How is RDII Storage Simulated in SWMM 5?

Subject:  How is RDII Storage Simulated in SWMM 5?

If you are using the SWMM 5 Rainfall Dependent Infiltration and Inflow(RDII)  feature you can also use the RDII storage parameters to change the RDII runoff by simulating the storage in the Sewershed.   The code in RDII.C as implemented by Lew Rossman of the EPA keeps track of used and unused initial abstraction or IA (Figure 1)

When there is rainfall the following actions are taken:

·         The raindepth available to be convoluted by the RDII unit hydrograph method is reduced by unused IA
·         The amount of IA used up is then updated 

When there is no rainfall

·         A portion of the IA already used is recovered using the recovery rate parameter and the variable IAUsed

Figure 1.  The long term effect of the RDII storage on the generated RDII Unit Hydrographs.  IA1, IA2 and IA3 are the Storage values for the short, medium and long term UH's.