SPATIOTEMPORAL CHANNEL-BED EVOLUTION PATTERNS OBSERVED FOR THE HUANGHE (YELLOW RIVER), CHINA: IMPLICATIONS FOR EVALUATING SYSTEM RESPONSE AND COMPLEXITY TO EXTERNAL PERTURBATIONS

The Huanghe (Yellow River), China, is an end-member fluvial system because it is characterized by very high sediment loads, variable water discharge over annual and decadal timescales, and substantial anthropogenic influences. Over the past several centuries, the channel bed of the Huanghe has aggraded significantly, resulting in several catastrophic avulsions. In order to mitigate channel-bed aggradation, in 2002, Chinese engineers implemented an annual Water and Sediment Discharge Regulation (WSDR) period when, over a two-week period, a controlled flood wave designed to erode the channel bed is triggered by releasing water from a network of upstream dams and reservoirs. Here, we present the results of an extensive time-series field survey that measured fluid and sediment properties to assess the effect of a WSDR flood wave on a portion of the river located ~80 km upstream of the ocean outlet. This controlled experiment represents a classic external perturbation to a fluvial-deltaic system; the morphodynamic responses, however, were anything but predictable. For example, multibeam bathymetric data illuminate the uniqueness of channel-bed properties, including a broad, flat, and shallow sand-bed devoid of a thalweg or bedforms in the straight-reach segments of the Huanghe. Alternatively, in bend segments, the channel-bed deepens significantly, and longitudinal dunes emerge as the dominant bed features. Interestingly, as the flood wave progresses over time, the channel bed in bend segments rapidly erodes upstream and downstream, thus propagating dunes in both directions. In order to further ascertain linkages between the fluid flow, sediment transport, and morphological evolution of the bed, we couple water velocity, suspended sediment concentration, and bedload transport datasets to standard physical models. The results are then used to constrain a numerical model that assesses stability of the channel bed over the duration of the flood wave. In effect, this study shows the inherent unpredictability that emerges between contemporaneously evolving “external” (allogenic) and “internal” (autogenic) processes. Finally, the results of this field and numerical investigation are used to constrain a prediction for the long-term stratigraphic development of the Huanghe fluvial-deltaic system.