Nature Based Solutions - Potential Site Identification - Floodplain Roughening

Abstract: The floodplain roughening layer identifies areas of floodplain which, based on land cover type, have low Manning’s n values. Land cover types with low Manning’s’ n values have a lower resistance to surface runoff than others, and are thus potentially suitable for floodplain roughening. Floodplain roughening can extend floodplain storage times by slowing flood waters attempting to re-enter the main channel. Lineage: For the purpose of this anaylsis, floodplains are defined as areas within the CFRAM and NIFM Mid-Range Future Scenario (MRFS) 1% AEP fluvial flood extents. GIS Layers from The National Land Cover Map for Ireland (NLC 2018) (Tailte Éireann and the Environmental Protection Agency, 2018) were assigned Manning’s n values as shown in Appendix B (Table 5) of the report. Urban extents (Tailte Éireann, 2022) were excluded. Areas with lower Manning’s n values were overlain with the floodplains and classified as areas suitable for floodplain roughening. 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.