Enhancing coastal function by sensible setback for open duned coasts


Healy, T.

Solutions to Coastal Disasters '02: 794-807

2002


As recently advocated by Healy and Dean (2000), assessment of appropriate development setback based upon identified coastal hazard along open duned coasts may be evaluated from 4 essentially independent factors, viz., historical shoreline change rate, decadal term "cut and fill", a sea level rise effect, and a dune stability factor. The setback delineated is based upon a 100 year planning horizon, and derived from the beach-dune profile. The initial setback determination is then subjected to 3 tests, viz., (i) is there sufficient sand in the beach dune reservoir? (ii) is the setback zone subject to storm surge flooding? and (iii) is the setback zone subject to tsunami inundation? The calculated setback zone may also function for other purposes including coastal landscape preservation, ecosystem protection, and preservation of cultural sites. Major difficulties arise when applying the scheme to already subdivided coastlines, when community views must be considered in an ICZM context. This methodology of determining one integrated hazard zone is considered superior to implementing sequential zones of supposed reducing hazard.

ENHANCING
COASTAL
FUNCTION
BY
SENSIBLE
SETBACK
FOR
OPEN
DUNED
COASTS
Terry
Healy,
PhD.,
M.IPENZ
Abstract:
As
recently
advocated
by
Healy
and
Dean
(2000),
assessment
of
appropriate
development
setback
based
upon
identified
coastal
hazard
along
open
duned
coasts
may
be
evaluated
from
4
essentially
independent
factors,
viz.,
historical
shoreline
change
rate,
decadal
term
"cut
and
fill",
a
sea
level
rise
effect,
and
a
dune
stability
factor.
The
setback
delineated
is
based
upon
a
100
year
planning
horizon,
and
derived
from
the
beach-dune
profile.
The
initial
setback
determination
is
then
subjected
to
3
tests,
viz.,
(i)
is
there
sufficient
sand
in
the
beach
dune
reservoir?
(ii)
is
the
setback
zone
subject
to
storm
surge
flooding?
and
(iii)
is
the
setback
zone
subject
to
tsunami
inundation?
The
calculated
setback
zone
may
also
function
for
other
purposes
including
coastal
landscape
preservation,
ecosystem
protection,
and
preservation
of
cultural
sites.
Major
difficulties
arise
when
applying
the
scheme
to
already
subdivided
coastlines,
when
community
views
must
be
considered
in
an
ICZM
context.
This
methodology
of
determining
one
integrated
hazard
zone
is
considered
superior
to
implementing
sequential
zones
of
supposed
reducing
hazard.
INTRODUCTION
Healy
and
Dean
(2000)
published
a
method
to
determine
coastal
setback
based
upon
nearshore-beach-dune
profile
site-specific
identified
and
measured
coastal
hazards
for
open
duned
coasts.
Komar
(1999)
has
also
advocated
the
need
to
base
setback
determinations
on
an
understanding
of
coastal
processes.
The
methodology
presented
in
this
paper
has
been
tested
and
applied
primarily
on
the
New
Zealand
northeast
coast,
but
has
general
application
to
open
duned
coasts
(Healy
1999).
It
has
been
published
in
a
number
of
peer
reviewed
papers
in
the
scientific
record
(Healy
1980,
1981;
Healy
and
Dean
2000).
More
specifically,
the
methodology
has
been
the
subject
of
a
special
peer
review
undertaken
by
the
National
Institute
of
Water
and
Atmosphere
(NIWA)
(Swales
and
Smith
1994).
Implementation
of
appropriately
derived
development
setback
based
upon
understanding
of
coastal
processes
of
beach
behavior
in
relation
to
the
role
of
the
frontal
dunes
(Condon
I
Research
Professor
of
Coastal
Environmental
Science,
University
of
Waikato,
Hamilton,
New
Zealand.
trh
®waikato.ac.nz
794
COASTAL
DISASTERS
'02
795
and
Barr
1968,
Healy
1975)
and
historical
shoreline
data,
provides
a
planning
mechanism
by
which
coastal
disasters
impacting
on
duned
coasts
may
be
mitigated
and
avoided.
And
a
wide
CHZ
setback
can
also
be
used
for
purposes
additional
to
that
of
hazard
protection.
Although
there are
a
variety
ways
in
which
one
can
quantitatively
evaluate
setback,
the
reality
is
that
many
sectors
of
the
global
coasts,
especially
those
of
the
western
hemisphere
nations,
are
already
developed
in
various
ways.
Thus
perplexing
issues
arise
as
how
to
implement
a
setback
upon
an
already
developed
coastal
zone.
Some
of
these
issues
are
discussed
following.
COASTAL
HAZARDS
Coastal
hazards
are
recognized
as
resulting
from
(Pilkey
et
al.
1983,
Bush
et
al.
1996,
Pilkey
and
Dixon
1996
):
coastal
erosion
i.e.
the
retreat
of
the
shoreline
due
to
wave
action
removing
sediment
offshore
or
alongshore,
or
tidal
current
action
scouring
in
some
locations;
storm
surge
flooding
and
damage
by
storm
waves
to
dunes,
structures
and
dwellings.
Flooding
may
be
exacerbated
by
contribution
to
the
flood
water
levels
from
adjacent
catchment
streams
and
rivers;
wind
effects,
which
may
be
direct
aeolian
processes
of
transporting
sand
onshore
and
inundating
structures,
or
indirect
effects
arising
from
loss
of
frontal
dune
sand
consequent
upon
landward
transgressive
dune
transport
depleting
the
reservoir
of
sand
in
the
frontal
dunes
the
natural
buffer
against
episodic
storm
wave
attack;
tsunami
overwash
and
inundation,
which
occur
in
parts
of
the
worlds
oceans,
particularly
the
Pacific,
and
may
be
expected
in
the
future;
and
human
modification
of
the
coast
inlet
mouth
jetty
or
sea
wall
construction,
for
example,
may
alter
the
natural
beach
erosion
processes
and
their
resulting
impacts
(Kraus
and
Mc
Dougal
1996),
by
exacerbating
and
accelerating
beach
lowering
and
dune
erosion
rate
upon
failure
of
a
sea
wall,
(as
occurred
at
Omaha,
New
Zealand,
during
the
July
1978
storm(Healy
1980));
or
vegetation
removal
may
enhance
the
loss
of
dune
sediment;
or
mining
of
beach
sand
may
induce
and
accelerate
dune
erosion
and
shoreline
retreat.
The
fundamental
determinants
for
sandy
shoreline
positional
stability
include
(i)
the
volume
of
sand
reservoir
in
the
frontal
dune,
(ii)
the
sediment
budget
(meaning
sand
volumes
delivered
to
and
from
the
beach)
and
(iii)
susceptibility
(meaning
exposure
and
frequency)
to
episodic
storms.
Where
the
frontal
dunes
are
low,
erosive
storm
waves
and
surge
washover
processes
may
easily
erode
the
relatively
small
volumes
of
sand
in
the
dune,
thereby
causing
rapid
retreat
of
the
duneline.
Such
sediment
loss
normally
occurs
by
the
mechanism
of
offshore
(diabathic)
transport
in
episodic
storms,
whence
it
may
then
be
carried
away
in
the
littoral
drift.
In
contrast,
dunes
of
higher
elevation
than
the
storm
surge
and
wave
run-up
elevations
will
not
be
subject
to
storm
wave
washover,
as
well
as
possessing
a
large
reservoir
of
sand,
and
accordingly
the
shoreline
remains
more
stable
positionally.
In
the
long
term
if
the
capability
of
the
waves
(or
human
activities)
to
remove
sediment
from
the
beach
system
exceeds
the
sediment
supplied
to
the
beach,
then
the
beach
and
duneline
will
undergo
retreat
Other
influences
on
shoreline
positional
stability
include
sea
level
rise
effects.
Long
term
erosion
from
sediment
budget
deficit
will
be
exacerbated
by
sea
level
rise
effects,
according
to
application
of
the
Bruun
Rule
(Komar
et
al.
1991,
Komar
1998).
This
well
796
COASTAL
DISASTERS
'02
known
effect
predicts
shoreline
retreat
for
a
given
rise
of
sea
level.
In
addition
research
by
Healy
(1987),
and
Stephens
et
al.
(1999)
has
shown
that
wave
energy
focusing
is
an
important
ancillary
process
which,
during
episodic
storms,
maximizes
the
erosive
processes
of
the
duneline
at
various
focus
zones
along
the
shoreline,
and
results
in
the
formation
of
arcuate
duneline
embayments
cut
within
the
duneline.
Storm
surge
flooding
is
another
hazard
experienced
along
many
duned
coasts.
This
hazard
typically
occurs
during
episodic
storms
and
hurricanes
when
storm
waves
wash
over
the
dunes
and
flood
the
low
lying
swales.
But
often
storm
waves
occur
during
meteorological
events
which
simultaneously
precipitate
intense
rainfall
onto
the
hinterland
catchments,
so
that
the
streams
draining
through
to
the
coast
contribute
and
exacerbate
the
storm
surge
flooding.
COASTAL
HAZARD
ZONES
AND
DEVELOPMENT
SETBACK
Usage
of
the
term
"coastal
hazard
zone"
(CHZ)
in
the
New
Zealand
literature
was
introduced
by
Gibb
(1981),
who
defined
the
CHZ
as
"an
adequate
width
of
land
between
any
development
and
the
beach".
By
implication
the
hazard
zone
was
defined
from
assessment
of
the
potential
for
identified
hazards
to
"damage
or
destroy
beachfront
property
and
assets"
landward
of
the
beach
(Gibb
and
Aburn
1986).
A
coastal
hazard
zone,
then,
may
be
defined
as:
a
sector
of
coastal
terrain
that
is
subject
to
hazards
from
the
marine
environment.
Mainly
the
hazards
become
manifest
as
storm
wave
erosion
of
the
frontal
dune,
storm
surge
and
flooding
of
coastal
lowland,
wind
erosion
of
the
dunes,
or
tsunami
wave
damage
and
washover
flooding.
Development
"setback"
as
a
concept,
is
subtly
more
general
than
a
CHZ
in
that
it
relates
to
providing
a
buffer
zone
between
the
beach
and
developments,
such
as
buildings
or
structures,
including
roads.
However
establishment
of
setback
may
include
considerations
other
than
coastal
hazard,
for
example,
preservation
of
'natural
character'
of
the
shoreline
environment
within
the
development
setback,
or
protection
of
sites
of
special,
ecological,
or
cultural
interest.
The
terms
"setback"
and
"coastal
hazard
zone"
are
often
used
more
or
less
synonymously
to
mean:
that
zone
measured
as
a
linear
distance
landwards
from
a
reference
feature,
usually
taken
as
the
toe
of
the
frontal
dune,
to
a
line
on
the
ground,
which
is
subject
to
hazards
from
the
coastal
marine
environment,
and
within
which,
on
the
balance
of
evidence
and
in
the
light
of
scientific
knowledge
of
the
moment,
it
would
be
prudent
to
restrict
development.
The
coastal
reference
feature
from
which
the
CHZ
is
measured
may
be
variously
identified
as
(i)
the
seaward
toe
of
the
frontal
dune
(Healy
1981;
National
Research
Council
1990);
(ii)
line
of
(dune)
vegetation;
(iii)
mean
high
water
mark;
or
(iv)
a
specified
elevation
contour.
Of
these,
the
seaward
toe
of
the
frontal
dune
is
the
most
easily
identifiable
in
the
field,
and
least
variable
through
time,
and
is
thus
the
preferred
reference
feature
from
which
to
measure
the
CHZ
(Healy
1981,
1993).
COASTAL
DISASTERS
'02
797
It
is
important
to
emphasize
that
having
established
the
CHZ
setback
line
there
is
still
no
guarantee
that
within
the
next
100
years
the
coast
in
question
could
not
be
pounded
by
a
series
of
destructive
storms,
or
some
other
processes
generating
a
long
term
loss
of
sediment
budget,
capable
of
removing
the
frontal
dune
and
sand
reservoir,
and
placing
the
development
in
peril.
Moreover,
it
must
be
realised
that
a
CHZ
line
does
not
of
itself
constitute
a
"magical"
safety
zone
immediately
on
one
side
of
it,
and
a
zone
of
"total
hazard"
or
"impending
destruction"
on
the
other.
Rather:
it
is
a
line
on
the
ground
beyond
which,
on
the
balance
of
evidence,
and
in
the
light
of
scientific
knowledge
of
the
moment,
it
would
be
prudent
to
restrict
development.
In
delineating
the
coastal
hazard
zone
it
is,
therefore,
better
to
err
in
the
direction
of
too
much
sand
in
the
coastal
setback
or
CHZ
than
too
little.
It
is
not
in
the
interests
either
of
the
people
who
in
good
faith
buy
into
such
developments,
or
of
ethical
developers
and
responsible
local
authorities,
for
the
developers
or
local
authorities
to
take
risks.
The
CHZ
is
therefore
the
area
in
which
property
is
likely
to
be
subject
to
coastal
hazards,
particularly
erosion
and/or
flooding
and/or
topographic
instability
in
the
reasonably
foreseeable
future,
and
within
which
some
planning
response
to
that
risk
should
be
made.
As
emphasized
by
Kaufman
and
Pilkey
(1983,
p.275)
"What
protects
a
building
is
not
its
distance
from
the
sea,
but
the
volume
and
mass
of
the
sand
between
it
and
the
sea",
while
Pilkey
et
al.
(1982)
state
that
"even setback
ordinances
-
which
require
that
structures
be
placed
a
minimum
distance
behind
the
dune
-
do
not
guarantee
long
term
protection.
Furthermore,
if
you
locate
on
a
primary
dune
you
should
expect
to
lose
your
home
during
the
next
major
storm".
In
a
similar
vein
Gibb
(1981,
p.47)
writes:
"Urban
development
should
be
strictly
controlled
in
the
coastal
hazard
zone
so
that
disastrous
consequences
are
avoided",
and
that
"Coastal
erosion
works
will,
in
time,
be
a
necessary
requirement
to
protect
the
assets
at
risk,
should
the
development
be
allowed
to
proceed
within
the
coastal
hazard
zone".
An
important
question
arises
as
to
what
constitutes
a
credible
coastal
hazard
risk?
i.e.
what
is
the
probability
of
occurrence
of
the
variously
identified
coastal
hazards
and
their
effect
on
the
coast?
In
this
case
one
may
identify
two
components
to
the
risk,
viz.,
the
trend
risk
imposed
by
the
long
term
trends,
and
the
rare
event
risk,
induced
by
the
extreme
rare
event,
storm
erosion
and/or
wave
flooding
or
tsunami.
In
this
respect
the
incredible
impact
of
rare
natural
hazard
events
of
low
probability
have
been
demonstrated
by
the
Mississippi
River
floods
of
the
early
1990s,
estimated
by
Dr
M.F.
Meyer
(pers.
comm.)
of
the
Natural
Hazards
Research
and
Applications
Center
of
the
University
of
Colorado,
as
demonstrating
flood
heights
of
between
a
1
in
100
and
1
in
500
year
event.
The
impact
of
those
floods
was,
of
course,
colossal.
The
coastal
zone
equivalent
might
be
envisaged
as
a
massive
dune
retreat
50-100
m
from
extraordinary
beach
and
dune
erosion
causing
MHWM
retreat
not
previously
experienced
in
recorded
history
along
a
coastline
otherwise
identified
as
"stable".
Such
a
scenario
along
New
Zealand's
Bay
of
Plenty
coast,
for
example,
could
result
from
a
series
of
severe
storms
over
a
prolonged period
of
time
in
a
prolonged
La
Nina
dominated
climatic
event.
There
is
evidence
that
the
Pacific
Decadal
Oscillation
has
recently
changed
to
a
La
Nina
phase,
so
that
the
Bay
of
Plenty
coast
may
expect
more
and
798
COASTAL
DISASTERS
'02
enhanced
stormy
erosive
conditions
over
the
next
25-35
years
or
so
and
this
would
be
expected
to
facilitate
major
shoreline
retreat
into
developed
areas.
Neither
can
the
potential
effect
of
a
large
tsunami
impacting
upon
certain
parts
of
the
coast
be
dismissed
(de
Lange
and
Healy
1999),
because
of
the
known
seismicity
and
crustal
fissures
across
various
continental
shelves.
Attention
is
drawn
to
the
Hokkaido-Nansei-Oki
earthquake
of
July
12,
1993,
which
occurred
in
a
'blank
spot'
outside
designated
seismic
danger
zones
where
even
seismologists
had
not
expected
it
to
hit.
In
general
it
is
difficult
to
assess
the
threat
of
tsunami
except
to
say
that
in
such
events
for
susceptible
coastlines
of
mainly
Pacific
coasts,
there
may
be
extensive
damage
to
property
and
potential
for
considerable
loss
of
life.
Clearly,
coastal
settlements
that
do
not
possess
massive
barriers
as
along
much
of
the
coast
of
Japan,
would
be
safer
with
a
wide
setback
and
a
restricted
development
zone.
In
summary,
for
open
duned
coasts
that
have
both
suffered
demonstrable
erosion
and
are
subject
to
both
trend
risk
as
well
as
rare
event
risk,
there
is
a
high
likelihood
of
continuing
retreat
and
substantial
damage
even
loss
of
life
from
future
low
probability
extreme
events.
Accordingly,
the
logic
of
the
National
Research
Council
(1990)
recommendation
is
clear
to
perceive:
where
it
is
possible
to
make
an
appropriately
conservative
coastal
hazard
zonation,
this
should
be
implemented.
KEY
ELEMENTS
OF
THE
METHODOLOGY
FOR
DETERMINATION
OF
COASTAL
HAZARD
ZONE
The
CHZ
implementation
is
based
upon:
application
of
CHZ
delineation
to
a
representative
profile
from
the
frontal
dune
toe
inland
across
the
dunes;
a
time
horizon
in
the
planning
context
taken
as
100
years
(on
the
basis
that
virtually
all
capital
developments
in
the
coastal
zone
remain
in
place,
or
are
replaced
and
rebuilt
over
that
length
of
time);
assessment
of
4
independent
parameters
of
:
R
"long
term"
erosion
(or
accretion)
rate.
In
the
New
Zealand
context
this
would
be
of
order
>50
years;
S
the
maximum
decadal
'cut
and
fill'
fluctuation
of
the
frontal
dune,
(or
vegetation
line)
i.e.
within
the
last
50
years;
X
the
sea
level
rise
effect,
for
example
as
predicted
by
application
of
the
Bruun
Rule
(Komar
et
al.
1991).
This
requires
determination
of
local
"closure
depth"
and
assumes
a
representative
beach
profile
and
nearshore
topography
out
to
the
depth
at
which
sediment
exchanges
regularly
with
the
beach;
and
D
dune
topographic
stability
factor,
which
allows
for
the
natural
stability
angle
of
dried
dune
sand,
which
is
typically
close
to
30°
so
that
a
dune
height
of
say
5
m
requires
a
ground
distance
of
—10
m
to
allow
for
stability
of
the
dune
face.
The
CHZ
is
then
given
as
the
summation:
CHZ
=
R
+
nS..
+
X
+
D
(1)
where
n
is
a
safety
factor.
COASTAL
DISASTERS
'02
799
As
each
parameter
is
independent
of
the
others
and
orthogonal
in
the
statistical
sense,
total
CHZ
setback
is
determined
as
the
summation
of
the
4
parameters.
The
safety
factor
"n"
in
the
New
Zealand
context
has
initially
been
evaluated
as
n
=
2
based
upon
maximum
likelihood
statistical
estimators
of
some
early
profile
data
of
storm
cut
from
Bowentown
Beach,
Bay
of
Plenty
(N.Z.).
However
that
site
is
not
necessarily
representative
of
all
coastal
beaches
or
beach
environments,
and
as
more
detailed
beach
profile
data
bases
become
available
the
appropriate
value
for
the
n
factor
will
become
better
established.
In
the
meantime,
however,
n
=
2
is
taken
as
the
best
estimate.
Data
required
to
evaluate
the
4
independent
factors
for
the
CHZ
includes
historical
survey
maps
and
air
photo
data
recording
the
position
of
the
toe
of
the
frontal
dune,
or
location
of
the
MHWM
if
it
is
known.
Regional
relative
sea
level
rise
data
is
required
to
apply
the
Bruun
Rule
for
shoreline
erosion
based
upon
mean
sea
level
rise,
or
failing
that
it
is
standard
practice
to
take
the
latest
available
Intergovernmental
Panel
on
Climatic
Change
(IPCC'01)
estimates
of
global
mean
sea
level
change
for
the
next
100
years.
A
knowledge
of
the
local
"closure
depth"
(i.e.
the
depth
out
from
the
beach
to
the
limit
of
regular
detectable
profile
change,
which
is
regarded
as
one
indicator
of
the
depth
of
regular
sand
exchange
between
the
nearshore
and
the
beach
(Kraus
1992)
is
required.
For
evaluating
D,
the
measured
angle
of
the
dune
face
is
needed,
otherwise
one
may
assume
an
approximate stability
angle
of
30°
for
dune
sands
which
classify
texturally
as
"fine-
medium"
sand.
The
initial
estimation
of
the
CHZ
setback
from
the toe
of
the
frontal
dune
is
then
subjected
to
3
tests
relating
to:
(i)
retaining
a
reservoir
of
sand
in
the
dunes
after
the
worst
expected
storm
cut
in
the
next
100
years,
expressed
as:
CHZ
>
+
400)
/E
(2)
where
4V.,,
is
the
maximum
storm
cut
volume
in
units
of
m
3
per
m
of
beach,
and
E
is
the
average
dune
elevation,
and
the
400
in'
per
m
of
beach
is
the
Dutch
recommended
frontal
dune
sand
reservoir
(van
de
Graaff
1986).
(ii)
effects
of
storm
surge
and
flooding,
which
is
a
summation
of
the
parameters
of
Figure
1.
Each
parameter
is
able
to
be
quantified
(see
Healy
and
Dean
2000):
highest
predicted
astronomical
tide
(above
MSL
datum)
barometric
set-up
(1
hPa
is
equivalent
to
1
cm
rise
in
mean
sea
level)
onshore
wind
stress
set-up
wave
set-up
wave
run-up.
The
calculation
of
these
components
is
based
upon
the
concept
of
a
"design
storm"
wave
effect,
and
calibrated
against
observed
storm
wave
run-up,
surge
and
flooding
events.
(iii)
tsunami
run-up
and
washover.
For
the
New
Zealand
North
Island
east
coast,
tsunamis
have
been
experienced
in
the
Bay
of
Plenty
and
Hauraki
Gulf
in
historical
times,
but
appear
not
to
have
exceeded
3
m
in
wave
run-up
except
for
the
Chilean
tsunami
of
1960,
which
had
an
amplitude
exceeding
7
m
at
Whitianga.
However,
for
many
open
duned
coasts
there
is
little
meaningful
historical
data
relating
to
tsunami
impacts,
and
accordingly,
this
test
may
be
difficult
to
quantify,
or
not
be
relevant.
800
COASTAL
DISASTERS
'02
Ancilliary
data
is
required
for
assessment
of
storm
surge
and
tsunami
run-up.
These
include:
-
design
storm
wave
height
and
period
-
design
storm
wind
speed
and
isobaric
pressure
-
offshore
sediment
mean
grain
size
-
estimate
of
maximum
1
in
a
100
year
storm
erosion
"cut"
of
the
frontal
dunes
-
average
frontal
dune
elevation
relative
to
MSL
datum
-
beach
slope,
13,
from
the
seaward
limit
of
the
offshore
bar
(i.e.
the
break
of
slope
at
the
bottom
of
the
outer
slope
of
the
offshore
bar)
to
the
dune.
Typically
for
the
Bay
of
Plenty
coast
13
is
close
to
0.01.
-
width
and
depth
of
shelf
edge,
or
depth
at
distant
shelf
fetch
limit.
-
beach
surveys
of
maximum
storm
cut
in
recent
decades.
OUTPUT:
Results
of
the
analysis
are
applied
to
the
dune-beach
profile
and
result
in
a
setback
distance
measured
from
the toe
of
the
frontal
dune,
or
the
vegetation
line.
In
earlier
(pre
GIS)
times
the
method
was
applied
to
a
specific
site
and
extrapolated
alongshore
as
far
as
the
beach-dune
cross
profile
was
representative
of
that
sector
of
beach
geomorphology
(Healy
1989;
Healy
1993).
With
modern
GIS
systems
it
is
possible
to
program
a
computer
to
calculate
a
line
on
the
ground
for
specific
beach
topographies
given
the
input
parameters.
SUPPLEMENTARY
ISSUES
As
noted
in
the
discussion
above,
the
CHZ
is
based
upon
4
parameters,
which
are
considered
as
orthogonal
or
independent.
This
assumption
is
arguable
if
one
considers
that
an
historical
erosion
rate
based
upon
the
last
100
years
presumably
has
incorporated
within
it
an
effect
for
the
sea
level
rise
of
the
past
100
years,
which
globally
has
been
of
order
20
cm.
This
is
in
addition
to
any
local
tectonic
uplift
or
downsinking
effect
of
the
land.
Thus
one
could
argue
that
there
is
a
partial
double
assessment
for
the
effect
of
sea
level
rise
within
the
methodology.
As
demonstrated
in
Healy
and
Dean
(2000)
one
can
take
out
the
effect
of
that
sea
level
rise.
However
Hicks
(1990)
notes
that
most
tectonic
influences
occur
as
sudden
tectonic
movements
during
earthquakes
and
therefore
it
is
meaningless
to
attempt
to
extract
an
average
tectonic
effect.
Recollect
that
the
determined
setback
is
based
upon
coastal
hazard
relating
to
the
scientific
and
survey
knowledge
of
the
time.
However,
scientific
knowledge
advances.
For
example
the
best
estimate
of
long
term
global
sea
level
rise,
assuming
the
"Business
as
Usual"
scenario
from
IPCC'90
was
0.67
m
rise
in
the
next
100
years.
However
the
IPCC
(1995)
"second
assessment"
was
for
0.5
m
rise,
and
the
most
recent
IPCC
mean
range
figure
is
for
0.49
m
sea
level
rise
during
the
next
100
years.
In
the
same
vein,
in
future
years
one
can
expect
the
values
of
n
(the
safety
factor)
and
S..
(maximum
storm
cut)
to
be
refined
as
more
research
and
longer
term
beach
and
shoreline
survey
data
become
available.
Establishment
of
the
delineated
CHZ
setback
line
on
a
map
should
be
regarded
as
"indicative"
and
not
as
absolute.
For
example
the
location
of
the
toe
of
the
frontal
dune
may
be
assessable
only
to
within
1-2
m
on
the
ground.
Thus
it
is
appropriate
to
"right
line"
the
CHZ
limits
within
a
few
meters
against
prevailing
property
boundaries
so
that
it
does
not
bisect
existing
houses.
This
allows
recognition
that
there
are
significant
uncertainties
and
COASTAL
DISASTERS
'02
801
the
methodology
requires
'experience
judgments'
in
its
application,
but
nevertheless
the
methodology
applies
the
latest
scientific
knowledge
of
the
moment.
Once
a
CHZ
setback
is
established
there
is
little
point
in
spending
vast
effort
to
reduce
a
CHZ
of
say
70
m
by
2.5
m;
over-emphasis
on
attempting
to
refine
the
CHZ
is
counterproductive,
and
it
is
preferable
to
be
conservative
in
philosophy
and
designate
an
appropriately
conservative
setback
zone.
ADDITIONAL
FUNCTIONS
OF
THE
CHZ
SETBACK:
IMPLICATIONS
FOR
SUSTAINABLE
MANAGEMENT
AND
NATURAL
CHARACTER
Although
the
CHZ
setback
is
determined
on
the
basis
of
actual
past
and
potential
future
coastal
hazard
impacts
for
a
planning
horizon
taken
as
encompassing
the
next
100
years,
one
cannot
deny
that
the
land
comprising
the
CHZ
may
have
functions
other
than
coastal
subdivision
and
development
as
argued
by
Healy
(1997).
An
important
consideration
of
contemporary
coastal
management
strategy
is
that
cognizance
should
be
paid
to
concepts
of
"sustainable
management",
safeguarding
ecosystems
and
avoiding,
mitigating
or
remedying
any
adverse
effects
on
the
environment.
Such
ideas
are
manifest
in
sections
5,
6
and
7
of
New
Zealand's
Resource
Management
Act
1991,
and
in
chapter
18
of
Agenda
21
of
the
1992
Rio
Earth
Summit
Conference.
More
recently
the
"European
Code
of
Conduct
for
Coastal
Zones",
officially
adopted
by
the
Council
of
Europe,
represents
an
effort
to
put
the
principles
of
sustainable
management
of
the
coastal
zone
into
practice.
While
a
coastal
hazard
zone
is,
as
the
name
suggests,
based
on
the
perceived
hazard,
a
development
setback
is
able
to
incorporate
both
the
concept
of
coastal
hazard
as
well
as
other
functions,
including
preservation
of
the
natural
character
of
the
coastal
environment,
outstanding
natural
features
and
the
protection
of
specialized
habitats
of
high
ecological
value.
A
suitably
broad
CHZ
can
also
provide
passive
recreational
space
on
the
dune
behind
the
beach.
To
date,
although
the
concept
of
the
'preservation
of
the
natural
character
of
the
coastline'
has
been
a
supporting
argument
for
determining
development
setback
(see
Healy
1980,
1981),
there
appear
not
to
be
specific
guidelines
for
the
quantitative
delineation
of
setback
relating
to
protection
of
outstanding
features
or
preservation
of
the
natural
character
in
either
state,
regional
or
territorial
local
authority
policy
statements
or
in
planning
schemes.
However,
a
development
setback
derived
from
the
methodology
advocated
in
this
paper
will
at
least
assist
in
preserving
the
natural
character
of
the
coast.
In
Healy
(1997)
it
is
argued
that
the
coastal
landscape
of
numerous
New
Zealand
beaches
and
dunes
constitute
"outstanding
natural
features"
and
that
greater
recognition
needs
to
be
accorded
landscape
architecture
criteria
in
the
planning
process.
The
rationale
behind
this
is
that
people
using
the
beach
for
amenity
purposes
should
not
be
imposed
upon
by
dwellings
overlooking
the
beach.
For
subdivisions
typical
of
many
"New
World"
coasts
to
date,
-
i.e.
single
family
houses
-
efforts
should
be
implemented
to
ensure
that
the
frontal
dunes
are
built
up
to
enhance
their
role
as
a
reservoir
of
sand
and
defence
against
coastal
erosion
and
dune
blowout
hazard,
as
well
as
enhance
their
dune
ecological
and
landscape
values,
and
thereby
maintaining
the
intrinsic
natural
character
of
the
coastline.
802
COASTAL
DISASTERS
'02
ECONOMIC
RATIONALE
FOR
MAINTAINING
WIDE
SETBACKS
Apart
from
the
obvious
reason
of
sensibly
avoiding
identifiable
coastal
hazard
at
the
design
stage,
there
is
good
long
term
economic
rationale
for
adhering
to
a
policy
of
wider
rather
than
narrower
setbacks
and
CHZs.
These
reasons
are
both
of
indirect
national
and
direct
local
significance.
For
example,
New
Zealand
is
carefully
projecting
and
nurturing
a
"clean
green"
and
attractive
landscape
image,
to
attract
tourists
from
the
world
metropolis.
Ultimately
subdivision
of
coastal
landscape
is
like
extractive
industries
such
as
mining:-
in
the
long
term
it
is
not
sustainable.
There
is
significant
infrastructure
cost
to
ribbon
and
'incremental'
development
of
the
coast.
The
solution
for
the
open
coast
context
such
as
New
Zealand's
Bay
of
Plenty,
is
to
enhance
the
frontal
dunes
and
their
landscape
values,
and
protect
a
resource
that
will
contribute
to
future
economic
sustainability
from
tourists
seeking
experience
of
a
"clean
green
coastal
environment".
Establishment
and
maintenance
of
a
suitable
coastal
hazard
zone
will
assist
this
goal.
Of
direct
local
economic
significance
is
the
point
that
implementation
of
a
suitably
wide
CHZ
will
minimize
the
risk
for
both
the
individual
landowners
and
the
local
community
being
required
to
fund
expensive
structures
to
prevent
shoreline
retreat
to
protect
the
coastal
properties.
A
wide
CHZ
and
development
setback
enhances
coastal
amenity
value
and
therefore
the
attraction
to
visitors
and
tourism.
For
the
case
of
already
developed
coastal
subdivisions,
restriction
on
development
in
the
identified
CHZ
will
minimize
future
local
authority
costs
to
protect
that
development
which
would
otherwise
require
expensive
remedial
actions
such
as
appropriately
constructed
sea
walls,
offshore
islands
and
reefs, and/or
artificial
beach
renourishments.
HAZARD
ZONE
IMPLEMENTATION
FOR
EXISTING
SUBDIVISIONS
The
issue
of
implementing
a
CHZ
upon
existing
subdivision
and
development
is
a
complex
and
emotive
one.
The
essential
question
to
be
addressed
is:
Does
the
density
of
settlement
the
buildings
and
infrastructure
(roadways
etc)
-
create
a
situation
of
less
coastal
hazard
than
if
one
was
implementing
a
coastal
hazard
zone
for
a
"greenfields"
situation
prior
to
coastal
subdivision?
The
answer
to
this,
clearly,
is
that
structures
on
dunes
will
not
materially
affect
the
coastal
hazard
processes.
For
example,
in
the
situation
of
a
reducing
beach
sediment
budget,
beach
erosion
and
duneline
retreat
will
occur
(exclusive
of
human
intervention)
and
the
erosion
effect
will
typically
become
manifest
in
major
storms.
Should
long
term
erosion
continue,
then
without
human
intervention
the
various
dwellings
would
become
undercut
and
collapse
into
the
sea.
For
a
short
time
and
without
human
intervention,
such
dwellings
would
locally
hinder
continued
dune
retreat,
but
eventually
the
sea
would
win
gnd
transport
away
the
debris
and
continue
the
dune
erosion
and
shoreline
retreat.
Likewise,
the
extent
of
storm
surge
flooding
and
damage
to
property
and
infrastructure
will
occur
regardless
of
the
dwellings
on
the
dunes
and
those
developments
below
the
storm
surge
flood
level
will
be
impacted.
Dune
'blowouts'
and
transgressive
sand
transport
from
the
frontal
dune
will
occur
regardless
of
what
is
on
the
dunes,
although
the
infrastructure
COASTAL
DISASTERS
'02
803
of
sealed
roadways
(along
with
likely
human
intervention)
will
likely
reduce
the
extent
of
blowout
and
landwards
sand
transgression.
And
tsunami
run-up
and
washover
will
occur
regardless
of
dwellings
on
the
dunes.
For
the
particular
case
of
Waihi
Beach,
Bay
of
Plenty,
human
interventions
have
contributed
to
the
hazard.
The
research
by
Steele
(1995)
showed
that
the
concave
deflated
beach
profile
in
the
beach
fronting
Shaw
Road
was
associated
with
marked
porewater
pressure,
most
likely
due
to
the
groundwater
recharge
from
the
Opawe
swamp,
and
that
this
recharge
has
been
exacerbated
by
human
actions
of
drainage
and
roadway
constructions,
and
increased
collection
of
surface
flows
into
the
Opawe
swamp.
The
human
response
action
of
constructing
and
maintaining
the
Shaw
Road
sea
wall
has
further
increased
the
hazard
because
of
the
well
known
effects
of
enhancing
scour
at
the
base
of
the
sea
wall
due
to
higher
wave
reflections
during
storms,
increasing
the
risk
to
houses
behind
the
seawall
should
it
collapse.
In
addition,
the
effect
of
diverting
Two
and
Three
Mile
creeks
to
the
sea
prior
to
1929
they
flowed
behind
the
frontal
dunes
parallel
to
the
shore
to
enter
the
Waiau
stream
and
discharge
into
Tauranga Harbour
has
exacerbated
erosion
of
the
frontal
dunes
within
100-200
m
from
the
stream
mouth
and
increased
coastal
hazard
in
those
areas.
Thus
as
the
identified
coastal
hazards
continue
to
exist
irrespective
of
the
developments.
The
sensible
and
rational
coastal
management
option
is
to
implement
a
coastal
hazard
zone
over
the
affected
existing
subdivided
areas
(as
well
as
over
areas
not
yet
subdivided),
with
strict
controls
to
ensure
that
the
objectives
of
the
coastal
hazard
zone
are
not
prejudiced,
and
the
territorial
local
authority
and
community
will
not
ultimately
be
put
to
great
cost
to
protect
against
the
hazards
in
the
future.
In
principle,
then,
one
applies
a
CHZ
to
an
existing
subdivision
as
if
there
was
no
subdivision
there
because
the
identified
hazards
of
erosion
and
storm
surge
flooding
etc
are
likely
to
occur
within
the
"planning
horizon"
of
100
years.
In
such
a
case
it
is
appropriate
to
"right-line"
the
hazard
zone
line
so
that
it
does
not
pass
through
houses
directly,
which
was
applied
for
the
1993
hazard
zone
delineation
for
Waihi
Beach
(Healy
1993).
Case
of
a
"Protected
Coast"
The
one
exception
when
the
above
philosophy
could
be
changed
and
a
much
reduced
coastal
hazard
width
applied
is
when
a
territorial
local
authority
undertakes
a
long
term
commitment
to
"protect"
a
section
of
coast
from
the
coastal
hazards,
i.e.
to
"engineer"
their
way
out
of
the
problem.
The
appropriate
methods
for
undertaking
this
may
be
varied
but
typically
include
such
options
as
constructing
and
maintaining
in
the
long
term
a
sea
wall.
Such
an
approach
often
needs
to
be
concurrent
with
a
"soft
engineering"
approach
of
artificial
beach
re-nourishment.
Other
modern
ideas
include
emplacement
of
offshore
islands
or
artificial
surfing
reefs,
as
has
been
implemented
at
the
Queensland
Gold
Coast.
For
the
case
of
an
appropriately
designed
and
maintained
sea
wall,
the
hazard
zone
line
could
reasonably
be
reduced
to
immediately
behind
the
sea
wall.
The
wall
would
need
to
be
of
sufficient
height
to
stop
both
beach
erosion
and
storm
wave
overwash,
and
also
allow
for
future
projected
sea
level
rise.
The
"protection"
of
a
coastal
sector
raises
many
difficult
issues
such
as
high
capital
cost,
apportionment
of
costs,
ongoing
maintenance,
and
the
likely
negative
impact
on
many
of
804
COASTAL
DISASTERS
'02
the
relevant
objectives
of
various
'sustainable
development'
policies.
Unless
there
are
appropriate
protection
structures
in
place
and
a
clear
commitment
to
maintain
them,
there
is
no
justification
to
rely
on
those
structures
to
reduce
what
would
otherwise
be
a
calculated
CHZ.
THE
ISSUE
OF
SEQUENTIAL
ZONES
OF
REDUCING
HAZARD
A
contentious
issue
is
the
establishment
of
a
sequence
of
zones
of
"degrees
of
hazard"
decreasing
inland,
for
example,
"an
extreme
erosion
risk
hazard
zone",
followed
by
a
"less
extreme
risk
zone",
and
so
on.
There
are
several
problems
with
this
approach
including:
(i)
the
boundary
of
the
"extreme"
hazard
zone
becomes
the
de
facto
boundary
of
the
hazard
zone
within
which
people
will
want
to
develop;
(ii)
there
is
undue
emphasis
on
erosion
hazard
compared
to
other
hazards
and
wider
coastal
management
issues
such
as
protection
of
landscape
and
coastal
environment
values;
(iii)
application
of
such
a
concept
does
not
allow
appropriate
recognition
of
the
need
to
safeguard
the
landscape
values
and
life
supporting
capacity
of
the
dune
ecosystems
(referred
to
in
Agenda
21
and
ICZM
principles)
(iv)
a
narrow
'extreme
hazard
risk'
zone
is
of
insufficient
width
to
allow
preservation
of
the
natural
coastal
environment,
or
maintenance
and
enhancement
of
amenity
values,
maintain
the
intrinsic
values
of
ecosystems,
and
maintain
and
enhance
the
quality
of
the
coastal
environment,
as
required
under
New
Zealand's
Resource
Management
Act
1991.
(v)
Neither
does
a
narrow
extreme
erosion
risk
hazard
zone
allow
for
spatial
re-location
over
time;
and
(vi)
it
is
difficult
to
determine
-
and
impossible
to
predict
-
the
maximum
extent
of
erosion
and
dune
retreat
in
an
extreme
event.
Proponents
of
a
multi-hazard
zone
concept
might
argue
that
in
an
eroding
situation
one
cannot
deny
that
a
house
in
a
seaward
location
will
fall
into
the
sea
before
a
house
further
landward.
However,
typically
beach
and
dune
erosion
events
in
which
houses
are
claimed
by
the
sea,
occur
in
episodic
extreme
storm
events,
and
it
is
not
possible
to
predict
how
much
dune
erosion
will
occur
for
any
given
storm.
An
extreme
storm,
especially,
if
it
follows
a
series
of
storms
leaving
the
beach
in
a
pre-existing
negative
sand
budget
depleted
condition,
will
cut
back
much
wider
than
a
typical
"extreme
risk"
erosion
zone.
Note,
for
example,
(i)
the
loss
of
houses
from
erosion
of
Ohiwa
Spit
(N.Z.)
in
May
1976
during
an
episodic
storm
event.
The
beach
width
in
the
early
1970s
had
previously
exceeded
200
m
at
that
point;
and
(ii)
the
effect
of
the
July
1978
storm
at
Omaha
beach
in
Northland
(N.Z.)
which
destroyed
the
sea
wall
and
rapidly
eroded
40
m
behind
the
sea
wall
during
that
event
(Healy
1980).
Fortunately
at
that
time,
in
the
early
days
of
the
subdivision,
there
was
only
one
house
affected,
and
that
was
manually
removed
to
safety
at
the
height
of
the
storm.
Should
the
same
event
occur
today
several
rows
of
houses
would
be
affected.
In
general,
should
an
"extreme
erosion
zone"
be
implemented
and
long
term
retreat
occur,
the
nearby
landward
dwelling
in
the
"moderate
risk
erosion
zone"
subsequently
becomes
part
of
the
new
"extreme
erosion
hazard
zone",
and
in
the
meantime
certain
developments
may
have
occurred
which
are
inappropriate
for
an
extreme
erosion
hazard
zone,
for
example,
development
of
high
rise
blocks,
which
as
a
result
of
such
capital
investment
then
require
the
local
authority
to
protect
them
by
major
coastal
protection
works.
COASTAL
DISASTERS
'02
805
There
is
considerable
danger
in
relying
on
implementing
a
narrow
extreme
erosion
risk
CHZ
from
assessing
just
one
factor,
that
of
S..,
because
(a)
this
becomes
the
"de
facto"
CHZ;
(b)
the
error
associated
with
determination
of
that
one
factor
is
relatively
large
and
therefore
the
risk
to
adjacent
property
is
large,
relative
to
combining
4
factors
in
a
single
zone.
Thus
the
margin
of
error
is
larger
and
the
margin
of
safety
is
reduced.
In
summary,
it
is
preferable
to
establish
one
CHZ
setback
based
upon
the
summation
of
the
4
independent
parameters.
There
is
then
only
one
line
on
the
planning
map
which
incorporates
analysis
for
all
known
coastal
hazards
on
the
given
duned
coast,
and
the
territorial
local
authority
and
community
will
not
be
expected
to
face
liability
for
expensive
ongoing
coastal
protection
structures
and
beach
replenishment.
CONCLUSIONS
(i)
The
terms
coastal
hazard
zone
[CHZ]
and
development
setback
are
defined
as
"that
zone
measured
landwards
from
the
toe
of
the
frontal
dune
to
a
line
on
the
ground
which,
on
the
balance
of
evidence
and
in
the
light
of
scientific
knowledge
of
the
moment,
it
would
be
prudent
to
restrict
development".
(ii)
The
methodology
for
CHZ
determination
outlined
here
is
based
upon
summation
of
4
essentially
independent
parameters
of
long
term
retreat
R
(or
progradation),
short
term
dune
cut-and-fill
(S,,.),
a
sea
level
rise
effect
(X),
and
a
dune
topographic
factor
(D).
These
parameters
are
calculated
for
a
representative
shore-normal
profile
across
the
dunes.
(iii)
Following
calculation
of
setback,
the
initial
CHZ
is
tested
to
ensure
that
(a)
there
is
sufficient
reservoir
of
sand
in
the
setback
distance
-
if
not
then
the
CHZ
setback
needs
to
be
extended
landwards;
and
(b)
the
designated
CHZ
setback
is
of
sufficient
elevation
to
allow
for
design
storm
surge
and
tsunami
flooding
if
not
the
CHZ
needs
to
be
extended
landwards
to
achieve
the
design
elevations
which
avoid
storm
surge
and
tsunami
flooding.
(iv)
For
the
case
of
determining
and
applying
a
CHZ
setback
to
already
existing
subdivisions,
the
setback
should
be
applied
as
if
the
development
does
not
yet
exist,
because
the
structures
on
the
dunes
will
not
materially
affect
the
hazard
arising
from
the
coastal
processes.
Restrictions
to
development
within
the
identified
CHZ
should
ensure
that
the
objectives
of
the
coastal
hazard
zone
are
not
prejudiced,
and
the
territorial
local
authority
and
community
will
not
ultimately
be
put
to
great
cost
to
mitigate
and
protect
against
hazards
in
the future.
(v)
Where
the
territorial
local
authority
commits
to
maintaining
a
suitable
coastal
protection
program,
which
may
include
a
sea
wall
above
the
storm
surge
and
tsunami
run-
up
levels,
and
the
amenity
value
of
the
beach
is
maintained
by
artificial
beach
renourishment,
perhaps
in
conjunction
with
structures
such
as
artifical
nearshore
reefs,
then
the
CHZ
setback
can
be
reduced.
However
such
an
approach
may
be
at
variance
with
sustainable
management
provisos
of
chapter
18
of
Agenda
21,
nor
does
it
accord
with
the
"Code
of
Conduct"
for
coastal
developments
recently
adopted
by
the
European
Council
of
Ministers.
806
COASTAL
DISASTERS
'02
(vi)
The
concept
of
CHZ
setback
should
concomitantly
incorporate
factors
promoting
the
preservation
of
the
natural
character
of
the
coastal
environment,
and
a
conservative
setback
is
generally
consistent
with
that
objective.
(vii)
The
implementation
of
a
CHZ
has
economic
implications
for
the
coastal
management
authority
as
it
aims
for
minimum
future
expense
to
the
community
for
coastal
hazard
mitigation
measures,
as
well
as
enhances
the
beaches
for
amenity
value,
and
is
likely
therefore
to
assist
the
local
tourism
industry.
(viii)
The
CHZ
methodology
presented
in
this
paper
is
robust
and
preferable
for
implementation
as
a
single
delineated
hazard
zone.
It
is
considered
that
this
methodology
is
superior
to
the
alternative
of
attempting
to
delineate
a
series
of
sequentially
less
hazardous
zones
for
each
potential
coastal
hazard,
because
we
do
not
have
the
scientific
ability
to
be
that
precise
in
predicting
coastal
hazards
over
narrow
slices
of
land.
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