I need to do a neighborhood scale drainage analysis. The analysis needs to accurately show the drainage area to each storm drain inlet. Using GRASS on QGIS 2.12.1 on OS X, I have been able to delineate basic watersheds. However, the DEM file does not appear to be precise enough to pick up curb/gutters, which significantly impact drainage patterns (the flow direction just runs over the curbs in many cases), nor does the analysis account for subsurface drainage networks, which collect drainage from one location and deliver it to another.
Is there a way to 'fine tune' the DEM to add curbs and/or a way to model a storm drain network in QGIS (on Mac)?