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Matarangi south Dr Willem de Lange.

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Presentation on theme: "Matarangi south Dr Willem de Lange."— Presentation transcript:

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


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