Revising the definition of fluid mud by establishing new protocols for rheological measurements

Fluid mud can be described as a mixture of water, organic matter and mainly cohesive mineral sediment that is usually found in estuaries and in rivers with low-intensity currents. It is in a transient state and can densify over time unless mixing (produced by mechanical and/or natural forces) prevents its gelling. Typically, fluid mud exhibits bulk densities between 1080 and 1200 kg/m3. It has a weak strength that, in low energetic conditions, develops over time forming a structured bed of considerably higher rigidity. The current definitions of fluid mud are based on the density of the sediment. These definitions are ambiguous since the proposed critical parameters do not account for the possible strength developments in fluid mud. We propose to use rheological properties for distinguishing the fluid mud layer from the suspension layer above and the consolidated mud layer underneath. The protocols involve the use of a rotational bob (Couette) rheometer that can perform oscillations, shear stress and shear rate controlled experiments. A number of natural mud samples were used for testing the protocols.