ESTIMATING BEDLOAD FROM GAGE DATA TO IMPROVE FLUX-BASED SEDIMENT BUDGETS

Measurements of sediment flux can be used to estimate rates of large-scale morphodynamic change. Recent advances in continuous suspended sediment monitoring through acoustic “surrogates” have reduced the error in flux estimates such that it is possible to resolve reach-scale gradients in sediment flux over multi-year timescales. Due to the difficulty of measuring bedload transport in large rivers, this method necessarily relies on untested assumptions concerning the relative magnitudes of bedload and suspended load. Though many of the world’s rivers are dominated by suspended load, bedload is potentially the largest source of systematic error in flux-based sediment budgets.

The partitioning of total sediment load into suspended load and bedload is difficult to predict, especially in large rivers with bed forms and high suspended sediment concentrations. The ratio of the two modes of transport varies nonlinearly as discharge, sediment supply, and transient reach morphologies interact with and modify bedform geometry and concentration-induced density stratification. These effects are difficult to quantify, precluding the use of direct physical models in most cases. We present an alternative approach for estimating bedload from suspended sediment data commonly generated by long-term sediment monitoring programs. This model is similar to that proposed by Rubin & Topping (2001) for estimating changes in bed grain size and shear velocity. The model requires empirical calibration, which is accomplished here using concurrent gage measurements of suspended sediment flux and bedload estimated from repeat bathymetric surveys of bed form migration.