Evaluation of hydraulic roughness coefficient from field and laboratory data

Rainfall simulation experiments were performed on a number of 1 m by 1 m plots in the field using a rainfall simulator at sites throughout New Mexico and eastern Arizona. Using a point frame device, data were collected on the roughness and cover of each plot. From equilibrium condition hydrographs, hydraulic roughness coefficients were calculated for each plot. The hydraulic roughness coefficient was then related to different surface characteristics and flow variables. Total surface cover density, surface random roughness, and flow Reynold's number were found to be the main variables affecting hydraulic roughness coefficient with total surface cover density as the single most important variable.;A number of laboratory experiments were performed on similar plots to more specifically investigate effects of the main three variables, and the extent of their contributions to the hydraulic roughness coefficient. Laboratory plots were covered with gravel of uniform size at a specific cover density. Laboratory experiments also showed that total surface cover has the most effect on the hydraulic roughness coefficient and would provide a reasonable estimate of hydraulic roughness. Surface random roughness and Reynold number of flow improve upon the original estimate.;Results of the field and laboratory experiments showed that hydraulic roughness coefficient is not constant and is varying with the Reynold's number of the flow. At the very low Reynold's numbers, during rainfall and when overland flow has not yet been developed, as Reynold's number increases, additional water on the plot surface provides a cushion and reduces the resistance effect of rainfall intensity. After overland flow starts developing, depth of flow increases and the upstream-projected area of roughness elements increases, thus increasing the hydraulic roughness coefficient to a maximum value. This maximum represents the point where all roughness elements are effectively covered with water. As Reynold's number further increases, with increase in depth of water, effective roughness decreases and so hydraulic roughness coefficient decreases also, following a pattern similar to the Moody's diagram.;Variations of hydraulic roughness coefficient with total surface cover and flow Reynold's number showed similar trends for both field and laboratory data but opposite trends with changes in random roughness. This difference was attributed to laboratory roughness consisting of nearly uniform gravel but field roughness consisting of nonuniform rocks and vegetation.;As a practical application, it was concluded that an estimate of Manning's n for a surface is equal to the total cover density on a fraction basis. This estimate could be improved by including the surface random roughness and the flow Reynold' s number.