The City of Charleston and its engineering consultant, Thomas and Hutton, have worked over the past several years to develop an efficient and cost-effective drainage improvement plan for the Forest Acres and 5th Avenue drainage basins. The Forest Acres drainage basin has experienced significant flooding of roads and yards, and in some cases, flood waters have entered homes causing significant damage. See the flooding experienced in the Forest Acres drainage basin on August 28, 2012 (PDF).
View a map illustrating the two drainage basins.
The two drainage basins, Forest Acres and 5th Avenue, are interconnected by the existing, man-made drainage system that drains the Forest Acres drainage basin. The existing drainage system includes a pump station (with a single pump) and a mile-long, 48-inch outfall pipe to the Ashley River. The pump station is located near the intersection of Playground Road and the West Ashley Bikeway. The outfall pipe is buried along the West Ashley Bikeway from Playground Road to the Ashley River.
Working within the urban and suburban environments found in the Forest Acres and 5th Avenue drainage basins provide additional challenges in addressing the drainage and flooding issues. These challenges include existing utilities (water, sewer, gas, electrical, communications, cable television, etc.) that must be avoided or relocated, limited City owned property or easements for the construction of improvements, and provisions for maintenance of traffic. The planning, designing, permitting, and the construction of the Forest Acres Drainage Improvements project is an involved and lengthy process.
To date, the following activities have been conducted to allow for the construction of Phase 1 of the Forest Acres Drainage Improvements:
- Existing Conditions Analysis - The combined 500-acre drainage basin was analyzed including approximately 2.7 miles of drainage systems (channels, pipes, etc.).
- Outfall Assessment - Viable outfall locations to the Ashley River were assessed, including potential impacts to wetlands and critical areas, and property/easement requirements.
- Alternatives Evaluations - Seven improvement alternatives were assessed, including rehabilitating the existing pump station, constructing a new pump station, and constructing a gravity drainage system.
- Recommended Improvements Assessment - Based on the alternatives evaluation, the recommended drainage improvements for the basins were developed. Based on various factors, including life-cycle cost, the gravity alternative was recommended for implementation. This step included the refinement of the proposed improvements and detailed cost analyses.
- Geotechnical Investigations - Various geotechnical investigations and analyses were conducted for the design and construction of the improvements.
- Hazardous Materials Investigations - The existing pump station, constructed in the 1960s, contains some building materials that may be hazardous, especially during demolition. These materials were identified and will be properly handled and disposed of when the pump station is demolished.
- Wetland and Critical Area Delineations - As with many areas in the Low Country, wetlands and critical areas (saltwater wetlands) are located in and around the project area. Wetland scientists examined various indicators in the project area (including soils, vegetation, and water flow) to delineate the wetlands and critical areas in the project area. These delineations are conducted per U.S. Army Corps of Engineers (USACOE) and SC Department of Health and Environmental Control - Office of Coastal Resources Management (SCDHEC-OCRM) regulations and policies. These delineations were surveyed and a legal survey document (called a 'plat') was prepared to depict the wetlands and critical areas.
- Wetland and Critical Area Jurisdictional Determinations - The wetland and critical area delineations were verified by the USACOE and SCDHEC-OCRM, and the various regulation jurisdictions that they fall within were determined. The wetland and critical area plat was approved and signed by the appropriate USACOE and SCDHEC-OCRM staff.
- Project Area Surveys - The project area was surveyed, including topography, roads, sidewalks, drainage pipes, etc. In addition, property, right-of-way, and easement lines were determined. The project area survey became the basis for the engineering design drawings.
- 30%, 70% and 95% Engineering Design Drawings - The engineering design drawings were developed with various levels of detail. At each percent completion, the plans were reviewed for constructability and other issues.
- Wetland and Critical Area Impact Permitting and Mitigation - The USACOE and SCDHEC-OCRM issued a joint wetland and critical area impact permit. As mitigation for the wetland impacts resulting from the project, the City of Charleston purchased 2.6 wetland mitigation credits from the Pigeon Pond Mitigation Bank.
- SCDOT Right-of-way Encroachment Permitting - Since the Phase 1 improvements are to be partially constructed within SC Department of Transportation (SCDOT) rights-of-way (5th Avenue, St. Andrews Boulevard, and Magnolia Road), an SCDOT encroachment permit was required. As part of this permit, temporary detours (5th Avenue and Magnolia Road) and temporary lane shifts (St. Andrews Boulevard) were approved.
- Local and State Environmental Permitting (NPDES Construction General Permit) - Like most construction projects in the Lowcountry, this project required a permit to ensure compliance with the National Pollution Discharge Elimination System (NPDES) mandated by the U.S. Environmental Protection Agency (USEPA). The NPDES permit requires review by the local jurisdiction and by SCDHEC-OCRM for consistency with the SC Coastal Zone Management Act.
- Utility Relocation Coordination - The proposed project improvements impacted various existing utilities. The project required the coordination of permanent and temporary utility relocations for water, sewer (both gravity pipes and force mains), electrical (buried and overhead power poles/lines), gas, communications (buried fiber optics/other cables and overhead wires), and cable TV (three different companies).
- Property and Easement Acquisition - The project required the acquisition of a small portion of a property and numerous easements over other properties. All property and easement acquisitions were accomplished through a fair market value analysis of the property/easements and negotiations with the owner.
- Project Bidding and Contract Award - The project was advertised for qualified construction companies to bid on. The bids were received and evaluated, and the project was awarded to the lowest, responsible bidder - Gulf Stream Construction.