Geoscience Reference
In-Depth Information
5-6 Nov. 1984
7-8 Nov. 1984
8-9 Nov. 1984
N
Macquarie
University
Turramurra
Ryde
Sydney
Randwick
Cronulla
Royal National
Park
A
B
C
< 50 mm
50-99 mm
100-149 mm
150-199mm
> 200 mm
Isohyets for three flash flood events in Sydney, Australia, 5-9 November 1984 (modified from Australian Bureau of Meteorology, 1985).
Fig. 6.9
spans under nine hours exceeded the 1:100-year event.
Many of the suburbs affected by this event had experi-
enced their previous 1:100 year rainfall only four days
previously. During this event, the road network sup-
planted natural drainage and took floodwaters away
from storm drains normally expected to carry runoff.
Because roadways have reduced frictional coefficients,
flood peaks were almost instantaneous throughout the
drainage system. Most of the damage during the third
flash flood was caused by floodwaters reaching
shopping centers at the base of steep slopes, sending
walls of water and debris - in some instances including
expensive cars - cascading through shops. In addition,
the Tank Stream, which runs beneath the lower central
business district of Sydney, was reactivated, flooding
the Sydney Stock Exchange and causing several million
dollars worth of damage. The State Library was also
flooded, damaging rare exhibits.
There are three points about flash flooding in urban
areas to be drawn from the events described above.
Firstly, the probability of occurrence of a high-
magnitude, localized rainfall event in a region can be
an order of magnitude greater than the probability of
the event itself. For example, the Dapto Flood was a
1:250-year event. The fact that any part of the Illawarra
can experience such a localized storm indicates that
the probability of a similar flood occurrence in the
Wollongong region - along a 40 km stretch of coast - is
about 1:25 years. Sydney experienced at least five
1:100-year events in the space of five days. In some
cases, emergency services had to respond to all five
events. Since then, Sydney has received the heaviest
48-hour rainfall on record. On 5-6 August 1986, an
east-coast low dropped over 430 mm of rain in three
days, causing rivers to flood from Bathurst, in the Blue
Mountains, to the Georges River, in south Sydney. At
least six people died, from drowning or being electro-
cuted by fallen power lines. Damage exceeded $A100
million, including over 3000 motor vehicles swamped
by floodwaters. In this case, rainfall with a recurrence
frequency of 1:100 to 1:200 years was experienced over
a large area, much of which had been affected by
sporadic 1:100-year events two years previously.
Secondly, urban flooding in most cases peaked
within half-an-hour of the onset of intense rain. Much
of the flash flooding was exacerbated by the structure
of the urban drainage system. In the less densely built
north-western suburbs of Sydney, surcharging of the
storm drains along existing natural drainage routes
caused flooding to homes and shops built in these
areas. In the more densely built up eastern suburbs,
the road network often took the place of the natural
drainage system, causing damage to areas that would
not normally receive this type of flooding.
Finally, it is very difficult to comprehend what the
increased frequency in flooding means. While some
