Subtidal sandbanks are permanently submerged sandy sediments, typically found at depths of less than 20m below chart datum. The Severn Estuary contributes approximately 3% of the UK Natura 2000 resource for subtidal sandbanks, by area.
In the Severn Estuary subtidal sandbanks have a high mobility due to the extreme tidal conditions, resulting in a reduced biodiversity. The sandbanks of the Severn Estuary are not only distinct from those in other UK estuaries, but they are considered to be some of the best. They are largely restricted to the middle and outer parts of the Estuary. The subtidal sandbanks in the Severn Estuary change their shape over time and many are ephemeral in nature, although some are relatively stable and long established. The sandbanks of the Middle and Welsh Grounds are relatively permanent sandbank features in the Severn Estuary, along with other long-established sandbank features at Cardiff Grounds and in Bridgwater Bay. The area of the more permanent subtidal sandbanks is approximately 1,300 ha and there are approximately 10,400 ha of ephemeral sandbanks.
Despite the significantly challenging conditions of the banks, they support very distinctive invertebrate communities, including a range of worms, shrimps, snails and bivalves. Their mobility and dynamic nature provides considerable potential for the transport of invertebrate larvae to other habitats across the Estuary. This offers the possibility for species to extend their geographical range across a wider area.
The subtidal invertebrate communities associated with these sandbanks play a key role in providing a food resource for the fish assemblage feature under the SAC and Ramsar site. Since the banks also occur in depths of less than 20 metres, they are easily accessible to feeding waterbirds and serve as an important nursery ground for an array of fish species in the Estuary.
For more information, please see the Severn Estuary EMS Regulation 33 Conservation Advice Package.