Moutoa floodgates open to relieve pressure on bottom stem of Manawatū River

Manawatū-Whanganui Emergency Management group controller Craig Grant says the floodgates, which are operated by Horizons Regional Council, were opened just after 1am today. The gates divert water down a spillway which acts as a shortcut to the sea.
 
“The bottom stem of the Manawatū River is windy and has a relatively shallow gradient, slowing water down. As this happens, the channel fills, risks significantly overtopping a 30-kilometre stretch and flooding adjacent properties.
 
“Diverting flood flows down the spillway reduces this downstream risk. However, this diversion has no impact on water levels upstream.
 
“There’s a common misconception opening the gates acts like a plug, draining upstream and dropping river levels near the likes of Palmerston North and the Tararua District.
 
“We also can’t operate the gates until the river is within the operating range of 8.2 metres at Moutoa as there needs to be enough flow left to not deposit heaps of silt into the lower channel.”
 
People who lease land along the Moutoa spillway were advised throughout Monday that a gate operation may take place, says Mr Grant.
 
“We appreciate the response these leaseholders take during flood events to move stock and drop fences, sometimes at short notice, before we operate the gates.”
 
While the weather situation has eased, people should still be alert to changes – especially the risk of landslips, says Mr Grant.
 
Road conditions will vary across the region, with many state highways and local roads closed.
 
Updated information on river heights and rainfall is available via Horizons’ 24-hour toll-free Waterline 0508 4 FLOOD (0508 435 663), or online at envirodata.horizons.govt.nz.
 
Updates are posted to the Manawatū-Whanganui Civil Defence Facebook page, while weather warnings are available at www.metservice.com/warnings.
 
An explainer video about how the Moutoa floodgates work is available here.

Image: A snip from our webcam network of the Moutoa floodgates at 6.30am Tuesday 17 February.