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A review of work done to protect Hokitika from the sea says neither rock groynes nor seawalls are likely to be effective in the long-term and the town's coastal hazard zone should be reviewed.
At the request of the West Coast Regional Council, BECA Ltd has updated its assessment of coastal erosion and the effectiveness of protection works done over the years, including recent emergency work.
BECA has concluded that only one of the groynes built since 1987 is functioning as it should to capture sand and sediment and stop the beach receding.
The report also says extending the existing rock seawall is likely to contribute to erosion from the end of the wall to beyond the Hampden Street groyne.
The regional council approved a budget of $500,000 last July to carry out short-term protection recommended by BECA to counter what it calls “aggressive erosion”.
It also asked for recommendations on improving the groynes, and managing the Hokitika River mouth.
The council has since completed short-term protection work over 300m of coastline at a cost of $218,000 including $26,000 of emergency works at Richard Drive.
BECA concludes that the three groynes at Weld Lane, Hampden Street and Tudor Street do not trap sand as intended because it is pushed out to sea by the Hokitika River and returned to shore further north.
Protection works to “train” the Hokitika River mouth would be unlikely to have any lasting effects on that process, the report found.
Only the Richards Drive groyne was functioning as expected, but the others were not having a detrimental effect and might be of use when the beach level was lower after storm events, BECA said.
The Hokitika Joint Seawall Committee, made up of regional and Westland District Council representatives, has asked council staff to design and cost an extension to the Hokitika seawall and apply for funding from the Crown Infrastructure Projects Fund.
The BECA report suggested councils should be mindful of other possible impacts of that project.
“The construction and completion of 1km of continuous rock seawall from the river mouth to Beach Road is likely to contribute to the present erosion evident from the end of the completed seawall to beyond the Hampden Street groyne ... this effect should be borne in mind when considering seawall extensions.”
The original architect of Hokitika's coastal protection project, Dr Jeremy Gibb, had identified boundaries for various levels of erosion risk back in 1987, BECA said.
Land and property to the west of Revell Street between Stafford Street and Richards Drive had been designated extreme and high-risk erosion zones.
Gibb had predicted the western side of Revell Street would be the natural shoreline position by 2050, based on sea-level rise projections of 0.6m at that time.
However, the council had set its present hazard zone about 30m closer to the sea than Gibb's proposed line.
The revised sea-level rise was now 0.36m for that timespan, possibly reduced by the groynes Gibb had proposed -- but was expected to continue indefinitely, with shoreline retreat beyond the identified hazard boundaries, BECA warned.
Long-term, neither seawalls nor groynes would protect the town, the report predicted.
Regional council general manager Mike Meehan said the issue of flood protections was more complex than scientific reports would suggest.
“The end effects of seawalls (i.e. erosion at the end) are well-known but we have communities to consider and being realistic, there isn't any flood protection work that isn't temporary -- just look at Edgecumbe.”
The council will vote today on whether to support an application to the Crown Infrastructure Projects Fund for money to extend the Hokitika seawall.
- Lois Williams