Consultants estimate the dam will block 260,000m3/year and without commercial extraction, most of this shingle would eventually reach the coast. With limited data, HBRC assessed the dam would reduce gravel input by a mere 1688m3/year. HBRC rounded this figure to 1700m3 and added another 1700m3 for the southern side. Dr Murray Hicks, principal scientist on river and coastal geomorphology at Niwa advised HBRC "coastal sediment delivery has been inadequately assessed".
HBRC convinced the Board of Inquiry that coastal erosion was pre-existing and any issues beyond the rivermouth did not need to be considered. During a recent public debate on the RWSS, Andrew Newman was asked a very fair question: "Who will accept liability when the dam is found to be a major contributor to coastal erosion on HB's gravel beaches". Mr Newman answered. "The issue is whether the dam over a long, long, long period of time reduces gravel loads at the mouth of the Tukituki River. The consent conditions require the HBRC to replenish gravel at the coast and we are replacing gravel at double the loss as assessed by the engineers as being the right level."
The crucial issue is replacing "double the loss" at 3400m3. This represents just 2 per cent of gravel volumes following Cyclone Bola, 3 per cent of assessment by ASR Consultants, and just 1.8 per cent of the average 187,000m3 extracted for commercial use each year.
Trucking the meagre 3400m3 of gravel from upstream where the material would otherwise naturally flow to the coast is a futile exercise. Moving gravel to replace gravel lost in the same river system is counter-productive and akin to "robbing Peter to pay Paul". Dr Murray Hicks from Niwa disagreed with council's methodology and advised several problems with council's simple gravel budgeting assessment to derive this 3400m3 sediment deficit. Napier has Awatoto, Napier South, the CBD, Ahuriri, Westshore, and Bay View formed from, or protected by three shingle spits. Spits maintain a constant state of accretion providing replenishment is greater than erosion. The dam will add to the risk of erosion and the eventual need for protection by "last resort" hard engineering. The choice will be "do nothing" and evacuate or build a $100 million 38km rock seawall.
Without impediments to the supply of gravel, these spits would be similar to atolls and continue to grow in height as determined by swells and the level of the sea. There will be fewer problems from climate change and sea level rise if councils did not interfere with material needed to maintain our unique shingle spits.
The integrity of gravel beaches between Clive and Tangoio is dependent on uninterrupted replenishment. Dams and Port shipping channels are impediments to natural replenishment for Napier's beaches. Napier councillors do not accept there is a problem with the dam or accept the Port is responsible for erosion on Napier's northern coast.
The detrimental effect this dam will have on the Napier coast is certain. It is inconceivable how Crs Dick, Pipe and Scott support this project while privy to information that outlines the risks to valuable property and city infrastructure.
-Larry Dallimore is a long-time Westshore resident and environmental campaigner.
-Business and civic leaders, organisers, experts in their field and interest groups can contribute opinions. The views expressed here are the writer's personal opinion, and not the newspaper's. Email: editor@hbtoday.co.nz.
