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

Jürgen Herget,Oliver Schlömer,Thomas Euler

doi:10.1002/esp.4793

Abstract

ABSTRACTObstacle 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. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Highlights

Obstacle marks are morphological features formed by coherent vortex structures induced by obstacles exposed to a current (e.g. Karcz, 1968; Richardson, 1968; Allen, 1984; Paola et al, 1986)
Submergence ratio and obstacle shape (Sh) are considered as primary control parameters defining the geometrical scale of the horseshoe-vortex system (HSV), because they control the adverse pressure gradients formed at the obstacle frontal face and determine the potential maximum size of the obstacle mark
This perspective is in contrast to prior investigations on local scouring at submerged structures, where different ratios of dw/Lo were modelled by changing the obstruction height of a cylinder protruding into flow, keeping the flow depth constant (e.g. Dey et al, 2008; Sarkar, 2014)

Summary

Introduction

Obstacle marks are morphological features formed by coherent vortex structures induced by obstacles exposed to a current (e.g. Karcz, 1968; Richardson, 1968; Allen, 1984; Paola et al, 1986). Obstacle marks are considered here as bed features of the fluvial environment, similar morphologies are reported from the seabed at shipwreck sites Studies of different scientific disciplines in Earth-science lead to synonymous expressions of erosional and depositional bed features induced by obstructions, including obstacle scour mark (Dżułyński and Walton, 1965), current crescent and current shadow (Peabody, 1947; Allen, 1984), comet mark (Werner et al, 1980), obstruction-formed pool (Buffington et al, 2002; Hassan and Woodsmith, 2004; Comiti et al, 2005) and vegetation-induced sedimentary structure (Nakayama et al, 2002; Rygel et al, 2004)

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Conclusion