Flow resistance in channels with large emergent roughness elements

Reducing uncertainty in flow resistance estimation in natural channels requires elucidation of contributing influences. Surface shear and form drag are the major contributors in channels containing large roughness elements under emergent flow conditions. The two effects can be accounted for in the Darcy-Weisbach and Manning equations by adding their associated, separate friction factors or taking the square root of the sum of the squares of the corresponding Manning coefficients. The friction factor for form drag can be estimated from the drag coefficient and areal density of the roughness elements and the flow depth. Predictions of the combined effect are tested against results of laboratory experiments with different arrangements of emergent cylinders on smooth and rough beds, using experimentally determined drag coefficients for the cylinders. The variation of the overall resistance coefficient with flow condition depends on the dominant influence, decreasing with flow depth when surface shear dominates, and increasing with depth when form drag dominates.