BEFORE THE HEARING COMMISSIONERS

IN THE MATTER OF The Resource Management Act 1991

AND The Proposed Natural Resources Plan for the Wellington

Region

AND Hearing 6 – Coast, Natural Hazards, Significant historic heritage values and Contaminated land, and hazardous substances

COASTAL HAZARDS EVIDENCE OF DR ROGER DUNCAN SHAND

ON BEHALF OF

THE MINISTER OF CONSERVATION

Dated 18 May 2018

Department of Conservation

PO Box 10 420, WELLINGTON

Counsel acting: Katherine Anton / May Downing

Contact email: kanton@doc.govt.nz / mdowning@doc.govt.nz

INTRODUCTION

1. My full name is Roger Duncan Shand.

2. I hold a Bachelor of Science Degree in both Mathematics and Physical Geography, a

Post Graduate Diploma in Geomorphology, and a PhD in Coastal Processes.

3. For the past 14 years I have been engaged as a coastal scientist (practitioner) with

Coastal Systems Ltd, a consultancy specialising in applied coastal geomorphology, and

coastal hazard assessment and management. A selection of more recent reports are

listed in Appendix 1

4. I have been studying, researching, teaching and working in pure and applied coastal and

fluvial science for over 40 years. With reference to coastal environments, I have acquired

process-based expertise in sand and gravel beach systems, sand dunes, sea cliffs and

inlets while working at Massey University, the Taranaki Regional Council, the Department

of Conservation, and Coastal Systems Ltd.

5. Through my work and research, I am familiar with the general characteristics and dynamics

of the coast administered by the Wellington Regional Council.

6. I am authorised to present this evidence for the Minister of Conservation in relation to

coastal hazards in the Wellington Regional Council’s Proposed Natural Resources Plan

(“the plan”).

CODE OF CONDUCT

7. I confirm that I have read the Code of Conduct for Expert Witnesses as contained in the

Environment Court’s Practice Note 2014. I have complied with the practice note when

preparing my written statement of evidence and will do so when I give oral evidence

before the Hearing Panel.

8. The data, information, facts and assumptions I have considered in forming my opinions

are set out in my evidence to follow. The reasons for the opinions expressed are also set

out in the evidence to follow.

9. Unless I state otherwise, this evidence is within my sphere of expertise and I have not

omitted to consider material facts known to me that might alter or detract from the

opinions that I express.

SCOPE OF EVIDENCE

10. I have been asked by the Minister of Conservation to prepare evidence in relation to the

submissions and further submissions they made on the Proposed Natural Resources

Plan (PNRP) for the Wellington Region. My evidence relates to Hearing 6 - Coast,

Natural Hazards, Significant historic heritage values and Contaminated land, and

hazardous substances. In particular, my evidence addresses technical aspects of coastal

hazard issues and implications for the PNRP under the following heading:

• Section 1: Types of coastal hazard;

• Section 2: Climate change and coastal hazards;

• Section 3: Hard engineering solutions;

• Section 4: Natural buffers, and

• Section 5: The risk-based approach to coastal hazard management.

11. In preparing this statement, I have read the following documents and evidence:

a. The original and further submissions by the Minister of Conservation.

b. The 1994 and 2010 Department of Conservation (DOC) New Zealand Coastal

Policy Statements (NZCPS).

c. The following primary technical reports:

i. The 2004, 2008 and 2017 Ministry for the Environment (MFE) reports on

Coastal Hazards and Climate Change: Guidance for Local Government.

ii. The 2017 DOC Guidance on the NZCPS 2010 with regards to Coastal Hazards

d. The following Wellington Regional Council Reports:

i. The Proposed Natural Resources Plan;

ii. The Section 32 report on Natural Hazards;

iii. The Section 42A report on Natural Hazards by Richard Sheilds, and

iv The Section 42A report on Management of the Coastal Marine Area by Mr Paul

Denton.

d. Technical literature used in preparing my evidence as listed in the Reference

Section.

12. Of particular relevance, the following two hazard guidance publications were released in

December last year (2017) - after the submission period:

• Coastal Hazards and Climate Change: Guidance for Local Government by MFE;

• NZCPS 2010 Guidance Note on Coastal Hazards by DOC;

These documents are pertinent to risk-based coastal management and thus to the PNRP

process in regard to natural hazards, but have eluded mention to date. MFE 2017 has

particular significance for Method M3: Wellington regional hazards management strategy,

and this will be discussed in the final section of my evidence.

SECTION 1 - TYPES OF COASTAL HAZARDS

Relevance to PNRP: Coastal hazards technical overview for Hearing 6

Relevance to NZCPS: Objective 5 and Policy 24

NATURAL EVENTS OF CONSEQUENCE

13. High energy atmospheric and marine forces regularly impact on the Coastal Marine Area

(CMA) and as this energy is directed landward the adjacent coastal environment also feel

its effects. These forces drive erosion of foredune and hillsides (cliffs), and inundation from

tsunami or storm surge. In addition, these phenomena are compounded by hinterland

flooding from rainfall, river flow and groundwater. However, such natural events only

become hazards when they have the potential to adversely affects our property, personal

safety or other aspects of the human-valued environment.

TSUNAMI

14. Tsunami are a series of waves generated when a large volume of water is rapidly

displaced. Events capable of generating tsunami include: earthquakes, volcanic eruptions,

coastal landslides and submarine slides, -and meteor impact. As the entire water column

is generally affected during a tsunami wave, tsunami contain large amount of energy which

is radiated outward from the generation source. Tsunami waves differ substantially from

shorter period wind (generated) waves in terms of waver volume, minimum energy losses

from shoaling and breaking, and greatly increased overland flow with maximum run-up

elevation typically exceeds offshore wave height by a substantial amount.

15. In New Zealand, tsunami with runup heights of 30 m have been found in the geological

record of the last 6000 years (GNS, 2005, 2013). New Zealand is affected by tsunami

generated both locally and remotely with 40 tsunami known to have affected the NZ coast

over the past 165 years. Of these tsunami, three resulted in runup heights exceeding 10

m at some coastal locations and two of these were generated by local earthquakes in the

Hikurangi Subduction zone just off the east coast (1855 in the Wairarapa and 1947 offshore

of Gisborne), and one by a large earthquake in South America (1868). The eastern and

southern Wellington region are particularly prone to tsunami hazard.

16. Following the 2004 Boxing Day Tsunami in Indonesia, GNS have been carrying out

ongoing modelling based on geological and historical event impacts, and more recently

continuous GPS monitoring at selected sites, with output being generated for each coastal

centre – down to a 100 year return period. This has comprehensively resulted in local

government incorporating such impacts into their plans and emergency response

procedures along with placement if large billboards indicating evacuation routes.

STORM INDUNDATION

17. Under storm conditions, several atmospheric and marine processes can elevate sea level

at the coast and thus increase the risk of inundation hazard on otherwise dry land. The

parameters which, in combination, define the inundation level at the coastal margin consist

include:

• Tides;

• Longer-term sea-level fluctuations;

• Storm surge, and

• Wave effects including run-up.

18. In addition, the extent of inundation is augmented by catchment runoff, freshwater

drainage and also topography which determines the flow path with constrictions that

further increase inundation elevation.

19. In New Zealand, tide tends to be the dominant control of static (sustained) water

elevation; however, wave runup can significantly further increase the reach of inundation

- albeit briefly.

20. The consequence of coastal inundation depends on the depth and frequency of flooding.

Very shallow flooding depths and occasional wave impacts are not likely to have major

consequences to property or personal safety. However, when they occur regularly the

cumulative damage can become significant. Greater water depths can cause more

significant damage to structures, particularly when combined with wave effects.

21. In the Wellington region, transport routes along the coastal margins and harbours are

particularly sensitive to storm inundation. Inlets are also hazard prone, providing an

opening for storm inundation to penetrate directly inland. Areas on the open coast without

foredune protection are also more prone to storm inundation.

22. Methods of measuring coastal storm inundation hazards have been summarised in

Appendix 2A. These increase in detail and accuracy and culminate with a fully

probabilistic computational approach by Tonkin and Taylor (2015). The development of

such a probability-based method was driven by the NZCPS 2010s use of the hazard-risk

concept which requires detailed consideration of likelihood when defining areas

potentially affected by coastal hazards and addressing risk-based management

(considered further in Section 5 of my evidence).

COASTAL EROSION

23. Coastal erosion occurs when sediment is lost from the shoreline resulting in landward

retreat of the coastal margin. Erosion can occur on all types of coast; sand and gravel

beaches on the open coast, enclosed estuarine shorelines and cliffed coasts. However,

each type of erosion results from a range of processes/mechanisms so rates vary

considerably.

24. On unconsolidated coasts (sand/gravel), shoreline fluctuations and wave cut into the

foredune toe or storm ridge accompany weather/marine events. However, in the medium

to longer-term, systematic erosion (also referred to as net long-term erosion, shoreline

retreat or passive erosion), or deposition (also referred to as shoreline accretion, coastal

progradation or shoreline advance) typically reflect changes to the sediment supply. Sea-

level (inundation level) during a storm event is particularly important as this controls the

elevation at which wave action is concentrated, with greater impact accompanying higher

the levels.

25. On consolidated coasts (hills/cliffs) of softer composition, episodic collapse follows wave-

driven toe erosion with material subsequently removed by wave action before another

cycle commences. On hard cliff coasts, erosion rates may be virtually imperceptible with

the land often plunging directly into the sea.

26. Vertical tectonic adjustments accompanying earthquakes can result in erosion either

following the earthquake or by subsequent marine processes acting upon the change in

sediment availability.

27. While the episodic nature of erosion events can threaten public safety, especially with cliff

collapse or earthquake, on non-cohesive coasts the shoreline response accompanies the

actual storm and the incremental (wave by wave) loss of land which lessens the safety risk

to the bystander. In this situation the hazard relates more to property.

28. Much of the Wellington regional coast is subject to coastal erosion, from the sandy

shoreline of the South Kapiti Coast in the west (up to 0.3 m/yr) to the sand and gravel

beaches of the Wairarapa coast in the east (up to 0.75 m/yr). While the bedrock cliffs of

the Wellington area are stable, the softer cliffs around the margin of Cape Palliser have

some of the county’s highest erosion rates - up to several metres per year in places (Gibb,

1978). Areas within Wellington and Porirua Harbours are also prone to erosion, and the

region’s numerous coastal inlets are particularly erosion-prone near their mouths as fluvial

and estuarine processes interact with marine processes to produce dynamic environments

with fringing shorelines easily capable of adjusting 10s of metres in a single event.

29. Methods of measuring coastal erosion hazards have been summarised in Appendix 2B.

These increase in detail and accuracy and culminate in a fully probabilistic computational

approach (Shand et al., 2015) that has recently been successfully applied in several

erosion hazard assessments. As with inundation, the development of a probability-based

method was driven by the 2010 New Zealand Coastal Policy Statement’s (NZCPS 2010)

use of the hazard risk concept which requires consideration of likelihood when defining

areas potentially affected by coastal hazards and addressing risk-based management

(considered further in Section 5 of my evidence).

SECTION 2 - CLIMATE CHANGE AND COASTAL HAZARDS

Relevance to PNRP: Objective O20, New DOC Objective, Policy 29 and Method M4

Relevance to NZCPS: Objective 5, Policy 24, and 27 (2c)

30. MFE Guidance manuals (2008 and 2017) begin with the following statements:

“Climate change will not introduce any new types of coastal hazards, but it will affect

existing coastal hazards by changing some of the hazard drivers”.

And further:

“Climate change will exacerbate coastal erosion and inundation in many parts of the New

Zealand coast, further increasing the impacts of coastal hazards on coastal development

from now on”.

31. Global Climate change predictions are published by the International Panel on Climate

Change (IPCC) about every 5 years. The 5th IPCC (2013) uses several representative

concentration pathway (RCP) scenarios of future radiative forcing to enable consistent

climate-change projections. IPCC information is then adapted/downscaled to regional and

local levels by government agencies to inform/guide practitioners and councils in hazard

assessment for planning and management decision making. The following four RCPs

have been selected for application in New Zealand climate-change inclusive hazard

assessments and decision making (MFE 2017):

• RCP2.6 peak and decline in global emissions would need to occur soon (within

the next decade), rapidly reducing to zero-net or negative global emissions by the

last quarter of this century, with a probable need for sequestration (storing) of

carbon from the atmosphere. World population peaks at around 9 billion later this

century.

• RCP4.5 moderate emission-mitigation pathways. RCP4.5 peaking around 2050

before declining. World population peaks at around 9–10 billion.

• RCP8.5 continuing high emission baseline scenario with no effective global

emissions reduction. An ‘RCP8.5 world’ would exhibit slow rates of economic

development, slow uptake of technology. World population estimated to reach

around 13 billion.

• RCP8.5+ has been added to reflect a world where a higher rate of sea-level rise

(e.g. from faster polar ice sheet melt) may be experienced in the latter part of this

century and beyond 2100. Such a scenario would primarily be used to assess

greenfield developments and adaptability of major infrastructure.

32. Climate change will affect coastal hazards in two main ways, (i) by sea-level rise (SLR),

and (ii) by changes in storm characteristics.

(i) SEA-LEVEL RISE EFFECTS ON COASTAL HAZARDS

33. Global mean projections and corresponding SLR values are given in Table 1. MFE (2017)

advise users to evaluate a range of pathways to address uncertainty and develop options

that can be evaluated for meeting particular objectives.

Table 1 IPCC global mean SLR projections for NZ for the 4 RCP scenarios from a 1986 to 2005 average.

Time frame RCP 2.6 RCP 4.5 RCP 8.5 RCP 8.5+

2020 0.01 0.01 0.02 0.02

2070 0.17 0.21 0.29 0.44

2120 0.31 0.43 0.82 0.87

2150 0.38 0.56 1.12 1.41

34. Relative sea-level rise: Global sea-level rise predictions refer to absolute changes in sea-

surface elevation. However, at any particular location, the land itself may be changing

elevation, from, for example, tectonic forces, sediment loading or even surface rebound

following ice melt from the previous Ice Age. If associated adjustments are made to the

global value this provides sea-level change relative to the land which is better suited to

hazard assessments. A current New Zealand-wide variation in sea-level from the global

average is available. However, the use of geologically determined average annual rates

are not incorporated into hazard assessments due to the episodic nature of such events –

large earthquake-driven surface change occurs once every few hundred years. Shorter-

term change in surface elevation is now being measured by an increasing network of

continuous GPS gauges which have been installed since 2002 (Beavan and Litchfield,

2012). MFE (2017) recommends such shorter-term change could be factored in where (i)

records are at least 10 years long and (ii) significant long-term change exists, i.e. beyond

+/- 0.5 mm/yr which equals the measurement error. However, substantial constraint to

making this adjustment is the limited number of gauge sites available and substantial inter-

site variation in output can occur. Indeed, in the Wellington region there are several gauge

sites and cGPS trends varying between -3.1 mm/yr (subsidence) and +1.3 mm/yr (uplift).

Because of these constraints, the use of the term “Relative Sea-Level Rise” is appropriate

if the qualifier “using reliable scientific data” is used rather than based on “the best available

information.”

(ii) STORM EFFECTS IMPACTING COASTAL HAZARDS

35. Coastal and estuarine environments will also be affected by changes in weather-related

coastal-hazard drivers such as storm surges, waves, winds, and the frequency, intensity

and tracking of storms. In particular, storm surge, extreme wave height and wind speeds

are predicted to increase by up to 10% (MFE 2017). However, trends and projections of

such future changes are not clear and consistent and are likely to exhibit local and regional

variations. Beyond 2100, SLR will tend to dominate over these secondary (storm-based)

climate change effects in coastal areas. However, subtle changes in these coastal storm

drivers in tandem with SLR may lead to substantial changes in shoreline erosion

processes.

36. The significance of climate change on coastal hazards is recognised in the NZCPS (2010),

Policy 24 which states that hazards are to be assessed by having regard to 7 separate

influences (a to g), and then (h) requires the assessor to specifically address the effects of

climate change on each these influences (a to g).

37. An informative summary of landform sensitivity to climate change is provided in Table 2.

Of note is the high sensitivity of foredunes, spits/barrier beaches and estuarine

environments, and weakly consolidated cliffs.

CLIMATE-CHANGE EFFECTS ON TSUNAMI

38. The geological causes of tsunamis (such as earthquakes, underwater landslides and

volcanic activity) will not be directly affected by climate change. The coastal effects of

tsunamis will be altered somewhat by SLR, but the most important variable controlling

tsunami impact is the size of the tsunami and the tide level. Estuaries and harbours may

also become more vulnerable to tsunami if entrance channels deepen in response to

greater tidal water volumes (tidal prism) from higher sea level.

CLIMATE CHANGE EFFECTS ON STORM INUNDATION

39. Sea-level rise will be the dominant direct influence on inundation and will also increase the

impact (reach) of the other drivers. Climate change will increase storm surge if there is a

change in magnitude, frequency or tracking of the storm regime. The effects of climate

change on longer-term sea-level fluctuations, and the effects on the wave regime, are

possible but remain uncertain. Climate change will contribute to coastal flooding where

the storm regime increases hinterland flooding and sediment yield (from hill erosion).

However, while climate change will not directly affect tides, impacts arising from the other

controls will be exacerbated for coastlines with smaller tide ranges (such as Wellington);

this is because elevated water levels will allow shoreline impacts to occur for longer

duration. Climate change effects on storm inundation hazards will increase over time.

Table 2 Landform sensitivity to changes in climate change drivers (MFE 2008)

CLIMATE-CHANGE EFFECTS ON COASTAL EROSION

40. Sandy beaches and backing foredunes are particularly sensitive to sea-level and hence

higher storm inundation levels enable more sustained storm-wave attack of the upper

beach and foredune. Climate change will enhance this instability by raising the base sea-

level along with the possible increase in storm surge and changes to the wave regime.

Coastal groundwater levels may rise as a consequence of SLR, further increasing the

potential for beach erosion. Change to the coastal sediment supply associated with

hinterland erosion may effect shoreline erosion. Of particular relevance is the dominating

foreland on the Kapiti Coast which is particularly sensitive to wave height/direction regime

and sediment supply – alteration of either being capable of significantly changing shoreline

behaviour.

41. Gravel beaches and barriers (e.g. Lake Onoke in the Wairarapa) can get rolled or pushed

back during wave overtopping regardless of how much sediment is supplied to them.

Climate change (SLR and wave regime) is likely to enhance this process unless substantial

increase in sediment supplies also occur. Under conditions of significant sediment deficit,

catastrophic failure of gravel barriers are possible.

42. Cliffs: the effects of climate change on cliffs depend on the resistance of the rocks the cliffs

are composed of, the water depth at the cliff toe and the morphology of any fronting

beaches or rock shore platforms. Cliff erosion is a one-way process, and erosion rates will

generally increase under SLR. Erosion rates will also increase if there is an increase in

storm frequency delivering wave forces that more frequently exceed rock resistance. While

climate change may cause erosion rates may double for soft rock cliffs, rates for hard-rock

cliffs are not expected to show much increase.

43. The SLR response component is often the largest contributor in erosion hazard modelling,

so this term is often the subject of intense debate amongst both scientists and the public.

A range of models for estimating response to changes in sea level have been developed

over the past 50 years and these methods range from the application of basic geometric

principles to more complex process-based analysis and are based on the well-accepted

concept that an elevation in sea level will result in recession of the coastline. A recent

paper (Shand et al., 2013) has reviewed existing shoreline response models including the

process assumptions, limitations, development and application history for open coast sand

environments, gravel beaches and cliffs. This review informed the SLR basis of

probabilistic modelling described in Appendix 2B

44. Inlets and estuaries have finely balanced forcing dynamics that will inevitably be

susceptible to climate change. Sea-level rise, ocean wave regime change, freshwater

flooding, catchment sediment increases and littoral sediment changes may well modify

inlet dimensions (tidal prism) and configuration. Within estuaries, raised water tables and

sea levels will increase the duration waves can attack the margin shorelines and recovery,

if it occurs at all, will be very slow. The small tide-range of the Wellington region make

estuaries here particularly vulnerable. To date only limited empirical-based modelling of

these environments to SLR has occurred. However, research is progressing on

comprehensive process modelling for climate change at different types of inlet/estuary (e.g.

Ranasinghe et al., 2013).

45. Climate change effects on coastal erosion in particular will increase over time.

SECTION 3 - HARD ENGINEERING SOLUTIONS

Relates to PNRP Objective 022, DOC’s proposed new Objective, and Policy 139 Relates to NZCPS Policy 25(e) and Policy 27

46. Mitigation of coastal hazards has traditionally focused on a range of engineering structures

(hard protection) which include seawalls, revetments, groynes on the coast, and jetties,

moles and guidewalls at the mouths of inlets and within estuaries/harbours. These

structures are composed of hard, rigid materials (wood, concrete, rock) and have two

purposes: to directly protect the land behind from erosion, or to control flow velocity (speed

and direction) either away from a vulnerable area, or to a designated area often with a view

to increase scour (bed erosion) for navigation purposes. So, to define hard (engineering)

structures as primarily to prevent erosion, as in the PNEP, is perhaps too broad.

Furthermore, the definition applies to any environment, for example, coast, lakes, and river

valleys.

47. The NZCPS 2010 definition of hard structures qualifies this to refer specifically to structures

used for protection, and further qualifies its location to be on the coast, i.e. to protect

against coastal hazards.

48. Focusing on protection structures, there are two main types.

• Groynes which protrude into the adjacent water body and trap (littoral) sediment -

thereby protecting the adjacent land; and

• Shore-parallel structures which intercept/block wave/current impacts. While

several types of shore-parallel structures exist (bulkheads [vertical structure face],

seawalls [moderately steep face] and revetments [low-slope structure face]), they

are often collectively referred to as “sea walls”.

49. Groynes rely on a supply littoral of sediment, which tends to be lacking on eroding coasts,

so seawalls are the more common structure.

50. If sea walls are correctly designed they will achieve their objective of protecting the land

directly behind them by resisting coastal forces. However, the life of such structures is

limited and they must be subject to regular inspection, maintenance and often

strengthening.

51. The environmental issues with seawalls stem from their introducing interactions between

waves, currents and sediment which do not exist in the original (natural) system, so beach

behaviour in the vicinity of such structures can be expected to show some difference to

that had the structure not been present. These interactions may produce problematic

responses in front of the structure including toe scour, profile steepening, reduced beach

width, and delayed beach recovery following storms. In addition, there will often be lateral

responses referred to as end-effect erosion which results in an embayment (of depth r in

Figure 1) which tapers away from the structure to pinch out at a distance alongshore (S in

Figure 1). Empirical relationships commonly used in design (McDougal et al., 1987) are S

= 0.7 * ls, and r = 0.1 * ls. Seawalls must be designed to account for end erosion by merging

with a hard structure/outcrop or being extended to where end erosion will not be

problematic, i.e. there are no assets of particular value therefore making loss of land more

acceptable.

Figure 1 Plan (bird eye) view of a seawall (length ls) and adjacent coast to right being affected by end-effect erosion (defined by alongshore length S and landward extent r), coupled with long-term shoreline retreat (e).

52. Seawalls can be more effective and/or have less environmental effects in some situations

than in others.

• If a coast is in long-term equilibrium then a seawall can act as a backstop against

episodic erosion that would otherwise impact on an adjacent landward asset. After

the storm event the coast will tend to naturally recover and the structure will

become buried.

• If the coast is subject to long-term net erosion, then the adjacent shoreline will

eventually migrate landward behind the structure (e in Figure 1), resulting in a

reduction or loss of dry beach width in front of the seawall and eventually

outflanking at the ends of the seawall.

• Seawalls located on long-term net eroding coasts are also increasingly subject to

ever-increasing wave and current impact and hence more frequent maintenance

and strengthening are required.

53. Such environmental issues are one of the reasons that hard protection should generally

only be used when other (softer) forms of protective management have been carefully

considered as addressed in NZCPS 2010, Policies 25, 26 and 27. Those policies do

recognise situations where seawalls may be required for particularly significant

regional/national infrastructure, or when relocation of existing development/infrastructure

that has national/regional significance is very problematic. The NZCPS policies also set

out particular considerations and criteria for hard protection structures that are proposed

for the protection of private property - by carefully weighing up the social and

environmental costs, and evaluation of alternative responses.

54. Where hard structures are unavoidable, particular care is required in designing and

locating the structure so as to minimize adverse effects. Appropriately qualified

professionals who are familiar with the geophysical characteristics of the site and will select

the most appropriate design and positioning of the structure. Examples of where these

principles have not been followed abound along with the consequential environmental

impacts.

55. Climate change will present serious problems for hard structures. SLR will cause

shorelines that are presently in dynamic equilibrium to experience long-term erosion trends

and shorelines presently experiencing long-term retreat will experience enhanced rates of

erosion. Hard protection structures will thus be subject to increasing financial and

environmental costs along more of the New Zealand coastline. With climate change,

structures will require even more maintenance and strengthening if they are expected to

continue to protect development, and there will be ever-increasing environmental effects

such as the loss of fronting beaches. Associated end-effect erosion will also translate

landward – likely into valuable property.

56. When existing seawalls break down or become otherwise problematic, consideration

should thus be given to relocation along with other hazard avoidance/reduction approaches

to be consistent with NZCPS 2010, Policies 25 and 27, rather than replacing or modifying

the structure.

SECTION 4 - NATURAL BUFFERS

Relates to PNRP Policy O20 and Policy 30

Relates to NZCPS policy 25e and Policy 26

57. Natural buffers or hazard defences include subtidal sandbars, intertidal areas, beaches,

foredunes, estuaries and coastal vegetation. The particular importance of foredunes and

estuary/inlet vegetation are addressed below.

58. Foredunes are a common feature backing sandy and sometime also mixed sand-gravel

beaches. Less extensive sand dunes may also occur in the vicinity of inlets and even

around estuary margins. These environments occur throughout the Wellington region.

59. Foredune growth is a product of vegetation type (with native grasses resulting in dune

shape that is more resistance to erosion onset, as well as enabling faster and more

extensive dune recovery following an erosive event, than achieved by introduced species),

sand supply from the beach, shoreline behaviour of the beach (erosion/stability/accretion)

and the onshore wind regime.

60. Systematically accreting shores are typically backed by a series of low dunes (perhaps up

to 2 m high), while stable shorelines tend to have a lesser number of moderately high

dunes (say up to 5 m), while eroding shorelines may have a single higher dune.

61. If vegetation cover is disrupted foredunes easily become unstable with wind-born sand

travelling inland at average rates of 10s or even 100s of metres per year. During

colonisation, introduced animals destroyed much of the foredune cover and approximately

130,000 ha of active sand dunes resulted. Significant government funding to firstly

experiment with conservation techniques and then carry out large-scale conservation

schemes resulted in the “sand problem” being largely controlled by the 1990s, some 100

years later. However, anthropogenic interference is not necessary to initiate dune

instability: past episodes of SLR bought marine sediment ashore and this buried vegetation

resulting in the extensive dune fields in Northland, the Manawatu and elsewhere.

62. Tsunami can effectively be blocked by the foredune if it is of sufficient height and lateral

extent. However, such events can result in significant loss of vegetation and sediment from

the dune itself.

63. Under storm inundation conditions, a vegetated foredune rapidly dissipates wave energy

with minimal loss of sediment occurring under return flows.

64. Storm waves, however, can cause substantial erosion of the lower dune and create a

scarp. However, this process relies upon the beach itself having a sediment deficit in which

case the foredune toe is effectively undermined. This condition can occur along several

hundred metres, or even kilometres of coast, at any one time. Where there is limited

foredune development, in terms of height or continuity, waves can overtop and penetrate

further inland.

65. As long as the dunes are not completely breached, and enough time elapses before the

next erosive (storm) episode, foredunes can naturally recover particularly where there is a

well established cover of native dune grasses such as pingao or spinifex. However, dunes

face increased pressure by foot and vehicle traffic. Conservation works (fencing to control

access or trap sand, planting with dune grasses and other species depending on the

location, and initial recontouring for more extensive projects) to assist dune establishment

and/or recovery are common in many areas are often planned and funded by councils with

substantial input by the community.

66. Climate change will introduce additional threats to coastal sand dunes:

• SLR increases the frequency with which storm waves can impact upon the

foredune, and increased wave heights further enhancing that impact.

• Sediment deficits along fronting beaches may become more common as the

shoreline retreat and this facilitates undermining of the dune toe and associated

dune scarping, and

• Wind regime changes may cause further problems by enhancing wave-induced

erosion and increasing landward volumes of wind-blown sand creating burial and

nuisance hazards further inland.

67. Foredunes offer effective protection from tsunami, storm inundation and erosion and as

they occur along about 50% of the New Zealand coastline they are a significant first-line

buffer. Maintaining vegetation cover is required to ensure dunes protect against seaward

originating coastal hazards, and to prevent wind-blown sand causing a further hazard to

landward. For many beach communities, maintenance and re-establishment of the

vegetation cover is a priority. With the increasing pressures from climate change,

conserving sand dunes may become a mandatory priority for local government.

68. Inlet and estuary shorelines are exposed to both erosion and inundation. Erosion is driven

by wave energy which enters the enclosed water body from the sea during high storm tides

and dissipates around the shoreline margin. Waves are also generated within the water

body by wind. Exposure to coastal hazards will increase under climate change - although

as noted earlier actual inlet/estuary response will be varied

69. In developed areas, hard protection already characterises much of these shorelines. Guide

walls limit lateral migration of the mouths and excavation (soft management approach) is

used to control channel configuration and outlet depth thereby reducing potential impacts.

As described in the previous section, there will be increasing need for such controls

measures under climate change with increasing pressure on the structures requiring

continual raising and strengthening.

70. By contrast, natural and lesser developed areas generally rely on ecological-based

controls. High water margin and backshore vegetation reduces wave runup and traps (if

available) wind-blown sand and estuarine muds thereby raising landward elevations.

Offshore vegetation (landward of the high water margin) is particularly effective in damping

wave energy.

71. Protection, maintenance and increasing the extent of such vegetation, both in natural/low

development areas and also, where practicable in more developed areas, provides modest

protection at relatively low cost that largely avoids consequential impacts

SECTION 5 - THE RISK MANAGEMENT APPROACH TO COASTAL HAZARDS

Relates to PNRP: Objectives O20, O21 and the new proposed DOC objective; Policy P28 and

Method M3

Relates to NZCPS: Objective 5 and Policies 24 to 27

72. The NZCPS (2010) ushered in a new approach to coastal hazard assessment and

management by focusing on hazard risk assessments and risk management with its

four policies summarised as follows:

Policy 24 sets out policies for the identification of areas potentially affected by coastal

hazards and carrying out hazard risk assessments with a view to identifying high risk areas;

Policy 25 sets out policies for all subdivision, use, and development in areas of hazard risk

including avoiding increasing the risk of social, environmental and economic harm;

encouraging development that will reduce risk, and discouraging hard protection while

promoting the use of alternatives.

Policy 26 addresses the protection, restoration or enhancement of natural defences such

as coastal vegetation, sand dunes and wetlands, to protect valued sites.

Policy 27 sets out strategies for protecting significant existing development from likely

hazard risk. It focuses on long-term sustainable risk-reduction approaches and includes

adaptation provisions such as removal/relocation, and hard protection where it is the only

option to protect nationally or regionally important infrastructure. It requires a focus on

approaches that reduce the need for hard protection over time, and, where hard protection

is considered necessary requires that its form and location be designed to minimise

adverse effects. The policy also requires strategy evaluations to recognise that the hazards

themselves may change over the planning timeframe, including from climate change.

73. The term risk, which occurs throughout these policies, first entered the coastal hazard

assessment/management process in the 1980s with the following UNESCO definition

(referenced in Gibb 1994):

Risk = Hazard (probability of occurrence) * Vulnerability (degree of associated loss).

74. This resulted in hazard zoning with a range of risk divisions from extreme, high, and

moderate based on temporal occurrence. Extreme erosion hazard risk typically involved

only short-term erosion, while high risk included long-term retreat plus the effect of

accelerated SLR, over a 50 year period, while moderate risk included long-term erosion

plus the effect of accelerated SLR, out to 100 years (Gibb, 1994, ARC, 2000). This was

consistent with the NZCPS 1994 which required statements and plans to recognise the

possibility of a rise in sea level.

75. As IPCC assessments began to show greater, more likely and more reliable climate

change projections and effect predictions, and the 2003 RMA amendment required SLR

effects be incorporated rather than considered, it became evident that greater emphasis

on consequence was required. The MFE (2008) guidelines focused on risk as a

combination of likelihood and consequence as did the 2009 draft revision and 2010 final

version of the NZCPS, referencing the Australian/New Zealand Standard Risk

Management AS/NZS 4360:204, August 2004, Third Edition, 2-6.

76. By 2012, DOC was producing guidelines to assist practitioners and councils implement the

new NZCPS provisions, but coastal hazard implementation guidance would be a further 5

years away (December 2017). In the mean time, Council Plans were expiring and as the

hazard management provisions in their Reviewed Plans had to give effect to the NZCPS

2010, practitioners and officers had to interpret the 2010 Policy Statement as best they

could. As noted earlier, practitioners went ahead and developed probabilistic hazard

assessment modelling – an implicit requirement of risk-based assessment. However,

implementation of the risk management aspects (NZCPS (2010) Policy 25 and 27) had to

rely on the 2009 international standard AN/NZS IOS 31000 – Risk Management: Principles

and Guidelines, a globally accepted framework of risk assessments and management.

(e.g. Russ and Shand, 2016).

77. The AS/NZA ISO 31000 (2009) standard defined risk as

“The effect of uncertainty on objectives,”

noting that an “effect” is a deviation from the expected, and that “objectives” are wide

ranging (such as financial, health and safety, environmental goals), can apply over a range

of scales (project, regional, strategic) and addressed the effects of uncertainty. In practice,

this expanded/detailed definition of risk is often simply expressed as the “combination of

the consequences of an event and the likelihood of its occurrence”, as used in NZCPS

2010 and MFE documents.

78. In December 2017, the Department of Conservation released its implementation guide

relating to hazard provisions in the New Zealand Coastal Policy Statement 2010: Policies

24, 25, 26 & 27. This detailed guide provides a line by line (in places word by word)

commentary on the terms, their interpretation, and how they relate to other provisions

within the policy statement. The interpretations are based on, amongst other things,

relevant legal input, practitioner developed best practice and reasoned analysis.

79. Of particular interest and applicability in DOC 2017 is the recommendation concerning the

identification of areas potentially affected by coastal hazards (the first step in NZCPS 2010

Policy 24 [1]) on the basis of the 1% AEP for inundation and the 5% (very unlikely)

probability of occurrence for erosion. The latter value having been the subject of

considerable debate in the past where some practitioners argued it should be of lesser

occurrence (the worst case) while some residents and academics argued it should be

based on a more likely erosional extent.

80. Also in December 2017, MFE released its update of their 2008 publication Coastal

Hazards and Climate Change – Guidance for Local Government. MFE 2017 provides a

step-by-step approach to assessing (hazard risk assessments), planning and managing

the hazard risks (risk-based management) facing local communities, along with an updated

synthesis of information, tools and techniques to underpin the process. The approach is

very much designed to facilitate inclusion/participation by the coastal communities

themselves.

81. MFE 2017 also use the “practical” definition of risk (likelihood combined with

consequence), as the level of management detail provided in their document (MFE 2018)

makes the use of the expanded ISO 31000 definition unnecessary.

82. The MFE 2017 guidance has been structured around the 10-step framework

diagrammatically illustrated in Figure 2.

Step 1 (preparation and context) is a necessary first step in any hazard assessment.

Step 2 (Hazard Assessment) is a necessary second step. Of particular note is the MFE

2017 recommendation for an initial (high level and quantitative) region-wide hazard

assessment for use in defining high hazard risk areas (NZCPS 2010, Pol 24 (1)) once the

hazard assessment (5% likelihood) is combined with basic asset/values screening

(consequences).

Step 3 (values) and Step 4 (vulnerability- for predisposition of infrastructure, property and

the like) addresses hazard consequences in detail, and when combined with a

detailed/probabilistic hazard assessment carried out for the high risk areas identified in

Step 2, provide the information base for the management planning process; in particular,

avoidance for new development (NZCPS 2010 Pol 25), and adaptation for existing

development (NZCPS 25 and 27).

Steps 5 to 8 (adaptation options, pathways and triggers) refers to response strategies in

developed high risk areas by accommodating, protecting, retreating or avoiding the hazard.

This approach recognises an initial response may apply for a limited time and a change to

an alternative path will subsequently be required when a predefined trigger point is

reached. For example, when shoreline erosion in front of an asset reaches a predetermined

location (the natural protection having been exhausted), removal may become the most

viable option. This should also assist in reducing the risk over time. These steps also

recognise the dynamic nature of coastal hazards over time.

Steps 9 and 10 refers to monitoring, review and adjustment

Figure 2 Decision making cycle (conceptual model) for risk-based management of coastal hazards (MFE 2017)

83. With the exceptions of Step 1 and Step 2 in Figure 2, parts of Steps 3 and 4 and much of

Steps 5 to 8 are largely conceptual and will be subject to extensive “application testing”

before practical methodologies emerge and efficient application is possible. As indicated

within MFE 2017, some of its approaches are perhaps better viewed as suggestions.

84. While MFE 2017 has been specifically designed to manage and adapt to the increased

coastal hazard risks posed by climate change and SLR, how long it will take to complete

the 10 Steps is unknown. But given the extensive community participation allowed for,

considerable time may elapse. Giving effect to the NZCPS 2010 must include actually

completing the required tasks within a reasonable/foreseeable timeframe.

85. From a practitioners (practical) perspective, progress to date is relevant. Most of the NZ

coast has been subject to regional or district-wide coastal hazard assessments and, if

necessary, these can be relatively easily and quickly updated with available data. When

broadly matched with (screened against) council assets and cultural/community values

(which are relatively easily acquired at a high level), the resulting hazard risk-assessment

is suitable to identify High Hazard Risk areas - the key initial requirement of NZCPS 2010

hazard provisions. This work fits within in MFE 2018, Steps 1, 2. These areas of coast can

then be subject to detailed hazard assessments which produce the full range of

probabilities of event occurrence at 10 m alongshore increments throughout the previously

defined high risk area. Acquiring this higher level of information is now relatively

straightforward and has been successfully produced for a few different areas (Northland,

Hawkes Bay, Christchurch and Poverty Bay by Tonkin + Taylor Ltd between 2015 and

2017). This suite of reliable and useful hazard information can then be web published as

the base information that gives effect to the NZCPS 2010. It will also be available for use

by developers, property owners/prospective purchasers, insurers and the councils

themselves when carrying out infrastructural/management functions. It is noted that in

some parts of NZ the hazard information presently available dates back to the 1980 making

it potentially as misleading as it might be useful. The various risk-based management

Steps in MFE 207 required to complete the NZCPS 2010 management provision can then

be addressed in the timely manner they will require.

86. Method M3 was not the subject of earlier submissions as there was no apparent need to

do so at that time. However, the recent release of extensive and pertinent guidance, in

particular MFE 2017, has the potential to create timing issues that have implications to

giving effect to the NZCPS 2010, as discussed above. I further note that this new

information could have been introduced in the Section 42 report but was not. This situation

can be addressed in part by inserting the following (bold) wordage into Method M3.

Method M3: Wellington regional hazards management strategy Wellington Regional

Council will work in partnership with city and district councils and key stakeholders

including the Department of Conservation, to develop and implement a Wellington

regional hazards management strategy. The purpose of the strategy is to facilitate a

consistent approach to managing hazards between local authorities in the region. The

strategy will set achievable staged objectives, outline industry-accepted/guidance-

based methodologies, and provide timing milestones to ensure an acceptable

progression to coastal hazard risk assessment and coastal hazard risk management

is achieved.

References

AN/NZS IOS 31000, 2009. International Standard– Risk Management: Principles and Guidelines,

ARC, 2000. Coastal hazard strategy: coastal erosion management manual. Auckland Regional Council

Technical Publication No 130.108p.

AS/NZS 4360:204. 2004. Australian/New Zealand Standard Risk Management. Third Edition, 2-6.

Beavan, R.J., and Litchfield, N.J., 2013 Vertical land movement around the New Zealand coastline:

implications for sea-level rise. GNS Science Report 2012/29. 41p.

Department of Conservation (2017): A guide to implementing the New Zealand Coastal Policy Statement 2010: Policies 24, 25, 26 & 27. 99p.

Gibb, J.G., 1978. Rates of coastal erosion and accretion in New Zealand. NZ Journal of Marine and Freshwater Research, 12 (4): 429-56.

Gibb, J.G., 1994. Standards and information requirements for assessing coastal hazard zones for New

Zealand. A report prepared for the Department of Conservation, 73p.

GNS, 2005. Review of Tsunami Hazard and Risk in New Zealand. Compiled by Kelvin Berryman.

Consultancy Report 2005/104.

GNS, 2013. Review of Tsunami Hazard and Risk in New Zealand Update. Compiled by William Power,

Consultancy Report 2013/131.

IPCC, 2013. 5th Assessment Report, WG I: The Physical Science Basis. Intergovernmental Panel on Climate Change.

McDougal W.G.; Sturtevant, M.A., and Komar, P.D., 1987. Laboratory and field investigations of the

impact of shoreline stabilization structures and adjacent properties. Proceedings of Coastal Sediments

’87, ASCE, 962-973.

MFE 2008. Coastal Hazards and Climate Change. A Guidance Manual for Local Government in New Zealand. Prepared for Ministry for the Environment. 127 p.

MFE 2017. Coastal Hazards and Climate Change: Guidance for Local Government. Prepared for

Ministry for the Environment. 279 p.

NIWA, 2012a. Coastal Adaptation to Climate Change: Mapping a New Zealand Coastal Sensitivity

Index. A report prepared for MBIE by the National Institute of Water & Atmospheric Research Ltd.42 p

NIWA, 2012b. Defining coastal hazards zones for setback lines: a guide to good practice. A report prepared for Envirolink by the National Institute of Water and atmospheric Research Ltd, 91p.

NZCPS 1994. New Zealand Coastal policy Statement. Department of Conservation. 26p.

NZCPS 2010. New Zealand Coastal policy Statement. Department of Conservation. 28p.

Ranasinghe, R., Duong, T.M., Uhlenbrook, S., Roelvink, D., Stive, M., 2013. Climate change impact

assessment for inlet-interrupted coastlines. Nature Climate Change. 3, 83–87.

Russ, M., and Shand, R.D., 2016. Climate change and coastal hazards: a risk-based approach that

connects science, engineering and planning. New Zealand Planning Institute Conference. 13p.

Carley, J.T.; T D Shand; A Mariani, and R J Cox, 2011.Technical Advice and Support Guidelines for Assessing and Managing the Impacts of Long-term Coastal Protection Works. WRL Technical Report 2010/32

Shand, T.D.; Wasko, C.D.; Westra, S.; Smith, G.P.; Carley, J.T., and Peirson, W.L., 2012. Joint Probability Assessment of NSW Extreme Waves and Water Levels. Report prepared by the Water Research Laboratory for the Office of Environment and Heritage, WRL Technical Report 2011/29

Shand, T.D.; R.D. Shand; R. Reinen-Hamill; J.T.Carley, and R.Cox, 2013. A review of shoreline

response models to change in sea level. Coast and Ports Conference.

Shand, T.D.; R. Reinen-Hamill; P. Kenc;, M. Ivamy; P. Knook, and B. Howse, 2015. Methods for Probabilistic Coastal Erosion Hazard Assessment. Coasts & Ports Conference, Auckland, New Zealand.

Tonkin + Taylor Ltd., 2015. Coastal Flood Hazard Zones for Select Northland Sites. Report prepared

for Northland Regional Council.

Appendix 1: Recent Consultancy Reports

• 2012 Kapiti Coast Erosion Hazard Assessment Update. Kapiti Coast District Council, Client

Report (CR) 2012-09

• 2012 Geomorphological Assessment of the Waikawa, Ohau, Hokio and Waitere Inlets,

Prepared for Horizons Regional Council, CR 2012/9B.

• 2012 Shoreline Retreat from Sea-Level Rise: Komar Approach c.f. Bruun Approach. Kapiti

Coast District Council, Client Report 2012-13C

• 2013 Coastal erosion hazard assessment for the Patea to Waverly coast. Trustpower Ltd.,

Client Report 2013-03b

• 2013 Coastal erosion hazard evidence: KCDC vs Weir Judicial Review. Kapiti Coast District

Council, Client Report 2013-05

• 2013 Waimeha erosion hazard reassessment. Kapiti Coast District Council, Client Report

2013-06

• 2013 Managed erosion hazard assessment for the Mangaone Inlet. Kapiti Coast District

Council, Client Report 2013-14

• 2014 Montgomery Reserve Holocene landscape evolution and land use zoning. Mrs V Cave,

Client Report 2014-01

• 2014 Patea Beach monitoring report. South Taranaki District Council, Client Report 2014-02.

• 2014 Site-specific erosion hazard assessment: Waikanae Inlet (Kotuku Parks frontage).

Kapiti Coast District Council, CR 2014-04

• 2014 Patea Beach drainage management options. South Taranaki District Council, Client

Report 2014-10

• 2014 Castlecliff Beach (Rangiora Street) sand management discussion paper. Whanganui

District Council, Client Report 2014-11

• 2014 Cooks estuary hazard assessment. Longreach Subdivision Ltd., Client Report 2014-

15.

• 2014 Preliminary erosion hazard assessment: Whangapoua Harbour fronting Matarangi

Drive, LOT 1 DP 405557. Client Report 2014-16

• 2015 Patea Beach Sand Management Project: Summary Report 2008 to 2015 including

Sand Management Guidelines. South Taranaki District Council, Client Report 2015-02

• 2015 Shoreline analysis for the Mininoa-Siumu coast. Client Report 2015-07

• 2015 Shoreline analysis for the Tafatafa coast, Samoa. Client Report 2015-08

• 2015 Shoreline analsyis for the Lano coast, Samoa. Client Report 2015-09

• 2015 Revised coastal hazard assessment for the Patea – Waverly coast. Trustpower Ltd.,

Client Report 2015-10B

• 2015 Patea Beach Management Plan Revision Issues. South Taranaki District Council,

Client Report 2015-14

• 2016 Geomorphological Assessment of the Manawatu Coast. Manawatu District Council,

Client Report 2016-01

• 2016 Shoreline change for the Happy Jacks Boat Harbour, Mahia Peninsula. Joan Ropia of

Ngai Tu ki Mahanga, Client Report 2016-02

• 2016 Environmental conditions and shoreline behavior at Waitatapia Inlet and the South of

Himitangi coast between 2011 to 2015. Ministry of Justice, Client Report 2016-03

• 2016 Historical survey shorelines for the Timaru to Otario coast. Client Report 2016-04

• 2016 Whanganui Harbour baseline study 1842 to 2015. Client Report 2016-06

• 2016 Cliff hazard assessment between the Patea Rivermouth and the Manawopou

Rivermouth. Meridian Energy, CR 2016-09

• 2016 Preliminary erosion hazard assessment: Waitara East Beach. Te Puni Korkori. Client

Report 2016-18

• 2016 Patea Beach Sand Management Project: 2016 Annual Report. South Taranaki District

Council, Client Report 2016-23

• 2017 Geomorphological Assessment and shoreline analysis from the Waipaoa Rivermouth

to Tuaheni Point, Poverty Bay. Client Report 2017-03

• 2017 Geomorphological assessment and shoreline change analysis for Waimea Inlet to

Cape Soucis, Tasman Bay. Client Report 2017-05

• 2017 Geomorphological assessment and shoreline change analysis from the Stony

Rivermouth to the Mokau Rivermouth, North Taranaki. Client Report 2017-07

• 2017 Geomorphological Assessment for Pukepuke Lagoon, Manawatu. Department of

Conservation. Client Report 2017-08.

• 2017 Patea Beach Sand Management Project: 2017 Annual Report. South Taranaki District

Council, Client Report 2017-11.

Appendix 2: Computational Methods for Coastal Hazards

A Computational methods for storm inundation:

• Sensitivity testing provides a very first order measure of relative (alongshore)

susceptibility to storm inundation by considering the elevation/topographic setting

(NIWA 2012a). This broad qualitative measure is unsuited to defining the hazard

potential (as required by the NZCPS 2010 Pol 24 and discussed in Section 5 of this

evidence) and subsequent land-use planning decisions

• The traditional quantitative method to storm inundation involves adding values for the

various contributary components (the building-block approach).

- The most basic building-block method adds high component values; however,

this tends to over-estimate the hazard as some of these components are inter-

related, e.g. wind drives both storm surge and wave height, so an element of

double dipping is incorporated.

- More refined building-block approaches use joint-probability analysis to separate

component contributions. However, this has now been found to introduce

considerable additional uncertainty (Shand et al., 2012) with the dependence

often biased by smaller events.

• Recently a fully probabilistic approach has been developed (Tonkin + Taylor, 2015),

that randomly combines values from each components distribution. This is the

preferred approach for detailed hazard assessments in high risk areas (NIWA, 2012b)

The development a fully probabilistic approaches for use in inundation assessments (and also

erosion assessments as described below) has been driven by the 2010 New Zealand Coastal

Policy Statement’s use of the hazard risk concept which requires consideration of likelihood

when defining areas potentially affected by coastal hazards and addressing risk-based

management (considered further later in my evidence).

B. Computational methods for coastal erosion:

• Sensitivity testing based on a range of variables including geological properties and

geomorphology, but excluding historical or current erosion, provides a very first order

measure of (alongshore) relative erosion potential (NIWA 2012a). However, as with

inundation, this broad qualitative measure is unsuited to defining the hazard potential

as required by the NZCPS 2010 Pol 24, and subsequent land-use planning decisions.

• The typical (building block or deterministic) approach for non-cohesive coasts is

based on the addition of component values for shorter-term (years) and longer-term

(decades to centuries)1 erosion, together with a post-erosion retreat adjustment for

the top of the erosion scarp as it attains a stable slope. An uncertainty measure is

also often included. Cohesive (cliffed) shorelines exclude the shorter-term

component. These components are essentially independent, thereby avoiding the

double-dip associated with inundation components. However, practitioners aim is to

achieve an output tending toward the worst case scenario, the output of which is used

to define hazard potential. As with inundation, two variants occur:

-While practitioners have tended to select high end values for all component, this can

result in excessively high (conservative) output.

Better alignment with a low probability-worst\case scenario can be achieved by

component value selection based on statistical analysis of component distributions

combined with the principals of probability combination.

• Recent development of a probabilistic (or stochastic) approach (Shand et al., 2015) is

based on identifying a distribution for each component (including its uncertainty), then

randomly combining values from each distribution to generate an erosion hazard

distribution from which the full suite of probabilities (likelihoods) can be extracted.

Since 2015, this method has been successfully applied for detailed assessments in

high risk areas in Northland, Hawkes Bay, Christchurch and Poverty Bay by Tonkin +

Taylor Ltd.