Processing area and calculating volume of polygon with known elevation from DEM in QGIS

Question

I got a DEM of a watershed and I need to create a dam at a fixed elevation (with a rectangular polygon corresponding at the top of the dam) and calculate it's area (including the slope (fixed value) between the perimeter of the top of the dam and the DEM) and the corresponding volume.

Do you have any idea of the tools I can use for:

• processing the real area of the dam (i.e. : including the area of the slope)?
• calculating the volume of the full dam

Answer 1

There wouldn't be a specific tool, but a work flow you could follow to do this. I have done something similar in ArcMap, and can describe the concept.

Firstly decide what the fixed elevation is of your dam. Then you can convert your rectangular polygon to a raster file (matching parameters such as cell size with your DEM). There should be a tool for this.

Now perform a raster function where you subtract your watershed from itself so that you have an empty watershed raster (raster calculator aka map algebra). This will be used later to construct the water elevation raster file. (because: empty raster + 100 = water elevation raster at 100 feet)

note: when creating your empty raster make sure to clip it to whatever you think will actually be the reservoir

Now you can display your DEM, and the water elevation raster on the map. If you perform more raster calculations you can generate a new raster that shows the depth of the water. (water depth would be the difference between the water elevation raster and the dem)

To find area you may consider the basics. area= # of cells * length of cell * width of cell so you could use the water elevation cell count and cell size for this input. Most cells are squares so the length and width values would be the same.

to find volume you can use raster calculator and use the water depth raster as an input then multiply by the length and width of the cells.

You may need to play around with this a bit, but that would be a basic processing flow for your needs.

Answer 2

Considering that calculating volume (V) using raster data has a simple algorithm which is:

     V = ap x h

where ap = area of all pixels within the lake, h = height or elevation

And

   h = vp
     

where vp = sum of the difference between h and the digital value of all pixels within the lake

Thus, the volume of the lake is the sum of all areas of the pixels (ap) multiplied by their respective distance to the top of the lake (vp). To achieve this, you can follow this procedure:

1. decide where your dam wall will be and its height or elevation (h)

2. create a polygon layer mimicking the surface of your lake barely following the height or elevation (h) over the DEM (you can do that more accurately using additional tools)

3. extract the raster corresponding to the DEM perimeter using a mask (Clip Raster by Mask Layer tool, that can be found in Raster >> Extraction)

4. in the Processing Toolbox select Raster analysis and choose Raster Surface Volume.

5. in the Raster Surface Volume window choose the following:

5.1. In Input Layer select your DEM

5.2. For Base Level type the height or elevation (h)

5.3. For Method select Count Only Below Base Level (B), because you want to calculate the volume of the lake from the surface (h) to the bottom.

5.4. Lastly for Output Parameter you can be save it to a file or in temporary file

5.5. Hit the Run button

Note that the result will show the surface area, the volume and pixel count. So, be mindful that the units of these results will be the same units as the ones used in your DEM. So, to make things easier, make sure your DEM has a metric coordinate system, such as UTM, rather than a Geographic coordinate system in degrees. If the area of the DEM is too large, I suggest using an Albers projection.