Unsteadiness effects on the morphological behaviour of gravel-bed river bifurcations

In recent works (Bolla Pittaluga et al. 2003a, Miori et al. 2005) the equilibrium configurations of a simple channel division and their stability have been investigated within the context of a one dimensional approach. Physically based relationships for sediment distribution have been employed by the authors at the channel division point, which allow to model the effect of transverse bed slope at the bifurcation on bed load transport. Both works are based on the assumption of steady forcing conditions. Recent experimental observations on gravel-bed channel divisions (Bertoldi et al. 2005) have shown that bar migration along the upstream channel induce time fluctuations in water and sediment feeding of the downstream branches, which significantly affect the morphological behaviour of the system. In the first part of this paper we extend the model proposed by Bolla Pittaluga et al. 2003a to account for such an effect. In the second part of the paper we also investigate the effect of water discharge fluctuations, which are invariably observed in gravel-bed streams in mountain regions, on the time evolution of a channel bifurcation. Results show that both sources of unsteadiness lead to significantly different and richer scenarios of the possible solutions of the system with respect to the steady case conditions.