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Physical Geography ­ Rivers and
Flood Management

Slide 2

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Surface run-off = Water that moves across the
surface of the land into streams rather than being
absorbed by the soil
Infiltration = the process in which water
percolates into the top layer of soil
Percolation = the vertical movement of water in
the zone of unsaturated rock below the water
Throughflow = the downslope of water with in
the soil
Groundwater = the store of water held beneath
the water table
Water table = the upper level of ground water
Interception = the process in which precipitation
is trapped on the surface of vegetation
Precipitation = water in any form that comes
from the sky
Surface Storage = water being held on the surface
eg: puddles
Soil water storage =the water has infiltrated into
the top layers and reached the water table…read more

Slide 3

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The balance of inputs and outputs in
the drainage basin
Soil moisture surplus = If precipitation
exceeds evapotranspiration and the
excess is not been used by plants
Soil moisture recharge = Replacement of
water lost during drier periods
Soil moisture Utilisation = where the
water is used in the soil
Soil moisture deficit = transpiration
exceeds precipitation
Field Capacity = the maximum amount
of water the soil can hold…read more

Slide 4

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Lag time = time it takes for max.
rain fall to reach river and for
discharge to peak
Peak discharge = Highest
flow/discharge in river at one point
Rising limb = increase in discharge
after rainfall
Falling Limb = discharge is falling
Base flow = what the river would be
normally flowing
Discharge = the volume of water
flowing in a river channel,
measured in cumecs (m3/s)…read more

Slide 5

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Factors which influence storm hydrographs
Vegetation cover ­ it varies seasonally, the type and amount will affect interception +
stem/throughflow. Overland flow is reduced and lag time rises
Lakes and Reservoirs ­ These will store floodwater and reduce lag time +control river response to
heavy rainfall.
Land use ­ impermeable surfaces in towns and cities will reduce infiltration and increase overland
flow. Different types of crops, eg crops with big leaves, will affect interception rates too.
Slopes ­ steep slopes will encourage overland flow and gentle slopes will slow down surface run off
Rock type ­ impermeable rock prevents groundwater flow and encourages overland
flow/throughflow. These rocks will decrease lag time. Permeable rock will have the opposite effect
Soil type and depth ­ deep soil can store more water, pipes in the soil will encourage throughflow.
Soil with small pore space will reduce infiltration and increase overland flow like clay.
Rainfall intensity and duration ­ intense rain will increase overland flow and reduce lag time. Gentle
rain over a longer time will allow for more infiltration
Climate ­ the distribution of rainfall over the year, temps, snowmelt and intense frost/drought
will affect lag times…read more

Slide 6

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Drainage density and basin
In circular basin:
-Equidistant from any point for
water to get into the river
-Shorter lag time and higher peak
In a Elongated Basin:
-It takes much longer for the
water to get into the river
-Longer lag time Tributaries ­ little streams within the basin joining the
main stream
Watershed ­ imaginary line separating basins (usually
*If it has imputs+outputs = open a high ridge of line)
Catchment area ­ area with in the watershed
system*…read more

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