Fast-tracking Floodplain Management in an Uncertain Climate: The Оpāwaho/ Heathcote River Story

The Ōpāwaho / Heathcote River catchment has a history of flooding and poor water quality. Floodplain management schemes have been developed in the past by the Christchurch Drainage Board (1985) and Christchurch City Council and Environment Canterbury (1992). These identified a range of options including storage in the upper catchment, in-channel works in the mid reaches, raising of some houses, and constructing a large bypass channel, the Woolston Cut. These schemes were partially implemented, but a drier period during the 1990’s and 2000’s meant that many of the key elements were not installed.

The Canterbury Earthquake Sequence (CES) dramatically changed the flooding situation for the Ōpāwaho / Heathcote River, with a general uplift in the lower reach combined with settlement in the mid and upper catchment. This had the effect of exposing 101 additional dwellings to the risk of flooding above the floor in a 2% annual exceedance probability event (AEP). However, it also increased the number of houses at risk of flooding above the floor in frequent events, and exposed significant stretches of riverside roads to additional flooding. Flood events in 2013, 2014 and 2017 resulted in some houses flooding above floor level multiple times, and repeated deep road and underfloor flooding.

Investigations into potential responses to the increased flood risk throughout the catchment began following the CES. This included asset condition assessment, surveys (landform and floor level), and updating flood models. Throughout this period there was a series of public meetings to update those affected by flooding to communicate the work underway and to convey the effort being expended to identify suitable response options. The focus of this work was on developing long term adaptive pathways focusing primarily on the 10% and 2% AEP events. The work of past studies was heavily relied upon for the likely suite of options suitable for the catchment.

The July 2017 flooding created a situation where the community needed a response plan to deal with the frequent flooding. Utilising the investigations undertaken to date, a floodplain management strategy was developed which focused on frequent (10% AEP) flooding with the current climate and sea level. This enabled rapid approval as it was affordable, could be readily implemented, met the immediate needs of relief from frequent flooding, and also did not compromise implementation of longer term measures. The toolbox of measures responded to a range of frequencies and time scales. It included voluntary purchase of the most affected houses, dredging (which was identified as providing benefit up to 0.25m sea level rise), bank stabilisation, upper catchment storage, and consultation on low stopbanks. The total package of works approved in November 2017 was approximately $80 million over a period of 5 years.

Some of the key findings were: the importance of good communication to allow decision makers and the public to understand the issues; the importance of quick wins to build confidence; utilising a short term response to achieve long term benefits; communicating the benefits achieved while