This waterway=dam line, and other similar features enclosing towns nearby this location, should probably be a man_made=dyke instead (http://wiki.openstreetmap.org/wiki/Tag:man_made%3Ddyke). Dams are for holding back a body of water, not flood protection like dikes.
Alternatively, a man_made=embankment might be used, if its main function is to carry a road at a higher level than the surrounding landscape.