Mud volcano risks identified in district

In 2018 this mud volcano erupted on the Uttings’ farm in Waimata Valley. A new report for the Gisborne District Council has flagged other areas at risk from similar eruptions. The report notes that this event at the Uttings’ was short-lived but violent, and capable of throwing large boulders over 100m. Drone picture by Ben Cowper

Homes on Wheatstone Road could be among those officially flagged as being at risk from mud volanoes, a new report reveals.

A staff report to go before Gisborne District Council’s Civil Defence and Emergency Management committee today (Wednesday) said the council started a comprehensive programme in 2016 to understand the location and risk the natural hazards pose.

“Our understanding of risk has been enhanced by an assessment of a new mud volcano eruptive centre on Uttings’ farm in the Waimata Valley and a more recent eruption on the adjacent Monowai Station,” the report said.

“It was not until the eruption of the Uttings’ mud volcano in December 2018 that it was clear that these features posed a potential risk to the community. Prior eruptions such as the earlier eruption of the Savages mud volcano on Monowai station in 2003 occurred in areas remote from dwellings.

“The Uttings eruption occurred within several hundred metres of a farmhouse and occurred at a site not previously recognised as being at risk.

“The Uttings eruption was short-lived but violent, capable of throwing large boulders over 100m, and was associated with methane gas and lesser higher gaseous hydrocarbons likely to includes volatile wet gases such as propane.

“A significant volume of debris was deposited over an area of over 2ha within a few hours.”

At that time, the initial site investigation by GDC principal scientist Dr Murry Cave found that rocks were ejected from about 10km under the ground. But with traces of boron found it could mean it potentially could have come from deeper than that —potentially from the Earth’s Crust.

A more recent LiDAR (light detection and ranging) acquisition, along with high resolution satellite imagery, has allowed GDC to map these features and understand the broader significance they pose in a hazard context.

“It has been established that the eruptive centres which have been long known to a varying degree of accuracy are merely the manifestation of a larger area of risk defined by mapped dome or cone-like features known as diapirs. In turn, diapirs occur along fault zones that incorporate zones of highly disturbed rock (melange),” the report said.

Two zones of risk have been identified —  the diapiric structures themselves, and the more widely extensive melange.

“We have identified specific areas of highest risk on which dwellings have been constructed as well as a broader area where there are no dwellings currently and a prohibition on construction would be advisable.

“Areas at risk with existing dwellings should have Land Information Memorandum (LIM) reports flagged to include the mud volcano risk and a zone defined where new builds are prohibited.

“Within Gisborne city and its environs, the Wheatstone Road-Sponge Bay area is an area of risk and is one where LIMs should be flagged. The Knapdale Eco Lodge and adjacent properties should be similarly flagged. as should several properties in the Waimata Valley, and between Pouawa and Tolaga Bay.”