Drag coefficients of boulders on a block ramp due to interaction processes

Block ramps are a common natural solution for bypassing large steps in riverbeds. The energy dissipation on these ramps was dealt with to develop formulas describing the hydraulic processes. Currently there are no detailed investigations available analysing the resulting forces for boulders on the ramp, although knowledge of these forces is necessary to guarantee an accurate and appropriate design. An investigation programme using a physical model was therefore conducted to describe the effect of boulder interaction processes with respect to forces and drag coefficients. The variable drag coefficients received a special focus because these influence the determination of mean water levels, mean velocities and forces on a single boulder. The physical model allows variation of both discharges and boulder arrangements. As a measurement technique, ultrasonic sensors and load cells were used to determine water levels and drag forces directly. New formulas for Reynolds number-dependent drag coefficients are developed.