Wainuiomata River

The Wainuiomata River catchment covers 133 square kilometres and a river length of about 22 kilometres. The location and shape of the Wainuiomata catchment make it susceptible to southerly rainstorm events.

Recent flooding events across New Zealand have shown the need for investment in critical flood protection infrastructure. The effects of flooding can be devastating for communities, and cause millions of dollars of damage.

Wainuiomata Flood Hazard Maps

Greater Wellington Regional Council, Hutt City Council and Wellington Water Ltd. are working on two flood hazard modelling and mapping projects in Wainuiomata.

In 2012, we produced flood hazard maps. The river’s meandering pattern has changed at some locations. We are planning to update the flood hazard maps over the next 18 months with the help of up-to-date computer modelling software, and more extensive data. To develop these maps, we need to undertake a flood hazard assessment.

The updated flood hazard maps will show areas that are likely to flood in case of a 1-in-100 year event that has a one percent chance of happening in any year. Once approved, these maps will help inform Lower Hutt’s District Plan review to help ensure new development is located and built in a safe manner, as well as, for the effective planning of infrastructure investment and for civil defense planning.

Developing Flood Hazard Maps

Flood hazard modelling is crucial in estimating the flood risk. It helps us to make better and informed decisions about how best to manage the risk. We want our flood hazard maps to be as up to date as possible to make sure they provide the best representation of what areas may flood.

We develop the flood hazard maps by using computer models. This process involves several steps including: 

• Collecting information such as any historic information or pictures, 
• Determining rainfall patterns in the catchment and the resulting river flows, 
• Building a computer model to understand where the river is likely to flood, and 
• Producing maps showing areas that are likely to be flooded.

Flood Hazard Modelling Standard

We have developed the Flood Hazard Modelling Standard to underline the process we will follow when we do any flood hazard mapping. This standard was developed to ensure all flood hazard modelling projects are done in a consistent and robust way in line with industry best practice. It is designed to be flexible enough to incorporate catchment or community-specific needs.