Measurement of the inner structure of turbidity currents by ultrasound velocity profiling

The inner velocity structure and particle concentration profile of opaque turbidity currents were mea- sured simultaneously by ultrasound velocity profilers. Currents consisting of a quartz particle suspension were generated by using the lock-exchange method in a flume to experimentally reproduce the quasi- steady state of a turbidity current. A pair of ultrasound transducers captured the horizontal and vertical velocities from Doppler frequencies, and the particle concentration profile was extracted from the echo amplitude. The data obtained were analyzed in terms of momentum conservation according to the two- fluid model. We found that: i) the viscous and Reynolds shear stresses balance in the top half of the current; and ii) the lower border of the stress balancing appears around the depth of the maximum ver- tical density gradient. These findings indicate that the reduction of flow resistance inside the body region of the turbidity current is maintained downstream, which enables the current to transport particles over a long distance.