Results of a high-resolution correlation current profiler

Sedimentologists have great interest in measuring flow velocity profiles close to the seabed, as the sediment is transported mainly in the half a meter above the seabed. Existing measurement instruments cannot measure that close to the seabed (e.g. ADCPs) or measure only in a single point. A technique named the `correlation technique´ is proposed. By means of volume back-scattering acoustic echoes of pairs of transducers, orientated vertically downward, are correlated. This technique is currently capable of providing a vertical resolution of 0.015 m and produces a flow velocity profile down to the seabed. The measurement range includes the complete lower meter of the vertical. As it is a remote sensing technique, no local disturbance of the flow is expected as it is with (multiple) point measurements in frames. The time resolution is at most 5 s, giving instantaneous estimates along the whole vertical at the given spatial resolution. This method has been tested under a variety of-experimental conditions over the last few years. This paper focuses on laboratory flume experiments. Flow velocity, concentration of suspended sand, turbulence intensity, acoustic bin size, type of transmitted signals and estimation times (time resolution) have been varied. The method works fine for small 0.015 m up to large 0.240 m acoustic bin sizes. Typical mean velocity standard deviation is better than 1.5%. Results are compared with an immersible laser Doppler anemometer. The technique proves to be insensitive to turbulence intensities. Different suspended sand concentrations lead to different correlation characteristics without degrading results. Estimates derived from clean water (tap water) results even are the most accurate. The methods will be tested and evaluated under nonstationary flow conditions and in the field in the near future