NERC-NSFGEO: The impact of cohesive material on critical shear stress in gravel bed rivers

Accurately predicting sediment transport is critical for infrastructure management, flood-risk assessment, river restoration, and predictions of habitat for aquatic organisms. Sediment transport predictions are often inaccurate partly because of uncertainties in estimating the onset of gravel motion. Cohesive material such as clay, microbial biofilms, and biological silks from aquatic organisms can affect the onset of gravel motion but are often neglected. A combination of laboratory experiments, river measurements, and modeling will be used to measure and predict the influence of cohesive material on gravel motion. This project will reduce uncertainty in sediment transport predictions, and produce tools that inform sustainable river management, flood mitigation, and restoration. Bedload transport in gravel-bed rivers is often predicted using the critical shear stress that causes the onset of gravel motion. Most practical applications select the critical shear stress for gravel from a nearly order-of-magnitude range or rely on simple functions, which can produce bedload transport predictions that have orders of magnitude in uncertainties. Cohesive materials found in riverbeds can affect gravel critical shear stresses but predictive equations for the effects of these materials are lacking. As part of a NERC-NSF collaboration, this project will: 1) measure the amount of biological and non‐biological cohesion in gravel‐bed rivers, 2) use laboratory experiments to understand how cohesive material alters gravel resisting forces, riverbed structure, and near-bed flow conditions to control critical shear stresses, 3) use refractive-index-matching experiments to understand how changes in riverbed structure from cohesive material affect within-bed flow conditions and near-bed turbulence, and 4) use these results to develop, test, and validate a new mechanistic model for gravel critical shear stresses that includes cohesive effects of clay, microbial biofilms, and biological silks. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.