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<br />e <br /> <br />,,,. <br /> <br />- <br />. <br /> <br />e <br /> <br />, <br /> <br />f <br /> <br />e <br /> <br />Chapter 2 <br />Introduction to River Hydraulics <br /> <br />2-1. Introduction <br /> <br />Proper use of this manual requires knowledge of the <br />fundamentals and laws of fluid mechanics. This chapter <br />provides an overview of the principles necessary .10 <br />perform river hydraulic slodies and provides some gwd- <br />ance for selecting appropriate methods for conducting <br />those studies. It must be supplemented with use of <br />standard textbooks such as Chow (1959), Henderson <br />(1966), and/or French (1985). Topics presented herein <br />include: flow dimensionality, the nature of water and <br />flood waves, an overview of defmitions and flow classi- <br />fications, and basic principles of river hydraulics and <br />geomorphology. <br /> <br />a. General. Rivers are complex and dynamic. It is <br />often said that a river adjusts its roughness, velocity, <br />slope, depth, width, and planform in response 10 human <br />activities and (perhaps associated) changing climatic, <br />geologic, and hydrologic regimes. These adjustments <br />may be rapid or slow, depending upon the source and <br />character of the forces spawning the adjustments. When <br />a river channel is modified locally, that modification may <br />initiate changes in the channel and flow characteristics <br />that may propagate both upstream and downstream and <br />throughout tributary systems. These changes may occur <br />over large distances and persist for long times. <br /> <br />b. Analysis techniques. Effective analysis of river <br />problems requires recognition and understanding of tbe <br />governing processes in the river system. There are two <br />basic items that must always be considered in river <br />hydraUlics analyses: the characteristics of the flow in the <br />river, and the geomorphic behavior of the river channel. <br />These two components are sometimes treated separately; <br />however, in alluvial channels (channels with movable <br />boundaries) the flow and the shape of the boundary are <br />interrelated. One-dimensional, steady state, fixed-bed <br />water surface proftles are often computed as part of <br />"traditioual" river hydraulics studies. However, some <br />floodplain management, flood control, or navigation <br />studies may require consideration of unsteady (time- <br />dependent) flow, mobile boundaries (boundary character- <br />istics that can change with flow and time), or multi- <br />dimensional flow characteristics (flows with nonuniform <br />velocity distributions) to properly perform the required <br />studies. <br /> <br />EM 1110-2.1416 <br />15 Oct 93 <br /> <br />c. Options. The analyst bas a number of options for <br />analyzing river flows and must choose one (or a combi- <br />nation of several) that yields sufficiently useful and <br />defensible results at optimal cost. There does not yet <br />exist defmitive criteria which can be routinely applied 10 <br />yield a clear choice of method. This manual serves as a <br />guide for thought processes to be used by the hydraulic <br />engineer studying a reach of river with the aim of pre- <br />dicting its behavior for a wide range of flows. <br /> <br />2-2. Flow Dimensionality Considerations <br /> <br />a. Realm of one-dimensionality. To decide if a <br />multidimensional study is needed, or a one-dimensional <br />approach is sufficient, a number of questions must be <br />answered. Is there a specific interest in the variation of <br />some quantity in more than one of the possible direc- <br />tions? If only one principal direction can be identified, <br />there is a good possibility that a one-dimensional study <br />will suffice. Let this direction be called the main axis of <br />the flow (e.g., stream wise); it is understood that that <br />direction can change (in global coordinates) along the <br />flow axis, as in a natural river. <br /> <br />b. limitations of one dimensionality. One- <br />dimensional analysis implies that the variation of relevant <br />quantities in directions perpendicular to the main axis is <br />either assumed or neglected, not computed. Common <br />assumptions are the hydrostatic pressure distribution, <br />well-mixed fluid properties in the vertical, uniform veloc- <br />ity distribution in a cross section, zero velocity compo- <br />nents transverse to the main axis, and so on. <br /> <br />c. Two-dimensional flow. It is possible that actual <br />transverse variations will differ so greatly from the <br />assumed variation that streamwise values, determined <br />from a one-dimensional study, will be in significant <br />error. If flow velocities in floodplains are much less than <br />that in the main channel, actual depths everywhere will <br />be greater than those computed on the basis of uniform <br />velocity distribution in the entire cross section. It is <br />possible that the transverse variations will be of greater <br />importance than the streamwise values. This is of partic- <br />ular importance when maximum values of water surface <br />elevation or current velocity are sought. For example, in <br />river bends, high velocities at one bank can lead 10 scour <br />that would not be predicted on the basis of average <br />stream wise values. Also, flow in a bend causes super- <br />elevation of the water surface on the outside of the bend <br />which may be a significant source of flooding from a <br />darn-break wave passing through a steep alpine valley. <br /> <br />2-1 <br />