Highways England’s shortlisted routes for the proposed Lower Thames Crossing could potentially increase the level of flood risk, according to an environmental appraisal which forms part of a number of background documents published at the start of a public consultation this week.

In 2013, two locations were shortlisted for a new bridge or tunnel across the river: one near the existing Dartford Crossing (known as Option A) and the other linking the M2 with the M25 via the A13 (known as Option C), with a possible further link to the M20 (Option C Variant).

Since then, Highways England has been carrying out detailed work with a wide range of stakeholders to assess the shortlisted options and develop possible routes at each location. Highways England has been considering a number of design options including a bridge crossing over the River Thames, a bored tunnel and an immersed tunnel.

The evaluation is now complete, and Highways England is recommending a new road crossing at location C through a bored tunnel. At Location C, three potential route options have been identified north of the river in Essex and two south of the river in Kent.

The proposed scheme would run from the end of the M2, crossing the river just east of Gravesend and Tilbury and joining the M25 between junctions 29 and 30. It will be the first new crossing of the Thames east of London since the Queen Elizabeth II bridge opened at Dartford 25 years ago.

The Pre-Consultation Scheme Assessment Report (SAR) published by Highways England brings together the engineering, safety, operational, traffic, economic, social and environmental appraisal of the shortlist routes for the Lower Thames Crossing.

Bored tunnel crossing option must not compromise TE2100 River Thames flood defence plans

Commenting on the bored tunnel option, the report says the main impacts would be during the construction phase. A completed tunnel would not impact the marine environment and the coastal/ terrestrial impacts would be greatly reduced in comparison to the construction of a bridge (where permanent effects for example from loss of habitat and shading effects could occur) or immersed tunnel (with very large impacts on habitats and species during construction).

A bored tunnel crossing option would also require a design that does not compromise Thames Estuary (TE2100) River Thames flood defence plans, including any defences along the Mardyke,  although it would have little impact on the River Thames itself.  During detailed design there would need to be consideration of piling on the flood defences.  A bored tunnel would also be at a higher risk of route closure due to high flood levels than a bridge option as the consequence of inundation of the tunnel portals which are located in the defended floodplain, the report says.  Again, the impacts would need to be addressed in the design of the crossing.

The report has also flagged a number of other significant issues related to the water environment, including adverse habitation and biodiversity impacts and the status of Water Framework Directive bodies.

The report also highlights a range of issues which will need to be considered by organisations charged with flood defence responsibilities, including the Environment Agency, Local Authorities and Internal Drainage Boards.

Water Framework Directive assessment required for all options

Commenting on Water Framework Directive status, the report says a WFD assessment would be required for all the options due to the potential for direct effects on biological, chemical and physical WFD parameters for both surface waters and WFD groundwater bodies. However, with appropriate mitigation, it is not anticipated that the impacts on the Mardyke or groundwater bodies would lead to a reduction in WFD status or would prevent the water bodies reaching good status or potential in the future. The appraisal has generally assumed that the target 2027 status of good applies, even though current status of most water bodies is poor.

Surface water drainage will need to be agreed with Lead Local Flood Risk Authorities

The report says surface water drainage strategy/ design (in accordance with Highways England guidance and standards) will also need to be agreed with the relevant Lead Local Flood Risk Authorities.

It also flags up the potential for the road design to act to reduce local flood risk, for example, by providing attenuation of road drainage, providing flood storage directly upstream of the road.

Bored tunnel is only viable alternative

The report concludes that the appraisal has demonstrated the risk of significant effects to European Sites with both the bridge and the immersed tunnel options. In this case a bored tunnel is the only viable alternative as it meets the scheme objectives and is the least damaging alternative - a conclusion which has been supported by advice provided by Counsel.

A WFD assessment would be required due to the potential for direct effects on biological, chemical and physical WFD parameters for both surface and WFD groundwater bodies. With appropriate mitigation, it is not anticipated that the River Thames crossing or impacts on the groundwater would lead to a reduction in WFD status or would prevent these water bodies reaching good status or potential in the future.

In relation to the bored tunnel option, the report says there would be no direct impact on the surface water environment of the River Thames. However,  a WFD assessment would be required to ensure assessment of any effects of changes in groundwater on WFD compliance. A tunnel option could also be at a higher risk of inundation due to high flood levels  i.e. through breach or overtopping of existing defences.

The report also says there are risks of “significant adverse effects” on the sites as a result of all options although they are greater with the bridge and immersed tunnel and more likely to be mitigated with the bored tunnel. In particular, for Location C Routes 2, 3 and 4, a bored tunnel crossing is the only option that does not directly affect the Thames Estuary and Marshes Ramsar site.

Article 6 (4) of the Habitats Directive states that where an Appropriate Assessment has been carried out and results in a negative assessment (where adverse effects on a European site (s) cannot be ruled out, despite mitigation measures), consent can only be granted if:

• there are no alternative solutions
• there are Imperative Reasons of Overriding Public Interest (IROPI)
• and compensatory measures have been secured.

Any alternatives put forward must be legally and technically feasible, including consideration of physical planning and timing considerations. The report also says:

“Greater cost or inconvenience are not necessarily reasons to rule an alternative out, although there will come a point where an alternative would be so much more expensive or inconvenient that it would be unreasonable to pursue it, at which point it could be rejected.”

There is no definitive guidance on what would constitute unreasonable additional cost or inconvenience.

In view of this the Lower Thames Crossing team had to consider which would be the least damaging alternative and as part of this process sought Counsel’s advice on the application of the Habitats Directive to the scheme and the selection of the proposed crossing type.

On the basis of the appraisal results and the subsequent advice from Counsel it was concluded that of the three crossing types under consideration, a bored tunnel, both at Location A and Location C, would be the least damaging options in terms of impacts on European Sites.

However, a crossing at Location A would perform poorly against a number of the scheme objectives and could therefore not be taken forward and be considered a viable alternative.

Therefore, of the crossing types at Location C it was determined that the bored tunnel would be the least damaging alternative based upon the assessment work completed to date.

"Bored tunnel faces least risk of being refused consent"

For this reason, it was determined that the crossing option at least risk of being refused consent in the context of the Habitats Directive was the bored tunnel and this was supported by Counsel.

The report concludes that in summary, a bored tunnel at Location C represents the only viable alternative that meets the scheme objectives and for which there are a wider and more practical array of mitigation measures that would increase likelihood of compliance with the Habitats Directive.

The report says that all three routes have the potential to unlock opportunities for housing and jobs and all offer high value for money. They each meet the transport objectives, although they offer different opportunities to connect with local roads.

“While there are important differences in the local and environmental impacts of each option, it is considered that all three routes are viable and should be taken forward to consultation.” the report concludes.

“Tunnel rather than bridge will minimise impacts on the environment”

Commenting on the proposed options, Highways England senior project manager, Martin Potts said:

“Deciding where the new crossing should go is a vitally important decision, and we’ve been working hard to identify solutions that strike the best balance between improving journeys, getting value for money and managing environmental impact. Our assessments have shown that Location C provides double the economic benefits of Location A as well as a clear alternative route to the Dartford Crossing, reducing congestion and improving resilience of the road network. And by choosing a tunnel rather than a bridge we can minimise the effects of the new road on the environment.”

“There are important choices to be made. As well as inviting comments from the public about our recommendations, we have identified three routes for the new road to the north of the river and two routes south of the river. We welcome views on them all.”

There will be 24 public exhibitions, held at venues across Kent and Essex. All responses will be taken into consideration before a final decision is made by the Government later this year.

According to the Government, once complete the new crossing could add over £7 billion to the economy by increasing investment and business opportunities, and create over 5,000 new jobs nationally. The scheme would cost between £4.3 billion and £5.9bn, with funding to be provided by the Government.

Deadline for submission of responses is Thursday 24 March – click here to access the consultation.

Click here to download PRE-CONSULTATION SCHEME ASSESSMENT REPORT – ENVIRONMENTAL APPRAISAL

Waterbriefing is media partner with the major Environment Agency Flood and Coast 2016 conference and exhibition in Telford from 23rd-25th February 2016. Click here for more information