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<br />weighted subsection conveyance. Subsection conveyances are summed <br />to get a total value for the reach which is used to calculate friction <br />loss. If more than three subsections are required or if overbank <br />n-values need to vary in the vertical, the optional format may be <br />used as illustrated in exhibit 5. The values may be chanQed at any <br />reach by inserting an n-value card (NC or NV) in front of the Xl- <br />card for the upstream end of that reach. <br /> <br />c. Reach Lenath. The cross section orovides two dimensions for <br />defining the geometric model~ the distance between cross sections <br />provides the third. Each subsection must have a reach length and <br />it extends from the present section (where entered) to "the downstream <br />cross section. <br /> <br />d. Manning n-Values. An n-value is required for each subsection. <br />It will be uti11zed at all cross sections unless chanaed. It is not <br />possible to automatically change n-values with respect to time. <br /> <br />Only one n-value can be specified for an overbank subsection, but <br />n can vary with either discharge or elevation in the main channel. <br />When n varies with discharge, the first one should be minus. In <br />all cases, code n-va1ues in sequence from high to low discharge <br />or elevation. <br /> <br />e. Bridges. This model has no provision for calculating flow at <br />bridges except by normal backwater calculations. Subtract pier <br />widths from the total width to reflect net flow area if detailed <br />scour is of interest at the bridge. Otherwise ignore bridges and <br />match water surface profiles by adjusting n-values to avoid the short <br />time intervals required for analyzing scour at bridges. <br /> <br />f. Weirs. The abrupt head loss at a weir may be reflected by calcu- <br />lating cr1tical depth or, if the weir is gated, an operating pool may be <br />specified (X5-card). When the operating pool option is used, a head <br />loss may be specified to simulate open river conditions at high flow. <br /> <br />The presence of a weir causes the program to establish an outflow <br />point there and sediment quantities are accumulated at that point. <br />Trap efficiency is calculated between each inflow and outflow point. <br />Up to 20 weirs may be entered (see "Operating rule or rating curve."). <br /> <br />g. Movable Bed and Fixed Bed. Part or all of the channel portion of <br />each cross section can be divided into movable and fixed bed portions. <br />Scour and deposition will cause the movable bed to fall and rise by <br />changing the cross section elevations after each cycle through the <br />program. (A cycle is illustrated in figure 1.) <br /> <br />10 <br />