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OF Additionally, many controls are not perpendicular to the channel, but lie diagonally across the channel. Figure 21 illustrates an example of a diagonal control. Velocity is a vector having both magnitude and direction. At low flows, the direction of flow is perpendicular to the control and, therefore, is not parallel to the banks of the channel. At high flows, the direction of flow is more closely parallel to the channel banks. Intuitively one wishes to install transects at right angles to the channel. If a right angle transect is placed over a diagonal control, virtually all of the eleva Lions measured will be incorrect. Therefore, the control transects must be parallel to the control, even though such practice-might be uncom- fortable to the i nvesti oator. _ __ _ _ J Figure 21_ Stream reach with Photo courtesy of diagonal hydraulic control (marked in white). Tim Cochnauer, Idaho Dept. of Fish and Game. Islands often provide unique transect either the head or the foot of the island, hydraulic control. If the flow around the f1ow-prof ile is started. at., the foot of'- the Tigur2 17). If the flow is supercritical, 1,e at the head of the island (Station 3+30 placement problems because or both, may act as a island is subcritical, the island (Station 1+65, the flow control point would in Figure 19). Channels with large, irregularly spaced roughness elements are most commonly represented by streams flowing through boulder fields (Figure 22). In this type of stream, identification of controls is usually no problem. The problem arises from segmentation of the cross section to define the velocity distribution. Manning`s n tends to fluctuate widely at specific ?. l oca .? ons in boulder strewn rivers. I f only the mean croszs-sectional velocity is desired, a single value of n may be used successfully for the