Towards Uniformity in Flood Mapping

For a country of New Zealand’s relatively small size and population, the lack of uniformity in development and presentation of flood maps across the country is striking. In parallel to efforts to promote country-wide uniformity in rainfall-runoff modelling, and drawing on examples and experience from around New Zealand and overseas, a case is presented for greater uniformity in flood mapping and clarity in communicating flood risk.

Flood maps are developed for a range of uses, including high-level hazard identification, integrated catchment management planning and District Plan hazards management. Terminology and the content of information presented on flood maps differ across the country.

Knowing that a map is showing the flood depth and extent does not in itself explain the level of modelling detail and reliability; was the model a simple 2D only rain-on-grid model, or a fully coupled model representing piped networks, open channels, structures and floodplains, and what were the underlying assumptions and constraints? Beyond the raw model output, different approaches are adopted for the inclusion of freeboard or identifying flood sensitive margins.

In addition to their use by stormwater practitioners and planners, the communities we serve are also interested, especially where they are at risk of flooding or it might affect property value and options. Flood maps are a key tool for communication, so communities need to understand the flood maps and have confidence in them.

For this they need to be accessible; an internet search for “flood maps” rarely delivers the desired result. Uniformity of flood mapping terminology and consistency of how councils make their flood maps available would assist, both for community understanding and to assist less well-resourced councils.

Moving towards a uniform approach would result in councils relinquishing local control of flood map specification, but should provide tangible benefits to the country as a whole.