Roughness and Energy Losses Induced by Mussel Growth on the Walls of Hydraulic Structures and Application to a Water Transfer Project

Mussel biofouling increases energy losses in hydraulic structures. The first contribution of this paper is the quantification of the mussel-induced equivalent sand roughness k_s as function of the mussel attachment density N and the shell length L. Laboratory experiments reveal that k_s/L ≈ 1.5 for a continuous regular layer of mussels, which is found for N L² > 1.2. For 0.5 < N L² < 1.2, the mussels form a continuous irregular roughness layer with increased values of k_s/L of up to 2.4. These geometrical irregularities are interpreted as macro-roughness elements, that is, roughness elements with a spatial scale larger than that of an individual mussel. For N L² < 0.5, the density of the irregularities is too low to act as macro-roughness elements leading to k_s/L < 1.5. The second contribution is the establishment of a threshold criterion for the importance of filtering activity on k_s based on data from the here reported experiments and data reported in literature in other configurations and/or with other mussel species. It is found that laboratory conditions are often close to the threshold value but that mussel filtering is always negligible in large hydraulic structures. The third contribution is the development of a method based on 3-D numerical simulations for estimating a Darcy-Weisbach friction factor f for walls that are only partially covered with patches of mussels. An application example illustrates how the thus obtained f can be used in a 1-D model for quantifying the additional energy losses in large water transfer projects.