Simple Modification of Manning-Strickler Formula for Large-Scale Roughness

Large-scale roughness renders the flow resistance underestimated with the Manning-Strickler (MS) formula. This can be corrected simply by considering the roughness-induced flow constriction. Two modified MS formulas have been therefore derived for pipe and open-channel flows, respectively. They are applicable for small- to large-scale roughness. Without making any adjustment of the constants involved, both formulas are successfully validated by experimental data, particularly for flows significantly affected by the large-scale roughness.