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Robert Dickinson
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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.
There are Four factors in Rainfall Dependent Infiltration and Inflow or RDII in SWMM 5:
1. The fractional response to Rainfall or R from 0 to 1
2. The Time Base of the Unit Hydrograph or T in hours * Dimensionless K Shape Factor
3. The Sewershed Contributing Area in acres or hectares and
4. The Maximum, Initial Abstraction and Recovery Rate for R on a Monthly Basis in units of inches, mm or mm/day,
5. The fifth and probably the most important factor is the Rainfall
Subject: How is the Soil Saturated Conductivity Used in SWMM 5 Green-Ampt?
How sensitive is the infiltration loss and rate to the Soil Saturated Conductivity parameter in the SWMM 5 Green-Ampt infiltration method (Figure 1). Figure 2 shows how the total infiltration loss and total loss rate vary as you change the soil saturated conductivity from 1 to 0.1 to 0.01 inches/hour. Internally, Ks is used to check saturation and in the computation of the soil infiltration rate. Two of the checks are:
· In low rainfall everything infiltrates as irate less than Infil>Ks and
· In the check to see if the soil is already saturated.
Figure 1. The three parameters for Green-Ampt Infiltration in SWMM 5
Figure 2. The sensitivity of the total infiltration loss to the soil saturated conductivity in a continuous simulation
Subject: How is Capillary Suction Head Used in SWMM 5 Green-Ampt?
How sensitive is the infiltration loss and rate to the capillary suction head parameter in the SWMM 5 Green-Ampt infiltration method. Figure 1 shows how the total infiltration loss and total loss rate vary as you change the suction head from 12 to 6 to 3 inches. Internally the suction head is used in infil.c of SWMM 5 by adding the suction head to the ponded water on the pervious area in the parameter c1 of the implicit Green-Ampt SWMM5 solution.
C1 = (Suction Head + Depth of Ponded Water) * IMD or Initial Moisture Deficit
Figure 1. The sensitivity of the total infiltration loss to the capillary suction head in a continuous simulation