Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 54

failure of dams. In 1963, the Vaiont Dam in Italy failed killing 2000 people and
in 1889, 2200 people were killed in Johnstown, Pennsylvania from a dam failure
flood (Bryant, 2005).
Characteristics of flood flows
Rivers are an important part of the hydrological cycle. They are a con-
duit for precipitation generated run-off where the water is transported to lakes,
swamps, dams and/or oceans. The extent of a rain-induced flood is dependent
upon the alignment of the storm to the basin, the rate of movement of the storm
and the spatial extent of the storm (Chapman, 1999). The passage of floodwaters
is governed by the configuration of the catchment including its shape, area,
slope and drainage density (Jones, 2002). The hydrological and biological char-
acteristics of the system are also important factors (Chapman, 1999). Catchment
area is the most important factor in controlling the volume of discharge (Jones,
2002); the larger the catchment, the longer it takes for the total flood flow to
pass a given point (Bell, 1999;Bryant, 1997). Also, the size of a catchment has
abearingontheamountofprecipitation it can receive. Larger catchments, for
example, can receive a considerably greater volume of rain than a small catch-
ment in a storm of large spatial extent.
The run-off to infiltration ratio of a catchment is another important factor
governing the behaviour and size of floods. Run-off is the surface water that
remains after evapotranspiration and infiltration into the ground and comprises
waterfrom surface sheet or channel flow; it is the major component of the flood
peak discharge during a precipitation event (Bell, 1999)(Fig.3.1). The total run-off
from a catchment is made up of direct precipitation into the stream channels,
surface run-off, interflow and base flow. The larger the response and travel time
(in terms of velocity) of surface run-off, the bigger the build up of peak flow.
The slower the run-off travel time, the greater the chance of that surface water
infiltrating the ground surface. This water will then take longer to reach stream
channels resulting in a more subdued flood peak. If the catchment contains a
large number of lakes or swamps, run-off rates are likely to be reduced as they
tend to absorb high peaks of surface run-off (Bell, 1999). High slope gradients
and drainage densities contribute to the speed of run-off; the shorter the length
of the hillslope, the faster that run-off will reach and contribute to flood flow
(Jones, 2002).
Measurement of flood flows
The peak discharge in a river is responsible for the maximum inun-
dation of an area. The discharge rate is the basis for most methods involved