Sediment discharge measurement and calculation: Techniques for use at river gauging stations

The terms 'wash load', 'suspended bed material load' and 'bed load' are not clearly defined but, if applied with care, provide a useful basis for sub-dividing the total discharge of sediment in a river for the purpose of measurement. The accuracy with which each of the three types of sediment transport must be measured will depend on their relative magnitudes and on the purposes for which data are being collected. This paper contains a proposed programme of measurements to be undertaken on a typical tropical river, one carrying appreciable wash load discharges and in which the suspended bed material discharges are large relative to the bed load discharges. The purpose of the programme is to provide data on the total discharge of sediment in such a river for studies of catchment erosion or reservoir sedimentation. In outline, the proposed programme is as follows: i) use an adequately calibrated turbidity monitor to provide a continuous record of wash load concentrations; ii) undertake a short intensive programme of pumped sampling to provide information about the relationship between suspended bed material discharges .and selected hydraulic parameters (principally river level); iii) apply this relationship to recorded flow discharge data to provide values of suspended bed material discharge over the period of available hydrological data; and iv) apply empirical and theoretical methods to provide an estimate of the magnitude of the 'bed load' (including unmeasured suspended bed material load close to the bed) relative to the measured suspended bed material load. The theoretical basis for this programme is examined in this paper. In particular, details of the assumptions made, and their implications, are provided. In addition, practical details of the field procedures and of the methods of analysis relating to pumped sampling in such rivers are described.