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Some time of concentration (Tc) methods require the use of an equivalent (or sometimes called the characteristic) width of the overland flow path for computing the sheet flow Tc component. An initial estimate of the characteristic width is given by the subbasin area divided by the average maximum overland flow length.
The maximum overland flow length is the length of the flow path from the inlet to the furthest drainage point of the subbasin. Maximum lengths from several different possible flow paths should be averaged. These paths should reflect slow flow, such as over pervious surfaces, more than rapid flow over pavement, for example.
Adjustments should be made to this width parameter to produce a good fit to measured runoff hydrographs. This value is generally a key parameter in calibrating peak flow and total runoff volume.
Some time of concentration methods, such as Kinematic Wave as is used for the EPA SWMM hydrology method, require that an equivalent, rectangular subbasin be determined from the actual subbasin. Using the method as described to compute an equivalent subbasin, StormNET then develops a rectangular subbasin in which overland flow contributions from both pervious and impervious areas are idealized as running down-slope off the subbasin to the subbasin outlet.
Please see the attached images for details.
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Linear Mode
