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29 <br />Equation (Eq. 2-2) for open channel flow and are used to describe the <br />conditions at a single transect (Stalnaker and Arnette 1976). <br />Q = 1 . A , R2/3 , S1/2 (2-2) <br />n <br />where Q = discharge (m3/s), <br />n = roughness coefficient, <br />A = cross-sectional area (m2), <br />R = hydraulic radius (m) = A/wetted perimeter, and <br />S = energy slope. <br />The Manning Equation can also be written as <br />V = 1 . R2/3 , S1/2 (2-3) <br />n ' <br />where V = mean velocity across transect (m/s). <br />Field data consisting of cross-sectional profiles (transverse <br />distance and elevation), velocity measurements, and water surface <br />slope in the vicinity of a transect are used to calculate the <br />roughness coefficient, n, in Eq. 2-2. Assuming the values of n and S <br />are independent of flow, an iterative computer program can then be <br />applied to find values for average velocity, wetted perimeter, <br />cross-sectional area, maximum depth, or hydraulic radius at discharges <br />other than that observed in the calibration data set. These data are <br />then used to derive a habitat-discharge curve (Sect. 2.4.1) with a <br />minimum of field work. The identification of critical areas along a <br />stream reach and proper transect placement remain important <br />prerequisites with this method. Also, significant errors can result <br />from using R-2 Cross to predict hydraulic conditions at flows greater <br />than 250% or less than 40% of the calibration flow (Bovee and Milhous <br />1978).