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Defending Poole Harbour

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Some people say that Poole Harbour is the second largest natural harbour in the world. Whether this is true or not, the harbour is clearly a lot bigger than most people would imagine. Although the overall tidal range is relatively small by UK standards, a large area of the harbour is drained at low water, revealing salt marshes and a rich array of mud flat ecosystems.

Spanning Poole Bay from east to west is a 1.4km, two tracked rail causeway, formed from a two-metre-high embankment. To enable tidal water to flow north to south, there is a three-span structure called Creekmoor Viaduct toward the east end of the causeway and another 3 spanned structure, Upton Viaduct, toward the west end of the causeway. The causeway itself dates back to 1842.

The Network Rail causeway crosses to the south of Pergins Island, a local nature reserve. Upton Lake and Creekmoor Lake, to the south-west and south-east respectively, form a southern harbour area known as Holes Bay.

For inspection purposes, the causeway is divided into six CERDs (Coastal Estuarine River Defence assets). They are, in fact, earthwork embankments which are protected on the southern face with pitching stone and mass concrete where most exposed to the tidal conditions. However, on the northern face within the bay, they are unprotected.

The surrounding land is very low lying and level, with very shallow slope angles typical of a brackish tidal basin. Tides are influenced by the Isle of Wight and a double tide is notable on the ebb, giving rise to a unique drainage system.

Challenging access

This means that the high tide periods last longer than the low tide periods. As a consequence, gaining access and carrying out maintenance and other construction work at low tide becomes far more challenging than experienced elsewhere in the UK. As would be expected at such a location, many repairs have been conducted over the years, including localised placement of rock armour, sprayed concrete render and installation of concrete wave-walls at the crest of the causeway in more exposed locations.

In January 2014, Bridgezone (now Xeiad) carried out a cyclic detailed inspection of the causeway on behalf of Network Rail. Gavin Baecke, senior asset engineer (structures) for the Network Rail Wessex route, explained that a substantial number of defects were identified, including voids that were penetrating into the embankment by up to two metres.

In addition, certain lengths of the structure were being subjected to locally generated wind-waves, creating scour and lowering of the foreshore that exposed the toe of the structure as well as the stone pitching. In the past, these areas had been repaired using sprayed concrete but these defects were deteriorating year-on-year and, therefore, it was decided that significant repair works were required.

Impact on the environment

Other sections of the causeway, that are less exposed to wave action because of higher foreshore levels, were heavily vegetated, with large trees and bushes growing which could undermine the stability of the embankments. Therefore, ahead of any major works, isolated trees had to be removed and the vegetation cut back so that the structure could be surveyed fully.

As this is a Site of Special Scientific Interest (SSSI) and a Wetland of International Importance (Ramsar convention on wetlands), Gavin pointed out that any planned works would have to be considered alongside their potential environmental impacts both during the repair works and for the long-term.

As a consequence, a detailed Environmental Impact Assessment had to be compiled for the Marine Management Organisation (MMO), including impact and mitigation measures for each stage of the repair work.

WSP, which is based in Exeter, was appointed as consultant to develop and review a wide range of engineering options taking into account environmental, engineering and economic merits. A short-list of options was compiled for Network Rail, presenting a series of possible approaches. Gavin explained that this piece of work, developed by WSP working closely with Network Rail and its framework contractor Osborne, has proved to be invaluable because they have now collectively developed a risk grading system on a scale of 1 to 5, which enables Network Rail to prioritise the work that needs to be carried out, with 5 being the highest priority.

Invaluable coastal expertise

The additional value that WSP injected into this risk-prioritising process was to offer knowledge about the coastal erosion generally alongside the impact that climate change is having on such locations. For example, without being too scaremongering what will the coast look like around Poole in years to come? Could the whole town be exposed to flooding? Farfetched you might say – but is it?

One option being considered is to work with the local council on an initiative to protect the harbour by developing a salt marsh regeneration programme designed to be environmentally friendly and to provide natural protection to the coastal area and to the embankments of the railway causeway.

With regard to the shopping list of work required to address the defects identified as Grades 4 and 5, the ‘One Team Wessex’ project delivery team, formed of Network Rail and Osborne with, on this occasion, WSP as the designer, developed the most productive and efficient way of carrying out the work over the four-day Easter period and for two days on the following weekend.

The approach they adopted was efficient, low-key and very successful. Two road-rail vehicles were hired for the duration of the work. They organised two, ten-hour shifts that would overlap by two hours, thus minimizing night-time work and, therefore, the associated risk that such working could present in such a potentially hazardous environment.

Creating flexible possessions

30 workers were allocated to each shift and a depot was established alongside the sidings at Poole station, about 500 metres from the causeway. The possession at night was from the station and across the causeway. This meant that the road-rail vehicles could move materials and equipment up to the causeway, creating worksite stockpiles.

Once everything was in place, the possession was shortened so that it only extended across the causeway. This meant that Poole station could then be fully operational for trains to run to Bournemouth and beyond, thus minimising disruption to passengers and the train operator – ‘One Team Wessex’ in action!

The Network Rail project manager for the work was Stuart Davis. He was clearly very proud that a team approach was adopted as this, and the form of contract adopted, enabled the scope of work to be completed, and emerging defects rectified, whilst ensuring that final costs were kept within budget.

The view was that this incentivised approach ensured that they were not only able to address all of the Grade 4 and 5 categories of work but also a significant amount of additional repair work as well. There were no accidents, either to people or the environment, which was pleasing given the location.

The work carried out on this isolated tidal location, with its intermittent rail and boat access included, the following:

  • Repairs to areas of mass concrete where the rendering had broken away and was missing;
  • Installation of stone masonry sets where the existing sets had been washed away, exposing the vulnerable embankment core;
  • Filling voids behind many of the masonry stone sets;
  • Repairing the revetment slopes around the abutments of the two viaducts that were in poor condition – material had been washed away causing the revetments to slump and hollow out so that they were no longer offering the protection that they were designed for;
  • Repairing and replacing loose flagstones offering toe protection to the embankment and mass concrete installed to the toe of the embankment, especially in the area of the two viaducts;
  • Placing of grout bags in washout areas, especially in those vulnerable areas where the embankment elevation is quite low;
  • Placing Portland Stone rock armour where areas of unprotected embankment toe had been damaged by scour;
  • Removal of trees and tree roots that had created weaknesses in the embankment shoulder;
  • Resetting masonry sets that have become dislodged by tide and vegetation.

Over this short period of intense work, 20 tonnes of limestone blocks were installed plus 840 Rock Rolls, 180 tonnes of granular fill and 390 tonnes of gabion rock.

Talking to Gavin, Stuart and their colleagues gave a reminder that, not only is Network Rail responsible for a significant number of high profile coastal sites, but it also has a significant mileage of lower-profile coastal railway that requires significant care and understanding.

It is also a reminder that, just as the industry needs to keep abreast with current technology for signalling, track and rolling stock, railway engineers also need to understand the potential harm that climate change can cause to the railways in years to come. Projects such as Poole Harbour’s Salt Marsh Regeneration Scheme should be essential reading and understanding for all concerned.

Written by Collin Carr


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