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HomeMy WebLinkAboutFB SOF Special Tactics Facility Fort Bragg - Summary Report*** ***FOR OFFICIAL USE ONLY June 13, 2016 Revised April 2018 Wilmington District prepared for Contract #: W912HN-14-D-0017- 0001 PN #: 76513 M&H Project #: 0142.01 Fort Bragg, NC SOF Special Tactics Facility, Phase 2 Summary Stormwater Report SOF STF Phase 2 Fort Bragg, Hoke County, North Carolina Mason & Hanger Page - a.1 - GENERAL INFORMATION The project is located within the Fort Bragg Army Installation at the Aberdeen Training Facility off of King Road. The site is Phase 2 of a multi-phase, multi-building project being developed within the existing training area. The site is currently undeveloped and covered with native vegetation. The site is bound on the north by either existing or proposed facilities or infrastructure and to the south by an existing wetland area. The project consists of three (3) new two story buildings totaling approximately 116,000 SF, privately owned vehicle (POV) parking, expansion of the pavement at the existing access control point to the training area and access drives throughout the site. The total site area within the limits of construction is approximately 23.32 acres. The site prior to construction is undeveloped, consisting of a combination of open area and vegetation. Vegetation consists primarily of coniferous trees 8 to 12 inches in diameter spread roughly 30 to 50 feet apart. The topography slopes generally towards the wetlands to the south, with an elevation range of approximately 55 feet. The soils consists primarily silty sands or clayey sands. Prior to construction, the government will harvest the area, with the remainder of the vegetation to be removed by the contractor. All drainage areas for the project are in the watershed of Wolf Pit Creek, Stream Index 18-31-3, Classification C: Aquatic Life, Secondary Recreation, Freshwater, in the Cape Fear River Basin. No water supply wells are within 100’ of the project site. No surface waters are within 30’ of the project site. No Class SA waters are within 50’ of the project site. METHODOLOGY All methodology used to determine both peak flow and storm event volumes follows the guidelines set by the Corp of Engineers Wilmington District and NCDEQ. As the project is a federal project which exceeds 5,000 square feet of footprint, Section 438 of the Energy Independence and Security Act or 2007 (EISA 438) mandates the use of site planning, design, construction, and maintenance strategies for the property to maintain or restore, to the maximum extent technically feasible, the predevelopment hydrology of the property. EXISTING CONDITIONS The existing site where the training facility and associated supporting infrastructure is to be constructed is approximately 23.32 acres. The site, prior to construction, undeveloped, consisting of a combination of open area and vegetation. Vegetation consists primarily of coniferous trees 8 to 12 inches in diameter spread roughly 30 to 50 feet apart. The topography slopes generally towards the wetlands to the south, with an elevation range of approximately 55 feet. The soils consists primarily silty sands or clayey sands. Prior to construction, the government will harvest the area, with the remainder of the vegetation to be removed by the contractor. For purposed of drainage calculations, the open site area is considered in fair to good condition, and the forested area moderately dense. Two existing drainage structures exist on the site to convey drainage underneath the perimeter access road. One pipe is a 15” CMP, the other a 24” CMP. These two pipe only convey drainage from a limited portion of the site. Most runoff from the site sheet flows or flows in shallow channels towards the wetlands. Due to the sandy nature of the existing soils, most first flush drainage permeates into the ground. For the purpose of the stormwater calculations, the site has been divided into three areas: Northern Area, Eastern Area and Western Area. The Northern Area is the area around the construction for the access control point. The Eastern Area is the area around the construction of Buildings 1 and 2. The Western Area is the area around the construction for Building 3. Each area has been subdivided into separate drainage areas, with no overlap. All drainage within each area is considered separately. Offsite runoff, if any will be routed around any onsite Best Management Practices (BMPs) installed for stormwater control. The existing soils on the site were obtained from a Subsurface Exploration Report performed by Soils Section, Geotechnical & HTRW Branch, US. Army Engineer District, Savannah. The field exploration was SOF STF Phase 2 Fort Bragg, Hoke County, North Carolina Mason & Hanger Page - a.2 - conducted between April 3rd, 2015 and July 28st, 2015. The report is dated June 2015, Revised September 2015. To calculate storm data, 24hr rainfall data to use for design was provided for by Fort Bragg. The rainfall data used are given in Table 1. For stormwater design, the 10yr event will be used. For temporary erosion control measures during construction, the 2yr event will be used. WinTR55 Small Watershed Hydrology software, developed for the USDA, was utilized to calculate the peak runoff for both predevelopment and post development runoff. This software has been used in the past for other projects located at Fort Bragg and for NCDEQ, and is an approved method for stormwater calculations. The composite runoff curve number calculations for each area were entered into WinTR55 and the results are given in Table 2. PROPOSED CONDITIONS To better control runoff at the source, and to better calculate strormwater runoff for drainage structures, the developed site was separated into 7 separate bioretention infiltration areas. These areas do not necessarily align with the original drainage areas, however the outflow of the post development basins is at the same location as the predevelopment basins. For all areas offsite and outside of the construction limits, the stormwater is routed around the site to its natural outfall location. STORMWATER MANAGEMENT As the project is a Federal facility over 5,000 square feet, the stormwater requirements of EISA Section 438 (Title 42, US Code, Section 17094) must be met. In accordance with the Department of Defense memo dated January 19, 2010 entitled DoD Implementation of Storm Water Requirements under Section 438 of the Energy Independence and Security Act (EISA) the designer of record shall implement the procedures for complying with EISA 438 as outlined in the EPA Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act. To provide for storage and infiltration requirements of EISA 438, a total of 7 infiltration areas have been designed to help infiltrate the runoff closest to the source. Each of these areas are bioretention area consisting of a volume of ponding storage, a section of engineered fill to promote infiltration and sediment removal, and a layer of open graded aggregate to provide for the storage (see Figure 2). All drainage for the construction portion of the site is routed to an infiltration area. The design of the bioretention areas is based upon design criteria from Sustainable Landscape Systems for Managing Storm Water 2nd Edition, developed by Ohio State University Extension. The bioretention areas are designed to infiltrate the 95th percentile storm event as close as practical to the origin of the drainage. All drainage enters the bioretention areas via surface flow, or via stormwater collection pipes with the outlet at the top of the pond area, allowing for sediment to be removed via infiltration. The storage required for each area and the amount of storage provided for infiltration is shown in Table 4. Storage is provided in the above ground ponding (maximum 12”), media and the open graded aggregate (minimum 2 feet). Full calculation sheets are provided in the appendices. For all storm events greater than the 95th percentile event, and for other events where the infiltration areas cannot sufficiently handle the storm event, excess runoff is bypassed directly to the outfall area. As the open aggregate fills with water, when the level reaches the bottom of the perforated pipe, drainage enters the pipe and is carried to either the next infiltration area, or to the outfall. In addition, if there is excess ponding due to either the saturation of the engineered fill or a large storm event, water is collected through the grate in the riser structure. This prevents flooding in areas surrounding the bioretention basins as the top of the grate is set at an elevation below the surrounding grade. The width of the bioretention areas within the parking lot are dictated by the Fort Bragg Installation Design Guide. To determine the storage volume to meet EISA 438, the Army LID Planning and Cost Tool was compared against what was calculated via the Direct Determination method. The database determines the volume required to be stored and infiltrated onside in order to meet EISA 438. The Army LID Planning and Cost SOF STF Phase 2 Fort Bragg, Hoke County, North Carolina Mason & Hanger Page - a.3 - Tool was developed to MILCON standards for projects within the jurisdiction of the Army. As such, the volumes determined using the Army LID Planning and Cost Tool are used for the design of the bioretention basins. In addition, the volume for the basins was checked for compliance with NCDEQ standards. The volume stored and infiltrated was checked against the first flush, or 1” storm event. In addition, the surface areas of the basins were checked for compliance with NCDEQ standards. The Minimum Design Criteria (MDC) for bioretention cells for NCDEQ was utilized to design the bioretention areas. These criteria are taken from the NCDEQ Stormwater Design Manual Section C-2. Bioretention Cell Revised 1-10-2018. OUTLET PROTECTION All excess stormwater from the infiltration areas, as well as outlet structures for stormwater routed around the project site are provided outlet protection at the headwalls. Outlet protection is provided in accordance with guidance from Chapter 8.06 Design of Riprap Outlet Protection of the North Carolina Division of Environmental Quality Erosion and Sediment Control Planning and Design Manual. Calculations for the outlet protection are shown in the appendix. WATER QUALITY To ensure the removal of Total Suspended Solids (TSS), all runoff is directed to the infiltration areas via either sheet flow or via storm collection pipes with their outlets at the top of the bioretention basins (protected with energy dissipaters). This includes runoff from all parking areas and roof drainage structures. This makes certain that the storm events up to the 95Th percentile event (and first flush for larger events) will have the runoff filtered through a minimum the landscape rock and a minimum of 24” of engineered soil. Additionally, in bioretention areas that do not have landscape rock, a 3’ rock strip is included to meet NCDEQ requirements for TSS removal at bioretention basins. In addition, a non-woven geotextile fabric is placed above both the gravel section of the bioretention basin and the perforated collection pipe to keep sediment and other fines from infiltrating the storage area. It is generally accepted that bioretention basins of this type will remove between 80-90% of TSS along with the removal of heavy metals. This design has previously been used on projects at Fort Bragg and has been approved by NCDEQ for TSS removal requirements. This method will also meet LEED 6.2 Stormwater Quality Control requirements. EROSION AND SEDIMENTATION CONTROL Separate erosion and sedimentation control during construction plans have been developed for submittal to NCDEQ for permit requirements. Refer to the Stormwater Report for more information, calculations and required forms.