Resistance to Shallow Uniform Flow in Small, Riprap-Lined Drainage Channels

Small drainage channels lined with rock riprap are often designed using Manning’s formula for uniform flow to calculate water depths and velocities. Resistance to flow is quantified by the Manning roughness coefficient n, which is strongly dependent on water depth when flows are shallow in comparison with the size of rock particles protecting the channel beds. To provide reliable formulas for evaluating n in riprap-lined channels, particularly for relatively shallow flows, 142 measurements of flow resistance carried out by others in several experimental studies were assembled and analyzed. Two formulas founded on a theoretical assessment of flow resistance are presented in which roughness heights are calculated based on measures of the distribution of riprap particle diameters. The formulas are compared with other equations that have been developed specifically to predict n in riprap-lined drainage channels, and they are shown to provide significantly more accurate estimates of flow resistance.