11.07.2011
Type: Link Language: Italian Author: Luca Ziliani
The study of channel adjustments and prediction of morphological channel evolution play a primary role in management and conservation river policies. Various techniques taken individually can reach a satisfactory degree of understanding of river morpho-dynamic, but an integrated approach is certainly desirable as it allows to develop a more robust knowledge of river processes and to make judgments with higher confidence levels. In this research an effort has been undertaken to integrate and combine two approaches very different in terms of methodological and conceptual base: GIS-survey analysis, and numerical morphodynamics modelling. The case study has been a reach of the Tagliamento River (Friuli Venezia Giulia, Italy) of about 49 km in length from the Pinzano gorge to San Mauro. This reach coincides roughly with the so-called Middle Tagliamento. Through the application of GIS techniques, topographic and geomorphological surveys it was possible to reconstruct the morphological adjustments undergone by Tagliamento River channel over the last 200 years. The quantification of the human activies also helped to highlight the existing link causes-effects with the processes of narrowing and incision started during the first half of the twentieth century, grew in intensity until the 1990s and followed by a recent morphological recovery phase. Numerical modelling was undertaken in order to support and complete the results of the historical analysis. Space-time scales involved by the historical analysis have led to choose a numerical model that can simulate long time periods on spatial contexts with dimensions similar to those of the study reach. The choice has fallen on a morphodynamic cellular model called CAESAR (Cellular Automaton Evolutionary Slope And River model), which fall into to the so-called Reduced Complexity Models group (RCM). This type of model necessarily required that a thorough sensitivity analysis was carried out before of model calibration and validation. Following validation the model was applied to estimate sediment budgets at reach scale and it was used in a "what-if scenarios" evaluation framework. It was thus possible to bring out the longitudinal variability of the average sediment transport yield, the strong influence of lateral erosion on the sediment transport dynamics and the lack of relevance on the planimetric trend of variation in sediment supply input on the upstream section of the reach. Integration of numerical evaluations and classic analysis results allowed to isolate the human impact that induced the historical channel adjustments and also to make quantitatively and conceptually based assessments about the likely future morphological evolution of the Tagliamento River.
Keywords: morphology, numeric model