During heavy downpours, a channel of the Cotton Mill Stream in Rock Hill, South Carolina, would become overwhelmed, causing increased velocity and bank erosion. In 2015, the existing culvert was upgraded and a section of stream downstream was restored; however, a stretch upstream was not restored. This project aimed to mitigate bank erosion, improve water quality, eliminate invasive species and increase dissolved oxygen in the stream.
An evaluation found that the existing creek banks were severely eroded and negatively impacting water quality. Additionally, an existing wetland area was within the project area. When construction began, sediment and erosion control best management practices were installed first, then five j-hooks were installed and flood benches were excavated. Some trees were removed, but others were kept to keep the bank stabilized and keep shade on the stream, which increased the capacity of the existing stream bed and reduced velocity by spreading out the flow and reducing bank erosion.
After overcoming various challenges, the team installed the retaining wall. Since completion, several storms have rolled through and the restored section functions more efficiently than before. The flood benches assists with slowing velocity, which reduces erosive forces, and j-hooks keep the main flow in the center of the channel adding dissolved oxygen into the water. The city will monitor the restored section in association with the South Carolina Adopt-A-Stream program.
“The project was important because it accomplished the goal of improving the water quality by reducing stream velocity, reducing/eliminating bank erosion, eliminating invasive species along the banks, and increasing dissolved oxygen to allow for a resurgence of fish and other species within the stream,” said David Dickson, city engineer with Rock Hill Public Works.