OPW: Office of Public Works
Oifig na nOibreacha Poiblí
What is Flood Risk?
What is Flood Risk?
Flood risk is the damage that may be expected to occur at a given location arising from flooding. It is a combination of the likelihood, or probability, of flood occurrence, the degree of flooding and the impacts or damage that the flooding would cause. It is typically expressed in terms of a quantity of damage expected to occur over a certain time period, for example, euros per year.
Flood risk is not the same as flood hazard. Flood hazard only describes the degree or severity of flooding, often referring to the extent and depth of flooding or the velocity of flood flow, but does not include an element of the damage.
The probability or likelihood of flooding is generally referred to in terms of the "Annual Execedence Probability" (AEP), which is the odds or probability (expressed as a percentage) of a flood occurring in any given year that is equal to or more severe than a given magnitude or severity. For example, a flood of a given severity that might typically be expected to occur only once every hundred years would have a one percent (or 1 in a 100) chance of happening in any given year, and would be the 1% AEP food event. Such an event is sometimes also referred to as the "hundred-year" flood. It should be noted that the occurrence of flood events is random, and so two 1% AEP events could occur within the space of just a few years, although this is statistically unlikely.
A detailed description of a range of terms often used in relation to flooding and flood risk management are provided in the Glossary of Terms.
Causes of Flooding
Flooding is a natural phenomenon arising from a range of sources including:
- Heavy rainfall that is unable to drain away and ponds
- Rivers exceeding their capacity and overtopping their banks
- High sea and tide levels inundating adjacent land
- Waves over the sea, estuaries and large lakes overtopping beaches, banks and coastal defences
- Groundwater rising up above ground level
- Turloughs swelling and flooding surrounding land
- Water from melting snow running off into swollen rivers
Human activities can increase the occurrence of flooding. For example:
- By covering green areas with impermeable surfaces (e.g. car parks), the amount of water that seeps into the ground is reduced. This increases the speed and volume of water run-off.
- Encroachment into, and blockage of, river channels reduce their capacity to carry water away, increasing water levels at times of high flow.
Climate change is expected to increase the frequency and severity of flood events, as current predictions for Ireland indicate increased rainfall in the winter and more frequent storm events. There is very strong evidence that sea level is already rising and will continue to do so, increasing the frequency and depth of flooding of coastal and estuarine areas. [EPA Report 2007]