As I was about to log out for the day, I was contacted by a customer who indicated that the analysis was not affected by the size or depth of the stormwater detention pond that they entered.

Well, the solution boils down to the definition of the storage curve data. The confusion arises when a user who had select Elevation from Project Options window for Elevation type and defines the Storage Curve data for the pond as an Elevation vs. Area. A quick and careful look at the Storage data type indicates that it requires Depth vs. Area as shown in the figure below:

Figure 1. Storage data of a pond defined in Depth vs. Area

One must take special care in defining the Depth column as and offset depth from the invert of the pond irrespective of the Elevation type selected in the Project Options dialog box as shown in the figure above.

You may want to refer to another blog post Using Civil 3D Stage Storage information in SSA by Matt Anderson for the possible workflow to incorporate Civil 3D stage storage information into the storage curve data in SSA.

SSA considers each stage as a horizontal section of prism and uses the traditional “Average-Area” method to determine the volume that is provided by the equation:

**V = (A1 + A2) h/2 **

Where

V = volume at the given stage (ft^3 or m^3)

h = height or depth of the section (ft or m) which is defined in the Depth column in the Storage Curve data type

A1 = area of the bottom section (ft^2 or m^2)

A2 = area of top section (ft^2 or m^2)

The volume at any stage is the cumulative volume of the pond. In the example noted above in Figure 1, at the depth of 4 feet, the pond contains 750 cubic feet of water.

Notice the difference in the computed volume when Elevation is used instead of Depth in the Storage curve data type:

Figure 2. Difference in computed volume when using depth vs. elevation in the Depth column

A very important point to be noted here; the software requires a unique depth values. In situations like safety ledge in the pond, one must adjust the depth values up or down by a small amount (e.g., 0.001 ft or m) as the computational engine could run into convergence issue if same depth is provided for different surface area.

I hope this helps in removing the confusion and clarify the reason for using the depth and not the elevation while defining the storage curve data.