Flow processes acting on the flood propagation in water bodies (3)

In stormwater pipes and channels the retention is only of minor relevance. Due to a compact cross section the flow velocity is nearly constant. Also the travel time of the water is reduced by a straight channel and a smooth surface along the wetted perimeter. This acceleration of flow reduces the translation time with the effect that flood waves, especially in storm water pipe networks and channels, reach the downstream end of their system without attenuation and much faster than the flood wave in the natural river. The more catchments are developed and drained by sewers and channels the more the flood waves will overlap with the negative effect of increasing the flood peaks in the central draining rivers. But this wave interference can be very complex especially in rivers where the flood waves from sewers and natural drains come together.

Urban area draninig into stream
Calculated discharge hydrograph up stream and downstream of stormwater outlet

Figure above shows a flood wave separated into a slow moving part coming from the natural watershed and the fast one from an urban area released through the sewer pipe network. They show all the characteristics and differences of flood waves between natural water sheds and urban areas. While the natural flood wave is long and broad with no distinct peak the flood from the sewer pipe is exactly the opposite, short and with a sharp peak. Short storm events with extreme rain intensities will cause the highest floods in urban areas. As they come fast and intensive (sometimes within 30 minutes) they are called flash flood. They cause overload of the pipe network with flooding of streets, cellars and deep parts of the urban area. The flooded area however is restricted to the surrounding of the pipes and does seldom comprise vast urban areas. In natural water sheds long lasting rain events with high total amount of precipitation but reduced intensity causes the critical floods. The larger the catchment of the river the longer will last the flood wave. For large rivers the floods can last several weeks as observed at the Elbe flood and Danube flood in 2006.

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Hydrology of Floods






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