Use of Bed-Surface Grain Size To Infer Equilibrium Bedload Transport Rates And Sediment Supply

Abstract:

Bed-surface grain size of gravel-bed rivers is responsive to sediment supply, a mechanism that allows attainment of equilibrium transport (input=output) by altering bed roughness that, in turn, affects the applied shear stress (driving transport) and the resistance to motion for particles traversing the bed. High supply causes deposition of fine material that smooths the bed and increases transport rates over that surface, while low supply causes winnowing of fine bed-surface grains, which coarsens and roughens the bed, leading to reduced bedload transport rates. Prior studies demonstrate that the degree of textural fining relative to a theoretical value of competence is related to the equilibrium transport rate and, thus, sediment supply. This suggests the possibility of being able to rapidly assess supply and transport rates from inspection of bed-surface grain size, providing a cost-effective alternative to lengthy bedload sampling and a means for assessing channel condition (supply relative to transport capacity) and response potential to altered discharge or supply. However, the approach was developed from flume data and has not been extensively field tested. Here, I use data from 33 mountain rivers in Idaho to explore the viability of the method. Results demonstrate that equilibrium transport rates are not uniquely related to the degree of textural fining; rather, transport rates are modulated by systematic downstream changes in the coefficient and exponent of the transport function, where the coefficient is controlled by basin-specific sediment supply and discharge, and the exponent is a function of the degree of supply-related armoring of the bed. Furthermore, results depend on whether the data are characterized by downstream fining vs. coarsening, an issue that is explored analytically. Consequently, use of bed-surface grain size to infer equilibrium bedload transport rates and sediment supply requires collection of regional data to identify downstream trends in parameter values, without which inferred equilibrium transport rates may be prone to substantial error.