Does Scale Matter? The Role of Vegetation in Controlling Morphodynamics in Large- and Small-Scale Delta Experiments

Abstract:

Experimental deltas are often braided systems with highly mobile channels that rapidly rework the topset surface. Plants can aid in decreasing channel mobility and creating more stable distributary networks. Here we compare two sets of experiments that used alfalfa as a proxy for delta vegetation to determine the effects of plants on delta and channel dynamics. One set of experiments was conducted with relatively low water and sediment discharge, and the second had an order of magnitude higher water discharge than the first. We found that the low discharge experiments had multiple small bifurcating channels that could easily be annealed by floods and were not capable of removing vegetation. This resulted in relatively uniform sediment delivery to the shoreline, thus forming a smooth shoreline planform pattern. The high discharge experiments had a more two-way feedback between the channels and plants, in which the plants constricted the flow into channels and the resulting strong channels were able to erode vegetated banks. The flow constriction created two dominant channels that were more persistent than in the low discharge experiments. This resulted in a highly irregular shoreline punctuated by a few active lobes of deposition. Much of this variability was a function of channel depth, since channels must be deeper than plant roots in order to effectively erode vegetated banks. Thus, the scale at which we choose to model deltas may give drastically different results, as our two sets of experiments showed a range of delta geometries, shoreline rugosity, and channel networks. For deltas in particular, it is important to understand this variability as we plan coastal restoration efforts and try to predict the optimal discharge conditions to build new deltas on the coast.