HomeMy WebLinkAbout08_PH 2 ENGINEERING REPORT Engineering Design Report Urban + Farm Phase 2 Infrastructure Improvements Bozeman Gallatin County, Montana November 20, 2023 Prepared For: Laurel Parkway LLC & Baxter Creek 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 2 Table of Contents November 2023 Page ii Version 11/21/2023 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 ...................................................................................................................... 5 5 Sewer System ..................................................................................................................... 6 5.1 Design Flow Rates ....................................................................................................... 6 6 Stormwater Network ........................................................................................................... 7 6.1 General Design ............................................................................................................ 8 6.2 Hydrologic Methodology............................................................................................... 8 6.3 Basin A Detention Pond ..............................................................................................10 6.4 Basin B Retention Pond ..............................................................................................11 6.5 Basin C Detention Chambers ......................................................................................11 6.6 Basin D Detention Pond ..............................................................................................11 6.7 Retention Pond E ........................................................................................................12 6.8 Retention Pond F ........................................................................................................12 6.9 Inlets & Storm Sewer ..................................................................................................12 6.10 Groundwater ...............................................................................................................13 6.11 System Maintenance ..................................................................................................13 6.12 Erosion Sediment Control ...........................................................................................14 6.13 Flooding ......................................................................................................................14
Engineering Report – Urban + Farm Phase 2 Introduction November 2023 Page 3 Version 11/21/2023 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 33-lot major subdivision. The proposed infrastructure has been designed accounting for future connections, as the property is bordered to the south and west by undeveloped land. 1.2 Scope Expansion of existing infrastructure (roads, 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 86.45 acres and is located on the western side of Bozeman and is bordered to the north by Norton East Ranch Phase 4 and J C Billion Auto Plaza Subdivision to the East, 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 REMU and B-2M. The nearby surrounding zoning is R-4, R-5, RO, and B-2. 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 2 Land Use November 2023 Page 4 Version 11/21/2023 3 Land Use 3.1 Existing The site is currently two vacant lots with six existing intersection tie-ins –South Eldorado Avenue, South Reliance Avenue, South Laurel Parkway, Pond Lily Drive, Water Lily Drive, and Competition Drive. The majority of the site has been left undisturbed. The subject area is comprised of 82.14 acres of B-2M zoning and 4.31 acres of REMU zoning. 3.2 Proposed The proposed major subdivision will consist of 33 developable lots & is proposed to satisfy section 38.410.130 by cash-in-lieu water rights payments, as well as existing irrigation wells. At this time, those cash-in-lieu of water rights fees will be deferred until future site plan or subdivision application. The lots will be comprised of commercial and residential uses. Through this development South Eldorado Avenue, South Reliance Avenue, South Laurel Parkway, Pond Lily Drive, Water Lily Drive, and Competition Drive will all be extended. Both water and sewer mains will connect to existing stub-outs from the improvements associated with surrounding developments. Water and sewer mains will provide service to the proposed lots through main extensions and service connections. Stormwater storage ponds will be sized to accommodate the runoff rates and volumes from this project, see section 6 of this report. 4 Water System The proposed water system will connect to and extend existing mains installed during the development of the East Norton Ranch Phase 4, Urban + Farm Phase 1, and to the west in Competition drive. The water system will be looped into the City of Bozeman water system at South Eldorado, South Reliance, South Laurel Parkway, Pond Lily Drive, Water Lily Drive, and Competition Drive. New water mains will be 8”, 12”, or 16” diameter, class 51 ductile iron pipe. Part of the proposed development will include the connection to the existing network south of Huffine Lane. As part of this connection a PRV will be installed on the south side of Huffine Lane. This PRV will be connected to the existing 10” main and will reduce the pressure coming from the south zone to the west zone. New 16” ductile iron pipe water main will be provided to the proposed connection point at Laurel Parkway. The 12” main in Laurel Parkway will be installed in a new casing bored under Huffine Lane to connect to the new 16” main along the south side of Huffine Lane. 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. Lots will be provided connection to city water by either a 2” type K copper water service and a 4” class 51 ductile iron pipe for fire service, or an 8” water main extended into the lot and capped for future main extensions and services. 4.1 Water Use Data Average Daily Flow Average day use B-2M = 705 gpd/ac Average day use REMU = 873 gpd/ac
Engineering Report – Urban + Farm Phase 2 Water System November 2023 Page 5 Version 11/21/2023 Acres B-2M = 82.14 Ac Acres REMU = 4.31 Ac 82.14 ac at 705 gpd/ac = 57,908.7 gpd 4.31 ac at 873 gpd/ac = 3,762.6 gpd Total Average Daily Flow = 61,671.3 GPD Peak Demand Peaking factor = 2.3 (peaking factor per City of Bozeman Design Standards) 61,671.3 gpd (2.3) = 141,843.99 gpd Total max day flow = 141,843.99 gpd / 1440 min/day = 98.50 GPM instantaneously Peak Hour Demand 61,671.3 gpd (3) = 185,013.9 gpd = 128.48 GPM 4.2 Fire Flow Required fire flow for B-2M and REMU zonings is 3,000 gpm. With a Peak Hour Demand of 128.48 gpm and a fire requirement of 3,000 gpm total flow requirement is 3,128.48 gpm. Fire hydrant data has been provided for surrounding hydrants demonstrating adequate fire flow for the development. Nearby hydrant #2673 has an available flow of 5,117 gpm at a residual pressure of 20 psi. The proposed development will be entirely in the West Pressure Zone. This pressure zone is a sub-zone within the South zone. The West Zone has an operating hydraulic grade line of 4,980. In order to model fire flow availability a WaterCAD model was built using reservoirs to supply water as well as pressure to the system. The proposed development will be supplied water from three major connection points, a 12” main in West Babcock, an 8” main in Fallon, and a new tie in location from Loyal gardens which will cross Huffine. The connections in West Babcock and Fallon are part of the existing network of the Norton Ranch developments and supply water to the entire area. In order to properly model these connections much of the existing network was included in the WaterCAD model. Using the operating HGL of 4,890 three reservoirs were placed at the three connection points. These reservoirs are the sole source of water as well as pressure for the model. System peak hour demands from the proposed development were distributed throughout the network. Required fire flow for the development is 3,000 gpm at 20 psi based on zoning. A fire flow analysis scenario was ran on the system to determine if adequate fire suppression flow rates are available. All hydrants in the system provide a minimum of 3,000 gpm at 20 psi. The City of Bozeman has provided hydrant data for hydrant #2673. As a check on the model this existing hydrant was also included as part of the network. In order to compare the hydrant, the model was ran with only the existing tie-in locations supplying water and pressure. This scenario models the existing system without the third proposed tie-in. When running the existing scenario, the predicted fire flow available from hydrant #2673 at 20 psi is 5,254 gpm. The provided data from the City of Bozeman claims hydrant #2673 can supply 5,117 gpm at 20 psi. This analysis check demonstrates that the model built for Urban + Farm Phase 2 is comparable to the model being used by the City of Bozeman.
Engineering Report – Urban + Farm Phase 2 Sewer System November 2023 Page 6 Version 11/21/2023 5 Sewer System The proposed development will extend existing sewer mains throughout the development to supply wastewater service to the proposed lots through either sewer services or main extension. Each lot will have wither a 6” sewer service or an 8” sewer main stubbed into the property and capped. The proposed development will be comprised of two sewer drainage basins. The west portion of the development will flow north into the existing 10” sewer main that was created during the construction of Phase 4 of Norton East Ranch. This existing 10” sewer main ultimately conveys wastewater to the Norton Ranch Lift Station. The Norton Ranch Lift Station is currently at capacity and requires upgrades. A long-term improvement plan is currently being created for the upgrade required for the lift station. Hyalite has been working with surrounding developers as well as HDR for the required upgrades and cost share. An analysis of capacity the existing sewer network has been conducted in previous analysis demonstrating adequate pipe sizing in existing infrastructure. The eastern portion of the proposed development will generally flow north east where the proposed sewer main network will connect to an existing manhole in Competition Drive. From this existing manhole wastewater will flow east down Fallon Street in an 8” main and will ultimately flow into South Cottonwood Road. According to the City of Bozeman the sewer mains that the east portion of the proposed development will contribute to are less than 50% obligated. 5.1 Design Flow Rates Average Daily Flow West Basin Average day use B-2M = 2,000 gpd/ac Average day use REMU = 1,456 gpd/ac Acres B-2M = 47.45 Ac Acres REMU = 4.31 Ac 47.45 ac at 2,000 gpd/ac = 94,900 gpd 4.31 ac at 1,456 gpd/ac = 6,275.36 gpd Total Average Daily Flow = 101,175 GPD Peak Flow West Basin Assuming 2000 gallons per acre and 89 gallons per capita = (2000 gal/ac) / (89 gal/capita) = 22.47 persons/acre (51.76 ac) x (22.47 persons/ac) = 1,163 persons Peaking Factor = (18+1.1631/2) / (4+1.1631/2) = 3.62 Peak Hour Flow = 3.62 x 101,175 gpd = 366,254 gpd Peak Hour Flow = 254.34 gpm. Including infiltration: 150 gallons/acre/day (51.76 acres) = 7,764 gpd = 5.39 gpm Peak hour flow (including infiltration) = 259.68 GPM
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 7 Version 11/21/2023 Average Daily Flow East Basin Average day use B-2M = 2,000 gpd/ac Acres B-2M = 34.67 Ac 34.67 ac at 2,000 gpd/ac = 69,340 GPD Peak Flow East Basin Assuming 22.47 persons/acre (34.67 ac) x (22.47 persons/ac) = 779 persons Peaking Factor = (18+0.7791/2) / (4+0.7791/2) = 3.87 Peak Hour Flow = 3.87 x 69,340 gpd = 268,346 gpd Peak Hour Flow = 186.35 gpm Including Infiltration: 150 gallons/acre/day (34.67 acres) = 5,201 = 3.61 gpm Peak Hour Flow (including infiltration) = 189.96 GPM The proposed 8” gravity main in the west basin are capable of flowing 259.68 gpm at minimum slope (0.40%) with a depth of 5.2 inches. The proposed 10” gravity main in the west basin is capable of flowing 259.68 gpm at minimum slope (0.28%) with a depth of 5 inches. A Manning’s friction factor of 0.013 was used to calculate normal depth. The existing sewer network that will be affected by the proposed development has been analyzed for capacity. The existing 10” sewer main in Reliance as well as the existing 15” sewer main in Laurel Parkway are adequately sized for the proposed development. The analysis was conducted during the design and permitting of Urban + Farm Phase 1 and was performed using more conservative methods than what is used in this report. The proposed 8” gravity sewer mains in the east basin are capable of flowing 189.96 gpm at minimum slope (0.40%) with a depth of 4.8 inches. A Manning’s friction factor of 0.013 was used to calculate normal depth. 6 Stormwater Network This section provides a design basis and hydraulic calculations for sizing storm water facilities for Urban + Farm Phase 2. The City of Bozeman Design Standards were used as the primary guidelines for this stormwater drainage design.
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 8 Version 11/21/2023 6.1 General Design The proposed development will be a combination of roadway, residential, and commercial lots as well as parks and open space. The proposed streets will be paved with curb and gutter. Stormwater runoff generated from the roadway and lots is designed to be collected by the curb and gutter and then directed toward storm drain inlets. The runoff will then either infiltrate or move through storm sewer system pipes to detention/retention ponds associated with each basin. The entire area within the proposed development was accounted for in the design of the stormwater infrastructure, including the lot areas outside of the publicly dedicated rights-of-way. The proposed stormwater piping, ponds, inlets, etc. have been sized appropriately for the entire development based on current zoning/anticipated land use (see included Storm Water Basins figure 1 of 2 and the corresponding Runoff Coefficient in Section 6.2 in this report), and analyzed in each of the stormwater basins described below. This methodology is practical for the majority of the development, however, due to existing topography, some lots/areas will have difficulty routing all stormwater to the common infrastructure and thus a portion of the stormwater runoff may need to be mitigated on-site. Future development on each of the lots created with Urban + Farm Phase 2 will be subject to stormwater design review through the City of Bozeman’s Site Plan review process. 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 Open Land 0.2 Dense Residential 0.5 Commercial 0.60 R.O.W. 0.73 The right-of-way coefficient was determined using quantities from the local street cross section detail (see sheet C5.16). Of the 60-foot right-of-way, 45-feet is impervious and the remaining 15-
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 9 Version 11/21/2023 feet is considered open land. Using those quantities and their corresponding C-values provides a cumulative C-value of 0.73. 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 conservatively 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: V = 7200Q Where V = Volume, cf Q = flow rate, cfs The City of Bozeman requires infiltration, evapotranspiration or capture for reuse of the runoff generated from the first 0.5 inches of rainfall from a 24-hour storm. A common methodology for estimating the volume of direct runoff from a drainage basin is the SCS method. This method requires basic data similar to the Rational Method: drainage area, a runoff factor, time of concentration, and rainfall. However, the SCS approach is more sophisticated in that it also
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 10 Version 11/21/2023 considers the time distribution of the rainfall and an infiltration rate that decreases during the course of a storm. The SCS method takes into account the Initial Abstraction (Ia) which is all losses before runoff begins. This includes surface depressions, water intercepted by vegetation, evapotranspiration and infiltration. Initial abstraction generally correlates with soil and cover parameters. Through studies of many small watersheds, Ia was found to be approximated by the following empirical equation: Ia = 0.2 x S [eq. 2-2 TR-55] Where S = (1000/CN) – 10 [eq. 2-4 TR-55] According to USDA’s Urban Hydrology for Small Watersheds, the Curve Number associated with soil type B and 1/8 acre residential lots is 85 (Table 2-2a). 1/8 acre lots was selected based on a high residential density. Using the standard SCS method, S = (1000/85) – 10 = 1.8 inches Ia = 0.2 * 1.8 inches = 0.4 inches 6.3 Basin A Detention Chambers Detention Pond A is designed to discharge stormwater runoff at a rate less than the predevelopment rate from the 10-year storm. This runoff rate was determined using the rational method per City of Bozeman Design Standards and was determined based on the remaining capacity in the Norton Ranch Phase 5 detention pond. The release rate will be controlled by an outlet weir which has been designed to reduce discharge to the predetermined rates for remaining capacity of the Norton Ranch Phase 5 detention pond and allow the 100-year event to overtop the weir and discharge at an unrestricted release rate. The detention pond will discharge into existing storm infrastructure that was installed & approved as part of Urban + Farm Phase 1. The 2.5 cfs release rate from Pond A was accounted for in the Urban + Farm Phase 1 storm pipe calculations, and the downstream infrastructure from Pond A’s outlet was analyzed in Section 6.4 of the Urban + Farm Phase 1 Engineering Report. The additional capacity of the existing 15” stormwater pipe (Norton Ranch Phase 3C) was calculated to be 7.61 cfs, with the flowrate from Urban + Farm Phase 1 being 4.68 cfs. The addition of the 2.50 cfs outlet from the proposed Pond A results in 7.18 cfs total. This existing infrastructure connects to the Norton Ranch stormwater network which ultimately discharges to a detention pond created with Phase 5 of Norton Ranch. The Phase 5 Norton Ranch Detention pond then discharges stormwater to Aajker Creek at a pre-development rate per separate approvals. Basin A was not originally planned as a contributing area to the Norton Ranch Phase 5 detention pond; however, the Norton Ranch Phase 5 detention pond was oversized leaving an additional storage volume of 10,649 cubic feet. This oversizing was originally for the potential groundwater infiltration that occurs in the area, but after subsequent fixes made by Norton Ranch, the then available capacity was able to be utilized for additional detention capacity. The Norton Ranch Phase 5 detention pond has been analyzed with the additional contributing area and is currently adequately sized. Detention Pond A is required to have a volume of 34,856 cubic feet. Using a 10-year 24-hour rain event in the City of Bozeman, the total precipitation (P) is 1.88 inches of which 0.4 inches is lost due to initial abstraction according to USDA’s SCS method. Therefore,
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 11 Version 11/21/2023 this drainage plan includes provision for the first 0.4 inches of rainfall from the 24-hour storm to be retained in surface depressions, intercepted by vegetation, or evapotranspired or infiltrated. The remaining 0.1 inches required to be captured is accounted for by setting the weir elevation above the bottom of the pond to retain the volume from the remaining 0.1 inches. The remaining volume from the first 0.5” is 18,769 cubic feet. The detention pond will utilize three separate, stepped, perforated underground storage sections, each being hydraulically connected and consisting of identical chamber layouts. The invert elevation of each of the chamber sections steps in elevation to maintain separation from seasonally high groundwater. Each of the chamber sections will be outlet controlled to ensure that the entire volume of each section will be used, with the furthest downstream outlet being held to 2.5 cfs. 6.4 Basin B Retention Chambers Basin B is centrally located on the subject property. Stormwater runoff from Basin B will be conveyed to a retention pond located along the north border of the site. Stormwater will be conveyed through the use of storm drain inlets and piping. Basin B is comprised of right-of-way, high density residential, and opens space. This retention pond is sized to handle the entire volume of a 10-year, 2-hour storm event. This retention pond will store runoff as it evaporates and percolates into the existing gravels on site. The required retention storage for Basin B is 45,903 cubic feet. The detention pond will utilize three separate perforated underground storage sections, each being hydraulically connected and consisting of identical chamber layouts. The invert elevation of each of the chamber sections steps in elevation to maintain separation from seasonally high groundwater. Each of the two upstream chamber sections will be outlet controlled to ensure that the entire volume of each section will be used, and no outlet will be installed for the furthest downstream chamber section. 6.5 Basin C Detention Chambers Basin C has been split into two separate basins: C1 and C2. Basin C1 is the southern half of the basin, with C2 being the remaining northern portion. All southern infrastructure in Basin C will convey stormwater runoff to the underground storage facility C1 located near the center of Water Lily Drive. The stormwater will then discharge into the northern infrastructure and will ultimately be conveyed through Pond C2 and into Baxter Creek. Each of the Ponds C1 and C2 are sized for their respective basin areas and will be controlled by an outlet weir which has been designed to reduce discharge to the predevelopment runoff rates. Using a 10-year 24-hour rain event in the City of Bozeman the total precipitation (P) is 1.88 inches of which 0.4 inches is lost due to initial abstraction according to USDA’s SCS method. Therefore, this drainage plan includes provision for the first 0.4 inches of rainfall from the 24-hour storm to be retained in surface depressions, intercepted by vegetation, or evapotranspired or infiltrated. The remaining 0.1 inches required to be captured is accounted for by setting the weir elevation above the bottom of the chambers to retain the volume from the remaining 0.1 inches. The remaining volume from the first 0.5” is 11,864 cubic feet. 6.6 Basin D Detention Pond
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 12 Version 11/21/2023 Detention Pond D is designed to discharge stormwater runoff at the predevelopment rate from the 10-year storm. This runoff rate was determined using the rational method per City of Bozeman Design Standard. The release rate will be controlled by an outlet weir which has been designed to reduce discharge to the predevelopment rates and allow the 100-year event to overtop the weir and discharge at an unrestricted release rate. The detention pond will discharge into Baxter Creek at the predevelopment rate. With the proposed development the required storage volume for the Basin D detention pond is 1,788 cubic feet. Using a 10-year 24-hour rain event in the City of Bozeman the total precipitation (P) is 1.88 inches of which 0.4 inches is lost due to initial abstraction according to USDA’s SCS method. Therefore, this drainage plan includes provision for the first 0.4 inches of rainfall from the 24-hour storm to be retained in surface depressions, intercepted by vegetation, or evapotranspired or infiltrated. The remaining 0.1 inches required to be captured is accounted for by setting the weir elevation above the bottom of the pond to retain the volume from the remaining 0.1 inches. The remaining volume from the first 0.5” is 1,258 cubic feet. The detention pond was sized using City of Bozeman Design Standards, and side slopes will be no steeper than 4:1. 6.7 Retention Pond E Retention Pond E is located within sub-basin 16B. Stormwater runoff from sub-basin 16B will be conveyed to the retention pond located along the north border of the sub-basin area. Stormwater will be conveyed over a flow over forebay manhole, into rip rap which leads into the proposed pond. Sub-basin 16B is comprised of right-of-way, high density residential, and opens space. This retention pond is sized to handle the entire volume of a 10-year, 2-hour storm event. This retention pond will store runoff as it evaporates and percolates into the existing gravels on site. The required retention storage for Pond E is 1,082 cubic feet. The retention pond was sized using City of Bozeman Design Standards, and side slopes will be no steeper than 4:1. 6.8 Retention Pond F Retention Pond F is located within sub-basin 17B. Stormwater runoff from sub-basin 17B will be conveyed to the retention pond located along the north border of the sub-basin area. Stormwater will be conveyed over a flow over forebay manhole, into rip rap which leads into the proposed pond. Sub-basin 17B is comprised of right-of-way and high density residential area. This retention pond is sized to handle the entire volume of a 10-year, 2-hour storm event. This retention pond will store runoff as it evaporates and percolates into the existing gravels on site. The required retention storage for Pond F is 292 cubic feet. The retention pond was sized using City of Bozeman Design Standards, and side slopes will be no steeper than 4:1. 6.9 Off-site Sub-basins Three street drainage sub-basins discharge directly off-site. The three sub-basins consist of the northernmost portions of Eldorado Avenue, Reliance Avenue, and Laurel Parkway rights of way, totaling 38,339 square feet in area. Per the City of Bozeman Design Standards, adequate on-site
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 13 Version 11/21/2023 stormwater detention shall be provided for design storm runoff exceeding the pre-development rate. As the contributing runoff for each of these sub-basins is much less than the pre-developed rate of the basin as a whole, on-site detention is not provided. Each of these sub-basins will runoff to the north and be captured within Urban + Farm Phase 1 infrastructure, and ultimately discharge in the Norton Ranch Phase 5 detention pond. With Basin A of the proposed development (described above), the required storage volume for the Norton Ranch Phase 5 detention pond is 47,185 cubic feet and the provided volume is 47,688 cubic feet. Of the 503 cubic feet of available storage, 319 cubic feet will account for these three street drainage basins, based on the above-mentioned first 0.1” of rainfall on these areas. 6.10 Inlets & Storm Sewer Sub-basins were first analyzed using the Rational Method to determine the contributing flow from each of the sub-basin areas. Next, the hydrologic and hydraulic analyses were modeled using AutoDesk Storm and Sanitary Analysis 2023. This program is a continuous rainfall simulation model that simultaneously models hydrologic storm events and routes runoff through multiple stormwater structures. The Bozeman IDF curve was imported into the model and used to determine catchment area flows, pipe conveyance calculations, and inlet capacity. User defined sub-basin information (area, runoff coefficient, time of concentration) are entered into the model to achieve a more accurate representation. Storm pipes are sized to convey the 25-year design storm event. 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 used for all storm pipe conveyance calculations. Each inlet grate has been located and deisgned to accommodate the 25-year storm event without exceeding a depth of 0.15’ below the top of the curb. Inlets and manholes will have a 9” sump for sediment collection. 6.11 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. Groundwater monitoring data has been included in this submittal. The hydraulic gradient across the site has been modeled using the highest recorded measurement at each of the monitoring wells. The modeled groundwater surface was used for design of all site infrastructure. Irregular groundwater behavior was observed starting in April of 2022 and reached a peak during May of 2022. During this time construction on a nearby development resulted in the accidental damaging of an existing drain tile. Due to the break in the existing drain tile groundwater irregularly rose on site due to the lack of subsurface drainage. Since this break, the drain tile has been repaired and is functioning as in its previous state. Due to this irregularity data from the peak 2021 season has been used to establish groundwater elevations. 6.12 Park G Recreation Pond Overflow
Engineering Report – Urban + Farm Phase 2 Stormwater Network November 2023 Page 14 Version 11/21/2023 The recreation pond located within Park G will be completely lined and fed by the nearby irrigation well. As the pond is man-made and does not have a natural outfall, the pond will discharge into an underground infiltration system that has been sized to accommodate the semi-continuous flow from the irrigation well. Percolation tests have been performed on-site to determine the infiltration rate of the system and infiltration system details have been included within the Storm Detail sheets. 6.13 System Maintenance Regular maintenance of stormwater facilities is necessary for proper function of the drainage system. All stormwater maintenance located outside of the public right-of-way will be owned and maintained by the property owners association. Maintenance items include removing debris from inlet grates and culverts, cleaning and flushing pipes, cleaning manhole sumps, and establishing ground cover after construction. 6.14 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. 6.15 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.