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I have a DEM of my study area, and polyline shapefile of the Dams in that area.
I have run the standard Hydrology tools on it (Fill, Flow Direction & Flow Accumulation).
I know that you can find the Watershed contributing to a pour point, but how do you use the Damlines, to get the catchment Area contributing water to the dam's reservoir?
I don't know of a function that does exactly what you ask, but I use Arc Hydro Tools to get the catchment areas and drainage lines. When following the drainage lines it is easy to determine which catchment areas that contribute to each water body (i have cut them out from my dem first, so that they are viewed as sinks in the calculations). After that, just select the relevant ones and export to a new shapefile, or whatever you want to do with it.
To get the drainage lines in AHT, you need to complete the following steps in the Terrain Preprocessing dropdown menu: Fill Sinks, Flow direction, Flow accumulation, Stream definition, Stream segmentation, Catchment grid delineation and Drainage line processing. For further information about each tool, visit the tutorial.
The same thing can probably be done with the standard Hydrology toolbox in Arcmap, but I'm not that familiar with it.
Conceptually I think the issue of a a dam line is misleading. Dams are built across an existing river valley, and will act to artificially truncate a watershed. So if you compute the stream network for your DEM using either archydro or the standard hydrology tools and find the point of intersection between the stream network and the dam line, you can use that as a pour point which will return the upstream contributing area of the area above the dam.
You can generate your pour point using the intersect tool, selecting the output feature as Point.
Another option would be to create the watershed for the entire dammed stream using the standard methods for delineating a watershed, and then simply clip the resulting polygon by the dam line to create the truncated watershed, although this seems like it would be more time and processor intensive than the above method.
You could try to put the point for delineation on the base of the dam. Then you could put other two points at the intersection of the upper part of the dam with the hillslopes.
There is also a scientific paper on this:
Holger Schäuble, Oswald Marinoni, Matthias Hinderer (2008) A GIS-based method to calculate flow accumulation by considering dams and their specific operation time, Computers & Geosciences, Volume 34, Issue 6, Pages 635-646.
