Q:

Generating multiple water level scenarios with only DEM data in QGIS

I want to depict certain water level scenarios (from -0,40mNN to +0,20mNN) by using a two dimensional hypsometric map. Only provided data are DEM-files of the AOI.
I managed to create the hypsometric map, but how do I do the water level scenarios with only the DEM data?
I am using the desktop version of QGis (Version 3.10.5). Here is a picture of what the map should look like as a result:

A:

If you just want to visualize it you can create contours. I use 20 m intervals then applied a filter to the output line layer to only show 20, 40 and 60 m.

A:

If there is only DEM as input, the most simple way is to style the DEM for the interval you need and mask (make transparent) everything above and/or below your desired interval. Then duplicate layer, and adjust colors for every scenario.
Example of styling - using Render type: Singleband pseudocolor on Band 1, set Min and Max to the values of your starting interval, and choose color ramp closest to what you like to see (you can adjust colors later). Then add one more value (green plus button), and set it tiny bit above your maximum interval of first scenario, e.g. maximum is 50, value for transparent color is 50.001.

You can set opacity (transparency) by double click on value and take down the opacity value to 0%. This way, as far as there is no higher value in color settings, starting 50.001 everything above will be transparent. Sure you can have multiple transparent intervals and many other variations, and simply adjust coloring of your duplicate. All depends how you set up your color value intervals.

