I'm looking for a way to create a buffer from a specific point that represents the catchment area in a selected time in QGIS. I've found many tools that allow me to do it using driving time or public transport. I need to do the same only using walking time in a natural environment using the geophysical information of a DTM as criteria. More in detail, I'm trying to reconstruct the catchment area of some different roman period settlements and to do that I'd like to create some isochrones of different walking times. I've searched all over the web but I have not been able to find a solution to this specific problem. I've spent time trying to use SAGA algorithm of Accumulated costs but somehow I'm not able to run this tool. I hope someone could help me or suggest me a proper tutorial.
you could use the r.walk.coords GRASS algorithm using processing (available in 2.18, may not be available in older versions).
According to the documentation this uses Naithsmith's rule which iant mentioned. It also takes into account the varying cost of uphill versus downhill for different slope ranges.
Not used it myself, but it looks like it'll do what you're after.
You'll need to create an additional friction raster (e.g. you could rasterize polygons for wooded areas to make walking progress slower through forests), although I suspect you could set all the cells to zero if you only have a DEM.