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HomeMy WebLinkAbout08_MayflyEngineeringReport Engineering Design Report May Fly Townhomes Infrastructure Improvements Bozeman Gallatin County, Montana March, 2023 Prepared For: Fallon First LLC 11 Lone Peak Dr Big Sky, Montana 59716 Prepared By: Hyalite Engineers, PLLC Brett Megaard 2304 N 7th Ave. Suite L Bozeman, MT 59715 Engineering Report – May Fly Townhomes Table of Contents March 2023 Page ii Version 03/29/2023 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 Basins .......................................................................................................... 7 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 – May Fly Townhomes Introduction March 2023 Page 3 Version 03/29/2023 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 46-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 5.48 acres and is located on the western side of Bozeman and is bordered to the north by Norton East Ranch Phase 1 and 2, Being Lots 1-4 of Norton East Ranch Phase 4 in the NW ¼ of the SE ¼ of Section 9, Township 2 South, Range 5 East, P.M.M., City of Bozeman, Gallatin County, MT. The existing zoning is R-4 and the nearby surrounding zoning is R-3, R-5, REMU, R-O, 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 – May Fly Townhomes Land Use March 2023 Page 4 Version 03/29/2023 3 Land Use 3.1 Existing The site is currently four vacant lots with one existing approach on May Fly Street. The majority of the site has been left undisturbed. There are currently three sewer main extensions running along the sites northern edge along with one water main extension. 3.2 Proposed The proposed major subdivision will consist of 30 town home lots and 16 condominium lots for a total of 46 units. All 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 the existing infrastructure from the improvements associated with Phase 4 of East Norton Ranch. The existing stormwater detention ponds north of the site will be used to detain runoff and release runoff at predevelopment rates. 4 Water System The proposed water system will tie into existing mains installed during the development of the East Norton Ranch Phase 2, 2A and 4. The water system will be looped into the City of Bozeman water system through the 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 ¾” and 1” diameter for all lots. The development will consist of 30 townhomes and 16 duplexes totaling 62 residential dwellings. 4.1 Water Use Data Average Daily Flow 62 dwellings at 2.17 persons/dwelling = 134.54 people 134.54 people at 170 gpd = 22,871.8 gpd Peak Demand Peaking factor = 2.3 (peaking factor per City of Bozeman Design Standards) 22,871.8 gpd (2.3) = 52,605.14 gpd Total max day flow = 52,605.14 gpd / 1440 min/day = 36.53 gpm instantaneously Peak Hour Demand 22,871.8 gpd (3) = 68,615.4 gpd = 47.65 gpm 4.2 Fire Flow Required fire flow for R-4 zoning is 3,000 gpm. Using nearby fire flow results from hydrant #2672 provided by the City of Bozeman a WaterCAD model was created to simulate fire flows from the proposed hydrant. The peak hour demand of 47.65 gpm was divided and distributed to nodes within the May Fly Townhomes Subdivision to create demands from the subdivision. Some of the existing surrounding water network was built into the model to more accurately model the proposed system. Pressure and flow in the system was simulated in the model by adding a reservoir and a pump to the model at a connection point near hydrant #2672. The reservoir Engineering Report – May Fly Townhomes Sewer System March 2023 Page 5 Version 03/29/2023 supplies the system with water and the pump is used to provided pressure to the network. The reservoir and pump were set as 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 created from the pump. Running a fire flow analysis of the system at 20 psi the hydrant is 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 smaller part of a larger network. 5 Sewer System The proposed development will utilize existing sanitary sewer infrastructure in South Laurel Parkway. The proposed development will have waste water flow north down the proposed alleys, then west down Anders Place and ultimately into a new sanitary sewer manhole located in South Laurel Parkway. This new manhole will be connected to an existing manhole in South Laurel Parkway. All proposed sewer main is to be 8” diameter SDR 25 PVC. Sewer services are to be ran to each lot and will be 4” diameter schedule 40 PVC. The proposed development is part of the Norton Ranch Lift Station basin. 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 exists. 5.1 Design Flow Rates Average daily Flow 62 dwellings at 2.17 persons/dwelling = 134.54 persons 134.54 persons at 64.4 gallons/day/person = 8,664.38 gpd Peak Flow Peaking Factor = (18+0.134541/2)/(4+0.134541/2) = 4.21 Peak Hour Flow = 4.21 x 8,664.38 gpd = 36,477.04 gpd The peak hour flow calculated above is 25.33 gpm Including infiltration: 150 gallons/acre/day (5.48 acres) = 822 gpd = 0.57 gpm Peak hour flow (including infiltration) = 25.90 gpm The proposed 8” gravity mains are capable of flowing 25.90 gpm at minimum slope (0.4%) with a depth of only 1.3”. 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 8” network located in South Laurel Parkway All sewer main piping will have uniform slope between manholes. 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 8” sewer main in South Laurel Parkway as well as the existing 10” sewer main in Fallon are adequately size for the proposed development as well as future developments. See Appendix E for sewer analysis. Engineering Report – May Fly Townhomes Stormwater Network March 2023 Page 6 Version 03/29/2023 6 Stormwater Network This section provides a design basis and hydraulic calculations for sizing storm water facilities for May Fly Townhomes. 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 alleys and residential lots. The proposed alleys will be paved asphalt with a two-foot concrete paving edge on both sides. The alleys will be paved with an inverse crown to convey stormwater runoff. Stormwater runoff generated from the alleys and residential lots is designed to be collected by the surrounding curb and gutter and then directed toward the existing storm drain inlets and one new inlet on Pond Lily Drive. The stormwater that is collected by the paved alleys will travel down the centerline of the alley and then to the curb and gutter and eventually flow north to either existing inlets or the new storm drain inlet. The runoff will then move through the storm sewer system pipes to the existing detention ponds constructed with Norton Ranch Phase 1. The detention ponds have been designed with an outlet structure to limit stormwater runoff from the developed site to the pre- developed runoff rates. The detention ponds that were built in Phase 1 of the Norton Ranch developments were increased in size due to creation of Phase 4 of Norton Ranch. This increase in detention pond size was originally designed to have built in capacity for this development. 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. Table 1 - Runoff coefficients used. RUNOFF COEFFICIENTS Undisturbed 0.2 Low-Med. Residential 0.35 Dense Residential 0.5 Alley R.O.W. 0.67 R.O.W. 0.74 Impervious 0.9 Engineering Report – May Fly Townhomes Stormwater Network March 2023 Page 7 Version 03/29/2023 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. 6.3 Detention Basins The stormwater runoff generated from the site will be conveyed to one of the existing detention ponds that was constructed with Norton Ranch Phase 1. Originally these two detention ponds were sized to detain runoff from Norton Ranch Phase 1 but, with the creation of more phases of Norton Ranch these ponds were expanded to detain larger areas. The most recent time being for Phase 4 of Norton Ranch. The proposed site is part of Norton Ranch Phase 4 and as such there is built in capacity in both detention ponds. Each detention pond has been analyzed to determine if the ponds have capacity for the proposed May Fly Townhomes development. In order to determine if the detention ponds are sized adequately an analysis of the detention basins needed to be done to determine if the proposed development generates more runoff than the ponds are sized for. The two factors that affect the runoff rate are the basin area and weighted C factor. Since the detention ponds were sized to accommodate the entirety of the May Fly Townhomes development the total area in the detention basin remains the same. May Fly Townhomes will have the same high density residential layout but with more impervious area from the proposed alleys which results in a higher weighted C factor. Engineering Report – May Fly Townhomes Stormwater Network March 2023 Page 8 Version 03/29/2023 The two detention ponds were analyzed separately. Each pond has its own contributing areas associated with it. With the creation of Norton Ranch Phase 4, Detention Pond 1 (the west pond) was expanded to have a capacity of 9,146 cubic feet of storage, per C&H Norton Ranch Phase 4 Stormwater Management Design Report. With the May Fly Townhomes development, the required capacity of detention pond 1 is 10,075 cubic feet. Coordination with the owners of detention pond 1 is in place to increase the existing pond’s capacity to have an additional capacity of 929 cubic feet to accommodate the May Fly Townhomes. Detention Pond 1 will detain stormwater runoff from Norton Ranch Phases 1-4 as well as from Urban + Farm Phase 1 and May Fly Townhomes. The detention pond outlet structure will not be changed and the weir will remain to provide a release rate of 1.43 cfs. With the creation of Norton Ranch Phase 4 Detention Pond 2 (the east pond) was expanded to have a capacity of 14,915 cubic feet of storage, per C&H Norton Ranch Phase 4 Stormwater Management Design Report. With the May Fly Townhomes development, the required capacity of Detention Pond 2 is 14,287 cubic feet of storage. The existing Detention Pond 2 is adequately sized to detain the stormwater runoff from the proposed May Fly Townhomes development. With the May Fly Townhomes development, drainage basins vary from the originally planned basins that were delineated during Norton Ranch Phase 4. The contributing basin that drains to Detention Pond 1 will be decreased by 12,207 square feet and a direct increase of 12,207 square feet will contribute to Detention Pond 2. The contributing basin for Detention Pond 1 also include areas of Urban + Farm Phase 1 and Norton Ranch Phase 3. The Fallon Apartments were included in the Norton Ranch Phase 4 calculations, but were not included in the May Fly contributing basins due to stormwater being retained on-site, per C&H Fallon Apartments Stormwater Management Design Report. In addition to detaining runoff from the surrounding developments both ponds were designed with built in retention to infiltrate, evapotranspire, or capture for reuse the runoff generated from the first 0.5 inches of rainfall from a 24-hr storm. Detention Ponds 1 and 2 retain this volume by setting the weir elevation high enough to prevent the release of this volume. In a 10-year 24-hour storm event, Detention Pond 1 is required to have 9,887 cubic feet of storage to retain the first 0.5 inches of rainfall. Detention Pond 1 is to be upsized to hold a capacity of 10,075 cubic feet, which satisfies the 0.5 inch requirement. In a 10-year 24-hour storm event, Detention Pond 2 is required to have 13,900 cubic feet of storage to contain the first 0.5 inches of rainfall. As mentioned above, Detention Pond 2 was expanded to have a capacity of 14,915 cubic feet with the development of Norton Ranch Phase 4, satisfying the 0.5 inch rainfall capacity. No further expansion of Detention Pond 2 is required with this development. The supporting calculations can be found in Appendix A of this report. 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 and analyzed 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 and analyzed 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 0.01 was used per manufactures design for the storm pipes within the model. Engineering Report – May Fly Townhomes Stormwater Network March 2023 Page 9 Version 03/29/2023 Existing inlets and storm pipes were analyzed for capacity with May Fly Townhomes runoff. The proposed inlet will connect to a proposed storm manhole which will connect to an existing 18” pipe as part of Norton Ranch Phase 2A. This 18” pipe currently has a capacity of 10.38 cfs at 17” full, the required capacity of the 25-year flow is 6.59 cfs. The 15” proposed storm pipe has a capacity of 6.39 cfs with a required of 1.67 cfs for the 25-year event. Table 2 - Pipe Capacity PIPE SIZE 25-YR FLOW RATE CAPACITY Existing Pipe Norton 2A 18” 6.59 CFS 10.38 CFS Pipe H 15” 1.67 CFS 6.39 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. Excessive runoff will be routed to storm inlets that will discharge to the existing detention ponds. Stormwater that overtops the ponds during a large rain event will flow through the emergency overflow and outlet to the adjacent wetlands north of the detention ponds. The existing detention ponds have been analyzed at a depth 1.5’, in the event of a larger than design storm the ponds have excess volume built in that will exceed a depth of 1.5’.