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The mechanisms of fluid mud formation and its characteristics are far from clear in the deep-dredged waterway in Xiamen estuarine bay. Some experiments were conducted in the muddy estuarine bay of Xiamen when Super Typhoon Meranti, which was the strongest typhoon to have affected the area over the last 70 years, struck and after it passed. Contrary to expectation, results reveal that typhoon-induced waves are not the main hydrodynamic forcing that transport sediment. The influence of fluvial flooding and ebb tidal flow results in high concentrations (greater than 14 kg/m3) of suspended sediment in a layer near the bed, which may either deposit or be released directly into the waterway to form fluid mud. Further, new indicators are proposed, namely, river discharge and fluvial flood duration, to describe quantitatively the conditions under which fluid mud forms; it is in fact likely to form in the waterway when a discharge greater than 1,000 m3/s lasts for more than one day. The new indicators may be applied as reference by waterway administrators in their operations. [ABSTRACT FROM AUTHOR]