Chaos and Morphological Predictions in Fluvial and Tidal Systems

Chaos and Morphological Predictions in Fluvial and Tidal Systems

Rivers have always been part of the complex relationship between mankind and the environment. We always tried to forecast floods and fluvial changes, as river systems represent both extraordinary opportunities and prominent threats for society. During the second half of 20th century, the scientific discipline of Fluvial Geomorphology began investigating the evolution of rivers over time. As we enter the century of complexity, with the ever increasing availability of data and development of computational power, many possibilities open for understanding of the complex relationships between fundamental processes in river systems, from fluvial morphology to eco-hydraulics at different spatial and temporal scales. In this context, ecosystems management plans that take into account long term morphological assessments might ensure better conservation and, in hot-spots of climate change such as tidal systems, more efficient adaptation measures.

Q1 - What are the fundamental processes that lead to chaos in river morphodynamics? Evidence seem to point to the fact that vegetation is among the main controls on the complex interactions with hydromorphodynamics that lead to chaotic behaviour. However, some numerical experiments indicate that in the case of braided rivers the system exhibits deterministic chaos even without the influence of vegetation for an infinitesimal perturbation of river bed morphology. For these reasons, further investigation of complex behaviour in models and analytical characterisation of key chaotic parameters in theoretical morphodynamics is required. This investigation will also build on the knowledge of critical thresholds (e.g. critical and resonant width-to-aspect ratios βc, βr ) in river evolution, as established by previous theoretical works.

Q2 - Can we develop models to predict the morphological trajectories? Once established the chaotic behaviour of eco-morphodynamic processes, the limitations of deterministic chaos to long term trajectories predictions would be relevant. However, the characterisation of potential long term multiple stable attractors is still something that might be well worth considering, along with the development of ensemble models dependent on some key global parameters (e.g. land use, climatic conditions, sea level rise), in a fashion similar to climate model predictions.

Q3 - Can we develop a common approach for fluvial and tidal systems? Another challenge would be to develop a common approach for riverine and tidal systems long-term geomorphic evolution, as the fundamental processes that influence morphodynamics would be the same but the forcing conditions of river estuaries are definitely more complex. However, if common system parameters (Q1) are found, this might be proven possible.

• Characterisation of the predictability skill on fluvial and tidal systems for long-term morphological trajectories forecasts based on key morphodynamic parameters.

• Development of long-term morphodynamic models to go beyond such a predictability barrier.

Both analytical and numerical