SEDIMENT TRANSPORT IN THE LOWER HUANGHE (YELLOW RIVER), CHINA: ASSESSING THE PHYSICAL CONDITIONS THAT PRODUCE HYPERPYCNAL FLOW EVENTS

The sediment deposits associated with hyperpycnal flows that emanate from river mouths represent an important process for growing ancient shelf margins, and these deposits also store information that can be used to reconstruct ancient river morphological properties. However, current studies of hyperpycnal flows are insufficient for addressing the conditions of fluvial input that produced the hyperpycnal flows. This probably occurs because, on one hand, the rock record reveals the existence of hyperpycnal flow on shelf-margins; however on the other hand, hyperpycnal flows are not common in modern rivers, and so it is difficult to assess the physical conditions that lead fluvial systems to generate hyperpycnal conditions, much less connect this to hyperpycnal flow deposits. These facts motivate us to examine the physics of hyperpycnal flows from the river side. Here, we use a water-sediment dataset of Huanghe (Yellow River), China, which is an end-member of fluvial system, with a frequent disposition to hyperpycnal flows. Arguably, the Huanghe is the ONLY large river in the world with reoccurring hyperpycnal flow. A very high sediment load throughout the lower reach of the Huanghe (> 800 km) provides data that enable for a study of the spatial changes in sediment transport capacity, in association with hydrological conditions. Our study indicates that: 1) suspended particle-laden flows, comprised primarily of fine sediment (silt and clay), possess a greater sediment carrying capacity than is otherwise predicted using conventional sediment transport formulae, e.g., Engelund Hansen formulation; 2) the spatial variation of sediment transport capacity suggests that no overall equilibrium hyperpycnal flow exists: the hyperpycnal flow conditions initiated upstream propagate downstream, with a tendency for sediment deposition to be enhanced near the delta due to backwater effects; and 3) the formation of hyperpycnal flows requires certain preferential compositions of discharge, sediment load, and grain size grading, which implies a particle grading signature must exist for the hyperpycnal flow deposit. Our study is helpful because it bridges river dynamics that produce hyperpycnal flow deposits, so that it is possible to link sediment stratigraphy found on the shelf basin with updip fluvial conditions.