Morphological evolution of bifurcations in gravel-bed rivers with erodible banks

In a recent paper Bolla Pittaluga et al. (2003a) proposed a physically based formulation for the nodal point conditions to be adopted, in the context of a one-dimensional approach, to describe channel bifurcations in gravel-bed rivers. The authors employed the nodal conditions to study the equilibrium configurations of a simple bifurcation and their stability, assuming erodible bed and fixed banks. With the present contribution we extend the model proposed by Bolla Pittaluga et al. (2003a) to the case of channels with erodible banks, i. e. channels which may adapt their width to the actual flow conditions. Such an extension is of a particular interest for river bifurcations in gravel-bed braided streams, in which bed evolution and bank erosion processes may occur over comparable time scales. Moreover, we investigate how the mechanism of generation of the bifurcation may affect its equilibrium configurations, showing that significant differences may arise if the bifurcation is formed through the incision of a new channel or as a consequence of the formation of a central deposit into a channel. In both circumstances, however, the common feature of the equilibrium configurations reached by the system is a strongly unbalanced partition of water and sediment discharges into the two branches. Such a finding agrees with experimental and field observations according to which stable symmetrical bifurcations are seldom observed in natural gravel-bed rivers.