Intermittent Channel Systems of a Low‐Relief, Low‐Gradient Floodplain: Comparison of Automatic Extraction Methods

Abstract

Intermittent floodplain channels are low-relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low-level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low-relief and low-gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom-hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission "error", augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km 2 with a detrended regional relief of 1.9 m. Further, these analyses allow us to shed light on the meaning and use of the term "low-relief landscapes".

Geomorphology of the Congaree River Floodplain: Implications for the Inundation Continuum

“…A DEM with 1 m resolution was generated from a 2010 lidar point cloud (South Carolina Department of Natural Resources) (Figure 1b). The lidar point cloud was reclassified to reduce misclassifications in ground returns, especially along floodplain channels (Xu et al., 2020). We then analyze the floodplain topography, through‐bank channel (TBC) profiles, and extract levees by a novel automatic approach.…”

“…For example, Fagan and Nanson (2004) found that channels occur over 83% of the Cooper Creek floodplain. Also, with the increased availability of high‐resolution lidar data channel networks from smaller floodplains have been assessed in agricultural (David et al., 2017) and in densely forested (Xu et al., 2020) locations. Hence, channel networks appear to be characteristic features of floodplains across many environments and spatial scales.…”

A Novel Method for Gaining New Insight on Flows Over Inundated Landscapes

“…The densely forested study site is 18.8 km long, and up to 5.9 km wide with a total area of 93 km 2 . Mean floodplain valley elevation declines from 35.0 to 24.5 m giving an average valley gradient of 4 × 10 −4 (Xu et al., 2020). The floodplain is bound by the Congaree River to the south and southwest and by bluffs to the north (Figure 1a).…”

“…The main channel along the CNP boundary is 39.1 km long with a gradient of 1.5 × 10 −4 . Xu et al (2020) showed that the bank elevation profile has a highly variable levee/bank crest height, thereby facilitating irregular along-channel inundation patterns. In particular, the levees in the upstream reach require a higher river stage for overbank inundation and therefore favor through-bank inundation, while in the downstream reach, the levees are lower and overbank inundation occurs at lower river stages (Xu et al, 2020).…”

“…Xu et al (2020) showed that the bank elevation profile has a highly variable levee/bank crest height, thereby facilitating irregular along-channel inundation patterns. In particular, the levees in the upstream reach require a higher river stage for overbank inundation and therefore favor through-bank inundation, while in the downstream reach, the levees are lower and overbank inundation occurs at lower river stages (Xu et al, 2020). The locations for sub-bankfull inundation are the 32 well-developed through-bank channels, channels that cut through the local bank or levee, with mouth widths ranging from 7 to 30 m and some through-bank channels extend several kilometers into the floodplain interior.…”