The implications for maintenance dredging costs could be profound. Estimates place it at around $4000 annually, per berth, based on a conservative siltation rate of 260mm a year.
Add to this the normal operating costs of between $2500 and $3500, and potential annual charges for each berth-holder could be as high as $6500 to $7500, making Sandspit the most expensive marina in Auckland.
Such spiralling costs could ultimately impact on the public purse should the marina operators - faced with an annual dredging bill of more than $500,000 - become insolvent.
"The marina developers based their assessment on theory and expert advice," said Mr Michael Taplin, a former chairman of the Sandspit Residents and Ratepayers Association who represented the community's interests at the resource consent hearings. "Similar expert advice has consistently underestimated maintenance dredging requirements at several other estuarine marinas."
Since December 2012, independent marine biologist Dr Roger Grace has monitored sediment traps in an experiment designed to simulate how silt will build up in the proposed marina basin over time. Preliminary results were presented to the Auckland Council Environment and Sustainability Forum last month.
These results show that, despite the summer drought, the amount of sediment washing into the Sandspit estuary has vastly exceeded expectation, and suggests maintenance dredging will begin within three years.
Although, under ideal circumstances, three years of data would be required to produce the most robust results, the disparity between the theoretical estimate and the empirical observations is great enough to "ring alarm bells".
Mr Taplin says without regular dredging, boats in the marina could be sitting in the mud at low tide within two to three years.
In comparison, siltation rates at Pine Harbour which, at 108,000sq m, is almost three times the size of the proposed Sandspit marina, is around 55mm annually. West Park is 131mm and Milford is 142mm.
One reason for the exceedingly heavy silt load at Sandspit is its rural location. The catchment area for both the Matakana and Glen Eden Rivers, which run into the estuary, is mostly farmland, forestry and bush. Ongoing logging, severe hillside erosion and subdivisions mean siltation is likely to increase.
Given that Sandspit berths are being advertised at almost three times the average price for the Auckland region the further underestimation of dredging costs could weigh heavily on the consumer, or on the council if berth holders fail to meet those costs.
"When you dig a hole in an estuary, it naturally fills up with mud much faster than the normal sedimentation rate on the open sand or mud flats. A marina dug in an estuary acts like a big sediment trap," said Dr Grace.
Quite apart from the running costs there is also the controversial issue of disposing of dredged spoil. According to the Sandspit Yacht Club Marina Society the 100,000cu m of capital dredgings will be disposed of offshore. But the only permit for offshore dumping for all marinas within the Hauraki Gulf is held by Maritime New Zealand and that is limited to 55,000cu m a year.
The simple answer, of course, is to stop locating marinas in heavily silting estuaries and build them in deep bays that require little or no maintenance dredging. The marine recreation zone at Goldsworthy Bay, a mere stone's throw from Sandspit, offers such conditions but has been neglected in favour of what might turn out to be a very costly alternative. How long before the lessons of West Park and Pine Harbour are learned and acted upon?
Allison Jones is a writer and environmental campaigner.
