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<br />9 <br /> <br />, <br /> <br />the water surface profile may be one of nine curves in three major <br /> <br />classifications according to slope. The slope classifications are <br />mild, critical and steep (Fig. 2-2). Classifications are based upon a <br />comparison of actual channel slope to that slope required to maintain <br /> <br />the given discharge at critical flow, where critical flow is defined <br /> <br />in Chapter 1. A mild slope is that which maintains a depth greater <br />than critical and a steep slope maintains a depth which is less than <br /> <br />critical. For each of the three types of slopes, there are three <br /> <br />designations of flow profile. These are shown on Table 2-1, together <br /> <br />with the type of flow encountered in each profile. <br /> <br /> <br />A determination of initial conditions is necessary prior to <br /> <br />beginning the solution of a backwater curve. This determination is <br /> <br />most easily accomplished at a control section. A control section is <br /> <br />defined as a section with a unique relation between stage and discharge. <br /> <br />This requirement is satisfied in a critical flow section. If flow in a <br /> <br />channel upstream of a control section is tranquil, flow influences at <br /> <br />the control section will be propagated in an upstream direction so that <br />solution of a water surface profile may begin at the control section <br />and proceed upstream. The Cl, Ml and M2 profiles are solved in an <br /> <br />upstream direction from a critical flow section. Flow influences at <br /> <br />a control section for rapid flows are propagated in a downstream <br /> <br />direction. Solution of the flow profiles for rapid flows must proceed <br />in a downstream direction from a control section. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br />