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Throughout the selection and design process, the designer should keep in mind that his primary objective is to protect the highway structure and adjacent area from excessive damage due to erosion. One way to help accomplish this objective is to return the flow to the downstream channel in a condi- tion which approximates the natural flow regime. This also implies guarding against overdesign or employing dissipation devices which reduce flow conditions substantially below the natural or no~l channel conditions. If scour computation indicates the need for an energy dis- sipator, a logical next step is to investigate the possible ways of reducing or eliminating this need by modifying the outlet velocity or erosion potential. This involves analyzing the effects of various alterations of the culvert characteristics-- changing slope, roughness, etc. These are discussed in the chapter on Outlet Velocity and design chapter VII. . The cost of the culvert alteration and its effects on culvert performance compared with the cost of providing an energy dissipator are all important considerations in this investigation. Preliminary energy dissipator selection is made by comparing the input constraints or design criteria--flow regime, debris, problems, location, channel characteristics, allowable scour, etc.--to the attributes of the various energy dissipators. This process may result in the selection of several energy dissipator designs or combination of designs which substan- tially satisfy the design criteria. Each situation is unique, however, and compromise between the various elements of the system and the exercise of engineering judgment will always be necessary. Flow transition design, the next step in the process, is an essential part of many dissipator designs. The Flow Transition chapter provides guidance for the selection and design of this important appurtenance. Most situations encountered involve supercritical flow, indicating transi- tions must be carefully designed in order to minimize wave and flow separation problems and to provide uniform flow conditions at the dissipator entrance. The individual dissipator designs have been qualified as to their area of application. The attributes delineated include: Froude number range for best performance~ discharge velocity or other limitations~ possible maintenance, opera- tional, or location problems~ maximum size~ limiting characteristics such as culvert slope or shape. 1-3