Influence of bank vegetation on alluvial channel patterns


  • Robert G. Millar


Numerous studies have suggested the importance of bank vegetation as a control of channel patterns; however, to date there is no conclusive evidence that vegetation does represent a significant control. An analytical model is developed in order to assess the influence of bank vegetation on channel patterns of alluvial gravel-bed rivers. Three channel types are considered: meandering, wandering, and braided. Bank vegetation effects are quantified in terms of a friction angle ϕ′. A new theoretical meandering-braiding transition criterion is formulated that includes ϕ′, median grain diameter D50, and bank-full discharge Q. The theoretical relation is tested against field data from 137 rivers and successfully discriminates between meandering and braided rivers. Wandering rivers show greater scatter. It is concluded that bank vegetation, as expressed in terms of ϕ′, does exert significant and quantifiable control on alluvial channel patterns. A simple planform stability diagram is developed to determine the sensitivity of gravel-bed rivers to changes in bank vegetation.