Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 56

in predicting the passage of a flood through a river system from a single storm
event (Bell, 1999). The rising limb of the hydrograph concaves upward reflecting
the infiltration capacity of the catchment (Fig. 3.1). A sharp rise in the hydro-
graph curve occurs as the infiltration capacity is reached and there is a sudden
surge of surface run-off (Bell, 1999). This is followed by a peak in the curve
that marks the maximum run-off from an event and occurs at some defined
time following the rainfall peak (Chapman, 1999). This period oftimebetween
the rainfall peak and flow peak isreferredtoasthelagtime(basinlag)of
the event. It ispossible that some basins may have two or more peaks in the
hydrograph curve for a single event depending upon the nature of the rain-
fall event and the catchment characteristics (Bell, 1999). The downward limb of
the curve represents the reduction of water into the system from surface run-
off and precipitation compared to the delayed delivery from interflow pathways,
regolith and groundwater storage (Chapman, 1999). Again, the slope of the reces-
sion limb is dependent on the physical characteristics of the catchment (Bell,
1999).
Floods as a natural hazard
A flood event is not considered to be a natural hazard unless there is a
threat to human life and/or property (Smith, 2001). The most vulnerable land-
scapes for floods are low-lying parts of floodplains, low-lying coasts and deltas,
small basins subject to flash floods, areas below unsafe or inadequate dams,
low-lying inland shorelines and alluvial fans (Smith, 2001). Rivers offer human
populations transport links, a water source, recreational amenities, flat often
fertile plains and are an attractive place for settlements. Floods then become
amajor natural hazard because of the high human population densities that
inhabit these lands.
The direct impacts of a flood are closely related to the depth of inundation
of flood waters. The extent of a flood has a direct relationship for the recovery
times of crops, pastures and for the social and economical dislocation impact to
populations (Chapman, 1999). In urban areas, the level of flood damage is related
to thetype of land use, the depth of water and period of inundation (ARMCANZ,
2000). In rural areas, the damage caused by floods is also dependent upon the
type and growth stage of the crops at the time of flooding. In addition to the
loss of crops and livestock, rural areas also suffer loss of topsoil and fertilisers,
with extensive damage to fences and increased weed infestation (ARMCANZ,
2000). Sediment loads, floating debris such as trees, pathogens and pollutants
in floodwaters also have an impact. The damage potential of floods increases
exponentially with flood velocity (Smith, 2001).