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Current status of Onetangi Beach - Waiheke

Project Focus - To provide a long-term management solution that will protect and enhance the existing values and qualities of Onetangi Beach for the enjoyment of the community.

Current status of Onetangi Beach - Waiheke

Onetangi Beach is approximately 1.6km long and is situated on the southern shore of Onetangi Bay. The beach is bounded by the western headland of Onetangi Bay and a rock promontory that is located approximately in the centre of the bay.

Onetangi Beach is dynamic and responds to periods of high and low wave energy. During high energy the beach erodes and during prolonged periods of low energy the beach accumulates.

The development of the Onetangi Beach Road has substantially modified the natural dune/beach processes by building over and seaward of the historic dune crest. The current buffering capacity of the dune system is low.

The major sediment movement process is cross-shore. Wave sheltering from the eastern headland appears to contribute to the variation in sediment dynamics along the beach.

At present the upper Onetangi Beach appears very healthy, with significant volumes of sand having accumulated on to the upper beach since monitoring began in 1998. The beach has accumulated approximately 50m3/m of sand over a 25-month period from 22 December 1998 to 26 March 2001.

Recent photographs, *(March 2002) show similar beach levels to those of *26 March 2001 and the rapid growth of isolated clumps of spinnifex plant.

The evidence of little beach change since 1998 supports the opinion that the beach appears to be dynamically stable with no clear evidence of systemic shoreline retreat.

The greater beach growth experienced at the eastern end of the beach also illustrates the more dynamic nature of this stretch compared to the more stable western end.

Future status of Onetangi Beach-Waiheke

Onetangi Beach has the potential to be affected by storm-induced erosion, erosion from climatic fluctuations, long term erosion trends and impacts from sea level rise.

The present healthy state of the beach represents an extreme beach state position, with the cyclical change between healthy beach-state and reduced beach levels from erosion likely to result in the future. Potential climate change effects may also intensify the erosion process over the healthy beach-state.

Action Plan

It is recommended that a management strategy be implemented to take into account the future deterioration of Onetangi Beach. The strategy is to develop a long-term approach to:

Beach access controls The provision of a dune vegetation buffer Preventing parking seaward of the existing road and provision of public parking outside the coastal hazard zones Catchment-based stormwater controls to reduce peak flows and concentrated stormwater discharges Planning methods to control increased development within the Coastal Hazard Zone Removal of Council structures within the current hazard zone. Sand transfer to increase the width of the upper beach area by transferring sand from the lower beach to the upper beach. Ideas for the protection of Onetangi Beach Idea 1: Dune Toe Protection Dune Toe Wall The dune toe protection option seeks to provide a reinforced dune toe along the most at risk areas of Onetangi Beach in order to restrict landward movement of the dune during stormy periods where beach lowering is likely. Considering available materials, consistency along the shoreline and cost, a gabion dune toe wall is an appropriate option.

Dune Planting

Remove the existing clay cap fill and replace with imported sand to facilitate the take of dune species. A dense/shrubby boundary to be established towards the upper part of the dune crest both to assist in ease of overland stormwater flow and to the control of beach user access.

Car Parking

Prevention of car parking on the seaward side of the road seal is proposed for the entire area, although initially maintaining some car parking adjacent to Forth Avenue. The prevention of parking to be reinforced by placing timber bollards, or similar, along the seaward edge of the road seal.

At the western end of the beach establish parallel parking on the landward side of the road reserve, adjacent to the existing property boundaries. This option may also be possible at the very eastern end of the beach.

Parking in the vicinity of Forth Avenue requires a larger land area for parking and a site outside the present coastal hazard zone area. Development of parallel angle parking along Third Avenue in the vicinity of the community hall is a potential alternative. Parking could be provided along some 140m of road reserve, providing more than 50 parking spaces. Stormwater

Overland flow is a particular issue along the entire beach. Where possible along the eastern end of the beach, overland flow would be diverted into the small stream running through Third Avenue Reserve.

Within the coastal hazard zones the provision of a wide reasonably shrubby vegetation buffer will assist in preserving the integrity of the dune. In addition, local road raising to remove dips that allow for stormwater ponding formation and focussing will be required.

Access ways

Access ways from the road reserve to the beach will require increased definition. This will be completed by providing breaks in the dune vegetation, with sand ladder type access along the western end. At the eastern end of the beach more steps will be considered. However, the toe of the steps will be designed to take into account the minimum beach levels, rather than the beach levels that presently exist.

Toilet Block

An alternative treatment system is required for the toilet block. A package system will be installed adjacent to the existing toilet block and the existing infiltration bed/septic tank is to be removed and the land area managed. Costs of Idea 1
Table 1: Costs for Idea 1
Item Cost
Design of walls $100,000
Consents for walls $100,000
Timber bollards at 2m centres $7,500
Replacement of wastewater treatment system for toilet block $75,000
Dune planting over 1400m based on $275/ linear metre, inclusive of removal of Unsuitables, reprofiling dune face, planting and wind fencing top and bottom $38,500
Dune toe wall (175m) based on $1,000 per linear metre $175,000
Dune toe wall (remaining 350m) $350,000
Accessways, assuming two step access ways at $10,000 each and two sand ladders at $3000 each $26,000
Carpark at Third Avenue based on use of grass paver type system at $30/m2 for 700m and concrete kerbing $35,000
Stormwater investigation, looking at Third Avenue Detention Pond and catchment treatment options $60,000
Monitoring over 3 years $40,000
Subtotal $1,007,000.00
Contingency (approx. 15%) $151,000
TOTAL $1,158,000
Option 2: Beach face dewatering This option includes the same proposals concerning: Access Stormwater Car parking Re-vegetation of the dunes as set out in Idea 1. The existing gabion structures are also maintained.

However, the threat of storm-induced erosion is lessened by a beach face dewatering system to be trial-tested along 200m of beach immediately to the west of Forth Avenue.

The trial would involve the installation of a 200m pipe drainage system situated around mean sea level on the beach profile. The pipe would drain to a gravity wet well with pressure discharge piping and submersible electric pumps. The pump station for the trial would be situated at the toe of the existing dune, to reduce the size of the structure.

This system would need to be evaluated over a period of at least 3 years, and preferably 5 years, prior to making a recommendation for the permanent installation of the system. During this period extensive monitoring will be required to evaluate the system’s performance.

Costs of Idea 2 Costs for the beach face dewatering trial of Idea 2 are included in the table below, including estimates for design, consents, capital works, annual maintenance and monitoring. The land based works costs, associated with dune vegetation, parking, stormwater, etc are as identified under Idea 1.
Costs for Idea 2
Item Cost
Design $100,000
Consents $100,000
Capital works (pump station/instrumentation, install pipeline and outfall) based on $2,000 linear metre $400,000
Annual operating cost (5% of capital cost) over 3 years $60,000
Annual monitoring (p.a.), over 3 years 60,000
Subtotal $720,000
Contingency (approx. 25%) $180,000
Total for beach face dewatering $900,000
Land based works from Option 1 including $25,00 design costs and approx. 15 % contingency $354,000
TOTAL $1,254,000

Idea 3: Beach Nourishment This option includes the same proposals concerning: Access Stormwater Car parking Re-vegetation of the dunes as set out in Idea 1. The existing gabion structures are also maintained. However, the threat of storm-induced erosion is lessened by placement of additional sand either dredged from the near-shore or imported from an approved sand extraction area.

A minimum volume of 30,000 cubic metres of sand would be required to enhance the beach. It is likely that an imported consented source would more closely match the existing beach sediment size as the near-shore sand is generally finer that the beach sand. Therefore, up to 50,000m3 of sand would be required to be placed should a near shore source be considered, to take into account the greater losses that could be expected.

The placed sand is likely to move, and over time be eroded from the beach. Therefore, regular maintenance is required to maintain sand volume. This is anticipated to involve the excavation of sand at the low tide limit of the beach and transfer back to the top of the beach and dune area. It is likely that this will be required on a regular (annual) basis, with up to 10,000m3 transferred each year from the offshore (imported) source and 15,000m3 for the near shore source.

Costs for Idea 3 Costs for the beach replenishment works of Idea 3 are included in the table below, including estimates for design, consents, capital works, annual maintenance and monitoring.

The land based works costs are as identified under Idea 1. Increased risk and uncertainty is included for the near shore source, due to the additional investigations required and the requirement to gain consent for the near shore source, while the offshore (imported) source is already consented.
Costs for Idea 3
Item Near-shore sand source (50,000m3) Offshore approved source (30,000m3)
Design $100,000 $50,000
Consents $200,000 $100,000
Capital works, $30/m3 for near-shore, $60/m3 for offshore plus 15% P&G $1,725,000 $2,070,000
Annual maintenance (based on $10/m3) over 3 years $450,000 $300,000
Annual monitoring over 3 years $40,000 $40,000
Subtotal $2,565,000.00 $2,590,000.00
Contingency (around 25% for near-shore, 15% for offshore) $640,000 $390,000
Total for beach replenishment $3,205,000 $2,980,000
Land-based works from Option 1 including $25,000 design costs and approx. 15 % contingency $267,000 $354,000
TOTAL $3,472,000 $4,234,000
Evaluation Ideas 2 and 3 have significantly higher capital costs than Idea 1 together with higher ongoing maintenance costs and uncertainties associated with retaining sand on the beach.

Idea 1 has the lowest capital cost and includes land-based work items that are common to all options. The toe-wall solution would not preclude dewatering or beach replenishment in the future. Work packages could be staged over several years to facilitate budgeting constraints.

At present, due to the current health of the beach and the likelihood of a reasonably stable (but fluctuating) beach system beach re-nourishment and/or dewatering are not required to maintain an acceptable beach system.

A suggested programme is set out below:
Proposed Works Programme for Onetangi Beach
Year Work Description Cost (incl. Contingency)
1 Design and traffic control (bollards along shoreline and carpark at Third Avenue) $75,000
Alternative wastewater treatment system for toilet block $75,000
Planting along 50m of dune at western end of beach and 1 sand ladder $20,000
Monitoring $15,000
Consents for dune toe wall $115,000
Subtotal for year 1 $300,000.00
2 Design dune toe wall $115,000
Planting along 50m of dune at western end of beach and 1 sand ladder $20,000
Stormwater investigation study $70,000
Monitoring $15,000
Subtotal for year 2 $220,000.00
3 Planting along 50m of dune at western end of beach and 1 sand ladder $20,000
Monitoring $15,000
Dune toe wall along 175m $202,000
Subtotal for year 3 $237,000.00
4 Planting along 200m of dune and 1 access-way $75,000
Assessment of monitoring and beach dewatering trial $25,000
Monitoring $15,000
Subtotal for year 4 $115,000.00
5 Planting along 200m of dune and 1 access-way $75,000
Monitoring $15,000
Subtotal for year 5 $90,000.00

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