A Conductivity Concentration Profiler for Sheet Flow Sediment Transport

Abstract

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 . 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.

Bed level motions and sheet flow processes in the swash zone: Observations with a new conductivity-based concentration measuring technique (CCM + )

“…These instances occur mainly during the uprush and during the middle/final stage of the backwash phase, which agrees with earlier observations for natural swash zones (Yu et al, 1990;Lanckriet et al, 2013;. The dynamics of the sheet flow layer for the lower swash in this experiment exhibit both similarities and differences with oscillatory sheet flow.…”

“…OBSs) fail at measuring the high concentrations during the early uprush and late backwash phase when water depths are shallow. The recently developed Conductivity Concentration Profiler (CCP; Lanckriet et al, 2013) is the only instrument capable of measuring sheet flow concentration profiles and bed level changes with high temporal (f > 1 Hz) resolution. The CCP measures a complete sheet flow concentration profile at once, which is a major advantage in many conditions.…”

“…However, it also has limitations, e.g. (i) sheet flow thicknesses smaller than 3.5 mm cannot be resolved (Lanckriet et al, 2013); (ii) the measuring window (30 mm) is limited, which may lead to a substantial loss of data when the bed is evolving ; (iii) the system is unable to measure particle velocities. Regarding the latter, it is stressed that to the authors' knowledge none of the existing instrumentation but the CCM + is capable of direct particle velocity measurements.…”

“…However, knowledge on swash zone sheet flow dynamics is limited since only few instruments are capable of obtaining high-quality measurements of sheet flow concentrations. Conductivity-based instruments showed the occurrence of sheet flow during early uprush and late backwash instances (Yu et al, 1990;Lanckriet et al, 2013;Lanckriet and Puleo, 2015). Vertical concentration profiles during isolated backwash events, measured with a recently developed Conductivity Concentration Profiler (CCP), showed strong similarities to observations for oscillatory sheet flow .…”

Sand transport processes in the surf and swash zones

“…These tanks were located at the bar crest at x = 53.0 m (at break point where wave starts to overturn) and at x = 54.5 m (between break point and plunge point). Over the high sand concentrations (100 -1600 kg/m 3 ) in the sheet flow layer, the measured conductivity of a water sand mixture is a linear function of sand concentration, which makes the conductivity based measuring principle highly suitable for studying sheet flow dynamics (Ribberink and Al Salem, 1995;Dohmen Janssen and Hanes, 2002;Lanckriet et al, 2013). The CCM + tanks in the present study are equipped with one single conductivity probe plus a combined double probe (for tank 1), or with one single probe (tank 2), and sample with a 1000 Hz data rate.…”