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

摘要

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. (c) 2020 John Wiley & Sons, Ltd.