In soft substrate inter-tidal and sub-tidal environments, blue mussels (Mytilus edulis) form raised beds of enhanced surface roughness.  These beds are rarely homogeneous, being typically characterised by multiple boundaries between regions occupied by mussels and regions of bare substrate.  This spatial structure is thought to enable a balance between the opposing controls of stress protection and food limitation.  Both of these controls are governed strongly by the interaction of the mussels with hydrodynamics, about which little is known.  We addressed this knowledge gap by studying the effects of mussel patch height and density and incident mean flow speed on the flow fields over and downstream of mussel patches via laboratory flume experiments.   Velocity profiles were measured at several along-flume locations in each run, from which several hydrodynamic parameters were calculated.  We found that mussel density is a strong determinant of flow structure over and downstream of mussel patches.  As the incident flow passes over a patch, its vertical profile adjusts to the new, raised and roughened bed conditions.  A second such adjustment occurs as the flow leaves the patch and re-finds the downstream bed.  Turbulent energy is generated primarily at these adjustments to new bed conditions, scales with U0² and was enhanced by increased mussel density to a greater-than-linearly-proportional extent.  This implies that there is advantage to higher density patches, in that they increase turbulent energy, enabling food to be mixed downwards to the mussels to such an extent that the amount of food per mussel increases.