The filter expression Elio Diaz is describing in the comments can be done via the Con (conditional) tool, either the standalone version or as part of Map Algebra. You would want to follow the examples for complex expressions described here, doing something like this:
Con(dam_layer == NULL, DEM, dam_layer)
(Where there are no dams use DEM, otherwise use the ...
If you look at the Raster > Analysis > Roughness index in QGIS 3.0, you will see the following (source https://docs.qgis.org/3.4/en/docs/user_manual/processing_algs/gdal/rasteranalysis.html#gdalroughness )
Outputs a single-band raster with values computed from the elevation.
Roughness is the degree of irregularity of the surface. It’s
Use the Sample raster values or Add Raster Values to Points tool to extract raster values:
Make sure each line has a unique attribute, eg "Line_ID".
Convert the lines into points
Use the Extract vertices tool if the vertices are frequent enough, or
Use the Points along geometry tool if you want more frequent/evenly spaced sampling points
Use the Sample ...
The joy of using the GDAL library is that you never need worry about popular formats again. If a format is at all popular then the will be a GDAL driver for it. Then all you need is to open the raster:
#include "cpl_conv.h" // for CPLMalloc()
poDataset = (GDALDataset ...
Here is one solution using ArcGIS:
1) Resample your 30-meter DEM using the Resample Tool. In the Output Cell Size you can write the path to your 8-meter resolution DEM. For the Resampling Technique choose either Bilinear or Cubic, as they are appropiate for continuous data. Also, before running the tool, click on the Environments... button and click on ...