Problem

When using the Elevation-Area method to define a pond, the results are drastically different as compared to using the Elevation-Volume method with the same pond.

Solution

First check to see if the Elevation-Volume curve and the Elevation-Area curve represent the same pond. Usually, the Elevation-Volume curve can be developed from the Elevation-Area / Depth-Area curve calculating the volume between areas corresponding to the elevation / depth intervals. If the Elevation-Volume and the Elevation-Area curves represent the same pond geometry then check to see if you are using the Explicit SWMM Solver in SewerGEMS or CivilStorm.

The Explicit SWMM Solver does not support Elevation-Volume curves and approximates them to a Depth-Area curve for calculations. You would likely see a user notification like;

"SWMM does not directly support elevation-volume curves. An approximate depth-area curve was generated and applied to the SWMM engine."

However, the way this happens is that SewerGEMS / CivilStorm converts the Elevation-Volume table to calculate the volume increment. From that it can find the diameter of a cylinder that has the same volume from depth “a” to depth “b”.  Then it exports the bottom and top areas of the cylinder for that interval.  

So, the volume linearly increases over that interval, but in reality the volume could change any which way from “a” to “b”. You could have a super tiny area at the bottom and then it gets really big at the top. That is why the method is an approximation and does not accurately reflect the volume which results in different results using the Elevation-Volume method to define your pond.

So, ideally use the Depth-Area / Elevation-Area curve method if you want a more precise solution or if you do use the Elevation-Volume curve; use lots and lots of points to capture how the volume changes.

See Also

Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver