The massive amount of gravel accumulating behind the dam would otherwise enter the Tukituki River, eventually reach the coast and replenish the entire coast between Clive and Tangoio. Demand for shingle from local sites along the Tukituki River will depend on gravel moving from the other two rivers. Reduced supplies will put pressure on established extraction sites and lead to considerable cartage costs from deposits at the upper reaches.
Somehow, the Board agreed with an expert claiming "the dam will have a lesser effect downstream towards the coast because there is surplus gravel in upstream rivers". The Board accepted this short-term situation and would not consider how Council's latest figure of 180,000m3 of gravel blocked behind the dam equated to the mere 1688m3 reduction at the river mouth, as calculated by the Regional Council engineers.
Engineers rounded up the 1688m3 then doubled the amount so they can put 1700m3 on each side of the river mouth, just in case a southerly coastal sediment drift appeared for the first time. Engineers would not explain the futility of excavating and carting 3400m3 of gravel from the Tukituki River to a river mouth within the same river system. This is counter-productive or akin to "robbing Peter to pay Paul" where the benefit to the coast is "net zero". This entire process is to mitigate any possible coastal erosion due to any interruption to beach replenishment caused by the Ruataniwha Dam.
During the three years following Cyclone Bola, 477,000m3 entered the coast but since 1993, annual input has been between zero and 28,000m3. All beaches between Clive and Tangoio are lacking replenishment therefore in the absence of major storm events, any interruption to supplies will exacerbate erosion during the coming decades. The long-term trend for erosion north of the Tukituki River mouth, measured from beach profiles between 1989 and 2012, is around 1.60 m/year. The dam will ensure this trend will extend north to Napier's Marine Parade and further north to Tangoio.
The Regional Council advised the Board of Enquiry that it did not need to consider pre-existing erosion on the coast. Even though erosion on HB beaches is solely due to lack of replenishment, they did not find reducing the only source of gravel for HB beaches was significant, let alone needing more than 1688m3 to mitigate potential erosion.
Prof Komar's report, which initiated the Coastal Hazard Strategy, noted "the gravel barrier ridge is vital defence and without it large areas would be regularly inundated". Providing these beach ridges (once known as shingle spits) are constantly replenished, they naturally grow in height and breadth and counter the effect of sea level rise due to climate change (Dr Cowell). Without a naturally raised beach ridge, without adequate natural replenishment, and without protection for the backshore, residents will be asking "who on earth created this mess"?
Any Councillor who supports the building of the dam while claiming there is a need to protect the coastal environment is obviously misguided.
Like any other council, Napier City Council should fully support water storage but supporting the Ruataniwha Dam will be a sorry legacy for future generations.
- Larry Dallimore is a long-time coastal erosion campaigner who will contest the Ahuriri seat in the upcoming Napier City Council elections.
- Views expressed here are the writer's opinion and not the newspaper's. Email: editor@hbtoday.co.nz
