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When rainfall takes place, a part of it infiltrates or percolates into the ground surface while the remaining flows over the land depending upon permeability of the ground, its surface slope and many other factors. The amount of water flowing over the ground surface, pavements, house roofs etc. is commonly known as ‘runoff or the storm water.’ This storm water is ultimately drained through the sewers; otherwise the streets, road etc would be flooded.
The amount of peak storm water flow may be several times more than D.W.F. In the case of combined system, the sewers are normally not designed for the peak storm water flow; otherwise the size of the sewer would be alarmingly large. During ordinary or normal rain, the sewers run full of storm water. However, during heavy rains, the peak storm water is allowed to accumulated for sometime, while the sewers continue to run full of storm water. After the rains are over the accumulated storm water is discharged gradually through the sewer.
The storm water is discharged gradually through the sewers. The storm water flow is also known as the wet weather flow and is abbreviated as W.W.F.
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