Fluid mud is a high-concentration colloidal suspension of fine sediment particles (< 63 µm, with often a high percentage of clay < 4 µm). It is formed by settling of mudflocs into a near-bottom suspended layer at a higher rate than the dewatering rate, or by fluidization and/or liquefaction of an underconsolidated mud bottom.
As long as the concentration is below the gel point (formation of an interconnected matrix of particle bonds) fluid mud can flow as a laminar near-bed layer, entrained by the flow or by pressure gradients of the overlying water mass. It can also flow as a gravity current along a slope. At rest, the fluid mud layer slowly consolidates - a process that can take a long time, proportional to the square of the layer thickness. The sediment concentration of fluid mud layers exceed 10 kgm-3 and can reach more than 100 kgm-3.
Fluid mud layers can arise from the pressure variations exerted by waves on an underconsolidated mud bottom. Once formed, the fluid mud layer absorbs part of the wave energy, so that wave action will be strongly damped.