CHARACTERIZING NATURAL LEVEE MORPHOLOGY FOR A SAND-BED COASTAL RIVER

A coastal river with changing morphodynamics is an ideal study area for understanding variations in levee morphology and deposition. Since levees are net depositional, they record sediment transport conditions for river-floodplain connectivity during floods. Therefore, morphological levee parameters encode variations in river-floodplain connectivity at times of flooding. This study uses airborne lidar covering a ~90km river reach to investigate the influence of both river and floodplain characteristics on levee morphology, particularly levee width and levee-crest elevation. The lidar data covers the lower Trinity River, Texas, USA, and includes a ~45 km downstream river segment defined by backwater hydraulics. From lidar and sparser bathymetric data we extracted the levee width, supraelevation (depositional thickness), levee crest elevation, detrended floodplain elevation, and bankfull river width and depth. We show that the largest levees widths correlated with the lowest detrended floodplains. This relationship is an indicator of floodplain drainage and water storage capacity, and highlights the importance of antecedent topography on levee widths. We also compared the correlation of elevations between levee crest and river bed to that of levee crest and water-surface of mean peak-annual flow. In the upstream river segment, all three possess approximately the same streamwise slope of 1.5x10^-4. However, in the coastal section of the dataset, the levee crest and bed elevation trends deviate. For this river segment, only water surface slopes during flooding more closely agree with levee crest slope. We hypothesize that the levee crest elevation is set by water surface elevation during the most common floods. With these insights into the controls on levee morphology, it will be possible to better understand sediment transport out of rivers onto floodplains and improve interpretations of levee as well as river-floodplain characteristics from the stratigraphic record.