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 of loose rock above, on esplanade reserve seaward of 21 Wairere Rd
Wainui
Wainui
Geological Context Wainui Stream Approximate location of where the back shore geology transitions from sand (to the north) to rock (to the Present Shoreline 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 of 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 often 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 of 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 occurring behind them Approximate “natural” position of shoreline if the seawall and gabion+rock were not present End effects of private seawall Gabions and erosion occurring behind them
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 on 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 overtopping and eroding the cliff • Cliff shoreline is highly prone to erosion
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