Wednesday, November 15, 2006 - 1:50 PM

Bank Erosion and Associated Sedimentation Dynamics along the Regulated Lower Roanoke River, NC.

Cliff R. Hupp1, Daniel E. Kroes1, and Jean Richter2. (1) U.S. Geological Survey, 430 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, (2) U.S. Fish and Wildlife Service, Roanoke River Natl. Wildlife Refuge, 114 W Water St., Windsor, NC 27983

The lower Roanoke River floodplain contains the largest contiguously forested wetland on the Atlantic Coastal Plain.  This floodplain experienced up to 7m of sediment deposition from post-settlement agricultural practices.  Following watershed reforestation and the construction of three dams (1950) that regulate water flow for 220km, substantial channel incision and bank erosion occurs.  Bank heights now range from 9m upstream and attenuate toward the mouth. The riparian zone supports sensitive ecosystems of concern to federal, state, and private agencies. We established 100 transects, in pairs from dams to mouth, where erosion is monitored through erosion pins, leveled channel surveys, and mass wasting surveys.  The objectives include determination of spatial and temporal erosion patterns along the channel and along bank transects (local), estimation of dam-release scenario impacts, and to provide erosion information to facilitate management alternatives for conservation of downstream ecosystems.  Results, to date (Sept. 2005 to April 2006) show that the banks of the lower river on average are eroding at about 5.4cm/yr through particle by particle erosion.  This erosion is greatest along the upper 2/3 reach and locally low banks.  Along the middle reaches where lateral processes peak, considerable mass wasting occurs by slab and rotational failure.  Up to 20% of the middle reach has been affected by recent bank failure; average slump-block depth is 2m. The floodplain along the lower 2/3 of the lower river annually traps about 1.5 x 106m3/yr of sediment (previous studies); presumably derived from upstream bed and banks below the dams. Flow regulation provides sustained higher low flows and the elimination of high peak flows.  These altered low flows have a stage elevation relatively high on the banks that affect bank morphology and associated sediment dynamics.  This prolonged stage may contribute to extensive particle by particle erosion and subsequent undercutting and bank failure.