Management of Fluid Mud in Estuaries, Bays, and Lakes. I: Present State of Understanding on Character and Behavior

Fluid mud is a high concentration aqueous suspension of fine-grained sediment in which settling is substantially hindered. It constitutes a significant management problem in rivers, lakes, estuaries, and shelves by impeding navigation, reducing water quality and damaging equipment. Fluid mud accumulations have been observed in numerous locations worldwide, including Savannah Harbor, U.S., the Severn Estuary, U.K., and the Amazon River Delta, Brazil. This paper describes the present state of knowledge on fluid mud characteristics, processes, and modeling. Fluid mud consists of water, clay-sized particles, and organic material and displays a variety of rheological behaviors ranging from elastic to pseudo-plastic. It forms by three principle mechanisms: (1) the rate of sediment aggregation and settling into the near-bottom layer exceeds the dewatering rate of the suspension; (2) soft sediment beds fluidized by wave agitation; and (3) convergence of horizontally advected suspensions. Once formed, fluid mud is transported vertically by entrainment and horizontally by shear flows, gravity, and streaming. If not resuspended, it slowly consolidates to form bed material. Quantitative relationships have been formulated for key fluid mud formation and movement mechanisms, but they rely on empirical coefficients that are often site- or situation-specific and are not generally transferable. Research to define general relationships is needed.