Modulation of outer bank erosion by slump blocks: Disentangling the protective and destructive role of failed material on the three‐dimensional flow structure

Abstract

The three‐dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion. Although submerged slump blocks and associated large‐scale bank roughness elements have both previously been proposed to divert flow away from the bank, direct observations of the interaction between eroded bank material and the 3‐D flow field are lacking. Here we use observations from multibeam echo sounding, terrestrial laser scanning, and acoustic Doppler current profiling to quantify, for the first time, the influence of submerged slump blocks on the near‐bank flow field. In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may also deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion. We use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

Monitoring Riverbank Erosion in Mountain Catchments Using Terrestrial Laser Scanning

“…Therefore, in order to cope with these issues, TLS has often been employed in combination with other techniques. A possible solution has been proposed in Vaaja et al [91] and Hackney et al [66], who employed a mobile laser scanning (MLS) system to map changes (erosion/deposition) in riverine topography. In case the MLS sensor is installed on a car, only vehicle-accessible sites are suitable for the application of this technology.…”

“…This certainly represents a strong limitation in mountain areas. An interesting alternative is given by the use of MLS sensors on more flexible platforms such as boats [66], Unmanned Aerial Vehicle (UAV), and backpacks [92]. Generally, if high quality positioning sensors (Global Navigation Satellite System (GNSS) and inertial measurement unit) are adopted and operated in proper conditions, the accuracy and spatial resolution offered by MLS is only slightly worse with respect to the one provided by static laser scanning.…”

“…Lyons et al [18] conducted a survey on a bank of meandering portion of a stream, indicating the relationship between high intense precipitation events and destabilization by desiccation-cracking. The technique was successfully used to assess the adequacy of erosion protection measures (slump blocks) [66]. The usefulness of TLS for the analysis of morphological changes in varied scales has been reported by Heritage and Hetherington [26] as well as Kociuba et al [24].…”

Flow Patterns and Morphological Changes in a Sandy Meander Bend during a Flood—Spatially and Temporally Intensive ADCP Measurement Approach

“…These include, for example, failed bank blocs which may inhibit the development of high near-bank velocities or, by contrast, increase the local shear stresses and rates of erosion [27]. These processes are, on the other hand, highly related to bank roughness and vegetation [27]. Ottewanger et al [24] demonstrated that major differences exist between the hydrodynamic processes in mildly and sharply curved bends.…”

“…They stated that even though bank erosion andchannel migration are governed by near-bank velocity and turbulence, the flow patterns are influenced by the morphological characteristics of the bend, which may vary considerably. These include, for example, failed bank blocs which may inhibit the development of high near-bank velocities or, by contrast, increase the local shear stresses and rates of erosion [27]. These processes are, on the other hand, highly related to bank roughness and vegetation [27].…”