Processes acting on the formation of surface runoff (2)

Loamy soil tends to form crusts at its surface through direct impact of heavy rain drops on the surface. It creates a sealing effect of the soil surface. Only through the remaining cracks the precipitation water can infiltrate into the ground(*4). These situations occur especially on farm land which are not covered by a vegetation layer.

In urban areas the infiltration capacity of the surface is reduced by streets, roofs and coated parking places. The sealing of the surface can vary considerably within urban areas. The runoff on these areas is nearly 100 percent. But not all of these sealed areas are connected to a storm water pipe network. The more they are surrounded by gardens and greens the more the sealed areas drain the rain water sideways into depressions, hollows or drains where it can infiltrate into the ground(*5) . Therefore Pasche et al (2004) differ the sealing rate of urban areas in total rate and connected to pipe rate.

Today’s policy of urban stormwater management fosters the extension of infiltration by diverting precipitation water into parks, on parcel detention by infiltration, unsealing of roads and places or retaining the water by green roofs and public detention ponds. These measures help to re-establish the natural surface runoff and infiltration of rain water into the ground.

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Hydrology of Floods




*4 Roth (1992)

*5 Sieker (1999)





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