Flood basins are an important part of water management systems, and their actual retention capacity can change over time as sediment accumulates or the basin surface changes. With Virtual Surveyor, you can use your drone data to measure the volume a flood basin can hold and compare its current capacity with design or as-built values.


In this workflow, you will prepare the basin terrain for an accurate calculation, determine the intended water level, create a water surface that represents the basin at capacity, and calculate the resulting volume. This can help you evaluate sedimentation in an existing basin or verify the capacity of a newly constructed pond or flood basin.


Note: This workflow works best with an empty flood basin. Photogrammetry cannot see through water, so submerged terrain cannot be accurately reconstructed from the drone imagery.


Overview


Drone Surface Cleaning

Before calculating the flood basin capacity, clean your Elevation Terrain so that it accurately represents the ground surface of the basin. Removing machinery, structures, and terrain noise prevents these features from affecting the final volume calculation.

  • Use Remove Object and Replace Terrain to remove machinery and other small structures that do not represent the basin surface. 
  • Use Modify Terrain for larger structures that may cover part of the basin floor, such as a bridge.
  • Small ponds or areas of standing water can create noise in lower portions of the basin. Flatten these noisy water surfaces so they do not distort the terrain used for the volume calculation.


Determining the Water Level

The water level represents the elevation at which you want to calculate the basin's retention capacity.


Use Create Water from the HOME tab to create a water level inside the basin. If you know the designed overflow elevation, create the Water directly at that elevation.


If the overflow elevation is not known, create Water at an elevation within the basin and adjust its level using the settings in the TOOLS for Water tab. Raise or lower the water until it represents the basin at capacity.


Position the water just below the overflow level. This allows the water surface to intersect cleanly with the surrounding terrain so you can create an accurate outline of the basin.


Creating the Water Surface

Once the water is positioned at the desired elevation, use it to define the area of the flood basin.


Select the Water and click Intersect Terrain from the TOOLS tab. Intersect Terrain creates boundaries wherever the water level intersects the Elevation Terrain across the dataset. Keep the boundary that represents the flood basin and remove any additional boundaries or lines created elsewhere in the project.


To quickly remove extra boundaries or lines inside the basin:

  1. Select the basin boundary.
  2. Open the TOOLS tab.
  3. Use Select Within to select the geometry inside the boundary.
  4. Press Delete.


Note: You can also use the Project View to select and delete multiple unwanted boundaries or lines at the same time. Afterward, renumber the remaining basin boundary if needed.


Once you have the basin boundary, you can hide the Water from the Project View. The water itself is no longer needed because the boundary now defines the area you will use to create the surface for the volume calculation.


You can also create a dedicated layer for the generated Cut/Fill geometry to keep the project organized.


Before creating the water surface, Simplify the basin boundary. Simplifying reduces unnecessary vertices in the boundary, which can help the Cut/Fill calculation process more efficiently.


Select the Boundary and open the TOOLS tab. Use the Simplify settings to choose the Method and Threshold that work best for your dataset while preserving the overall shape of the basin.


With the boundary prepared, select the flood basin boundary and open the TOOLS tab. Use Triangulate Within to create a flat surface at the selected water elevation.


This surface represents the water filling the basin to the capacity level you established earlier.


Volume Calculation

The final step is to compare the created water surface with the existing basin terrain.


Select the water Surface and open the TOOLS tab. Click Cut/Fill to calculate the difference between the water surface and the basin terrain. A progress bar appears in the lower-left corner while Virtual Surveyor performs the calculation. When the calculation is complete, the results appear in the Selection box.


The Fill value represents the flood basin's retention capacity because it measures the volume required to fill the existing basin terrain up to the water surface you created.


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