Collaborative Research: Fate of Irrawaddy and Salween River Sediment: Relative Importance of Oceanographic and Tectonic Controls

有5亿人生活在河流三角洲，由于地面沉降加剧、增长率下降和海平面加速上升，其中许多地区现在被认为处于危险之中。这些沿海地区下沉的土地大大增加了风暴潮和洪水的威胁。例如，2008年，纳尔吉斯气旋横扫缅甸伊洛瓦底江三角洲，造成13.8万多人死亡，2015年，同一地区的强季风降雨导致低洼地区洪水泛滥，导致20万人流离失所。为了了解和预测河流三角洲的未来演变，有必要了解河流的沉积物是如何运输和输送到三角洲的陆上和海上区域的，因为这是三角洲形成的物质。该项目将提供对缅甸伊洛瓦底江-萨尔温江三角洲（I-S）的了解，这可能是地球上最后一个我们缺乏完整图景的主要三角洲系统。具体而言，该项目将研究控制沉积物输送的构造和海洋学因素，以及三角洲目前的生长模式和速度。这项研究的结果将为一个拥有5000万人口的敏感三角洲环境的未来发展提供信息，并增加我们对全球三角洲的一般认识。该项目还将涉及对美国本科生和研究生的指导和培训，并将吸引缅甸科学家和学生，为与美国同行的培训和合作提供机会。据估计，每年由I-S系统携带的600公吨河流沉积物中的大部分被输送到安达曼海北部广阔的大陆架，该大陆架受强潮、季风条件和周期性气旋的影响；然而，这种材料的命运是未知的。现有的水深数据和稀疏的初步调查表明，陆架上存在一种水下三角洲发育模式，与之前研究过的其他主要三角洲形成鲜明对比。特别是，陆架在表面形态和沉积物结构上表现出戏剧性的双边（东西）不对称：伊洛瓦底江东部河口处有一个平缓的沙坡道，而在邻近的马尔塔班湾西部，陆架内侧到中部有一个泥泞的斜形。实地和基于模型的实验将使研究小组能够对比海洋分选、运输过程和潜在构造在创造这种不寻常的三角洲表达中的相对作用。地球物理观测、取心和实验室研究将详细描述其结构，并量化全新世和新近沉积在陆架上的沉积物的数量、来源和分布。为了探索沉积物从河流源头到沉积汇的运输路径，将使用一个数值模型来解释河流输入、波浪、潮汐和风力驱动的水流对沿海海洋内河流沉积物扩散的影响。模式运行将代表东南和西北季风期间以及高流量时期的情况。该模式还将用于利用气旋产生的波浪和风场的表示来探索在大风暴期间加强输送的可能性。

Half a billion humans live on river deltas, and many of these areas are now considered at risk as a result of increasing land subsidence, decreasing growth rate, and accelerating sea-level rise. Sinking land in these coastal areas greatly increases the threat from storm surge and flooding. For example, Cyclone Nargis tracked across the Irrawaddy delta in Myanmar in 2008 resulting in over 138,000 fatalities, and heavy monsoon rains in this same area in 2015 displaced 200,000 people as a result of flooding in low lying areas. To understand and predict the future evolution of river deltas, it is necessary to know how sediment from the river is transported and delivered to the onshore and offshore regions of the delta, as this is the material from which deltas arise. This project will provide an understanding of the Irrawaddy-Salween (I-S) delta in Myanmar, perhaps the Earth's last remaining major delta system for which we lack a complete picture. Specifically, the project will examine the tectonic and oceanographic factors that control the delivery of sediment, and the present pattern and rates of delta growth. This results of this study will inform the future development of a sensitive deltaic setting home to 50 million people, and add to our general knowledge of deltas worldwide. The project will also involve the mentoring and training of US undergraduate and graduate students, and will engage Myanmar scientists and students, providing opportunities for training and collaboration with US counterparts.Much of the estimated 600 Mt of river sediment carried by the I-S system annually is delivered to a wide continental shelf in the northern Andaman Sea that is influenced by strong tides, monsoon conditions, and periodic cyclones; however the fate of this material is unknown. Existing bathymetric data and sparse initial surveys hint at a mode of subaqueous delta development on the shelf which contrasts markedly from other previously studied major deltas. In particular, the shelf exhibits a dramatic bilateral (east-west) asymmetry of the surface morphology and sediment texture: a gently sloping sandy ramp off the eastern mouths of the Irrawaddy, and a muddy inner- to mid-shelf clinoform in the adjacent western Gulf of Martaban. Field- and modeling-based experiments will enable the research team to contrast the relative roles of oceanographic sorting, transport processes, and underlying tectonics in creating this unusual deltaic expression. Geophysical observations, coring and laboratory studies will detail the architecture and quantify the amounts, source, and distribution of Holocene and recent sediment accumulating on the shelf. To explore transport pathways of sediment from the fluvial source to depositional sinks, a numerical model will be used to account for the effects of river input, waves, tidal- and wind-driven currents on the dispersal of river sediment within the coastal ocean. Model runs will represent conditions during the SE and NW monsoons, and during times of high discharge. The model will also be used to explore the potential for enhanced transport during large storms using representations of the cyclone-generated wave and wind field.