5 Nov 2015

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Detailed design work on two further schemes to mitigate flood risk may go ahead following a Christchurch City Council committee recommendation today.

Two staff reports from the Council's Land Drainage Recovery Programme were considered at today's meeting of the Infrastructure, Transport and Environment Committee. Councillors recommended that the preferred remediation option, identified for both Bells Creek in Woolston and Knights Drain in Bexley, progress to the detailed design stage.

For the Bells Creek scheme, the preferred remediation option is expected to reduce the frequency and severity of flooding in a 50 year rainfall event and benefit at least 31 properties that are at risk of above floor flooding, with a further 88 properties experiencing reduced flood levels. Proposed Stage 1 work includes:

  • A pump station located on Ferry Road near the Heathcote River.
  • Minor re-grading of a short length of Richardson Terrace.
  • A new detention pond within the northern half of Edmonds Park (where the sports fields are currently located. This will be dual-use, with the park flooding only in large rainfall events, and otherwise still operating as sports fields).
  • A new detention pond within Linwood College's lower fields.
  • Upgrade of pipes in the Mackworth Street area.

The Stage 1 area was identified as an early works package to speed up construction. Further investigations are currently underway to identify options for reducing flood risk in the Moorhouse Avenue, Ferry Road, Mackenzie Avenue and Tabart Street areas (Stage 2).

The preferred remediation option for Knights Drain will see six of the 13 homes affected by above floor flooding benefit in a predicted 50 year rainfall event. A further 70 of an estimated 230 properties at risk of underfloor flooding will also see flood levels reduce. Another 60 homes will be protected by the proposed pump station during extreme high tide events. Proposed work includes:

  • A pump station and pressure main, which is a pipe where the water in it is pumped under pressure, usually, to higher ground (Stage 1).
  • Remediation of the existing stormwater basin (Stage 1).
  • Modifications to Knights Drain (Stage 1).
  • Using a small part of Wainoni Park as detention storage (Stage 2).

Stage 1 work can be fast tracked as these structures are either already in place or are on Council-owned land. However, as Stage 2 is in a small part of an existing park wider community involvement is required ahead of any proposed work.

The proposed detention basins for both schemes will be dual-use and will only be used as detention basins during large rainfall events. Edmonds Park sports fields will continue to operate and the Wainoni Park area will be designed to incorporate picnic facilities. 

For both schemes, in extreme rainfall events the preferred option aims to return these areas to their pre-earthquake level of flood risk. However, there will still be flooding of streets, properties and some homes.

Committee Chairman Phil Clearwater says, "Remediation work will improve the capacity of the stormwater drainage network. This means we can mitigate against homes in these areas flooding by adding or upgrading infrastructure such as pump stations and detention basins. We can also enhance these schemes in future." 

Land Drainage Recovery Programme Working Party Chair Councillor Pauline Cotter says, "Damage to land and stormwater infrastructure in these two areas poses a significant flood risk to homes. The remediation options provides us with a clear way forward to reduce this risk. 

"While these two schemes are important, we haven't forgotten other areas of the city also affected by flooding and staff continue to investigate and address the issues faced by residents affected by flooding," she says.

The Council is expected to make a decision on the Bells Creek and Knights Drain schemes at its next meeting on Thursday 12 November 2015. Following this, Council staff will be refining the schemes' designs by surveying floor levels, validating modelling and talking to residents. In the coming months, there will be opportunities for residents to be involved and provide feedback on these schemes as part of this work.

The estimated cost for the Bells Creek Stage 1 works is $9 million and Knights Drain $11 million.