STORMWATER MANAGEMENT REPORT FOR. Chick-Fil-A S Ferdon Blvd, Crestview FL. February 10 th, 2017

Transcription

1 STORMWATER MANAGEMENT REPORT FOR Chick-Fil-A 3000 S Ferdon Blvd, Crestview FL February 10 th, 2017 PROJECT #

2 Table of Contents Introduction.1 Project Information. 1 Site Conditions.1 Land Use Calculations.2 Curve Number Calculations...3 Methodology Stage Storage Water Quality..5 Pre Vs. Post..5 Conclusion 7

3 Introduction Project Information The project consists of one (1) lot located at 3000 S Ferdon Blvd in Crestview, Florida. The lot folio number is 29-3N , located in the jurisdiction of the City of Crestview and is currently zoned C-1 Commercial. The lot contains 2.85 acres in total. The project proposes the construction of a four thousand nine hundred eighty-nine (4,989) square foot Chick-Fil-A restaurant with a drive thru and one hundred twenty-nine (129) inside seating and twenty (20) outdoor seating. Site Conditions Existing Conditions The existing 2.85-acre site currently has an existing free standing restaurant with a three thousand three hundred-five (3,305) square foot drainage structure and an eleven thousand five hundred forty-one (11,541) square foot storm water management area. The site is approximately twentyseven percent (27%) pervious, generating a Runoff Curve Number (RCN) of 73. Proposed Conditions The proposed improvements include the construction of a four thousand nine hundred eighty-nine (4,989) square foot Chick-Fil-A restaurant with a drive thru and one hundred twenty-nine (129) inside seating and twenty (20) outdoor seating. A new drainage system will be constructed consisting of catch basins and trench drains at the front of the site, that will be connected to the existing drainage structure, to keep runoff from leaving the site. Under proposed conditions, the site will be forty-seven (47%) percent impervious and forty-three (43%) percent pervious grass and open space and generates a RCN of 63. Water Table The geotechnical report enclosed describes there to be no water table elevations encountered during the soil borings and exfiltration rate studies done on the site. It should be recognized that fluctuation of groundwater level will occur due to seasonal variations in the amount of rainfall, runoff and other factors not evident at the time the borings were performed. 1

4 Exfiltration Rates Geotechnical borings were performed and tests based on the constant head exfiltration test methodology were used to estimate K values. Based on the field data, a K value of 2.01 x 10-5 cfs/sqft-ft of head was found. Please refer to attached geotechnical report. Flood Elevations The FEMA FIRM map 12091C0255-H indicates the site to be in FEMA Flood Zone X, which has determined no Base Flood Elevation. Zone X is also known as Areas of 500-year flood; areas of 100-year flood with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 100-year flood. FEMA panel number 12091C0255-H has been enclosed for reference. Land Use Calculations Existing Conditions Area Description Square Feet Acreage % Building / Sidewalk / Paving 78, Grass / Open Area 34, Storm Management Area 11, Total Area 124,

5 Proposed Conditions Area Description Square Feet Acreage % Building / Sidewalk / Paving 59, Grass / Open Area 52, Storm Management Area 11, Total Area 124, Curve Number Calculations: CN= Σ CN x A SUBAREA A TOTAL whereas, Impervious CN = 98 and Open Space CN = 39 Existing Conditions CN CN= Proposed Conditions CN (98 x 1.80 ac)+(39 x 0.78 ac) = ac METHODOLOGY CN= (98 x 1.36 ac)+(39 x 1.22 ac) = ac Water Quality Per North West Florida Water Management District design criteria, water quality treatment is required for the first inch of rainfall to be stored on site. This was calculated by calculating the Composite Runoff Coefficient (C) using Equation (1) below; 3

6 Equation (1): 0.59 The required treatment volume was calculated by inserting the Composite Runoff Coefficient found in Equation (1) into the Treatment Volume equation for one inch by using Equation (2) below; Equation 2: The required water quality treatment volume of 0.14 ac-ft will be met within the existing drainage structure onsite. The existing drainage structure, with a storm structure top elevation of , currently provides a total storage volume of 0.11ac-ft. The existing storm structure top will be raised to elevation which will provide total storage volume of 0.15ac-ft thus providing the required one inch of water quality treatment. Water Quality Treatment Required = 0.14 ac-ft Water Quality Treatment Provided = 0.15 ac-ft Pre vs. Post Analysis A Pre vs. Post analysis has been performed to model the existing condition and compare it to the proposed condition. Please see Appendix A for calculation outputs. In the pre-development condition, the site was modeled by calculating the amount of volume that was being treated in the first one inch of runoff and the rate at which the runoff is currently discharging to the concrete block drainage structure. There are four (4) discharge points that connect to the concrete drainage structure. There is the existing curb inlet at the northwest section of the parking lot, the existing curb inlet at the southwest section of the 4

7 parking lot, the curb inlet at the main entrance of the site along S Ferdon Blvd and the storm inlet south of the property. In the post-development condition, the site was modeled by calculating the amount of volume that will be treated in the first one inch of runoff and the rate at which the runoff is will be discharging to the concrete block drainage structure. There are six (6) discharge points that connect to the concrete drainage structure. There is the proposed Type S curb inlet in the bus parking area, a proposed Type S curb inlet in the west section of the parking lot, the Type 4 curb inlet in the north drive thru lanes, two Type 9 curb inlets in the south drive thru lane, and a trench drain at the entrance of the site. Discharge Rate Storm Event Pre (cfs) Post (cfs) 25-year - 3-day (1) 0.14 (1)

8 CONCLUSION In summary, raising the existing storm structure top to elevation 162.9, in the existing concrete block drainage structure, the North-West Florida Water Management District water quality treatment requirement will be met. Water quality treatment volume is being met by providing 0.15 ac-ft of storage volume in the existing concrete block drainage structure, when only 0.14 ac-ft is required. In addition, due to there being more pervious space and less impervious space provided in the post developement condition versus the pervious and impervious space provided in the pre-development condition the discharge rate dropped by 0.02 cfs. The proposed finished floor elevation is raised to William Pfeffer, P.E. Florida License No Bowman Consulting Group, LTD. Board of Professional Engineers Certificate of Authorization No

9 Appendix A 7

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12 Appendix B 8

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