DISTINGUISHING BED-LOAD AND BED-MATERIAL-LOAD FLUXES WITH REPEAT BATHYMETRIC DATA

Quantitative understanding of bed-load and bed-material-load fluxes in sandy rivers would afford greater understanding and prediction of channel form, river behavior, and habitats of river corridor biota. However, practical difficulties and cost ineffectiveness often exclude bed-sediment measurements from studies and monitoring efforts aimed at estimating sediment loads in rivers. An alternative to direct sampling is through the measurement of evolution of bed topography constrained by sediment-mass conservation. Historically, the topographic-evolution approach has been limited to systems with negligible transport of sand in suspension. We show that by loosening the constraint on mass conservation (that is, allowing divergence of sediment flux to vary temporally when averaged over a bed form length) bed load and bed-material load can be distinguished by their effects on the evolution of bed topography. As was shown decades ago, pure bed load transport is responsible for the mean migration of trains of bed forms when no sediment is exchanged between individual bed forms. In contrast, the component of bed-material load that moves in suspension is responsible for changes in the size, shape, and spacing of evolving bed forms; collectively this is called deformation. The sum of the effects of deformation and translation on bed topography reflects the total movement of bed material. Similarly, the difference between bed-load flux and bed-material-load flux equals the flux of suspended bed material. This approach is demonstrated using a set of repeat multibeam sonar bathymetric surveys coupled with point-integrated suspended-sediment profiles and acoustic Doppler velocimetry.