The Calhoun West/Beaufain drainage basin has been a historically flood-prone area. The low elevation and flat topography of the lots and streets comprising the district make stormwater conveyance by conventional gravity systems nearly impossible under certain tide conditions. Maps of Charleston indicate that in the 1880s most of the area west of Gadsden and President Streets was made up of ponds and saltwater marshes of the Ashley River. This marsh area was filled in over the course of time and now includes Halsey Boulevard, Lockwood Drive, the extension of Calhoun Street to the James Island Connector, and the James Island Connector approaches. Based on the 1984 Master Drainage Plan (MDP), this approximately 300+ acre basin bounded by King Street, Cannon Street, and Beaufain Street has only one primary outfall to the Ashley River, a four-foot wide box culvert under Calhoun Street.
Business & Residences Impacts
The Calhoun West/Beaufain basin contains the Medical University of South Carolina (MUSC), the College of Charleston, Roper Hospital, and many businesses and residences that are impacted by frequent flooding. Flooding of streets poses many problems including restricting access to hospitals, diverting traffic around accumulated water, and damage to vehicles parked along flooded streets. Calhoun Street is one of the principal east/west vehicular corridors through the City. It also has a significant amount of pedestrian traffic due to the locations of MUSC, Roper Hospital, and the College of Charleston. Economic impacts, both direct and indirect, may be significant.
The City of Charleston is currently conducting a study for improving drainage in the Calhoun West/Beaufain drainage basin and alleviating many of the existing drainage problems. Ultimately, the project will increase the capacity of the stormwater collection and conveyance system as well as provide means to convey stormwater directly into the Ashley River during storms and tidal events via pumping systems.
The MDP provides some information for beginning a study of the basin. The MDP attempted to delineate various drainage basins within the City and map all accessible drainage pipes (twenty-four inches and larger) within each basin. It also made assumptions on the condition and capacity (blockages) of the existing system. Using these findings, the MDP identified proposed solutions based on criteria developed as part of the study. The Rational and Manning’s formulae were used for sizing inlets and pipe systems.
These methods of calculating flows and head losses in the system do not account for reverse flows that can occur due to tidal fluctuations or the timing and response of the system under design rainfall events. Due to the age of the study and changes within the basin, a reassessment of the existing system, design parameters, and the proposed recommendations should be conducted as part of this study.
The City of Charleston is working with our consultant team of Davis and Floyd and Black and Veatch on the data collection and conceptual design phases of the project. Data collection for this drainage project includes a variety of surveys and studies to determine the condition of the existing drainage system and the possibilities and constraints for improving the system. Topographic and hydrographic surveys were conducted to establish limits of wetlands and critical areas in the study area as well as to establish elevations and other design data needed for the project. Subsurface utility engineering (SUE) was used to determine the location and ownership of existing utilities.
Natural resources surveys and a limited environmental assessment were also conducted to verify the presence of environmental conditions or past land uses that may impact the permitting and development of the project. These surveys and studies have been compiled in a series of technical memos issued by Davis and Floyd March 11, 2016.
Stakeholder Advisory Meetings
The first of our stakeholder advisory meetings was held May 25, 2016. A second stakeholders meeting was held June 2, 2016. These meetings were held to inform the hospitals, schools, businesses, and other entities in the study area of the scope of the work and progress of the project and to solicit input and comments to improve the project. Two meetings were conducted on July 12 and 13, 2016, at the Charleston County Library for additional public input. Invitations were sent to the neighborhood associations informing them of the place and time for these meetings.
The preliminary findings indicate that a deep-tunnel system routed to a pump station is the only viable approach to meet the design goals for the project. This method of using surface collection, drop shafts, tunnels, and pumps has proven effective on other City drainage projects. The project team is currently identifying the possible locations of the access shafts, tunnels, and pump station to serve the basin. Many factors are considered in the siting of these structures including:
Operation and Maintenance Requirements for the Systems
Once the proposed locations are identified, an evaluation matrix is used to rank the sites based on the mentioned criteria. Public input will be sought in the final selection of the sites.
As we continue through the conceptual design stage of this project, we will schedule additional meetings to keep you informed of our progress and to receive your comments and suggestions. This project website will be updated periodically to reflect the project’s progress. An email address has also been established to receive any comments and/or suggestions for improving the project.