Abstract
Unstructured block ramps are fish-friendly structures to stabilize a riverbed, characterized by large macroroughness elements randomly placed on the riverbed. Physical model tests were conducted to investigate their stability and their behavior in case of overload. Different parameters describing the block and the base material were tested to find an optimal combination in terms of ramp stability with a maximum ramp slope typically in the range of 1–3%. A model for the determination of the ramp stability is presented, where the equilibrium slope of the ramp is related to a dimensionless specific discharge including information on the block size, the block placement density, and a characteristic grain size of the base material.