Reach-scale controls on local deposition: modeling river natural levees as internal deltas

Abstract:

Natural levees, deposits that flank riverbanks, grow during overbank flooding, and their growth effects future river-floodplain connectivity. We propose that this sedimentation is controlled by changes in the patterns of overbank flow as the floodplain becomes increasingly inundated. Difference maps derived from time-lapse lidar analyses capture successive episodes of levee growth on the Trinity River, Texas. Growth is dominated by deposition focused at the downslope ends of levees that often terminate in low-relief slip faces. This style of deposition is similar to that of a delta building out into standing water. Here we model levees as internal deltas across which relatively steep and shallow levee channels convey floodwater and sediment downslope into deeper, slower-moving water at lower elevations on the floodplain. A 1-D morphodynamic model, originally written to describe delta formation, is successfully used to investigate levee growth. Model results illustrate how relatively small changes in water depth at the levee toe produce larger changes in extension rates. The widening of levees by the amounts observed in our difference maps is tied to values of topset aggradation in the model results that are at or below lidar detection. At the scale of an individual river bend, lidar difference maps document a correlation between the total amount of sediment eroded and deposited within the river channel and the volume of sediment added to its levee. At the river-system scale, there is a substantial reduction in both moving toward the coast as loss of prominent point bars in the backwater zone leads to increasing levee continuity. In summary, our results document that (1) in-channel sediment availability predicts levee deposition, and (2) water levels on the floodplain during floods determine the rate of levee growth through widening. Using a framework that moves across scales within the river system has produced a clearer understanding of how water flow and sediment transport contributes to levee-scale depositional patterns and floodplain formation.