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Flood hazard modelling

http://www.gw.govt.nz/flood-hazard-modelling

Flood hazard modelling

Updated 6 August 2021 4:33pm

Image of flooding along the banks of the Hutt RiverGot photos like this one? 

Bring these and your stories from previous flood events in the Hutt Valley to one of our drop in sessions between 9 August and 22 August 2021. 


Understanding the flood hazard helps us to make informed decisions about the best ways to manage the risk. One method of achieving this is to ensure development is carried out in a way that is suitable for the flood hazard. For future planning, excluding sensitive land uses (i.e. residential development, hospitals and schools) from high hazard areas will help to minimise the risk.

Flood hazard modelling is a crucial tool to understand the threat. The models produce a series of flood hazard maps and data for various situations. A range of possible flooding scenarios and the danger associated with these events are considered. Our investment and emergency management decisions are based on these modelling results.

Our Flood Hazard Modelling Standard

We have developed our Flood Hazard Modelling Standard to outline the protocols for new flood hazard modelling projects. The protocols ensure a robust and consistent approach in line with the accepted industry practice. It allows flexibility because the best approach may be dependent on the catchment or the project.

Developing flood hazard maps

We want our flood hazard maps to be as up to date as possible to make sure they provide the best representation of which areas may flood.

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

  • Stage 1: Collecting information such as any historic information or pictures 
  • Stage 2: Determining rainfall patterns in the catchment and the resulting river flows 
  • Stage 3: Building a computer model to understand where the river is likely to flood
  • Stage 4: Producing maps showing areas that are likely to be flooded

Current Flood Hazard Modelling Projects

We are currently working alongside local councils, iwi and community members to upgrade flood hazard maps for different rivers across our region. Once approved, these maps will help inform District Plan reviews 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 defence planning.

Below is an outline of the different rivers we are currently modelling and what stage of the process they are at.

Hutt Valley

Waiwhetū Stream

We are working with Wellington Water to create a combined model to understand the flooding from stormwater and the Waiwhetū Stream. From the model, we have produced draft maps simulating the flood events from February 2004 and November 2016. We have previously collected information from you about the 2004 flood. We now need your help to make sure the maps that have come out of our models match your real-life observations of flooding. We want you to share any photos, stories, or other information you have of the 2004 and 2016 events to help us refine the model.

View the draft maps.

Image of flooding along the banks of the Waiwhetu River

Te Awa Kairangi Hutt River

Our Te Awa Kairangi Hutt River modelling is at Stage 4. We now have three final draft maps that show the following:

  • Flooding from a river flow of 1900 cumecs.
    This is a 1-in-100 year event, which has a 1% chance of occurring in any year.
  • Flooding from a river flow of 2300 cumecs.
    This is a 1-in-100 year event that includes an allowance for climate change, i.e. 1% chance of occurring in any year including climate change allowance.
  • Flooding from a river flow of 2800 cumecs.
    This is a 1-in-440 year event, which has a 0.2% chance of happening in any year.

We are planning to share these maps with you over the month of August 2021 at various drop-in sessions throughout the Hutt Valley.

View the draft maps.

Wainuiomata River

In 2012, we produced flood hazard maps for the Wainuiomata River. The river’s meandering pattern has changed at some locations. In July 2021 we began undertaking a flood hazard assessment for this catchment. It involved collecting information such as stories and old photos. The next step in our modelling process is to look at rainfall patterns and resulting river flows. To do this, we will be engaging a consultant to do some hydrological modelling. In the meantime, we are still keen to continue collecting your information, photos and stories of flooding from the Wainuiomata River – so make sure you keep sending them through!

For more information