Old riverbed could ease flood risk

The Team Reporter from The Press

From local democracy reporter Brendon McMahon:

Revegetating the historic Buller River overflow channel to lessen the risk of floodwaters spilling into the Orowaiti River and hitting the eastern side of Westport township is being proposed.

It is one part of the proposed $26 million Westport flood protection scheme.

In the 19th century the Buller River was straightened using a bypass, taking out a meandering section, creating Organs Island upstream of Westport.

The original meandering section remains, but the Westport Joint Rating District Committee last week heard the new scheme proposes to take back the leased Organs Island area and to revegetate it.

Using it as a riparian buffer would reduce the "split" in flow from the river's main channel during floods, consulting river engineer Gary Williams said.

The idea was to redirect more flow away from the Orowaiti and lessen the ongoing need for rock maintenance at Organs Island.

Williams said the area required "constant management" with a lot of rock put there over the years to keep the Buller River in the 19th century diversion.

Adding more vegetation would "split the flow" more down the main channel "without having a strong fixed rock wall over that long length".

"The idea is in the end that nature will do most of the regeneration," he said.

Hydrological consultant Matthew Gardner said vegetation could act as "a significant buffer" against flow.

"What we found with the model, the Buller River is at capacity and for every increase in flow more comes down the Orowaiti. This vegetation actually reduces that."

It was in fact going back to the method put forward in the 19th century to manage the area, Gardner said.

The other aspects in the scheme are the proposed realignment of Abattoir Creek, east of Westport, which would reduce flood flows from the east towards the town.

Ruled out is dredging the lower Buller River, making a direct cut to the sea from the Orowaiti Estuary, excavating a causeway to protect the Snodgrass Road residential area, adding culverts in the Westport to Ngakawau railway at Stephens Rd, and constructing culverts on the embankment near State highway 67 near the Orowaiti Bridge.