Minimum floor levels tipped to rise
People living in low-lying areas of Christchurch may need to raise the floor levels of their homes if they are repairing or rebuilding under new city council rules aimed at reducing the risk of flooding.
The council will today release flood-modelling information it has collected since the September 2010 earthquake to enable it to set floor levels for building work across the city.
The information was released to insurers in August but has not been available to property owners until now.
It potentially affects more than 10,000 properties.
Council regulation and democracy services general manager Peter Mitchell said the quakes had caused significant land damage, and ground levels across large areas of the city had dropped an average of 200 to 300 millimetres.
For some months the council had been working to better understand the extent of the damage and what needed to be done to protect properties from flooding.
Much of the work had focused on the Avon River catchment, but some investigations had been completed in the Styx and Heathcote catchments and in Sumner.
The investigations showed that of the 160,000 properties in Christchurch, 10,361 in the Avon, Styx and Heathcote catchments had the potential to flood in a 50-year rainfall event - 769 more than before the quakes.
Most of those properties were in already identified flood-management areas, but some were in adjoining areas not previously considered at risk of flooding.
"Each time the city has experienced a major [seismic] event, the land has been surveyed to ensure the council has a thorough understanding of what has been happening with the land," Mitchell said.
The Press understands that minimum floor levels in some areas are likely to change as a result of the data collected. They will come into effect immediately and could affect rebuilding plans and insurance.
Minimum floor levels of 11.8 metres above the Christchurch City datum have been in effect in flood management areas since January 2011, when variation 48 of the city plan became operative.
However, today's data release is likely to see requirements for floors to be built to that level, and possibly higher, in other areas that have sunk.
The existing flood-management areas are around the Styx, Avon and Heathcote rivers, in Lansdowne Valley and in some low-lying coastal areas, including Redcliffs and Sumner.