City Scene - November 2000: Holding back the flood

November 2000

Stimulating Senses

Cricket World Coming To City

Gardening Raised To New Heights

Port Hills - Keeping The Dream Alive

The People's Palace

Holding back the flood

A police officer wades through flood waters to check a home near the Heathcote River during the big storm in October.

Extreme flooding in Christchurch would result in tens of millions of dollars in damages and affect thousands of people.

To help reduce future damages, the Council is considering amending the City Plan. Prior to the proposed formal notification of the variation, Council staff are keen to get residents' views.

The proposed measures for new developments seek to reduce potential damages in Christchurch from flooding by waterways or the sea. Flood damages increase significantly when water enters buildings (average cost being $60,000 and $65,000 per dwelling).

Some areas of the City are vulnerable to flooding. At times of extreme flooding, over 200 houses could currently be flooded. These areas are located near the estuary and coastal margins, as well as on the flood plains of the main river systems.

Accelerated rise in sea levels, as well as increased severity and frequency of storms, is predicted under global warming, therefore it is important to plan now to reduce future damage. This approach is consistent with policies in the New Zealand Coastal Policy Statement and the principles of the Resource Management Act.

The measures proposed to be included in the variation have been assessed in detail in the flood plain management investigations by Christchurch City Council and Environment Canterbury. They are:

Waimakariri River

100m setback for future buildings adjacent to the primary stopbank

Addition of a new policy to the plan which seeks to avoid high risk development within the overflow channels.

Estuary and Coastal Margins, and the Avon/Heathcote/Styx, Sumner, and Landsdowne Valley Flood plains

Addition of a new rule requiring floor levels to be set at a minimum level of 11.8m CCC Datum in tidal areas. This figure incorporates 0.5m for sea level rise being the best estimate from IPCC (Inter-Government Panel on Climate Change over the next 100 years.

In non-tidal areas it is proposed that floor levels be set at a minimum of a 1 in 200 year event (0.5% chance of happening in any one year). Note: over a 70 year period there is a 33% chance of an event of this magnitude occurring. It is proposed that the areas where this rule is proposed to apply will be identified on the planning maps. Non-compliance with the rule would require approval through theresource consent process.

Additional restrictions on filling are also proposed within these areas.

In some locations the minimum floor level rule will be difficult to apply at the minimum section size for the zone. For this reason it is proposed to increase the section sizes for new lots in these locations to make compliance with the minimum floor level rule easier.

The investigations by the Council indicate minimum floor levels are a cost-effective way of reducing the level of damages from flooding over time. They will also help reduce the dependency on engineering measures.

In some places, however, engineering may be required to reduce flood damages to existing houses. It is suggested the planning measures proposed need to be put in place now considering the life of a building is 50 to 100 years.

The Council is seeking feedback on the proposed measures, outlined in a discussion document, by the end of January (2001). Copies of this document can be obtained by phoning the Council numbers below.

Meetings are being arranged with residents in the areas affected by the proposed rules and for people in the building industry. For more information contact
Water Services planning engineer
Tony Oliver on 371-1394, email
tony.oliver@ccc.govt.nz, or the
City Plan Team on 371-1631.