Carmarthen Bay & Estuaries SAC includes large areas of intertidal mudflats and sandflats. These range from open coast sandy beaches at Tenby and eastwards towards Worms Head, including the wide beaches of Pendine Sands, Cefn Sidan Sands and Whiteford Burrows. The most extensive are the wide expanses of Llanrhidian Sands, Cefn Padrig and Dafon Sands, in the lower and middle estuary of the Burry Inlet. The mudflats and sandflats cover around 7,000 ha, thus comprising 2.4 % of the UK resource and approximately 10 % of the area of the SAC.

There is a gradation within the distribution of sediments, from mud in the upper, more sheltered regions of the estuaries, to sand at the more wave-exposed mouths of the estuaries. 

Large areas of the intertidal mudflats and sandflats are dominated by bivalves. In areas of fine sand, common cockles Cerastoderma edule are abundant, along with other bivalves, amphipods and worms. In muddier sediments the sand-gaper Mya arenaria, peppery furrow-shell Scrobicularia plana and mud-snail Peringia ulvae are also found in large numbers. The lower Loughor Estuary is one of the few places in the UK where the worm Ophelia bicornis has been found. There are also beds of the nationally scarce dwarf eelgrass Zostera nolteii.