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. This work, published in Geophysical Research Letters in 2015, used our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

In summary, the role of submerged slump blocks in modulating the near-bank 3-D flow field is far more complex than previously thought. Failed material may act to both protect the bank from erosion as proposed in past work [Wood et al., 2001; Parker et al., 2011; Motta et al., 2014] but may also enhance bank erosion by deflecting flow up, and onto, the bank as the geometric properties of the slump block change. It is thus clear that in order to develop better predictive models of bank erosion, all of these effects must be considered, and that future work needs to parameterize the influence of slump block flow-form interactions at different stages of embayment evolution. Although the our work illustrates one case example, our novel data suggest the possible differential influences of slump blocks at various times in their life cycle. Further research is needed to constrain these process dynamics across a range of flow stages that determine the magnitude and distribution of shear stress [Papanicolaou et al., 2007; Guo and Julien, 2009; Nikora and Roy, 2012]. Additionally, more work is needed to quantify the effects of slump block size, orientation, shape and position relative to the bank, and their role in enhancing or reducing bank erosion, in a similar way to past studies of flow around groins and bendway weirs [Przedwojski, 1995; Abad et al., 2008].