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At a given section, Q = VA; thus equation 1 becomes
 Because the hydraulic depth D = A/T and _A = T _Y, the above equation may be written The above equations are all forms of the continuity equation for unsteady flow in open channels. For a rectangular channel or a channel of infinite width, equation 1 may be written where q is the discharge per unit width. Equation of Motion In a steady, uniform flow, the gradient,
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, of the total energy line is equal in magnitude to the "friction slope" 
=
/(C
R) where c is the chezy coefficient and r is the hydraulic radius. Indeed this statement was in a sense taken as the definition of 