Silverstream homes at risk

Up to 300 new homes in Silverstream Estates may be flood prone due to an error in the original survey. Residents were expected to receive a letter this week about the potential problem.

A similar problem affecting five homes rebuilt in South Brighton was discovered a few weeks ago and the Christchurch City Council is working with affected owners.

Prime Minister John Key opened the 1115-section subdivision in June 2012 and since then about 700 titles have been created and 300 homes built.

The problem with benchmark flood levels set by Davis Ogilvie engineers was discovered three weeks ago by a surveyor working for a building company.

The council subsequently obtained confirmation from the developer, Waimakariri District Council chief executive Jim Palmer said.

The levels appeared to be about 150mm too low, Palmer said.

‘‘We are unable to comment on what affect this could have on values or insurance.’’

Foundation levels were based on one-in-50-year flood modelling, which recommended a level between 4.3m and 4.4m above sea level depending on the individual property.

But the level also includes an additional council safety margin (freeboard) of 300mm.

‘‘It doesn’t automatically mean every building foundation is too low because of the way the subdivision was contoured plus the extra freeboard,’’ Palmer said.

The 2014 flood in the area was a one in 60-year event and none of the houses built at that time had water in them, Palmer said.

The council would consider mitigation after it obtained all the information.

‘‘It’s the only subdivision we’re aware of that’s affected,’’ Palmer said. Davis Ogilvie civil engineers, which carried out the initial surveys, is working with the council.

Davis Ogilvie director and licensed surveyor Russell Benge did not believe many sections would be affected.

A saleswoman at the Silverstream development said she knew nothing about it and referred inquiries to the council.

The incorrect levels became a topic of conversation in business circles last week after builders seeking building consents were told about the height issue, which will be noted on land title records.