Sunday, November 10, 2013

The Effect of the LID Percent Impervious Treated in SWMM5

The Effect of the LID Percent Impervious Treated in SWMM5

If you use the option percent impervious treated for LID's on a Subcatchment then a percent of the impervious flow is routed to the LID.  For example, 50 percent of roof runoff to a LID Swale.  It has the impact of reducing the peak and total runoff fro the whole Subcatchment but increases the LID only flow.  From the SWMM 5 help file  "The percent of the impervious portion of the subcatchment's non-LID area whose runoff is treated by the LID practice. (E.g., if rain barrels are used to capture roof runoff and roofs represent 60% of the impervious area, then the impervious area treated is 60%). If the LID unit treats only direct rainfall, such as with a green roof, then this value should be 0. If the LID takes up the entire subcatchment then this field is ignored"
Figure 1.   LID's get 50 percent of the impervious flow which reduce the overall flow from the Subcatchments.

Figure 2.  No Impervious Flow to the LID. 

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