# InfoWorks ICM RiskMaster: Calculation of Event Probability and Annual Damage March 30, 2017 | Innovyze staff

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ICM RiskMaster allows 2D hydraulic results from InfoWorks ICM to be combined with economic data to allow damages to be quantified.

Traditionally flood management policies have been based upon the design standard philosophy, where policy makers decide on an appropriate protection level to be achieved within the flood system which is used to design and manage hydraulic infrastructure. In contrast with this approach and following the guidelines specified in the EU Floods Directive 2007/60/EC on the assessment and management of flood risks, flood management policies based on risk rather than system performance have been developed in recent years. Flood risk management policies are based on the evaluation of the consequences generated by flooding events and the alleviation measures on the expected flood impacts over a given time period.

Risk-based analysis methods can be used in order to assess and manage hydraulic infrastructure which protects assets from flood events. A flood-risk methodology analyses a hydraulic system based on the evaluation of the consequences derived from the service of the hydraulic infrastructure rather than system performance. Thus, in contrast with traditional performance methods, in which the hydraulic system is expected to service a specific loading level, a flood risk approach should take into account all types of event based on their probability of occurrence. The results of the analysis provide a comprehensive view of the performance of the hydraulic system and the consequences derived from flood events.

The calculation of risk involves multiplying the damage result for each receptor in a simulation with the probability of occurrence of the event simulated. The total damages are calculated at each damage receptor by taking the 2D flow depth and using the depth-damage curve to associate this with a damage value. This is then summed for all damage receptors to provide a total damage value for each return period. This in turn is multiplied by the event probability, to provide the annual damage.

Event Probability

For each return period simulated, the probability of non-exceedance is calculated using the following equation: Where RPi is the return period under consideration.

The event probability is calculated by discretising the probability of non exceedance curve, where the largest zero-damage event return period, specified by the user in the Risk Analysis Run View gives the probability for zero damage. Table 1: Calculation of Probability of Non-Exceedance and Event Probability

Depending on the return periods simulated, the event probability does not necessarily increase. For example, consider the following return periods, 1, 5, 20, 30 and 100. The table below gives the probability of non-exceedance and the event probability. At first glance, it appears that the event probability for the 100 year event is greater than the 30 year event which wouldn’t make much sense. However, what the event probability is suggesting is that the probability of an event having a return period of between 30 to 100 years is greater than a return period of between 20 to 30 years. Table 2: Calculation of Probability of Non-Exceedance and Event Probability-Alternative Discretisation

The expected annual damage is the average of flood damages calculated over a number of events. The total damages for each event is then multiplied by the event probability to provide the annual damage.  This is then summed for all return periods to provide the Expected Annual Damage for all events.  The expected annual damage value is the sum of the annual damage for each of the event probability bands which is the integral of the area under the damage probability curve. Table 3: Calculation of total event damages and Estimated Annual Damage

These damage costs can be used to then analyse the performance of proposed mitigation measures and approaches.

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