Intermittent channel systems of a low-relief and low-gradient (LRLG) fluvial-tidal floodplain

Abstract:

Floodplains are low-relief and low-gradient (LRLG) landscapes often with perennial and ephemeral surface channel systems. These channels are only a fraction in size of the main channels, but they provide important hydrological and biological functions such as facilitating sub-bankfull inundation and movement of aquatic species. Although floodplain channel systems are ubiquitous features of many landscapes most existing studies focused on large perennial river systems. This study investigated the ephemeral channel systems of Santee River floodplain, SC. We compared the position of Santee River prior to 1940s on USGS topographic maps and on the 2017 lidar DEM. We then automatically extracted the channels from 1 m × 1 m lidar DEM (2017) using the bottom-hat transform approach, a well-established method in digital image processing. Finally, we attempted to find evidence of the fluvial-tidal transition from the change in channels and floodplain topography.

Overall, the regional gradient of the floodplain surface drops from 2.3 × 10^-4 m/m in the upstream to 9.9 × 10^-5 m/m in the downstream. We found that the Santee River experienced negligible lateral migration since 1940s after the construction of the Santee Dam. Numerous subtle channels are identifiable in the DEM, including where land use was changed to commercial tree plantation. The floodplain is highly channeled and local drainage density is over 10 km/km² in the fluvial dominated area, among the highest in reported values. The dense channel network suddenly declines as Santee River becomes tidal dominated.