Effects of Freezing and Thawing Processes on Bank Stability

The majority of scour-related studies in rivers and streams in Iowa and the Midwest indicate that bridge pier protection is strongly interrelated to bank stability. Findings from a recent survey conducted as part of a project performed for the IHRB (Iowa Highway Research Board) and HCA (Hungry Canyon Alliance) suggest that accelerated riverbank degradation has resulted in an estimated $1.1 billion in damages to bridge infrastructure and the loss of agriculture lands since the turn of the century in parts of the western United States, including western Iowa. Yet, a robust technique that systematically quantifies bank erosion on a continuous basis is lacking. This is because conventional, manual, field monitoring methods, typically erosion pins, cross-section resurveys or terrestrial photogrammetry, merely reveal net change in the position of a bank surface since the previous measurement. They do not quantify the precise temporal distribution of that change. This means that erosion event timing, and the precise bank response to individual flow or flow hydrograph changes, is generally uncertain. Because of the limitations of existing measurement methods, little knowledge has yet emerged of the dynamics of bank erosion and deposition events at a time resolution comparable to that available for flow discharges. Clearly, bank erosion monitoring will be more securely based when (a) the full episodicity of bank change is detected, including event timings and (b) magnitude/timing information for specific erosion and deposition events can be related to continuous information on the temporal fluctuations in the suspected hydraulic forces.