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HomeMy WebLinkAbout6.10_Gallatin Subaru_Civil Engineering Report_3-3-2020 Engineering Report Gallatin Subaru 31910 E. Frontage Road Bozeman Gallatin County, Montana March, 2020 Prepared By: Hyalite Engineers, PLLC 2304 N 7th Ave. Suite L Bozeman, MT 59715 ENGINEERING REPORT – Gallatin Subaru Contributors March 2020 Page ii Version 3/3/2020 Contributors Sarah Johnson, PE Project Engineer, Hyalite Engineers, PLLC, Bozeman, MT Brett Megaard, PE Project Engineer, Hyalite Engineers, PLLC, Bozeman, MT Brian Van Rooyen, EI Staff Engineer, Hyalite Engineers, PLLC, Bozeman, MT Mike Stenberg, PE Principle Project Manager, Hyalite Engineers, PLLC, Bozeman, MT ENGINEERING REPORT – Gallatin Subaru Table of Contents March 2020 Page iii Version 3/3/2020 Table of Contents Contributors ................................................................................................................................ ii Table of Contents ....................................................................................................................... iii List of Tables ............................................................................................................................. iv List of Figures ............................................................................................................................ iv List of Appendices ...................................................................................................................... iv 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 3.3 Expected Water Usage ................................................................................................ 2 3.3.1 Irrigation ................................................................................................................ 2 3.3.2 Building Water Use ............................................................................................... 3 3.3.3 Total Water Use .................................................................................................... 3 4 Sanitary Sewer Improvements ............................................................................................ 3 4.1 Existing Infrastructure .................................................................................................. 3 4.2 Proposed Improvements .............................................................................................. 3 5 Potable Water Improvements .............................................................................................. 4 5.1 Existing Infrastructure .................................................................................................. 4 5.2 Proposed Improvements .............................................................................................. 4 6 Storm Water Improvements ................................................................................................ 5 6.1 Existing Conditions ...................................................................................................... 5 6.2 General Design ............................................................................................................ 5 6.3 Hydrologic Methodology............................................................................................... 6 6.4 Extended Detention Ponds .......................................................................................... 7 6.5 System Maintenance ................................................................................................... 8 6.6 Erosion Sediment Control ............................................................................................ 9 6.7 Flooding ....................................................................................................................... 9 References ...............................................................................................................................10 ENGINEERING REPORT – Gallatin Subaru List of Tables March 2020 Page iv Version 3/3/2020 List of Tables Table 1 - Calculated water usage summary. .............................................................................. 3 Table 2 - Runoff coefficients used. ............................................................................................. 6 List of Figures Figure 1 - Vicinity map. .............................................................................................................. 1 List of Appendices A) Pre-development and Post-development Drainage Basins B) Hydrologic calculations C) Pond Calculation & Summary D) Stormwater Maintenance Plan ENGINEERING REPORT – Gallatin Subaru Introduction March 2020 Page 1 Version 3/3/2020 1 Introduction 1.1 Purpose of Report This report is intended to serve as the design document for site/civil improvements associated with the construction of a new Subaru dealership and service center at 31910 E. Frontage Road, in Bozeman, MT. 1.2 Scope Sanitary sewer service, fire and potable water service, and storm water design are within the scope of this report. All improvements analyzed in this report are within the property or directly adjacent to it. No off-site improvements are expected or considered. 2 Location and Site Information The property occupies 4.24 acres and is located within the eastern extents of the City of Bozeman between Interstate 90 and Frontage Road, in the SE ¼ of Section 8 and the SW ¼ of Section 9, Township 2S, Range 6E, P.M.M., Gallatin County, MT. The existing zoning and the nearby surrounding zoning is M-1. The property is within the service area for municipal water and sewer from the City of Bozeman. Figure 1 - Vicinity map. ENGINEERING REPORT – Gallatin Subaru Land Use March 2020 Page 2 Version 3/3/2020 3 Land Use 3.1 Existing The site currently functions as a Subaru dealership and service center with the majority of the lot already being paved. The northern and southernmost extents of the lot are the only areas that have vegetated areas. The existing building is served by city water and sewer but no storm water facilities exist. Currently the paved parking and building occupy approximately 4.0 acres of the total 4.24 acres. 3.2 Proposed The site is proposed as entirely new with all existing buildings, parking, and utility services being removed, abandoned or relocated. The facility will continue to serve as an automotive dealership and service center. The building, as proposed, covers a total of 30,077 square feet and a curb & gutter parking lot (including parking and auto display/storage) occupies 114,997 square feet. The remainder will be landscaped area. The following design criteria are used with respect to utility sizing. Irrigated Turf Area: 6,527 square feet. Service Employees: 51 full time. Sales/Office Parking Spaces: 57. Fixtures: 26, assume 4 GPM instantaneous each. 3.3 Expected Water Usage Water usage can be estimated using Montana DEQ Circular 4, DNRC guidelines, and building design parameters. Total water usage is a combination of interior domestic use from the dealership and service center, and irrigation. Average daily demand (ADD) and peak instantaneous demand (PID) are considered in the design of water and sewer infrastructure. 3.3.1 Irrigation Irrigation demand in the summer months from turf grass can be estimated to be equal to net potential evapotranspiration (ET). Potential ET is the amount of ET that would occur if there was sufficient water, which is applicable to irrigated areas. The Climate Atlas of Montana was used to determine a point value of 9.04 inches for the month of August at the location of interest (Mu, Zhao, and Running 2013). This translates to a daily water use depth of .13 inch (0.011 feet) per day during the peak month of August. The average daily irrigation water usage can then be calculated by multiplying by the irrigated turf area ADD TURF = (6,527 ft 2)(0.011 ft)(7.48 gal/ft 3) = 522 GPD. The landscape designer estimates a total water use from drip irrigation (trees, shrubs, etc.) to be 1,676 gallons per month, or 56 GPD (see landscape plan). The total average day demand from ADD IRIG = 522 GPD + 56 GPD = 578 GPD Peak instantaneous irrigation demand can be assumed as the flow rate of a typical irrigation system. There are no large turf grass areas requiring high flow rate heads and most irrigation will be via drip irrigation so a reasonable assumption would be PID IRIG = 10 GPM. ENGINEERING REPORT – Gallatin Subaru Sanitary Sewer Improvements March 2020 Page 3 Version 3/3/2020 3.3.2 Building Water Use Standard values for average daily water use can be used as follows to calculate non-commercial water use (bathrooms, break room, hand washing, etc.) (“Circular DEQ 4 - Montana Standards for Subsurface Wastewater Treatment Systems” 2013). Automobile Service Station = (12 GPD/Employee)(50 Employees) = 600 GPD Retail = (2 GPD/Parking Space)(67 Spaces) = 134 GPD TOTAL = ADD BLDG = 734 GPD Peak instantaneous demand for building uses (PID BLDG ) is best calculated in this case by analyzing the number for fixtures and other mechanical process demands. A standard design parameter for building service sizing is the 99th percentile water demand. A reasonable conservative approach for estimating 99th percentile water demand is assuming 50% of building fixtures are running (Omaghomi and Buchberger 2014) as follows; PID BLDG = (26 fixtures)/2 x (4 GPM/Fixture) = 52 GPM 3.3.3 Total Water Use Total water demand and the associated expected wastewater volumes are summarized below. Table 1 - Calculated water usage summary. Building Irrigation Total Average Day Demand (GPD) 734 578 1312 Peak Instantaneous Demand (GPM) 52 10 62 Wastewater Average Day Flow (GPD) 734 734 Note that final design of the building is not complete so the above water usage estimation should be considered approximate in lieu of final design. 4 Sanitary Sewer Improvements 4.1 Existing Infrastructure A 10-inch diameter City of Bozeman sewer main exists within Department of Transportation right- of-way approximately 40 feet south of the existing Frontage Road edge of pavement. The existing 10-inch main is PVC and laid at 0.52% slope. The main was installed in 1984. The existing main ultimately terminates at a lift station near the I-90/Frontage/Main St. interchange where wastewater is pumped under Interstate 90 and into the Hyalite Interceptor Sewer, ultimately flowing to the Bozeman Wastewater Treatment Facility. The City of Bozeman has confirmed that capacity exists for the proposed use. Either coring into an existing neighboring manhole or installing a new “Inserta Tee” service connection is acceptable for the proposed connection. 4.2 Proposed Improvements A simple service connection is the only sanitary service improvement required. The proposed service design can be summarized as follows: ENGINEERING REPORT – Gallatin Subaru Potable Water Improvements March 2020 Page 4 Version 3/3/2020 Length = 279 feet Slope = 0.60% Nominal Diameter = 6-inch Minimum Depth = 3 feet (insulation required) Maximum Depth = 7 feet Pipe Material = SDR 35 PVC (0.010 Roughness Coeff.) Cleanout Spacing = 200-foot maximum. The flat nature of the site requires the service to be oversized (6” rather than 4”) in order to enter the proposed building at an acceptable depth. Minimum slope, rather than hydraulic capacity controls the design. As proposed, the 6” service has a hydraulic capacity of 254 GPM. This can be compared to the instantaneous building water demand of 52 GPM. For the purposes of estimating downstream system impact, from Section 3.3.2, the average day sewer flow is estimated to be 734 gallons per day. The peak hour flow can be estimated using a peaking factor approach where Peak Hour Flow = Average Day Flow × Peaking Factor, Peaking Factor = (18 + √P)/(4 + √P). where P denotes population in thousands (“Circular DEQ-2 Design Standards for Public Sewage Systems” 2016). The expected total population of the site is 73 employees and customers, yielding a peaking factor of 4.3 and a peak hour flow of 131 gallons per hour. 5 Potable Water Improvements 5.1 Existing Infrastructure A 10-inch City of Bozeman water main exists within Department of Transportation right-of-way approximately 10 feet south of the existing Frontage Road edge of pavement. The main was installed in 1993 and city of Bozeman GIS information does not make any note of deficiencies in the area. Hydrant testing results nearby obtained from the City of Bozeman showed a 120 PSI static pressure, and 1,405 GPM measured out of a single 2” hose nozzle. 5.2 Proposed Improvements One (1) 2-inch domestic service and one (1) 4-inch fire services is proposed to serve the property and is to be installed per City of Bozeman standards. The proposed service design can be summarized as follows: Domestic Length = 301 feet Nominal Diameter = 2-inch Material = Type K Copper (City Requirement, 0.011 Roughness Coeff.) Design Flow = 60 GPM (See Section 3.3.2) Design Pressure Loss = 0.06 psi/ft (@ 60 GPM) Design Pipe Velocity = 5.9 ft/s Building Standpipe Pressure @ Design Flow = 101.94 psi (pressure reduction required) ENGINEERING REPORT – Gallatin Subaru Storm Water Improvements March 2020 Page 5 Version 3/3/2020 Fire Length = 301 feet Nominal Diameter = 4-inch Material = Class 51 Ductile Iron (City Requirement, 0.012 Roughness Coeff.) Design Flow = To be determined by final fire sprinkler design. Capacity = 392 GPM (10 ft/s average pipe velocity) Pressure Loss @ Capacity = 0.08 psi/ft Building Standpipe Pressure @ Capacity Flow = 95.92 psi The existing services to the property should be abandoned per the direction of City of Bozeman staff as they are unsuitable for use with the proposed improvements. 6 Storm Water Improvements This section provides a design basis and hydraulic calculations for sizing stormwater facilities for Gallatin Subaru. 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 Existing Conditions There are three major stormwater basins within this site as shown on the “Pre-development Storm Basin” figure in Appendix A. The north portion of the site drains to the northwest toward the existing roadside ditch along Frontage Road and the south portion of the lot drains into an existing ditch on the south side of the lot. To the east of the lot there is an existing commercial building and parking area with stormwater management ponds. To the west of the lot is an RV campground that is predominantly grass lawn and gravel. The surrounding lots make the offsite run-on to the project site insignificant. The slope across the existing site varies from 0.5% to 1.5%. 6.2 General Design The proposed development will be a 30,077-sf structure and an asphalt paved parking lot. Five soil borings were drilled during the geotechnical investigation and the results show that the in-situ soils are generally lean clay with sand to approximately 10-12 feet in depth. Groundwater was encountered in all of the borings and was measured to be approximately 5 feet below existing ground. Clay soils and high groundwater are a factor in this design that make infiltration difficult. Due to the size of the site and the large amount of impervious surface it is not feasible for this site to infiltrate, evapo-transpirate or capture for reuse the entire first ½” of rainfall so the design will incorporate low impact development by treating the stormwater runoff. The volume created from this first half inch of rainfall will be referred to as the Runoff Treatment Volume (RTV). Various types of treatment systems were considered during the design process including biofiltration swales, extended detention basins, wet detention basins, and proprietary treatment devices. Given the site characteristics, the best option for this site is to utilize extended detention basins (EDBs). This type of treatment pond is designed to detain and slowly release storm water over an extended period of time following a rainfall event. This BMP is similar to a detention basin used for flood control except it uses a smaller outlet that extends the drawdown time to improve pollutant removal. The primary characteristics of and extended detention basin are: ENGINEERING REPORT – Gallatin Subaru Storm Water Improvements March 2020 Page 6 Version 3/3/2020 · EDBs consist of an inlet, a pretreatment device, a main treatment cell, a micropool and an outlet structure. · An EDB has a minimum 48-hour drawdown time for the RTV facilitation removal of 80% TSS. The extended detention basin has been designed to provide both runoff treatment and flood control. The goals of the stormwater design are to limit the peak runoff of the 10-year storm to the pre-developed rate and to treat the Runoff Treatment Volume for removal of 80% TSS. 6.3 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 2 - 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)D 1/2 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 ENGINEERING REPORT – Gallatin Subaru Storm Water Improvements March 2020 Page 7 Version 3/3/2020 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. A pond summary table and output from Hydraflow Hydrographs can be found in Appendix C. 6.4 Extended Detention Ponds The Montana MS4 Post-Construction BMP Design Guidance Manual was used to design the extended detention basins. An extended detention basin is a constructed basin designed to capture and treat stormwater runoff. Runoff is detained for a minimum of 48 hours, providing time for pollutants to settle out prior to discharge. An extended detention basin is expected to remove 80% of TSS. There will be three basins located on this site; one on the south side of the lot, one on the north side of the lot, and one in the northwest corner. The basins have been sized for 100 percent of the Runoff Treatment Volume (RTV) and also for flood control to match the pre-development 10-year peak flow. The RTV begins at pond bottom elevation and extends to the invert of the flood control rectangular weir. The additional storage needed to meet the flood control volume is stacked on top of the RTV and the outflow is controlled by the rectangular weir. The Runoff Treatment Volume was calculated for this site using the following equation as referenced in The Montana MS4 Post-Construction BMP Design Guidance Manual: RTV= PR vA 12 Where RTV= Runoff Treatment Volume P= Water Quality Rainfall Depth (0.5”) Rv= Dimensionless Runoff Coefficient, R v = 0.05 + 0.9(I) I= Percent Impervious, decimal A=Area of Site, acres There are four major components to an EDB, a pretreatment device, a main treatment cell, a micropool, and an outlet structure. 1. The pretreatment device use in this application will consist of a dry-well inlet with a 4-foot sump. The runoff that collects in the curb will flow over the inlet grate and fill the sump with the initial “first flush” volume. This volume is known to carry the highest amount of TSS. The suspended sediment will then settle out within the pretreatment device. This helps preserve the capacity of the main treatment cell. Any runoff beyond the capacity of the pre-treatment device will overtop the grate and flow past the inlet to the main treatment cell or vegetated swale. 2. The main treatment basin is 1.5 feet deep with 4:1 side slopes. This is sized to hold the entire RTV volume and drawdown in 48 hours to allow for settling of TSS. The Guidance Manual recommends a trickle channel in the main treatment basin. A concrete channel does not seem effective for this short distance and would be difficult to mow around. The runoff will be conveyed to the micropool along the bottom of the pond similar to a “soft- bottom” trickle channel. Short circuiting is the term used when stormwater inflow to the pond flows directly to the outlet with little or no dispersion resulting in reduced residence times and increased ENGINEERING REPORT – Gallatin Subaru Storm Water Improvements March 2020 Page 8 Version 3/3/2020 sediment in the pond outflow. This may occur particularly in Pond A and B due to shorter distances from the inlet of the pond to the outlet. The micropool and slow release rate of the pond should assist in preventing short circuiting. 3. The micropool is a small pool located in front of the outlet structure designed to prevent sediment resuspension and protect the orifice from clogging. This design will include a micropool that is filled with gravel. The gravel will provide additional filtering and keep the orifice free from leaf litter and other debris. The gravel will also eliminate the potential hazard of a permanent pool of water allowing the design to meet the City’s 1.5-foot max pond depth rule. 4. The outlet structure for each pond will have an orifice for extended release of the RTV and a rectangular weir sized to release runoff at the predeveloped 10-yr peak flow rate. The design includes a small orifice outlet that will drain the pond in 48 hours. The concern of a small outlet orifice is the potential for clogging. To minimize the risk of clogging, the EDB has a gravel filter in the micro pool and a perforated drain pipe to remove sediment and debris. Care will need to be taken to keep the orifice free from debris. In the event that the orifice clogs, runoff will flow over the weir to the outlet pipe. Additional information regarding maintenance of the orifice is found in the System Maintenance section of this report. 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. 6.5 System Maintenance The Extended Detention Basin system will require more maintenance than a standard stormwater detention basin. The system must be inspected and maintained at regular intervals to increase performance and longevity. Along with this narrative, an operation and maintenance document can be found in Appendix D. The design objective of the pond is to treat stormwater by settling out suspended solids. This means that the solids such and gravel, sand, and fine sediment will build up in the system components over time and will need to be removed or cleaned out. The pretreatment inlets and vegetated swale must be inspected annually after spring runoff. Pretreatment inlets will need to be vacuumed out when approximately 9” of sediment collects in the bottom. Sediment should be removed from the vegetated swale as it builds up. The vegetated swale will be seeded with grass and will be mowed regularly. It is crucial for the swale to maintain adequate vegetation cover so that the treatment is effective and the bottom of the swale does not erode. If vegetation dies the swale will need to be re-seeded. The ponds are placed in a location that will be visible to the owner/ maintenance staff. If the ponds do not drain within 48 hours after a storm event, then the system may be clogged and will require immediate maintenance. The outlet structure lid should be removed and the orifice should be inspected. If the orifice is clogged debris should be removed from the outlet structure. If the orifice does not appear to be clogged and the system is not draining, then gravel micropool bed is likely clogged. In this event, the gravel should be removed along with any sediment build up. Clean washed gravel of similar size should be replaced. ENGINEERING REPORT – Gallatin Subaru Storm Water Improvements March 2020 Page 9 Version 3/3/2020 6.6 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. Care will need to be taken during construction to keep sediment laden stormwater from clogging the pond underdrain and outlet structure. Any visible sediment must be removed from the pond prior to completing construction. 6.7 Flooding Excessive runoff from a large storm event (significantly exceeding the design storm, i.e 100-year) will be routed such that it does not inundate buildings, drainfields or over top the roadway. The stormwater infrastructure including ditches, culverts, and detention pond outlet structures have been analyzed for the 100-year storm. Stormwater that overtops the ponds during a large rain event will flow though the emergency overflow and outlet to the respective ditches. ENGINEERING REPORT – Gallatin Subaru References March 2020 Page 10 Version 3/3/2020 References “Circular DEQ-2 Design Standards for Public Sewage Systems.” 2016. “Circular DEQ 4 - Montana Standards for Subsurface Wastewater Treatment Systems.” 2013. Mu, Q., M. Zhao, and S.W. Running. 2013. “Algorithm Theoretical Basis Document: MODIS Global Terrestrial Evapotranspiration (ET) Product (NASA MOD16A2/A3) Collection 5.” Omaghomi, T., and S. Buchberger. 2014. “Estimating Water Demands in Buildings.” Procedia Engineering 89: 1013–22. doi:10.1016/j.proeng.2014.11.219. Peterson, Matt, Spencer Savage, and Vern Heisler. 2017. “Montana Post-Construction Storm Water BMP Design Guidance Manual.”