What is the best way to model a pond that has both surface storage capacity and subsurface storage capacity? For example, a 2 foot deep pond (100% void ratio) underlain by 1 ft of soil (20% void ratio) which is underlain by gravel (40% void ratio).
I don't see a way to vary void ratio with each elevation increment. One alternative is to reduce the pond footprint by 80% and then by 60% at each corresponding elevation. This is not ideal though, and leaves room for error.
How else can it be done?