The so-called ‘100-year’ (or 1% Annual Exceedence
Probability) flood has two main uses in the public domain in
Australia. Both are problematic, though not intentionally so
on the part of those whose professional activities are bound
by its use.
At one level its use in the
media tends to mislead people about flood frequency by
seeming to minimise the likelihood of a near-term repetition
of a significant flood, and at another it provides a
less-than-optimal basis for land use decision making in
areas that are liable to flooding. I dealt with the first of
these points in an earlier post on Pearls and Irritations;
now I examine the second.
New South Wales (and indeed
all the other Australian states) operates to a rule as
regards the construction of new dwellings in areas that are
prone to floods. Floors must be above the height estimated
to be reached by the 1% AEP flood, plus an allowance (50
centimetres in NSW) for freeboard. The freeboard element was
introduced to cover uncertainties in the estimation of the
height likely to be reached by the 1% flood.
The rule
In NSW came into being in 2007. Before that, a ‘merits
approach’ applied in which decisions about the level below
which floors should not be built were made using judgements
about individual cases. These took into account social and
economic considerations and the risk to life which is
associated, amongst other things, with the hazard (the
product of velocity and depth of flood flows, which can
differ greatly between locations). Earlier again, going back
to the 1970s, the highest known flood in a locality was
‘deemed’ to be the 1% AEP flood and permissible floor
levels were tied to this height.
In effect, the 2007
decision took the state back to its original height-related
regulation, but with the addition of the element of
freeboard which in part recognised that calculations were
based on estimations from data series which were quite
short. With our limited history of reliable flood records
(less than a century in many areas), not to mention the
complexifying factor of climate change, it is not possible
to be certain about the accuracy of estimation of the 1% AEP
event.
There are various things wrong with the current
rule, but the principal one is that it takes no account of
risk. It is a purely statistical standard and the degree of
risk is not the same in the 1% flood at different locations.
The rule gives the illusion of an equality of both standard
and risk between locations.
This can be demonstrated
by taking the difference between the environments of the
west of NSW, where the ‘boundless plains’ provide
enormous storage for floods and restrict the heights which
they can reach, and the environments of the eastward-flowing
rivers with their much narrower and more restricted
floodplains.
In places like Bourke, Nyngan and Forbes
on the western plains, the difference between the assessed
1% flood level and the level likely to be reached by the
highest flood possible (the Probable Maximum Flood) is small
perhaps of the order of a metre or a little more. In eastern
NSW the difference between the 1% event and the estimated
PMF is invariably much greater: at Windsor, on the
Hawkesbury, it is about nine metres. This is because the
topographical constriction (‘choke’) of the Sackville
gorge downstream of Windsor impedes drainage, allowing
floods to reach great heights when inflows from tributaries
entering upstream exceed the outflows through the gorge.
Other river valleys in the state also have such
constrictions, though they are less dramatically
‘effective’ in limiting drainage than in the Hawkesbury
case: two instances are the Georges River, which drains much
of south-western Sydney, and the Bega River in the state’s
far south.
Even where there is no choke, as at
Lismore, floods can rise far above the level of the
estimated 1% event: Lismore’s PMF is of the order of four
metres higher than would be reached in the 1% flood and at
least a metre and a half above the level recorded in the
extremely severe flood of earlier this year. At Maitland, on
the Hunter River (where the storage capacity of the
floodplain is greater than at Lismore), the assessed PMF is
nearly three metres higher than the 1% level.
It
follows from all this that houses whose floors are, say, a
metre above the 1% AEP level in the western towns mentioned
could be flood-free even in an extreme event, whereas the
same is not true in the towns and suburbs on the rivers that
flow to the Tasman Sea. The consequence is that large-scale
housing development in a valley such as that of the
Hawkesbury will put many houses in danger of inundation in
floods larger than the 1% event. Indeed many thousands of
houses, developed over recent decades, are at such a risk in
the Hawkesbury and the number is increasing rapidly. This
is, after all, edge-of-metropolis territory where the
pressure to develop to accommodate Sydney’s growing
population is intense.
What should we do about this?
Firstly, we should recognise the real difference between the
‘western’ and ‘eastern’ flood regimes, and indeed
the differences between river valleys generally as regards
individual locations in terms of the potential reach and
hazardousness of floods in extreme events. Recognising this
would lead us to implement a different (indeed higher and
more restrictive) standard for house floors for each
location. The 1% standard would be seen as inappropriate and
we would revert to something like the ‘merits approach’
of former times, consistent with society’s adaptation to a
risk-based approach to the habitation of our
floodplains.
Such a change would probably be resisted
by technical services staff on councils. Understandably,
they prefer the ‘cookbook’ approach which is based on a
reasonably easily-calculated 1% AEP level. Applying a simple
statistical standard is easier than having to make a complex
judgement. Reverting to the merits approach would not be
without its difficulties, therefore, and new guidelines
would be needed for council planning staff to operate from.
Much assistance would be required from the state government
through its Department of Planning, Industry and Environment
as the lead organisation for floodplain management in the
state.
The current approach is simply untenable.
Without doubt it is leading to very large numbers of people
being placed in positions of great risk come rare (but
nevertheless inevitable) floods reaching higher levels in
some cases much higher levels than the 1% event would
produce. In all likelihood the number of houses being built
in NSW with floor levels between the 1% level and levels
well below those likely to be reached by PMFs is greater now
than it has ever been in the past.
Our regulatory
framework, in this instance, is not fit for purpose. It is
in fact a substantial contributor to the problem that is
being created on floodplains. The case of the
Hawkesbury-Nepean is not the only instance of this, it is
merely the most spectacular instance in the valleys of the
eastward-flowing rivers of the state. The equivalent problem
does not exist west of the Great Dividing Range where the
topography is different and the demand for new housing is
with few exceptions relatively small and there is less
developmental pressure on floodplains.
NSW, and indeed
all states, needs to re-think radically its approach to
setting minimum floor levels for new houses. The current
standard is logically flawed and not sufficiently
conservative given the serious outcomes it must at some
stage produce. Worse yet, climate change will almost
certainly make its deficiencies even more apparent than is
now the case as what is currently thought to be the 1% AEP
flood is experienced more frequently.
It is abundantly
clear that we need to restrict housing (and indeed
commercial and industrial development) on floodplains in
eastern NSW to a greater degree than we do at present. In
essence we need to move the ‘lines’ below which
development is not permitted to higher levels on
floodplains: we need to confine development to
flood-compatible land uses not only on the ‘lower’
floodplains as we do now but also on their middle
levels.
Ideally, one might argue, we should go even
further and prevent all development apart from agricultural
and recreational uses on floodplains. Politically, though,
applying a blanket restriction on development on the higher
parts of floodplains (which would be inundated in only the
most extreme flood events with recurrence intervals of
thousands of years), would be impossible. It would be seen
as unduly harsh on development interests and would have many
unintended adverse effects. Redevelopment to allow
rejuvenation of ageing housing stock, for example, would
have to be prohibited.
That point aside, there is a
horrible inevitability that awaits if changes are not made
to our regulations on floodplain development, and soon: a
genuinely big flood on almost any of our eastward-flowing
rivers will enter many new and relatively new houses. In the
Hawkesbury, many thousands of dwellings will be affected.
Happening there, the economic and other costs imposed upon
the community of the time, the insurance industry and
government will be massive. We need to act, not timidly but
decisively, to stop the problem escalating even further than
it already
has.
ENDS
Chas Keys is
a former academic and emergency management practitioner. He
was from 1997 to 2004 the Deputy Director General of the NSW
State Emergency Service, with responsibility for planning to
warn and evacuate people from approaching
floods.
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