Planning to avoid flood damage - pdf article

Water & Atmosphere 15(3) 2007
Weathering the Storm
Planning to avoid flood damage
Alistair McKerchar and Graeme Smart explain how
new models are refining the process of predicting
damaging floods.
To avoid flood damage we need to keep people and
property out of the path of floods. There are well known
strategies to do this:
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control flood flows using flood detention dams
protect vulnerable areas with engineering works
such as stopbanks or floodways
exclude property development from vulnerable
areas and
give warnings of pending floods (see ‘Flood forecasts
for New Zealand communities’, pp. 14–15).
These strategies all require that we know the size of the
flood that is to be controlled, protected against, avoided, or
forecast. This means we must estimate flood peak flow rates
and how often these floods can be expected.
Predicting flow
Hydrologists define the likelihood of flood peak flows by their
annual exceedence probability (AEP). A so-called 100-year
flood does not mean that there is exactly one flood of this size
every 100 years. It means that there is a 1 in 100 chance in any
given year that a flood of this size or bigger will happen; it is
therefore more correctly called a 1% AEP flood. In any given
year, there is about a 65% chance that there will be at least
one 1% AEP flood in populated catchments of New Zealand.
Where sufficiently long records of flood flows are available
for a river or stream, hydrologists can estimate the AEP of
different flood sizes by determining how often floods of a given
size occur. When records of previous floods are not available,
we use two alternative methods to estimate flood sizes. The
regional method looks at flood magnitudes for other similar
streams with longer records, and design estimates are scaled
to match the site in question. The second method, the rainfallrunoff
model, uses information about rainfall frequency over
a drainage basin (or catchment) to estimate a corresponding
peak flood flow. The model translates the rainfall information
into flood information.
Staying high and dry
• Floods are our most frequent and damaging hazard.
• Past records tell us how often to expect damaging
flood flows.
• Inundation modelling shows areas that are at risk and
need long-term protection from floods.
In our experience, regional methods that interpolate
flood frequency information are generally more reliable than
rainfall-runoff models. With funding from the Foundation for
Research, Science & Technology, NIWA scientists are revising
and updating flood estimates for all New Zealand regions.
Calculating the frequency, size, and duration of flood flows
in rivers is not sufficient to know whether particular areas will
be flooded or not. We still need to determine how far waters
may spread when floods spill out of their normal river course;
this is known as inundation.
Predicting inundation
Traditionally, the extent of flood inundation has been
estimated by recalling data on historic flood levels, or else
calculated by surveying floodplain sections and assuming that
these have horizontal water surfaces. These methods may
lead to considerable errors in predicted areas and depths of
inundation. While the prediction of peak flow and timing of
river floods is a relatively mature science, precise calculation
of inundated areas and local flood depths is a relatively new
undertaking.
Reliable inundation calculations require massive amounts
of very detailed topographic data and large computing
resources. Airborne surveys using LiDAR (laser radar) can now
provide maps of the ground surface that have a ground level
measurement for every square metre of terrain over areas of
tens of square kilometre. Because such datasets contain
enormous volumes of data (typically, a model uses 5 000 000
ground elevations) and the data are collected with little manual
intervention, identification of any errors in the data is difficult.
In addition, parts of the topography, including the river
bed, can change during the passage of a flood, due
to scour and deposition, and the resistance to flow
caused by vegetation along the river bank or sediment
movement increases or decreases as floodwaters rise
and fall.
Photo: Otago Regional Council
The 1995 flood in Alexandra
highlights the costs of flooding.
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NIWA

Water & Atmosphere 15(3) 2007
High-resolution modelling
can find the weakness in a
town’s flood defences.
Even with these complications, we have completed several
high-resolution inundation models. Before they can be used
to predict future inundation depths, the models must be tested
to see whether they can accurately reproduce water levels
measured in historical floods.
Model results
We can see the accuracy of our models when we compare
predicted inundation with actual flooding. The river images
below show the Mosley Road meander of the Matau Branch
of the Clutha River in Otago. The first image shows computerpredicted
water levels for a river flow of 5100 cumecs (m 3 per
second; the average flow in this part of the river is 600 cumecs).
The second image is an aerial photo of the Clutha River taken in
November 1999 during a flood when the flow was 5100 cumecs.
The average difference between computer-predicted and fieldmeasured
water levels was less than 20 cm.
An example of application of a high-resolution inundation
model to a New Zealand town is shown above. Water levels
were calculated every 4 m throughout the town. Grey
areas show the surface of the ground, houses, and trees as
detected by LiDAR. Rainbow-coloured areas represent the
inundation from a 1% AEP flood. The red areas are deepest
and occur in the river channel. Green and blue zones are
progressively shallower. The model indicates that this town’s
flood protection banks would be overtopped in the event of
a 1% AEP flood.
Putting the models to use
In New Zealand, the current Building Act (2004) specifies
that surface water from 2% AEP events (50-year floods) shall
not enter housing. However, there is about a 90% chance
that there will be at least one 2% AEP flood in populated
catchments of New Zealand in any given year. Given these
statistics, it’s not surprising that we often hear news of flood
disasters in our country. Though we can’t stop the rain, new
technologies and our improved models can help refine flood
predictions. With more accurate information, councils and
other land managers can be prepared for the inevitable. W&A
Photo: Otago Regional Council
Close match between modelled (top image) and actual flooding
in a bend of the Clutha River.
Dr Alistair McKerchar and Dr Graeme Smart
study hydrology and hydraulics; they are based
at NIWA in Christchurch.
www.niwa.co.nz 13