Damage functions

The experience gained from former flood events has shown, that there is a relation between flow depth or rather velocity and the expected loss. Normally, floods cause damages only in cellar and ground floor. This is why it doesn’t make sense to calculate damage arising in the whole building. Therefore a function/factor is needed which reduce the asset value of the appropriate land use type to the amount of actually expected loss.

In Germany damage functions are available. These functions represent the relation between hydraulic parameters (water depth) and damage (vulnerability). They give information on the amount of damage in percent for different water depths while considering the respective land use type. The determination of expected losses on agricultural or forestal used land is less detailed. Often the factors differ from zero, zero point five to one.

In other countries, like for example Switzerland or Austria, there are also factors available derived from real damages and/or datasets from insurances which describe the vulnerability of buildings or other land use types in dependency on the impact (flow depth, velocity, pressure).

The damage function is a function between 0 (no damage) and 1 (total loss of asset). But even in cases of extreme flood there will be no complete loss of all material assets. Damage functions are characterised by the parameters:

  • first occurrence of damage 
  • maximum damage value 
  • shape of the function

There are two types of stage-damage curves

  • one is based on actual damage costs 
  • the other type is based on synthetic costs (used for benefit-cost analyses) 

The development of residential synthetic cost stage-damage curves needs the following steps: 

  • Representative classes of houses are selected in the area, usually based on building size. 
  • A sample of houses is selected in dwelling class. For each room the contents are checked, values are noted or the height above floor level can also be noted. Preferably, a qualified quantity surveyor or value should undertake this step. 
  • Values are averagedacross each sample for each class of house and stage-damage curves are constructed. 

The damage function based on synthetic cost method describes potential damage, not actual damage. It also can be used for constructing actual cost stage-damage curves soon after a flood.


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Module
Flood Risk Assessment
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TUHH

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