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Muna E. Faltas, Ph.D. Michael Baker, Jr., Inc. Consulting Engineers Alexandria, VA. 22314 , APPLICATION OF HEC-2 TO SUPERCRITICAL CHANNELS . INTRODUCTION According to FEMA's 1985 Guidelines and Specifications, the delineation of a special Flood Hazard Area (SFHA) in areas of natural streams with supercritlcal flow, should be based upon the critical flow profile. Because the water surface elevation at critical depth Is usually several feet higher than that at supercritical depth, communities affected by this approach had often objected to this practice and conslde~ed It to be too conservative. This paper provides the rationale for this approach through a detailed analysis of the supercritical flow regime, and in view of the limitations of the HEC-2 program. A case . study involving a natural unimproved stream is used to illustrate this analysis. CLASSIFICATION AND CRITERIA OF OPEN- CHANNEL FLOW REGIMES The critical state of flow Is defined as the condition for which the Froude number is equal to unity, which means that the inertial and the gravitational forces acting on the flow are in equilibrium. When the Froude number Is less than unity, the gravitational forces docninrt{> the inertial forces and the flow regime Is classified as subcrltical, or tranquil. Conversely when the Froude number Is larger than unity, the Inertial forces acting on the flow dominate the gravitational forces, and then the flow regime Is classified as supercrltical, or rapid. Steep streams are defined as streams in which the normal depth is smaller than the crItical depth. There are three profile types associated with steep streams; namely, s" s2.' and s3 curves. S2 and s3 are supercritical curves that are controlled from upstream and can be analyzed by HEC-2 as supercritical flow profiles. SupercrltiCal curves can also occur on mild, horizontal, critical, or adverse slopes. Each of the M3' H3, C3, and .