Potential controls of alluvial bench deposition and erosion in southern Piedmont streams, Alabama (USA)

- We determined conditions conducive to alluvial bench erosion with empirical modeling.
- Flow simulations revealed the current flow regime incapable of bench erosion.
- Vegetation enhanced deposition and limited erosion to bench edges.
- Reverse flow was neither sustained nor widespread in any of the simulated flows.
- Alluvial bench persistence better explained by flow regime than localized hydraulics.

Benches are bank-attached channel deposits occurring at an elevation between the channel bed and top of banks. 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. Hydrodynamic modeling software (River 2D), in combination with sediment particle size analysis and total station topographic surveys, was used to simulate flow conditions needed to erode and deposit the D84, D50, and D15 particle sizes of concave and lateral benches in two rivers (Talladega and Hillabee creeks) in Alabama. 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 all discharges meeting and exceeding benchfull flow stage, likely owing to its overall smaller sediment particle sizes. At both sites, the presence of vegetation increased the bench area subjected to deposition but, somewhat counterintuitively, also helped influence the location of erosion by limiting flow vectors. In contrast with previous research findings, the occurrence of reverse flow was neither sustained nor widespread at either site. These findings provide new insight into alluvial benches, suggest that the study benches are relatively stable features under the prevailing hydrologic regime, and that in some temperate climate settings, such as the southern Piedmont, localized hydraulic controls on bench formation can be superseded in importance by hydrologic flow regime, even in the case of concave benches and where flow regulation is not a factor.