Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 55

Peak

Basin
lag

Recession limb

Base flow
separation

Rising
limb

Base flow

Time

Figure 3.1. Flood hydrograph.

in predicting flood magnitude (Bell, 1999). The peak discharge of a flood can
be used to determine the depth and area of the flood as well as the duration
and velocity of the flood waters (Bell, 1999). The probability of a flood event
occurring is determined by fitting a frequency distribution to a sample of flood
observations from a given stream. This helps to determine the recurrence inter-
val of a particular magnitude event (Chapman, 1999). To measure the amount
of damage potential a flood may have it is necessary to consider the stream
power of an event and the amount of energy developed per unit time along a
river boundary (Bryant, 2005). The largest calculated stream power was from the
late Pleistocene Missoula floods in the USA. These floods reached velocities of
30 m s − 1 and depths of 175 m with a stream power in excess of 1 000 000 W m − 2
(Bryant, 2005).
Flood flows in river systems can be analysed via the discharge hydrograph.
The hydrograph is a curve describing the volume of discharge with time at a
point along the stream channel (Fig. 3.1). A hydrograph can be used to deter-
mine the total flow, base flow and periods of high and low flow showing yearly,
monthly, daily or instantaneous discharges (Bell, 1999). Hydrographs are useful