Strategic Flood Risk Assessment - South Worcestershire ...

4 How Flood Risk is Assessed4.1 Definitions4.1.1 FloodSection1 (subsection 1) of the FWMA defines a flood as:‘any case where land not normally covered by water becomes covered by water’.Section1 (subsection 2) states ‘it does not matter for the purposes of subsection (1) whether aflood is caused by –(a) Heavy rainfall(b) A river overflowing or its banks being breached(c) A dam overflowing or being breached(d) Tidal waters(e) Groundwater, or(f) Anything else (including any combination of factors).Note: Source does not include the following – flood from any part of a sewerage system, unlesscaused by an increase in the volume of rainwater, entering or affecting the system, or a floodcaused by a burst water main.4.1.2 Flood RiskSection 3 (subsection 1) of the FWMA defined flood risk as:‘a risk in respect of an occurrence assessed and expressed (as for insurance and scientificpurposes) as a combination of the probability of the occurrence with its potential consequences.’Thus it is possible to define flood risk as:Flood Risk = (Probability of a flood) x (Scale of the Consequences)On that basis it is useful to express the definition as follows:ConsequencesFloodRiskFloodReceptorProbabilityHazard Presence= X X XMagnitudeReceptorVulnerabilityUsing this definition it can be seen thatIncreasing the probability or chance of a flood being experienced increases theflood risk. In situations where the probability of a flood being experienced increasesgradually over time, for example due to the effects of climate change, then themagnitude of the flood risk will increase.The scale of the consequences can increase the flood risk.oFlood Hazard Magnitude: If the direct hazard posed by the depth of flooding,velocity of flow, the speed of onset, rate of risk in flood water or duration ofinundation is increased, then the consequences of flooding, and therefore risk, isincreased.o Receptor presence: The consequences of a flood will be increased if there aremore receptors affected, for example with an increase in extent or frequency of2012s5947 S Worcestershire Level 2 SFRA Update FINAL Report v1.0.doc 22