A STABILITY ASSESSMENT OF STREAM RESTORATION SITES IN THE COASTAL PLAIN OF MARYLAND

During the spring of 2010, we assessed the stability of 25 stream restoration projects in urban and suburban areas of Anne Arundel County, Maryland. The restoration sites used a variety of techniques including Natural Channel Design, fish passage structures, Regenerative Stormwater Conveyance (RSC) systems, stormwater management pond retrofits, and riparian wetland creation projects. The purpose of the study was to identify and prioritize maintenance needs at the sites, which ranged in age from 1 to 12 years old. However, the close proximity of the sites provides a unique opportunity to compare restoration types and identify restoration challenges specific to the geologic setting of Anne Arundel County in the Western Shore Uplands region of the Coastal Plain physiographic province. We collected physical stability data using a combination of longitudinal profiles, cross sections, bed material characterization, rapid-assessment structure performance analysis, and visual observations.

Of the 25 sites examined, only 16% showed signs of severe instability and 40% of the sites were essentially stable. Instability documented at the sites was often related to challenges associated with the fine-grained sediments endemic to the Coastal Plain. Side cutting of in-stream structures was a problem at 8 sites, particularly at cross vanes and RSC weirs. Use of local, fine-grained sediments as structure backfill during construction also resulted in piping and undercutting beneath structures, particularly in areas with limited access to the floodplain. At six of the sites, high sediment loads resulting from urban stormwater discharges through the highly erodible Coastal Plain sediments had contributed to the burial of in-stream structures and pools designed to dissipate stream energy and provide habitat diversity. Transport of placed streambed riffle materials that were intended to provide static grade control was problematic at three sites. Groundwater seeps related to contacts between sandy and clay strata that were unidentified prior to construction and inadvertently exposed during construction contributed to significant bank erosion and floodplain headcuts at two sites. These observations provide some insight into the importance of understanding geologic constraints in the restoration site design process.