The Planning System and Flood Risk Management - Offaly County ...

10Flood risk2.13 Understanding flood risk is a key step in managing the impacts of flooding.Flood risk is a combination of the likelihood of flooding and the potentialconsequences arising. These guidelines recommend a staged approach toflood risk assessment that covers both the likelihood of flooding and thepotential consequences.2.14 Firstly, it is important to define the two components of flood risk in order toapply this guidance in a consistent manner.Planning Guidelines• Likelihood of flooding is normally defined as the percentage probabilityof a flood of a given magnitude or severity occurring or being exceededin any given year. For example, a 1% probability indicates the severityof a flood that is expected to be exceeded on average once in 100 years,i.e. it has a 1 in 100 (1%) chance of occurring in any one year.• Consequences of flooding depend on the hazards associated with theflooding (e.g. depth of water, speed of flow, rate of onset, duration,wave-action effects, water quality), and the vulnerability of people,property and the environment potentially affected by a flood (e.g. theage profile of the population, the type of development, presence andreliability of mitigation measures etc.)2.15 Flood risk is then normally expressed in terms of the following relationship:Flood risk = Likelihood of flooding x Consequences of flooding2.16 Flood risk can be measured in monetary terms, as a potential for loss of lifeor injury, as environmental impacts, or as combinations of all these indicators.Guiding proposed development to areas, where the risks arising from inundationin terms of human impact, economic loss or potential environmental damageis minimised, is a key tenet of this guidance.2.17 The vulnerability of development to flooding depends on the nature of thedevelopment, its occupation and the construction methods used. For example,a sheltered housing complex would be more vulnerable than a retail unit. Abroad classification of vulnerability has been developed as shown in theGlossary of Terms. The classification of different land uses and types ofdevelopment as highly vulnerable, less vulnerable and water-compatible isinfluenced primarily by the ability to manage the safety of people in floodevents and the long-term implications for recovery of the function and structureof buildings (see Glossary of Terms).2.18 Transport and strategic utilities infrastructure can be particularly vulnerableto flooding because interruption of their function can have widespread effectswell beyond the area that is flooded. For example, flooding of primary roadsor railways can deny access to large areas beyond those directly affected by