Letchworth State Park, Gibsonville - Stream Restoration

A stream in Letchworth State Park was diverted through a culvert in the aftermath of Hurricane Agnes in 1972. By 2012, the culvert had deteriorated and hydraulic forces continued to cause stream bank erosion downstream of the culvert. The stream had eroded into the valley side slope, thereby destabilizing the bottom of the slope below the main Park Road. 

 

A slope stability analysis indicated that excavation for replacing the culvert would require a shoring system to minimize the potential for slope movement. The dense glacial soil present at the site dictated that the shoring system would require drilled soldier piles, which was deemed cost prohibitive. Furthermore, the U.S. Army Corps of Engineers preferred that an ecologically friendly solution be implemented.

 

McMahon & Mann's design repair included abandoning the culvert in-place and restoring the stream to surface flow. Elements of the design included regrading the stream valley to help slope stability, restoring a stream channel that resembled the geomorphic characteristics of nearby reference stream reaches, and building a drop structure that conveys the stream flow non-erosively to the return channel.