The Evolution of Fluvial Sediment Delivery from Asia’s Rivers to the Oceans: Trends and Causes

Project Rationale:

This project aims to quantify and explain variations (1985-2020) in the discharge of fluvial sediment to the oceans, with a focus on the continent of Asia.

River-borne sediments help determine the character and evolution of coastal environments. For example, the delivery of fluvial sediment to the coast affects coastal retreat and relative sea-level adjustments of the world’s deltas [1]. Unfortunately, estimates of the ways in which sediment flux varies remain highly uncertain because of large biases in data availability. The issue is not that individual measurements are unreliable, but rather that such measurements are rarely made. Further, such measurements are often point samples, rather than continuous monitoring setups. Less than 10% of the world’s rivers are monitored for their sediment delivery to the coastal zone, and these issues are more acute in developing nations where resources are highly pressured. Meanwhile, the Earth – and Asia in particular, which dominates the global sediment budget - is undergoing a period of unprecedented anthropogenic influence, with dam building, climate change, deforestation and agricultural practices all thought to have had a dramatic impact on the movement of water and sediment from land to sea in recent decades. These global changes are anticipated to continue, affording a clear imperative to determine the effects of these major transitions on fluvial sediment flux.

Methodology:

Prior studies have demonstrated that satellite data can estimate surface suspended sediment concentrations (SSC) in rivers via calibration of surface reflectance to field based measurements [2]. We will use the global archive of Landsat and Sentinel satellite imagery to construct time series of SSC for Asia’s rivers. New computing tools, such as the cloud-based Google Earth Engine, now enable this process to be applied across many river reaches [3], making it feasible to work at unprecedented (as per the continental focus of this project) spatial scales. To derive estimates of total sediment flux, it is also necessary to model the vertical variation of SSC through the water column to convert surface SSC to a depth-averaged value. This will be achieved through analysis of field data from the Mekong River to develop concentration profiles of suspended sediment as a function of the so-called Rouse number, which is the ratio of the sediment settling velocity and the shear velocity of the flow. Finally, quantified trends in sediment flux will be linked to climatic and catchment changes in Asia’s rivers, so that we will be able to attribute and understand the key controls driving this changing flux.

Training:

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted within the School of Geography and Environmental Science, based at Highfield Campus. The supervisors have extensive field experience in the Mekong River and are experts in the field of fluvial measurement, monitoring and remote sensing. Full training will be provided in the use of field-based hydro-acoustic measurement techniques (such as Acoustic Doppler Current Profiling; ADCP), as well as the remote sensing and data processing techniques needed to estimate suspended sediment concentrations from satellite imagery.