The effects of annual and extreme fluvial processes on river dynamics in hydrologically different regions Seminar

Event details

The relative importance of both frequent and infrequent fluvial processes as channel modifiers has been widely discussed. Extreme fluvial processes, such as flash floods in arid regions, have been shown to cause great river channel changes, but also in some cases they have had only little or highly temporary effects on the channel morphology. In addition, the channel evolution during the different phases of medium and low magnitude discharge events is still not well known, for example in gravelly ephemeral rivers, and there has been insufficient knowledge of the effects of peak flows on both point bar and bank evolution during hydrologically different years. In addition to rain-induced fluvial processes, snow-melt, frazil ice and ice dams are significant causes of flooding in subarctic and temperate latitudes, but only recently the general consensus on the importance of ice on river channel morphology has emerged. Therefore, the aim of the post-doctoral research is to detect the effects of different extreme fluvial processes, compared to annual processes, on river dynamics in subarctic, temperate, arid and subtropical environments. This project provides new data particularly with the means of Acoustic Doppler Current Profiler (ADCP), multi-temporal mobile and terrestrial laser scanning, and multidimensional simulation approaches of hydro- and morphodynamics. The results are based on published works from Finland (Pulmanki, Tana and Kokemäenjoki Rivers), and Spain (Rambla de la Viuda). Also ongoing studies from Australia (Brisbane River) and USA (Arizona: Bronco Creek) will be discussed. Based on the case studies performed in Finland, the frequent low discharges of typical magnitude may be even more important for fluvial transport and channel modifications than annual discharge peaks particularly in a sandy braided reach, but great transportation can also occur during low discharges in a river consisting of cohesive material. In a sandy meandering river, on the other hand, the magnitude and duration of annual snow-melt discharge events control the morphological changes: the greater the magnitude and duration of the water stage peak, the greater the erosion at the banks and the deposition on top of the point bars. However, in coarser reaches of a sub-arctic river, ice has the most significant role, greater than that of flowing water, in erosion of coarse sediment from the channel bed and gently sloping banks. The low flows and both rising and falling stages of different magnitude discharge events are shown to be important for bar movement and channel evolution also in the ephemeral Spanish study reach. Despite the great channel evolution during the first hours of discharge events, the morphodynamic simulations show that erosion and deposition continue to be great during the long-lasting receding phase. This causes more channel changes in total during receding phase than in the beginning of medium and low magnitude discharge events. Noteworthy is that the difference between the timing of sediment transport peak and the timing of discharge peak depend on the magnitude of the discharge event in question.