Summary
Horizons Regional Council has had to reassess its worksprogramme on the lower Manawatu schemefollowing this years flooding. Anna Wallis reports.
LIKE a wriggly snake, the Manawatu River has been difficult to get a grip on this year. Since the February flood it has been changing channels, levels lowering and rising and generally not staying still for very long.The state of flux means even extraordinarily generous specifications for flood protection made now by Horizons Regional Council might, in six months, be barely adequate, as Cr Paul Rieger put it at a council meeting in November.The Moutoa sluicegates, built 42 years ago, were opened six times in 2004. The gates have been opened only 33 times in total and, until this year, were opened only twice a year on average.The February flood level was a 70-year high. Another in August was considered a once-in-a-decade event, while Septembers was a three- year flood. There were also minor floods during the period.The February flood had a discharge of 3500 cumecs (cubic metres per second), placing it behind floods in 1880 (4000 cumecs) and 1902 (3800 cumecs).The February flood saw the design parameters of the river altered from a 100-year flood estimate of 3450 cumecs to the revised figure of 3700 cumecs.For cost reasons, the council has decided to raise stopbanks rather than remove silt from the river, except in the reach in front of the Moutoa sluicegates. Even this wont come cheap. To date the council has budgeted $3 million to rework 23.8km of stopbank. More work will follow.Raising stopbanks is not a permanent solution. Experts are mindful of the experience from post-flood work along the Yellow River in China, where the stopbanks were elevated to the point where the riverbed is above a surrounding plain.To find out how much sediment there is, including both the bed load of gravel and sand and the suspended load of sand and silt, the Manawatu River channel has been re-surveyed. Most of the rural sections have been completed and Palmerston North reach will be calculated next year.The reports findings:Ashhurst to Palmerston NorthIn this reach, most of the problems exist downstream of Te Matai Road. A 300-metre section of the riverbank has been severely eroded and a rock lining for the new works will be completed over summer.In November Cr Vern Chettleburgh questioned if the Palmerston North City Council had discussed with Horizons a second bridge at Te Maitai and housing developments in the area. Preliminary discussions had been held, he was told.Palmerston NorthThe city survived the flood comfortably, with the river at its height still being a metre lower than the top of the stopbanks. The flood protection standard was higher for the city than for rural areas.However with additional flow in the Manawatu River, the flood capacity of this reach is to be reassessed, and some sections of the stopbank may be raised.The stopbank had its effectiveness impaired by erosion, particularly of the rock lining near Buick Crescent.The report noted a shortage of rock for repair work was making the council prioritise work.There was also erosion of the stopbank between the Fitzherbert Bridge and the Massey University College of Education. There was no immediate threat.Longburn to Jackeytown RoadOf the 7km of stopbank on this reach, 5.6km had less than 450mm freeboard but there was no erosion. The report recommends a new stopbank upstream of the railway bridge at Longburn.Jackeytown Road to Downstream End of Gravel ReachGravel deposits raising the river bed was more serious here than anywhere else on the river. The problem area was from Jackeytown Road to a point about 2km upstream from the Opiki Bridge, while 2km down-stream of Jackeytown Road the build up was such that the rivers alignment has been very unstable.The council believed it would have to remove between 30,000 and 50,000 cubic metres of gravel soon, and about 2000 cubic metres every year thereafter over and above the current extraction rate.Hamiltons Line SpillwayLowering of the full length of the spillway was no longer required but some work was needed. The loss of channel capacity would require scrutiny and it might be necessary to raise the spillway at some point.Downstream End of Gravel Reach to Oroua River ConfluenceThe Poplar Road stopbank height on this reach was a major concern, with only 50mm of freeboard in some places.Another 12.1km of stopbanking has less than 450mm freeboard.Of high priority was a 600m length of stopbank on the right bank downstream of the SH56 bridge. The flood crossed this section in February. The report said that, if a breach were to occur, water would flow north and potentially flood 500-1000 hectares of land in the area within Glen Orua, Rangiotu and Himatangi (the Sluggish Drainage District).Rangiotu SpillwayPlans to build a structure on the spillway to reduce its volume during smaller flood events have been abandoned. Prior to the flood, a $400,000 structure to control the operation of the spillway was planned to reduce the amount of water that was tipped into the Taonui Basin during less than 50 year floods.The February flood was the largest recorded in the Oroua River. It has a discharge of 900 cumecs downstream of the Makino confluence, south of Feilding. Subsequently the 100 year flood estimate has been shifted from 500 to 700 cumecs a 40-percent increase, much more than the 7 percent increase for the Manawatu.The larger volume of water means the proposed spillway structure would cost a lot more than $400,000 probably several million dollars.The benefits of such work have also dropped with the volume to be discharged nearly trebled and the new discharges changing the hydraulics of the river and spillway markedly.Oroua Confluence to Moutoa SpillwayThe most important job would be to remove 100,000 cubic metres of silt ahead of the sluicegates, with a subsequent lowering of flood levels in the river up to 12km away.The report said it was likely 2-4km of stopbank will also need to be raised along the length of the reach, though little of this work was of high priority.Downstream of MoutoaThe Kara Creek stopbank which failed has been fixed but other low points are worrying, particularly Richmond fellmongery, which the report noted could have to close if it flooded again.The stopbank protecting Foxton Beach township would be reworked in two locations. Flooding in October had severely eroded a stopbank. The river had changed course and monitoring was required to ensure the stopbank remained secure. Another stopbank is planned.nSee the full content of this document
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Taming the River
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