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HomeMy WebLinkAbout2.0_Engineering Report Engineering Design Report 6590 Davis Lane Infrastructure Improvements Bozeman Gallatin County, Montana July, 2024 Prepared For: S&S Davis LLC 640 Taylor Street Suite 2200 Fort Worth, Texas 76102 Prepared By: Brian Van Rooyen, PE Hyalite Engineers, PLLC 2304 N 7th Ave. Suite L Bozeman, MT 59715 Engineering Report – 6590 Davis Lane Master Site Plan Table of Contents July 2024 Page ii Version 07/31/2024 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 ........................................................................................................... 5 4.2 Fire Flow ...................................................................................................................... 7 5 Sewer System ..................................................................................................................... 7 5.1 Design Flow Rates ....................................................................................................... 7 5.2 Buoyancy ..................................................................................................................... 7 6 Stormwater Network ........................................................................................................... 8 6.1 General Design ............................................................................................................ 8 6.2 Hydrologic Methodology............................................................................................... 9 6.3 Detention Basins .........................................................................................................10 6.4 Inlets & Storm Sewer ..................................................................................................11 6.5 Groundwater ...............................................................................................................11 6.6 System Maintenance ..................................................................................................11 6.7 Erosion Sediment Control ...........................................................................................11 6.8 Flooding ......................................................................................................................12 Engineering Report – 6590 Davis Lane Master Site Plan Introduction July 2024 Page 3 Version 07/31/2024 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 multi-family mixed-use apartment complex with 300 individual apartment units in five multi-story buildings. The proposed infrastructure has been designed accounting for future connections from the south, as the project will connect to the existing sewer main within the Davis Lane right-of-way and will extend water from the existing dead-end water main near the intersection of Davis Lane and Galloway Street. 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 8.55 acres and is located on the northwestern side of Bozeman bordered to the west by Davis Lane, in the SW ¼ of Section 35, Township 1 South, Range 5 East, P.M.M., City of Bozeman, Gallatin County, MT. The property has recently been annexed into the City as B-2M zoning and is within the service area for municipal water and sewer from the City of Bozeman. Figure 1 - Vicinity map. Engineering Report – 6590 Davis Lane Master Site Plan Land Use July 2024 Page 4 Version 07/31/2024 3 Land Use 3.1 Existing The existing property contains a single-family dwelling that is serviced with an on-site well and wastewater treatment system. The home, wastewater treatment system, and well will all be properly abandoned and removed from site. City of Bozeman water and sewer mains will both be extended to meet the demands of the proposed development. Stormwater runoff will be captured and detained on-site utilizing underground storage with the adjacent Section Line Ditch ultimately being the outfall location. There are jurisdictional wetlands located along the Ditch and the northern property boundary, which are being coordinated with the ACOE on next steps related to the impacts associated with the development. 3.2 Proposed The proposed development will consist of 5 separate apartment buildings, each 4-stories with the building along Davis Lane frontage having 7,315 square feet of ground level commercial space. Galloway Street will be continued through its existing intersection with Davis Lane, and will dead- end near the property’s eastern border. In addition to the Galloway Street access, a partial access (right in, right out) will be constructed in the northern portion of the property. A public access easement will be provided for the future Windward Avenue along the eastern boundary. The entire Davis Lane frontage will be improved to a two-lane urban minor arterial standard with a raised median. Due to the Davis Lane improvements, the existing Section Line Ditch along the property will be piped. An 8” sewer main will be extended from the existing 24” main within Davis Lane, that will provide service to the entire development. The 8” water main that currently terminates at the intersection of Galloway Street and Davis Lane will be extended into the property to provide domestic and fire service to all five buildings within the development. The water main will be extended both north and south within the property’s frontage of Davis Lane. Both the water and sewer mains will be terminated near the eastern property boundary for future connection. All stormwater storage and treatment will utilize underground infiltration facilities via piped storage. The stormwater runoff within Galloway Street will be collected and detained, with a weir- controlled outlet connecting directly to the proposed Section Line Ditch piping along the western boundary. Underground storage chambers are proposed under the central courtyard area and along the northern parking area that will outlet into the existing Section Line Ditch outfall near the northern property border. 4 Water System The proposed water system will tie into the existing main installed during neighboring Sundance development improvements. The existing water main stub is located at the current intersection of Galloway Street and Davis Lane. The water system will dead-end near the property’s eastern Engineering Report – 6590 Davis Lane Master Site Plan Water System July 2024 Page 5 Version 07/31/2024 boundary, with a stub-out provided for future development. A fire hydrant will be provided near the proposed water main termination for flushing purposes. 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. Ductile iron water services will extend 8’ past the easement line to each of the 5 buildings proposed. 4.1 Water Use Data Average Daily Flow Residential: 300 residential units at 2.5 persons/dwelling = 750 people 750 people at 170 gpd = 127,500 gpd Neighborhood Commercial: 7,315 sq ft commercial space = 1,200 gpd * (7,315/43,560) = 202 gpd (Table V-2 in City of Bozeman Design Standards) Total Daily Flow = 127,702 gpd Maximum Day Flow Peaking factor = 2.3 (per City of Bozeman Design Standards) 127,785 gpd (2.3) = 293,906 gpd Max day flow = 293,906 gpd / 1440 = 204.1 gpm Peak Hour Demand Peaking factor = 3.0 (per City of Bozeman Design Standards) 127,702 gpd (3.0) = 383,106 gpd Peak hour flow = 383,106 gpd / 1440 = 266.05 gpm Main Sizing 205 gpm (Max Day) + 2000 gpm (Fire Flow) = 2205 gpm 2205 gpm in 8” water main ≈ 14 ft/sec (adequate for fire flow) 267 gpm (Peak Hour) = 1.7 ft/sec in 8” water 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 wells being used for all on-site irrigation purposes, the above calculations are conservative. Domestic Service Sizing Considering Building 3 has the highest flow rate of the development, it will be analyzed for the domestic service sizing using the same factors as above. 64 residential units at 2.5 persons/dwelling = 160 people at 170 gpd = 27,200 gpd Peak hour flow = 27,200 gpd (3.0) = 81,600 gpd / 1440 = 56.7 gpm Building 3 will consist of 32 two-bedroom units and 32 one-bedroom units. As a check to the above flow, the number of fixture units were estimated as outlined on the next page: Engineering Report – 6590 Davis Lane Master Site Plan Water System July 2024 Page 6 Version 07/31/2024 Appliance Fixture Units X # Fixtures = Total Dishwasher 1.5 64 = 96.0 Clothes Washer 4.0 64 = 256.0 Lavatory Sink 1.0 96 = 96.0 Kitchen Sink 1.5 64 = 96.0 Water Closet 2.5 96 = 240.0 Shower 2.0 32 = 64.0 Combo Tub/Shower 4.0 64 = 256.0 Total = 1104 Based on the above fixture units (2021 Uniform Plumbing Code - Chart A 103.1(1)), the peak design flow is estimated around 220 gpm. The proposed 4” ductile iron pipe is capable of flowing 220 gpm at approximately 6 ft/s. Irrigation Demand The proposed irrigation use for this development will be provided by individual groundwater wells. A Certificate of Water Right will be obtained through DNRC via Form 602. The following flow rate will be used for the permitting process: Assume 1”/week for irrigation of turf areas Assume 4-month irrigation season Engineering Report – 6590 Davis Lane Master Site Plan Sewer System July 2024 Page 7 Version 07/31/2024 Total irrigated turf / open space areas = 0.80 acres 1”/week (16 weeks) = 1.33 ft of irrigation over 0.80 acres Total irrigation = 1.06 ac-ft/yr = 46,350 cu. ft. (7.48 gal/cu. ft.) = 346,682 gal./year Over 120-day irrigation season, 15 hours of irrigation per day 346,862 gal./year (1 year/120 days) (1 day/15 hrs) (1 hr/60 min) = 3.2 gpm 4.2 Fire Flow The City of Bozeman performs annual hydrant testing, and the fire hydrants in the area have been tested and observed to flow at or above the required 2,000 gpm for available fire flow. 5 Sewer System As part of the Meadow Creek Subdivision Project, a 24” sewer main was installed within the Davis Lane right-of-way along the subject property’s frontage. A new manhole connection is proposed near the north end of the property, to connect to the existing 24” main. The grade of the existing sewer network allows for the proposed sewer main to be extended near the SE corner of the property, at the proposed termination of Galloway Street. 5.1 Design Flow Rates The average daily flow rate calculated above is 127,500 gpd. The peak hour flow calculated above is 267 gpm. Including infiltration: 150 gallons/acre/day (8.5 acres) = 1275 gpd / 1440 = 0.89 gpm Peak hour flow (including infiltration) = 268 gpm The proposed 8” gravity mains are capable of flowing 268 gpm at minimum slope (0.4%) with a depth of only 5.3” (67% 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 24” trunk main located in Davis Lane. All sewer main piping will have uniform slope between manholes. Sewer mains have been designed at nearly the same slope as the finished grades across the site. Individual sewer services will be provided for each of the 5 buildings within the development, and are sized according to the peak hour flow for Building 3 calculated above. Building 3 Peak Hour Flow = 56.7 gpm A 6” gravity sewer service on a minimum ¼”/foot grade is capable of flowing 56.7 gpm at a depth of only 1.6” (27% full). 5.2 Buoyancy In areas of high groundwater, dewatering will be required during construction. The localized lowering of groundwater will prevent buoyancy during construction. Engineering Report – 6590 Davis Lane Master Site Plan Stormwater Network July 2024 Page 8 Version 07/31/2024 Sewer Manholes Buoyancy Check: Assume Fully Submerged and 7.5 foot bury depth Concrete Wt = 150 pcf Soil Wt = 100 pcf Water Wt = 62.4 pcf Volume Displaced Eccentric Cone = (3.14 x 23” 2) x 4ft = 46.14 cf MH Section = (3.14 x 2.4’2) x 3.5ft = 63.33 cf Base = (3.14 x (35/12)2) x 0.5 ft = 13.36 cf Uplift= 122.83 cf x 62.4 pcf = 7,665 lbs Concrete MH Weight Eccentric Cone = ((3.14 x 23” 2) – (3.14x18” 2)) x 4ft = 17.89 cf MH Section = ((3.14 x 2.41’2) - (3.14x2’ 2)) x 3.5ft = 19.88 cf Base = (3.14 x (35/12)2) x .5 ft = 13.36 cf 51.13 cf x 150 pcf = 7,669 lbs Weight Soil on Base = ((3.14 x 2.922) – (3.14 x 2.42 2)) x 7.5 ft = 62.91 cf 62.91 cf x 100 pcf = 6,291 lbs Soil Weight on Base, Concrete Wt, and Side Friction (13,960 lbs) > than uplift (7,665 lbs) 6 Stormwater Network This section provides a design basis and hydraulic calculations for sizing the storm water facilities. 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, drive accesses, sidewalks and parking areas for the apartment buildings. The Galloway Street extension will contain angled back in parking with perimeter curb and gutter. The road crown will continue into on-street parking areas, with all sheet flow from the road being captured by the standard curb and gutter system. All drive accesses, landscaping islands, and building sidewalks will have curb and gutter. Stormwater runoff generated from the roadway and apartment buildings is designed to be collected by curb and gutter and then directed toward storm drain inlets. The stormwater runoff within Galloway Street will be conveyed to curb inlets and into underground storage piping beneath the curb. All stormwater runoff within the site will also be conveyed by curb and gutter to the underground stormwater chambers located either under the central shared open space area or in the northwest corner of the parking area. Engineering Report – 6590 Davis Lane Master Site Plan Stormwater Network July 2024 Page 9 Version 07/31/2024 All stormwater areas will utilize the nearby Section Line Ditch as the outfall location. The existing ditch flows north along the subject property’s western property boundary, where it then ponds up near the northern property boundary. The watercourse then continues through the existing culvert found under the neighboring driveway to the north. 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 initially calculated using a weighted percentile of impervious and pervious area. As the calculated coefficients were similar to the “Dense Residential” value found in the COB Design Standards, this value was used for each basin. The coefficient used are shown in the table below. Table 1 - Runoff coefficients used. RUNOFF COEFFICIENTS Undisturbed 0.2 Dense Residential 0.5 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 to 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 SCS method, as discussed below, was also used for further analysis of the initial rainfall volume (i.e. the runoff treatment volume). Engineering Report – 6590 Davis Lane Master Site Plan Stormwater Network July 2024 Page 10 Version 07/31/2024 The method outlined in Appendix C of the City of Bozeman Design Standards and Specifications was used to size each of the stormwater facilities for the 10-yr flood control volume. Each of the underground detention facilities will be outlet controlled to only allow discharge at a rate equal to the pre-developed runoff rate. The hydrology calculations and pond sizing spreadsheets can be found in Appendix B. 6.3 Detention Basins 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 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 (TR-55), the Curve Number associated with soil type B in urban districts is 90 (Table 2-2a). Using the standard SCS method, S = (1000/90) – 10 = 1.1 inches Ia = 0.2 * 1.1 inches = 0.2 inches Using a 10-year 24-hour rain event in the City of Bozeman the total precipitation (P) is 1.88 inches of which 0.2 inches is lost due to initial abstraction according to USDA’s SCS method. Therefore, this drainage plan includes provision for the first 0.2 inches of rainfall from the 24-hour storm to be retained in surface depressions, intercepted by vegetation, or evapotranspired or infiltrated. The remaining 0.3 inches will be captured and treated to remove 80% TSS prior to being discharged from the respective storm basin. This allows the remainder of the first 0.5 inches of rainfall after initial abstraction to be retained or treated. Storm basins A, B, and C will all utilize underground detention to capture and release runoff at the pre-developed flow rate for the 10-year storm. Considering the poor soils and relatively high groundwater, retention/infiltration is not a practical approach. Rather, the stormwater systems are designed with a shallow profile to maintain separation from groundwater. As each of the underground facilities will be outlet controlled and will not have any standing water, 80% TSS removal will be achieved prior to discharge. Hydrodynamic separation is proposed downstream of each of the proposed outlet structures for all three basins. 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 Engineering Report – 6590 Davis Lane Master Site Plan Stormwater Network July 2024 Page 11 Version 07/31/2024 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. In the event of a 100-year storm, the runoff will overtop the weirs in the outlet structures and bypass the hydrodynamic separators. This will allow the increased runoff to be conveyed directly through the systems without treatment and reach the outfall location. The Section Line Ditch outfall has sufficient capacity for the 100-year storm event. 6.4 Inlets & Storm Sewer 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 and manholes will have a 9” sump for sediment collection. Storm pipes were sized to accommodate the 100-year 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 0.009 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 soils investigation report was completed for the property by C&H Engineering in 2021. The report states that the seasonally high groundwater is near 2.5 feet below ground surface in the northwest corner and near 3.5 feet at the southern end of the property. In addition to the test pit observations, groundwater monitoring wells were installed on site this year (2023) and are a good representation of the approximations in the report. Monitoring data and a map of the three wells can be found in Appendix C, with the most shallow depths ranging from 3 feet in Well 1 to 1.5 feet in Wells 2 & 3. 6.6 System Maintenance Regular maintenance of stormwater facilities is necessary for proper function of the drainage system. Annual inspections are best practice for all underground systems. The detention systems shall be cleaned when an inspection reveals accumulated sediment or trash is clogging any part of the overall system. Accumulated sediment and trash can typically be removed via the manhole or inlet. 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 underground detention facilities will be maintained by the property owner’s association. 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. Engineering Report – 6590 Davis Lane Master Site Plan Stormwater Network July 2024 Page 12 Version 07/31/2024 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 fills the underground chambers during a large rain event will overtop the weir structure outlet to the respective outfall location.