Geoscience Reference
In-Depth Information
stream in which no rain has fallen. Second, after about
one hour of continuously heavy rainfall, rough catch-
ments receive 10-25 per cent more rainfall than
smooth basins. That babbling brook running through a
property with hilly topography is incredibly susceptible
to flash floods. Finally, the volumes of rain that can fall
within small catchments can be awesome. Figure 6.1 is
a conservative depiction of the amounts of catastrophic
rain that have been measured by our inadequate
network of rain gauges. Within pristine catchments
typical of the hilly topography of Sydney or Los
Angeles, over 500 mm of rain can fall within the space
of one hour. Thankfully rainfalls generating flash floods
rarely continue for longer periods. When they do, the
effects are catastrophic.
Urb
an flash floods
(Nanson & Hean, 1984; Australian Bureau of Meteorology,
1985; Riley et al., 1986a, b)
Urban areas exacerbate flash flooding. The breaking of
the long drought in eastern Australia following the
1982-1983 ENSO event heralded the onset of
extraordinary flash flooding in the Sydney-Wollongong
region, as Walker circulation 'turned on' again over the
following two years. The flooding was very localized
and can be separated into two components: the Dapto
flood of 17-18 February 1984 and the Sydney thun-
derstorms of 5-9 November 1984. In each case, slow
moving or stationary convective cells developed in
association with east-coast lows. Neither the localized
convection nor the east-coast lows was unusual.
Initially, the events represented the rare concurrence
of two commonly experienced meteorological
processes. Unfortunately, the pattern has occurred
with alarming frequency throughout the remainder of
the twentieth century.
The Dapto flood event began in the late evening of
17 February 1984 as a cold front pushed through the
area in front of a high pressure cell centered south-east
of Tasmania. As a result, moist onshore air flowed into
the area at the same time as a small east-coast low
developed south of Newcastle. This low tracked down
the coast and then stabilized over the 500 m high
Illawarra escarpment, north of the town of Dapto, at
7 am on 18 February. This was followed by develop-
ment, during the remainder of the morning, of a
complex, upper-level trough over the Wollongong
region. These lows produced extreme instability and
caused marked convergence of moist north-east
and south-east airflow into the Wollongong area.
Orographic uplift along the high escarpment dumped
copious amounts of rain over the Lake Illawarra
drainage basin over 24 hours (Figure 6.7). In a one-
hour period on 18 February, 123 mm fell west of
Dapto. Over a nine-hour period, 640 mm and 840 mm
were recorded, respectively, at the base and crest of
the escarpment. The heavier rainfall had a 48-hour
recurrence interval of 1:250 years and is the greatest
nine-hour rainfall recorded in Australia.
The resulting flooding of Lake Illawarra, however,
had a 1:10-year frequency of occurrence. This case
supports the United States evidence that flash flooding
with severe consequences can occur in parts of a
drainage basin with areas less than 50 km
2
. Figure 6.7
Flas
h flood events
(Bolt et al., 1975; Cornell, 1976; Maddox et al., 1980)
Flash flooding in the United States tends to be associ-
ated with summer storms and landfall of tropical storm
systems. The Rapid City flood in the Black Hills of
South Dakota, 9-10 June 1972, exemplifies the former
aspect. One metre of rain fell in the space of six hours,
an amount that had a recurrence interval of 1:2000
years. Flash floods swept down all major streams
including Rapid Creek, where an old dam - built forty
years earlier as a Depression relief project - collapsed,
sending a wall of water through the downtown section
of Rapid City. Over 230 people lost their lives and 2900
people were injured. Property damage exceeded
$US90 million and included over 750 homes and 2000
cars. Texas is affected by the intrusion of tropical
cyclones and easterly waves embedded in trade winds.
Strong orographic uplift of these westerly moving
systems can occur along the Balcones Escarpment. As
a result, Texas has the reputation for being the most
flood-prone region in the United States, and has
recorded some of the highest rainfall intensities in the
world (Figure 6.1). For example, during the Pecos
River flood of June 1954, Hurricane Alice moved up
the Rio Grande Valley and stalled over the Balcones
Escarpment. The hurricane interacted with an upper
air disturbance and turned into an intense, extra-
tropical, cold-core cyclone. Over 1 m of rain was
dumped in the lower drainage basin, resulting in flood-
waters 20 m deep, with a recurrence interval of 1:2000
years. To date, this is the largest recorded flood event
in Texas.
