Surface water management planning: guidance (2018)

Appendix 5 Example of estimating flood risk damages to properties

This example shows a simplified method for using SEPA data to estimate the flood damages avoided when implementing an option to reduce the probability of flooding to an assumed level or ‘standard of protection’. The approach, an example of ‘approach 2’ described in Section 7.5.1, does not require further modelling and uses the regional pluvial risk data (baseline appraisal) derived from SEPA’s regional pluvial hazard mapping.

Figure A5.1 is an example of an output from regional pluvial hazard and risk data showing flood extents from the 1:30 and 1:200 year return period rainfall events. Coloured dots indicate the residential properties at risk from surface water flooding.

Figure A5.1 SEPA regional pluvial hazard and risk data showing properties predicted to flood

Step 1: Table A5.1 is an extract from the regional pluvial risk data summarising the direct flood damages ^[20] for the 33 properties highlighted in Figure A5.1. Damages are reported for five return periods (10, 30, 50, 100, 200 year) based on the predicted depth of flooding in each property, the type of property and its plan area. The final column presents the calculated Annual Average Damage; this is the damage that might be expected annually given the probability of each constituent event occurring. Summing the Annual Average Damages for the whole area over all return periods gives the total Annual Average Damage. The baseline Annual Average Damage across the area is £49,227. In the baseline situation, all properties flood in the 1:200 year event but only 13 flood in the 1:10 year event.

Table A5.1 Illustration of calculating the baseline Annual Average (direct) Damage from SEPA regional pluvial risk data

Step 2: This step requires an understanding of the likely flood mechanisms in order to propose a ‘standard of protection’ that is achievable under a flood risk management option. Engineering judgment and experience should be used to determine this. In this example, the proposal is that local improvements are made to maintain highway drainage and the capacity of the sewer network in order to delay the onset of flooding at all properties until after the 1:30 year event. In this simplified methodology, a new direct damage is calculated by removing the 1:10 and 1:30 year damages from the calculation. The impact of this change is illustrated in Table A5.2. The revised total Annual Average Damage is now reduced to £21,159. Note how the simplified approach conservatively assumes that the impact of less frequent floods remains unaltered.

Table A5.2 Illustration of calculating Annual Average (direct) Damage for a flood risk management option by manipulating SEPA regional pluvial risk data (option removes all damages up to 1:30 year event, as highlighted)

Box A5.1 Calculating Annual Average Damage

The formula for calculating Annual Average Damage (AAD) at each property from the data is:

AAD =
((DDMG10) + (DDMG30))/2*(1/10-1/30) +
((DDMG30) + (DDMG50))/2*(1/30-1/50) +
((DDMG50) + (DDMG100))/2*(1/50-1/100) +
((DDMG100) + (DDMG200))/2*(1/100-1/200) +
((DDMG200) + (DIRINFIN))/2*(1/200-0)

Where:

DDMG 10, 30, 50, 100, 200 is the direct damage for each return period event; and
DIRINFIN = (DDMG200) + ((DDMG200) - (DDMG100)) *((1/200-0)/(1/100-1/200))

This notation is used in the baseline appraisal information. The formulae can be pasted from this guidance directly into a spreadsheet.