Morphodynamics of a cyclic prograding delta: the Red River, Vietnam

Abstract

River deltas are inhabited by over 60% of the world population, and are, consequently, of paramount agricultural and economical importance. They constitute unique wetland envi ronments which gives river deltas ecological importance as well. Additionally, many deltas contain large accumulations of oil, gas, and coal, which further increase their economical value. More than 50% of the world population lives in Asia, most of which on the large deltas of the Ganges, Brahmaputra, Irrawaddy, Chao Praya, Mekong, Pearl, Yangtze, Yel low, and the Red River. The high population density that characterizes these deltas implies a large pressure on the natural resources, resulting in a decrease in sediment supply (dam construction, irrigation works), subsidence (lowering of the groundwater table), and loss of the natural sea defence (destruction of mangroves). Global climate change may provide additional threats through accelerated sea-level rise and changing weather patterns. In South-East Asia, river floods and typhoons are expected to occur more often and as a direct result, coastal erosion and flooding will become increasingly common. Like many other Asian deltas, the delta of the Red River, in the North of Vietnam, is char acterized by an alongshore alternation of irregular but rapid accretion and severe erosion. Locally, coastal erosion has been so severe (exceeding 2 km in the past decades) that entire villages have been lost to the sea; in contrast, the coastline experienced accretion up to 5 km elsewhere. The central objective of my research was to under stand the mechanisms that regulate the medium term (time scales of decades to centuries) patterns of erosion and accretion of the Red River Delta (RRD), and in particular that of the main branch of the Red River, the Ba Lat.

The growth of the Ba Lat delta is cyclic, with a barrier that forms alongside the river mouth every 100 years. The present day barrier is about 30 km long. Alternating phases of seaward delta progradation and subsequent development of barrier-spit systems are commonly observed near many river mouths. However, the explanations offered in literature for this cyclic progradation vary considerably. The classical model for barrier development is that developed for the Mississippi delta. In their model, delta lobe switching results in a period of sediment starvation, during which former river mouth sediments are reworked into a barrier. This appears to be more or less the same for the Ebro and the Huanghe delta, but this type of barrier formation may not be very representative of other delta systems worldwide. For example, barriers on the Mississippi form on a retreating delta, while in many delta systems (including the Ba Lat) barriers are associated with actively growing delta systems. In the Ba Lat Delta, there are no indications that there was a change in sea level, wave activity, or sediment supply within the period of barrier formation. Additionally, the evolution of other branches of the Red River delta such as the Tra Ly and Day rivers is also cyclic: the Day River is just developing barriers while the Tra Ly is in-between the Ba Lat and the Day river. It seems that the cyclic growth pattern of the Ba Lat is regulated by internal dynamics of the system rather than to changing external forcing.