Flow processes acting on the flood propagation in water bodies

The concentration and formation of the runoff ends at rivers, storm water pipes, channels or any other water body. Within these transport elements the runoff is subjected to translation and retention.

The translation corresponds to the travel time of the water which is dependent on the flow velocity and the length of the water course. The flow velocity is influenced by the gradient of the bed of the water course, the water depth and the flow resistance.

Inflow/Outflow hydrograph of a flood wave

The retention is caused by temporal storage of water within the water course. This occurs either at rising water when the river itself is filled with water or when overbank flow causes inundating flood plains. Especially in natural rivers the retention has a strong effect on the propagation of the flood wave. Here wide flood plains, meandering river beds and wooden vegetation cause high flow resistance and vast areas of inundation. They have a positive effect on the retention at flood by dampening the flood wave and attenuating its peak. However they are very complex and long not fully understood.

A strong impact on the retention has not only the size of the flooded water body but also the variation of the flow velocity along the wetted perimeter within this water body. If the velocity is nearly constant all water particles reach the downstream end of the river section at the same time. In this case no retention occurs. The more the velocity profile varies over the cross-section the more the water will be retained in the slower sections of the river.

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Hydrology of Floods






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