Subject: Buildup/Washoff from a Subcatchment in SWMM 5
A pollutant concentration from a subcatchment in SWMM 5 can come from the precipitation, event mean concentration (EMC) in the washoff, rating curve washoff as a function of the surface runoff or power/exponential washoff from pollutants built up on the land surface. The pollutant washoff is mixed in the node and links for the network and can undergo first order decay, BMP or LID removal and CSTR mixing.
Figure 1: The COD buildup in pounds or kilograms from the initial loading, dry day loading and interevent loading.
Figure 2: The COD Washoff based on the amount built up and the amount of runoff.
Figure 3: The COD concentrations washing off the watershed based on the washoff equation, the runoff and the amount of buildup.
Figure 4. COD Exponential Washoff and the Runoff from one Subcatchment.