Boundary conditions and dynamics of fluvial obstacle mark formation

Abstract

Obstacles like boulders, riparian vegetation, and man-made structures such as bridge piers are common in fluvial environments. When exposed to flow, they form characteristic geomorphological and sedimentary structures known as “obstacle marks”, consisting of an upstream scour hole and a downstream sediment ridge. These structures result from three-dimensional turbulent vortices, which increase flow velocity and shear stress, mobilising sediment even below the threshold for general bedload movement. The processes that control the formation of obstacle marks depend on environmental conditions such as flow characteristics, sediment properties, and obstacle shape. While hydraulic engineering research has focused on local scour around infrastructure such as bridge piers, natural obstacles such as boulders and riparian vegetation remain understudied. In particular, the effects of submergence variations on local scour and the influence of obstacle permeability on vortices formation require further investigation. <br /> This thesis presents results from four experimental campaigns with over 190 flume experiments, in which key control parameters were systematically varied. Laboratory results on obstacle mark geometries are compared with field observations from the Colorado River (USA) and the Loire River (France). These results improve the understanding of obstacle mark dynamics and contribute to the reconstruction of past hydrological conditions based on preserved sedimentary structures and help to refine indicators of past flood magnitudes based on persistent obstacle mark geometries. <br /> Key findings of this thesis include <br /> - A new framework identifies 13 non-dimensional control parameters that influence obstacle mark formation, categorised by their effects on geometric length scales and formation thresholds. <br /> - Flow depth relative to obstacle size is a primary control parameter, maximising obstacle mark size at comparable depths but limiting formation at greater depths. <br /> - Factors such as alluvial depth and boulder tilt limit geometric length scales, while highly permeable obstacles (e.g. shrubs) prevent obstacle mark formation beyond a critical porosity threshold. <br /> - A quantitative model describes local scour hole expansion under steady and unsteady flow conditions and shows scale-invariant similarities between laboratory and field results. <br /> - The dominant vortex size at the base of the obstacle correlates with flow depth during formation, allowing an empirical estimate of past flow conditions. This approach, applied to preserved obstacle marks in the Rambla de la Viuda (Region of Valencia, Spain), provides the first quantitative use of local scour holes as flow depth indicators.