Bridge-blocker completes Waitara flood defences

A new removable barrier is ready for use if extreme rainfall ever results in the Waitara River threatening to rise above the deck of the Town Bridge.

The temporary fixture will protect the township by sealing the gap between the permanent flood barriers on each side of the bridge. It completes a multi-year upgrade of the Lower Waitara River Flood Control Scheme by the Taranaki Regional Council.

“It’s the final piece in the jigsaw puzzle,” says the Council’s Rivers Manager, Daniel Harrison.

“All the flood barriers through the township have been raised but the bridge would have been a weak link if we didn’t have a way of containing the flow there.”

He says the temporary barrier may not look high but it offers significant extra protection considering the river’s depth, width and speed if it was flooded to such an extent. “A smallish rise in the flow above the bridge would represent a considerable volume of extra floodwater.”

A barrier is needed only on the town side of the bridge, as the eastern side is higher and floodwaters would not overtop at that end.

“The temporary barrier is stored nearby as a lightweight kitset, and can be installed easily and quickly by just one person, if necessary, with no special equipment needed,” says Daniel.

As well as raised floodwalls and stopbanks, the Waitara upgrade also included better and more rock lining and rock groynes on the riverbanks, to help prevent erosion that might damage the floodwalls and stopbanks, and floodgates to prevent floodwater travelling up stormwater pipes into the township.

Daniel says climate change was taken into account for the upgrade. “Waitara now has protection from floodwaters up to 3,840 cumecs (cubic metres a second), well above the currently calculated ‘one in 100-year’ flow of 3,200 cumecs.” But he says the old ‘one-in-100 year’ definition can be misleading, as nature doesn’t work to a scheduled timetable. “It’s more accurate to say there’s a 1% chance of such a flood in any given year, regardless of when the last one was.”

He says a small risk remains that a very large flood might weaken the new defences, and residents should be aware of this risk. “However, the risk factor today is extremely low compared with other risks to property, such as damage from high winds. We keep a careful eye on how the scheme performs during very heavy rainfall.”

To find out more about the Waitara scheme and other Taranaki Regional Council flood protection schemes, go to www.trc.govt.nz/you-and-your-river/

Waitara Town Bridge flood barrier

The new Town Bridge flood barrier is 'the final piece in the jigsaw puzzle'.