Modeling flow through a Rock Sandwich beneath driveway

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Does anyone have experience modeling flows through a Rock Sandwich?

I'm interested in routing a small swale beneath a driveway using a rock sandwich and am not sure how to define this feature in HydroCAD. I'm thinking of the technique used for infiltration basins where we are able to define the % Voids in the stone layers below, but I have not encountered a technique for doing this with horizontal flow.

Thanks for any advice you may have!