ORC natural hazards analyst Julion Wright said there was still work to be done on how to mitigate the town’s flood risk but it was a consequence of the valley’s creation.
‘‘It’s a natural process. The very land that the area sits on is built by these processes.
‘‘It is one of those things — you are here in this beautiful landscape because of these sort of intense processes. It’s important to remember that.’’
ORC and WSP staff were on hand along with people from the Central Otago District Council (CODC) and Civil Defence to discuss the ORC’s latest research into alluvial fans around the town’s 13 creeks.
ORC manager natural hazards Jean-Luc Payan said the study was an extension of work done for the CODC spatial plan.
The Teviot Valley was a really good example of land formed by alluvial fans, Dr Payan said.
‘‘Steep terrain, flat land and the creeks that drain those steep hills, when they come on the flat land, because of the slope easing, they start to deposit material.
‘‘They spread it in a fan, but also with time, the channel kind of goes one way, deposits material, shifts to the other, to another side.’’
The resulting fan shapes were flat land in rough terrain and had good, fertile soil, he said.
However, because of the way they were formed during intense rain, such as during a thunderstorm, debris including large boulders and rocks could wash down the hill.
Roxburgh had experienced that in 1978 and 2017.
The water and rock coming down the hill were like a concrete slurry with boulders and rocks heavy enough to block culverts and damage infrastructure such as roads, buildings and power poles, Dr Payan said.
It was difficult to predict such weather events and they were often brief.
‘‘The time between the rain starting and the debris flowing is very, very short.
‘‘We’re talking less than an hour sometimes, so our response time is very limited sometimes.’’
Being prepared at all times for civil defence response was crucial given the speed with which an event could happen.
While there were some engineering options such as deflection bunds, widening and concreting channels and sediment ponds, non-structural measures such as planning intervention and civil defence preparedness would also reduce the risk to residents and property, Dr Payan said.
