Urban drainage and flooding

Urban drainage accentuates flooding.

Urbanisation aggravates the flood problem. It is obvious that when land is covered with an impermeable material, to make roofs or pavements, then more water runs off the land instead of seeping into the ground. Accurate calculations can be done for small areas but it is difficult to predict, or measure, the consequences of largeï¾­scale urbanisation. Dunne and Leopold, using American data, predict that urbanisation of a oneï¾­square mile drainage basin will increase the peak discharge by a factor of between 1.5 and 6, depending on soil conditions, the extent of the storm water drainage system, and the proportion of the area which is rendered impermeable. They take 'complete urbanisation' as approximately equal to 50% of the area being impermeable (Dunne & Leopold 1978). In Britain, the Flood studies report reached similar conclusions and stressed the difficulty of making accurate predictions (Packman 1981). The traditional method of dealing with storm runoff from impermeable surfaces is to install an underground drainage system. This led Geddes to exclaim that 'drains are for cities, not cities for drains'(Geddes 1917: 3). Engineering and landscape students are taught how to predict the volume of runoff and how to size drainage pipes to accommodate the flow. They forget that because the waterï¾­courses into which the drains discharge are rendered more liable to erosion and flooding, they will have to be turned into concrete channels, as described in the previous section.