The ratio of measured to total sediment discharge

Abstract

It is important to examine the ratio of measured to total sediment discharge to determine the error in measured sediment transport rates from depth-integrated samplers. The ratio of measured to total sediment discharge as well as the ratio of suspended to total sediment discharge are examined based on the Modified Einstein Procedure. Both ratios reduce to a function of the ratio of shear velocity, u*, to the fall velocity, ω, of suspended material, u*/ω, and the ratio, h/ds, of flow depth, h, to the median grain size of bed material, d50. In rivers transporting fine material (such as silt or clay), the ratio of suspended to total load is a function of the ratio, h/d50. In this study, it is found that the ratio of measured to total load becomes a simple function of flow depth. For fine sediment transport, with a Rouse number (Ro) <0.3, at least 80% of sediment load is in suspension when h/d50>15, and at least 90% of sediment load is measured from depth integrating samples when h>1m. Detailed measurements from 35 river stations in South Korea demonstrate that sand sizes and finer fractions predominantly are transported in suspension. Also, at least 90% of sand and finer fractions are transported in suspension in gravel and sand bed rivers when the discharge is larger than the mean annual discharge.