Flow and vegetation interactions in organic-debris laden rivers: new insight from dye-dilution, laser scanning and structure from motion techniques

Project Rationale

This project aims to deliver a step-change in our understanding of flow interactions with complex vegetative structures in rivers.

Large Woody Debris (LWD) has a significant effect on river flows and channel morphology. These effects are conceptually well understood: the flow resistance introduced by the presence of LWD leads to changes in flow velocity structure and flow depth that, in turn, modulate sediment transport causing erosion and deposition. Yet, attempts to quantify these processes, for example in numerical models of channel flow and morphodynamics, remain elusive due to difficulties in accurately estimating the altered flow. This represents a major limitation as LWD accumulations are common in rivers with forested floodplains (the natural state of many rivers prior to large scale human disturbance), and the (re)introduction of LWD is often now undertaken as part of natural flood management and river restoration schemes. This project will address these limitations using state-of-the-art dye dilution and Terrestrial Laser Scanning (TLS) techniques to measure hydraulic roughness as a function of flow retardation and the physical structure of the LWD. Unmanned Aerial Vehicles (UAVs) will be used to monitor spatial distributions at a reach level to allow upscaling of the findings.

Methodology

A key challenge in quantifying the roughness of LWD accumulations is its inherent structural complexity. This project will employ novel high-resolution surveying techniques [1] to characterise the 3D structure of debris dams. Specifically, the student will deploy Southampton’s Terrestrial Laser Scanners (TLS) to evaluate the 3D structure of in situ debris dams across a range of representative field sites. In addition, dye-dilution techniques will be used to estimate hydraulic roughness based on flow retardation using the Aggregated Dead Zone (ADZ) approach [2]. TLS will be used to characterise the frontal area of debris jams as flow discharge varies, the frontal area being a key variable in determining the drag exerted by the LWD. The project fieldwork will also evaluate the other components contributing to the forces exerted on the studied LWD, using a range of additional flow velocimetry techniques (aDcp). The resulting data sets will be synthesised to provide a novel physically-based relationship between LWD structure and resulting flow resistance. UAVs and Structure from Motion (SfM) techniques will be used to asses reach-scale (and beyond) distributions of LWD to allow upscaling of the findings from the high resolution techniques, with key applications in Natural Flood Management and river restoration generally.

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 at Highfield Campus. Opportunities are available for the student to engage with key applied and research stakeholders via our excellent links, including with the Environment Agency. The supervisors are experts in the field of fluvial geomorphology and high resolution survey techniques. Full training will be provided in relevant measurement, monitoring and remote sensing techniques.