1. Matarangi south
Dr Willem de Lange

2. Geomorphology Barrier spit with parallel dune ridges and overwash fans
Affected by anthropic vegetation disturbance over last 800 years
Evolution of spit shows:
Punctuated accretion
Accretion still underway
Implies storms & tsunami are most important
No strong link to climate change or sea level changes identified
Slightly fewer storms during warm periods
Sea level rise tends to increase accretion
FPCL assumes opposite will occur

3. General model Assumed for Matarangi that no sediment is being added
Observations & theory demonstrate that sediment continues to move onshore during highstands & transgressions
Continuing to do so at Matarangi

4. Omaha
Dune development
Punctuated progradation is evident in NZ coastal barrier deposits
Due to storms, tsunami & anthropic disturbance
Scale is 100 m, & note effect of marram on dune shape

5. Decadal scale processes
Increasing evidence of interannual (ENSO) & decadal scale (PDO) changes in coastal processes
Decadal-scale cut & fill cycles ( years?)
Increase in storm frequency/magnitude = offshore sediment transport = erosion
Decrease in storm frequency/magnitude = onshore sediment transport = accretion
Variations in storm tracks or wave approach directions causes beach rotation (particularly for pocket beaches)
Ranasinghe et al 2004.
Marine Geology 204: 273–287.

6. Shoreline changes

7. Climate variability Proxy data indicate no clear relationship between
Longer term climate oscillations & coastal evolution
Sea level changes & coastal evolution during Holocene

8. Matarangi

9. Climate
NZCPS (2010) requires consideration of effects of climate change over next 100 years
50 year - mostly predictions & indicate little to no change from historical patterns
100 year – only projections
Sea level continue to rise with most likely value ~0.4 m
Storminess unclear
Australian studies indicate a decline in severe storms & waves
NZ assessments suggest decline in frequency but slight increase in magnitude
Assessment should be based on a range of scenarios & relative likelihood should be estimated

10. Bruun Rule
Bruun (1962) proposed model for shoreline response to sea level rise
Known as Bruun Rule
Multiplies assumed sea level rise (S) by a scale factor (1/tanβ = nearshore slope)
Only predicts erosion for rising sea level
Response predicted depends only on assumed values for
Sea level rise
Nearshore slope
Flatter slopes = more erosion
Hence type of beach is important
This method is not appropriate for forecasting future shoreline changes
Reflective (pocket) beach
Dissipative (dune barrier) beach

11. Future sea level
IPCC (2013) projections indicate reduction in future rate of sea level rise, except for worst- case RCP8.5 scenario
RCP8.5 assumes burning 2-16x known reserves of fossil fuels & no mitigation measures
Suggested that RCP8.5 be taken as minimum for planning purposes
MfE (2008) at least 10 mm.y-1 by AD 2100
NZCPS 2010 requires most likely

12. Recent global sea level trends
Multiple studies using different techniques have measured a slowing in rate of sea level rise during21st Century
Coastal tide gauges
Satellite altimetry
Ocean mass
Measured rate is below assumed initial rate for IPCC (2013) projections
But, there are significant regional variations
Should consider local sea level
Purkey et al, in press. Journal of Geophysical Research: Oceans, DOI /2014JC010180

13. Ruakaka sea level Dahm & Gibberd (2009) – MfE (2008)
Dahm & Munro (2002)
IPCC projection mid-point values plotted above assume a normal distribution of projections

14. IPCC projections
IPCC combines all computer model projections into an ensemble, & presents them as if they are normally distributed (ie. all computer models are equally valid)
They are not
More projections lie below mid-point of range than above
Hence, likely projected sea level is below the mid-point
MfE (2008) guidance note was based on a single extreme projection for worst-case scenario
CMIP5 sea level projections used to assess risk for New York City, USA. Extreme outliers were omitted as their probability of occurrence was too low to consider. IPCC (2013) ensemble results for the same CMIP models were a range of 45-82 cm, with a midpoint of 63 cm. Larger difference are evident at regional scales.
Little et al, in press: Uncertainty in 21st century CMIP5 sea level projections. Journal of Climate. doi: /JCLI-D

15. Alternative approaches
There are alternatives to Bruun Rule
Simpler approach is to assume no change from historical trends
More complicated approach is probabilistic modelling using process-based models
Eg. SimCLIM2013 developed originally at University of Waikato (CLIMPACT)
CLIMsystems SimCLIM2013 model results for shoreline changes at Papamoa for different scenarios in response to RCP8.5 projections of climate change.
Demonstrates that mitigation of sea level rise impacts for worst-case scenario is not difficult to achieve