Probabilistic Streambank Erosion Curves for Advancing Assessment And Prediction in the Context of Climate Change

Streambank erosion is a ubiquitous morphodynamic process in riverine environments. However, when pronounced, high rates of streambank erosion are detrimental to the stability and function of streams. Over time, such high rates can lead to excessive channel-bed erosion and deposition, increased loading of fine-grained material and nutrients, and poor water quality and habitat conditions. The overarching goal of this research project was to advance assessment and prediction of streambank erosion in the context of climatic fluctuations in streams’ water-sediment regimes. The project was organized into two phases at answering the following research questions: (1) how streambank erodibility parameters vary in response to rapidly changing hydro-environmental conditions? and (2) how do different geomorphic characteristics and hydrological fluctuations control the probability of exceeding annual erosion rates? To answer the posed research questions, the following specific objectives were pursue: (1) to characterize the temporal variability of streambank erodibility parameters (i.e., τc – critical shear stress, K – erodibility coefficient, and α- numerical exponent greater than zero) under changing physical, hydrological, and hydro-environmental conditions; and (2) to develop a probabilistic streambank erosion curve for quantifying the probability of exceedance of streambank erosion rates under variable conditions. The technical approach integrated two components. The first component consisted of field measurements and laboratory analyses across seven streambanks located along reaches of Crabtree Creek and Walnut Creek in Raleigh, North Carolina. The second component coupled high-resolution hydro-environmental monitoring and process-based probabilistic modeling at a selected streambank in Crabtree Creek. Monthly Jet Erosion Tests (JET) normalized by instantaneous measurements of hydro-environmental conditions were performed to characterize the temporal variability of streambank erodibility parameters. At the selected streambank in Crabtree Creek, data from a soil sensor that recorded high-frequency (5 min.) soil moisture content and temperature for almost a year was combined with streamflow data from nearby USGS gages to construct a site-specific probabilistic streambank erosion curve. Results from this project revealed that: (1) streambank erodibility parameters are dependent on rapidly changing hydro-environmental conditions, showing variability with changing moisture content and soil temperature at the same streambank; (2) the degree of variability of streambank erodibility parameters with changing moisture content and soil temperature is dependent on the streambank composition, with the percent of fine material playing an important role in modulating the magnitude and the direction of such degree of variability; (3) the shape of probabilistic streambank erosion curves provides a direct indication of a streambank susceptibility to experience erosion over given time horizon as dictated by concurrent changes in erodibility parameters and hydro-environmental conditions; and (4) the overall shape of the probabilistic streambank erosion curve is controlled by the probability distributions of hydro-environmental conditions and flow rates, highlighting the importance of the frequency and magnitude of erosion-generating hydrological events.