‘Our stopbanks are not up to the job’
Farmer commissions engineer’s report
There are major
inherent weaknesses in the levees [stopbanks] protecting the flood plains under study.
Hawke’s Bay stopbanks have “major inherent weaknesses” as they are built from material susceptible to a “high degree” of erosion, according to an engineer’s report.
Hastings farmer Peter Raikes commissioned the report into breached stopbanks after Cyclone Gabrielle last February. He shared the report with Hawke’s Bay Today, saying he’s concerned about the “erodable” materials used to construct the stopbanks.
The report cost him $13,000 and was carried out by Royal HaskoningDHV, a company based in the Netherlands, with a long history of flood risk management.
Cyclone Gabrielle floodwaters breached stopbanks in 30 locations around Hawke’s Bay mostly along the Tūtaekurī and Ngaruroro Rivers (in the wider Napier and Hastings area) as well as the Waipawa River.
Hawke’s Bay grower John Bostock referenced the report during a recent Hawke’s Bay Regional Council meeting, calling on the council to urgently improve the stopbanks and “stop them breaking down”.
The regional council is in charge of the stopbank network and has received a copy of the report.
The 50-page study stated its main objective was to “record observations of damage” to the stopbanks but it was “not a comprehensive levee [stopbank] audit”.
A Royal HaskoningDHV engineer visited four locations last March, where serious stopbank breaches occurred. The report took particular interest in the make-up of the stopbanks — noting in general the core was shingle/gravel and silt, overlaid with a silt capping.
The engineer noted those materials were susceptible to a “high degree of erodability . . . during large flood events” — demonstrated by a serious breach next to the Ngaruroro River near Fernhill. An image from that breach showed a large amount of shingle/gravel and silt strewn out the side of the stopbank.
The study concluded there was “an urgency” to further investigate and improve the stopbanks.
“There are major inherent weaknesses in the levees [stopbanks] protecting the flood plains under study.
“These weaknesses will inevitably lead to future failures during similar flood events with potentially disastrous consequences.”
Raikes said he commissioned the report out of his own pocket after hearing of the large number of breaches.
He had wanted an independent expert opinion and the findings were concerning.
“Our stopbanks, the way they are constructed, are just not up to the job. Shingle is an erodable material and silt is as well.”
Raikes wanted to be helpful, rather than critical of the regional council, and hoped the report would lead to action, including investigating other materials to strengthen the stopbanks — such as a clay capping over the stopbanks.
“What I would like to see done, and it needs to be done urgently, is a plan put together and all the engineering investigation [undertaken].”
He said the current flood protection was “inadequate” and we need “to get stuck in and fix these stopbanks before it comes again”.
Raikes’ own land was flooded, but he said that was caused by overtopping of stopbanks. He said his concern was for the entire region, not just his own properties.
A Hawke’s Bay Regional Council spokesman said the report had been passed to its technical team “for future consideration”. He said stopbanks were generally constructed with “locally available materials”.
“In general, all stopbanks are a mixture of gravel and silt, capped with silt and finished with topsoil and grass. But this may vary by location.”
The spokesman said the key recommendations from the report had been acted on. “We started flood scheme reviews of the Heretaunga and Upper Tukituki catchments in July last year and we have engaged Tonkin and Taylor as the first step.”
An in-depth councilcommissioned flood review — the