Bay of Plenty Regional Council Rangitāiki River and Flood Plain Model Build

The Challenge

To build a scenario assessment tool for the Rangitāiki River and Flood Plain.

The Solution

Awa and HARC teamed up to build a consolidated 300 km² floodplain model and a detailed model of the Rangitāiki River.

The Impact

The Council now has a powerful simulation capability to assess hydraulics, understand the performance of flood assets, and plan the longer-term future of the area.

Project Summary

Nestled between Whakatāne and Maketū, the Rangitāiki Plains have a long and rich cultural, environmental, agricultural, engineering and flood event history. The low-lying plains were once wetlands, forests, rivers and meandering waterways, covering an area of approximately 300 km². Now, the landscape is made up of complex hydraulic features including multiple rivers, drainage schemes, stopbanks and pump stations.

Awa Environmental, alongside Hydrology and Risk Consulting (HARC), was given the opportunity to build two new models as a scenario assessment tool for the Council. The project aimed to consolidate five legacy models that covered different sections of the Plains, as well as to update the existing models with new information and detail. The key activities in the project were:

• a coincident rainfall analysis upstream and downstream of Matahina dam;
• the development of rainfall boundary conditions for historic, current and future climate events;
• building HEC-HMS hydrological models for 170 contributing hill sub-catchments;
• building a two-dimensional river model, aimed at understanding the detailed hydraulics within the river channel; and
• building a two-dimensional flood plain model covering 300 km² with 430 km of rivers and drains represented in 1D.

Through rigorous attention to detail and quality control, the models were calibrated to the measurements of the devastating 2017 Edgecumbe flood event, which severely damaged more than 250 homes. The calibrated models were then successfully verified against the 1998 and 2004 major flood events. The extensive reporting of the model build, assumptions and limitations have provided a record of considerations when using the models in the future.

The final report can be downloaded from Council's website.