17/03/2015

Coastal‐storm inundation and ‐ level rise Scott Stephens and Rob Bell

Talk outline

• Using MHWS to define the Coastal Marine Area boundary • Coastal inundation by storm‐ and waves • How big and how often? • Mapping coastal‐storm inundation • Sea‐level rise –the elephant in the room • What do we do about it? rtoonStock.com • not considered Dave Allen, Ca Dave Allen, ©

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Coastal Marine Area Boundary

• The coastal marine area (CMA) landward boundary delineates a jurisdictional limit for rules under New Zealand’s Resource Management Act (RMA). • It is defined legally by the line of mean high water springs (MHWS). • The CMA boundary defines the landward boundary for activities that require a coastal permit, and the coastal boundary for land‐based planning. • An important component of Auckland Council’s new Unitary Plan will be its graphical representation of the coastal margin (coastline).

The mapped coastline in 2012

Okura Estuary

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Projecting levels onto the land

Tide MHWS level Land

Defining mean high water spring tide •Discuss MHWS‐10 again, from Rob’s work

•Do we want to go into this discussion? (m) Natural land‐sea boundary markers 0

= Perigean spring tide M + S + N

• Probably a good idea, just briefly 2 2 2 MSL Nautical spring tide M2 + S2

4000 PPNautical spring tide M + S Kaikoura 2 2

3000

2000 NF NF

1000 (mm) level Sea 0

-1000

Foxton -2000 05-Sep 12-Sep 19-Sep 26-Sep 03-Oct 10-Oct 17-Oct 24-Oct-96 Date

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Updated coastline in Council GIS viewer

Okura Estuary

Coastal‐storm inundation

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A nice place to be

Milford Beach

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Milford Beach –July 2008

Photo: Rob Waardenburg

Tamaki Drive –April 2014 cyclone Ita

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Maraetai – cyclone Ita April 2014

NZ Herald

Anatomy of a storm tide

L Total sea‐level = MSL + sea‐level anomaly + tide + storm‐surge + and runup Wind

Wave setup and runup

Storm surge

Storm‐tide High Tide Sea‐level anomaly (climate) Mean sea level Local vertical datum

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Anatomy of a storm‐tide –neap tide

L Total sea‐level = MSL + sea‐level anomaly + tide + storm‐surge + wave setup and runup Wind

Wave setup and runup

Storm surge Neap Tide Mean sea level

Anatomy of a storm‐tide –

L

Wind Wave setup and runup

Storm surge

King Tide

Mean sea level

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Hauraki Plains: May 1938

RNZAF Coastal stopbanks burst Gale‐force winds + rain + very high tide Depth on land = 0.5 to 1.2 m £1000’s damage 1600 ha flooded and houses

Whitianga: 23 August 1989

Also sizeable coastal inundation events in 1936, 1968, 1972, 1978

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Feb 2014 King Tide –Ohiwa

Feb 2014 King Tide –Ohiwa

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Feb 2014 King Tide –Ohiwa

Auckland awash –23 Jan 2011 –highest storm‐ tide on record at Auckland

NZ Herald NZTA: AMA

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Storm surge height: EcoConnect forecast

Forecast storm surge = 0.33 m in Inner Gulf [Inverted barometer =1015 − 995 hPa= 0.2 m] Actual storm surge = 0.4 m at Port of Auckland

Auckland awash –how often in future?

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Extreme sea level analysis: Auckland

100‐year ARI

1‐year ARI

Climate change and sea‐level rise

• Coastal‐storm inundation and erosion is a problem both now, and historically. • What changes will occur in a changing climate that includes sea‐level rise? • NIWA’s Waves and Storm Surge Projections project suggested that the magnitude of the largest waves and storm surges would change by only a few percent by the 2090’s, and would have negligible impact on coastal‐storm inundation over the next 100 years, relative to the effect of SLR.

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Storm‐tide & Tsunami wave inundation inundation

Groundwater Salinity intrusion table & drainage

Coastal/estuarine Ecological effects shoreline erosion

MHWS+ tidal flooding

Sea‐level Rise

Changing state vs past “static” environment paradigm

Port of Auckland: Annual MSL (1899‐2014) 2

1.9 (m)

Datum 1.60 mm/yr 1.8 Chart

MSL: AVD‐46 1.743 m

1.7 Annual

1.6 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Linear trend for Auckland is 1.6 cm/decade (NZ average rate is 1.7 cm/decade compared with global average of 1.8 cm/decade)

Since 1900, the mean sea level at Auckland has risen by 0.2 m 2014: annual MSL has hit 0.2 m above AVD‐46

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Business‐as‐usual (high) emission scenarios: SLR projections for IPCC reports

MfE (2008):

0.8 m by 2090s

0.5 m by 2090s

Extreme sea level analysis: Auckland

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Expected number of exceedances in 100 years

2011 storm tide 1‐year ARI storm tide

Effects of sea‐level rise

Essentially, only a 0.45 metre SLR is 0.5 m sea‐level rise required for a 23‐ January‐2011 type event to be 2011 storm tide exceeded a few 0.3 m sea‐level rise times per year

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Annual number of sea‐level > MHWPS “In Auckland, flooding events became twice as frequent during the 20th century as a result of sea‐level rise”

Equiv. Note: MfE SLR values (2115) incl. a further 0.05 m due to SW Pacific difference in projections MfE 2008 above the global mean (2090s)

IPCC “likely range” (33% probability may lie outside)

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Annual number of sea‐level > 100‐year ARI In 2045 – thirty years from now – flooding, such as that that occurred in Auckland in January 2011, is likely to occur about once every ten years. By 2070 such flooding is likely to be a yearly event if the world takes no action to reduce greenhouse gas emissions

This analysis was included in “Changing climate and rising : Understanding the science”. Parliamentary Commissioner for the Environment (2014).

Climate change and sea‐level rise

• Coastal‐storm inundation and erosion is a problem both now, and historically • As sea levels continue to rise, the frequency of coastal storm inundation events that reach damaging elevations will dramatically increase. • The future 1% AEP event will produce deeper overland inundation and have much greater consequences than the 2011 event.

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Mapping coastal‐storm inundation for the Auckland region

NZCPS policies:

1. Consider climate‐change & hazard effects for at least 100 years (Policies 10, 24‐25, 27)

So need to venture beyond the 2100 cut‐off in IPCC projections

Need to be looking out to 2115+

2. Not just SLR –other hazards as well incl. storm tide, erosion, tsunami (Policy 24)

3. Different approach signalled for greenfields (Objective 5, Policies 3, 25) vs existing development (Policy 27)

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Extreme sea level analysis: Auckland

Hydrodynamic harbour models

• Tidal amplification • Wind‐driven storm‐surge • Meteorological record

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Extreme storm‐tide elevations ‐ Waitemata

Open‐coast –joint probability analysis: storm‐ tide + –WASP

WASP: 30‐year hindcast of waves and storm‐surge around New Zealand

Pakiri Beach

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Open‐coast –joint probability analysis: storm‐ tide + wave height –WASP

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Mission Bay

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What do we do?

• We have coastal‐storm inundation. • The sea is rising. • What can we do about it?

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The rising sea‐level challenge: a risky business!

Best or most likely estimate

(relative to baseline)

Selection of sea‐level rise to adapt to, is to be undertaken in the context of a risk management approach (NZCPS and MfE Guidance)

The rising sea‐level challenge: a risky business!

Risk peaks for higher SLR within a timeframe, hence need for erring on upper range. For infrastructure, also capital & maintenance costs

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Adaptation to sea‐level rise

• Do nothing • Retreat • Adapt • Protect

Policy: Reducing natural‐hazard risk •RMA  “Risk‐reduction” not explicitly defined but: . Section 7(i) … "have particular regard to" the effects of climate change . Section 31 … "the control of any actual or potential effects of the use, development, or protection of land, including for the purpose of— (i) the avoidance or mitigation of natural hazards” •NZCPS:  reducing risks in relation to managing existing development in areas subject to coastal hazards (Policy 25 (c) and Policy 27).  Avoid increasing the risks –avoid redevelopment –locate new development away from areas prone to coastal risks (Policy 25, Objective 5) • Defining risk? Measuring success of “do not increase risk”  Direct damage, disruption, clean‐up, loss of utility, indirect impacts  Increasing population, house values, infill, infrastructure

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Adaptation: adjusting to changes & rising risk •Land‐use planning  Zoning e.g., Closed Residential zone in Mapua/Ruby Bay (PC22)  Minimum floor and/or ground levels for buildings  Restrictions on re‐development  Erosion trigger‐points (removable buildings)

• Adaptive management approach:  Develop master plan for staging based on defined tipping points  Adopt current best‐estimate SLR trajectory to initial phasing  Monitor & review –amend plan if required (forward or delay)  More suitable for infrastructure or urban areas e.g., London, NY, Auckland, Wellington

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Local coastal adaptation pathway •Engaging with communities (small is more do‐able) •Determine “lived values” that the community holds to  Whitianga and hapu place‐based studies • Issues:  External/expert framing for what is a local problem  Community differences & local politics ‐ mandates  Climate‐change skepticism • Negotiate a decision strategy that provides certainty  Manageable steps (e.g. Lakes Entrance‐VIC, Barnett et al. 2014)  Focus on social/environmental triggers relevant to the locals e.g. o 1:20 yr event leads to main street flooding –so Trigger 1 is when this occurs 5 days in a year o 1:100 yr flood –Trigger 2 when 2 of these floods occur in a year  No SLR values in pathway strategy (but inherent)  Monitoring frequency and consequences of inundation events!

Improving resilience: the challenge

•What’s at stake? What is the time‐varying risk profile? How to measure it? •Adaptation approaches & evaluation of options: scale of community – different pathways for different folks • Implementation in councils –where possible best mainstreamed with other drivers and processes e.g. major upgrades, 30‐yr asset management plans, RPS/Unitary Plan – but also long‐term strategic planning needed with resilience at the core • Policy changes & guidance e.g., NES?, RMA?, MfE, SOE reports • Governance will be critical to achieve a paradigm shift: • managing existing development: LIM reports, development restrictions, life of dwellings, property rights, council roads • ultimately retreat from low‐lying coasts vs protection, •multi‐sector approach (govt / insurance/ business/ science/ engineering/ social/ economics)

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