978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

Base flow/groundwater flow – this is the ‘normal’ level of water in the
channel determined by the groundwater flow prior to a rainfall event.
Lag time – this is the period between the peak precipitation and the peak
discharge.
Peak flow/discharge – this is the maximum river discharge for any given
event measured in cubic metres per second m 3 s -1 (cumecs).
Rising limb – this is the part of the graph that initially rises, indicating
the increasing level of water as determined by the combined rate of
surface runoff, throughflow and groundwater flow following a precipitation
event.
Storm flow – this is the additional discharge created as a result of a
precipitation event.
Falling limb/recession – this is the part of the graph that shows the
discharge decreasing and river levels falling back towards base level.
As the rain falls within the catchment it takes a variety of routes before
some of it enters the river (see Figure 1.5). As that water joins the river
the volume of water increases, thus increasing the discharge. Water that
rapidly flows into a river will have a more rapid rise in discharge. Water
that travels slowly to the river will have a more gradual effect on the level of
discharge.
Catchment hydrology
Catchment hydrology refers to the movement, distribution and quality of water
within a drainage basin. Whilst drainage basins vary in form there are common
principles that will shape the response of the area to any given event.
Infiltration rate
Infiltration is the flow of water (precipitation, irrigation) through the soil surface
into a porous medium under gravity action and pressure effects. The maximum
rate of infiltration for an event is the infiltration capacity.
Several factors control the rate of infiltration within the catchment/drainage
basin.
The morphology of the drainage basin
affects discharge in a number of ways.
The larger the drainage basin the greater
potential discharge but longer lag time
as precipitation is caught over a wider
area. Roughly circular shaped basins are
more likely to result in a ‘flashy’ rapid
response as precipitated water is more
likely to reach the river at the same
time having travelled an equal distance.
Steeper drainage basins will have a short
lag time as the influence of gravity will
increase the rate of flow to the river.
Types of precipitation
Flooding most frequently occurs after prolonged periods of rainfall when soil
stores are full and there is less drainage possible. The conditions preceding a
rainfall event can be referred to as antecedent conditions.
During cold conditions, water may be temporarily stored as snow or
ice. This means there is less water circulating through the system. It also
means that there may be a sudden release of water during times of thaw.
Annual flooding in Bangladesh is largely attributed to the combined
effects of monsoonal rain and seasonal snow-melt from the Himalayas to
the north.
There has been much speculation on the effects of climate change. Though
storms are not necessarily increasing in frequency, there does seem to be a
correlation with an increasing intensity. Intense storms are more likely to cause
floods as the ground is unable to absorb high quantities of water in a limited
amount of time.
Relief
The size and shape of the land affects the rate at which water can flow down
it. Slopes with an angle of less than 5 o will have significantly greater rates of
infiltration. The greater the gradient, the greater the rate of surface runoff as
there is less opportunity for infiltration. Higher in the catchment, rivers may cut
steep incised valleys acting under the influence of gravity (as they seek to reach
the lowest point). As they travel downstream this influence is lessened and rivers
erode laterally creating flat, wide floodplains.
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Hydrology and fluvial geomorphology