Resolving Conflicts in Hydrologic Design Criteria: A Case Study in Maryland

Abstract

Maryland law requires that all proposed changes to the course, current, or cross section of any non-tidal waters of the State must first obtain a waterway construction permit from the State. Also by law, the waters of the State are defined to include the stream channel as well as the 100-year floodplain. The Maryland State Highway Administration (SHA), therefore, is required to obtain waterway construction permits from the Maryland Department of the Environment (MDE)for every proposed bridge, culvert, or other roadway feature that encroaches into the 100-year floodplain. As part of each permit application, the SHA must present the potential impacts from the proposed projects for the 2-year, IO-year, and 100-year floods. In most, but not all, situations the hydrologic analysis must address ultimate development conditions in the affected watershed. For a variety of reasons, the MDE historically required, or at least preferred, the application of the TR-20 watershed model to estimate peak flow rates under both existing and ultimate development conditions. Because sufficient data generally is not available to calibrate the TR-20 model, significant time and resources frequently were expended by the two agencies merely to obtain agreement on the flow rates. The SHA felt that other hydrologic methods, such as gauging records, regression equations, or other models, should be considered where appropriate. In general, the SHA believed that the approved flow rates were too high and, thus, resulted in over-designs. Significant conflict arose between the agencies.