Dr Willem de Lange

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


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
Omaha

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

Increasing evidence of
interannual (ENSO) & decadal
scale (PDO) changes in coastal
processes
◦ Decadal-scale cut & fill cycles (2060 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.


Proxy data
indicate no clear
relationship
between
◦ Longer term
climate
oscillations &
coastal evolution
◦ Sea level changes
& coastal
evolution during
Holocene
7

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

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

IPCC (2013) projections
indicate reduction in
future rate of sea level
rise, except for worstcase 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

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 10.1002/2014JC010180
Dahm & Gibberd (2009) – MfE (2008)
Dahm & Munro (2002)
IPCC projection mid-point values plotted above assume a normal
distribution of 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:10.1175/JCLI-D-14- 00453.1



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