The Factors that Influence the Flood Hydrograph

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.