全新世以来亚洲七个主要河口三角洲的生长极限

Modeling the growth limit of seven major Holocene river deltas in Asia

由河流入海泥沙在河口及其邻近区域不断堆积而逐渐形成的河口三角洲存在地貌学意义的生长极限。针对入海泥沙显著减少的背景,依据概念几何模型刻画了亚洲7个主要河口三角洲的发育规模,并探讨其滞留指数、临界输沙率和生长极限。结果表明:河口三角洲的规模与沉积物入海通量显著正相关;全新世期间,各河流均有超过一半的泥沙向外海逃逸;目前各河流入海泥沙通量低于保持其陆上面积的临界输沙率;在无海岸防护工程前提下,未来长江、珠江、红河、湄公河和印度河的三角洲平原面积和体积都将较目前大幅度减少,而伊洛瓦底江和恒河-布拉马普特拉河将增加。基于概念几何模型刻画河口三角洲的系统行为对其地貌演化规律的研究具有重要参考和指导意义。

River deltas grow in response to sediment discharge from the land,and there is a limit of growth.A conceptual geometric model is used in this paper,based on the principle of mass conservation,to investigate the impact of the significant decrease in sediment supply to the growth of deltas taking seven large Asian river deltas as examples.Furthermore,the sediment retention index for the Holocene period,the critical river input for maintenance of a delta and the growth limit without human interferences are discussed.Preliminary analyses of these deltas indicate that the area of delta plain is roughly in an order of 103~104 km^2,and the volumes of 101~103 km^3,and both are significantly positively correlated with the fluvial flux before the large numbers of dams being constructed.The range of the Holocene retention index varies between36%~54%,with an average of 45%,indicating that more than half of the sediment escaped offshore and supported the deposition of shelf mud or deep sea fan.For the Yangtze,the Pearl,the Red,the Mekong and the Indus rivers,their sediment discharges are now below the level required to maintain the area of the sub-aerial deltaic plain and the volume of the entire deltaic deposits,except for the Irrawaddy and the Ganges-Brahmaputra rivers,of which the river discharges are higher than the critical level required to maintain their total volumes,but lower than the critical level required to maintain deltaic plain areas.Without coastal protection projects,the area and volume for the former five deltas would drop sharply,but increase for the latter two.The present study has established the interrelationships between the growth magnitude,Holocene sediment retention index,the critical fluvial sediment flux and the growth limit of the seven Asian major river deltas,demonstrating that the method based on the conceptual geometric model has a great potential to predict the long-term evolution of river deltas.