Impact of Vegetation on Bed Load Transport Rate and Bedform Characteristics

The impacts of aquatic vegetation on bed load transport rate and bedform characteristics were quantified using flume measurements with model emergent vegetation. First, a model for predicting the turbulent kinetic energy, k(t), in vegetated channels from channel average velocity U and vegetation volume fraction phi was validated for mobile sediment beds. Second, using data from several studies, the predicted k(t) was shown to be a good predictor of bed load transport rate, Q(s), allowing Q(s) to be predicted from U and phi for vegetated channels. The control of Q(s) by k(t) was explained by statistics of individual grain motion recorded by a camera, which showed that the number of sediment grains in motion per bed area was correlated with k(t). Third, ripples were observed and characterized in channels with and without model vegetation. For low vegetation solid volume fraction (phi <= 0.012), the ripple wavelength was constrained by stem spacing. However, at higher vegetation solid volume fraction (phi = 0.025), distinct ripples were not observed, suggesting a transition to sheet flow, which is sediment transport over a plane bed without the formation of bedforms. The fraction of the bed load flux carried by migrating ripples decreased with increasing phi, again suggesting that vegetation facilitated the formation of sheet flow.