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Purpose of the Report 

To assess the impact of diverting floodwater into Whakawiriwhiri Stream and design upgrades to manage increased flow and flood risk. 

Geographic Area 

The report covers the Whakawiriwhiri Stream located in the Wairarapa region of New Zealand, including Barton's Lagoon and surrounding floodplain. 

Summary 

This report presents the hydraulic modelling and design work undertaken for the Whakawiriwhiri Stream in the Wairarapa region. The primary objective was to evaluate the effects of diverting water from the Tawaha Floodway into the stream at Barton's Lagoon and to design mitigation measures. A MIKE FLOOD model was developed using a combined 1D/2D approach, incorporating MIKE 11 for stream channels and structures and MIKE 21 for floodplain analysis. Topographic and LiDAR survey data were used to construct the terrain model, and various flow scenarios were simulated including pre- and post-diversion conditions, high tide, and climate change sensitivity.

The proposed design includes modifications to culverts and bridges at multiple cross sections (XS30 to XS47), such as lowering inverts and installing wingwalls. The stream channel between XS30 and XS39 will be deepened and widened to a trapezoidal shape with a 2-meter base width and 2:1 side slopes. At Barton's Lagoon, the outlet culvert will be replaced with a 900mm diameter pipe and lowered to an invert of 11.8 meters to improve drainage. 

Modelling results indicate that the upgraded design significantly reduces water levels in the stream and floodplain. Floodwaters are expected to recede below the weir crest within 10 hours and return to base levels within three days. Environmental impacts on the lagoon are minimal due to steep banks and shallow depth changes. Bed erosion risk is generally low, except downstream of Pahautea Road Bridge where velocities reach approximately 1.2 m/s. 

The report concludes that the current stream configuration is under-capacity for the additional flow. The proposed upgrades are cost-effective and environmentally sound. Variable grades are preferred over a consistent slope for practical construction. Increasing the culvert diameter at Barton's Lagoon improves drainage and reduces flood duration. Appendices include LiDAR metadata, final design drawings, lagoon depth impact analysis, and landowner database.