Preliminary study of the interference of surface objects and rainfall in overland flow resistance

Linear superposition approach, which states that composite resistance of different types of roughness elements equals to the sum of individual resistance, has been widely adopted in the study of overland flow hydraulics. The approach assumes that different roughness elements act independently in the flow. This seems to be implausible because roughness elements in shallow overland flow are often close to and interfering with each other. The interference, such as that between rainfall impact and stone generated vertex, may strengthen or restrain flow resistance on one another. To examine this possible interference in flow resistance, a set of flume experiments using surface objects and rainfall as roughness elements were conducted. Since the flow resistance due to each element cannot be measured directly, the linear superposition equation had to be rewritten using friction factor increment as variable. The experiments measured different friction factor increments in flows where Reynolds number was less than 2000 and Froude number less than 2. The result shows that the interference between rainfall and surface roughness is significant and the composite resistance does not statistically equal to the sum of individual resistance. The linear superposition approach is hence invalid in the case of rainfall and surface roughness elements in overland flow resistance.