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Whanganui, a city on the west coast of the North Island of New Zealand, has a history of repeated flooding in low-lying areas, particularly in areas adjacent to the Whanganui River.

The inlet of Whanganui River is subject to tidal influence with high silt content and has caused flooding and damage to the nearby properties and roads.

Climate change modelling tells that heavier and more frequent downpours will cause localised or large-scale flooding in this area of the country. With this knowledge, the residents had to be prepared to evacuate themselves, their pets and important items to a safe location in the case of flooding.

One of the causes of flooding is backflow from the river, through the stormwater outfall system. To prevent backflow, WaStop Inline Check Valves were installed on the outlet of these very flat DN1350, and DN450 concrete pipes. While the DN1350 pipe was a new line draining an industrial subvision catchment, there was no backflow device installed on the DN450 line so this was upgraded at the same time.

With the WaStop in place backflow flooding has been eliminated, and the stormwater system can be used for attenuation during periods of high rainfall coinciding with high river levels.

The solution has provided peace of mind for the residents and property owners by protecting their properties from flood damage.

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