Building owner fears rising water table
Christchurch central-city building owner Richard Owen fears a neighbouring new deepwater bore heating system has raised the water table and threatens his building but officials suggest high winter rainfall may be to blame.
A recently completed Environment Canterbury (ECan) report has revealed potential flooding affecting sewerage and drainage systems, and making central-city land more prone to liquefaction.
Owen said basement flooding in the Wynn Williams building on the corner of Hereford and Montreal streets began in June, two months after the deepwater bore system was switched on at Nga¯i Tahu Property’s new office building on the former King Edward Barracks site.
However, Roger Sutton, project manager for the Nga¯ i Tahu building’s heating system, said a test had been carried out by switching it off during Labour Weekend and it made no difference.
ECan groundwater science manager Carl Hanson said high rainfall in winter was a major cause. He was discussing the report findings with consent holders and landowners.
Future resource consent applications would need to be considered more closely, Hanson said.
Building owner Owen said the weekend trial was too brief. His structural engineers had warned that mounding pressure of water could affect the foundations of the relatively lightweight wooden office building.
The potential problem arose because the heatpump system took water from the Linwood aquifer at 150 metres deep, and to avoid thermal mixing and contamination, the water was returned to the shallower Riccarton aquifer at 40m.
The bus interchange, justice and emergency services precinct, Arts Centre and ECan used similar systems.
The city council was planning a groundwater source heatpump for the new central library, and O¯ ta¯ karo has included one in the design for the convention centre but final decisions had yet to be made and they would require resource consent.
An O¯ ta¯ karo spokesman said the metro sports facility designers had abandoned the idea of a groundsource heatpump and would harness heat from a city council wastewater pipe in St Asaph St.
The area around the planned metro sports facility has some of the shallowest water table depths between 1m and 0m, the report said.
The new ECan report confirmed re-injecting water to the shallower Riccarton aquifer has the effect of raising the water table and provided conditions for problematic saturation in localised patches of the central city, especially from Moorhouse Ave to Hagley Park.
Water-table mounding would lead to higher moisture levels in cellars or underpasses, saturation of soils, surface ponding, flooding of buried sewers, and increased liquefaction risk.
The Colombo St crossing of Moorhouse Ave and slightly east was the principal area of potential surface saturation. The report recommended further investigation.
A recent Land & Water Regional Plan change made pumping for groundwater heating systems and discharge to the Riccarton aquifer a permitted activity.
‘‘Questions arose following the plan change due to the belief that unconstrained proliferation of heating systems discharging to the Riccarton Gravel could give rise to unanticipated consequences that would be more than minor.
‘‘Regulatory controls over ground-source heating systems should be explored in concert with any reviews of Land & Water planning documents,’’ the report concluded.