A TIDAL BARRIER TO PROTECT CHRISTCHURCH? A CONTROVERSIAL PROPOSITION

As part of the Land Drainage Recovery Programme the Christchurch City Council undertook a pre-feasibility study of a tidal barrier across the entrance of the Avon-Heathcote Estuary. A barrier has been considered at this location for over 50 years. Its purpose for Christchurch would be two-fold: firstly to mitigate fluvial flood risk by artificially holding back the advancing tide during high flows from the Avon and Heathcote Rivers. This would allow the rivers to drain more freely into the estuary. Secondly, it would hold back exceptionally high tides to protect low lying land.

In the face of increased flood risk as a result of the earthquake sequence, and impending climate change, it was considered pertinent to consider a tidal barrier. This paper briefly explains the tidal barrier design proposition while the major focus is on the Council and community reaction to the barrier proposal.

The study concluded that a barrier is technically feasible. It would cost in the range of $300 - $350M, with operations and maintenance ranging from $2 to $7M per year. This would protect a considerable area of public and private assets.

Council requested commentary from its key stakeholders on the proposition. Strongly worded responses were forthcoming but their direction was mixed. Some stakeholders were highly opposed to a hard engineering solution and considered it represented a false security. There were widespread concerns about the perceived lack of consultation. The environmental effects were raised, but paradoxically so were the concerns that the estuary would be fundamentally changed through sea level rise. Several responses suggested a city-wide integrated approach to flood management would be more pragmatic.

The Council considered the proposal, feedback and alternatives and voted not to proceed with a full feasibility study. The paper discusses the implications for other Councils in presenting engineering options to mitigate climate change.