Discussion of “Management of Fluid Mud in Estuaries, Bays, and Lakes. I: Present State of Understanding on Character and Behavior” by W. H. McAnally, C. Friedrichs, D. Hamilton, E. Hayter, P. Shrestha, H. Rodriguez, A. Sheremet, and A. Teeter

In two companion papers, the ASCE task committee on fluid mud management presented their findings. While the papers provide a comprehensive updated overview, this discusser feels that some important aspects of fluid mud formation and transport behavior are overlooked. The present discussion is primarily focused on the first paper and highlights the formation and transport of fluid mud in a well-mixed oscillatory tidal and estuarine environment.

As discussed in the paper, and also in a volume edited by Dronkers and van Leussen (1988), the flocculation behavior and formation of fluid mud depend, among others, on the textural composition and mineralogy of mud grains, and salinity of ambient water. Such behavior has been studied in an experiment conducted at the Land Reclamation Project laboratory of Delft Hydraulics and Bangladesh Water Development Board (Barua 1991a). The mud samples were taken from Sandwip Channel, a tide-dominated estuary located on the eastern side of the Ganges-Brahmaputra river mouth. The channel is floored by thick mud, and high sedimentation rates have been observed in sheltered areas (Barua 1990, 1991b). The results of the experiment were modeled as two relations of decreasing suspended sediment concentration with time and salinity:andwhere C is suspended sediment concentration in g/L as a function of salinity and time, s is salinity in ppt, and t is time in min. The textural compositions of the samples were: median diameters from $6\text{to}11\mu \mathrm{m}$ , and clay contents from 14.9% to 34.1%. The clay-size mineralogy was dominated by illite (52% illite, 23% kaolinite, 15% smectite, and 10% others). To avoid hindered settling effect (Owen 1970), the initial concentration $\left({\mathrm{C}}_{\mathrm{o}}\right)$ was set at a value lower than $10\mathrm{g}∕\mathrm{L}$ . The settling behavior of the mud presented in Eqs. (1) - (2), has been used to explain the formation of fluid mud layer at the channel bottom on tidal and seasonal time-scales. The formation mostly occurs during high salinity season of the year around neap tide slack water that has duration of about $15\min$ .

The second most important mechanism of estuarine sedimentation is the process of residual transport. Originally proposed by Postma (1967), this process explains resuspension and net transport of finer sediments upstream into a well-mixed estuary primarily due to tidal asymmetry, scour lag, and settling lag. This results in the well-known turbidity maximum zone in such estuaries. Such a process has been observed in the Ganges-Brahmautra River mouth and explains settling of finer sediments in tidal flats (Barua 1990). The process is further enhanced in a wide estuary such as the Ganges-Brahmaputra river mouth estuary that is horizontally stratified into distinct flood and ebb channels. Net import of resuspended fine sediments was observed through flood channels that are characterized by long-term sedimentation problems (Barua 1990; Barua et al. 1994).