Ripple dynamics over various microtopographical roughness elements and their implications for river management

Abstract

Specific problems in connection with the sediment regime have arisen in the granite and gneiss area in the northern part of Austria due to the increased sediment supply of coarse sand and fine gravel as a product of rock weathering. One of the physical characteristics of these sediments is their high mobility in form of ripple bedforms along the surface. In these rivers, the microtopographical structures of the bed surface are hydraulically active, especially in the initial stages when ripples have not overtopped the largest elements of the bed's roughness elements. There is a lack of detailed studies of the impacts of these microtopographical structures on ripple dynamics. Thus, the aim of the present study was to investigate the impact of various microtopographic roughness elements on ripple movement. This study was performed using a 1:1-scaled physical laboratory experiment. The results showed, in all repetitions of the experiment with variable grain sizes, that the higher the microtopographical variability of the river substrate and turbulence was, the higher the measured transport velocities of the ripples were. The expected hidden impacts on sediment transport were not documented. Important implications for sediment management were discussed, as river sections with high accumulation rates of coarse sand and fine gravel are prone to additional degradation as downstream transport decreases over smooth ripple-dune morphologies. Additionally, structural elements such as exposed rocks may increase the downstream transport due to the increased variability in turbulence.

Evolution of artificial spawning sites for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta): field studies and numerical modelling in Aurland, Norway

“…Prediction of spawning habitat life span wash out of the deposited substrate (see supplementing material). These, findings are supported by flume experiments ofHauer et al (2019) in which an increase in turbulence intensity (%) was identified as a driving parameter for magnitude in sediment transport. Similar to those scaled experiments, the surface substrate in the Aurlandselva contains high variability in largest grain sizes (d 90 ) (compare toFig…”

“…Furthermore, in the model no corrections according to El Kadi Abderrezzak et al (2016) were carried out with regard to exposure/protection; and therefore, sometimes in the very small test areas (about 50 m 2 ), an increased expression of re-arrangement processes occurs. Nevertheless, in recent years, the role of turbulence and fluctuating parameters such as unsteady velocities, pressures, drag and lift forces has been widely acknowledged (Schmeeckle et al, 2007;Smart & Habersack, 2007;Diplas et al, 2008;Celik et al, 2013;Sindelar & Smart, 2016;Hauer et al, 2019). Mainly in physical laboratory studies, research on the sediment transport dynamics (e.g.…”

“…6) and thus a high variability in micro-topographic roughness. This has a strong impact on the near-bed turbulence intensity (compare to Hauer et al, 2019) and thus the erosional forces for sediment transport like the deposited spawning gravel. This might be the reason why the model prediction of gravel distribution failed for most of the sites in comparison to good examples of two-dimensional depth-averaged numerical modelling in solely fluvial environments with lower largest grains (d 90 ) of surface layer composition.…”

“…The presented ratio (BCR) between (i) mean diameter of artificial introduced spawning gravel and (ii) the micro-topographical roughness of the river bed (d 90 ) allows to reflect more detailed information on this substrate variability and can be seen as an indicator for how high turbulence as well as possible hiding effects (compare to Parker, 1990) might impact artificially implemented spawning sites. Based on Hauer et al (2019) a BCR [ 3-5 (depending on the height of gravel deposits) might be seen as a threshold criteria where local turbulence may impact the stability (positively and negatively) of spawning habitats and when numerical modelling reaches invalidity. However, these numbers are just based on a single physical laboratory experiment and need to be validated and probably adjusted in future.…”