Fingal East Meath Flood Risk Assessment and Management Study

Hydrology

Hydrology concerns the occurrence and movement of water in the environment. For assessing flood risk, we are particularly interested in the effects of surface water hydrology, which looks at the relationship between rainfall on the land surface and runoff into water bodies (streams, rivers and lakes).

The rate and quantity of water reaching a water body
depends on a number of factors:

duration and intensity of rainfall;
size of the catchment;
topography and geology of the catchment - catchments with steep slopes and impermeable rocks and soil will have a faster rate of surface water runoff;
land use - urban areas have lots of impermeable surfaces which causes the transfer of water to a water body more quickly than rural farm lands, and
previous weather - if the ground is already saturated, then any new rainfall will have a faster rate of run-off.

Hydrological analysis

The hydrological analysis for the FEM FRAM Study uses the scientific findings of the Flood Studies Report (NERC, 1975) and Flood Estimation Handbook (IoH, 1999) together with recorded river flow and rainfall data to estimate rates and quantities of river flows in the study area water bodies. These rivers flows will be inputted into hydraulic computer models of the rivers to generate water levels for a range of Annual Exceedence Probabilities (AEP).

The AEP is a statistical measurement indicating the likelihood of a flood event of a certain intensity occurring or being exceeded in any given year. Thus a 10% AEP describes a flood event which has a 10% chance of occurring or being exceeded in a given year. Design river flows will be produced for the 50%, 10%, 5%, 2%, 1%, 0.2% and 0.1% AEP. Design river flows will also be produced for future scenarios taking into consideration climate change and land use changes.

To ensure the correct focus for the hydrological analysis, the FEM FRAM study area is first divided into more than 24 individual river catchments. These river catchments are further divided into more than 300 smaller areas based on natural sub-catchments. These sub-catchments act like a funnel, collecting all the water within the area covered by the sub-catchment and routing it into the nearest water body.

The hydrological analysis is being carried out in three stages:

Stage 1 involves collecting information on historical flooding in the study area and sourcing river flow (hydrometric) data and rainfall (meteorological) data in the study area and neighbouring catchments. This data is available from gauging stations, the location of which are shown in the map at the following link.
Stage 2 involves a statistical analyses of the hydro-meteorological data to define design rainfall and river flows at the gauging stations; a joint probability analysis to determine the appropriate combinations of fluvial (river) and tidal events; and a review and analysis of historic floods in the study area.
Stage 3 involves further analysis on rainfall and river flows to define the design river flows in the catchment watercourses and an assessment of the effects of climate change and future land use changes on the existing flooding scenarios will also be investigated.

The outputs of the above stages will be documented in a Hydrology Report which will be available on this website later this year.

Review and analysis of historic floods

A review of available flood records relevant to the study has been undertaken. Information was made available from a number of sources including Fingal County Council, Meath County Council and the OPW. The OPW National Flood Hazard Mapping website www.floodmaps.ie contains information on past flood events in the form of various reports, documents and photos. The results of this analysis will inform the calibration and validation of the computer models of the rivers at a later stages of the project.

Joint probability analysis

The rivers being studied as part of the FEM FRAMS are both tidally and fluvially influenced where they discharge to the Irish Sea. Therefore the flood risk along this coastal zone can be from either fluvial, tidal or a combination of both fluvial and tidal. A joint probability analysis has been carried out to determine the chance of two or more conditions occurring at the same time to produce a high water level e.g. high river flow and high tidal level.