A document by Oliver Schlömer. Click on the document to view its contents.

Local scouring is typically a bed load transport phenomenon that is well studied at bridge piers while less attention is given to the enlargement processes of local scour holes at natural instream obstacles like boulders. This specifically applies to changing hydraulic boundary conditions at the obstacle in the course of flood hydrographs while physical modelling in flumes offers the advantage that hydraulic boundary conditions can be systematically varied. This second companion paper yields novel experimental data on the role of hydrographs of different shape and flow intensity onto enlargement processes of local scour holes at boulder-like obstructions and evaluates the impact of hydrographs chronology on the local scour hole geometry. In total 48 unscaled process-focussed flume experiments at a range of flows (subcritical, clear-water and live-bed conditions) were performed. Experimental results revealed that (1) the enlargement of local scour hole length and width depends on scour hole depth rather than on actual flow conditions while an empirical model is presented that mechanistically predicts enlargement in local scour length and width based on (i) scour depth, (ii) the inclination of scour slopes and (iii) the planform area of the scour hole bottom. (2) In the course of subsequent hydrographs memory effects (i.e. the dependence on antecedent sizes of local scour depth, length, and width) have to be considered. This contribution improves the understanding of local scouring at boulder-like obstacles exposed to hydrographs while it is speculated that the identified memory effect may be utilized for hydraulic interpretation at field condition.

Local scouring is a bed load transport phenomenon related to different scientific disciplines in Earth-science and hydraulic engineering. However, less attention is given on the enlargement processes of local scour holes at natural instream obstacles like boulders that are exposed to unsteady flow conditions in the course of a flood hydrograph. Experimental investigations in laboratory flumes offer the advantage that hydraulic boundary conditions can be systematically varied. This contribution yields novel experimental data on the impact of submergence ratio, hydrograph skewness, and flow intensity onto local scour holes at boulder-like obstacles and evaluates the effect of discharge chronologies, i.e. subsequent hydrographs in time onto the geometry of local scour holes. In total 48 unscaled flume experiments for subcritical, clear-water and live-bed conditions were performed. Experimental results reveal that (i) local scouring dominantly occurred at the rising limb when flow depth is comparable to the obstacle size so that the obstacle is unsubmerged. (ii) Up to 80% of the scour hole size at the end of experimental runs was reached in only 10% of the experimental runtime while the skewness of the hydrograph is important. The steeper the rising limb the quicker the local scour hole matured. (iii) Local scouring is history dependent and memory effects are present that describe the dependence on antecedent sizes of the scour hole. It is speculated that speculate that local scour holes found at boulders in the field may be used for the interpretation of the minimum duration of the beginning stage of a flood.