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HomeMy WebLinkAboutStormwater Study_091421 Prepared For: Smart Foods 101 South 200 East, Suite 200 Salt Lake City, Utah 84111 (801) 521-4781 Smart Foods - Bozeman 1601 West Main Street Bozeman, Montana Anderson, Wahlen & Associates, Inc. 2010 North Redwood Road Salt Lake City, Utah 84116 Phone: (801) 521-8529 Fax:(801) 521-9551 Date: August 19, 2021 Job #: 21-097 Table of Contents Introduction .................................................................................................................................................. 1 Location ........................................................................................................................................................ 1 Methodology ................................................................................................................................................ 1 Analysis ......................................................................................................................................................... 2 Stormwater Treatment ................................................................................................................................ 4 Low Impact Development ............................................................................................................................ 4 Conclusion .................................................................................................................................................... 5 Appendices  Project Location  Lag Time Calculations  Runoff Coefficients for Modified Rational Method  Modified Rational Calculations  Bozeman Design Specification  Bozeman Precipitation Depths INTRODUCTION Smart Foods grocery store is proposing to construct a grocery store in Bozeman, MT on the southwest corner of North 15th Avenue and West Beall St. The address is 1601 West Main Street. The store will be approximately 22,000 square feet. Site construction activities will include but is not limited to asphalt paving, curb and gutter, and grading. The purpose of this report is to examine the following: · Determine the overall allowable storm water release rate to the public storm drain system. · Determine the required detention using the 10-year, 2-hour storm event. · Determine the required retention based on Bozemen City LID requirement (DSSP Section IIA(4)) of 0.5 inches/acre. · Explain how storm water will be treated prior to being released into the public system. · Explain how the Low Impact Development (LID) best management practices are implemented. LOCATION The commercial development is located on an existing lot with various retail stores. The location of the proposed Smart Foods grocery store will be directly east of the Harbor Freight store. See Appendix for project map. METHODOLOGY The 10- year, 2-hour storm depth was provided in the Bozeman Storm Water Facilities Plan. This value is 0.73 inches. Additionally, a LID retention requirement of 0.5 inches/acre was provided by the City of Bozeman. It is required that the post development runoff does not exceed that of the existing conditions. The modified rational method is one of Bozeman City’s approved methods for calculating stormwater runoff and is used throughout this report. The lag time and corresponding rainfall intensity were also calculated and can be found in the appendix. ANALYSIS Contributing Areas The development of this area will result in a single drainage basin of 2.21 acres that will drain to an on- site above ground stormwater storage pond. This pond will be located directly north of the Smart Foods. on the north of the property. The proposed development will consist of approximately 70% impervious area with the remaining 30% being landscaping. Allowable Release Rate As previously mentioned, the release rate cannot exceed that of the existing conditions. To determine the allowable release rate, the modified rational method was used. The lag time for the condition was calculated to be 5.27 minutes. The figure below shows the intensity calculations approved by the City of Boseman. i = (2.21 acres) * (5.27/60)-0.65 The rainfall intensity was calculated to be 0.66 inches per hour for the existing condition. This value was then used to calculate the runoff. The runoff was calculated using the Modified Rational Method shown below. For the existing conditions, Q=(0.24) * 0.75 * 2.21 Q=0.4 Therefore, the allowable release rate for the developed condition is 0.4 cfs. Storage Requirements An above ground detention basin will be constructed with the capacity to detain the 10-year, 2-hour storm. This results in a required detention volume of 5,856 c.f. The basin is designed to not exceed the allowable release rate. This is accomplished with the use of a wier that will restrict flows. To meet the LID requirement of 0.5 inch per acre will be retained and infiltrated back to the surrounding groundwater table. The onsite basin will be constructed to provide a minimum of 5,856 c.f. of detention storage. To meet the LID requirement a portion of this 5,856 c.f. will be treated and infiltrated back to the groundwater table. The total volume to be infiltrated is 4,011 c.f., which is 0.5 inches per acre. The outfall structure will consist of a wier sized for the allowable release rate, as well as an emergency overflow for any larger storms. Storm Drain Pipes The stormwater will be conveyed from the site to the basin via a trench drain that runs along the site from north to south. Storm drain pipes will connect the trench drain from the parking lot to the detention basin. The water within the basin will sheet flow to the trench drain. Per city requirements, on-site storm drain design must be sufficient to convey runoff from the 25-year storm. The depth of the 25-Year, 2-Hour storm is 0.88 inches (see Appendix). This will require storm drain pipes to convey 1.0 cfs in order to meet the city’s requirement. STORM WATER TREATMENT Per the City of Bozeman, it is required to infiltrate, evapotranspire, and/or capture for reuse the post- construction runoff generated from 0.5 inches/acre. To meet this requirement, the onsite storage basin will have an outfall structure with an invert elevation above the bottom of the storage basin. Considering that the onsite basin will provide stormwater treatment for an area of 2.21 acres, a volume of 4,011 c.f. will be treated by infiltration. Providing this much retention meets the reduction requirement entirely and no other treatment means are necessary. One of the requirements for the infiltration is that retention basin must have a minimum area of 145 s.f. per 1 cfs. This requirement is met because the pond bottom has a surface area of 200 s.f. and our peak flow into the basin is 1.0 cfs (10-Year, 2-Hour). LOW IMPACT DEVELOPMENT The purpose of Low Impact Development (LID) requirements is to minimize the impact of the proposed development on the natural environment, as it pertains to stormwater runoff. A pretreatment manhole will be constructed at the outfall of the detention pond. A Hydrodynamic separator within the manhole remove 80% of total suspended solids. This will effectively treat the detained stormwater that is being released back to the public system at the allowable release rate of 0.4 cfs. The Barracuda Max Model S3 is sufficient, and details can be referenced in the appendix of this report. Flared end sections with rip rap pads will be installed at the inlet of the detention pond so as to prevent erosion. CONCLUSION Detention: The above ground stormwater storage system designed for Smart Foods will be able to detain stormwater from the 10-Year, 2-Hour Storm. It will also not exceed the pre-existing release rate of 0.4 cfs with the use of a wier. Retention: The above ground stormwater storage system will have an outfall structure raise above the pond bottom, which will retain the entire 0.5 inches of rain over a 24-hour storm. Water Quality Sediment Control: The stormwater storage has a pond bottom area of 200 feet which exceeds the required pond surface area for 1 cfs (The peak on-site flow for 10-Year, 2-Hour storm). Rational Method was used for determining runoff volumes and peak runoff rates. A pretreatment manhole will be constructed to remove TSS from the 0.4 cfs of stormwater that is being discharged back to the public system. Installation of this storm drain system will result in a positive impact on the public storm drain facilities located downstream. By implementing LID methods, the stormwater entering the public facilities will have less debris. Additionally, this development will not contribute anymore flows to the public storm drain system than it did in the existing condition. VOLUME 1 APPENDICES Final tc Design: CN K Area (ac)Area (mi2)Length (ft) Slope (%)ti (min)Overland Flow Only? Length (ft) Slope (%) Velocity (fps)tt (min)Total Length (ft)tc=(L/180)+10 (min)min Lag Time (hr) Remarks Existing Site 94 0.851 2.21 0.00345 300 4.0 4.92 NO 85 4.0 4.040 0.35 385 12.14 5.27 0.053 Developed Site 87 0.758 2.21 0.00345 300 4.0 6.74 NO 182 4.0 4.040 0.75 482 12.68 7.49 0.075 Lag Time Calcs - Smart Food Bozeman Company: Anderson Wahlen & Associates Project Name: By: Grayson Smith Description: Drainage Basins Lag Time Calcs Date: 2021/05/20 File Name: Lag Time Calcs Drainage Basin Data Initial/Overland Time (ti) Travel Time (tt)tc Check (Urbanized Basins) Smart Foods Bozeman Developed Condition - 70/30(Asphalt/Landscape) Area Type Area (ft^2)C Landscaping 28,338.0 0.35 Asphalt/Hard Surfaces 66,123 0.9 Subtotal=96,461 Total=2.21 acres C average=0.72 Frequency: 10-Year Duration: 2-Hour Tc= 7.49 (See Lag Time Calculations) Intensity(in/hr)= A*(Tc/60)^-0.6 0.66 Q(cfs)=CiA= 1.1 Storm Water Calculations - Developed Conditions Existing Condition - Meadow - (Soil Class D) Area Type Area (ft^2)C Meadow (Soil D) 96,461.0 0.24 Subtotal=96,461 Total= 2.21 acres C average=0.24 Frequency: 10-Year Duration: 2-Hour Tc= 5.27 (See Lag Time Calculations) Intensity(in/hr)= A*(Tc/60)^-0.6 0.82 Q(cfs)=CiA= 0.4 Storm Water Calculations - Existing Conditions “” ADS® BarracudaTM Max The Barracuda Max is market-changing stormwater quality technology. This high-performance vortex hydrodynamic separator is designed to remove total suspended solids in order to protect our precious receiving waters. The Barracuda Max is also an outstanding value that offers multiple pipe configurations, and quick installation. The “Max” version of the Barracuda is built on the base platform of the original ADS Barracuda with improved removal efficiencies and installation components. Features • Single manhole design • No elevation loss between the inlet and outlet • Variable inlet/outlet angle configurations (not just 180 degree orientation) • Internal bypass for inline installation (where applicable) • Revolutionary, patent-pending “teeth” mitigate turbulence in the sump area to prevent re-suspension of captured contaminants and an added deflector plate and bowl extension enhance the unit’s removal capabilities Benefits • Internal components are in stock for quick delivery • The S3, S4, S6, and S8 can be installed in a standard 36” (900 mm), 48” (1200 m), 72” (1800 m), and 96” (2400 m) precast manhole, respectively • The S3 & S4 can be provided factory installed within a 36” (900 mm) and 48” (1200 mm) ADS HP manhole and delivered to the jobsite • The Barracuda Max “teeth” and deflector plate apparatus are fabricated and designed for quick and easy field assembly • Designed for easy maintenance using a vacuum truck or similar equipment. • Inspection and maintenance are performed from the surface with no confined space entry Barrucuda Specification Materials and Design • Concrete Structures: Designed for H-20 traffic loading and applicable soil loads or as otherwise determined by a Licensed Professional Engineer. The materials and structural design of the devices shall be per ASTM C857 and ASTM C858. • 36” (900 mm) and 48” (1200 mm) HP Manhole Structures: Made from an impact modified copolymer polypropylene meeting the material requirements of ASTM F2764. The eccentric cone reducer shall be manufactured from polyethylene material meeting ASTM D3350 cell class 213320C. Gaskets shall be made of material meeting the requirements of ASTM F477. • Separator internals shall be substantially constructed of stainless steel, polyethylene or other thermoplastic material approved by the manufacturer. Performance • The stormwater treatment unit shall be an inline unit capable of conveying 100% of the design peak flow. If peak flow rates exceed maximum hydraulic rate, the unit shall be installed offline. • The Barracuda Max unit shall be designed to remove at least 80% of the suspended solids on an annual aggregate removal basis. Said removal shall be based on full-scale third party testing using OK-110 media gradation or equivalent and 300 mg/L influent concentration. Said full scale testing shall have included sediment capture based on actual total mass collected by the stormwater treatment unit. - OR - The Barracuda Max unit shall be designed to remove at least 50% of TSS using a media mix with d50=75 micron and 200 mg/L influent concentration. - OR - The Barracuda Max unit shall be designed to remove at least 50% of TSS per current NJDEP/NJCAT HDS protocol. • The stormwater treatment unit internals shall consist of (1) separator cone assembly, and (1) sump assembly, which includes the “teeth”. Installation Installation of the stormwater treatment unit(s) shall be performed per manufacturer’s installation instructions. Such instructions can be obtained by calling Advanced Drainage Systems at 800-821-6710 or by logging on to www.adspipe.com. Barracuda Max Model Manhole Diameter NJDEP (50% removal)OK-110 (80% removal) S3 36” (900 mm)0.85 CFS (24.1 L/s)0.86 CFS (24.1 L/s) S4 48” (1200 mm)1.52 CFS (43.0 L/s)1.52 CFS (43.0 L/s) S6 72” (1800 mm)3.40 CFS (96.3 L/s)3.42 CFS (96.8 L/s) S8 96” (2400 mm)6.08 CFS (172.2 L/s)6.08 CFS (172.2 L/s) ADS “Terms and Conditions of Sale” are available on the website, www.ads-pipe.com . The ADS logo, Barracuda logo, and the Green Stripe are registered trademarks of Advanced Drainage Systems, Inc. © 2021 Advanced Drainage Systems, Inc. #11051 7/21 CS adspipe.com 800-821-6710 ADS N-12® ST IB Pipe (per ASTM F2648) * Peak bypass flows are dependent on final design