Geometry of obstacle marks at instream boulders—integration of laboratory investigations and field observations

Obstacle marks are instream bedforms, typically composed of an upstream frontal scour hole and a downstream sediment accumulation in the vicinity obstacle. Local scouring at infrastructure (e.g. bridge piers) is well-studied phenomenon hydraulic engineering, while less attention given to time-dependent evolution holes boulders their geometric relations (depth width, length ratio). Furthermore, comparison between laboratory studies field observations rare. Therefore, morphodynamic importance such features fluvial transport morphological change largely unknown. In this study, obstacle boulder-like obstructions were physically modelled 30 unscaled process-focused flume experiments (runtime per experiment ? 5760 min) range flows (subcritical, clear-water conditions, emergent submerged water levels) boundary conditions designed represent setting (i.e. tilting, limited thickness alluvial layer). Additionally, geometries 90 in-situ (diameter 1 m) located 50-km segment Colorado River Marble Canyon (AZ) measured from m-resolution digital elevation model. Flume reveal similar local scouring, irrespective controlled by incision, whereas layer tilting into evolving limit incision. Three temporal phases—(1) rapid (2) decreasing (3) widening—are identified based on statistical analysis spatiotemporal bed time series. A quantitative model presented that mechanistically predicts enlargement width (1) depth, inclination slopes, planform area bottom. The results demonstrates scale invariance geometry, which implies similitude processes form rather than equifinality.