Larry Dallimore: The flow-on effects of the dam

Larry Dallimore.

Martin Williams (Talking Point, August 12), overlooked my full support for dams and water storage and focused on his novel version, "the dam will destroy gravel beaches".

My view remains, anybody who cares for the coastal environment and supports building a dam that contributes to starving replenishment for the HB gravel coast, is misguided.

My Talking Point (August 5) simply repeated expert evidence "the dam will block a minimum 180,000m3 of gravel and substantially reduce flood flows that move gravel to the Tukituki River mouth".

Mr Williams quoted Dr Fisher: "Gravel trapped by the dam would end up in gravel beds and not make it to the coast for beach replenishment anyway".

This statement is short-sighted and defies natural processes, because gravel will not stay on the gravel beds forever. Erosion caused by reduced replenishment will occur gradually and possibly over decades.

His letter asserted "much of the surplus gravel is currently mined". According to HBRC records, there has been no surplus gravel in the Tukituki River system since 1993.

According to the Komar Report, the sediment balance for the coast is 46,000m3/year but, since Cyclone Bola, between zero and 28,000m3/year has reached the river mouth.

Any gravel held up on river beds, trapped by a dam, or mined for construction will increase the deficit until massive reserves at the upper reaches flow freely to the coast.

He quotes: "Gravel lost to the river mouth from the dam will be replaced each year by double the amount needed".

Gravel input to the coast was calculated at 1688m3 per year, doubling to 3400m3 to top-up each side of the river mouth to appease those who might believe in a southerly longshore drift.

HBRC appear to have underestimated the impact of the dam on the coast by underestimating the net volume of gravel blocked by the dam.

The Makaroro River supplies 180,000m3 of gravel to the Tukituki River system but engineers insist the dam will reduce that volume to just 1688m3 (less than 1 per cent) at the river mouth. Surely, without mining or a dam, most of this 180,000m3 would eventually reach the coast.

An eminent expert from NIWA also does not agree with HBRC calculations. Mr Williams could refer to the Peer Review of Dr Fisher's RWSS Sediment Assessment dated April 4, 2013.

I refer to the closing statement by Dr Murray Hicks, principal scientist. "I retain my views of 5 March regarding the issues with the HBRC bedload calculations but also that these are likely to have delivered conservative results at the coast".

Mr Williams' agreement to "it is vital we focus on the real causes and issues" is admirable but unfortunately that is foreign to the HBRC.

The council consistently advocates all erosion is due to natural causes.

If that was the case, I would tend to agree with slowing down the erosion process by "managed retreat" and agree with the preferred solution.

At Westshore, council's adopted solution is to form a weak shingle seawall so the next storm collapses the bank to allow loose stones to partly replace eroded sand.

This dismal solution has been an expensive failure.

- Larry Dallimore is a longtime 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