Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 52

followed a major El Nino event, as the Walker Circulation intensified resulting
inaLaNina event bringing two years of the heaviest rain in recorded history to
theregion (Bryant, 1997).
Meltwater from snow and ice is also a source of floods. Meltwater flooding is
aproblem in areas draining mountains and places with significant winter snow
accumulation (Bryant, 1997). Melting snow is responsible for flooding in the late
spring and early summer on the continental interiors of Asia, North America and
some parts of Eastern Europe (Smith, 2001). Some of the most threatening events
are from rainfall on snow combined with rapid spring warming resulting in
large water flows in a short period of time (Smith, 2001;Bryant, 2005). Meltwater
floods can also be compounded by ice jam flooding, which occurs through an
accumulation of large pieces of floating ice that build up in the river system,
often at bridges, resulting in a damming effect (Smith, 2001). Large ice masses in
snowmelt can damage buildings, destroy trees and dislodge houses (Smith, 2001).
The Romanian floods of 1970 were a result of heavy rain from a deep atmospheric
depression during annual snowmelt from the Carpathian Mountains (Smith,
2001).
Flash floods are short-lived extreme events. They most commonly occur under
slow moving or stationary thunderstorms when rainfall exceeds the infiltration
capacity of an area resulting in rapid run-off (Bell, 1999). They tend to last less
than 24 hours and are destructive due to the high-energy flow. Flash floods are
particularly hazardous due to the quick onset of floodwaters and the lack of time
for adequate warning systems to be activated (Blaikie et al ., 1994). Flash flooding
in semi-arid and urban areas can be more destructive because extensive sheet
flow can occur within very short distances due to a lack of vegetation to defuse
raindrop intensity (Bryant, 2005). This results in a high run-off to infiltration
ratio and as a consequence the water can enter the channel quickly resulting in
ahigh, rapid onset, flood peak.
Prolonged rainfall events are the most common cause of flooding worldwide.
These events are usually associated with several days, weeks or months of con-
tinuous rainfall. Prolonged rainfall over large drainage basins can be associated
with monsoonal activity, tropical cyclones or intense depressions of the mid-
latitudes (Smith, 2001). Heavy rain can form in the large vertical development
of storm clouds with strong updraughts associated with large-scale low-pressure
systems, or as a result of large volumes of atmospheric moisture sucked into con-
vection cells or orographic uplift along coastal mountain ranges (Jones, 2002).
The longer the duration of the rainfall event the greater the amount of surface
run-off entering a river channel (ARMCANZ, 2000). A prolonged rainfall event in
1861 in Cherrapunji, India, was caused by monsoonal winds carrying unstable
moist air from the Bay of Bengal up over the Himalayas. This system produced