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HomeMy WebLinkAbout5B.1_Storm Engineering ReportProfessional Consulting Engineers and Land Surveyors Equal Opportunity Employer | www.hyaliteeng.com Stormwater Engineering Report: for 705 S Church Ave. Bozeman, Montana August, 2025 Prepared For: Landern West LLC Prepared By: Hyalite Engineers, PLLC Brett Megaard, PE 705 S CHURCH AVE STORMWATER ENGINEERING REPORT August 2025 This section provides a design basis and hydraulic calculations for sizing stormwater facilities for the residential site. 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. 1. Existing Conditions The site is divided by Bozeman Creek that runs north-south through the middle of the site. The existing site is native grasses, that slopes 1% to 5.5% to Bozeman Creek. There is an existing house on the property, that is proposed to be demolished and a townhouse is proposed to be built. 2. General Design The proposed development will be a, 4,650 -sf townhome and an asphalt paved driveway to each garage. There has been geotechnical investigation and report for this site. 5 soil test pits were excavated during the geotechnical investigation and the results show that the site consists of 1-2 feet of driveway fill material with 1-2 feet of topsoil and native silt/clay below, followed by native sand and sandy gravel to the bottom of the test pits (8 feet). Groundwater was encountered in all of the test pits. Due to the topography of the site and Bozeman Creek in the middle of the site, retention was chosen to be the method of capturing storm water. Each lot will capture and retain its own stormwater on-site. The storm ponds will capture the entire first ½” of rainfall so the design will incorporate low impact development by treating the stormwater runoff. 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 = CfCiA Where C = Weighted C Factor Cf = correction factor for infrequent storms i= Storm Intensity (in/hr) A = Area (acres) Q = Runoff (cfs) The storm intensities were developed from table 6.5.2 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 Asphaltic and Concrete 0.95 Unimproved area 0.10 Heavy Soil (flat) 0.17 Heavy Soil (average) 0.22 Drives, Walks and Roofs 0.95 705 S CHURCH AVE STORMWATER ENGINEERING REPORT August 2025 Time of concentration was determined using the following equation: Tt = Where Tt = Sheet flow travel time (min) n= roughness coefficient L= Flow length (ft) P2= 2-year, 24-hour rainfall depth (inch) S= Slope of Basin, (ft/ft) Ku= Unit conversion constant, 0.42 in CU 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. Retention volumes were calculated using the modified rational method. The modified rational method uses peak flow rates to determine volume. The peak flow rate is determined using the rational method, that peak flow rate is conservatively held constant for the duration of the storm. The volume of retention volume required is calculated by multiplying the peak runoff by storm duration. City of Bozeman Standards requires both minor and major storm event be evaluated. In this case a 10-yr 2-hr storm was evaluated for the minor storm event and both the 100-yr 2-hr and 100-yr 24-hr were evaluated for the major storm event. The intensities are developed from table 6.5.2 of the City of Bozeman Design Standards and Specifications. Runoff coefficients are taken from Table 6.6.4. Retention volume is determined using the following equation: V = 7200Q Where V = Volume, cf Q = flow rate, cfs 4. Basin 1 Basin 1 is located on the north side of the site. Stormwater runoff from Basin 1 will be flow out of roof drain downspouts and around the northern driveway to a retention pond in the northern proportion of the site. The retention pond is 39’ long by 30.5’ wide with 4:1 side slopes. Basin 1 is comprised of asphaltic and concrete, unimproved area, roof, and heavy soils/landscape areas. This retention pond is sized to handle the entire volume of a 100-year, 24-hour storm event. This retention pond will store runoff as it evaporates. The required retention storage for Basin 1 is 1,517 cubic feet. 5. Basin 2 Basin 2 is located on the south side of the site. Stormwater runoff from Basin 2 will be flow out of roof drain downspouts and around the southern driveway to a retention pond in the southern proportion of the site. The retention pond is 58’ long by 24’ wide with 4:1 side slopes. Basin 2 is comprised of asphaltic and concrete, unimproved area, roof, and heavy soils/landscape areas. This retention pond is sized to handle the entire volume of a 100-year, 24-hour storm event. This retention pond will store runoff as it evaporates. The required retention storage for Basin 2 is 1,705 cubic feet. 705 S CHURCH AVE STORMWATER ENGINEERING REPORT August 2025 6. Groundwater Groundwater is known to be high in this location. The soils investigation report was completed for the property by Allied Engineering in 2022. The report states that groundwater was observed during all the excavation of the test pits. From this report states groundwater was encountered between 4-6 feet below the existing ground. 7. System Maintenance Regular maintenance of stormwater facilities is necessary for proper function of the drainage system. All stormwater maintenance will be the responsibility of the property owners. Maintenance items include removing debris and cleaning ponds and establishing ground cover after construction. The infiltration facilities must be inspected after every storm event to ensure that they are draining properly. 8. 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 and on the down gradient side of Bozeman Creek outside of the floodplain. 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. Any visible sediment must be removed from the pond prior to completing construction. 9. 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 has been analyzed for the 100-year storm. Both retention ponds are sized to hold a 100-year storm.