Local widening of river beds

The transport capacity and the morphology of the river bed depend on the width of the channel. Wide rivers, i.e. rivers with a large ratio of bed width to flow depth, tend to a braided morphology (Fig.1) and their equilibrium slope is steeper than that of narrow and straight channels with the same bed load transport capacity (Fig.2). This phenomenae could be used to stabilize eroding rivers and to give them back part of their earlier dynamics.

The effect of local widenings on the morphology and the sediment balance of gravel bed rivers has been inverstigated by means of hydraulic experiments with constant and time dependend discharge and with numerical simulations. Special attention has been given to the effects in the transition between narrow and wide cross sections (Fig.3).

The main morphological results can be summarized as follows:

1. The slope in the wide reach is steeper than the slope in the narrow one. The bed level in the upstream reach increases.
2. For reasons of continuity and energy conservation, the mean bed level is stepped vertically.
3. Flow concentration causes intense scouring at the constriction.
4. Braiding within the wide river reach increases the variability of flow parameters (flow depth and flow veloctiy) and instream habitat conditions. On the other hand, the hydraulic load on the banks increases due to cross currents.
5. The retain of sediment within the widening causes temporare downstream erosion.