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Purpose of the Report 

To update and refine the hydraulic model of the Waiohine River and floodplain for improved flood hazard assessment and planning. 

Geographic Area 

The report covers the Waiohine River and its surrounding floodplain in the Wairarapa region, including Greytown and the confluence with the Ruamahanga River. 

Summary 

The Waiohine Flexible Mesh Modelling Report (2018) was prepared by Land River Sea Consulting Ltd for Greater Wellington Regional Council to improve the hydraulic modelling of the Waiohine River and its floodplain. The updated model was developed in response to a peer review and community feedback, aiming to enhance the accuracy and reliability of flood hazard assessments for the region.

The modelling area includes the Waiohine River from the Tararua Ranges to its confluence with the Ruamahanga River, encompassing Greytown and surrounding rural areas. The updated model integrates a flexible mesh 2D component using MIKE 21 and a 1D river channel component using MIKE 11, linked through MIKE FLOOD. This approach allows for improved resolution and representation of floodplain features, including stopbanks, culverts, and bridges. 

Key improvements include the use of updated LiDAR data, refined floodplain roughness values based on land use, and the inclusion of detailed structures such as the SH2 and railway bridges. The model was calibrated using three historical flood events (1990, 1998, and 2004), with debris marks and aerial imagery used to validate flood extents and depths. The 1990 flood, in particular, provided valuable calibration data due to its magnitude and community recollections. 

Design flood scenarios were simulated for 20, 50, and 100-year events, including allowances for climate change. The model results showed reduced flood extents compared to previous versions, aligning more closely with observed flood behaviour and community expectations. Interim flood maps were produced using the 100-year event with climate change and a modelled freeboard allowance. 

The report highlights the sensitivity of the model to changes in riverbed levels and recommends further geomorphological assessment to account for future changes. It also suggests a more systematic approach to model uncertainty and freeboard in future updates. Overall, the updated model is considered more robust and suitable for informing land use planning, flood risk management, and community engagement in the Waiohine catchment.