The
Menai Strait (, the "River Menai") is a narrow stretch of shallow tidal water about 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
mediaeval fish traps - now no longer used.
The strait varies in width from 400 metres from Fort Belan to Aber Menai Point to 1,100 metres from Traeth Gwyllt to Caernarfon Castle. It then narrows to about 500 metres in the middle reaches (Port Dinorwic and Menai Bridge) and then it broadens again. At Bangor Pier it is 900 metres wide. It then widens out, and the distance from Puffin Island to Penmaenmawr is about 7.5 km. 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. 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
school 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 Anglesey side at this point is
Fort Belan, an 18th-century defensive fort built in the times of the
American War of Independence.
Tidal effects
Glyn Garth flats on the banks of the Menai Strait opposite Upper
Bangor The tidal effects observed along the banks of the strait can also be confusing. 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.
Theoretically it is possible to ford the strait in the Swellies at low water, spring tides when the depth may fall to less than 0.5 metres. However, at these times a strong current of around 9 km/h is running, making the passage extremely difficult. Elsewhere in the strait the minimum depth is never less than 2 metres until the great sand flats at Lavan sands are reached beyond
Bangor.
Ecology
Marine
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.
Land
The same unique ecology and geo-morpholgy has let to a number of designations of
SSSIs along the strait including
Glannau Porthaethwy, the
ivy -
oak -
ash woodland on the southern shore (
Coedydd Afon Menai) and
Lavan sands (Traeth Laven).
Much of the land on Anglesey at the eastern end of the strait is designated as an
area of outstanding natural beauty.
Places on the Strait
Just north of the rail bridge on Anglesey is the famously named
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch.
Further links
Eifion Jones.
