Flood protection not in time

Planned flood-prevention basins would have kept more than 300 Olympic swimming pools’ worth of stormwater out of the Heathcote River if they had been ready before the weekend’s storm.

The basins are part of $77 million worth of work planned for the river’s catchment by the Christchurch City Council following the March 2014 floods.

Construction of two at Wigram and Hendersons basins is under way, work on two in the Cashmere-Worsley valley and the Curletts stream area should start later this year, and two small basins were built last year as the first stage of the Hendersons project.

Together the storage areas will hold back 800,000 cubic metres of water – enough to fill 320 Olympic pools.

Based on 2014 figures, the work would halve the number of flooded homes along the river. It is designed mostly to help ease flooding of the upper and mid reaches of the river, as the lower stretches are affected by tides. Residents along the river were evacuated on Saturday afternoon as two months’ worth of rain came down in two days.

The council has already spent about $25m in the Heathcote catchment area and will spend about $52m more in the next 12 months. Spending beyond that will be determined by the long-term plan, to be debated by the council early next year before public feedback is sought.

Keith Davison, manager of land drainage for the council, said the programme was being fast-tracked. He said the two wetland basins already built had been working well in heavy rain.

The work is being done as part of the Land Drainage Recovery Programme, after the 2014 floods revealed how badly the earthquakes increased the flood risk in many parts of the city.

In other parts of the programme, the council is stabilising riverbanks, especially in the suburbs of Beckenham, St Martins and Opawa between Colombo St and Wilsons Rd, and just west of Barrington St. A new pump station is being built in Richardson Tce, along with two storage basins to support the Bells Creek area in Woolston. These are expected to be in use early next year.

Flood-mitigation work has already been done in the Heathcote Valley.

Davison said the council had installed 75 river and rain gauge monitors since the 2014 floods.The underground pipeline built as past of the council’s $48m Dudley Creek flood remediation plan was in operation in time for the weekend floods.

Other parts of the scheme – designed to protect more than 585 properties in the Flockton Basin, which spans St Albans and Mairehau – are still being completed.