Currently, two formulas are being used in drainage pipe diameter design; the first being based on postulating the presence of uniform flow while the second one derived by considering non-uniform flow conditions. For a given set of conditions the conduit carrying capacity computed based upon the second formula exceeds the result calculated on the basis of the first by about 70%. While same hydraulic assumptions have been considered in deriving either formula, once such discrepancy arises, which formula should be relied on and to what extent? In this research, the carrying capacity of the corrugated drainage pipes was experimentally determined in such a way that the above assumptions be valid. The results, then, were compared with those obtained when using common formulas. Data compiled based on the discharges, computed by taking into account uniform flow formula and using the applied longitudinal slope, indicated the presence of free surface flow along the pipes, while discharges determined based on non-uniform flow formula run with excess internal pressure especially for steeper longitudinal slopes. This implies that a pipe diameter determined based on uniform flow formula is capable of carrying discharges higher than the predicted design discharge, while it cannot deliver the design discharge with a desirable flow condition if the nonuniform flow formula is employed. Since the increase in a pipe carrying capacity and the presence of excess internal pressure, attributed to the use of uniform and nonuniform flow formulas respectively, are affected by the longitudinal slope, the effects of longitudinal slope as well as clogging conditions on pipe carrying capacity were also studied and their practical results were reported.
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