The City of Charleston has completed construction on the Phase 1 drainage improvements within the Forest Acres and 5th Avenue Drainage Basins.
The drainage improvements are designed to alleviate various drainage and flooding issues including recurring structural flooding. With the completion of Phase 1, the City of Charleston and its consultant, Thomas and Hutton, will be moving immediately into Phase 2.
Previously, the Forest Acres Drainage Basin was drained by open channels and culverts that flow to a stormwater pump station near the corner of Playground Road and the West Ashley Bikeway. The existing pump station consisted of a wet well with a single vertical pump (originally from 1968) discharging through a single 48-inch concrete pipe. The stormwater pump station is powered by electricity from overhead power lines and has no emergency backup generator. The station pumped stormwater approximately one mile east, through a gravity flow pipeline located in the West Ashley Bikeway, to where it discharged into the Ashley River.
It was determined that the system can be safely and efficiently drained using a gravity system. The new system consists of wide stormwater channels and dual 10-foot by 5-foot box culverts along the bikeway, with crossings under Magnolia Road, St. Andrews Boulevard, and 5th Avenue. This new system offers the same level of protection to the properties within the drainage basin as a modern pumped system with the added benefit of not relying on electric power. Therefore, if power is lost during a rain event, the system will continue to drain under the force of gravity.
A 40-foot wide drainage channel, approximately 700 feet in length, parallels the new bikeway path. About halfway between Playground Road and Magnolia Road, the drainage channel flows into a dual 8-foot by 5-foot box culvert. The culvert, approximately 1,550 feet in length, will carry flow under Magnolia Road towards St. Andrews Boulevard Approximately 350 feet west of St. Andrews Boulevard, the culvert discharges into another drainage channel, 30 feet wide and 156 feet in length. This section of drainage channel is bounded by a reinforced concrete retaining wall along the south bank.
Stormwater is then carried under St. Andrews Boulevard via dual 10-foot by 5-foot box culverts approximately 490 feet in length. Approximately 200 feet downstream of St. Andrews Boulevard, stormwater discharges into a 40-foot wide drainage channel and flows for about 450 feet towards 5th Avenue. Stormwater is then conveyed under 5th Avenue via a dual 10-foot by 5-foot box culvert, 41 feet in length. The 5th Avenue culvert crossing, which is the downstream limit of the proposed drainage improvements, discharges to a tributary to Oldtown Creek and ultimately to the Ashley River.
The drainage improvements includes:
Handrails, where necessary, for safety precautions
A debris screen at the 5th Avenue culvert crossing to keep trash from entering the Ashley River
An articulated block mat outfall at the 5th Avenue culvert crossing to prevent potential erosion and minimize impact to adjacent wetlands.
In addition to the drainage improvements, the affected portion of the West Ashley Bikeway has been significantly improved and landscaped for a more aesthetically pleasing final product. The final product includes a 10-foot wide crowned, paved bike path (two 8-foot wide lanes, where separated by a landscaped area), detectable warning surfaces at all locations where the bike path crosses a road, and landscaping consisting of more than 100 assorted shrubs and 60 trees throughout the length of the project. Wherever practical, the existing mature trees are remained in place.