CHARTERED SURVEYORS

Project 2: Acoustic Surveys of Sub-Tidal Reefs in Flamborough Head
European Marine Site Client: English Nature

Designated as a European Marine site of considerable importance, the sub-sea area off Flamborough Head is unique in terms of the total area of submerged chalk reef systems that it supports. These are valued for their rarity and the particular types of faunal associations and habitats that exist amongst them.

The near-shore littoral and circa-littoral zone has bands of intact bedrock with submerged sea caves, underwater ravines and tall vertical faces. The offshore area exhibits extensive low-lying reef areas with terraced bedrock and rich faunal turf

The sub-littoral reef system was known to extend out to beyond 10 km offshore, but the extent of coverage in terms of area and zonation was not known precisely. In order to provide a basis for quantitative monitoring of the site's conservation status, English Nature required detailed maps of the reef system and the various seabed habitats and biotopes associated with them.

Hydrosurveys were awarded the contract to survey a sub-sea area of some 10,000 ha, using a combination of sidescan imaging, acoustic ground discrimination (AGD), and single beam echo-sounding.

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After the field surveys had been completed, the immense task of interpreting and bringing together the various data sets commenced. Seabed bathymetry around the Flamborough headland (above) was interpreted in conjunction with the acoustic ground discrimination plot of habitat zones (above right). Sidescan images (right) were used for simultaneous visual assessment of the geology and physical nature of the reef system.

The figure to the right shows a small part of the overall sidescan mosaic and is taken from the rectangular area inset above.

Dive and video 'drop' surveys were used to provide ground-truth data for discrimination of different substrates and habitat types within both the AGD footprint and the sidescan images.



Shoreline features to the north of Flamborough Head