ....................................................................................................................... 9 Engineering Report – Urban + Farm Phase 1 Introduction August 2022 Page 3 Version 08/30/2022 1 Introduction 1.1 Purpose of Report This report is intended to serve as the design document for the infrastructure improvements associated with the construction of a 31-lot major subdivision. The proposed infrastructure has been designed accounting for future connections, as the property is bordered to the south by undeveloped land which will be part of future phases of this project. 1.2 Scope Expansion of existing infrastructure (water and sewer mains) and storm water design elements are within the scope of this report. All improvements analyzed in this report are within the property with proposed connections to existing infrastructure. No off-site improvements are expected. 2 Location and Site Information The property occupies 8.7 acres and is located on the western side of Bozeman and is bordered to the north by East Norton Ranch Phase 3, in the S ½ of Section 9, Township 2 South, Range 5 East, P.M.M., City of Bozeman, Gallatin County, MT. The existing zoning is R-5 and the nearby surrounding zoning is R-3, R-4, REMU, and B-2M. The property is within the service area for municipal water and sewer from the City of Bozeman. Figure 1 - Vicinity map. Engineering Report – Urban + Farm Phase 1 Land Use August 2022 Page 4 Version 08/30/2022 3 Land Use 3.1 Existing The site is currently a vacant lot with three existing intersection tie-ins –South Eldorado Avenue, South Reliance Avenue, and Fallon Street. The majority of the site has been left undisturbed. There is currently a sewer main extension running along the sites southern edge from Fallon Street and then turning north running up to South Reliance Avenue. 3.2 Proposed The proposed major subdivision will consist of 21 single family lots and 10 multi-family lots for a total of 64 units. Through this development South Eldorado Avenue, South Reliance Avenue, and Fallon Street will all be extended. All centralized lots will be alley-loaded with water and sewer mains installed within the provided 30’ alley right-of-way. Both water and sewer mains will connect to existing stub-outs from the improvements associated with Phases 3C and 4 of East Norton Ranch. The existing stormwater retention pond in the northwest corner of East Norton Ranch Phase 5 will be sized to accommodate the runoff from this project see section 6 of this report. 4 Water System The proposed water system will tie into existing mains installed during the development of the East Norton Ranch Phase 3C and 4. The water system will be looped into the City of Bozeman water system at South Eldorado, South Reliance, and Fallon. The water main will also be installed within the provided alley right of ways. All new water mains will be 8” diameter, class 51 ductile iron pipe. Fire hydrant leads will be 6” diameter ductile iron pipe as well. All mains will be wrapped in V-Bio Enhanced Polyethylene Encasement per DIRPA recommendations. Type K copper water services will extend 8’ past the property line and will be 1” in diameter for the single-family lots, 1.5” diameter for the four-plex lots, and 2” diameter for the seven-plex lot. 4.1 Water Use Data Average Daily Flow 64 dwellings at 2.17 persons/dwelling = 138.88 people 138.88 people at 170 gpd = 23,610 gpd Peak Demand Peaking factor = 2.3 (peaking factor per City of Bozeman Design Standards) 23,610 gpd (2.3) = 54,302 gpd Total max day flow = 54,302 gpd / 1440 min/day = 37.7 gpm instantaneously Peak Hour Demand 23,610 gpd (3) = 70,830 gpd = 49.2 gpm 4.2 Fire Flow Required fire flow for R-5 zoning is 3,000 gpm. Using a nearby hydrant’s flow data provided by the City of Bozeman a WaterCAD model was created to simulate fire flows from the proposed hydrants. A peak hour demand of 49.2 gpm was divided and distributed to nodes within Urban + Engineering Report – Urban + Farm Phase 1 Sewer System August 2022 Page 5 Version 08/30/2022 Farm Phase 1 to create demands for the subdivision. As the proposed subdivision will be connected to the existing pressure network some of the existing system was included in the model. The nodes that are part of the existing subdivision to the north were given the same demands as the nodes within Urban + Farm Phase 1 as they are very similar residential densities. The pressure and flow within the proposed system was simulated in the WaterCAD model by adding a reservoir and a pump to the model at a connection point. The reservoir supplies the system with water and the pump is used to provide pressure. The reservoir and pump were set to the same elevation as the tie-in location as to not create any head on the system from the reservoir. All head in the system is generated from the pump. Running a fire flow analysis scenario for each hydrant at 20 psi both hydrants are capable of supplying over 3,000 gpm while also satisfying the demands of the subdivision. WaterCAD results are attached. The existing hydrant used to create pressure/flow for the system was also modeled as a check. At 20 psi the model predicts a lower available flow rate than the supplied hydrant data. This is due to the model being a small part of a larger network. This lower flow rate is a result of a conservative model. 5 Sewer System During construction of Phases 3C of East Norton Ranch, 8-inch sewer main stubs were extended from a manhole on South Eldorado and South Reliance. During construction of Phase 4 of East Norton Ranch a 10” sewer main was ran west through the property along the south edge of the property from Fallon and turns north running to South Reliance. This sewer main will collect all sewer from the property, the entire sanitary sewer network within Urban + Farm Phase 1 will drain to this existing sewer main and flow to the Norton Ranch lift station. The Norton Ranch lift station upgrades are currently being designed by HDR and the development group is working with them and nearby developments to ensure adequate capacity does exist. 5.1 Design Flow Rates Average daily Flow 64 dwellings at 2.17 persons/dwelling = 138.88 persons 138.88 persons at 64.4 gallons/day/person = 8,944 gpd Peak Flow Peaking Factor = (18+0.138881/2)/(4+0.138881/2) = 4.20 Peak Hour Flow = 4.20 x 8,944 gpd = 37,565 gpd The peak hour flow calculated above is 26.1 gpm. Including infiltration: 150 gallons/acre/day (8.7 acres) = 1305 gpd = 0.91 gpm Peak hour flow (including infiltration) = 27.0 gpm The proposed 10” and 8” gravity mains are capable of flowing 27 gpm at minimum slope (0.28% and 0.4% respectively) with a depth of only 1.5”. This capacity calculation was performed using a Manning’s friction factor of 0.013. As mentioned previously, the gravity network will then discharge into the existing 10” network located in South Reliance Avenue. Engineering Report – Urban + Farm Phase 1 Stormwater Network August 2022 Page 6 Version 08/30/2022 All sewer main piping will have uniform slope between manholes. Sewer mains have been designed at a slope to accommodate for future development in the area. Individual sewer services will be provided for each lot within the development. The 4” Schedule 40 PVC sewer services will be extended 8 feet past the property line of each lot. A 4” gravity sewer service on a ¼”/foot grade is capable of supporting flows from each lot. The existing sewer network was analyzed for the proposed development as well as for future development. The existing 10” sewer main in Reliance as well as the existing 15” sewer main in Laurel Parkway are adequately size for the proposed development as well as future developments. See Appendix E for sewer analysis. 6 Stormwater Network This section provides a design basis and hydraulic calculations for sizing storm water facilities for Urban + Farm Phase 1. The City of Bozeman Design Standards were used as the primary guidelines for this stormwater drainage design. 6.1 General Design The proposed development will be a combination of roadway and residential lots. The proposed streets will be paved with curb and gutter. The alleys that split the interior lots will be paved with an inverse crown. Stormwater runoff generated from the roadway and residential lots is designed to be collected by the curb and gutter and then directed toward the storm drain inlets. The stormwater that is collected by the paved alleys will travel down the centerline and then to the curb and gutter eventually flow north to either existing or new storm drain inlets. The runoff will then move through the storm sewer system pipes to the existing detention pond constructed with Norton Ranch Phase 5. A small area will discharge storm runoff to an existing stormwater retention pond in Norton Ranch Phase 1. The detention pond has been designed with an outlet structure to limit stormwater runoff from the developed site to the pre-developed runoff rates. 6.2 Hydrologic Methodology The rational method was used to determine peak runoff rates. The rational formula provides a peak runoff rate which occurs at the time of concentration. Q = CiA Where C = Weighted C Factor i= Storm Intensity (in/hr) A = Area (acres) Q = Runoff (cfs) The storm intensities were developed from the IDF curve found in Figure I-2 of the City of Bozeman Design Standards and Specifications. Runoff coefficients for each basin were calculated using a weighted percentile of impervious and pervious area. The coefficient used are shown in the table below. Engineering Report – Urban + Farm Phase 1 Stormwater Network August 2022 Page 7 Version 08/30/2022 Table 1 - Runoff coefficients used. RUNOFF COEFFICIENTS Undisturbed 0.2 Low-Med. Residential 0.35 Dense Residential 0.5 Impervious 0.9 Time of concentration of overland flow was determined using the following equation: Tc = 1.87(1.1-C)D1/2 S1/3 Where Tc = Time of Concentration, minutes S= Slope of Basin, % C= Runoff Coefficient D= Length of Basin, ft Time of concentration for gutter flow was determined using the following equation for shallow concentrated flow velocity: V = KukS0.5 Tc = D/(60V) Where V = velocity, fps Ku = unit conversion, 3.28 k = intercept coefficient S = slope, % Tc = Time of Concentration, minutes D= Length of Basin, ft V= Velocity, fps The rational method was used to compute runoff flow rates. This method can be used for storm durations equal or greater than the time of concentration. This method assumes the maximum runoff rate occurs at the time of concentration and continues to the end of the storm. Maximum runoff rates for durations greater than the time of concentration are less than the peak runoff rate because average storm intensities decrease as duration increases. Retention volumes were calculated using the modified rational method. The modified rational method uses peak flow rates to determine volume. The peak flow rate is determined using the rational method, that peak flow rate is conservatory held constant for the duration of the storm. The volume of retention volume required is calculated by multiplying the peak runoff by storm duration. City of Bozeman Standards requires that retention volumes be calculated for a 10-yr 2- hr design storm. The intensities are developed from the IDF curve found in Figure I-2 of the City of Bozeman Design Standards and Specifications. Runoff coefficients are taken from Table 1. Retention volume is determined using the following equation: Engineering Report – Urban + Farm Phase 1 Stormwater Network August 2022 Page 8 Version 08/30/2022 V = 7200Q Where V = Volume, cf Q = flow rate, cfs 6.3 Detention Basin The stormwater runoff generated from the site will be conveyed to the existing detention pond that was constructed with Norton Ranch Phase 5. Originally this detention pond was sized to detain runoff from Urban + Farm Phase 1 when it was planned as part of the Norton Ranch developments. In order to determine if the detention pond is sized adequately an analysis of the detention basin needed to be done to determine if the changes to the original design generate more runoff than the pond is sized for. The two factors that affect the runoff rate are the basin area and weighted C factor. Since the detention pond was only sized to accommodate 6.4 of the 8.7 acres of Urban and Farm Phase 1 the total area in the detention basin increases by 2.27 acres. Urban and Farm Phase 1 will also have a higher density residential layout which results in a higher weighted C factor. The detention pond was originally calculated to require a storage volume of 37,018 cubic feet; however, the pond was sized at a storage volume of 47,688 cubic feet per C and H Norton Ranch Phase 5 Design Report. Leaving an additional 10,670 cubic feet of storage. With the increase in area as well as a higher weighted C factor the entire detention basin, which includes Phase 3 and 5 Norton Ranch and Urban Farm Phase 1, requires a storage volume of 39,028 cubic feet. Coordination with owners of the existing detention pond is in place to expand the detention pond to have an additional capacity of 2,010 cubic feet of storage to accommodate Urban + Farm Phase 1. All drainage basins in Urban + Farm Phase 1 flow to the new Norton Ranch Phase 5 detention pond except Basin E. Basin E will flow into an inlet just north in Laurel Parkway where it will discharge into a pond built with Phase 1 of Norton Ranch. This pond was initially sized to accommodate a larger area of contribution from Urban and Farm Phase 1. Originally the drainage area was designed for a volume of 3,217 cubic feet, with Urban + Farm Phase 1 the contributing volume is 2,779 cubic feet. No further expansion of this pond is required. 6.4 Inlets & Storm Sewer Using the Rational Method, a contributing flow to each inlet in the system was determined. The inlet grates were designed to accommodate the 25-year storm event without overtopping a depth 0.15’ below the top of curb or spreading greater than 9 feet. Inlets and manholes will have a 9” sump for sediment collection. Storm pipes were sized to accommodate the 25-year storm event and were modeled in Flow Master. Pipes are sloped to maintain a minimum velocity of 3 ft/s when flowing full to prevent sediment deposit. A manning’s N of 0.013 was conservatively used for the A-2000 Storm pipe within the model. Both existing inlets and storm pipes were analyzed for capacity with Urban + Farm Phase 1’s runoff. The two proposed inlets will connect to an existing 15” storm pipe which was stubbed out as part of Norton Ranch Phase 3C. This 15” pipe currently has a capacity of 4.96 cfs at 14” full, the 25-year event flow rate generated from Urban + Farm Phase 1 is 4.86 cfs. The 12” storm pipe that connects the two proposed inlets has a capacity of 5.4 cfs, the required capacity for the 25- Engineering Report – Urban + Farm Phase 1 Stormwater Network August 2022 Page 9 Version 08/30/2022 year event is 1.03 cfs. The existing 15” pipe connected to the existing inlet on Eldorado has a capacity of 7.25 cfs, the required capacity for the 25-year event is 4.10 cfs. Table 2 - Pipe Capacity PIPE SIZE 25-YR FLOW RATE CAPACITY P1 New S. Reliance 12” 1.03 CFS 5.40 CFS P2 Existing S. Reliance 15” 4.86CFS 4.96 CFS P3 Existing S. Eldorado 15” 4.10 CFS 7.25 CFS 6.5 Groundwater Groundwater is known to be high in this area. A geotechnical report was done in April 2021 that included borings near and on the property. Test Pit 1 was dug on site and Test Pit 2 was just south of the site. Groundwater was found at 4.8’ in Test Pit 1 and at 5.1’ in Test Pit 2. Test Pit 1 had a monitoring well installed and ground water depth has been recorded. Test Pit 1 had a maximum groundwater height of 2.56’ below the surface on May 20th, 2021. 6.6 System Maintenance Regular maintenance of stormwater facilities is necessary for proper function of the drainage system. Maintenance items include removing debris from inlet grates and culverts, cleaning and flushing pipes, cleaning manhole sumps, and establishing ground cover after construction. 6.7 Erosion Sediment Control During construction, stormwater pollutant controls will include silt fencing, straw wattles, rock check dams, and straw bales. Silt fence, straw waddles, or other perimeter protection will be installed on the down gradient edge of disturbed soil. Straw wattles, straw bales, or other erosion protection will be placed near existing and newly installed culverts. Temporary erosion control measures will be installed and continuously maintained for the duration of construction. This project will require acceptance of a Stormwater Pollution Prevention Plan (SWPPP) permit for stormwater discharge associated with construction activity prior to starting any construction. Protection during and immediately after construction, will be controlled in accordance with this permit and the Montana Sediment and Erosion Control Manual. Permanent erosion control will consist of implementation of seeding disturbed areas and placing riprap at pond inlet/outlets. Any visible sediment must be removed from the stormwater system prior to completing construction. 6.8 Flooding Excessive runoff from a large storm event (significantly exceeding the design storm, i.e 100-year) will be routed such that it does not inundate buildings, drainfields or over top the roadway. The stormwater infrastructure including ditches, culverts, and detention pond outlet structures have been analyzed for the 100-year storm. Stormwater that overtops the ponds during a large rain event will flow through the emergency overflow and outlet to Aajker Creek.