Pluvial flood risk modelling, Fingal and East Meath, Ireland

The Fingal and East Meath region in Ireland have suffered significant flooding for more than a decade, resulting in considerable flood damage. In order to develop integrated and pro-active flood risk management, meeting the requirements of the EU Floods Directive, a Flood Risk Assessment and Management Study was undertaken for the Fingal and East Meath region. The study focused on urban areas known to have experienced flooding in the past and areas subject to significant development pressure both now and in the future.

Flood Risk Assessment and Management Study undertaken.

Flood Modeller’s 2D FAST solver was used due to its speed.

2m grid of LiDAR terrain data was used.

Key facts

Model results were mapped at a scale of 1:50,000.

The 2D FAST solver in Flood Modeller, which is designed to allow quick assessment of flooding using simplified hydraulics, was used as it was able to rapidly provide results when compared to traditional 2D approaches.

Use of the 2D FAST solver to simulate pluvial flooding involved the following three stages:

Stage 1 automated pre-processing of digital terrain data - to identify topographic depressions where water may pond and the flood pathways which connect them. Elevation-storage relationships are calculated for the depressions.

Stage 2 rainfall applied and routed between depressions - taking account of the elevation-storage relationships, the higher ground surrounding depressions, and the flood water already within adjacent depressions. 

Stage 3 post-processing of results to generate flood depths by subtracting ground levels from calculated water levels.

For this study, a 2m grid of LiDAR terrain data was used, both to generate the elevation-storage relationships and for post-processing; producing a pluvial flood depth dataset on a 2m grid. A single representative extreme rainfall depth of 50mm was applied to the study area. The model set-up and simulation was extremely quick to undertake which was only possible when using the 2D FAST solver. 

The model results were mapped at a scale of 1:50,000 with pluvial flood depths shown colour-coded to enable easy identification of shallow and deeper flooding. These maps were then used as part of a consultation workshop with Fingal County Council, Meath County Council and the Office of Public Work. The maps were compared with a database of historic flooding and with the local knowledge of the workshop attendees. 

A workshop provided valuable feedback confirming that the pluvial flood maps were, in general, representative of expectations and knowledge of the area whilst also identifying additional areas of potential pluvial flood risk. 

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