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Southwest Regional Coastal Monitoring Programme

Bathymetric Survey Techniques

Swath Bathymetry

The historical approach to nearshore bathymetric monitoring varies within the region. Differential GPS (DGPS), digital echo sounders and sophisticated hydrographic surveying software allow large volumes of data to be collected and surveyed far more efficiently than has previously been possible. Where little bathymetric survey data has been collected previously, detailed baseline surveys can be conducted efficiently, allowing future programme flexibility, identification of areas for regular monitoring and preparation of grids for numerical modelling purposes.

The use of DGPS enables survey points to be collected with a positional accuracy of +/- 10cm. Straight profiles can rarely be steered to an accuracy of better than 1-2m. The use of DTMs to produce profiles is advantageous in these circumstances, since survey error will be reduced. Vertical accuracy varies enormously, depending upon sea state, but is typically +/-0.25m on the open coast.

Traditionally, bathymetric data has been gathered with a single beam echo sounder, gathering one profile line at a time. These provide poor spatial resolution and may fail to identify large and potentially important seabed features, even when the line spacing is fairly close (50m). As an alternative to single line bathymetric surveys, swathe bathymetry can be used to provide high-resolution surveys across a wide area. Swathe (or multibeam) bathymetry becomes more cost effective in deeper water conditions, since the width of the swathe is proportional to water depth. The swathe width that can be measured equates to approx. eight times the water depth - some recent systems have the capability of wider-angle swathes but these are not widespread. Such systems are expensive and require a sophisticated gyro system in conjunction with DGPS. Tidal correction can be applied automatically through the use of kinematic GPS. This approach would provide the best technical solution to the problem, and would enable a detailed terrain model to be developed across the whole region. The data can be viewed as contour plots, grey scale surface maps or three-dimensional images that allow easy visualisation of the survey areas.

A survey vessel carrying out a single beam bathymetric survey.