Tangimoana flood response put to test

An action plan to protect Tangimoana from the impact of flooding has been put to the test by Horizons Regional Council.

The Tangimoana settlement was inundated with floodwaters in 2004 and as a result substantial improvements to flood defenses were given a high priority. Horizons group manager operations Allan Cook says those improvements were completed in 2010-12 and now protect the settlement from a 1 in 50 year flood.

“Part of these works included the construction of a new stopbank between Anga and Kina Streets to prevent floodwaters from backing back up from the old river loop and estuary into Tangimoana,” says Mr Cook.

“As the new stopbank runs across Tangimoana Beach Road, a removable flood gate was included in the design to be erected once flood waters reach a certain level. However, the prevention of flood waters from entering the settlement from the downstream end poses a problem if a flood over the design standard of 1 in 50 year level overtops or breaches a stopbank further up-river.”

Mr Cook says in that situation floodwaters would pond within the settlement to a greater depth than in previous flood events.

“For that reason, we have developed a flood action plan that ensures emergency contingency measures are taken at the appropriate time. Those measures include the deliberate breaching of a section of the stopbank near Tangimoana Beach Road, if necessary, to allow for an outlet of flood water before it is able to pond.

“The exercise we undertook last week tested parts of that action plan,” Mr Cook says.

Mr Cook says once the gates are installed flood water levels both upstream and downstream of the settlement will be monitored onsite to ensure timely decisions are made.

“As part of the flood action plan, council staff will use an excavator digger to install the gates once a prescribed trigger level is reached. That machine will then be retained on site for as long as the potential need to open up the stopbank exists.

“If a stopbank overtopping or breach is in fact predicted, then a key part of the flood action plan provides for informing all residents and initiating an evacuation phase.”

The flood action plan was developed as a joint effort between Horizons and Manawatu District Council staff. Horizons emergency management and operations staff reviewed the plan at Horizons’ Marton service centre before heading to Tangimoana to practice the installation of the temporary floodgates.

“It was really good to get all our staff who could be involved in the same room to iron out any kinks in the plan,” says Mr Cook.

“We intend on holding another exercise with Manawatu District Council staff in the near future to ensure they also know how the action plan works.”
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