Geography Reference
In-Depth Information
high precipitation during the summer but experience a marked dry
season during the winter. Evaporation and transpiration is high at
all times so that the streamflow mirrors the seasonal pattern of
rainfall.
Land management change can dramatically alter the flows in
river systems. Building dams and changing or diverting stream
courses alters river regimes. Deforestation, or intense grazing may
result in a large reduction of the infiltration capacity of a soil and
decrease in transpiration rates. Covering more of the landscape
with concrete and tarmac, which are impermeable, and then chan-
nelling flow into drains which feed streams may lead to increased
flood risk. These processes lead to more water flowing to rivers
with shorter lag times and therefore potentially higher flood peaks
(Box 4.2).
Box 4.2 Floods
Flooding is a natural phenomenon and should be expected. Each year
flooding causes hundreds of deaths around the world and significant
impacts on infrastructure. However, it is staggering when there are
flood events such as that in 1998 on the Yangtze River in China which
left 14 million people homeless. Floods commonly occur when rivers
overtop their banks. However, flooding can also occur through a very
high tide at the coast, often made worse by a storm event and high river
flows. Flooding may also happen when very heavy rainfall cannot
escape from an area (e.g. an urban area where the drainage system
cannot cope with the excess surface water) or when there is a lot of
saturation-excess overland flow. Flooding can also bring benefits to
farmland through supplying nutrients. There is always a flood occurring
somewhere in the world (see http://floodobservatory.colorado.edu for
maps of current floods).
Often our response to flooding is to build larger and better flood
defences around our towns and cities. However, the flood water has to
go somewhere and sending the water more quickly through one part of
the river system by building large embankments or straightening and
deepening river channels simply reduces the lag time downstream and
increases the overall flood peak there. Working floodplains mitigate
against worse flooding downstream by acting as a temporary store of
water. However, historic and continued building or farming on the flat
