Ōtorohanga stopbanks get okay

WAIKATO Regional Council has provided assurances to questions about the performance of the Ōtorohanga stopbanks, that were read out at the recent Ōtorohanga District Council meeting.  

Honikiwi resident David Sellars said between ex-cyclones Hale, which struck the East Coast on January 13 to 16, and Gabrielle last month, there was another un-named event between January 27-30 that delivered 230mm of rain at Honikiwi – more than either of the former cyclones.  

During that event, the Ōtorohanga District Council monitored the Waipā River stopbanks and closed the stopbank walkway.  

David wanted to know what the rainfall and flood assumptions were in the mid-1950s that determined the height and width of the stopbanks, and how valid those assumptions were today.  

Regional council communications advisor Inger Voss said Ōtorohanga stopbanks had not overtopped since their construction in the late-1960s. The stopbanks were part of a three-stage flood control.

In general, the river channel carries annual floods, while larger events breach the river banks and flow over the berms and floodplains, she said.

The stopbanks were designed to withstand a flood magnitude equivalent to that of February 1958, which was considered a 100-year flood, Inger said.  

Data from the WRC hydrology team’s analysis of rainfall and river levels from January 18 to February 3 showed lots of rain but comparatively little flooding.    

“It should be noted that a 100-year rainfall doesn’t necessarily generate a 100-year flow, especially in complex catchments,” she said.

“The rainfall in Te Kūiti (253mm) was assessed to be in excess of a 250-year event, yet the river level was assessed to be around a 20-year level.

“The Waipā River level at Ōtorohanga was assessed as a five-year to 10-year level, and less than that downstream.

“The Waitomo stream at Aranui Caves Bridge was a 50-year to 100-year level.”

During Cyclone Gabrielle, rainfall and river levels were lower than those of the Auckland Anniversary weekend event, she said.

The Ōtorohanga stopbanks were constructed with 0.61m (2ft) freeboard above the design flood level to allow for wave action, debris blockages, and other uncertainties.

The floodway capacity – flow area between stopbanks – within the Ōtorohanga township is about 50% larger than the 1958 flood.

The Ōtorohanga scheme was designed and built to withstand a specific large event, which was nominally assumed to be the 100-year event.

The assurance should be based on three factors including, maintaining the height and structural integrity of the stopbanks and associated infrastructure, maintaining the capacity of the river channel and floodway and review of the scheme design parameters taking into consideration changes in hydrology, river channel capacity and catchment changes in land use.

The district council is responsible for ensuring the structural integrity of the stopbanks and associated infrastructure.

The regional council is responsible for maintaining the river channel capacity.

Inger said WRC was not aware of any issues with the stopbanks that were not attended to by ŌDC. Also, the river channel was being inspected and maintained as necessary.  

Hydraulic reviews were generally undertaken every 10 years.

The most recent hydraulic model was in 2015, and the results were that the stopbank design levels remained appropriate to accommodate the 100-year flood.

The next review will take into account climate change effects on flood levels.