Group Leader: James Brasington
Braided rivers are challenging environments to study empirically. By definition, they are wide, topographically subtle and yet complex. While, much of the active river bed is often accessible at low flow, the shallow nature of the remaining anabranches, particularly in gravel bed streams, often precludes boat-based measurements, while at the same time high turbidity, turbulence and velocities make wading difficult. In floods, the same rivers may expand over their entire braidplainmaking spot samples unrepresentative limiting access to infrequent bridging points and confounding sampling strategies with boundaries that shift over time. On top of this, the high mobility of braided rivers means that irregular snapshots of river behaviour are difficult to link coherently through time and without a ready means to monitor their distributed fluxes and forms continuously, connectingmorphology and process is like completing a complex jigsaw puzzle with most of the pieces missing.
Despite this, recent technological advances have enabled the development of strategies to address many of these challenges and created significant new insights into the character of braided rivers and the dynamical processes that drive their evolution. Progress has been delivered on a range of fronts, but in particular through the advent of new digital survey and remote sensing methodologies that provide a means to quantify river morphology rapidly and at high spatial resolutionand precision. The development of laser based ranging methods (airborne and terrestrial lidar), 3d photogrammetric reconstruction and in large rivers, multi-beam echo sounding, have enabled a step-change in the way we can characterise river form. This has seen a dimensional shift in standard data models, from sparsely sampled cross-sections to continuous digital elevation models. These new datasets have transformed our appreciation of the complexity of braided rivers and have provided seamless information across a spectrum of scales, from measurements of individual grains and their packing, through to full river reaches.
Digital Elevation Models (DEMs) of braided rivers have also facilitated improved measurement of bed level changes and with appropriate information on boundary sediment fluxes and DEM uncertainty, can be used to estimate reach-scale bedload transport rates indirectly. While this approach offers new insights into the distribution of channel changes and by inference sediment sources and pathways, it is hindered by frequency of resurvey and in particular by morphological changes that go unobserved during floods. Direct measurements of sediment transport in braided rivers remain difficult due to their size, however significant progress has been made through the development of proxy or surrogate methods, in particular using bottom-track bias measurements from GPS-coupled acoustic Doppler current profilers, active and passive radio tracers, and acoustic and seismic arrays.
These technical developments are opening a series of new pathways for empirical research, which can provide a data rich framework to parameterize, calibrate and validate numerical morphodynamic models whilst at the same time, themselves demand a radical rethink of traditional (planform) metrics we use to describe and classify the form and structure of braided rivers.
Looking to the future, themes that this workshop might address include:
- Approaches for cost-effective, high resolution, synoptic remote sensing through low-altitude aerial photography from UAVs;
- Hybrid survey methods using data from multiple sensors, e.g., lidar, sonar, optical and thermal imagery;
- Error models used to quantify DEM uncertainties and propagate these into related products;
- Representation of river form in fully three-dimensions, through technologies such as TLS, SfM-MVS and methods to manage and compare 3d point-cloud datasets.
- Improved methods to characterise river bathymetry both at low flow and in floods;
- Techniques to study sediment transport and morphological response during floods – e.g., aDcp surveys from low draft tethered or remote-control boats;
- Continuous methods for quantify sediment flux, e.g., the potential of seismic or acoustic arrays to provide proxy signals of transport rate.
- The potential to develop field observatories that provide for continuous monitoring of braided rivers, quantifying and disseminating in real-time, information on their form and fluxes.