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Description
Taken from the Anglesey side of the Menai Straits, Wales. 23-10-10.

The Menai Strait (Welsh: Afon Menai) is a narrow stretch of shallow tidal water about 16 miles long, which separates the island of Anglesey from the mainland of Wales.

The strait is bridged in two places - the main A5 road is carried over the strait by Thomas Telford's elegant iron suspension bridge, the first of its kind, opened in January 1826, and adjacent to this is Robert Stephenson's 1850 Britannia Tubular Bridge. Originally this carried rail traffic in two wrought-iron rectangular box spans, but after a disastrous fire in 1970, which left only the limestone pillars remaining, it was rebuilt as a steel box girder bridge. Between the two bridge crossings there is a small island in the middle of the strait, Ynys Gored Goch on which is built a house and outbuildings and around which are the significant remains of fish traps - now no longer used.

The strait varies in width from 1,300 ft from Fort Belan to Aber Menai Point to 3,600 ft from Traeth Gwyllt to Caernarfon Castle.
The differential tides at the two ends of the strait cause very strong currents to flow in both directions through the strait at different times, creating dangerous conditions. A rising tide approaches from the south-west causes the water in the strait to flow north-eastwards as the level rises. The tide also flows around Anglesey until, after a few hours, it starts to flow into the strait in a south-westerly direction from Beaumaris. By the time this happens the tidal flow from the Caernarfon end is weakening and the tide continues to rise in height but the direction of tidal flow is reversed. A similar sequence is seen in reverse on a falling tide. This means that slack water between the bridges tends to occur approximately one hour before high tide or low tide.
One of the most dangerous areas of the strait is known as the Swellies (or Swillies – Welsh: Pwll Ceris) between the two bridges. Here rocks near the surface cause over-falls and local whirlpools, which can be of considerable danger in themselves and cause small boats to founder on the rocks. This was the site of the loss of the training ship HMS Conway in 1953. Entering the strait at the Caernarfon end is also hazardous because of the frequently shifting sand banks that make up Caernarfon bar. On the mainland side at this point is Fort Belan, an 18th-century defensive fort built in the times of the American War of Independence.

Because the strait has such unusual tidal conditions, coupled with very low wave heights because of its sheltered position, it presents a unique and diverse benthic ecology. The depth of the channel reaches 15m in places, and the current can exceed 7 knots. It's very rich in sponges. The existence of this unique ecology was a major factor in the establishment of the School of Ocean Sciences at Menai Bridge, part of the University of Wales, Bangor as well as its status as a special area of conservation with marine components. The waters are also a proposed Marine Nature Reserve.

Description
Taken from the Anglesey side of the Menai Straits, Wales. 23-10-10.

The Menai Strait (Welsh: Afon Menai) is a narrow stretch of shallow tidal water about 16 miles long, which separates the island of Anglesey from the mainland of Wales.

The strait is bridged in two places - the main A5 road is carried over the strait by Thomas Telford's elegant iron suspension bridge, the first of its kind, opened in January 1826, and adjacent to this is Robert Stephenson's 1850 Britannia Tubular Bridge. Originally this carried rail traffic in two wrought-iron rectangular box spans, but after a disastrous fire in 1970, which left only the limestone pillars remaining, it was rebuilt as a steel box girder bridge. Between the two bridge crossings there is a small island in the middle of the strait, Ynys Gored Goch on which is built a house and outbuildings and around which are the significant remains of fish traps - now no longer used.

The strait varies in width from 1,300 ft from Fort Belan to Aber Menai Point to 3,600 ft from Traeth Gwyllt to Caernarfon Castle.
The differential tides at the two ends of the strait cause very strong currents to flow in both directions through the strait at different times, creating dangerous conditions. A rising tide approaches from the south-west causes the water in the strait to flow north-eastwards as the level rises. The tide also flows around Anglesey until, after a few hours, it starts to flow into the strait in a south-westerly direction from Beaumaris. By the time this happens the tidal flow from the Caernarfon end is weakening and the tide continues to rise in height but the direction of tidal flow is reversed. A similar sequence is seen in reverse on a falling tide. This means that slack water between the bridges tends to occur approximately one hour before high tide or low tide.
One of the most dangerous areas of the strait is known as the Swellies (or Swillies – Welsh: Pwll Ceris) between the two bridges. Here rocks near the surface cause over-falls and local whirlpools, which can be of considerable danger in themselves and cause small boats to founder on the rocks. This was the site of the loss of the training ship HMS Conway in 1953. Entering the strait at the Caernarfon end is also hazardous because of the frequently shifting sand banks that make up Caernarfon bar. On the mainland side at this point is Fort Belan, an 18th-century defensive fort built in the times of the American War of Independence.

Because the strait has such unusual tidal conditions, coupled with very low wave heights because of its sheltered position, it presents a unique and diverse benthic ecology. The depth of the channel reaches 15m in places, and the current can exceed 7 knots. It's very rich in sponges. The existence of this unique ecology was a major factor in the establishment of the School of Ocean Sciences at Menai Bridge, part of the University of Wales, Bangor as well as its status as a special area of conservation with marine components. The waters are also a proposed Marine Nature Reserve.