Fluid resistance of composite roughness

A series of flume experiments was conducted to determine the composite resistance of different types of roughness opposing the flow of water through an open channel. Each of the roughness types investigated possessed a distinctive geometrical shape and each type contributed resistance to the flow. The different types of roughness were acting on the same surface and each could be assumed to be uniformly distributed over the surface. The combined or composite resistance was sought in terms of the resistances which the individual roughness types exerted. Within the range of the variables tested, it was found that the total resistance exerted by combined types of roughness is equal to the sum of the resistance forces exerted by each type individually.