For flooding assessments, it is common to see the addition of a freeboard allowance to account for uncertainties and other effects above a calculated water level. A uniform freeboard allowance is often globally applied, even when the magnitude of these uncertainties and presence or absence of other effects can easily be shown to vary significantly.
In this paper variation of model uncertainty is compared to freeboard offsets that are commonly applied across New Zealand. Multiple modelling scenarios have been used to investigate the sensitivity of outputs. The use of fuzzy maps is demonstrated to show how these can support a more detailed assessment of the freeboard - risk relationship.
This approach is compared with current practice across New Zealand and the pros and cons discussed. The implications of freeboard choices are considered in the context of urban flooding, river stop banks and coastal flooding situations.