Climatic and Autogenic Controls on the Morphodynamics of Mega-Rivers: Modelling Sediment Flux in the Alluvial Transfer Zone

世界上最大的河流每年运送约190亿吨沉积物，其中很大一部分被隔离在占世界人口14%的大三角洲中。这些大型三角洲中的大多数（约70%）正受到海平面上升、地面沉降和三角洲建设所需的河流沉积物供应减少的威胁。然而，虽然许多三角洲存在海平面上升和下沉的测量和预测，但量化河流沉积物供应历史变化的数据很少，这限制了我们对三角洲建设与气候波动之间关系的理解。这种情况反映了控制河流泥沙负荷的复杂性，其中包括气候和高地地区土地利用变化的影响、大坝建设以及世界主要河流的低地河段（“泥沙转移带”）的广大洪泛区内洪水驱动的泥沙储存和再动员。该项目将首次全面量化世界上最大的河流之一（湄公河）对河流泥沙通量的控制，从而对气候变化、河流过程和流向三角洲带和海洋的泥沙通量之间的关系产生新的一般性认识。为了实现这一目标，我们将开发一种新的通用模拟模型，该模型将首次量化气候和形态控制对大河冲积沉积物转移预算的所有单个组成部分的影响，并以次年分辨率进行量化。该方法是使用水文模型来预测从集水区到河流泥沙转移河段（连接上游泥沙源区和下游泥沙汇的河流部分）的泥沙供应。在转移河段内，该模型将考虑河床和河岸侵蚀以及河漫滩沉积等关键形态动力学过程，这些过程要么为转移河段提供物质，要么将物质储存起来供以后释放。该模型将使用我们将在本提案中收集的目标现场数据进行参数化和验证。我们将运行该模型来探索包括过去50多年在内的几十年周期内河段内沉积物通量的历史趋势。从我们的模拟模型中得到的数据将是独一无二的：第一个每年解决的巨型河流沉积物预算，包括几十年的时间。这些数据将使我们能够探索一系列具体的研究问题：冲积转移河段内泥沙交换对湄公河泥沙负荷的净影响是什么？与洪泛区储存和河岸侵蚀相关的泥沙通量是否促进了转移带流出的净增加或减少？这种调制效应在绝对和相对方面有多大？这种调制的年际变化有多强？又是什么因素造成的？事实上，我们预计年际变率将反映与控制每个组成部分的特定气候指数相关的预算的各个组成部分变化的净效应。这将通过测试以下特定假设来探讨：(i)气候变率的特定模式（印度洋偶极子和El-Niño南方涛动）在调节沉积物转移中的作用；（ii）极端事件（与热带气旋有关）控制河岸侵蚀和漫滩沉积的方式。预测世界上最大的河流之一的河流沉积物转移是一项新颖、及时和重要的科学挑战。解决这一挑战将提高我们预测沉积物从“源头到汇”转移的能力，从而有助于(i)解释洪泛平原沉积记录，（ii）了解沉积物、营养物质和碳通量如何响应气候，（iii）评估三角洲洪水风险的变化，以及（iv）大河生态系统服务维持的物理过程。

The world's largest rivers transport ~19 billion tonnes of sediment each year, with a significant fraction being sequestered in the large deltas that are home to 14% of the world's population. Most (>70%) of these large deltas are under threat from rising sea levels, ground surface subsidence & declining riverine sediment supply required for delta construction. However, while measurements & projections of sea level rise & subsidence exist for many deltas, data quantifying historic changes in fluvial sediment supply are sparse, limiting our understanding of how delta building is related to climatic fluctuations. This situation reflects the complexity of controls on river sediment loads, which include the influence of climate & land use change in upland areas, dam construction, & flood driven storage & remobilisation of sediment within the extensive floodplains that characterise the lowland reaches ("sediment transfer zones") of the world's major rivers. This project will provide the first comprehensive quantification of these controls on riverine sediment fluxes for one of the world's largest rivers (the Mekong), leading to new generic understanding of the relationships between climatic variability, fluvial processes & sediment flux to deltaic zones & the ocean.To meet this aim we will develop a new generic simulation model that will, for the very first time, quantify the effects of climatic & morphological controls on all individual components, & at sub-annual resolution, of the alluvial sediment transfer budget of a large river. The approach is to use a hydrological model to predict sediment supplied from the catchment to the head of the river's sediment transfer reach (the part of a river that links sediment source areas upstream with sediment sinks downstream). Within the transfer reach the model will account for the key morphodynamic processes of river bed & bank erosion, & floodplain sedimentation, which either supply material to the transfer reach, or store the material for later release. The model will be parameterised & validated using targeted field data that we will collect in this proposal. We will run the model to explore historical trends of within-reach sediment fluxes over a multi-decadal period encompassing the last 50+ yrs. The data derived from our simulation model will be unique: the very first annually resolved mega-river sediment budget encompassing a multi-decadal period. These data will enable us to explore a series of specific research questions: What is the net effect on the Mekong sediment load of sediment exchanges within the alluvial transfer reach? Do sediment fluxes associated with floodplain storage & bank erosion promote a net increase or reduction in efflux from the transfer zone? How large is this modulating effect in both absolute & relative terms? How strong is the interannual variability in this modulation, & what factors drive this? In fact, we expect interannual variability to reflect the net effect of changes in the various components of the budget linked to specific climate indices that control each component. This will be explored by testing specific hypotheses concerning (i) the role of specific modes of climate variability (Indian Ocean Dipole & the El-Niño Southern Oscillation) in modulating sediment transfer, and; (ii) the ways in which extreme events (associated with tropical cyclones) control river bank erosion & floodplain deposition. Predicting fluvial sediment transfer through one of the world's great rivers is a scientific challenge that is novel, timely & significant. Addressing this challenge will improve our ability to predict sediment transfer from 'source-to-sink' thereby aiding (i) interpretations of floodplain sedimentary records, (ii) understanding of how sediment, nutrient & carbon fluxes respond to climate, (iii) assessment of changes in flood risk within deltas, & (iv) the physical processes by which ecosystem services within large rivers are sustained.