Coupling of Ultrasonic and Photometric Techniques for Synchronous Measurements of Unconfined Turbidity Currents

Richard Wilson,Heide Friedrich

doi:10.3390/w10091246

Abstract

By synchronizing data collection, such as photometric and ultrasonic Doppler profiling (UVP) measurement techniques, new insights can be obtained into environmental flows, such as highly dynamic turbidity currents. We introduce a combined experimental setup, which ultimately allows a time reduction in testing programmes, and discuss the measurement advances with the help of four surface conditions we tested for unconfined turbidity currents: (a) a smooth surface; (b) a smooth surface with an obstacle present; (c) a rough surface; and (d) a rough surface with an obstacle present. We show that data from both measurement techniques indicate that a rough surface reduces global current velocities and the magnitude of turbidity current phenomena, including Kelvin-Helmholtz instabilities and lobe-and-cleft formation. However, by coupling the techniques, photometric data give valuable insight into the spatial development of instabilities, such as the grouping of lobe and cleft formations. The presence of an obstacle causes local regions of an increased and decreased velocity, but does not affect the global current velocity. Additionally, the obstacle created three local intensity maxima upstream, dissipating to two maxima downstream, supporting the presence of local eddies. The study shows that the combination of UVP and photometry is an effective way forward for obtaining detailed qualitative and quantitative insights into turbulent flow characteristics and we highlight the potential for future research.

Highlights

Sediment-laden gravity currents, commonly referred to as turbidity currents, are found in sedimentary environments such as seas, deep oceans, reservoirs, and lakes, where they are significant contributors to sediment transport [1]
By using a coupled Ultrasonic Doppler velocity profiling (UVP) and photometric measurement approach, new data to study the effect of surface roughness on the propagation of turbidity currents is presented
It was found that the rough substrate reduces the turbidity current maximum velocity, umax, by a mean of 27% through the analysis area

Summary

Introduction

Sediment-laden gravity currents, commonly referred to as turbidity currents, are found in sedimentary environments such as seas, deep oceans, reservoirs, and lakes, where they are significant contributors to sediment transport [1]. Triggering events can cause these currents to mobilize sediment off the subaqueous bed, causing a catastrophically erosive current [2]. Turbidity currents pose potential environmental hazards, such as submarine cable damage and reservoir sedimentation, and are important to engineers [3]. Turbidity currents are important to scientists as they are one of the main agents of geological sea-bed formation and stratigraphy, which in turn host some of the largest oil reservoirs [4]. Complex numerical solutions of turbidity current mechanics are desired to enable the prediction of long-term events of turbidite formation [2]. In order to validate such models, laboratory experiments are needed, focusing on the flow characteristics of small-scale turbidity currents, e.g., [5]

Methods

Discussion

Conclusion