Engineered Step-Pool Sequences: Geometry, Failure Modes, and Stability

Engineered step-pool sequences made from natural blocks to imitate the step-pool morphology of steep streams represent a nature-based alternative to concrete check dams. They potentially improve ecology and reduce construction cost; however; their stability and failure mechanisms are not fully understood. Therefore, flume experiments were conducted to investigate scour, failure modes, and bed stability of these systems for initial bed slopes S0=0.04−0.08 and jamming ratios W/D=4−8 with W = channel width and D = step-forming block diameter. An empirical relation is presented to predict the resulting step heights. The step failure modes were classified into processes related to direct entrainment, scouring, internal erosion, and pool filling. An individual step failure potentially triggered an upstream migrating erosion pattern destroying all upstream steps and leading to severe bed incision. Finally, the mean block weight, bed-load transport conditions, and jamming effects were found to increase step stability. Overall, the findings contribute to a better understanding of the complex step-pool systems, improving future design recommendations for practitioners.