Abstract:
Benches are bank-attached channel deposits occurring at an elevation between the channel banks and the bed. Their occurrence in a variety of geologic and hydrologic settings has led to confusion about the mechanisms driving their formation, which in turn contributes to difficulty identifying the active floodplain, bankfull stage, and the determination of environmental flows in some rivers. This research sought to identify the hydrodynamic and hydrologic processes involved in bench formation and use this information to predict bench stability. Sites at Talladega and Hillabee creek were selected and are located in the southern Piedmont (USA). Using hydrodynamic modeling software (River 2D) in combination with sediment particle size analysis and topographic surveys, the flow conditions necessary for the entrainment and deposition of the d84, d50, and d15 particle sizes were simulated. Results suggest that bench erosion requires flows at least 150% larger than benchfull stage at the Talladega site, while the Hillabee site experienced erosion at much smaller discharges due to its smaller mean sediment size. An analysis of USGS historical flow data showed the recurrence interval for erosive flows at the Talladega site has increased from 1.8 to 2.3 years since 1985. An analysis of precipitation data and historical landuse show this change in hydrologic regime likely being driven by reforestation, and to a lesser extent a change in precipitation timing, intensity, and yearly totals. This muted flow regime experienced by the study streams creates an optimum environment for vertical accretion, leading to the maintenance of alluvial benches. This research suggests that benches are stable (actively accreting) features functioning as incipient floodplains in over-widened rivers of the southern Piedmont. It also supports the idea that the main valley flat is not always the best field indicator of bankfull stage, and that bench surfaces are important forms of channel complexity that should not be neglected by stream management and restoration planning.
