The Council’s flood control schemes at the lower Waiwhakaiho River in New Plymouth and the lower Waitara River have undergone major upgrades in recent years and offer ‘one in 100-year’ protection incorporating allowance for climate change.
Waitara flood protection scheme
A three-year, $3 million upgrade of the scheme was completed in July 2016. Waitara is now protected by a resilient protection scheme designed to withstand the high and fast-moving flows of a one-in-100-year flood, up from the old one-in-30-year safeguard.
As well as strengthening the flood protection, the project has resulted in the amenities along the river front being greatly enhanced.
Stopbanks were first constructed in the early 1970s and after flood damage to parts of the river bank and stop banks in 1990, the Council undertook a significant amount of work to strengthen the channel banks and improve the channel alignment. The main channel alignment works were constructed downstream of SH3 to direct the river flow away from the stopbanks to the south of the town centre. The last strengthening of the channel was completed in 2013.
To future-proof the town, the flood defences have been raised by up to 1.8m, with a variety of methods being used in different locations.
Waiwhakaiho flood protection scheme
The standard of flood protection has been doubled in the lower Waiwhakaiho Valley (‘The Valley’) in New Plymouth as a result of a three-year, $1.5 million project completed in 2014.
The project increased the level of protection from a one-in-50-year flood to a one-in-100-year flood in an area that has become a popular retail hub, attracting thousands of shoppers a day. The work included:
- Raising the height of stopbanks along the Waiwhakaiho River and along the lower reaches of the Mangaone Stream, between SH3 and its confluence with the Waiwhakaiho.
- Raising the headwalls of Mangaone Stream culverts under SH3 and Katere Road to allow more water to flow through them, and raising the height of the wall of gabion (rock-filled) baskets beside the stream.
The engineering design work took into account the likely impact of climate change over the coming half century.
Although the project was completed on time and within budget, it was not without its engineering challenges. Increasing the level of flood protection along the Mangaone Stream between SH3 and Katere Road was especially challenging due to the nature of the channel and the existing protection works and culverts. The initial plan was to control the flood flow by building detention dams further upstream. Two potential sites were ruled out because their cost benefit ratios were too low, and the third would have required a 12m high dam wall, the cost of which would have blown the budget out by more than 100%.
So after additional surveying, hydrological and hydraulic modelling and detailed engineering analysis, it was decided to increase the flood capacity of the culverts under SH3 and Katere Rd and raise the existing gabion basket wall along that section of the stream.
The overall project was completed with minimal disruption to the public or businesses. At times, the contractors had an audience of thousands as they worked on raising the Waiwhakaiho River stopbanks alongside Rifle Range Rd behind The Valley retail centre. But apart from a few minor hold-ups, everyone was able to come and go, and carry out their business.