Description

Currents can be measured at set locations using impellor-driven, Doppler or Electro-magnetic (EM) current meters.

The Acoustic Doppler Current Profiler (ADCP) is the latest equipment in used for current measurements. The ADCP is an extremely flexible instrument. It can be deployed as part of a conventional oceanographic mooring to record time series of the current profile, or used as a direct reading instrument to investigate the spatial variability of currents in an area. Typically this is used to collect transect data across rivers for model validation, although the technique is equally valid offshore to assist in the preparation of Tidal Atlas plots or to replace conventional current profiling at sites where a number of profiles are required over a small area. The ADCP can also be used to measure the level of suspended sediment in the water column; a typical application would be to monitor the movement of dredge spoil.

Current regimes are routinely measured to determine:

• Sediment transport characteristics
• Engineering construction design (cables, platforms, piles etc)
• For navigation purposes
• Mooring forces on ships.

ADCP's are particularly pertinent to oceanographic current profiling and monitoring dredging pollution

In many cases where detailed information on the flows and other physical parameters are required at a specific position, the use of profiling techniques from a moored vessel is the most cost effective technique. Profile data, typically current speed and direction, together with salinity, temperature and suspended solids would be monitored, although additional data such as PAR, Dissolved Oxygen, pH etc. can also be included. Data would normally be collected every half hour over a full tidal cycle, and can be presented in a number of ways.
Standard plots include time series of specific parameters at key depths; contour plots of measured values against time and depth or as simple data listings of the measured data.

Conventional current meters are now being supplanted by either bottom or vessel mounted ADCPs, which provide considerably more information from the whole of the water column, without the risk of material being wrapped around impellors.

Of particular significance is the ability of the ADCP, using careful calibration techniques, to measure directly sediment plume densities using acoustic backscatter measurements.