Geography of the Bristol Channel
The International Hydrographic Organisation defines the western limit of the Bristol Channel as "a line joining Hartland Point in Devon (51°01′N 4°32′W) to St. Govan's Head in Pembrokeshire(51°36′N 4°55′W)".[1]
The upper limit of the Channel is between Sand Point, Somerset (immediately north of Weston-super-Mare) and Lavernock Point (immediately south of Penarth in South Wales). East of this line is the Severn Estuary. Western and northern Pembrokeshire, and north Cornwall are outside the defined limits of the Bristol Channel, and are considered part of the seaboard of the Atlantic Ocean, more specifically the Celtic Sea.
Within its officially defined limits, the Bristol Channel extends for some 75 miles (121 km) from west to east, but taken as a single entity the Bristol Channel - Severn Estuary system extends eastward to the limit of tidal influence near Gloucester. The channel shoreline alternates between resistant and erosional cliff features, interspersed with depositional beaches backed by coastal sand dunes; in the Severn Estuary, a low-lying shoreline is fronted by extensive intertidal mudflats.[2]
The Severn Estuary and most of the embayments around the channel are less than 10m in depth. Within the channel, however, there is an E-W trending valley 20 to 30m in depth that is considered to have been formed by fluvial run-off during Pleistocene phases of lower sea level. Along the margins of the Bristol Channel are extensive linear tidal sandbanks that are actively dredged as a source of aggregates.
[1],[2] Bristol Channel / Severn estuary. River Avon. (2017, April 26). In Wikipedia, The Free Encyclopaedia. Retrieved 12:47, April 26, 2017, from https://en.wikipedia.org/w/index.php title=Bristol_Channel&oldid=777310392 http://www.severn-bore.co.uk/surfing.html
The River Avon
The River Avon is an English river in the south west of the country. To distinguish it from a number of other rivers of the same name, this river is often also known as the Bristol Avon. The name "Avon" is a cognate of the Welsh word afon, "river".
The Avon rises just north of the village of Acton Turville in South Gloucestershire, before flowing through Wiltshire. In its lower reaches from Bath to the Severn Estuary at Avonmouth near Bristol, the river is navigable and known as the Avon Navigation.
The Avon is the 19th longest river in the UK at 75 miles (121 km) although there are just 19 miles (31 km) as the crow flies between the source and its mouth in the Severn Estuary. The catchment area is 2,220 square kilometres (860 sq mi).
Bristol Pilots LLP
Bristol Pilots LLP provide pilotage services for Bristol city docks and the navigable and tidal river below the Cumberland Basin and the approach to Portishead Pier in addition to pilotage in the Severn & Bristol Channel.
The Severn Estuary
Definitions of the limits of the Severn Estuary vary. A narrower definition adopted by some maps is that the river becomes the Severn Estuary after the Second Severn Crossing near Severn Beach, South Gloucestershire, and stretches to a line from Lavernock Point (south of Cardiff) to Sand Point near Weston-super-Mare. The definition used on Admiralty Chart 1179 and the Bristol Channel and Severn Cruising Guide is that the estuary extends upstream to Aust, the site of the old Severn Bridge. The estuary is about 2 miles (3.2 km) wide at Aust, and about 9 miles (14 km) wide between Cardiff and Weston-super-Mare.
The Estuary forms the boundary between Wales and England in this stretch. On the northern side of the estuary are the Caldicot and Wentloog Levels which are on either side of the city of Newport; and, to the west, the city of Cardiff together with the resort of Penarth. On the southern, English, side, are Avonmouth, Portishead, Clevedon, and Weston-super-Mare. Denny Island is a small rocky island of 0.24 hectares (0.6 acres), with scrub vegetation, approximately three miles north of Portishead. Its rocky southern foreshore marks the boundary between England and Wales, but the island itself is reckoned administratively to Monmouthshire, Wales.
The estuary has one of the highest tidal ranges in the world — about 50 feet (15 m). The estuary's funnel shape, its tidal range and the underlying geology of rock, gravel and sand, produce strong tidal streams and high turbidity, giving the water a notably brown coloration.
West of the line between Lavernock Point and Sand Point is the Bristol Channel, which in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The islands of Steep Holm and Flat Holm are located close to that line, in the middle of the estuary.
Sometimes the term Severn Estuary is used to include the tidal upstream stretch between Gloucester and Aust. During the highest tides on the upper reaches of this stretch, the rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river current.
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