Bank erosion processes in streams in the U.S. Midwest

Rivers in the U.S. Midwest are dynamic systems that can be natural laboratories for understanding the different modes of bank erosion, namely fluvial erosion, mass erosion, and mass failure. Fluvial and mass erosion are hydraulically driven and semicontinuous, whereas mass failure is episodic and often catastrophic. Being catastrophic, mass failure and its driving mechanisms have received considerable attention comparatively to mass and fluvial erosion. However, the linkage between hydraulically driven erosion and mass failure has not been examined fully. We hypothesize that fluvial and mass erosion affect the memory and response of the system by creating favorable hydrogeomorphic conditions for mass failure.

This dissertation addresses three major shortcomings in the bank erosion literature, including the confusion surrounding critical erosional strength values for mass and fluvial erosion (τ_c,m and τ_c,f, respectively). The herein results clearly show that these two parameters are different, with τ_c,m being three to five times greater than τ_c,m. Therefore, excluding mass erosion estimates from sediment budgets or stability analyses can lead to significant errors in quantifying or predicting bank retreat and channel geometry. In addition, this study offered a methodological improvement for measuring the τ_c,m in-situ using Photo-Electric Erosion Pins, which semi-automatically measure mass erosion to generate erosional strength and erodibility values that are currently missing in the literature. This study also addressed the preconceived notion in morphodynamic modeling that bank soil profiles are homogeneous and universal strength/ cohesion parameters adequately represent the bank soil profile. This study shows that bank soil heterogeneity is present and significantly affects bank stability.

Therefore, heterogeneity along a bank face must be assessed in at least three locations to provide adequate input data for bank erosion models. Finally, this study suggests that Factors of Safety for mass failure must be complemented with those for fluvial and mass erosion to avoid underestimating mass failure by as much as 30%. Hence, this study provides agencies like the U.S. Department of Agriculture key data regarding the total contributions from the different modes of bank erosion and channel, itself, to the stream sediment load for strategic targeting of Best Management Practices and in-streams stabilization structures.