Advantage of the loose rip rap is that the riverbed is not densely covered with stones. Between the stones, there are gaps with free gravel bed, which is important for the ecology of the river. If properly dimensioned and constructed, this gravel bed is not eroded during floods.
Practical experience with loose rip raps is currently only available for Froude numbers (Fr) up to about 1. There is no experience of how the open revetment design behaves in the case of supercritical flow (Fr > 1). For the same bed shear stress, greater mean flow velocities result with supercritical flow than with subcritical flow. This also suggests higher flow velocities near the bottom. Thus, higher flow velocities and turbulence could occur around the revetment stones, leading to erosion of the underlying gravel. In the areas with a change from Fr > 1 to Fr < 1, hydraulic jumps (undulated hydraulic jump) and thus increased shear stresses at the river bed are to be expected.
View of canal
View of canal
Project team members: C. Havel, M. Hengl, P. Stockhammer, M. Urbanek, K. Wölzl
Public client: Office of the Federal Government of Vorarlberg