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

process is essential to ensure the level of assessment is commensurate withthe risks and the available datasets. The scoping phase should include:611. Confirmation of sources of flooding that may affect a plan area;2. An appraisal of the availability and adequacy of existing information;3. Potential impact of development on flooding elsewhere;4. Scope of possible mitigation measures.The scoping process may identify that sufficient quantitative information isalready available to complete a FRA appropriate to the scale and nature of thechanged land use or proposed development. If not, then the FRA will need tocarry out such information gathering as part of the assessment process.Step 3 - Assessing Flood RiskThis is the stage at which the actual Flood Risk Assessment (FRA) is carriedout.It is important that a FRA should consider both the actual and the residualrisks.Actual flood risk is the risk posed to an area, whether it is behind defences orundefended, at the time of the study. This should be expressed in terms ofthe probability of flooding occurring, taking into account the limiting factors,both natural and manmade, preventing water from reaching the development.Residual risks are the risks remaining after all risk avoidance, substitutionand mitigation measures have been taken. Examples of residual flood riskinclude:• The failure of flood management infrastructure such as a breach of araised flood defence, blockage of a surface water channel or drainagesystem, failure of a flap valve, overtopping of an upstream storage area,or failure of a pumped drainage system; and• A severe flood event that exceeds a flood design standard such as, butnot limited to, a flood that overtops a raised flood defence.Assessment of flood defence breaching should generally be undertaken onthe basis of a design event of the appropriate design standard (1% AEP 1 forriver flooding and 0.5% AEP for flooding from the sea), including an allowancefor climate change 2 . Assessment of overtopping of flood defences shouldgenerally be undertaken on the basis of the 0.1% AEP event, including anallowance for climate change.1 AEP – Annual Exceedance ProbabilityPlanning Guidelines2 See http://www.opw.ie