High-resolution near-seabed velocity and sediment transport profiling

Abstract

In the Netherlands morphological studies are quite important because of the dynamics of the Dutch coastline and rivers. These types of processes require accurate and high resolution (both in time and space) measuring equipment, which so far was unavailable. Conventional Doppler profiling techniques (ADCPs) are hampered by the fact that the bottom 10-15% of the depth range cannot be measured. Non-acoustic methods for 'in-situ' and point current and/or concentration measurements disturb the actual phenomena under study. By this new acoustic method, based on incoherent cross-correlation techniques in the time domain, profiles now can be measured in these last few meters near the seabed. This remote sensing technique uses various pairs of vertical downward looking active acoustic transducers. This method has been developed and extensively tested for various conditions in the MAST VERTPARSE project. The experimental ranges varied for mean particle sizes from 50 /spl mu/m up to 500 /spl mu/m and for flow velocities in the range from 0.5 m/s up to 2.0 m/s. The experiment was focused on an upper river situation. In a dynamical process starting from a horizontal bed with a water flow velocity of 1.0 m/s sand dunes occurred with a typical height of 0.3 m and horizontal dimensions of 3.0 m. During this process particles with a mean diameter of 480 /spl mu/m were suspended by the current and observed by this method. The flume experiment provided a unique facility to observe these realistic morphological processes instantaneously. Additional measurements under realistic field conditions have already been preformed in a wave tunnel and in the Dutch coastal area near the Rotterdam Harbour. Velocity and concentration profiles have been measured with a vertical resolution of 0.015 m down to the very seabed. The mean and standard deviation of the flow velocity profiles are in agreement with reference measurements. The concentration profiles have not been checked yet in an absolute sense, but are looking very realistic compared to what is to be expected from theory and other experiments. This method proved itself robust for flow velocity measurements, so it can be implemented as a standard instrument in operational field experiments. The advantage of spatial resolutions to be selected by the observer and the facility of measurements down to the seabed including bottom tracks, give new opportunities in the development and calibration of morphological models.