The role of vegetation and large wood on the topographic characteristics of braided river systems

Riparian vegetation and large wood play a crucial role in shaping rivers. On the one hand, vegetation tends to increase bank stability, reduce channel width and reduce the braiding index in gravel-bed rivers. Conversely, large wood tends to increase channel dynamics and promote avulsions. The effects of vegetation and large wood have been rarely simulated together in flume experiments. In this paper we present a series of experimental runs conducted in a large flume facility, using cylindrical wooden dowels and alfalfa seedlings to represent logs and vegetation, respectively, in order to investigate their role on determining the topographical nature of braided river systems. A terrestrial laser scanner was used to measure in detail the topography of the channels and bars, and distribution of the elevations and second-order structure functions were used to explore the topography that large wood and vegetation imposed on the braided pattern in the flumes. Results show that vegetation reduced the braiding index and produced wider and deeper channels. The standard deviation of the bed elevations was higher than in flumes without vegetation. Large wood alone was not effective in changing the braiding index or the standard deviation of the elevations. When vegetation and large wood acted together in the flume, the braiding index reached a minimum value, and the skewness of the distribution of elevation increased, revealing shallower pools if compared with runs conducted with only vegetation.