TURBIDITY CURRENTS GENERATED AT RIVER MOUTHS DURING EXCEPTIONAL DISCHARGES TO THE WORLD OCEANS

A marine hyperpycnal plume is a particular kind of turbidity current o ccurring at a river mouth when the concentration of suspended sediment is so large that the density of the river water is greater than the d ensity of sea water. The plume can then plunge and possibly erode the seafloor to become self-maintained for a particular period of time (ho urs to weeks). Frequency of hyperpycnal plumes emanating from river di scharge can be predicted with knowledge of rating curve characteristic s, particularly during flood conditions. Examples of these curves are shown for middle-sized North American rivers. Semi-empirical relations hips among average discharge, average sediment concentration, and the discharge during flood are proposed and applied to 150 world rivers. R esults show the importance of small and medium sized rivers in their a bility to trigger underflow at their mouth. There are at least nine '' dirty'' rivers that may trigger underflows during one or more periods of the year. Most other rivers are cleaner and have hyperpycnal plumes only during floods. Large rivers do not generate underflows at their mouth because sediment retention within their expansive coastal flood plains effectively reduces the upper limit of the suspended concentrat ion. Underflow transport may be an important process in marine-delta c onstruction and should be considered in sedimentary basin-fill modelin g. Proposed equations and nomograms may assist engineers in infrastruc ture design seaward of a river mouth.