A significant advance in theoretical geomorphology that explains how and why rivers self-adjust and why this has biased Earth’s alluvial stratigraphic record
About the Event:
Least action principle provides a radically different paradigm for river research by identifying the attractor that controls river channel self-adjustment and evolution. It links advances in theoretical physics to an improved understanding of fluvial geomorphology and hydraulic engineering while also explaining the profound biases that exist in Earth’s longterm stratigraphic record. Because meandering is an energy-shedding mechanism these intensively studied sinuous rivers, extremely common today, sequester very little sediment. Indeed, all the known examples of meandering-river alluvial sequences are limited to a few tens of metres thick and a few kilometres wide. In contrast, low-energy braided and anabranching disequilibrium systems have sequestered sediment in sequences sometimes more than a kilometre in thickness and tens to hundreds of kilometres wide. For climatic and topographic reasons most rivers globally are substantially overpowered and consequently they adjust to a dynamic equilibrium state to minimise their instability. Many bedforms (ripples, dunes bars etc.) are generated to create roughness in the channel and to thereby assist in consuming part of this excess of energy. Like meanders, they contribute to channel stability. This begs the question; why do bedforms occur in rivers that are underpowered, unstable and accreting (sequestering) their sediment? It is apparent there are two types of bedforms; those in rivers that have excess energy (insufficient load) where the bed is markedly deformed so as to generate turbulence and obstruct the flow in contrast to those that are more tabular and create less turbulence but readily migrate so as to store bed-sediment that occur in rivers with insufficient energy (excess load). Least action provides, for the first time, a universal, rational explanation for why alluvial rivers behave as they do, changing their width/depth ratios, planforms and bedforms to achieve equilibrium in response to imposed sediment loads and energy conditions. This self-adjustment limits the globe’s alluvial stratigraphic record to those river systems that were not able to achieve dynamic or stationary equilibrium and, as a consequence, this disqualifies many contemporary rivers as suitable analogues for studying the ancient record.
About the Speaker:
Professor Gerald Nanson obtained a BSc (Hons) from the University of Otago in 1970, and MSc from the University of Alberta in 1972 and a PhD from Simon Fraser University in 1977. Early in his career he pioneered research into river channel migration, developing the first rational and statistical models for explanation and prediction. He has led the description and detailed analysis of anabranching rivers, demonstrating that, along with meandering and braiding, they can represent a stable equilibrium form. Recently he developed a quantitative rational physical model for predicting the least action state of stable (stationary) equilibrium in alluvial rivers and over 30 years has revealed in detail the geomorphology, palaeohydrology and Quaternary history of the rivers and dunefields of central Australia. He has published over 134 refereed articles in diverse fields including alluvial sedimentation, loose-boundary hydraulics, theoretical geomorphology and hydraulics, river-pattern classification, river-channel migration, river management, desert-dune dynamics, and Australia’s Quaternary environmental change. His papers are commonly used in textbooks and encyclopaedias In terms of international distinctions Professor Nanson has been awarded the Geological Society of America El-Baz Award for Excellence in Desert Research in 2006, the Linton Medal in 2013, the highest honor from the British Society for Geomorphology for his outstanding international contribution to geomorphology, and also in 2013, the Australia and New Zealand Geomorphology Group medal as ‘Distinguished Geomorphologist’.