Agenda item

To consider a joint report by the Council’s Corporate Director:  Environment & Economy, the Head of Highways & Environmental Services, and the Risk & Asset Manager (copy attached) which seeks the Committee’s feedback to help inform the next steps in relation to the project to replace Pont Llannerch.

11am – 12pm

The Lead Member for Environment & Transport alongside the Corporate Director: Environment & Economy, the Head of Highways & Environmental Services, the Risk & Asset Manager and the Senior Engineer - Bridges & Construction presented the Committee with a report (previously circulated) updating members on the Pont Llannerch project.

Due to the complexities involved with this specific subject the Head of Highways & Environmental Services guided members through the report's contents.  He also welcomed Dŵr Cymru Welsh Water’s Head of Water Production to the meeting to answer the Committee’s questions on their operations in the bridge’s vicinity and how any proposals may impact on those operations. 

Following the collapse of Pont Llannerch during Storm Christoph in January 2021, Highways & Environmental Services has been working on a project to build a replacement bridge. The project was split into three stages: Optioneering, Detailed Design, and Construction. It had now reached the end of the Detailed Design stage. The detailed design stage had been complicated and lengthy and has raised significant challenges. The main challenge being the consideration of the foundations required for a new bridge. This discussion had been complicated because Pont Llannerch was located above a freshwater aquifer, which was within a layer of weathered sandstone, and because Dŵr Cymru Welsh Water (DCWW) had a freshwater abstraction site situated directly next to where the old bridge was located. This aquifer and freshwater abstraction site provided water to around 85,000 homes in the region, therefore, the freshwater aquifer and water abstraction assets were extremely important to many DCWW customers. Consequently, the Council would need to ensure that it did not compromise these assets when building a new bridge.

A preferred design using raft foundations had been chosen because raft foundations worked by distributing the load over large areas of ground. However, significant risks still existed because construction would require sheet piling – essentially drilling into the rock below where the foundations would sit. This drilling created a risk of compromising the assets. Provisional ground investigations at the pioneering stage determined that this weathered sandstone sat between 12 and 36 metres below ground level and extended as far upstream as Ruthin. Further ground investigations during the detailed design stage showed the water level to be as close as 10 metres beneath ground level. There was a standard for assessing the predicted scour depth of a bridge, and this showed that all 10m of the riverbed gravel in this location would likely erode in the future. Therefore, to protect a new bridge and to ensure it was not undermined, the newly constructed sheet pile curtain containing the new raft foundation would need to be drilled/driven into the area beneath where the foundations were placed, and this would penetrate the weathered section of sandstone in which the aquifer sits. Drilling into the aquifer could potentially compromise water quality at the DCWW abstraction point at Llannerch Park. Compromising the site would present a risk of supply loss to 85,000 DCWW customers. The project team had been working closely with DCWW throughout the Detailed Design Stage, and the DCWW view was that all risk assessments had failed to provide suitable evidence that the drilling work would not introduce risk to this critical asset.

DCWW has stated that drilling into the aquifer would ultimately create a pathway for the risk of water supply contamination, compromising the aquifer local to the abstraction point could lead to several factors related to risk and safeguarding for their customers. An introduction of significant risk associated with the water supply had the potential to introduce a public health risk with wide-reaching consequences. DCWW has also stated that rectifying an issue created by drilling into the ground would not be straightforward and would be extremely costly to resolve. DCWW had also indicated that it may not even be feasible to repair it if a physical pathway was driven into the aquifer. For these reasons, DCWW had assessed the bridge's construction to be a high-risk activity.

Members were also guided through a technical presentation illustrating further details regarding the project's design stage and all of the proposed designs for the bridge.

Responding to Members questions the Lead Member, Officers and DCWW’s representative – 

• clarified that due to the existence of the aquifer building a bridge of any size to accommodate vehicle traffic was not feasible without risk.
• confirmed that the concerns were not solely confined to the weight capacity of the bridge but to the potential velocity and power of the flood waters running underneath the bridge.  Due to these factors, a strong foundation would be needed regardless of the size and weight capacity of the bridge, it was the foundations that would cause risk for the aquifer.
• confirmed that the Council had a 10-year bridge maintenance plan in place with a view to safeguarding such structures.  As part of this plan restrictions had been placed on the stone bridge over the River Clwyd in to Rhuddlan.  However, this bridge was built on a rock foundation, which was not the same as the Llannerch bridge. 
• confirmed that the potential impact on the aquifer had not been fully assessed until the detailed design stage.
• advised that it was hard to quantify the impact of the loss of the bridge on local communities, however, roughly 1,600 vehicles were thought to use the road daily before the bridge was swept away.
• confirmed that the Council had strived to carry out as much correspondence with residents as possible throughout the whole design process for a potential replacement structure.
• emphasised that they fully understood the value the bridge had to residents and the potential of erecting a temporary bridge had been explored, but the risks would remain the same as it would for a permanent structure as both would require strong foundations.  The crux of the problem was the foundation and the potential risks that posed to the aquifer.
• confirmed that it would not be possible to erect a replacement bridge either a few miles up or downstream because the issue would remain as the aquifer ran along approximately 22km below the riverbed.
• highlighted the advice received from the Council’s insurers (paragraph 10 in the covering report) if it proceeded to build a replacement bridge which resulted in damage caused to the aquifer.
• provided assurances that, with a view to improving connectivity between the communities effected, improvement work on the current diversion routes was being considered with funding from the Welsh Government.

The Committee, following a comprehensive discussion:

Resolved:  to communicate to the Project Team and Cabinet -

(i)   that having considered the report and received the presentation summarising the detailed design stage, attached at Appendix A to the report, emphasised that its desire was similar to that of local residents to have a replacement bridge built to connect both communities and ease travel between them.  Nevertheless, based on the expert engineering and risk management advice received the delivery of this project did not seem viable as its construction would likely pose significant risk to critical infrastructure in the area which supplies water for circa 85,000 homes;

(ii) acknowledging the impact the loss of the bridge had on local communities and their residents that the Council should make every effort to reduce the impact on connectivity between the communities affected by improving other local transport links and highway routes in the area; and

(iii)                to confirm that, as part of its consideration of this matter, it had read, understood and taken account of the Well-being Impact Assessment, Appendix B to the report.