Borth Y Gest – Flood Alleviation Scheme
|Client:||Welsh Government / Gwynedd Council|
|Completion Date:||October 2018|
Borth y Gest is a small coastal community near Porthmadog, Gwynedd. The village is a popular tourist destination during the summer months which also includes a significant number of people utilising the Coastal Path that follows the bay. Over recent years Both y Gest suffered a number of flooding events. The floods were a result of the sea breaching the footpath and flowing along the highway and into both residential and commercial properties.
The initial stages of the project was to carry out a Project Appraisal Report for the village, this included identifying the source and pathway of the flooding, development of potential options, risks, costs and hydraulic modelling of the bay.
Public engagement was also undertaken with local residents, local councillors, Natural Resources Wales and Gwynedd County Council.
Both wave over topping and hydraulic modelling was carried out to inform the design height of the new wall. The standard of protection post construction would be up to and including a 1 in 100 year event. The design of the walls was such that if the wall height required increasing in future years in line with climate change estimations and sea level rise, that the foundations would be able to accommodate this. As the bay of Borth y Gest is a designated SSSI, detailed environmental assessments and strict working methods (including relevant consents and licences) were included within the works.
The scheme consists of ~170m new sea wall, extending the existing main wall in the centre of the bay to encompass the bay of Borth y Gest. YGC designed the sea defence to join onto the main wall that has existed in the bay since the 1800s. The new wall was designed to be able to withstand hydrostatic pressures from the sea and wave action.
The walls were constructed from reinforced concrete clad in local slate (from Blaenau Ffestiniog). The walls were cast in situ using bespoke formwork for each section of the wall. The wall also included two flood gates at the slipway and pedestrian access, along the wall there are a number of drainage holes with non-return valves through the wall to allow any surface water to be drained away.
One of the key elements of the wall design was to ensure that it was in fitting with the local area and blended with the existing wall. This was achieved by using local slate to clad the wall. Included within the design was additional features, these included two built-in benches and three slate information plaques.
In combination with the wall extensions to protect against tidal flooding, a non-return gate was also installed on the inlet of the Afon Llety watercourse. This was to provide protection to the village from tidal waters backing up through the culvert and posing a flooding risk.