Flood-risk areas may be widened
New houses in low-lying parts of Christchurch may need to be built with higher floor levels because of a proposed expansion to the city's flood management areas.
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As part of its review of the natural hazards chapter of the District Plan, the city council has been mapping low-lying areas across greater Christchurch where there is a risk of flooding and looking at what land use rules should apply in these flood management areas.
It is looking at increasing the size of flood management areas by about 30 per cent but that figure could go even higher depending on whether it decides to factor in a half-metre sea level rise or a one-metre sea level rise.
Nearly 2500 Christchurch properties were found to have been exposed to new flood risk because of the city's earthquakes when the revised flood management area was released in October 2012.
In affected areas, all new homes and commercial buildings will be required to have their floor levels at a height that protects them from a one-in-200-year flooding event.
That means their floors will need to be substantially higher than in other parts of the city where the risk of flooding is seen as minimal as they only have to comply with the Building Act, which specifies that floor levels must be above a one-in-50-year flooding event.
Existing homes within the flood management areas will only be affected by the new rules if they want to extend beyond their existing footprint and require a resource consent.
Quantity surveyor Stewart Harrison, of Harrisons Quantity Surveys, said the floor level requirements in the flood management areas would not necessarily add considerably to the cost of building.
Additional costs would be incurred if land remediation or extra foundation work was required as a result of geotechnical investigation, he said.
Council senior planner Fiona Eunson said when the council last looked at rules around flood management it only looked at certain areas of the city, such as those near rivers that were likely to be affected by tidal flooding.
This time it was looking across the whole city and taking into account the land level changes that had occurred since the quakes, as well as sea level rise.
"These flood management areas integrate a half-metre sea level rise but we are also mapping for a one-metre sea level rise," Eunson said.
Community feedback on whether to plan for a half-metre sea level rise or a one-metre sea level rise is being sought.