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HomeMy WebLinkAbout4._EngineeringReport_08162021 Engineering Report – Parkview Crossings Major Subdivision Table of Contents April 2021 Page ii Version 04/16/2021 Table of Contents Table of Contents ........................................................................................................................ ii 1 Introduction ......................................................................................................................... 1 1.1 Purpose of Report ........................................................................................................ 1 1.2 Scope .......................................................................................................................... 1 2 Location and Site Information ............................................................................................. 1 3 Land Use ............................................................................................................................ 2 3.1 Existing ........................................................................................................................ 2 3.2 Proposed ..................................................................................................................... 2 4 Water System ..................................................................................................................... 2 4.1 Water Use Data ........................................................................................................... 2 4.2 Fire Flow ...................................................................................................................... 3 5 Sewer System ..................................................................................................................... 3 5.1 Design Flow Rates ....................................................................................................... 4 6 Stormwater Network ........................................................................................................... 4 6.1 General Design ............................................................................................................ 4 6.2 Hydrologic Methodology............................................................................................... 4 6.3 Detention Basin ........................................................................................................... 5 6.4 Inlets & Storm Sewer ................................................................................................... 6 6.5 Groundwater ................................................................................................................ 7 6.6 System Maintenance ................................................................................................... 7 6.7 Erosion Sediment Control ............................................................................................ 7 6.8 Flooding ....................................................................................................................... 7 References ................................................................................................................................ 9 Engineering Report – Parkview Crossings Major Subdivision Introduction April 2021 Page 1 Version 04/16/2021 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 44-lot major subdivision. The proposed infrastructure has been designed without accounting for future connections, as the property is bordered to the west by the Gallatin County Regional Park. 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 7.0 acres and is located on the northwestern side of Bozeman and is bordered to the east by Vaquero Parkway, in the NE ¼ of Section 3, Township 2 South, Range 5 East, P.M.M., City of Bozeman, Gallatin County, MT. The existing zoning and the nearby surrounding zoning is R-3. The property is within the service area for municipal water and sewer from the City of Bozeman. Figure 1 - Vicinity map. Engineering Report – Parkview Crossings Major Subdivision Land Use April 2021 Page 2 Version 04/16/2021 3 Land Use 3.1 Existing The site is currently a vacant lot with two existing intersection tie-ins – one off of Vaquero Parkway and the other a continuation of Tschache Lane. The majority of the site has been left undisturbed, with parking for the Gallatin County Regional Park just to the south. Of the 7.0-acre property, 0.3 acres is an existing stormwater pond site that was used for the Crossings 2 at Baxter Meadows Phases 4G & 4H. 3.2 Proposed The proposed major subdivision will consist of 44 townhome lots: nine 4-plex units, two 3-plex units and one duplex. A road will be constructed that connects to the intersection of Tschache Lane and Vaquero Parkway and loops to the north to connect back into Vaquero Parkway. The road will surround a majority of the property and will be bordered by the park to the west. All centralized lots will be alley-loaded with water and sewer mains installed within the 30’ alley right- of-way. The exterior lots will be serviced by the loop road. As mentioned previously, both water and sewer mains will connect to existing stub-outs from the improvements associated with Phases 4G & 4H of the Crossings 2 at Baxter Meadows. The existing stormwater pond to the north will be extended to account for the proposed development, with the proposed storm sewer network also connecting to existing stub-outs from previous improvements (Phase 4G & 4H). 4 Water System The proposed water system will tie into existing mains installed during the development of the Crossing 2 at Baxter Meadows Phases 4G & 4H. The water system will be looped into the City of Bozeman water system at two places along Vaquero Parkway. At each end of the exterior loop road (north and south) the water main will continue past the proposed alley and each will end at a fire hydrant near the N-S leg of the loop road. The water main will also be installed within the provided alley right of way. 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. Type K copper water services will extend 8’ past the property line and be ¾” in diameter. 4.1 Water Use Data Average Daily Flow 44 residential lots at 2.5 persons/dwelling = 110 people 110 people at 170 gpd = 18,700 gpd (425 gpd/lot) Peak Demand Peaking factor = 2.3 (peaking factor per City of Bozeman Design Standards) 18,700 gpd (2.3) = 43,010 gpd Total max day flow = 43,010 gpd / 1440 = 29.9 gpm instantaneously Main Sizing 29.9 GPM (Max Day) + 2000 GPM (Fire Flow) = 2029.9 GPM 2029.9 GPM in 8” Water Main ≈ 14 ft/sec (adequate for fire flow) Engineering Report – Parkview Crossings Major Subdivision Sewer System April 2021 Page 3 Version 04/16/2021 29.9 GPM = Negligible velocity within an 8” main Note: The Integrated Water Resources Plan (2013) showed that the typical domestic water usage in a City of Bozeman home was around 45 gpcd. The Bozeman Water Facility Plan (updated 2017) noted that “The domestic use parameter is consistently lower than 70 gpcd for the non-irrigation season”. With irrigation being accounted for in the next section, the above calculations are conservative. Irrigation Demand Assume 1”/week for irrigation of turf areas Assume 4-month irrigation season Building Areas: Nine 4-plex units = 9 (7820 sq. ft.) = 70,380 sq. ft. Two 3-plex units = 2 (6800 sq. ft.) = 13,600 sq. ft. One duplex unit = 4340 sq. ft. Total = 88,320 sq. ft. = 2.03 acres Above calculations include driveway and sidewalk for each unit. Total developable lot area = 4.07 acres Irrigated turf = 2.04 acres 1”/week (16 weeks) = 1.33 ft of irrigation over 2.04 acres Total irrigation = 118,187 cu. ft. (7.48 gal/cu. ft.) = 884,040 gal./year Over 120-day irrigation season, 15 hours of irrigation per day 884,040 gal./year (1 year/120 days) (1 day/15 hrs) (1 hr/60 min) = 8.2 gpm At 8.2 gpm during the summer months 8.2 gpm (60 min/hr) (24 hr/day) = 11,808 gpd Peak hour flow during the summer/irrigation season would be 38.1 gpm. 4.2 Fire Flow Within the design report for The Crossing 2 at Baxter Meadows, the fire hydrants tested for the fire protection plan were each at or above 2,000 GPM for available fire flow. As of April 12, 2021, the City Water Department fire tested the nearby hydrants with the following information; FH#2389 had a static pressure of 90psi and a residual of 82psi and FH#3033 having a pitot pressure of 68psi while flowing approximately 1385 GPM. This results in an available fire flow of approximately 4,460 GPM at 20 psi. See attached for the City’s fire flow documentation. 5 Sewer System During construction of Phases 4G & 4H of the Crossing 2, an 8-inch sewer main stub was extended from a manhole on Vaquero Parkway. This sewer main stub is located at the northern loop road tie in for the proposed development. As this is the low point of the property, the entire sanitary sewer network within Parkview Crossing will drain to this point, and connect to the existing 8” sewer mains in Vaquero Parkway. Engineering Report – Parkview Crossings Major Subdivision Stormwater Network April 2021 Page 4 Version 04/16/2021 5.1 Design Flow Rates The average daily flow calculated above is 18,700 gpd (13 gpm). The peak hour flow calculated above is 38.1 gpm. Including infiltration: 150 gallons/acre/day (7 acres) = 1050 gpd = 0.73 gpm Peak hour flow (including infiltration) = 38.83 gpm The proposed 8” gravity mains are capable of flowing 38.83 gpm at minimum slope (0.4%) with a depth of only 1.9” (24% full). 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 Vaquero Parkway. All sewer main piping will have uniform slope between manholes. Sewer mains have been designed at nearly the same slope as the road centerline grades. 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 single-family townhome. 6 Stormwater Network This section provides a design basis and hydraulic calculations for sizing storm water facilities for the Parkview Crossings Major Subdivision. The City of Bozeman Design Standards and Specifications Policy and the Montana Post-Construction Storm Water BMP Design Guidance Manual (Peterson, Savage, and Heisler 2017) 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 loop road that extends from the intersection of Tschache Lane and Vaquero Parkway will be paved with curb and gutter. The alley that splits 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 alley will travel down the centerline and to the curb and gutter to the north. The runoff will then move through the storm sewer system pipes to the detention pond. 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) Engineering Report – Parkview Crossings Major Subdivision Stormwater Network April 2021 Page 5 Version 04/16/2021 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 Impervious 0.9 Time of concentration 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 The modified rational method approach was used to compute runoff volume. 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. The total runoff volume is computed by multiplying the duration of the storm by the runoff rate. The method outlined in Appendix C of the City of Bozeman Design Standards and Specifications was used to size the detention pond for the 10-yr flood control volume. Hydraflow Hydrographs Extension for AutoCAD was also used to calculate this volume and develop a stage-storage graph for the pond. The program gave a more conservative volume for the 10-year design storm so the weir was sized using this program. The hydrology calculations and pond sizing spreadsheets can be found in Appendix B. 6.3 Detention Basin 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 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 Engineering Report – Parkview Crossings Major Subdivision Stormwater Network April 2021 Page 6 Version 04/16/2021 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). Using the standard SCS method, S = (1000/85) – 10 = 1.8 inches Ia = 0.2 * 1.8 inches = 0.4 inches 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. Pond 5B was designed to be oversized with the improvements for Phases 4G & 4H to account for future development along Vaquero Parkway. However, the exact use of the future development was not known at that time. The density of the proposed property is higher than previously accounted for; thus, the pond will be extended and larger volume provided. As the existing calculations still remain true for Pond 5B (outlet structure, weir, outlet pipe) the calculations will be expanded on and updated to reflect the larger volume. The pond is designed with an outlet structure to limit stormwater runoff from the developed site to the pre-developed runoff rates for the 10-year storm and to not overtop during the 100-year storm. The outlet structure for the pond will be re-located, but the outfall location will remain the same. The original stormwater design (Phases 4G & 4H) accounted for a basin area of 9.48 acres to determine the pre-development flow rate that would be controlled by the outlet structure. This differs from the current 10.71 acre basin area that is proposed. Considering the increase in basin area from the original to the current design and the fact that the outlet structure will not be altered, the detention release rate will be less than the pre-development rate for the larger basin. The method outlined in Appendix C of the City of Bozeman Design Standards and Specifications was used to size the detention pond for the 10-year flood control volume. This method assumes the outlet structure release rate is always equivalent to the pre-development runoff rate, and thus provides a good starting point for overall pond volume. Hydraflow Hydrographs Extension for AutoCAD was also used to calculate the pond volume and develop a stage-discharge graph for the outlet structure. The stage-discharge graph better represents the changing weir flowrate that will occur with the varying water surface elevations in the pond. Per the report generated from the Hydraflow Extension, the weir flow rate will equal the pre-development flow rate at a pond depth of approximately 18 inches (the top of the rectangular weir). In the event of a 100-year storm there is an emergency spillway that will prevent the pond from over topping. This spillway will be lined with riprap to prevent erosion and will outlet directly into the respective outlet ditch. 6.4 Inlets & Storm Sewer Engineering Report – Parkview Crossings Major Subdivision Stormwater Network April 2021 Page 7 Version 04/16/2021 Using the Modified 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 at sag points were modeled using 50% clogging. Inlets and manholes will have a 9” sump for sediment collection. Storm pipes were sized to accommodate the 100-year storm event and were modeled in Storm and Sanitary Analysis. Pipes are sloped to maintain a minimum velocity of 3 ft/s when flowing full to prevent sediment deposit. The Hydraulic Grade line was plotted throughout the system and pipes were sized to keep the HGL within the pipe during the 25 yr storm and 100 yr storm. A manning’s N of 0.009 was used for the A-2000 Storm pipe within the model per the manufacturer’s specifications. 6.5 Groundwater Groundwater is known to be high in this area. A geotechnical report was done in 2001 that included borings near the pond location. ST-29 near Pond 5B showed depth to groundwater at 3 feet below existing ground. Due to the high groundwater the maximum pond excavation depth for Pond 5B is 2 feet. The depth of the existing Pond 5B will not change, only the overall area of the pond will increase. 6.6 System Maintenance Regular maintenance of stormwater facilities is necessary for proper function of the drainage system. In general, regular mowing of detention pond areas and removal of debris from the outlet structure will be required to maintain functionality of the system. Sediment removal from the pond may be required over time to restore the detention pond volume. Additional maintenance items include removing debris from inlet grates and culverts, cleaning and flushing pipes, cleaning manhole sumps, and establishing ground cover after construction. The detention pond will be maintained by the HOA. 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 Engineering Report – Parkview Crossings Major Subdivision Stormwater Network April 2021 Page 8 Version 04/16/2021 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 the respective ditches. Engineering Report – Parkview Crossings Major Subdivision References April 2021 Page 9 Version 04/16/2021 References Peterson, Matt, Spencer Savage, and Vern Heisler. 2017. “Montana Post-Construction Storm Water BMP Design Guidance Manual.”