I'm with you. I observed a year before I went to action. I guess at least one time through all the seasons is a must, when one wants to really understand a property.
Make shure to position your swale on the highest, longest contour line on your property. Is your main drive on contour? Also I would advice you to have the swale on the lower side of your driveway so that the water infiltration into the soil will not undercut your road.