Calculating Storage Basin Volumes with Terra Power Tools
Terra Power Tools includes a feature for floodplain analysis which can be used to calculate the volume of water stored in a basin if the water surface elevation is known, and conversely can calculate the elevation of the water surface if the volume is known. Let’s take a look at the following model:
This is an arbitrary basin created using the AutoCAD Civil 3D ‘Grading-4.dwg’ tutorial dataset as a starting point. It has been modified by first leveling the feature line to z=765.0’, then by adding an interior slope at -2:1 to a distance of 25’, and finally an infill was added. We also created a copy of the feature line, and exploded it to form a closed 3d polyline that will be used later.
In Terra Power Tools, we first import the surfaces and then adjust the display style and color of the EG surface so we can clearly see our grading surface. Next, we import the polyline as a boundary – this will be used to limit our calculations to the interior of the basin.
From here, we use the ‘Floodplain’ command in Terra Power Tools to bring up the Floodplain Volume Result Editor window:
In order to limit the calculation to the interior of the basin (vs the extents of the surfaces), we select the option for a ‘bounded’ volume computation and select the boundary:
We also need to set the surface used for the computation to the grading surface:
We can now use the Floodplain Elevation panel to set an elevation for which we want to calculate a volume and watch the model and the calculation results update in real time:
The results of the volume calculation are shown in the Results panel (the result we are after here is the ‘flooded’ volume, which is above the referenced surface and below the water elevation; the ‘not flooded’ volume is below the surface and above the water – this is typically not used):
The process is simple, fast, accurate and reliable. It also does not require the creation of special surfaces in your drawing – the water surface exists only virtually, so your drawing remains uncluttered.
If instead you know the volume you want to store, and need to simply calculate the elevation of the water surface given that volume, the process is just as simple. Instead of modifying the elevation of the floodplain using the elevation field or the slider, you can use the ‘Targeted Volumes’ subpanel to specify the target volume and a tolerance:
Once set, click on the ‘Compute Elevation’ button and the results in the panel will be almost instantaneously updated with an elevation calculated to yield the targeted volume within the specified tolerance, and the results panel will be updated with the final ‘actual’ calculated volumes:
The 3d model in the graphics viewport will also be updated to reflect the final values calculated above: