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HomeMy WebLinkAboutSW6110103_Stormwater Narrative_20221220cul A=COM JSOC - Microgrid Installation and Integration Post -Construction Stormwater Management Permit Package Controlled by: JSOC Controlled by: USACE SAW CUI Category: CONTRACT Distribution Statement D POC: USACE FOIA (foia-saw@usace.army.mil) USACE Project No.: PN97470 AECOM Project No.: 60652522 October 17, 2022 Delivering a better world cui cul JSOC - Microgrid Project number: 60652522 Prepared for: U.S. Army Corps of Engineers, Wilmington District Prepared by: AECOM 10 South Jefferson Street Roanoke, VA 24011 aecom.com Copyright © 2022 by AECOM All rights reserved. No part of this copyrighted work may be reproduced, distributed, or transmitted in any form or by any means without the prior written permission of AECOM. AECOM cul JSOC - Microgrid Project number: 60652522 Table of Contents Attachment A — Stormwater Permit Application................................................................ Attachment B — NCDEQ Supplement and O&M Forms .................................................... Attachment C — Land Development Drawings.................................................................. Attachment D — Hydrology Report.................................................................................... AttachmentE — USGS Map.............................................................................................. Attachment F — Erosion & Sediment Control Checklist..................................................... Attachment G — NRCS Websoil Survey............................................................................ Attachment H — Geotechnical SHWT and Infiltration Report ............................................. AECOM cul JSOC - Microgrid Project number: 60652522 1. Introduction The JSOC Microgrid project provides for the construction of a new microgrid system to integrate new and existing diesel generators to the JSOC Compound at Fort Bragg in order to provide emergency backup power independent of the Fort Bragg utility grid. The project scope generally consists of the following: • New 2 megawatt (MW) diesel generator • Integration of four (4) existing 2 MW generators • A new fuel supply system consisting of three (3) 50,000 gallon above ground, double wall, protected diesel fuel tanks, pumping and polishing equipment, and underground fuel distribution to the existing and new generators. • Pavement improvements to include a new fuel delivery access road and contained fuel delivery area. • Supporting utility connections to include medium voltage and low voltage electrical feeders, underground telecommunication and fiber optic connections, domestic water for fire protection, and stormwater drainage infrastructure. 2. Site Description The project site is generally divided into three project areas: Fuel Storage Tank Yard Building 3-1549 Vicinity Utility routing from Building 3-1549 to Malvesti Street The proposed Fuel Storage Tank Yard site is located approximately 90.0 feet to the east of Building 138 and outside the JSOC compound perimeter fence. Approximately 70-percent of the site is wooded with a mix of pines and hardwoods. A new bulk fuel oil facility will be located at this location to include above ground double wall fuel storage tanks, fuel truck unloading facilities, and paved access road for fuel truck delivery and fire truck access. The existing topography at this location generally slopes from the west to the east. This site is bounded by Building 138 to the west, Building 137 to the north, and the existing JSOC perimeter fence to the south and east. Proposed work in the vicinity of Building 3-1549 (within the JSOC compound) will consist of new electrical switchgear and generator installation, with supporting utility infrastructure. A portion of the existing parking lot to the south of Building 3-1549 will be removed for installation of the new electrical equipment. This area is already developed with paved access roads, parking, and numerous underground utilities. Stormwater runoff is collected within storm drains and discharge either into an underground infiltration trench (primarily serving Building 3-1549) or into existing detention basins, with an existing outfall to the southwest of the parking lot. The remaining work area is linear in nature for the installation of underground electrical feeders and communication cable from the proposed microgrid switchgear equipment to the existing SUS electrical switches. AECOM cul 1 cUl JSOC - Microgrid Project number: 60652522 3. Site Layout The new fuel tank storage yard will be located approximately 90.0 feet to the east of Building 138. The fuel storage tanks, and ancillary equipmentwill be housed on a reinforced concrete pad within a closed security fence. A new 20-foot asphalt access road will be provided to support fuel delivery and fire truck access to the fuel tank storage yard and to maintain access to Building 138. A fuel truck unloading area will be provided adjacent to the fuel storage tanks to facilitate unloading of fuel to the new fuel tanks. The unloading area will consist of rigid concrete pavement and containment curbing to collect any potential spills by sloping the pavement to a central grate inlet, which drains to a new containment tank. The containment tank has sufficient volume to contain a potential spill from the largest anticipated delivery vehicle. The new electrical switchgear and generator will be located within the existing parking lot to the south of Building 3-1549 and will require the removal of existing parking spaces. A maintenance roadway will be provided to allow for maintenance vehicle access to the new equipment. 4. Stormwater Management Storm drainage and stormwater management is designed to meet all applicable requirements from the North Carolina Department of Environmental Quality (NCDEQ) Stormwater Design Manual. NCDEQ: Fort Bragg maintains a MS4 permit with NCDEQ. Consequently, this project is subject to North Carolina Stormwater Rule 5A NCAC 02H.1017 and will require a North Carolina Post -Construction Stormwater Permit since there is more than one acre of disturbance. This project will modify the existing stormwater permit (SW6110103) that was granted in connection with the C4 Building construction. The existing permit information and some of the proposed modifications are summarized as follows: Table 1. Summary of Proposed Modifications to Existing Permit Item Existing Permit Proposed Modification Project Limits / Limit of Disturbance (associated with land disturbance permit of respective project) 4.78 acres 3.58 acres Project Area 2.49 acres 3.10 acres Drainage Area 1 Total Area 85,000 sf (1.95 acres) No Change BUA 58,400 sf (1.34 acres) 60,221 sf (1.38 acres) Drainage Area 2 (modification) Total Area N/A 26,654 sf (0.61 acres) BUA N/A 12.337 sf AECOM cUl 2 cul JSOC - Microgrid Project number: 60652522 As stated in more detail above, this project is generally comprised of three areas: Fuel Storage Tank Yard (designated as "Site 1" elsewhere) Building 3-1549 Vicinity (designated as "Site 2" elsewhere) Utility routing from Building 3-1549 to Malvesti Street Site 1 is quantified and presented as "Drainage Area 2" in the existing permit modification. Site 2 includes new "built -upon area" (BUA, as defined by NCDEQ) that will contribute to the existing infiltration trench drainage area, which is noted as "Drainage Area 1" in the existing permit. The entire project disturbs roughly 3.6 acres, largely due to utility trenching to Malvesti Street. That utility routing pertains to the land disturbance permit but not the Post - Construction Permit since there are no new impervious areas associated with that work. New site development and changes to the BUA are only present in the first two areas — Site 1 and Site 2, which have a combined area of less than one acre. SITE 1: Only Site 1 has an increase in the BUA. A bioretention basin with internal water storage (IWS) serves as the stormwater control measure (SCM) and is sized to treat the NCDEQ one -inch treatment volume for the entire BUA. The bioretention surface area will be approximately 1,260 square feet with a ponding depth of 12 inches and will provide approximately 1,525 cubic feet of treatment volume — exceeding the Site 1 minimum required treatment volume of 1,028 cubic feet. Runoff from the containment drainage area is conveyed to a containment tank. The primary purpose of the tank is to capture potential fuel spills from a tanker truck during delivery. The containment tank will also store the initial first -flush of storm runoff until a visual inspection can be performed to ensure there is no sheen present and that it is safe to drain the contained volume to the bioretention SCM. The tank can store the runoff volume from a 2-inch rainfall event before overflowing to the bioretention SCM. The containment tank storage volume is not counted as part of the treatment volume for NCDEQ compliance but does provide some actual stormwater benefit. SITE 2: Site 2 has an overall decrease in development (i.e. a net decrease in BUA). However, a portion of Site 2 will increase the developed area contributing to an existing underground stormwater management infiltration trench under an existing stormwater permit. Based on as -built information, this existing SCM required 4,750 cubic feet of treatment volume and provided 6,842 cubic feet— resulting in a treatment volume surplus. The amount of added treatment volume required for the additional BUA in Site 2 will be less than 200 cubic feet and can, therefore, be accommodated by the existing underground SCM. AECOM cul 3 CUI JSOC - Microgrid Project number: 60652522 5. Stormwater Conveyance New storm drainage ditches and piping are designed with capacity for the 10-year design storm. Regarding existing drainage capacity, the fuel tank storage yard (Site 1) and the parking lot area to the south (Site 2), currently drain eastward to an existing ditch along the railroad tracks to an existing 30" reinforced concrete pipe (RCP) culvert, which crosses the railroad approximately 500 feet south of the site. The existing drainage area that contributes to this culvert crossing is approximately 45 acres. Construction of the fuel tank storage yard area at Site 1 will increase impervious areas. The effect of this increase will be reduced by the new bioretention SCM at Site 1 and by reductions in pavement that are associated with the Site 2 work to the south. The reductions in imperviousness are accomplished by replacing existing pavement with #57 gravel fill. Existing stormwater management systems will not be modified. Site development associated with this project is less than 1 acre and represents about 2 percent of the total drainage area that contributes to the existing ditch and pipe culvert crossing. The post -development peak runoff from Site 1 is controlled and sufficiently reduced using a bioretention SCM so that the increase in 10-year runoff to the existing ditch is minimized. The adequacy of the existing culvert for the 10-year design storm and possible correction of any inadequacy is outside the scope of this project. However, due to the proportional area of this project with respect to the whole, this project will not result in significant changes to the watershed and will, therefore, not adversely affect the existing culvert. AECOM CUI 4 cul aecom.com