The barrage includes locks and bridges, sluice gates, and a fish pass. As the estuary has a high tidal range, the sector lock gates are up to 16m high to enable the boats to pass through at all stages of the tide. The inner cills are at –0.5m OD, but to minimise salt water intrusion, beams may be placed on the cills to prevent as little estuary/saline water as possible from entering the Bay. Any estuary/saline water entering the Bay is contained in a sump and is then flushed out via a 1,200mm gravity discharge pipe.

The wide variety of environments, with tidal ranges outside the barrage in excess of 12m and water depths in the rivers of less than 1m, require a comprehensive range of survey techniques and vessels.

Coastline Surveys Limited own an expanding fleet of inshore and coastal survey vessels ranging from the 24m ‘MV FlatHolm’, suitable for a wide range of geotechnical surveys including vibrocoring, CPT and grab sampling, to smaller vessels including an 11m twin screw launch and a highly manoeuvrable 8m twin engine/twin hydrojet aluminium survey vessel.

The 'Coastline Surveyor II', adapted from a NATO Combat Support Boat, provides a robust platform for complicated inshore surveys. It's monohedron aluminum hull with self bailing and full floatation design was also built for the U.S. military. Transported by a  purpose adapted rigid HGV fitted with a 26 ton HIAB crane, the Coastal Surveyor II can be deployed in small harbours and marinas without cranes or hoists. With MCA Category 3 certification, it has seen service across the UK and can be quickly mobilised.

Powered by twin 212hp Sabre turbodiesel engines coupled to Dowty Hydrojets, the NATO based design has a carrying capacity of up to 1 ton depending on configuration and a bollard pull of 2 tons. The twin jets allow accurate ‘hovering’ over grab or core sampling stations in a tideway without anchoring and infinitely variable speeds to suit deployed equipment up to a transit speed of 19 knots.

Uses have included oceanographic sampling, recovery of seabed instruments, bathymetric surveys, including swathe, and deploying a full suite of geophysical instruments. Importantly, the gross weight of 4,500 kg provides a stable platform for accurate measurements and surveyor comfort.