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When used to estimate discharge, the water depth is measured at some distance from a control (such as weir or free outfall) and other variables are either calculated or measured. Backwater profile calculations are iterative and are performed with a guessed value of discharge that is adjusted until the calculated depth at the known distance from the control matches the measured depth. For a more complete explanation with examples of backwater profile calculations, see an open channel flow text or manual, (e.g., Sturm 2001).ManningвЂ™s Equation: Robert Manning developed ManningвЂ™s equation (equation 4.2) in the 19th century to estimate discharge for uniform open channel flow using cross-sectional area, hydraulic radius, energy grade-line slope, and an empirically defined roughness coefficient (n) (Sturm 2001).The potential measurement uncertainty in roughness coefficient, however, is large, and it is recommended that ManningвЂ™s equation be used only as a last resort to estimate discharge in stormwater applications. Selection of a discharge measurement method is dependent on many factors, including accuracy, cost, range of discharge, and site conditions. For further discussion of individual factors, see Chapter 4 in the вЂњWater Measurement ManualвЂќ (U.S. Bureau of Reclamation 2001).ДЄll the discharge measurement principles listed above require a measurement of water depth and a known channel (or pipe, etc.) geometry to calculate discharge. In the case of a weir, the water depth is measured behind the weir and weir equations (discussed in detail below) convert depth to an estimated discharge over the weir.