Appendix C - Assessment of Corridor B
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APPENDIX C - ASSESSMENT OF CORRIDOR B C1 DESCRIPTION OF CORRIDOR Corridor B is the second of the options to be assessed (taken in order from the west). It is shown in Drawing Number. 49550/G/02, and an extract is given below showing the corridor. It is defined at its western boundary by the oil pipeline which crosses the Firth between Bo’ness and Torry Bay. The eastern boundary of this corridor is formed by the gas pipelines which cross the Firth between the area west of Blackness Bay and Ironmill Bay ©Crown copyright. All rights reserved. License Number 100019139 C2 INFLUENCE OF GEOLOGY East of Grangemouth, between Preston Island to the north and Bo'ness to the south, it is suggested that rockhead plunges to an anticipated depth of 190m in the middle of the Forth. The maximum depth to rockhead is anticipated to decrease eastwards, although conjectural depths have not been mapped. The log of one borehole in Corridor B, near the northern edge of the current shipping channel, was extended to over 100m without reaching rockhead. This suggests that in this area there is generally a considerable depth of sediment. In this corridor the overlying sediments are also classified as ranging from fair to unpredictable. In the absence of more detailed information, the sediments are judged unsuitable as founding strata for the main towers of a large suspension bridge. A cross section of the Firth of Forth within this corridor has been prepared to illustrate the approximate geology. This has been done using available information and is shown in Figure C.1 below. Figure: C.1 Geological Cross-Section at Alignment BB1 C3 TRANSPORT PLANNING A test of Corridor B’s operational performance has been undertaken using the TMfS. This test is representative of both potential tunnel and bridge options in this corridor. In this test the crossing is connected to the M9 at Junction 3 (Linlithgow). On the north side of the Forth, it connects to the A985 to the west of Cairneyhill. At this point options are available for the subsequent connection to the M90 corridor. Two options have been considered for the connection to the strategic road network in Fife. The first involves upgrading the A985 eastwards to the junction with the A823(M) as was assumed for Corridor A. The other option creates a new bypass around the north of Dunfermline joining the M90 at Hill of Beith. For the purposes of the modelling tests the former option was used as it was considered that this would meet the demands of the expected users better. This test has been run in two different scenarios. The first assumes that the new crossing is simply added to the existing network and, working with the existing Forth Road Bridge, there are, consequently, two crossings available to vehicles. The performance of Corridor B in this instance is relatively poor. Although this corridor is closer to the existing Forth Road Bridge than Corridor A, it is still some distance from the existing Forth Road Bridge. Once again the results from the TMfS reflect this. In the first model scenario around eight per cent of traffic diverts from the existing Forth Road Bridge in each of the three forecast years. The second scenario modelled assumes that the existing Forth Road Bridge is closed to all traffic and therefore only the new crossing is available for cross Forth trips. This latter case is representative of the situation that might exist should the existing bridge be closed for maintenance purposes. This test has been run for 2012 only. In the scenario when only this new corridor is available, there is an increase of one per cent total daily vehicle hours and four per cent increase in daily vehicle kilometres across the network. This increase is unsurprising given the extra distance that all vehicles are forced to travel and the additional time incurred as a consequence. It is considered, as a consequence of these results, that Corridor B would have little value in providing support to the Forth Road Bridge during periods of major maintenance. As such it would not satisfy some of the key objectives identified for the study. The origins of southbound peak hour traffic on the existing bridge showed that 19 per cent came from the M90 north of Junction 3 (Halbeath) 23 per cent came from the A92 East Fife Distributor Road, 29 per cent came from Dunfermline town, 20 per cent came from the south Fife coastal routes and five per cent from Rosyth. More importantly the destinations of this traffic saw only three per cent heading for the M9 corridor and 19 per cent for the M8 corridor. It is therefore not surprising that this corridor does not adequately cater for traffic movements. With both crossings available (the first scenario) the daily traffic flow on the Forth Road Bridge is envisaged to be around 67,000 in 2012, rising to 74,000 in 2022. Given these levels of increase, it is clear that the objective of maintaining cross Forth transport links to at least the level of service offered in 2006 would not be met should a new crossing be provided in Corridor B. The increase in total distance travelled and extra travel time incurred during closure of the Forth Road Bridge would also result in additional economic costs. In addition, there would be consequential environmental impacts resulting from the additional distance travelled. Furthermore, although this corridor is closer to the Forth Road Bridge and the Forth Bridge than Corridor A, it is still remote from the main public transport routes across the Forth. The opportunity to integrate enhanced public transport services into a new crossing on Corridor B would be reduced as a consequence. There would be little prospect of new LRT modes being usefully incorporated into a crossing in this corridor. As with all the corridors public transport services could be given priority on the existing Forth Road Bridge. In summary, therefore, Corridor B performs poorly against the transport planning objectives for this study. It is not recommended for further consideration. C4 BRIDGE CROSSING OPTIONS C4.1 Detailed Summary of Constraints Along the north shore within Corridor B, the Fife coast is dominated by large areas of mudflats extending east from Preston Island through Torry Bay. This area is a SSSI and also a SPA. Given their designations, these environmental features represent a major constraint to construction. A firing range protected area adjacent to Crombie Jetty is also located on the north shore. This extends well into the Firth and areas of land in possession by the MOD. These areas also represent a major constraint to construction. Along the south shore, the town of Bo’ness occupies the potential landing sites in the western part of the corridor. East of Bo’ness, the mudflats which exist along the shore are designated as a SSSI and a SPA. Immediately inland of Bo’ness and the mudflats are several Scheduled Ancient Monuments (SAMs) including the Roman Fort at Carriden, Muirhouses Roman Temporary Camp and Carriden Fort. These mark parts of the Antonine Wall, which runs east-west through the area. It is possible that the Antonine Wall will be listed as a World Heritage site, with consequent changes in its status and protection. The combination of urban, environmental and historic buildings represent major constraints to construction in the area to the southern end of Corridor B. Within the Firth, the main navigational channel is located closer to the north shore. Information received from the Forth Ports PLC indicates that the width of the channel in this area is 450m. Within the navigation channel the water depth is a maximum of 14 metres. From admiralty charts, it can be seen that there are six mooring circles located near the middle of the Firth. These represent potential constraints to any crossing in this location. The location of the oil and gas pipelines around the line of Corridor B has been determined using the current admiralty charts. The accuracy of the locations of these pipelines is not absolutely known and it has been assumed that constructing foundations close to the pipelines would not be permitted due to the high risk of damage. Therefore, these pipelines are considered to be major constraints on the location of a bridge crossing in this corridor. C4.2 Bridge Option (Refer to Drawing 49550/B/02) Several bridge alignments have been reviewed to determine if it is possible to construct a bridge crossing within Corridor B. The alignment was initially selected as it represented a relatively short crossing of the Firth. It had the advantage of avoiding the eastern extremity of Bo’ness. This alignment also had a minimum likely impact on the SSSI and SPAs on both sides of the Firth. On the southern landfall, the alignment of the bridge approach viaduct is located between Muirhouses and the Fort at Carriden. At the north landfall the crossing passes close to Crombie Point. Although the alignment avoids the SAMs and minimises the impact on the SSSI and SPA, it was considered that the environmental impact during construction and operation would not be acceptable due to the damage that would occur. It is possible to provide curved alignments for the approach viaducts such that the viaduct avoids the SAMs but this will increase the complexity and cost. Possible schemes for the bridge superstructure were initially considered on the assumption that ground conditions would be adequate to support piers and other structural elements. Following consideration, it was concluded that a possible construction form for this alignment would consist of a suspension bridge with a main span of approximately 1500m.