Hundreds more flood-prone in Christchurch

PETER MITCHELL
Last updated 07:35 10/10/2012

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The Christchurch City Council will publish updated floor levels today - the heights above ground level that buildings must be constructed in a city prone to flooding. Council regulation and democracy services general manager PETER MITCHELL explains what's been happening with the city's land and drainage network as a result of the earthquakes, and the work being undertaken by the council.

Christchurch has always been a flat, low-lying city with areas prone to flooding. This flooding has tended to be shallow and more of an inconvenience rather than a serious threat to life or property.

The Canterbury earthquakes, as most homeowners are aware, caused significant damage to land throughout the city.

The worst hit areas tended to be those closest to riverbanks and waterways, with ground levels across large areas of the city dropping on average 200 millimetres to 300mm.

For some months now, the Christchurch City Council has been working to better understand the extent of the damage and what work is needed to help recover the city's land drainage/flood protection network and what measures need to be put in place to protect properties from future flooding.

Much of this work has focused on the Avon River catchment, but some investigations have also been completed in the Styx and Heathcote river catchments and in Sumner.

Though the earthquakes have extended some of the boundaries of the areas prone to flooding, resulting in an increase in the number of properties that are now likely to flood, there are also properties that no longer flood as a result of the changes to the land.

Of Christchurch's 160,000 properties, 10,361 in the Avon, Styx and Heathcote river catchments have the potential to flood in a 50-year rainfall event. This is an increase of 769 properties since the Canterbury earthquakes.

What is important to remember when we consider the risk of flooding in Christchurch is that the greatest potential for damage from flooding in the city remains the Waimakariri River bursting its banks and discharging what would be large volumes of water across the city.

The stopbanks that protect the city from such an event were damaged by the earthquakes but have since been repaired by Environment Canterbury to provide protection from a one-in-500-year event.

A secondary stopbank under construction by ECan will significantly lower this risk to somewhere in the order of one in 10,000 years.

The city's own stopbanks along the Avon were also damaged during the earthquakes and have been repaired and raised in the worst-hit areas close to the rivers.

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Council staff are regularly inspecting these and undertaking additional repairs as required.

Many low-lying areas near streams and rivers are experiencing impeded drainage and base-flow water levels higher than in the past as a result of the lower ground levels.

This is annoying for residents but these will be addressed by repairs to the drainage system being undertaken by Scirt, the Stronger Christchurch Infrastructure Rebuild Team, and the council is about to undertake floor-level surveys in areas at risk.

The city's land drainage network has been moderately tested since last February's earthquake without any serious consequences or identifying any new areas of risk. There have been no extreme storms during this time, the biggest event being about 95mm over a 72-hour period in August. The intensity and depth was a one-in-three-year event. There was surface flooding in areas that have always been susceptible to flooding - it was no worse than in recent times.

From a tidal perspective, the biggest test to our land drainage network was in July last year when the stopbanks contained the high tide, preventing any tidal flooding of the low-lying mostly red- zone suburbs around the lower Avon. This tide did not coincide with a significant storm. The predicted 50-year tidal event is about six centimetres higher than this.

The city has developed a programme of work for recovery of the land drainage network. This work is prioritised with the first projects to be delivered before the end of this year. This includes understanding the effects of the earthquakes on the Avon, Heathcote and Styx rivers and the flood plains in the tidal reaches, new river/ stormwater modelling to identify and manage any new drainage issues and a full assessment of the Port Hills stormwater network.

The council began work in the 1990s to identify flood management areas in the city. This was first notified in 2003 as Variation 48 to the city plan. This variation was made operative, becoming part of the city plan, on January 31 last year.

Flood management areas are those that are prone to flooding, as a result of a major tidal or rainfall event, and that are vulnerable to the effects of climate change as a result of rising sea levels. These areas were identified to help reduce future damage to the city from flooding.

Christchurch's flood management areas are around the Styx River (lower areas), Avon and Heathcote Rivers, in the Lansdowne Valley and in some low-lying coastal areas, including Redcliffs and Sumner.

Today the council will issue new land information data that has been collated since the first earthquake in September 2010. Each time the city has experienced a major event, the land has been surveyed to ensure the council has a thorough understanding of what has been happening with the land. This new data will show property owners the accumulative effect of the earthquakes and aftershocks on ground levels, and what this means for the rebuild.

- © Fairfax NZ News

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