A Conductivity Concentration Profiler for Sheet Flow Sediment Transport

摘要

A new sensor for measuring sediment concentration under sheet flow conditions is presented. Electrical conductivity, measured using a four-electrode method, is used as a proxy for sediment concentration. The relationship between conductivity and sediment concentration was calibrated using known masses of neutrally suspended sediment in a heavy liquid and agrees well with existing linear and power-law relationships $(r^{2} > 0.98)$. A 29-point conductivity profile at 1-mm resolution is generated by multiplexing through a vertical array of 32 plate electrodes. Numerical simulations of the voltage field around the sensor indicate that the horizontal extent of measurement volume is 1.5 times the sensor width. The finite extent of the measurement volume leads to smoothing of the vertical concentration profile. The sensor resolves sheet flow layers with a thickness greater than 3.5 mm, and a correction formula is introduced to correct the measured sheet thickness for the smoothing effect. Initial field results in the swash zone of a natural beach quantify sheet flow processes with unprecedented detail. Short-lived sheet flow with a maximum thickness of 19 mm was observed during the uprush, and a longer duration sheet flow with a maximum thickness of 8 mm was observed during the backwash.