Final Report Summary - FLOODSETS (Flume study of flood history effects on sediment entrainment and transport in gravel-bed rivers)
Floods are among the most effective geomorphic force featuring the landscape, and also one of the most hazardous natural phenomena. The timing and the sequences of flood events have major impact on fluvial morphodynamics, river management and flood protection planning, especially under a perspective of climate change. However, the effects of different sequences of events in terms of duration and magnitude - the so-called flood history - on sediment entrainment and transport are still poorly understood, thereby limiting the accuracy of predicting fluvial adjustments and the associated flood hazards.
The project addressed the hypothesis that the grain size and roughness conditions of the river bed and sediment transport fluctuate with flood history in a manner that can be predicted. The overall objective of the research was to quantify the influence of flood history on the mobility and transport rate of mixed sand-gravel river beds, by means of laboratory experiments.
A series of laboratory flume experiments under conditions of zero sediment feeding and sediment recirculation were conducted in order to identify the temporal evolution and the 3D properties the surface sediments. Being the surface layer of sediment coarser than the subsurface, this is usually called armoured and has remarkable implications for bed load transport estimation, hyporheic flow exchange, and macro-invertebrates and fish habitats. The experimental setup allowed to develop both static and mobile armours, which are different for sediment transport supply and thus sediment transport conditions. Analyses of detailed laser scans of the bed revealed that static and mobile armour are different in terms of roughness, level of structuring, imbrication and exposure of coarsest particles. Overall, the results suggest that detailed elevation models of surfaces of gravel-bed rivers could provide important insight into changes in surface texture and structure, allowing inferences to be made relating to the sediment supply and transport conditions, and flow regimes in gravel-bed rivers.
In a further set of laboratory experiments, three types of hydrographs with contrasting durations and magnitudes were simulated under sediment recirculation conditions. The results indicate that sediment transport during the falling limb is lower than during the rising limb in all of the three types of hydrographs. Interestingly, this is not related to a change in surface or sediment transport grain size, but to a change in the organisation and complexity of the surface sediments (bed restructuring during the falling limb of hydrographs). The same dynamic of sediment reorganisation have been identified and assessed in a set of experiments where various hydrographs were simulated in different sequences. The competence of high-magnitude floods to disrupt the surface sediment structures and thus increasing sediment availability for following flood events was quantified, as well as the ability of long and low-magnitude events in re-establishing the armour layer.
The project represents the first attempt of exploring the influence of sequences of floods with different magnitude and duration on bed load transport under sediment recirculating conditions. A such, it provides major progress to our physical understanding of the effects of flood history on bed mobility, and further insights on the complex inter-relationships between bed surface grain size, organisation, and bed load transport in gravel-bed rivers. The results have strong implications for the evaluation of climate change effects on the stability of gravel-bed river, in the widely accepted scenario of long drought periods followed by flashy floods. Moreover, the improved understanding of bed mixture composition after floods of different magnitudes and frequencies is expected to refine the methods currently used in the field to interpret ancient alluvial deposits and their use for inferring the future impacts of future climate change.
The project addressed the hypothesis that the grain size and roughness conditions of the river bed and sediment transport fluctuate with flood history in a manner that can be predicted. The overall objective of the research was to quantify the influence of flood history on the mobility and transport rate of mixed sand-gravel river beds, by means of laboratory experiments.
A series of laboratory flume experiments under conditions of zero sediment feeding and sediment recirculation were conducted in order to identify the temporal evolution and the 3D properties the surface sediments. Being the surface layer of sediment coarser than the subsurface, this is usually called armoured and has remarkable implications for bed load transport estimation, hyporheic flow exchange, and macro-invertebrates and fish habitats. The experimental setup allowed to develop both static and mobile armours, which are different for sediment transport supply and thus sediment transport conditions. Analyses of detailed laser scans of the bed revealed that static and mobile armour are different in terms of roughness, level of structuring, imbrication and exposure of coarsest particles. Overall, the results suggest that detailed elevation models of surfaces of gravel-bed rivers could provide important insight into changes in surface texture and structure, allowing inferences to be made relating to the sediment supply and transport conditions, and flow regimes in gravel-bed rivers.
In a further set of laboratory experiments, three types of hydrographs with contrasting durations and magnitudes were simulated under sediment recirculation conditions. The results indicate that sediment transport during the falling limb is lower than during the rising limb in all of the three types of hydrographs. Interestingly, this is not related to a change in surface or sediment transport grain size, but to a change in the organisation and complexity of the surface sediments (bed restructuring during the falling limb of hydrographs). The same dynamic of sediment reorganisation have been identified and assessed in a set of experiments where various hydrographs were simulated in different sequences. The competence of high-magnitude floods to disrupt the surface sediment structures and thus increasing sediment availability for following flood events was quantified, as well as the ability of long and low-magnitude events in re-establishing the armour layer.
The project represents the first attempt of exploring the influence of sequences of floods with different magnitude and duration on bed load transport under sediment recirculating conditions. A such, it provides major progress to our physical understanding of the effects of flood history on bed mobility, and further insights on the complex inter-relationships between bed surface grain size, organisation, and bed load transport in gravel-bed rivers. The results have strong implications for the evaluation of climate change effects on the stability of gravel-bed river, in the widely accepted scenario of long drought periods followed by flashy floods. Moreover, the improved understanding of bed mixture composition after floods of different magnitudes and frequencies is expected to refine the methods currently used in the field to interpret ancient alluvial deposits and their use for inferring the future impacts of future climate change.