A sinkhole has been found in the face of a flood detention dam in Elsthorpe but the nearby community is not at risk, says the Hawke's Bay Regional Council.
The sinkhole was discovered during one of the regional council's regular dam inspections on May 31, in the face of the largest dam on Kokatewai Road. The local community has been informed and engineering specialists have visited the site.
Five Makara and Atua Valley flood detention dams were constructed in the early 1980s after severe flooding in the previous decade.
Regional council group manager asset management Mike Adye said the situation was that the bottom of the corrugated steel pipeline at the base of the dam had rusted.
When the pipe was flowing full, as it did in April 2011 and again in January 2012, turbulence caused fill material from around the pipe to be sucked out, which made a void around the outside of the pipe. The top face of the dam has subsequently given way and dropped down into the cavity, creating a sinkhole in the dam face.
"Our first priority is the safety of people downstream of the dam," he said. "However, there is no risk to them while the weather remains fine with no significant rainfall events forecast. There is no current risk as there is no water in the dam."
Engineers have three diggers on standby should severe weather be forecast. The diggers would be used to reduce the height of the dam to ensure there was no risk to public safety.
"If the dam is reduced in height there will however be a potential risk to farmland and property downstream of the dam," he said.
In normal weather conditions a flood detention dam is empty, with in-flowing water exiting through a pipe in the face of the dam. During storm events the capacity of the pipe is exceeded and water builds up behind the dam.