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<br />e <br /> <br />Throughout the selection and design process, the designer <br />should keep in mind that his primary objective is to protect <br />the highway structure and adjacent area from excessive damage <br />due to erosion. One way to help accomplish this objective <br />is to return the flow to the downstream channel in a condi- <br />tion which approximates the natural flow regime. This also <br />implies guarding against overdesign or employing dissipation <br />; devices which reduce flow conditions substantially below <br />the natural or normal channel conditions. <br /> <br />If scour computation indicates the need for an energy dis- <br />sipator, a logical next step is to investigate the possible <br />ways of reducing or eliminating this need by modifying the <br />outlet velocity or erosion potential. This involves analyzing <br />the effects of various alterations of the culvert characteristics-- <br />changing slope, roughness, etc. These are discussed in <br />the chapter on Outlet Velocity and design chapter VII. <br />The cost of the culvert alteration and its effects on culvert <br />performance compared with the cost of providing an energy <br />dissipator are all important considerations in this investigation. <br /> <br />e <br /> <br />Preliminary energy dissipator selection is made by comparing <br />the input constraints or design criteria--flow regime, debris <br />problems, location, channel characteristics, allowable scour, <br />etc.--to the attributes of the various energy dissipators. <br /> <br />This process may result in the selection of several energy <br />dissipator designs or combination of designs which substan- <br />tially satisfy the design criteria. Each situation is unique, <br />however, and compromise between the various elements of the <br />system and the exercise of engineering judgment will always <br />be necessary. <br /> <br />, <br /> <br />Flow transition design, the next step in the process, is <br />an essential part of many dissipator designs. The Flow <br />Transition chapter provides guidance for the selection and <br />design of this important appurtenance. Most situations <br />encountered involve supercritical flow, indicating transi- <br />tions must be carefully designed in order to minimize wave <br />and flow separation problems and to provide uniform flow <br />conditions at the dissipator entrance. <br /> <br />,e <br /> <br />The individual dissipator designs have been qualified as <br />to their area of application. The attributes delineated <br />include: Froude number range for best performance; discharge <br />velocity or other limitations; possible maintenance, opera- <br />tional, or location problems; maximum size; limiting <br />characteristics such as culvert slope or shape. <br /> <br />I-3 <br />