Restoration efforts at storm water outfall control erosion
A storm water facility that outfalls at the top of a hill inevitably will cause erosion. However, there often are several competing priorities during the design phase that force the placement of the outfall. This was the case for Delaware Department of Transportation’s (DelDOT) Best Management Practice (BMP) No. 21-a wet pond with approximately 0.90 acre-ft total storage that flows into Jenny Run, a tributary of the White Clay Creek.
The Right Solution
The White Clay Creek watershed is located along the fall line of the Piedmont and Coastal Plain physiographic provinces. The outfall placement was confined by space and influenced by elevation change.
The Piedmont is characterized by gently rolling uplands and is underlain with metamorphic and igneous rocks. The project site has an elevation change of about 5% from the roadway to the receiving waters. This combined with an increase in impervious cover within the 18.7-acre drainage area, resulted in severe erosion at the pond inlet and outlet. The overall erosion rate for the outfall channel was between 26.8 and 48 cu yd per year.
In 2013, DelDOT worked with engineering consultant WSP (formerly Parsons Brinkerhoff) to develop a solution that addressed the erosion concerns and improved the quality of water entering Jenny Run. This drainage area is of particular importance because it drains to the White Clay Creek near the city of Newark, Del.’s water supply reservoir. Avoiding erosion from the BMP has the potential to directly improve drinking water quality and reduce water supply treatment costs.
The final design included installing a sediment forebay near the inlet and stabilizing the outfall channel through a series of rock sills and cascades. The channel bed was raised between 2.1 and 4.5 ft to compensate for the erosion over time. Due to the steepness of the channel, subterranean clay blocks were installed in conjunction with three of the rock sills to avoid subsurface flow piping. The project helps reconnect the hyporheic zone and provides important riparian habitat through the use of native plants. A second option of relocating the outfall off of the ridgeline and into the valley was explored, but tabled because of the significant earthwork required, removal of mature trees and potential to disturb virgin ground.
While there was a clear need for major maintenance to restore the BMP functionality as designed, there also were construction constraints. The BMP abuts an electric transfer station and private property. A permanent access road was built to avoid an entrance near the transfer station. Access to the far end of the construction limits was an additional obstacle because of mature trees and a sensitive wetland area.
Tree removal was minimized, which required precision equipment movement and operation by the construction contractor. Four trees were tagged for removal, but only two were removed during construction. One of the four tagged for removal-a Tulip poplar-ultimately was incorporated into the project design as a habitat tree. Although the majority of the tree was dead, only a portion of the tree was removed. The remaining tree was left in place to provide critical habitat for birds, such as woodpeckers and owls. An aggressive planting plan mitigated the tree removal. To ensure recruitment, animal enclosures were placed over the tree and shrub saplings. Impacts to the wetland area were minimized by smart construction sequencing, as well as laying down a wooden boardwalk for equipment movement.
Construction began Aug. 3, 2017, and was completed in less than two months. Barksdale Nursery in Elkton, M.D., won the construction contract. The first stage of construction was brush clearing to open access to the storm water pond. Next, an eroded inlet was protected by riprap, and the sediment forebay installed. Finally, the outfall channel was reconstructed to safely bring the water down the 15-ft drop in elevation, over 200 ft in length. The construction cost was approximately $220,000, and the overall project cost was approximately $452,000. The successful project is a reminder to designers that they cannot discharge at the top of a hill and expect the water to tie in safely unless proper measures are implemented to assist the water in its travels.