Deciphering the dominant drivers of contemporary relative sea-level change: Analysing sediment deposition and subsidence in a vulnerable mega-delta

世界主要河流三角洲正面临着重大的可持续性危机。这是因为它们正面临着被海平面上升“淹没”的威胁，这可能会给在那里生活和工作的5亿人带来严重后果。在定性层面上，我们对推动海平面上升的过程有了相对较好的理解。当海平面上升和地面沉降的总速率不能与地表高程的增加相平衡时，三角洲地表高程就会发生变化，而地表高程的增加是由上游河流集水区提供的沉积物沉积造成的。持续和重大的环境变化似乎正在推动这些因素的更大不平衡：由于人为气候变化，海平面上升，而沉积物压实在三角洲自然发生的地面沉降，在许多情况下，地下水和/或碳氢化合物开采大大加速了地面沉降。因此，唯一可能抵消三角洲表面高程损失的因素是三角洲表面的沉积物沉积。不幸的是，由于人类活动，如在上游的支流集水区采砂和筑坝，许多三角洲也在失去河流沉积物的供应。准确估算海平面上升、沉积物供应速率、地表沉积和地面沉降是一项重大挑战。在短期内，这种平衡中最重要的因素是泥沙沉积和沉降（在较长期内，海平面上升的变化将变得相对更重要）。然而，在估计泥沙供应方面的一个特殊问题是，以前的研究主要集中在三角洲顶端的泥沙负荷上，而在三角洲顶端下游的三角洲分流河道网络中几乎完全缺乏可靠的数据。此外，迄今为止，在确定导致海平面相对上升的其他驱动因素时所涉及的相关学科专业知识的多样性，共同抑制了对系统解决这一问题真正必要的综合综合。湄公河是世界第三大三角洲，是东南亚的粮仓和2000万人口的家园，但由于经济的快速发展，最明显的是上游筑坝和人为沉降，它正面临环境风险。因此，湄公河不仅代表了世界三角洲面临的许多问题，而且迫切需要可靠的数据来帮助制定可持续的管理计划，为三角洲居民提供安全的经营空间。在我们NERC资助的工作中，我们开发了新的方法来估计供应给三角洲顶点的河流沉积物的近期历史和未来趋势。然而，在三角洲顶端下游的三角洲分流网络内的沉积物流动，才是控制三角洲地表局部沉积速率的最关键因素。在这一建议中，我们将与芹苴大学和越南水文局合作，获取存档的沉积物运输测量数据。利用我们在上游集水区现有工作中开发的新方法，我们将“解锁”并将这些数据转化为三角洲分流网络内部和整个网络的沉积物负荷的最初估计。与此同时，我们还将与其他国际组织合作，这些组织一直在开发新的模型来模拟三角洲表面沉积（波茨坦）和地面沉降（乌得勒支）的速率。通过共同努力，我们将把这些数据汇总在一起，对全球重要的、标志性的三角洲地区近期海平面相对上升的驱动因素进行首次综合评估，为全球其他脆弱的三角洲地区的类似分析提供模板。

The world's major river deltas are facing a major sustainability crisis. This is because they are under threat from being 'drowned' by rising sea levels, with potentially severe consequences for the 500 million people who live and work there. At a qualitative level we have a relatively well developed understanding of the processes that are driving these rising sea levels. Changes in delta surface elevation occur when the summed rates of eustatic sea level rise and ground-surface subsidence are not balanced by gains in surface elevation, the latter being caused by the deposition of sediments supplied from river catchments upstream. Ongoing and major environmental changes are seemingly driving greater imbalances in these factors: eustatic sea levels are rising as a consequence of anthropogenic climate change while ground-surface subsidence, which occurs naturally in deltas as a result of sediment compaction, is in many cases being significantly accelerated by groundwater and/or hydrocarbon extraction. As a result, the only factor that could potentially offset these losses in delta surface elevation is sediment deposition on the delta surface. Unfortunately, many deltas are also being starved of their supply of river sediments as a result of anthropogenic activities, such as sand mining and damming, in the feeder catchments upstream. Estimating precise values of eustatic sea-level rise, sediment supply rate, surface deposition and ground-surface subsidence, is a significant challenge. In the near term the most significant factors in this balance are sediment deposition and subsidence (in the longer term eustatic changes will become relatively more significant). However, a particular issue in estimating sediment supply is that previous studies have focused on the sediment loads at the apices of deltas, with an almost complete absence of reliable data within the delta distributary channel network downstream of the apex. Moreover, the diversity of relevant disciplinary expertise involved in determining the other drivers contributing to relative sea-level rise has thus far conspired to inhibit the integrated synthesis that is really necessary to tackle the problem systematically.The world's third largest delta, the Mekong is SE Asia's rice basket and home to 20 million people, but it is being exposed to environmental risks as a result of rapid economic development, most notably through upstream damming and anthropogenic subsidence. The Mekong is therefore not only representative of many of the issues facing the world's deltas, but reliable data are urgently needed to help inform the sustainable management plans required to provide a safe operating space for the delta's inhabitants. In our NERC funded work we have developed new methods to estimate recent historical and future trends in the river sediments supplied to the apex of the delta. However, it is the flows of sediment within delta distributary networks, downstream of the delta apices, that are most critical in controlling local rates of delta surface deposition. In this proposal we will collaborate with Can Tho University and the Vietnamese Hydrological agency to access archived sediment transport measurements. Using novel methods developed in our existing work in the catchment upstream we will 'unlock' and translate these data into the very first estimates of sediment loads within and across the delta distributary network itself. Meanwhile, we will also work with other international groups who have been developing novel models to simulate rates of delta surface deposition (Potsdam) and ground-surface subsidence (Utrecht). Working together we will draw these data together to build the first integrated assessment of the factors driving near-term relative sea-level rise in a globally significant, iconic, delta, providing a template for similar analyses in other vulnerable deltas worldwide.