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Coastal Hazard Management at Plimmerton Community Meeting July 2019 Prepared for the Porirua City Council By Jim Dahm Bronwen Gibberd Focus Resource Management Group
Coastal Hazard Management
at Plimmerton
Community Meeting
July 2019
Prepared for the Porirua City Council
By
Jim Dahm
Bronwen Gibberd
Focus Resource Management Group
Please note…
This presentation was prepared to provide information and outline options for
coastal hazard management at a community meeting in July 2019.
The hazard maps presented here are draft maps for consultation and do not
yet reflect changes that may be made as a result of feedback received
during the workshops.
The slides are reproduced here to encourage further community discussion
and feedback.
Please direct any questions or feedback to:
Objectives
Porirua District Plan review – natural hazards
• Identify areas vulnerable to coastal erosion and coastal flooding
• Consider impacts of future sea level rise
• District-wide recommendations for coastal hazard management
• Detailed assessments of difficult/complex high risk areas
This presentation presents draft hazard areas and management recommendations. PCC is looking for feedback.
Information Gathered Information from community:
Historic coastal erosion
Community values
Coastal hazard management issues
Wide range of observations
Analysis of aerial photographs dating from 1940s
Photos of beach and dune from early 1900s (including
data held by Pataka and National Library)
Early descriptions and maps of the area
Published resources – books, reports and studies
Sub-surface geology – field investigations
Elevation data
Sea Level Rise
In addition to existing risk (i.e. with existing sea
level), required to consider potential future sea
level rise
National Policy - must plan for at least 100 years
including climate change
National guidance
1.6-1.7 m of SLR is inevitable, not if – but WHEN.
Huge uncertainty, cannot simply extrapolate
past trends
Must use “plausible scenarios”
IPCC emissions scenarios (Representative
Concentration Pathways or “RCPs”)
Scenario 2070 2120
Low (RCP 2.6)
Lower bound
“surprise”
0.32 m 0.55 m
Intermediate (RCP
4.5)
0.36 m 0.67 m
Transitional 1.00 m
High+ (RCP8.5)
(85th percentile)
0.61 m 1.36 m
National Guidance (MfE 2017)
The MfE (2017) national guidance recommends Councils apply the following sea level
rise values for coastal hazard planning where an adaptive plan is not yet in place:
Greenfields developments or major new infrastructure: avoid all risk and use only
RCP8.5 H+.
Intensification or change in land use of existing development: Any intensification/land use change requires a full adaptive management planning process using the four sea
level rise scenarios (at the scale appropriate to the scale of the intensification).
Existing exposed development: use a transitional sea level rise of 1.0 m (2120) until a full
adaptive management plan is complete.
Low-risk non-habitable works and activities, particularly those with a functional need
to be near the coast: use transitional value of 0.65 m by 2120, which aligns with RCP4.5
(2120).
Coastal Hazard Areas
Three coastal erosion and coastal flood hazard zones:
Current coastal erosion/flood hazard - the areas vulnerable to coastal erosion
and/or flooding with existing sea level and coastal processes.
Future coastal erosion/flood hazard 1.0 m sea level rise - the areas potentially vulnerable to coastal erosion/flooding over the period to 2120, assuming sea
level rise of 1.0 m.
Future coastal erosion/flood hazard 1.4 m sea level rise - the areas potentially
vulnerable to coastal erosion/flooding, assuming sea level rise of based on
RCP8.5 H+ (1.36 m to 2120).
Management Options and Philosophy: National and Regional Policy (NZCPS 2010, RPS)
Risk avoidance - Managing land use in hazard risk areas to avoid risk.
Landward relocation of assets to a safe location
Risk reduction - Managing land use and development to reduce existing risk
exposure over time
Living with coastal erosion/flooding – accepting where appropriate
Mitigation of coastal hazards through protection and restoration of natural
buffers (e.g. beaches, dunes, wetlands)
Soft engineering measures which mitigate coastal hazards using natural
buffers (e.g. beach nourishment)
“Hard” engineering structures, including new or existing sea walls or rock
revetments. Traditional emphasis – now last resort for most areas. Significant challenge
Decre
asin
g p
refe
rence
Coastal Structures – Effects on Beaches
When erosion occurs on a natural
beach, the whole profile moves landward
A wide high tide beach will re-
establish after erosion
Erosion does not destroy beaches
as long as the erosion encounters
sand
Coastal Structures – Effect on Beaches
Building an effective seawall protects the
land behind the wall
A seawall does not stop the processes
driving erosion in front of the wall
The beach continues to erode, and over
time the beach disappears
Recreational values are lost, and public
access alongshore also adversely affected
So, generally not an acceptable long-term
solution on sandy shorelines.
Likely Management Approaches
Development setbacks and associated controls – to avoid and reduce risk and to
inform on existing and longer term risk
Minimum floor levels to protect from flooding
Controls/guidance on use of structures in the coastal margin
Coastal restoration and soft approaches as relevant – e.g. restoration and
enhancement of beaches, dunes and coastal wetlands, working with nature
Adaptive management strategies and triggers will be required at some sites
Requires a decision-making framework that is neither “business as usual” nor a radical
(expensive, impractical, etc.) “overnight” fix/change, but which will ensure a transition to
more sustainable management over time.
Many of these measures and controls new to PCC.
Will be very challenging in some areas and require time.
Adaptive Planning – Decision Cycle
Five key stages:
1. Building a shared understanding (processes, hazards, community
resilience)
2. Exploring the future and how communities are affected and
identifying objectives
3. Building adaptive pathways
4. Implementing the strategy in practice
5. Monitoring the strategy using early signals and triggers (decision
points) for adjusting between pathways.
This study focusing on Items 1-3
District-wide Coastal Compartments:
Geomorphic Shoreline Types
Beaches
Cliffs
Low-Lying Land
Land use
Nearshore Roads and Rail
Infrastructure and Services
Coastal Roads & Rail
Rail corridor incl. North Island main trunk
Titahi Bay Road
SH 58 – Paremata to Pauatahanui
Grays Road & Motukaraka Point
Moana Road & Sunset Parade
Ocean Parade, Pukerua Bay
SH 1 north of Pukerua
Plus various services/infrastructure
Of district and sometimes regional & national
significance
Many roads close to coast at risk without protection; vulnerability increased with sea level rise
Road is often a key district asset and sometimes regionally significant - assume will be protected in some way
Encourage transition towards softer approaches over time – considerable potential even with existing knowledge and experience
Where hard structures are required, guidelines for low impact design, to minimise adverse impacts and to work towards softening where practicable
Will put pressure on asset managers (council, Kiwi Rail, NZTA) to lift game over time but opportunity for Council to lead and set examples
Titahi Bay Road, Area 6 and Dolly Varden will provide useful examples
Coastal Roads & Rail
BeachesDifferent beach types: cheniers and estuary beaches, open coast sandy and gravel beaches.
Estuary beaches: e.g.
- Dolly Varden
- Browns Bay
- parts of Golden Gate shoreline
- upper harbour cheniers (Pauatahanui –historically Porirua Harbour)
Open coast beaches: e.g.
- Plimmerton and parts of Karehana
- Titahi Bay
- Ngatitoa
- Whitireia Park beaches
- Hongoeka
- Pukerua
BeachesErosion issues already at many sites and these will generally significantly worsen with sea level rise.
Coastal inundation also an issue at some sites and will significantly worsen
Beaches are typically important community assets and these values will generally need to be protected
Hard protection works will generally not provide appropriate long-term solutions (though may be part of mix in some cases and during transition)
Significant implications for nearshore infrastructure and property, including setbacks and development controls
Very complicated issues at some sites (e.g. Plimmerton, Browns Bay, etc.)
Need to work through with community, including wider community and adjacent property owners
Beaches
Most developed sites will likely require site specific adaptive management strategies which transition to more sustainable management over time
Mix of measures will vary from site to site but will include some or all the following
➢ Development controls
➢ Soft strategy for managing erosion
➢ Living with hazards
➢ Dune management
➢ Nourishment
➢ Retreat
➢ Hard structures
Setbacks and development control likely to be required at most developed sites.
Low Lying Coastal
Margins
Vulnerable to coastal storm inundation
Often also subject to other flood hazards (e.g.
river flooding, stormwater, tsunami)
Managed differently to coastal erosion
Key contributors:
Storm surge
Wave overtopping
River/coast interactions
Even small amount of sea level rise will greatly
increase severity and frequency of flooding
Sea level rise will also aggravate river and
stormwater flooding (downstream control)
Effect of Sea Level
Rise on Return Period
of Storm Inundation
Events
100-year event becomes an
annual event with just 30 cm of
sea level rise
Coastal Storm Inundation
Karehana Area - 1894
Karehana Area (incl. Sunset Parade and Moana Road)
Sand and gravel beaches
overlying rock reefs and shore
platform
Original coastal plains (dunes
and gravel ridges) now
covered in development
Road commonly extends out
onto former beach with sea-
walls and/or fill often placed on
seaward side
Some limited development also
on seaward side of road (e.g.
fire station and properties) and
some underground services
Karehana Area (including Sunset Parade and Moana Road)
Existing beaches fluctuate over time, but no
evidence in historic data of long-term erosion
Projected future sea-level rise is likely to lead
to permanent beach retreat and severely
aggravate existing beach loss and coastal
squeeze effects
Sea level rise will increase wave energy
reaching the shoreline and severely
aggravate existing coastal inundation,
including much more frequent and severe
wave overtopping
Karehana Area – Management of Coastal Hazards
Road is an important access and likely to continue to be protected in short-medium
term. With sea level rise will eventually need to be raised.
However, potential severity of future effects means that landward relocation and/or
narrowing cannot be ruled out in medium-longer term (depending on scale of sea
level rise)
Severe beach loss with sea level rise may be able to be offset with beach nourishment
but cannot presume that at this stage.
No erosion areas or associated controls will be required in hazard areas on landward
side of road. Future intensification not desirable.
Coastal Inundation
Existing risk largely limited to road
Much more widespread and serious issues will
occur with future sea level rise
Wave effects including wave overtopping will
become much more severe close to coast
Existing stream and stormwater flooding issues
will also be aggravated
Rise in groundwater also likely to be an issue
Minimum floor levels, adaptable foundations
and development controls will be key
measures
In areas of stream/stormwater flooding will
require complex adaptive management
strategies integrating coastal, stream and
stormwater flooding issues – range of
measures likely to be required
Plimmerton
Popular recreational beach of regional significance
North of Taupo Stream
Beach was subdivided very early (1896 north of stream) and property boundaries were
placed on the active beach
Over time, boundary fences became sea walls of increasing height
Beach now lined with sea walls and little or no high tide beach north of Taupo Stream
Waves severely impact seawalls during storms and some overtopping
South of Taupo Stream, houses are on lease hold land and seawalls
are set back further from the sea – providing for a high tide beach
with existing sea-level
Drilling has confirmed that sand extends back to at least Steyne
Avenue
History of Plimmerton Beach: 1895
View of the beach at Plimmerton, showing Plimmerton House on the left. Creator of Collection Unknown: Photographs of Plimmerton. Ref: 1/2-004016-G. Alexander Turnbull Library, Wellington, New Zealand. http://natlib.govt.nz/records/22784142)
History of Plimmerton Beach: 1920
Overlooking the beach, and houses on the waterfront, at Plimmerton. Smith, Sydney Charles, 1888-1972: Photographs of New Zealand. Ref: PAColl-3082-14. Alexander Turnbull Library, Wellington, New Zealand. http://natlib.govt.nz/records/22667220
History of Plimmerton Beach: 1951
Plimmerton, Porirua. Whites Aviation Ltd: Photographs. Ref: WA-28587-F. Alexander Turnbull Library, Wellington, New Zealand. http://natlib.govt.nz/records/22494505
Coastal Erosion
Properties extend into active coastal zone.
Current erosion risk:
Without sea walls erosion could extend up to 10 m
inland with existing sea level
No evidence of any permanent erosion – beach
levels simply fluctuate
Loss of high tide beach more a consequence of sea
wall
Future erosion risk:
At least 20 m of erosion for each 1.0 m of sea level rise
Will considerably aggravate erosion hazard
Will significantly aggravate beach loss if sea walls
remain in current location
So, significant risks to both public and private values
and potential for serious conflicts
Coastal Flood Hazard Areas
Current flood hazard risk
some nearshore areas vulnerable to
extreme sea levels and wave run-up
during coastal storms – can affect existing
properties and some dwellings
potential flooding upstream Taupo Stream
Future coastal flood risk with 1.0 m SLR
will become more frequent, widespread and serious with sea level rise
includes much more serious wave effects,
aggravation of stream flooding
minimum floor levels will be required to
manage risk
Plimmerton Very complex and difficult issues with significant
conflicts between public and private interests
Will require complex adaptive management
strategy and considerable time to transition to
more sustainable situation
Seawalls in existing location not likely to be an
appropriate long- term solution
Nourishment would likely be expensive and high
maintenance, would likely require major engineering
structures to retain sand (e.g. groynes)
Retreat would also be very complex and difficult at
this site
No easy options
Setbacks and development controls required with
implications for private properties
Potential to enhance areas with small adjustment
– Council may need to lead by example.
Discussion/Questions
