You can enter a third storage definition for the volume above the stone.  When you define this volume, be sure to remove the check mark from "Allow Exfiltration", otherwise the area of the upper and lower volumes will be added together for the exfiltation calculations.

Thank you PSMART,

The stone is not under the chambers but around the perimeter.  The chambers are put in the pit on native soil and then backfilled with stone.  I could place a layer of stone under the chambers but for now I just want to understand how to model the backfilled stone which extends 6 inches around the perimeter of the field.

If one embeds a storage with 100% voids inside of the prismatoid with 40% voids, HydroCad will credit the embedded storage.

                           Pond 1P: (new Pond)
 [43] Hint: Has no inflow (Outflow=Zero)
 Routing by Sim-Route method
Peak Elev= 0.00' @ 0.00 hrs   Surf.Area= 0.000 ac   Storage= 0.000 af
Plug-Flow detention time= (not calculated)
Center-of-Mass det. time= (not calculated)
Volume     Invert  Avail.Storage  Storage Description
   #1      50.00'       0.107 af  50.00'W x 100.00'L x 5.00'H Prismatoid
                                  0.574 af Overall - 0.306 af Embedded = 0.268
   #2      50.00'       0.306 af  60.0"W x 48.0"H x 5.00'L Parabolic Arch  x 2
                        0.413 af  Total Available Storage

jtowne,

All you need to do is give the dimmensions of the total volume that will contain the stone and chambers.  Set the voids at 40%.  This is volume #1 in TerryScan's example.

Then define the chambers as volume #2 and set "embed inside volume #1"  Be sure to leave the chamber voids at 100%.

Now check the storage calculations in the pond summary.  This will show the exact storage calculation, including the contribution from the stone and chambers.