A Sediment Budget for the Regulated Lower Roanoke River, NC

Dam construction and its impact on downstream fluvial processes may substantially alter sediment budgets. Three high dams (completed between 1953 and 1963) occur along the Piedmont portion of the Roanoke River, North Carolina; just downstream the lower part of the river flows across largely unconsolidated Coastal Plain deposits. Bank erosion rates along the lower Roanoke River were measured along 66 bank transects using erosion pins; floodplain deposition was measured along 50 bottomland transects using artificial markers. Additionally, discrete measurements of channel bathymetry, turbidity, and presence or absence of mass wasting were documented along the entire study reach (153 km). A bank erosion-floodplain deposition sediment budget was estimated for the lower river. Bank toe erosion related to consistently high low-flow stages may play a large role in increased mid- and upper-bank erosion. Present bank erosion rates are relatively high and are greatest along the middle reaches (mean 63 mm/yr) and on lower parts of the bank on all reaches. Erosion rates were likely higher along upstream reaches than present erosion rates, such that erosion rate maxima have since migrated downstream. Mass wasting and turbidity also peak along the middle reaches; floodplain sedimentation systematically increases downstream in the study reach. The lower Roanoke River is net depositional (on floodplain) with a surplus of about 2,800,000 m3/yr. Results suggest that unmeasured erosion particularly mass wasting may partly explain this surplus.