The Welsh Grounds are an extensive area of mud and sand flats on the northern side of the Severn Estuary. The flats are bounded to the southwest by the Welsh Hook. Sailing Directions dating to 1884 note that they occupy '... fully two-thirds of the water of the head of the Bristol channel; on an average they dry out from about 3 1/4 miles, and are 12 miles in length. The southwest extremity, called the SW spit, which in 1847 was a detached shoal seperated by a channel of from 2 to 4 fathoms, is a sharp point, bold-to on both sides, extending from that portion of the sand named Welsh hook; its northwestern edge trends in a straight line towards Gold cliff, or NE by E for 2 miles, and turning eastward for a distance of nearly 4 miles, forms a shallow bight in connection with the Usk patch... The highest part of the Welsh grounds, about midway between the Denny and SW spit, is near the southern edge, where it dries up to a height of 30 feet at low water springs, or over which at high water there is a depth of 13 feet'.

Sources include:
Admiralty; 1884, Sailing Directions for the Bristol Channel, 4th Ed, pg144-5
Historic Admiralty Chart 1183-A2, RCAHMW Digital Collections sourced from the UK Hydriographic Office and first published in 1839

Maritime Officer, RCAHMW, January 2010.