#### User’s Question:

*“Jim, I have a quick question regarding the Waterbalance Modelling program and I was wondering if you could help me? I’m trying to model a grass swale next to a road, however, my results seem almost ‘too good to be true’.”*

*“I added an infiltration swale without underdrain within the “Design Source Controls – Surface Enhancements” tab. And then added the grass swale area and impervious paving under ‘Apply Source Controls’ tab.”*

*” Is this the correct route to go if I just want to model a grass swale adjacent to a road segment?”*

#### Jim’s Response:

“Your results may be too good to be true.”

“The swale without storage acts just like any other grassy area in that it only captures the rainfall that directly lands upon it. If you apply a grass swale to a paved area then what the model will do is replace a portion of the area of the paved area with the area of the grass swale. An effective way of reducing imperviousness but I suspect that was not your intended method of reducing runoff.”

“If you wish to direct surface runoff to a Source Control then you must apply a Source Control with Storage. There is again the little quirk in the model for which we are contemplating a revision but for this version you still need to keep in mind. The surface area of the swale will replace an equal area of impervious pavement. This means the impervious area needs to be increased to allow the model to reduce it when it applies the surface area of the source control.”

“This should allow you to model the system and obtain reasonable results.”