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Purpose of the Report: 

To assess flood capacity and optimise flood protection in the Lower Ruamahanga River system. 

Geographic Area Covered: 

The report covers the Lower Ruamahanga River and surrounding floodways, including Lakes Wairarapa and Onoke in the Wairarapa region. 

Summary of the Report: 

This report presents a detailed hydraulic modelling study of the Lower Ruamahanga River, conducted to evaluate the performance of the Lower Wairarapa Valley Development Scheme (LWVDS). The scheme includes extensive flood protection infrastructure such as stopbanks, floodways, and control structures, designed to manage flood risks across a large area of the Wairarapa region. The modelling was performed using MIKE 11 software, which simulates unsteady open channel flow and incorporates complex features like floodway connections, culverts, and the barrage gate structure at Lake Wairarapa. Calibration was based on five historical flood events from 1992 to 2003, using observed water levels and flow data. The model was refined to reflect current riverbed conditions and floodway configurations. Sensitivity tests were conducted to assess the impact of channel roughness, sill elevations, and riverbed aggradation on flood levels. Design simulations for 20-year and 50-year flood events showed that most of the system meets expected protection standards, although some areas near Waihenga may experience overtopping in a 20-year event. The study also highlighted discrepancies in the existing Waihenga rating curve and recommended updates based on model predictions. The report concludes that the LWVDS generally performs well but could benefit from further optimisation, including updated sill surveys and potential use of 2D modelling with LIDAR data for improved accuracy.