Tauranga extra-vulnerable to rain slips

A "very large number" of houses throughout the Tauranga area are vulnerable to landslides in intense rain, says an independent report into the slips caused by last month's flood.

"This is the way the city has developed," the report warns, "and similar situations exist in many - if not most - New Zealand cities and towns."

Engineering geologist Bernard Hegan and geotechnical engineer Laurie Wesley said the main cause of the May 18 slips was a one-in-100-year storm. Residents spoke of large "rivers" and "waterfalls" on their properties.

"Of almost equal significance is the fact that New Zealand is a very young country geologically, and the slips and erosion are part of the natural processes occurring throughout the country.

"There are probably few areas in the North Island that are not susceptible to slip damage to some extent in the event of severe rainfall storms," they said in a preliminary report to the Tauranga City Council.

They said Tauranga appeared to be particularly vulnerable because of the prevalence of steep slopes - the remains of sea cliffs formed by erosion - where volcanic ash soils were in danger of being weakened by rain.

Many of the latest landslides were where ancient or relic slips had happened in the past, the report revealed.

"However, some are first-time slips initiated by a combination of the local geology and the local intensity of the storm rainfall.

"Human activities have to some extent contributed to the probability of such slips, but in our opinion are of secondary importance."

The geologists said few of the slides they inspected were surprising or unusual, given the general steepness of the topography. But "this is not to say they could be predicted with any certainty."

It was virtually impossible, for both technical and economic reasons, to make houses totally secure, the experts said.

Limited measures could be taken, such as getting rid of soak holes (which drain off rainwater into underground pits), especially those close to the edge of steep slopes. Equally effective would be ways to intercept large concentrations of surface run-off down concrete driveways and cul-de-sacs, flowing toward the edge of vulnerable slopes.

The report did not find any direct relationship between soak holes and the slope failures.

Nor could the influence of ground reshaping be determined. It depended on what the aim was, and how it was carried out.

"The construction of pole walls retaining significant quantities of fill at the top of slopes is clearly undesirable as it places an added load," the geologists said.

On several sites, the slips had taken walls and fill with them.

To protect house buyers it was possible to identify "with limited accuracy" houses at risk of slips.

Mr Hegan and Mr Wesley said they were aware the Tauranga council had "quite rigorous" procedures in place to ensure sites met modern criteria guarding against landslips.

However, they suggested last month's slips warranted a review, citing the "surprisingly large travel distances" of several slides and the number of houses damaged.

The preliminary report is part of a wide-ranging council investigation into the damage, expected to take six months.

Flood update

* 299 homes and 719 people evacuated in Tauranga

* 741 properties inspected for flood damage

* 145 flood-damaged properties at present uninhabitable

* 326 properties inspected for land slip

* 60 landslip properties still uninhabitable