Flowing water at the land surface - Fundamentals of the Physical Environment - page 433

generated by input. The shorter the lag time, the steeper the rising limb, the higher the
PDV and the more likely that any particular flood threshold will be reached. The curve is
thus 'elastic', and human activity in the catchment can be assessed in terms of whether it
will reduce the lag time and enhance the flood risk or extend the lag time and thereby
reduce it.
Figure 2 Deliberate and inadvertent human influences on
hydrological processes.
Source: After Ward (1975).
Flood adjustment measures commence with prediction of the scale and frequency of
future floods. Assessment of historical records and future forecasting generates the
probable recurrence interval of any particular critical discharge, or the mean annual
flood or maximum probable flood . The latter, worst-case scenario assumes that the
atmosphere 'dumps' all available moisture in a single event and that antecedent
catchment conditions are favourable. Prediction permits measures to be taken to reduce
the flood risk but the ability to do so depends on the technological status and economic
wealth of the population. Knowledge of recurrence intervals alone is not enough, and the
response depends on ability to forecast the next event of a particular size within the return
period from the monitoring of catchment conditions and weather forecasts.
There are three principal forms of flood protection or mitigation. Hard engineering
options are most likely in urban or industrial areas and are covered in the box on p. 300.
Soft options permit the flood to develop but, through a warning system, evacuate people
and partially shut down or protect installations to reduce losses. Structures can fail or be
bypassed, leading to more serious if less frequent flooding. Passive options do nothing to
reduce lag times or raise critical thresholds but sustain economic damage as a less