How to model a pond with void ratios that vary with elevation?

Question

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?

Sushma Choure · Answer

Hello, 
 Which product are you using PondPack or SewerGEMS? 
 Which type of pond volume type you are using? Did you try using storage chamber type of pond where you can enter the pipe geometry, chamber arrangement, stone bed geometry and void ratio. 
 Modeling Underground Storage Chambers 
 https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/28970/what-is-the-chamber-system-fill-void-space

Jesse Dringoli · Answer

As Sushma mentioned the storage chamber system enables you to model storage space with surrounding fill at a specific void space percentage. 
 For a case where you have a standard above-ground pond with available storage from void space in the material below it, there are two approaches you could consider: 
 1) You could model this with the pond element, using an Elevation-Volume curve to represent the overall available storage. At each depth increment, you would specify the available cumulative volume, taking into consideration the void space. You would need to determine this curve externally but this is how it could be modeled in PondPack (or SewerGEMS/CivilStorm). 
 2) Consider modeling only the actual pond using an elevation-area or elevation-volume curve with the pond element, and model the sub-surface soil and gravel using one of the available infiltration method. This would assume that there is a certain rate at which water that enters the pond will seep into the ground. If it is not possible to model the variation in this rate (as the void space in the gravel fills for example - see this ), consider the first option above.