Protection Works Resource Consent Applicant Team Neil Daykin, GDC - - PowerPoint PPT Presentation

Proposed Wainui Beach Protection Works Resource Consent

Applicant Team

• Neil Daykin, GDC Land, Rivers

& Coastal Manager

• Dr Tom Shand, T&T Coastal

Engineer

• Reuben Hansen, T&T Principal

Environmental Planner

Proposed Works at Wainui Beach

• Replacement of failing rail, log and

rock seawall with new sloping Rock Revetment at Tuahine Crescent

• Retrospective authorisation of

gabion basket structure and removal

• f loose rock above, on esplanade

reserve seaward of 21 Wairere Rd

Wainui

Wainui

Geological Context

Wainui Stream Present Shoreline Approximate location of where the back shore geology transitions from sand (to the north) to rock (to the south)

Wainui Beach Erosion Strategy (WBEMS)

• Developed and First adopted 2003
• Updated in 2014
• Sets out GDC’s strategy for managing coastal erosion at Wainui Beach.
• Broad vision to conserve & enhance the environment for future generations.
• Developed through a stakeholder engagement process
• 28 properties identified within the coastal hazard zone ‘Extreme Risk Area’ -

i.e. they are potentially at risk from erosion resulting from one storm.

• Strategy was guided by the Resource Management Act 1991 and the New

Zealand Coastal Policy Statement.

• Notably, the latter identifies “Wainui-Stock Route – Pines – Whales” in the list
• f surf breaks of national significance.

• Short term (next 10-20 years) options promoted focus on managing the risk

associated with existing sea level and coastal processes:

• Development control via resource management plans
• Promoting the restoration of dunes through planting
• Responding to erosion events that threaten dwellings by ‘pushing-up’ of sand to

reform dunes

• Constructing a new rock revetment north of the concrete groyne at Tuahine

Crescent

• Maintaining structures around the streams (Hamanatua & Wainui)
• Retaining other existing structures subject to removal of features that present

safety concerns e.g. rusted iron, etc.

• Medium term (20-30 years) to longer term (next 100 years) options

promoted in response to aggravated erosion due to projected sea level rise and other climate change factors include:

• Removal of structures
• Managed retreat of structures and property

Philosophy Underlying the Strategy

• 1. Risk avoidance: preclude any further subdivision in risk areas
• 2. Risk reduction: preclude renovations that increase a buildings footprint or new

dwellings in risk areas

• 3. Mitigate erosion by protecting and restoring dunes & beach
• 4. Use soft engineering measures which mitigate erosion using

natural buffers:

• stream training walls (e.g. Hamanatua) that do not significantly impact on

beach processes and that help maintain wider beach and/or dune than would otherwise occur

• 5. Living with erosion – accepting erosion where appropriate
• intervention occurs if land at risk, e.g. beach scraping (sand push-ups)
• relocation or removal of dwellings if at risk
• 6. Landward relocation of assets to a safe location
• 7. “Hard” engineering structures, including new or existing sea

walls or rock revetments are generally not suited as long term solutions on high natural and amenity value open coast beaches such as Wainui

Summary for “Hard” Structures

The following approaches are promoted:

• Retain structures that are performing and which have only

minimal or infrequent adverse effects on beach use and values – remove if this changes.

• Leave for the meantime structures (i.e. gabions) which may

not be serving a particularly useful role (other perhaps than psychological) and are not having significant adverse effects.

• Retain consented structures which have significant adverse

effects on beach values BUT prepare for their removal.

• Long term reliance should NOT be placed on existing

structures.

• Avoid new structures, except for the short-to-medium term

management of erosion AND plan for their eventual removal.

• It is important to note that there are NO existing structures

anywhere on Wainui Beach that are likely to be suitable long term solutions due to adverse effects and/or engineering limitations.

Strategy for Area 2 – Tuahine Cres

• Replace the existing rail and rock wall north of the groyne

ending in the vicinity of the Tuahine Crescent beach access way with a more robust structure.

• Consider complete removal of the rock revetments from

the coast once their consents expire.

• Note: cliff landslide hazard still present

Strategy for Area 5 – Wairere Rd

• Implement development controls.
• Restore dune vegetation
• Progressively remove existing sea walls (i.e. gabions),

though these features can be left in the immediate future

Withdrawal of Beach Scraping / Sand Push-Ups

• The rationale for amending the proposal was principally around

the concerns expressed by submitters regarding this erosion management method, as well the lack of certainty as to what effect the beach scraping may have on beach and nearshore morphology.

• While linear beach scraping (pushing straight up the beach) is
• ften used on Coromandel beaches and alongshore relocation

at other beaches (i.e. Orewa and Eastern Beach, Auckland), the specific combined scraping and alongshore movement proposed for Wainui is less well documented.

• While the approach has the potential to mitigate erosion without

the use of permanent structures, some uncertainty inherently remains around its effect on sand bar morphology, and therefore surf break quality.

• Owing to this uncertainty and the substantial cost of achieving

more certainty, including Dr de Lange’s recommended surveying and monitoring programme, compared to cost of the actual proposed works, and the value of surf breaks at Wainui, this component of our application has been withdrawn.

Gabion Structure Retention & Rock Removal on Council Reserve Seaward of 21 Wairere Road, Wainui

Gabions & Geotextile Bags

• Both Gabions & Geotextile Bags are “structures”
• Both are only for temporary use for coastal erosion protection
• Both only tolerate small waves (typically less than 1.5m) before being

displaced and damaged.

• Gabions at Wainui are coming to the end of their useful life
• Gabions have lasted longer than their design life (5-10yrs in coastal
• environment) as they were buried for extended periods.
• Gabions are increasingly being exposed for longer periods due to ongoing

erosion and climate change, thus accelerating their deterioration.

• Some Gabions can be repaired to extend their life temporarily
• Damaged Gabions will be removed when they pose a H&S risk or are

beyond reasonably cost effective repair

• Gabions will NOT be replaced

Function of Gabions and Geotextile Bags:

• They are a form of shoreline structure
• For Temporary use only
• They are the Final line of protection after the overlaying sand has been

eroded

Effects of these engineering structures:

• Visually and aesthetically unsightly and easily damaged.
• Impermeable to varying degrees, they will not absorb wave energy like a

beach, and may cause local beach scour to accelerate.

• Interfere with the natural interchange of sand between beach and

dune.

• Influence the longshore transfer of sand, modify dune habitats, disrupt

the natural landform and potentially result in localised dune face scour at their terminal ends (end effects).

Gabions and Geotextile Bags

Esplanade Reserve seaward

• f 21 Wairere Road

21 Wairere Road Erosion hazards

Why Needed:

• An August 2016 Swell Event destroyed

part of an old and private seawall located on GDC land

Before After Emergency Works Note new gabion wall has no rail iron support or tie-backs

Gabions and erosion

• ccurring behind them

Gabions and erosion

• ccurring behind them

End effects of private seawall Approximate “natural” position

• f shoreline if the seawall and

gabion+rock were not present

Rock Removal

T&T have assessed the rock to be:

• Not a conventional revetment design
• Poorly constructed
• Uncertainty around extents of geotextile under the rocks
• Mix of rock sizes. Many too small that are then likely to be

displaced on to the beach during storm events

• Poor transition between gabions and rock including rock
• n top of the gabions that increases risk of rock

dislodgment and damage to the gabions

• No rail iron support for gabions
• H&S risks to beach users (walkers, cyclists, horses, SLSC

vehicles etc.) from falling & dislodged rock on the beach

• GDC Engineer who oversaw works won’t provide post

construction certificate

• Inconsistent with NZCPS & WBEMS

Conclusion of Gabion Structure Component of Consent:

• Proposal for retrospective

consent to keep the gabions

• Gabions consistent with existing gabions
• Temporary structure
• Removal of the rock
• To remove the H&S hazard
• Revetment poorly designed & constructed
• Potential to re-use the rock at Tuahine Crescent

revetment

Removal of existing rail, log and rock seawall & Replacement with a new Rock Revetment at Tuahine Crescent

Erosion hazards

Why Needed:

• Existing rail, log and rock seawall is failing,

with rusting rail, loss of logs and rocks spilling on to the beach and waves

• vertopping and eroding the cliff
• Cliff shoreline is highly prone to erosion

Revetment Site

Revetment South of Concrete Groyne showing overtopping

Rock crest low – may lead to scour behind structure or to the

• cliff. May lead to cliff

landslides

GDC Report: Wainui Beach Management Strategy Detailed Screening of Potential Options

Options for Coastal Protection between Concrete Groyne & Tuahine Beach Access Steps - 2016 update

Proposed Rock Revetment

Public Access

• Proposed sloping design improves public

access compared to existing vertical wall

• Forecast climate change will increasingly

reduce viability of access structures

• No guarantees GDC will maintain in

perpetuity public access at Tuahine Crescent

• Continuation of access will be based on

demand, H&S, feasibility & cost-benefit

• Alternative access only 300m & 500m

away

• Cliff geology is harder “rock”

with limited sand i.e. proposal won’t impact sand supply to beach

• GDC reserve to the north of

proposal has ability to absorb any end effects

Conclusions

• The gabions, rock removal and

new rock revetment are consistent with the NZCPS & WBEMS

• Provides a Medium Term Solution
• Balances socio-economic-

environmental outcomes