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Home My WebLink AboutSW1220301_Stormwater Narrative_20220322Section 1 PROJECT OVERVIEW 1.1 Narrative The Asheville Regional Airport Terminal Expansion — Contractor Staging Area project includes the construction of a new parking lot, staging areas, and associated grading, drainage, landscaping and erosion control. This report includes the computations that show compliance with the NCDEQ High Density Project. A Primary SCM (Bioretention cell ) will be used to treat the stormwater runoff from the proposed site. 1.2 Introduction and Site Description The site is located at Asheville Regional Airport in Fletcher, NC, south of Asheville, North Carolina near Airport Road (35° 26' 17.43" N. 82° 32' 13.40"W). The area that includes airport related operations encompasses approximately 580 acres. For a larger vicinity map, see Appendix A. The Asheville Regional Airport Terminal Expansion — Contractor Staging Area project site is located on an existing pad graded area in the eastern portion of the airport, located along Airport Road. For hydrologic planning purposes, the site is comprised of Hydrologic Soil Group (HSG) B soils. Appendix A contains the NRCS Soil Survey Maps for the parking lot. The existing land use of the project _ o Asheville Regional a,....a.m o Airport is grass, gravel, forest, and impervious. The proposed land use will impervious (parking lot) with a small amount of grass for landscaping and bioretention. As shown in the FEMA FIRM 3700964300K (January 6, 2010) in Appendix A, the disturbance is not located within 200ft of a FEMA floodplain. 1.3 Construction Sequence and Cost As explained later in the report, a bioretention cell is proposed as a primary stormwater conveyance measure (SCM). The rough outline of the permanent bioretention area will occur during the grading phase of the project and a portion of it will be used as a temporary sediment trap to protect downstream receiving stormwater systems from construction activity. After the site is stabilized, the sediment trap will be cleaned out and re -graded to final bioretention design grades. No mechanical stabilization will occur. The cost of the bioretention area is unknown at this time and will be finalized during bidding. The Asheville Regional Airport will be the end -user and responsible for annual inspection, maintenance and repairs of the system. Section 2.3 discusses the operation and maintenance of the cell. 9 AVL TERMINAL EXPANSION -CONTRACTOR STAGING AREA INITIAL SUBMITTAL: MARCH 2O22 STORMWATER MANAGEMENT REPORT Section 2 HYDROLOGY 2.1 Project Methodology In order to qualify for a primary Stormwater Conveyance Measure (SCM) for a Bioretention, several minimum design criteria (MDC) must be met. The recommendations will also be met to maximum extent practical. MDC 1 Separation from the SHWT Boring logs for the parking lot did not observe water levels at 10ft depths so the season high water table is assumed to be deeper than 10ft below ground. Therefore, the lowest point of the bioretention cell will be a minimum of two feet above the SHWT. MDC 2 Maximum Ponding Depth for Design Volume The maximum ponding depth for the bioretention cell is 12 inches. MDC 3 Peak Attenuation Volume The maximum depth of the entire cell is 18 inches for peak attenuation. See plans with spillways at the 12 inch depth and the overall dam height of 18 inches. MDC 4 Underdrain An underdrain with internal water storage is designed. See plan view on SW-01 and details on SW-02 and SW-03 showing the underdrain location and details. MDC 5 Media Depth The media depth is 36 inches, see detail on SW-02 showing media depth. MDC 6 Media Mix The media shall be a homogeneous soil mix engineered media blend with approximate volumes of: (a) 75 to 85 percent medium to coarse washed sand (ASTM C33, AASHTO M 6/M 80, ASTM C330, AASHTO M195, or the equivalent) (b) 8 to 15 percent fines (silt and clay); and (c) 5 to 10 percent organic matter (such as pine bark fines). See detail on SW-02 showing media mix. MDC 7 Media P-Index The phosphorus index (P-index) for the media shall not exceed 30 in NSW waters as defined in 15A NCAC 02B .0202 and shall not exceed 50 elsewhere. MDC 8 No Mechanical Compaction The media shall not be mechanically compacted. It is recommended to either water it or walk on it as it is placed. This note is on the detail, sheet SW-02. AVL TERMINAL EXPANSION -CONTRACTOR STAGING AREA INITIAL SUBMITTAL: MARCH 2O22 STORMWATER MANAGEMENT REPORT MDC 9 Maintenance of Media The bioretention cell shall be maintained in a manner that results in a drawdown of at least one inch per hour at the planting surface. This is accomplished by not installing until the end of the project phase, installing the correct soil type as outlined above, and not mechanically compacting. MDC 10 Planting Plan Not applicable due to sod. MDC 11 Mulch Water will enter the bioretention at less than 3 fps (see Recommendation 1 narrative below). Therefore, sod is proposed in the bioretention area. MDC 12 Clean -Out Pipes A minimum of one clean -out pipe shall be provided on each underdrain line. Clean out pipes shall be capped. As seen on plan and details, sheets SW-01 through SW-04, a cleanout is provided in the bioretention cell. Recommendation 1 Dispersed Flow or Energy Dissipation The parking lot runoff will enter the bioretention via pipe flow from the parking lot into a proposed forebay. The forebay spillway is spread over 40ft at 6inches depth, resulting in an outlet velocity into the bioretention of less than 3 fps. Recommendation 2 Pretreatment The parking lot runoff will enter the bioretention via pipe flow from the parking lot into a proposed forebay. The forebay will be 24 inches deep. The Rational Method was used for the parking lot design on the development. The Intensity Duration Frequency (IDF) curve for Fletcher, NC below was used for the calculations: „ _■■■■■■■■■■ � ■■■■■■■■■■ ■■■■■■■■■■■ , ■.�■■■■■■■■■■ . ME III E 11110 1 0 11111 9 AVL TERMINAL EXPANSION -CONTRACTOR STAGING AREA INITIAL SUBMITTAL: MARCH 2O22 STORMWATER MANAGEMENT REPORT The following runoff coefficients were also used for hydraulic (storm inlet, pipe) analysis of this project: c = 0.30 (Grass, HSG B) c = 0.95 (Impervious area) Design Volume The design volume for a bioretention cell is equivalent to the volume that is contained above the planting surface to the invert of the bypass mechanism for the design storm. See below for volume calculations: Design Volume - Required Drainage Impervious IA Rv Rp (Design Design Volume Basin Area Area (Impervious (0.05 * 0.9 Storm Depth) (3630 * Rp * Rv * A) (a c) (ac) Fraction) * In) (in) (cf) Bioretention 1 1.69 1.36 0.80 0.77 1.0 4,750 Design Volume - Provided Design Volume Surface Area Depth Design Volume Basin Required Provided Provided Provided cf sf ft cf Bioretention 1 4,750 5,332 1.0 5,812 2.2 Drainage & Erosion Control Design 15" and 18" RCP are proposed along the curb and gutter of on the west and east sides of the parking lot. Both lines collect water via curb inlets and travel south. At the southern edge of the parking lot, the eastern line turns west to tie into the western line. From there, an 18" RCP sends all of this parking lot drainage to a proposed forebay which will then spill into the bioretention area. Additionally, an 18" RCP is proposed from the bioretention outlet control structure. This 18" RCP will tie into an existing inlet. These pipes have been designed to safely convey the 25-year storm. Pipe profiles and Hydraflow Storm Sewers results can be found on sheet SW-02 of the stormwater plans (Appendix C1). 2.3 Operation and Maintenance Manual The bioretention area requires maintenance in order to work properly. An O&M plan has been developed and can be found in Appendix D. 2.4 Design Software and Manuals The following design software programs were used in analysis and development of the stormwater and documents: • Hydraflow Hydrographs for AutoCAD Civil 3D 2021 • AutoCAD Civil 3D 2021 • Microsoft Office 2021 The following design manuals and literature were used in analysis and development of the stormwater documents: • NCDEQ Stormwater Design Manual— 2017 Edition 6 AVL TE RM I NAL EXPANSION —CONTRACTOR STAGING AREA INITIAL SUBMITTAL: MARCH 2O22 STORMWATER MANAGEMENT REPORT