Towards a Simplified, Step-Length-Based Morphodynamic Model for Gravel Bed River Evolution

Predicting the evolution of gravel bed river morphology and in-channel habitat over decadal to centennial timescales is integral to making informed stream management and restoration decisions. Due to these longer timescales of change, fieldbased studies are often unable to fully capture morphologic shifts. Scenario-based morphodynamic modeling has emerged as a viable means of quantifying gravel bed river evolution, yet current models fall short with regard to their ability to predict changes in stream morphology over the timescales in question and with adequate spatial resolution, a problem due largely to the computational overhead they require. While the computing power required to quantify sediment transport has hindered previous modeling efforts, field-based research suggests a potential improvement: sediment is often mobilized downstream with characteristic step-lengths. Here we introduce a morphodynamic model which drives sediment transport using a steplength based approach, negating the need for frequent recalculation of sediment dynamics in the flow, and correspondingly reducing computational overhead. Upon application of this model to a study reach along the River Feshie (UK), we observe that it accurately reproduces many bed morphologies observed over an eight-year study involving annual RTK-GPS and terrestrial lidar surveys. Subsequently, we utilize this new model to quantify morphologic outcomes under various discharge and sediment regimes. By utilizing simple step-length based sediment transport distributions, the evolution of bed topography can be accurately modeled with less computational overhead than previously possible. The knowledge gained from such scenario-based modeling may aid in the management and restoration of gravel bed streams under shifting discharge and sediment regimes