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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />2b illustrates that the water surface and energy grade line profiles will be different for the trickle or low- <br />flow and the main channel flow zones. <br /> <br />e <br /> <br />After review of the crest and upstream hydraulics, the analysis proceeds to the supercritical flow and the <br />hydraulic jump downstream. It is here that the designer should give special consideration to the potential <br />of reverse rollers and avoid them in boatable channels and, where practicable, in grass-lined channels. <br />Little flow dispersal from the trickle or low-flow zone to the main zone occurs through the supercritical <br />portion of the drop. (Flow expansion is more likely downstream of the jump.) Therefore, unit discharge <br />determined at the crest for either the trickle channel or the main portion of the drop is assumed to remain <br />constant. The required basin length varies between these zones. Baffle chutes are the only type of drop <br />where this distinction is not significant because the baffles break up the flow patterns and spread the flow <br />more evenly over the width of the channel. <br /> <br />With the exception of baffle chute drops, separate analysis should be performed to evaluate the main <br />drop and trickle or low-flow channel zones, as follows: <br /> <br />Critical depth, Y" is determined for the entire section area. The subscript (,) or (m) is added to refer to the <br /> <br />trickle or low-flow zone or main channel zone, respectively. For example, in the main channel zone: <br /> <br />Yom = El, - Elm <br /> <br />(HS-1) <br /> <br />e <br /> <br />Similarly, In the trickle or low-flow channel zone: <br /> <br />Yet =El, -El, <br /> <br />(HS-2) <br /> <br />in which: <br /> <br />EI, = critical water surface elevation <br /> <br />Elm = elevation of the main channel at the drop crest <br /> <br />EI, = elevation of the trickle or low-flow channel at the drop crest <br /> <br />The remaining hydraulic parameters, such as critical velocity, V, (ftIsec), energy grade line, EGL, and unit <br />discharge, q( cfs/ft), are determined separately for the main and trickle or low-flow channel zones by <br />equations of the form: <br /> <br />v =( y)1I2 <br />c g, <br /> <br />(HS-3) <br /> <br />v' <br />EGL = Y, + -"- + Elm (or El,) at the drop crest <br />2g <br /> <br />(HS-4) <br /> <br />e <br /> <br />HS-12 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />