Distinct patterns of bank erosion in a navigable regulated river

Abstract

Distinct bankline patterns appeared after the removal of protection works along a navigable reach of the Meuse River. A series of oblique embayments now dominate the riverine landscape after ten years of bank erosion, but their location and asymmetry cannot be explained yet. This work analyses and integrates field measurements of flow, ship waves, bank composition, bed topography and historical maps to explain the observed patterns along two reaches of the river. An extraordinary low-water-level event generated by a ship accident provided the unique opportunity to also analyse the subaqueous bank topography.The results indicate that the formation of oblique embayments arises from the combination of floodplain heterogeneity, structured by scroll-bar deposits, and the regulation of water levels, resulting in ship-wave attack at a narrow range of bank elevation for 70% of the time. Substrate erodibility acts on the effectiveness of trees to slow down local bank erosion rates, which is possibly enhanced by a positive feedback between woody roots and cohesive soil. The strong regulation of water levels and the waves generated by the intense ship traffic produce an increasingly long mildly-sloping terrace at the bank toe and progressively dominate the bank erosion process. This study demonstrates the important role of floodplain and scroll bar formation in shaping later bank erosion, which has implications for predictive numerical models, restoration strategies, and understanding the role of vegetation in bank erosion processes.

Experimental and Numerical Modeling of Overhanging Riverbank Stability

“…During stage III, the above processes keep shaping bank region, gradually forming the observed multi-arched-shape bank line. Although other processes, such as flow deflection induced by slump blocks (Hackney et al, 2015) and floodplain heterogeneity (Duró et al, 2020), may also result in the presence of a series of oblique embayment, the present study highlights the importance of a three-dimensional representation of failure surfaces.…”

Bank Erosion Processes in Regulated Navigable Rivers

“…Vegetation could hardly grow in this context, due to either the relatively fast erosion of the upper-bank toe or the frequent high shear stresses at this area. The presence of trees on the floodplain reduces bank retreat rates, but the contribution of trees depends on substrate erodibility at the terrace and upper-bank toe elevation range (Duró et al, 2019).…”

“…Lithological characterization along the terrace was done taking eight cores at 10 cm from the soil surface, seven of which were already presented by Duró et al (2019). All samples were tested with direct shear box to obtain the internal friction angle and cohesion, using a range of normal loads of 17, 36, and 73 Pa and shear rates of 0.01 mm/s.…”

“…Erosion during low-flow periods is associated to wave action only, given the very low flow velocities at the bank toe, absence of seepage erosion, and rainfall events only producing few local and isolated failures (Duró et al, 2019). During flood events, the flow currents may produce significant shear stresses to contribute to bank erosion (Duró et al, 2019), in addition to ship wave action and water-level fluctuations. From km 153.860 to km 153.885, more bank erosion occurred during periods with flood events than during low flows.…”

“…Traditionally, bank erosion is controlled by means of hard measures, such as riprap, but recent approaches searching to balance technical and ecological requirements (Boeters et al, 1997; Heibaum & Fleischer, 2015) allow for some natural processes. Furthermore, in some cases, river banks are allowed to erode again, as for instance along the Meuse River near Gennep, in the Netherlands (Duró et al, 2019). However, banks are allowed to retreat even if there is no comprehensive understanding of ship‐induced riverbank erosion and without knowing the final extend of bank retreat.…”