Boundary condition control of fluvial obstacle mark formation – framework from a geoscientific perspective

Abstract

Obstacle marks are sedimentary bedforms, typically composed of an upstream local scour hole and a downstream sediment accumulation in the vicinity of an obstruction that is exposed to a current. However, specific morphologies are variable in fluvial, coastal and submarine environments. Although obstacle marks and the phenomenon of local scouring are subject to different scientific disciplines, the objectives of investigations are rather incoherent and no systematic framework for analysing and evaluating boundary condition control exists yet, especially concerning limited knowledge of the cause and effect relationship of obstacle mark formation at instream boulders or vegetation elements in variable environmental conditions. Thus, a parameter framework is developed which identifies a spectrum of extrinsic and intrinsic boundary conditions that control the major process dynamics of obstacle mark formation. The framework is composed of dimensionless control parameters that are separated by a hierarchical order regarding their significance for obstacle mark formation. Primary control parameters determine the geometrical scale of flow field at the obstacle, and therefore control the potential maximum size of the obstacle. Secondary control parameters affect the dynamics of the flow field in geometrical scale and limit the potential maximum size of the emerging sedimentary structure if thresholds are crossed. The framework is supposed to be a foundation for subsequent quantification and determination of thresholds by systematic laboratory studies. To elucidate this, flume-based research is presented, evaluating the influence of different flow levels at boulder-like obstacles of different shapes. The results show that obstacle mark dimensions were maximized at shallow flow depths compared to obstacle dimensions, while deep flows at submerged boulder-like obstructions caused considerably smaller obstacle marks. In interdependency with a rounded and more streamlined obstacle shape, deep flows even cause a deviation of morphology if the flow depth above an obstacle exceeds 1.6 times the obstacle's dimensions.

Comments from Paul A. Carling

“…In addition, boulders protruding from the flow and subject to obstacle-induced standing waves, have radically different entrainment conditions in contrast to fully-submerged boulders (Carling et al, 2002a & b). Scour in gravel surrounding boulders is far more intense for large protruding boulders than for deeply-submerged boulders (Schlömer et al, 2020) promoting undermining and motion of large protruding boulders (Clark, 1996). Boulder protrusion and undermining might be a particular issue, where there is a 'dramatic topographic gradient' as indicated by the authors resulting in steep channels subject to rapidly changing periods of aggradation and degradation of alluvial bed levels, also noted by Huber et al (2020).…”

River, Coastal and Estuarine Morphodynamics Selected papers from the 10th anniversary of the RCEM Symposium

“…() and Schlömer et al . () focus on the complex interactions between morphodynamics and biological processes, reflecting the interdisciplinary spirit invoked in RCEM symposia. Calvani et al .…”

“…The 17 collected contributions include four state-of-science papers concerning the most recent advances in computational morphodynamic modelling of coupled flow-bed-sediment systems (Shimizu et al, 2019), a critical analysis of existing data on vegetation-flow-sediment interactions obtained through both laboratory experiments and field campaigns (Tinoco et al, 2020), a review of existing moving-boundary theories of shorelines with two extensions to allow inclusion of firstorder effects of waves and tides (Voller et al, 2020), and an overall assessment of the role played by wave forcing on the hydro-morphodynamics in shallow nearshore areas and at river mouths (Brocchini, 2019). The other 13 papers cover different topics about morphodynamics, spanning multiple environments, tackling concepts and processes with the aid of refined theoretical and numerical tools (Redolfi et al, 2019;Tambroni et al, 2019), grounding the results on laboratory data (Finotello et al, 2019;Geng et al, 2019;Matoušek et al, 2019, Porcile et al, 2020 and field observations (Fogarin et al, 2019;Tommasini et al, 2019), and making use of interdisciplinary approaches (Calvani et al, 2019;Chen et al, 2019;Pivato et al, 2019;van de Vijsel et al, 2020) also to develop new conceptualizations (Schlömer et al, 2020).…”

“…Finally, the papers by Calvani et al (2019), Chen et al (2019), Pivato et al (2019) and Schlömer et al (2020) focus on the complex interactions between morphodynamics and biological processes, reflecting the interdisciplinary spirit invoked in RCEM symposia. Calvani et al (2019) analyse the interactions between river morphodynamics and vegetation properties at the reach scale by using field observations and theoretical modelling.…”

“…Using this model, Pivato et al (2019) highlight the crucial role of biostabilization on the morphological evolution of shallow tidal systems and the related strong seasonal variations. Finally, Schlömer et al (2020) propose a unifying hierarchical theoretical framework for 'obstacle marks', sedimentary bedforms, created by the interaction between an obstruction exposed to a current. The framework identifies the boundary conditions and parameters controlling the major process dynamics of obstacle mark formation, and is illustrated with reference to laboratory experiments.…”