Through a holistic analysis of all flood risk mechanisms using hydraulic modelling, we have designed a scheme that combines natural flood risk management measures with minimal hard engineering in Llanmaes to provide a sustainable solution to the village’s frequent flooding problems.
Llanmaes sits in a catchment with a very high surface water runoff due to underlying strata of hard rock being very close to the existing surface. As a result of reduced permeability within the catchment, a greater percentage of overland flow reaches the rural village of Llanmaes than would otherwise be anticipated compared to a catchment with a greater capacity to absorb rainfall. As such, the village has a history of flooding from rapid response surface water runoff which regularly exceeds the capacity of the existing drainage network, with flood events causing varying degrees of disruption, including widespread damage to properties and closure of highways to vehicles and pedestrians.
Previous proposals for a solution to overland flooding considered hard engineering solutions close to the locations where the impacts of overland surface water flooding were felt greatest, however, these also presented a number of significant financial and constructability risks. We undertook a review of flood risk mitigations options to determine if a more bespoke and holistic solution could be achieved.
Following a review of the existing proposals and additional refinement of optioneering through hydraulic modelling, it became apparent that the most effective way to reduce flooding was to intercept overland flow as close to the source as possible, upstream of the village, using natural flood management techniques attenuating flow and routing it carefully away from the key population areas of Llanmaes. The new preferred option also eliminated the need for a large culvert under the primary highway, and in doing so avoiding unnecessary significant costs and major disruption to the community and underground utilities.
The detailed design is still underway and although overland flow cannot be eliminated in its entirety throughout the catchment, the upstream contributing region which previously conveyed overland flow through the heart of Llanmaes will become significantly reduced as a result of the proposed option. As such, any residual overland flow which does reach Llanmaes will be carefully encouraged away from key risk receptors through the provision of highway improvements and minor targeted flood wall protection.