The Waipā River breached its banks on January 29 with flood waters lapping the base of the Ōtorohanga stopbank.
ANSWERS to questions about the performance of the Ōtorohanga stopbanks will be found, mayor Max Baxter said.
He was commenting on questions from Honakiwi resident David Sellars about the vulnerability of Ōtorohanga township to flooding from the Waipā River.
In a letter read by councillor Kit Jeffries in the public forum of Tuesday’s district council meeting, David said between ex-cyclones Hale which struck the East Coast on January 13/16 and Gabrielle this month, there was another un-named event between January 27-30 that delivered 230mm of rain at Honakiwi – more than either of the former cyclones.
During that event, the Ōtorohanga District Council monitored the Waipa River stopbanks and closed the stopbank walkway.
David asked what the rainfall and flood assumptions were in the late 1950s that determined the height and width of the stopbanks, and how valid those assumptions were now.
“The stopbanks were clearly designed to handle certain river depths and floods. What is the relationship between those river depths and flows and any given amount of rainfall?” he asked.
Factors would include the duration and amount of rainfall and soil saturation in the catchment. During Cyclone Gabrielle, the Gisborne ranges received 488mm of rain and the Hawkes Bay ranges 337mm, David said.
He wanted to know if the stopbank design information was known by the council, and what were the answers to his questions.
“And if not, what is the council going to do to ensure that these questions can be answered,” he said.
Mayor Max Baxter said they were all relevant questions and with what had happened across the country, he had been asking a similar question.
“I’m not going to put any council staff on the line at the moment but I’m sure as a consequence of this event and the devastation across the country, our staff will be having conversations with the regional council in terms of what their modelling is.
“I think we need reassurance as councillors as well, that we can talk at a level of confidence further down the track should there be an extreme weather event. And we know what we are in for and know what mitigation is in place to ensure the stopbanks aren’t inundated.
“And I think if we look at the devastation where the stopbanks in this last event were breached and the flood levels had risen so quickly, that that is where the real danger is. So good questions by David Sellars and I thank him for that.
“Obviously we don’t have the answers at the moment but I’m sure this investigation will go on because we as councillors want that confidence, so thank you.”
