Environmental Engineering Reference
In-Depth Information
FLOODS AND FLOOD CONTROL
human impact
Flooding is the inundation of land beyond the normal confines of a channel or coastline,
either by overflow of excess water or its influx via shallow subsurface or low-lying
routes. Coastal flooding, which occurs through abnormal tidal surges or waves, often
enhanced by the coincidence of storms and river discharge, is of less concern here. All
other forms of flooding involve water temporarily unable to enter a channel, in the case
of
depression storage
and sheetflow, or re -enter it after an
overbank discharge
. Floods
originate from extreme meteorological events and their indirect and geophysical
consequences (Figure 1). Flooding is the most frequent and widespread form of 'natural'
disaster, affecting more humans than any other physical hazard. It appears to be on the
increase yet takes a surprisingly small toll of human life, although this is not the enigma
it may seem. Flooding is so common that people who live in flood-susceptible areas
rarely do so unawares. Risk is accepted either by choice, in pursuit of economic or
aesthetic gain, or of necessity because of the pressure on, or the particular attributes of,
the land. Historically the latter have including flooding itself as a means of natural
irrigation and nutrient replenishment (e.g. in farmland along the Nile).
Figure 1
Controls on processes leading to flooding.
Risk assessments of varying degrees of formality are undertaken and means of
evading, avoiding or mitigating flood losses are developed. Unfortunately, human
occupation of floodable land - and other activities apparently unconnected with it
elsewhere in the catchment - usually adds to the frequency and intensity of flood hazard
(Figure 2 and box on pp. 299-300). In essence, over-bank flooding is an undesired
discharge level and its inevitability is seen in the form of flood or annual hydrographs.
The 'area under the curve' of the rising and receding limbs represents the total discharge
