Describe and Explain the Factors that Influence the Flood Hydrograph, with particular reference to Rural and Urbanised Drainage Basins.

Flood hydrographs

A flood or storm hydrograph shows the relationship between rainfall and discharge. Studying this relationship in different drainage basins in different weather conditions can help to predict when and where flooding might occur. In this essay I am going to concentrate on the storm hydrograph of Tokyo before and after urbanisation, which will therefore help me describe the difference between rural and urban drainage basins using the factors.  

Figure 4 below shows a storm hydrograph with labels.

Factors that influence the storm hydrograph

        The discharge of rivers in some drainage basins rises rapidly after a storm often leading to flooding. Rivers in other drainage basins rarely flood.

        There are six main factors that influence this:

• Angle of slope: on steep slopes overland flow can be very high. This helps explain the lag time between precipitation and the arrival of water in stream channels. On low slopes these rates will be much reduced.
• The nature of the drainage network: the main factor here is the density of drainage.  This is defined as total area of basin divided by the length of active channels. The higher the drainage density the more likely flooding will occur because water moves more quickly in river channels than it does on the land surface or in the soil or ground.
• Geological rock type and soil: impermeable rocks such as clays and shales, hard igneous and metamorphic rocks promote more surface overland flow. This moves much more swiftly than water flowing in the soil or ground. These rocks produce quick and high flood peaks on hydrographs. Chalk sandstones, limestone’s and other permeable rocks have more regular hydrographs.
• Vegetation cover: forests can regulate flow by intercepting rain, reducing rainfall intensity and increasing Evapotranspiration. This will reduce any dramatic flood peaks on the hydrograph. Urban areas produce high overland flow from their impermeable fabric. Drain and storm sewers accelerate the movement of run-off into river channels.
• Rainfall intensity: where rainfall exceeds the infiltration capacity of soil leads to rapid surface or overland flow. This obviously results in more water arriving into the channel quickly and this increases the flood peak on the hydrograph. As rainfall intensity increases, the % of overland flow increases.
• Seasonality: infiltration capacities of soil vary depending on the time of the year and the state of the soil moisture deficit. This is the amount of rainfall needed to return a soil to its field capacity. In dry weather infiltration is normally high; after wet weather overland flow is more likely.

The impact of urbanisation on Tokyo storm hydrograph (figure 5 shows this graph, separate sheet)

        From looking at figure 5 I can see that before urbanisation the river responded to the storm before any development took place in the area much quicker, as the land was not used for industry, buildings, roads/pipes, therefore there was no interception so peak discharge was low, 650 cumecs, with a lag time of 7 hours. As there was no interception, the water infiltrated into the ground and reached the channel slower.

        Whereas, after urbanisation, there was a major impact on the flood hydrograph. The peak discharge was much higher; 1400 cumecs, with a shorter lag time of 5 ½ hours. The peak discharge is much higher as the land has been built on. It is impermeable therefore water infiltrated less and reached the channel quicker.

        The problems that might be caused for people living in Tokyo by these changes in the way the river responds to rainfall are increased risk of localised flooding so roads will easily become flooded and cause problems to traffic movement, which results in a snowball effect leading to economic difficulties. People’s quality of life will be affected; due to localised flooding they would have less access. You suddenly get gutters, sewers and other drainage systems blocked, due to over flow of water.

Conclusion

        Flooding is less likely when there is a lot of vegetation to intercept 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.