Laserfiche WebLink
<br />e <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />Chapter 1 <br />IntroductiDn <br /> <br />1-1. Purpose <br /> <br />This manual presents procedures for the design analysis <br />and criteria of design for improved channels that carry <br />rapid and/or tranquil flows. <br /> <br />1-2. Scope <br /> <br />Procedures are presented without details of the theory of <br />the hydraulics involved since these details can be found in <br />any of various hydraulic textbooks and publications avail- <br />able to the design engineer, Theories and procedures in <br />design. such as flow in curved channels. flow at bridge <br />piers. flow at confluences. and side drainage inlet struc- <br />tures, that are not covered fully in textbooks are discussed <br />in detail with the aid of Hydraulic Design Criteria (HOC) <br />charts published by the US Anny Engineer Waterways <br />Experiment Station (USAEWES). The charts and other <br />illustrations are included in Appendix B to aid the desig- <br />ner. References to HOC are by HOC chart number. The <br />use of models to develop and verify design details is <br />discussed briefly. Typical calculations are presented to <br />illustrate the principles of design for channels under vari- <br />ous conditions of flow. Electronic computer program- <br />ming techniques are not treated in this manual. However, <br />most of the basic hydraulics presented herein can be <br />adapted for computer use as illustrated in Appendix D. <br /> <br />1-3. References <br /> <br />References are listed in Appendix A. <br /> <br />1-4. Explanation of Terms <br /> <br />Abbreviations used in this manual are explained in the <br />Notation (Appendix I), The symbols employed herein <br />conform to the American Standard Letter Symbols for <br />Hydraulics (American Society of Mechanical Engineers <br />1958) with only minor exceptions. <br /> <br />1-5. Channel Classification <br /> <br />In this manual, flood control channels are considered <br />under two broad classifications: rapid. and tranquil. flow <br />channels, The most important characteristics that apply to <br />rapid and tranquil flows are listed below: <br /> <br />a. <br /> <br />Velocities. Rapid flows have supercritical <br /> <br />EM 1110-2-1601 <br />1 Jul 91 <br /> <br />velocities with Froude numbers greater than I (F > I), <br />and tranquil flows have subcritical velocities with Froude <br />numbers less than I (F < I). <br /> <br />b. Slopes. Invert slopes in general are greater than <br />critical slopes (So> Sc) for rapid flow and less than criti- <br />cal slopes (So < Sc) for tranquil flow. <br /> <br />c. Channel storage. Channel storage is usually <br />negligible in rapid flow. whereas it may be appreciable in <br />natural rivers with tranquil flow. <br /> <br />d. Discharge. All discharges are normally confined <br />within the channel for rapid flow (no overllank flow). <br /> <br />Other characteristics such as standing waves, surges, and <br />bed configuration that differ under the influence of rapid- <br />or tranquil-flow conditions should be recognized and <br />considered as the occasion demands. Rapid and tranquil <br />flows can occur within a longitudinal reach of a channel <br />with changes in discharge, roughness. cross section, or <br />slope, Channel improvements may bring about changes in <br />flow characteristics. <br /> <br />1-6. Preliminary Investigations for Selection of <br />Type of Improvement <br /> <br />The investigation required in selecting the type of channel <br />improvement to be adopted involves three considerations: <br />physical features of the area. hydraulic and hydrologic <br />aspects, and economy. <br /> <br />a. Physical features. The topography of the area <br />controls in a general way the channel alignment and <br />invert grades. Of prime importance. also. are width of <br />available right.of-way; location of existing channel; and <br />adjacent existing structures. such as bridges, buildings, <br />transportation facilities. utility structures, and outlets for <br />local drainage and tributaries. Invert slopes may be con- <br />trolled by elevations of existing structures as well as by <br />general topography. elevations at ends of improvements, <br />and hydraulic features, <br /> <br />b. Historical and observed elements. The flow char- <br />acteristics noted in historical records and indicated from <br />detailed observation of existing conditions will usually be <br />basic to the selection of type of improvement or design, <br />With the flood discharges determined, the interdependent <br />factors that determine improvement methods and general <br />channel alignment are slope of invert, width and depth of <br />flow, roughness coefficient, the presence or nature of <br />aggradation and degradation processes. debris transporta- <br />tion. bank erosion, cutoffs. and bar formations. <br /> <br />1-1 <br />