River widenings are used to control general bed degradation and to improve river ecosystems. Re-widening of channelized rivers may restore the early braiding patterns and the original morphological dynamics of gravel bed rivers. Aquatic habitat conditions will become more variable, which increases the density and variety of fish and invertebrate population.
Extensive experiments were performed to investigate the morphological processes in rivers with variable bed width. The investigation lead to design criteria for widenings of gravel bed rivers.
In local widenings with limited length, the flow conditions and the channel forms are influenced by the transition zones between narrow and wide cross sections. At enlargements, flow expands smoothly with an expansion angle that increases downstream. In the wake zone of the expanding flow, the hydraulic load on the banks is neglectable and bank protection measures may be omitted. A empirical approach to predict the wake length is proposed.
A braided pattern forms only after a transition zone, which is approximately twice as long as the wake zone. Thus, in short local river widenings no dynamic morphology will form.