Direct measurement of roughness of bed covered with gravel and ripples

In this study a series of experiments were conducted on different beds and under different flow conditions. The tests included a series of uni-directional near wall measurements of mean velocity. These tests were conducted in a channel covered with two types of roughness elements: i) bed covered with spherical aggregates with a diameter between 4 and 9.5mm ii) bed covered with triangular elements (an idealization of rippled beds) with a height of 13.6mm. The result shows that equivalent sand roughness associated with aggregate is of the order of the magnitude of the grain size. For rippled-like beds, the result of this study shows that this type of roughness can be adequately described with only one length scale (such as Nikuradse equivalent sand roughness) and that Dirling's empirical relation can best predict this length scale