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Last modified
11/23/2009 10:40:51 AM
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Floodplain Documents
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Statewide
Title
River Hydraulics
Date
10/15/1993
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />EM 1110-2-1416 <br />15 Oct 93 <br /> <br />These are so-called "long waves" that are, in fact, gradu- <br />ally varied unsteady flows in open channels. The term <br />"unsteady" implies that measurements of water velocity <br />at one point in such a channel will show time variance at <br />a scale larger than turbulent fluctuations. "Varied" <br />means that, at any instant, velocities at different points <br />along the channel are different "Gradually varied" <br />means that the pressure distribution in a cross section is <br />hydrostatic. <br /> <br />(4) Wave speed. The analyst must be cognizant of <br />the fact that the response of water in a river to a flood or <br />other disturbance is a wave which propagates at some <br />speed and influences water levels consecutively, not <br />simultaneously. While it may be possible to ignore that <br />fact under certain circumstances, it should never be done <br />mechanically without careful consideration of the specific <br />conditions. Only if the travel time of the wave is small <br />compared 10 the time for a boundary condition 10 change <br />substantially can the water in a reach be assumed 10 <br />behave as a unit without regard for the wave motion. <br />The kinematic wave speed, that is, the speed of propaga- <br />tion of the main body of the flood. is strongly dependent <br />on the channel slope and roughness and must be consid- <br />ered (Ponce 1989). <br /> <br />2.4. Flow Classification <br /> <br />To determine which principles apply 10 a particular situa- <br />tion in river mechanics, it is necessary to properly class- <br />ify the flow. Various categories of flow are amenable to <br />different simplifying assumptions, data requirements, and <br />methods of analysis. The first step in the analysis of <br />river hydraulics situations is classification of the state, <br />type, and characteristics of the flow. Once the presumed <br />flow characteristics have been categorized, the engineer <br />can identify the data, boundary conditions, and simulation <br />techniques appropriate for the situation. The following <br />sections present definitions and flow classifications that <br />lead 10 selection of analysis techniques. <br /> <br />a. Effects of channel boundaries. Water may be <br />conveyed in two types of conduits: (l) open channels <br />and (2) pressure conduits (neglecting ground water). The <br />extent 10 which boundary geometry confmes the flow is <br />an important basis for classifying hydraulic problems. <br />Open channel flow is characterized by a free (open to <br />atmospheric pressure) water surface. Pipe or pressure <br />flow occurs in conduits, pipes, and culverts that are flow- <br />ing completely full and, therefore, have no free water <br />surface. Flow in a closed conduit, however, is not <br /> <br />2-4 <br /> <br />necessarily pipe or pressure flow. If it is flowing par- <br />tially full and has a free surface, it must be classified and <br />analyzed as open channel flow. <br /> <br />e <br /> <br />(1) Figure 2-1 shows that the same energy principles <br />are valid for both pressure flow and open channel flow. <br />The dynamic forces, however, in steady pressure flows <br />are the viscous and inertial forces. In open channel flow <br />the force of gravity must also be considered. Flows are <br />more complicated in open channels because the water <br />surface is free to change with time and space; conse- <br />quently, the water surface elevation, discharge, velocity, <br />and slopes of the channel bottom and banks are all inter- <br />related. Also, the physical conditions (roughness and <br />shape) of open channels vary much more widely (in <br />space and time) than those of pipes, which usually have a <br />constant shape and roughness. Because this manual <br />covers only river hydraulics, little empbasis is placed on <br />methods of solving pipe or pressure flow problems unless <br />they pertain directly to river hydraulics, such as pressure <br />flow through bridge crossings or culverts (see Chapter 6). <br />Chow (1959, chap. I) discusses many of the similarities <br />and differences between pipe and open channel flow. <br /> <br />" <br /> <br />.- <br />. <br /> <br />(2) Flow in an alluvial channel (a channel with <br />movable boundaries) behaves differently from flow in a <br />rigid boundary channel. In alluvial channels (most natu- <br />ral rivers) rigid boundary relationships apply only if the <br />movement of the bed and banks is negligible during the <br />time period of interest. Once general mobilization of bed <br />and bank materials occurs, the flow characteristics, <br />behavior, and shape of the channel boundaries become <br />interrelated, thus requiring far more complex methods for <br />flow analysis. Chapters 4, 5, and 6 of this manual are <br />directed primarily at rigid boundary problems. Chapter 7 <br />presents the theory and methods for analyzing movable <br />boundary river hydraulics. Delails of sediment investiga- <br />tions are provided in EM 1110-2-4000. <br /> <br />e <br /> <br />b. Effects of viscosity (laminar and turbulent flow). <br /> <br />\ <br /> <br />(I) The behavior of flow in rivers and open channels <br />is governed primarily by the combined effects of gravity <br />and fluid viscosity relative to inertial forces. Effects of <br />surface tension are usually negligible for natural rivers. <br />The three primary states of flow are laminar flow, transi- <br />tional flow, and turbulent flow. <br /> <br />t <br /> <br />(2) A flow is laminar, transitional, or fully turbulent <br />depending on the ratio of viscous 10 inertial forces as <br />defined by the Reynolds number: <br /> <br />e <br />
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