Surface Roughness for Shallow Overland Flow on Crushed Stone Surfaces

Results of laboratory experiments carried out to determine the effective surface roughness for shallow overland flow as a function of the runoff rate, roughness element height, and underlying soil condition are presented. This work was conducted to extend the understanding of the mechanics of shallow overland flows based on the relationships between the Reynolds number, Froude number, and surface resistance over a wide range of conditions. Results exhibit a strong dependence of the effective roughness on the ratio between the depth of flow and the height of the roughness element, and a strong inverse relationship was found between Manningʹs n (and the Darcy-Weisbach friction factor f) and the Froude number. Three distinct subcritical flow regimes were identified: (1) Submerged flow ( < 300); (2) sheet flow ( > 1,200); and (3) transitional (partially submerged) flow (300 < < 1,200). Analysis and synthesis of laboratory data allowed relationships between the Reynolds and Froude numbers, and Manningʹs n, which are suitable for practical engineering applications, to be established in this study.