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The Factors that Influence the Flood Hydrograph

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The Factors that Influence the Flood Hydrograph

Introduction

In this essay I aim to find out the ways in which they affect the
flood hydrograph, with particular reference to rural and urbanised
drainage basins of Tokyo.

A drainage basin is an area of the land's surface from which a river
receives its supply of water. An imaginary line can mark the edge of a
drainage basin. This is called the watershed. The other main features
of a drainage basin are shown in figure 1.

The drainage basin relies on the atmosphere for its inputs of water,
whilst water passes through the drainage basin leaves the system
either to return to the atmosphere or to become an input into the
coastal and ocean systems. A single drainage basin is one part of the
whole hydrological cycle, but the hydrological processes taking place
within it are most likely as those operating at the global scale.
Figure 2 shows the hydrological processes taking place in a drainage
basin, with its outputs, inputs, stores and processes. (On separate
sheet).

The river flow out of the drainage basin is determined by the amount
of precipitation, the losses in evaporation and Evapotranspiration and
the gains or losses from the storage areas: surface storage, soil
moisture and groundwater storage.

Using climate, it is possible to construct a water budget graph.
Figure 3 (on a separate sheet) shows water budget graphs for a.
Birmingham, UK and b. Athens, Greece. This graph lest us know more
about the processes that work in the drainage basin and it also allows
us to compare different countries, I have used the examples of UK and
Greece. Looking at figure 3, I can see ...


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rainfall. In arable farm areas, interception is greatly reduced when
fields are bare in winter. Urban land use increases the risk of
flooding because water reaches river more quickly through underground
drains and by flowing off impermeable surfaces, such as tarmac and
concrete, roofs and gutters.

Sandy soils have large pore spaces allowing rapid infiltration, so
rainfall is soaked up quickly. Clay soils have small pore spaces
slowing down infiltration and increasing the risk of flooding. If the
soil has been compacted flooding is also more likely. Permeable rocks
allow water to pass through reducing the flood risk.

Flood hydrographs for rivers in urban areas are different to those for
rivers in rural areas. They have a shorter lag time and a higher peak
discharge; the rising and recession limbs are steeper.


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