I've been asked to produce a risk map at work which will show the possible flood route from sewers that surcharge (flood) during heavy rainfall. This is basically where I know the x-y coordinate (OS National Grid Ref. as I'm in the UK) of a number of manholes we know sometimes flood, and I have a 2 metre x 2 metre Digital Elevation Model (Raster) from the UK Environment Agency that covers the area in question.
Basically I need a method that looks at the elevation at the point each manhole occurs at, and then looks at the elevation of each raster square around that point. If a square is at the same or lower elevation then assume that square is at risk of flooding, and for each of THOSE squares, look around again and identify further squares at the same or lower elevation. Keep going until there are no more directly connected squares at the same or lower elevation, and this is the total area that could be at risk.
It doesn't need to account for volumes or the speed at which this might happen, just literally all connected raster squares at the same or lower elevation as the origin point.
I know how to use QGIS fairly well and it's all I have access to at work.