PROPOSED WORK: The proposed work consists of impacts to a total of 4.382 acres of jurisdictional aquatic resources (4.17 acres permanent impacts; 0.212 acre temporary impacts). This includes impacts to 4.33 acres of freshwater aquatic resources (4.13 acres permanent; 0.200 acres temporary) and 0.052 acres of tidal wetlands (0.04 acres permanent; 0.012 acres temporary). In detail, the following work is proposed as summarized by the applicant:
Interior Roads
The work in waters of the United States, including wetlands, will require 2.01 acres of impact for construction of road crossings through 404 jurisdictional freshwater aquatic resources, adequately engineered to accommodate post construction hydraulics so as to maintain appropriate flow through the wetlands. According to the applicant, the project engineer, Civil Site Environmental (CSE), has designed appropriately sized culverts to maintain the functional characteristics at each road crossing as part of the comprehensive Stormwater Plan for Harmony Subdivision. These impacts include approximately 2.00 acres of permanent impacts and 0.01 acres of temporary impacts to freshwater aquatic resources. Approximately 1.06 acres of the impacts for road construction are associated with improvements to an existing road (impacts H, I, L8 and L9). Side slopes of 2:1 are as recommended by project engineers to minimize impacts and costs while providing a stable and maintainable slope.
Surge Protection
The City of Charleston's Church Creek Basin Study conducted by Weston and Sampson recommends storm surge protection for the basin during coastal storm events (tropical storms/hurricanes) to prevent storm surges from inundating storage within the basin thereby preserving storage within the basin. The HRD project site is identified as a location for implementation of these measures (page 7, Church Creek Drainage Basin Final Report, Weston & Sampson) and the Applicant has worked to coordinate these efforts with the City and the City's consultants. As such, the use of culverts (pipe and box) is proposed at wetland crossings CC1/CC2, H &E to provide points where flood controls (self-regulating tidal gates, flap gates, etc.) can be installed, accessed and maintained. Bankfull flow adjacent to the crossing at Church Creek is provided for by additional box culverts with flap gates placed at existing grade in the overbank areas adjacent to the creek.
Proposed improvements to the existing dirt road crossing Church Creek include replacing the existing pipes with larger box culverts. The box culverts will include self-regulating tide gates on the downstream side to prevent storm surge from coastal storm events from inundating areas upstream and reducing available storage capacity during those storms. These tidal gates will still allow upstream tidal flow under normal tide conditions. During storm surge events, the gates will close to prevent surge from proceeding further upstream. Once the downstream water levels return to normal, the tide gates reopen and return to allowing passage of tidal flows upstream. Per page 16 of the Weston & Sampson report, surge protection is recommended as a top priority element to improve the overall condition within the Church Creek Basin when it comes to flood events. Proposed improvements do not create increases in Water Surface Elevations on typical rain events.
To accomplish the recommended storm surge protection at road crossing E, six 4´x 4´ box culverts with downstream flap gates will be installed as depicted on sheets 5 and 18 of revised project plans. No additional impacts to aquatic resources are required to provide storm surge protection at proposed road crossing E.
To accomplish the recommended storm surge protection at road crossing H, a total of forty-two 4´x 4´box culverts with downstream flap gates will be installed. These culverts will be installed at five locations along the roadway in groups of six, nine and twelve culverts as depicted on sheets 5 and 23 of revised project plans. No additional impacts to aquatic resources are required to provide storm surge protection at this proposed crossing.
To accomplish the recommended storm surge protection at road crossing CC1/CC2, the following specific activities are proposed:
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Construction of a temporary diversion channel through 404 jurisdictional freshwater aquatic resources to re-route the flow of Church Creek during construction of the storm surge protection structure. The temporary channel will have a bottom width of 15´and a top width of 25´as depicted on sheets 15 and 32 of revised project plans. Rip-rap will be placed temporarily at bends within channel and silt fence will be installed around the perimeter. Temporary impacts of 0.13 acres to freshwater wetlands are required to accomplish this work.
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Installation of a sheet pile coffer dam to provide protection for the work area and dewatering as depicted on sheet 16 of revised project plans.
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Removal of the four existing 36ʺ HDPE culverts as depicted on sheet 16 of revised project plans.
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Installation/construction of three 5´x 5´box culverts with concrete headwalls and rip-rap toe protection placed over non-woven geotextile fabric at both upstream and downstream ends of the culverts. Two of the box culverts will include self-regulating tide gates on the downstream end. These structures will provide for normal exchange of tidewater during normal tides and are designed to close, preventing storm surge inundation upstream during coastal storm events when downstream water surface elevation exceeds 5.0 feet. The third box culvert will include a flap gate on the downstream end. Revised project drawing sheets 13-17 and 24-30 provide more specific details of the proposed storm surge protection structure. Temporary impacts of 0.06 acres to freshwater wetlands and 0.012 acres to tidal saltwater wetlands and permanent impacts to 0.04 acres of tidal saltwater wetlands are required to accomplish this work. The 0.04 acres of permanent impact to tidal saltwater wetlands is specifically required for rip-rap toe protection of the structure.
Upon completion of the work described in items 1-4 above, the sheet pile coffer dam will be removed, the temporary diversion channel will be backfilled to original grades and all temporary impact areas will be stabilized and restored. In total, installation/construction of the storm surge protection at CC1/CC2, requires permanent impacts to 0.04 acres of tidal saltwater wetlands, temporary impacts to 0.012 acres of tidal saltwater wetlands and temporary impacts to 0.19 acres of freshwater wetlands.
Residential Lots and Stormwater Management
The land design for HRD includes 2.13 acres of permanent impacts to 404 jurisdictional freshwater aquatic resources to accommodate construction of residential building lots and stormwater management facilities. This impact acreage includes 0.60 acres of depressional wetlands and 1.48 acres of man-made ditches/swales.
Project Utilities
As part of the Applicant’s attempt to avoid and minimize impacts to aquatic resources, the project engineer has planned for the installation of the site’s utilities without additional loss of wetlands. All applicable utility lines will be included in the footprints of proposed wetland road crossings or installed by directional bore.
Development Timeline
DR Horton plans to construct the HRD over time, subject to economic conditions and market fluctuations, with anticipation that development will continue over the next ten years. Therefore, the Applicant requests the USACE to consider this a request for a ten (10) year permit life.
Compensatory Mitigation
The Applicant proposes to mitigate for proposed permanent and temporary impacts to 404 jurisdictional freshwater aquatic resources in accordance with the USACE Charleston District Compensatory Mitigation Guidelines dated October 7, 2010 (Guidelines). The Guidelines define a requirement of 44.3 mitigation credits to compensate for the proposed impacts. HRD also agrees to perpetually preserve 15.07 acres of upland buffers (average of 25’ in width with a minimum of 15’ in width) adjacent to the remaining/preserved 45.29 acres of on-site 404 jurisdictional freshwater wetlands through deed restrictive covenants. In accordance with the current Guidelines, the proposed on-site perpetual freshwater wetland preservation with buffers affords a 25% reduction (11.1 credits) in wetland mitigation credits that will have to be purchased from a USACE approved mitigation bank. After the 25% reduction, HRD will purchase 33.2 wetland mitigation credits from an approved mitigation bank.
Direct compensatory mitigation is not proposed for the permanent (0.04 acres) and temporary (0.012 acres) impacts to tidal wetlands, as the benefit of providing storm surge protection for the upstream Church Creek drainage basin is believed to offset this minimal environmental impact. The permanent impact of 0.04 acres is for the purpose of providing rip-rap toe protection for the storm surge protection structure previously described above. Additionally, although not proposed as compensatory mitigation, the land plan for HRD preserves 23.17 acres of tidal wetlands and 2.92 acres of associated buffers within the project site.
Stormwater
The stormwater retention and drainage system at HRD will appropriately accommodate stormwater through implementation of various stormwater conveyance structures including pipes, swales and existing ditches and will be designed to meet all applicable local and state regulations. Specifically, these conveyances will be engineer designed and constructed to accommodate existing and anticipated future water flows without adversely affecting adjacent upstream or downstream properties or wetland systems. A Stormwater Masterplan for the Harmony Subdivision has been prepared by CSE, the project engineer.
The purpose of the proposed work as stated by the applicant is “to construct and complete an economically viable, long term, residential development that will meet rising housing needs in proximity to existing and future infrastructure, civic services and employment opportunities within the City of Charleston.”
PLEASE NOTE: A similar project for this site was previously advertised on public notice. That project was revised to incorporate previous comments received, resulting in the current project. ONLY COMMENTS IN RESPONSE TO THIS CURRENT PUBLIC NOTICE DATED 31 OCTOBER 2018 WILL BE CONSIDERED.