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Onetangi Beach protection
Current status
| Options
Options
There are 3 options being considered for the protection of Onetangi Beach:
Option 1: Dune toe protection
| Dune toe wall |
Provide a reinforced dune toe (bottom of dune) 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. Gabion structures are wire baskets designed to contain stones, to form a retaining
wall. |
| Dune planting |
Remove the existing clay cap fill and replace with imported sand to encourage growth of dune plant species. A dense/shrubby
boundary to be established towards the upper part of the dune crest, to assist overland stormwater flow and control of beach user access. |
| Car parking |
Prevention of car parking on the seaward side of the road is proposed for the entire area, although initially maintaining some
car parking adjacent to Fourth Avenue. Parking to be prevented by timber bollards or similar.
At the western end of the beach, establish parallel parking on the landward side of the road, 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 Fourth Avenue requires a larger land area for parking and a site outside the present coastal hazard zone. The
coastal hazard zone is the area of land under threat from sea water inundation due to high tides, storms and global warming increased sea
levels.
Development of parallel angle parking along Third Avenue in the vicinity of the Community Hall is a potential alternative. Parking could be
provided along 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 planting of a wide reasonably shrubby vegetation buffer will help preserve the dune. In addition,
local road raising to remove dips that allow for stormwater ponding formation and focussing will be required. |
| Accessways |
Accessways from the road reserve to the beach will need to be more defined. 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
| Item |
Cost |
| Design of walls |
$100,000 |
| Consents for walls |
$100,000 |
| Timber bollards @ 2m centres |
$7,500 |
| Replacement of wastewater treatment system for toilet block |
$75,000 |
| Dune planting over 1400m based on $275 per 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 accessways @ $10,000 each and two sand ladders @ $3000 each |
$26,000 |
| Carpark at Third Avenue based on use of grass paver type system @ $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 |
| Contingency (approximately 15 per cent) |
$151,000 |
| TOTAL |
$1,158,000 |
 Option 2: Beach face dewatering
This option includes the same proposals concerning
- access
- stormwater
- car parking
- revegetation of the dunes
as set out in Option 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 trialled along 200m of beach immediately to the
west of Fourth Avenue. Dewatering means the use of a pipe drainage system under the beach to remove water from the sand.
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
Costs for the beach face dewatering trial of Option 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 identified under
Option 1
| Item |
Cost |
| Design |
$100,000 |
| Consents |
$100,000 |
| Capital works (pump station/instrumentation, install pipeline and outfall) based on $2,000 per linear metre |
$400,000 |
| Annual operating cost (5 per cent of capital cost) over 3 years |
$60,000 |
| Annual monitoring (per annum), over 3 years |
60,000 |
| Subtotal |
$720,000 |
| Contingency (approximately 25 per cent) |
$180,000 |
| TOTAL FOR BEACH FACE DEWATERING |
$900,000 |
| Land based works from Option 1 including $25,000 design costs and approximately 15
per cent contingency |
$354,000 |
| TOTAL |
$1,254,000 |
Option 3: Beach nourishment
This option includes the same proposals concerning
- access
- stormwater
- car parking
- revegetation of the dunes
as set out in Option 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 30,000m3 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 than the beach sand. This means, if a near shore
source was considered up to 50,000m3 of sand would be required 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
Costs for the beach replenishment works of Option 3 are included in the table below, including estimates for design, consents, capital works,
annual maintenance and monitoring. 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.
The land based works costs are identified under Option 1
| 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
per cent 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 |
$2,590,000 |
| Contingency (around 25 per cent for near-shore, 15 per
cent 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 approximately 15 per cent contingency |
$267,000 |
$354,000 |
| TOTAL |
$3,472,000 |
$4,234,000 |
Evaluation
Options 2 and 3 have significantly higher capital costs than Option 1, together with higher ongoing maintenance costs and uncertainties
associated with keeping sand on the beach.
Option 1 has the lowest capital cost and includes land based work items that are common to all options. The toe wall solution would not rule
out dewatering or beach replenishment in the future. Work packages could be staged over several years to facilitate budgeting constraints.
Due to the current health of the beach and the likelihood of a reasonably stable (but fluctuating) beach system, beach renourishment 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 |
| 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 |
| 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 |
| 4 |
Planting along 200m of dune and 1 accessway |
$75,000 |
| Assessment of monitoring and beach dewatering trial |
$25,000 |
| Monitoring |
$15,000 |
| Subtotal for Year 4 |
$115,000 |
| 5 |
Planting along 200m of dune and 1 accessway |
$75,000 |
| Monitoring |
$15,000 |
| Subtotal for Year 5 |
$90,000 |
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