Llanfairfechan and the Kinmel Bay area have been flooded in recent decades, with coastal defences overwhelmed. WHS studied the risks for the future.

Please click here to watch a video of the flooding of Towyn, North Wales in 1990:

Conwy County Borough Council (CCBC) commissioned WHS to assess both Llanfairfechan and The Kinmel Bay area.

WHS Study Methods

The complex study combined detailed offshore wave calculations with floodplain characteristics to provide a comprehensive understanding of coastal flood risk and probability mapping of coastal flooding at the two locations.

We calculated wave overtopping of coastal defences in three stages, to provide an inflow boundary into the floodplain:

• analysing offshore joint probability accounting for wave and sea-level data, wind speed and direction, bathymetry and tides
• transposing wave conditions from offshore to near-shore using the SWAN wave transformation model, which accounts for physical processes such as refraction, shoaling and wave breaking
• determining wave overtopping rates for each section of defence using EuroTop and EuroTop II manuals.

We then constructed TUFLOW floodplain models to determine the impact of wave overtopping in both study areas. The models took into account:

• coastal defences and structures on the floodplain such as underpasses, culverts and buildings
• discrete sections of defence to allow for variations over higher or lower sections of sea wall and different types of sea wall
• breach locations to allow for sections of defence collapsing partially under extreme conditions.

The study outcomes

• Kinmel Bay – This study provided a valuable update to the 2004 flood maps and allowed for a greater understanding of flooding, in particular key flow routes through the floodplain and latest climate change allowances.
• Llanfairfechan – We used the modelling data to undertake a cost-benefit analysis for spending on the town’s flood defences. While the analysis showed limited benefits for present-day flooding events, the influence of climate change on overtopping rates risks significantly increased flooding, justifying defence works.

The next steps will be to prioritise flood-risk spending in the area using the recommendations gleaned from these studies.

The last word

“CCBC have recently employed WHS on several flood investigation, coastal improvement and tidal flood risk assessment projects and have found the team extremely helpful in the delivery of the schemes. Their outputs have been used to support our applications to the Welsh Government and we would have no hesitation in employing the WHS team again on similar projects.”

Principal Engineer, CCBC