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, kt, in vegetated channels from channel average velocity U and vegetation volume fraction ϕ was validated for mobile sediment beds. Second, using data from several studies, the predicted kt was shown to be a good predictor of bed load transport rate, Qs, allowing Qs to be predicted from U and ϕ for vegetated channels. The control of Qs by kt 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 kt. Third, ripples were observed and characterized in channels with and without model vegetation. For low vegetation solid volume fraction (ϕ ≤ 0.012), the ripple wavelength was constrained by stem spacing. However, at higher vegetation solid volume fraction (ϕ=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 ϕ, again suggesting that vegetation facilitated the formation of sheet flow.
Bibliographical noteFunding Information:
The work was supported by NSF grant EAR 1414499. Judy Q. Yang was supported by the Fellowship for Sustainability funded by the Martin Foundation. The data in this study are tabulated in Judy Qingjun Yang's thesis, which will be available on MIT D-Space (https://dspace.mit.edu/). The authors thank Milani Chatterji-Len and Yinghao Zhang for their assistance with the experiments and John Trowbridge for his insightful comments.
- bed load
- bedform migration
- sediment transport
- sheet flow
- turbulent kinetic energy