Relating stream‐bank erosion to in‐stream transport of suspended sediment

Abstract

We seek an improved and quantitative understanding of the sources and transport of sediment and attached phosphorus in upland catchments and downstream reaches of the Namoi River in New South Wales, Australia. Study of the sources of phosphorus and related sediment was motivated by severe problems with blooms of blue-green algae and toxic by-products in the Darling and Namoi Rivers. Using atmospheric fall-out of radionuclides as tracers, Olley et al. (1996) concluded that much of the sediment deposited in the lower reaches came from subsoil rather than topsoil. With this insight, we focus on quantifying sediment sources from stream bank erosion, especially in seasonally erosional reaches of Cox's Creek and the Mooki River. The approach presented here integrates interdecadal aerial photography, interseasonal field measurements of bank erosion processes, continuous monitoring of stream flow and turbidity and event sampling of suspended solids and phosphorus, with an analytical model of in-stream suspended sediment transport. We compare a lateral source term in the calibrated transport model with field-based and aerial measurements of stream bank erosion. Calibration of the in-stream model is illustrated for two reaches of the Mooki River, with the changes in parameter values being related to aspects of the hydraulic geometry and particle size. The processes of stream flow and bank erosion due to undercutting, desiccation, block failure and mass wasting of aggregated particles interact to produce instream fluxes of suspended sediment that are transported and redeposited downstream. The combined approach demonstrated here has potential for predictive spatial modelling of sediment concentrations and loads. Copyright © 1999 John Wiley & Sons, Ltd.