Geometry of Local Scour Holes at Boulder-like Obstacles during Unsteady Flow Conditions and Varying Submergence

Abstract

Local scour holes are erosional bed structures that are related to different scientific disciplines in Earth science and hydraulic engineering. Local scouring at naturally placed boulders is ubiquitous, but many competing factors make it difficult to isolate the effects of a given variable. This is especially true for local scouring at natural instream obstacles that are exposed to unsteady flow conditions in the course of flood hydrograph experiments. Experimental investigations in laboratory flumes offer the advantage that boundary conditions can be systematically varied. We present novel experimental data on the impact of the submergence ratio, hydrograph skewness, and flow intensity on local scouring at boulder-like obstacles during unsteady flow and evaluate the effect of discharge chronologies. In total, 48 flume experiments on subcritical clear-water conditions and channel degradation were performed. The experimental results reveal that local scouring dominantly occurred at the rising limb when flow depth was comparable to the obstacle size, so the obstacle was unsubmerged. The steeper the rising limb, the quicker the local scour hole matured. The experimental results are relevant for the hydraulic interpretation of local scour holes found at boulders in the field. They may be utilized as a proxy for the minimum duration of the beginning stage of a flood.