A 100-year flood refers to the flood that occurs on average once every hundred years. Note that the loss return period is not the same as the event return period, since more than one event can contribute to loss. The convolution of exposure , hazard and vulnerability . For natural hazards, we can only attempt to reduce the risk not the hazard, either by controlling exposure to hazards or their vulnerability. The susceptibility to losses due to exposure to hazard .
Vulnerability reflects the extend of losses for any given hazard.