Fingal East Meath Flood Risk Assessment and Management Study

Hydraulic computer modelling

Hydraulic computer modelling involves the use of computer software to solve advanced mathematical equations, based on a variety of parameters, to provide an estimate on water levels, flows and velocities along a river system.

A total of 23 hydraulic river and estuary models are being developed for the watercourses in the FEM FRAMS study area using ISIS 1 dimensional (1D) and 2 dimensional (2D) computer modelling software. 1D models simulate the flow of water in one dimension only; along the direction of the river flow. 2D models simulate the flow of water in two dimensions: both along and across the floodplains.

Some of the parameters used by the computer model when solving mathematical equations include the dimensions of the river channel, estuaries and floodplains, flows in the river channel, coastal water levels and the physical make up of the river channel and floodplains, i.e. gravels, rocks, trees, and grass.

Information on the dimensions of the river channel and floodplain has been gathered as part of the FEM FRAMS topographical surveys. Flows in the river channel and coastal water levels have been assessed as part of the hydrological assessment. The physical make up of the river channel and floodplains is determined from site visits and photos.

Hydraulic model verification

In order to ensure the hydraulic models are working within acceptable limits, they are being checked against historic flood event data wherever data is available. This data, in the form of recorded rainfall, river flow and tidal levels, is being used in the hydraulic models to replicate the flooding which occurred in reality. If the hydraulic model is not performing within acceptable limits, the hydraulic computer model parameters are adjusted until a level of acceptable agreement between the model results and observed data is reached. Information on historical flood events is available from the OPW's National Flood Hazard Mapping website.

Hydraulic model outputs

Outputs from the hydraulic river models will provide data on water levels, flows, depths and velocities for a range of annual exceedance probability (AEP) events and will be available for each of the 19 watercourses. This data will be presented graphically in the form of flood maps.