Analyzing Pond Volumes using Terra Power Tools
Estimating pond volumes and water surface elevations in AutoCAD Civil 3D® can be a tedious process.
If the water surface elevation is known, the volume can be obtained using the surface volume tools by modeling the water surface elevation as a ‘flat’ surface which is compared to the existing ground surface in a cut/fill analysis. It works, but it requires quite a few steps, and results in extraneous surfaces in your drawing.
If the runoff volume is known and the water surface elevation must be calculated, the problem gets a whole lot more complicated. The solution here is to use an iterative technique wherein you ‘guess’ a starting elevation and analyze surface volumes to determine the actual volume at that elevation; you then adjust the elevation up or down by a reasonable amount and repeat until you close in on the target volume. This is time consuming as a manual process and is therefore best left to automation…the only problem is that AutoCAD Civil 3D doesn’t provide a mechanism for this.
Thankfully, there is now a much easier way to perform both of these tasks – the solution is found in the Floodplain Analysis feature in Terra Power Tools.
Let’s see how this works.
First, let’s look at the analysis process when the elevation is known. For this, we first select the surface we want to compare against. If a boundary is needed (and available), we select the option to do a ‘bounded’ volume computation and select our previously imported boundary. We then set the target elevation using the Floodplain Elevation controls and press the ‘Compute’ button. The results are shown graphically in the viewport and as tabulated values in the Results pane of the Floodplain Volume Result dialog:
Note that we did not have to create extra surfaces or any of the other extra steps required to reach a similar result using the tools available in AutoCAD Civil 3D.
Now, let’s turn the problem around and assume that we know the volume that must be stored in the pond basin, but not the final water surface elevation. To calculate this value, we use the Targeted Volumes control in the Floodplain Elevation pane. We set the Target Volume and the tolerance, and then click ‘Compute Elevation’. The elevation controls are adjusted to reflect the result of the calculation, and the graphics in the viewport (and the tabulated results) are updated accordingly:
For a more detailed discussion of this topic, please check out our previous post on calculating storage basin volumes.