Nature Based Solutions - Potential Site Identification - Run off Reduction

Abstract: Surface runoff is the flow of water overland outside a stream or river channel. This map shows areas which have been identified as being sensitive to run off i.e. areas where rainfall generates a high proportion of surface runoff rather than attenuation or infiltration. NbS measures can be implemented in these areas to reduce and/or slow down run off and thereby increasing the lag-time between rainfall events and resulting in–stream peak water level.

Lineage: The method used is based on the Environment Agency’s (UK) method for identifying areas for run off reduction in England and Wales (EA, 2008). SEPA (Scotland) has also adopted this method (SEPA, 2015). The method considers the PCDs that have the greatest influence on runoff sensitivity, which are land cover, average annual rainfall, soil drainage class, and slope and assigns a sensitivity score to each of them (see Appendix A). Combining the sensitivity scores using Equation 1, below, allows a given area to be assigned a run off sensitivity score, thereby identifying areas where run off reduction measures would have the greatest impact in flood risk management. Areas that have the potential for run off reduction measures are mapped as either ‘High Opportunity’ or ‘Medium Opportunity’. The country is divided into 250m x 250m grid squares and each square is assigned a combined run off sensitivity score by adding the 4 sensitivity scores together. Areas with a combined sensitivity score of 4 are classified as having ‘High potential’ and areas with a score of 3 are classified as having ‘Medium potential’.

Purpose: The Climate Adaptation and Strategic Assessment (CASA) Section of the OPW is responsible for the development of nature based solutions (NbS) in flood risk management. This project is the OPW’s first nation-wide attempt to map potential areas of opportunity for the implementation of NbS to mitigate against flooding in Ireland. The maps offer a useful preliminary indication of NbS suitability at local and catchment level.