Second Severn Crossing

The Second Severn Crossing (Ail Groesfan Hafren) is the M4 motorway bridge over the River Severn between England and Wales, inaugurated on 5 June 1996 by HRH The Prince of Wales to augment the traffic capacity of the original Severn Bridge built in 1966. The bridge marks the lower limit of the River Severn and the start of the Severn Estuary.

Its location is farther to the south than the old bridge and, being more in line with the landward sides of the M4 motorway, is a shorter journey when travelling between England and South Wales. The junctions at each end are designed for most traffic to use this crossing – to use the old Severn Bridge crossing one has to leave the M4 and join the M48 motorway either at Aust or near Magor. The new crossing carries more traffic than the Severn Bridge, which is still in use. It is wider than the Severn Bridge, having three lanes and narrow hard shoulder each way, compared to the two lanes, cycle path and footpath of the original crossing. The path taken by the bridge is close to that of the Severn Tunnel which has carried the railway line beneath the river bed since 1886.

Much of the estuary is mudflats at low tide, but at high tide can be covered by up to 14 metres of water. This presented the engineers with a unique set of constraints with packets of work being scheduled at low tide and being completed with the short windows allowed by the tides.

The concession given to the consortium who financed, built and operate the bridge required them to take over the outstanding debt on the original Severn Bridge and to operate the two bridges as a single entity. Tolls are set by the government and on the expiry of the concession the consortium are required to hand the bridge over into public ownership.

Background
The Severn Estuary presented a barrier between the Bristol area and South Wales. The estuary has a maximum tidal range of 14.5 metres, the second highest in the world and during a rising or falling tide, strong currents of up to 8 knots (4 m/s). Much of the estuary is mud flats that are exposed at low tide that have been designated a Special Protection Area. The central part of the estuary is a navigable channel which, at the site of the bridge, is known as "The Shoots". The bridge is upstream from Avonmouth and the Port of Bristol, but downstream from the Port of Sharpness. The Gloucester Harbour Trustees have responsibility for controlling navigation in the estuary's tidal waters upstream from the bridge.

Until 1966, road travellers had the option of either using the Aust Ferry which had operated since medieval times (and as a car ferry since 1926) or by making the 92 km detour via Gloucester. In 1966 the first Severn road bridge, a four-lane suspension bridge was opened carrying the M4 motorway from England into South Wales. By 1984 traffic across the first Severn Bridge had tripled and it was projected that by the mid-1990s the old bridge would be running at capacity. A study was commissioned into the building of a second crossing - either a tunnel or a bridge. The consultants reported back in 1986 recommending that a new bridge be built downstream from the existing bridge.

In 1988 it was announced that tenders would be invited from private consortia to fund, build and operate the bridge for as specified period. The consortium would also take over the management and the £100 million debt of the old bridge. Tenders were invited in 1989 and in 1990 the concession to build the bridge was awarded to Sev­ern River Cross­ing plc. Construction work started on 26 April 1992 and the bridge was opened by the Prince of Wales on 5 June 1996.

Design
The bridge which has portals close to Sudbrook, Monmouthshire on the Welsh side and Severn Beach in South Gloucestershire on the English side has three principal sections - a 25 span viaduct on the English side of length 2103 m, a 24 span viaduct of length 2077 m on the Welsh side and the bridge itself, a 948 m structure with a 37 m navigational clearance, giving a total of 5128 m. The central section, called the Shoots Bridge, is of cable-stayed design and the central span (between the bridge pylons) is 456 m in length. The approach viaducts are of a segmental bridge design. The crossing forms a very slight "S" curve – the roadway having an approximate east-west alignment at each of the portals, while the central bridge follows an alignment which is approximately WNW to ESE.

The Severn Railway Tunnel passes under the estuary bed on a line which is generally about 500 m upstream of the bridge, but which passes under the line of the bridge close to the English shore.

The deck, which carries three lanes of traffic in each direction is 34.6 m wide. The sides of the bridge are fitted with 3 m baffle plates to reduce lateral wind loads coming from the Severn Estuary onto the traffic and this has reduced the number of times that speed restrictions have been needed. The overall design of the new crossing makes it more resistant to high winds than the old Severn Bridge.

Construction
The crossing was built by a business consortium under a public-private partnership. A company called Severn River Crossing plc, led by John Laing plc and GTM-Entrepose, was formed to build the new crossing. This company also took over the responsibility of managing and maintaining the old Severn Bridge crossing, as well as managing and maintaining the new crossing. The cost of constructing the new crossing was expected to be paid for by tolls collected from motorists using the two crossings. Work on the new crossing began in 1992. Completion was in 1996.

Sub-assemblies for the bridge were constructed onshore and then shifted by a large tracked vehicle (similar to that used to move the Apollo and Space Shuttle at Cape Kennedy) onto a barge (the SAR3), prior to being floated out on the high tide to the appropriate site. The 37 bridge pier foundations on the approach viaducts are 98.11 m apart, and consist of open concrete caissons weighing up to 2,000 tonnes, which were founded on the rock of the estuary bed. The decking consists of 3.6 m post stressed match cast sections, weighing 200 tonnes each.

The cable stayed section of the crossing is over 1044.7 m long, consisting of a 34.6 m wide deck made from steel plate girders with a composite reinforced concrete slab. These were prefabricated on shore and put in place using balanced cantilever methods. There are two high twin leg, reinforced and pre-stressed concrete pylons carrying 240 cables which support the bridge deck rising to a height of 149 m above the river bed or 101 m above the bridge deck. Cable vibrations were experienced during construction and secondary cables were added to eliminate this. To avoid detracting from the aesthetics of the primary cables, the secondary cables are very slender and are not very noticeable.

During the summer of 1994 the bridge deck launching gantry fell onto the Gwent viaduct, causing the 200 tonne bridge deck unit to fall onto the deck below. Although this delayed the project by ten weeks, the builders were nevertheless able to complete the bridge on schedule two years later.

Environmental impact
Prior to the bridge's construction, environmentalists raised numerous concerns and criticisms, chiefly about the immediate damage from construction work and the effects of long-term pollution from a projected increase in car traffic.

The crossing passes over mudflats in the Severn Estuary with part of the eastern approach viaduct sited on the English Stones, a rocky outcrop uncovered at low tide. The estuary wetlands are home to migrating birds such as the Ringed Plover, Redshank and Whimbrel, while the Eurasian Curlew, Dunlin and Grey Plover winter in the area. The birds feed on ragworm, lugworm and other invertebrates. Saltmarsh is found along the fringes of the coast. Beds of eelgrass occur on the more sheltered mud and sand-banks. In 1976, in recognition of the importance of the estuary as a wetland used by migrating birds, an area of 247 square kilometres was designated a Ramsar site. In 1988 the Severn Estuary was designated a Special Protection Area. The same year an area of 99 square kilometres was notified as a SSSI designated as such the following year; in 1995, it was extended to cover the whole of the Ramsar site.

The construction process resulted in a temporary increase in turbidity of the Severn’s waters. Although eelgrass is reasonably tolerant to short-term high turbidity and consequent loss of light, the bed of eelgrass in the Severn was observed to greatly decline during the period of construction. The construction of the approach roads and toll plaza resulted in the permanent loss of some wet pasture land.

Field surveys carried out in 2003 and 2004 reported that the bridge had little effect on the flight paths of the various birds – it was noted that when Curlew, Dunlin and Lapwing approached the bridge, they would change course to gain height and fly over the bridge, but that Oystercatchers and Turnstone would fly under the bridge. The surveys also showed that the bridge had minimal impact on the roosting habits of most birds, though a flock of Mallard were seen to be roosting directly under the bridge with many positioning themselves on the dry concrete base on one of the pillars. Likewise, it was noted that the bridge had little impact on the birds' feeding habits – Turnstone and Lapwing were recorded as feeding on mudflats directly under the bridge while the absence of other waders was attributed to the rocky nature of the foreshore.

Finances
The consortium consisting of two civil engineering firms and two banks that funded and built the bridge have a 30-year concession to redeem their outlay from tolls collected from users of the bridge. At the end of the concession period, the bridge will pass into public ownership.

Tolls
Tolls are collected from west-bound traffic near Rogiet, some 2.1 mi from the Welsh portal of the bridge. Tolls can be paid by cash, major credit or debit cards, or by use of the Severn TAG system, which is a wireless electronic toll collection system that does not require vehicles to stop. Cash can be paid at the manned booths or coins can be used at the automated coin gates.

Tolls charges are based on a three tier pricing system:

Motorcycles and UK disabled badge holders travelling in a vehicle are exempted from paying a toll, although these vehicles must stop at the toll booth to confirm their eligibility.

The toll prices are updated every year on 1 January in accordance with the Severn Bridges Act of 1992. In late 2008, UK VAT was reduced from 17.5% to 15%, but the bridge toll remained unchanged. For reasons of convenience the tolls are kept at round multiples of 10p (the bridge toll is largely paid in cash), but the 13p VAT reduction was not passed on immediately. In 2009, the planned toll for cars would have been £5.50, as the announcements listed, but the actual toll charged was £5.40.

Funding and ownership
The bridge was built at a cost of £380 million and is owned by the company Severn River Crossing Plc. As of, Severn River Crossing Plc was owned
 * 35% John Laing, British developer infrastructure operator
 * 35% Vinci, French concessions and construction company
 * 15% Bank of America, American multinational banking and financial services corporation
 * 15% Barclays Capital, British multinational investment bank

The company's 2011 annual report showed the same companies still owned, through subsidiary companies, all the issued ordinary share capital of the Plc.

Ownership of the crossing and the original Severn Bridge will return to the UK government when the project's required revenue, as defined in the Concession Agreement with the Secretary of State for Transport, has been collected. In 2010, the concession was expected to end in 2017. In 2012, changes were made to the agreement to reflect the effect of changes in VAT and Corporation Tax and the costs of installing credit card handling systems. The net effect was to increase the required revenue from £995.83 million to £1,028.91 million in 1989 prices.

Closures
On 6 February 2009, the bridge was closed by weather for the first time since its opening after three vehicles were struck by falling ice, damaging windscreens. Bad weather, and again falling ice, repeated this in December 2009. Temporary speed limits are put in place due to heavy wind or fog, with drivers informed of this by the electronic signs. Because of the more advanced aerodynamic design of the later bridge, the Second Crossing is far less prone to such restrictions caused by crosswinds than the first Severn Bridge.