$16.5m to buy flood-risk homes

The Christchurch City Council expects to spend $16.5 million buying up to 35 homes at risk of flooding along the Heathcote River.

Between 25 and 35 houses along the river are at risk of frequent flooding and some have flooded above the floor four times since since 2010-11 earthquakes. Before then it was estimated only seven houses faced a similar level of risk.

The council is proposing a raft of measures, expected to cost $55m, to reduce flooding in the area, but the full benefits will not be achieved for several years, so staff are recommending voluntary buyouts to provide homeowners with ‘‘timely relief’’.

A council report, to be discussed at the Infrastructure, Transport and Environment Committee meeting on Wednesday, recommends the council continue work on four new storage basins in the upper Heathcote, dredge the river downstream of Hansen Park, stabilise banks, and apply its flood intervention policy (FIP) to buy out homes.

The cost of the entire scheme was $72m plus or minus 30 per cent. About $28m has already been approved and another $44m would be needed from the council’s Land Drainage Recovery Programme.

Once the works were complete computer modelling expected five properties to remain at risk, but it would take several years for the full benefits to be achieved even with maximum funding, the report says. ‘‘While the proposed works will be effective for the majority of houses . . . it is not considered that the works will be timely.’’

Under the policy the council can also offer to complete localised drainage work and raise homes, but staff said those options were not recommended in this case because residents would still be affected by frequent underfloor flooding and could still be at risk of not obtaining insurance.

The policy was used for the first time in 2016 to buy seven properties at risk of flooding in the Dudley Creek catchment area. The council spent $2m buying and demolishing those properties.

The council was also proposing changes to the policy including increasing the buyout cap from

$600,000 to $750,000 for individual houses and from $400,000 to

$500,000 per unit.

The existing caps did not provide sufficient flexibility for rolling out the policy across the city, the report said.

Residents have called on the council to do something about the flooding in the area, with many reporting high levels of anxiety when it rains. At least two homeowners have reported being unable to get insurance for flooding unless the council could demonstrate it was undertaking works to reduce the risk.

Flooding along the Heathcote River has got worse since the earthquakes, the council said. Between Colombo and Radley streets the number of homes at risk of frequent flooding (greater than a one in 10 year event) was now six times greater than before the earthquakes (23 compared to four) and those at risk in an extreme event (one in 50 year) had almost doubled from 85 to 152.

The number of homes at risk upstream of Colombo St had reduced since the earthquakes.

The quakes have reduced the size of the river channel in places due to bank slumping and liquefaction. Drainage at the river mouth had reduced because the land there lifted in the earthquakes.

Dredging the river was expected to cost $14m and would remove about 60,000 cubic metres from the riverbed. Building low stopbanks was not part of the flooding mitigation options planned, but staff have asked the council to investigate the technical feasibility of low stopbanks.