WMS:Rational Method Outlet Data

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Some of the information for outlet points is automatically determined from the contributing area upstream, while other parameters are entered separately. The area is determined by summing the area of all upstream sub-basins. The runoff coefficient is computed from the upstream basins using the area-weighted equation shown below.

C_{OUTLET} = \frac{\sum A^iC_{BASIN^i}}{A}
COUTLET = The runoff coefficient for the outlet.
CBASINi = The runoff coefficient for the ith upstream basin.
Ai = The area of the ith upstream basin.
A = The total upstream area at the outlet/confluence.

The time of concentration is determined by computing the longest combination of upstream time of concentration and channel travel time to the given outlet point. For example if a given outlet point had two contributing sub-basins the time of concentration for the outlet would be the longest time of concentration of the two upstream sub-basins. If there were other outlets upstream of the given outlet then the travel time would be added, and again the longest time or combination of times would be used as the time of concentration for the outlet.

The rainfall intensity value should be supplied separately for the outlet in the same way it is for a sub-basin. However, it is a function of the time of concentration and can be determined from an IDF curve relationship.

There are two ways routing of a hydrograph can be accounted for using the WMS implementation of the rational method. The first is to simply apply a time of travel between outlets. When hydrographs are computed (only with the summing method) at downstream outlets they are lagged by the travel time and added with other contributing basins. In addition, simple level-pool reservoir routing may be performed on an outlet hydrograph before it is routed downstream. Again, both of these options are available only when choosing the summing method of hydrograph generation rather than the traditional method where a peak flow and resulting hydrograph are determined from the time of concentration (and therefore rainfall intensity) at the outlet point.

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