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HomeMy WebLinkAbout07 Stormwater Engineering Report Engineering Report East Frontage Road Commercial Development Bozeman Gallatin County, Montana October, 2020 Prepared By: Hyalite Engineers, PLLC 2304 N 7th Ave. Suite L Bozeman, MT 59715 12/19/2022 Engineering Report – East Frontage Road Commercial Development Contributors October 2020 Page ii Version 10/9/2020 Contributors 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 – East Frontage Road Commercial Development Table of Contents October 2020 Page iii Version 10/9/2020 Table of Contents Contributors ................................................................................................................................ ii Table of Contents ....................................................................................................................... iii 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 Storm Water Improvements ................................................................................................ 2 4.1 Existing Conditions ...................................................................................................... 2 4.2 General Design ............................................................................................................ 2 4.3 Hydrologic Methodology............................................................................................... 3 4.4 Biofiltration Swales ....................................................................................................... 4 4.5 Detention Basin ........................................................................................................... 5 4.6 System Maintenance ................................................................................................... 6 4.7 Erosion Sediment Control ............................................................................................ 6 4.8 Flooding ....................................................................................................................... 6 References ................................................................................................................................ 7 Engineering Report – East Frontage Road Commercial Development Introduction October 2020 Page 1 Version 10/9/2020 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 “pad-ready” site on East Frontage Road, approximately 1000 feet east of the I-90 interchange in Bozeman, MT. 1.2 Scope Overall site development/layout and storm water design elements 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 4.4 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 – East Frontage Road Commercial Development Land Use October 2020 Page 2 Version 10/9/2020 3 Land Use 3.1 Existing The site is currently a vacant lot without any public access. The majority of the site has been determined to be non-jurisdictional wetlands, a wetland delineation map is attached with this submittal. Of the total 4.4-acre property, 3.94 acres are non-jurisdictional wetland. The total wetland impact from the construction of the pad-ready site will be approximately 3.32 acres. 3.2 Proposed With this phase of site development, an MDT approach will be constructed to provide access to and from the site off of East Frontage Road. The entire site will be built up to create a property that is ready for building construction. No sewer or water services will be installed at this time. As the exact use of the building is not yet known, an office building was used in all preliminary design calculations including parking spaces to be provided. The approximate building footprint used is 22,000 square feet with a full curb and gutter parking lot. A portion of the site along the western property line will not be developed and will remain in its current state, including the wetland areas and the aspen/snowberry upland buffer areas. 4 Storm Water Improvements This section provides a design basis and hydraulic calculations for sizing storm water facilities for the East Frontage lot. 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. 4.1 Existing Conditions There is one major storm water basin within this site as shown on the “Pre-development Storm Basin” figure in Appendix A. The entire site drains to the northwest toward the existing roadside ditch along Frontage Road. To the west of the property are two more vacant lots located within a wetland area. To the east 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 1.0%. 4.2 General Design The proposed grading of the site is based around a centrally located 22,000-sf structure with a surrounding paved parking area and full curb and gutter. The parking area will be graded such that all storm water drainage will be routed toward curb cuts located in each of the four drainage sub-basins. The runoff will then discharge into biofiltration swales and end up in the natural wetland portion of the site. All storm water detention is accounted for in the pond near the northwest corner of the property. The detention pond will utilize the existing wetlands and all storm water will be detained using an earthen berm surrounding the wetland area. The earthen berm will be constructed near the northern and western property boundaries that will act as the boundary of the storm pond. No excavation will be needed for the pond bottom, as the existing topography will remain. Engineering Report – East Frontage Road Commercial Development Storm Water Improvements October 2020 Page 3 Version 10/9/2020 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 7-9 feet in depth. Groundwater was encountered in all of the borings and was measured to be approximately 3-4 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 biofiltration swales that discharge into the existing wetland area. Bio-swales are designed to provide runoff treatment while slowly conveying storm water runoff. The primary characteristics of biofiltration swales are: · Biofiltration swales consist of an inlet structure, level spreader, main treatment channel, and outlet structure. · A minimum hydraulic residence time of 9 minutes for the RTF rate facilitates the removal of TSS. The biofiltration swales have been designed to provide both runoff treatment and flood control. The swales will outlet into a large detention facility that will slowly release the stormwater runoff. 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. 4.3 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 1 - Runoff coefficients used. RUNOFF COEFFICIENTS Undisturbed 0.2 Impervious 0.9 Engineering Report – East Frontage Road Commercial Development Storm Water Improvements October 2020 Page 4 Version 10/9/2020 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. 4.4 Biofiltration Swales The Montana MS4 Post-Construction BMP Design Guidance Manual and the NRCS TR-55 Graphical Peak Discharge Method were both used to design the biofiltration swales. The swales will have trapezoidal cross-sections and longitudinal slopes of approximately 1.0%. All four of the swales will have bottom widths of 2.5 feet. Each swale will have side slopes of 3:1 (H:V). The hydraulic residence time must be a minimum of 9 minutes to achieve 80% TSS removal. To ensure the hydraulic residence time in each swale meets the minimum, the length of each swale is calculated using the flow velocity at the Runoff Treatment Design Flow Rate (RTF) for each respective drainage sub-basin. The RTF rate was determined by first calculating the runoff curve number and time of concentration of each drainage sub-basin. When those parameter are known, the initial abstraction can be calculated and the unit peak discharge can then be found (Exhibit 4-I from TR-55). Existing topsoil on site will be stockpiled during site preparation and re-used as cover for the swales. The native soil will have the ability to establish thick and healthy vegetation to reduce runoff velocities which increases the likelihood of sediment removal. The design flow depth for each swale is 4 inches, as the swales will be mowed infrequently. Each swale will have a maximum depth of 6 inches, to allow for emergency overflow on the wetland-side of the swales. There are four major components to a biofiltration swale: an inlet structure, a level spreader, a main treatment channel, and an outlet structure. 1. The inlet structure used in this application will be a curb cut to act as a LID stormwater inlet. Rip rap will be used from the curb cut to the bottom of the swale to control runoff velocity entering the swale. Each curb cut will be sized to completely capture the linear Engineering Report – East Frontage Road Commercial Development Storm Water Improvements October 2020 Page 5 Version 10/9/2020 flow traveling along the gutter, as well as the perpendicular flow from the paved parking area. 2. The curb cut will also act as the level spreader in this instance. The entire width of the curb cut lip will be level to ensure the flow entering the swale is evenly distributed. This and the riprap placed at the swale inlet will help to mitigate erosion of the bottom of the swale as flow from larger storm events enters and is routed through the swale and into the wetland area. 3. The main treatment channels will be constructed with 3:1 side slopes and bottom widths ranging from 2-3 feet. Considering each of the channels runs alongside the parking area, the bottom of the channel will meander away from the parking lot to maintain the 3:1 slope as the distance between the channel bottom and top of curb increases. The side of the channel nearest the wetland area will have a height of 6 inches and will match the running slope of the channel bottom. This will ensure each channel will have a maximum depth of 6 inches, and larger flows will overtop this side of the channel and discharge directly into the wetland area. 4. There will be no outlet structure within the biofiltration swales, as each of the swales will naturally discharge into the wetland area and end up in the main detention pond that is controlled with a concrete outlet structure. Each of the swales act as pretreatment devices for the detention pond that will reduce the amount of sediment that reaches the pond. 4.5 Detention Basin The standard detention basin has 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= PRvA 12 Where RTV= Runoff Treatment Volume P= Water Quality Rainfall Depth (0.5”) Rv= Dimensionless Runoff Coefficient, Rv = 0.05 + 0.9(I) I= Percent Impervious, decimal A=Area of Site, acres. The biofiltration swales will act as pretreatment devices for the main treatment basin. The main treatment basin is 1-foot deep at the northern end and tapers to the south to eventually tie into the existing grade. This is sized to hold the entire RTV volume and drawdown in 48 hours to allow for additional settling of TSS. The outlet structure for the 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. 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 Engineering Report – East Frontage Road Commercial Development Storm Water Improvements October 2020 Page 6 Version 10/9/2020 to the outlet. 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. 4.6 System Maintenance The Biofiltration Swale system will require standard maintenance procedures. 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 F. The curb cuts, main treatment channels, and detention pond outlet structure must be inspected annually after spring runoff. Sediment should be removed from the biofiltration swale as it builds up. The swale will be seeded with grass and will be mowed only during peak growing seasons. 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 swales and detention pond are placed in locations that will be visible to the owner/ maintenance staff. If the ponds do not drain within 48 hours after a storm event or the swales do not efficiently convey runoff, then the system may be clogged and will require immediate maintenance. If the pond outlet orifice is clogged, debris should be removed from the outlet structure. If the curb cut is forcing runoff to pond near the swale inlet, all debris or accumulated sediment shall be removed. 4.7 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. 4.8 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 overtops the ponds during a large rain event will flow through the emergency overflow and outlet to the respective ditches. Engineering Report – East Frontage Road Commercial Development References October 2020 Page 7 Version 10/9/2020 References Peterson, Matt, Spencer Savage, and Vern Heisler. 2017. “Montana Post-Construction Storm Water BMP Design Guidance Manual.”