In this paper, we present a detailed investigation of the size, scale and dynamics of macro-turbulent flow structures in gravel-bed rivers. We used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel-bed river. The array was deployed successively in various configurations in order to quantify the vertical, lateral and longitudinal extent of the flow structures and to estimate their advecting velocities. To depict the spatial and temporal properties of the flow structures, we used space–time velocity matrices, space–time correlation analysis and coherent-structure detection schemes. The results show that the large-scale turbulent flow structures in a gravel-bed river occupy the entire depth of the flow and that they are elongated and narrow. The length of the structures is 3 to 5 times the flow depth while the width is between 0.5 and 1 times flow depth. In spite of the high roughness of the bed, these values are similar to those reported in the literature for laboratory experiments on large-scale turbulent flow structures. The dynamics of the large-scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near-bed region. Our evidence suggests that large-scale flow incursions trigger ejections in the near-bed region that can develop into megabursts that can reach the water surface.

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Abstract:River systems are increasingly under stress and pressure from agriculture and urbanization in riparian zones, resulting in frequent engineering interventions such as bank stabilization or flood protection. This study provides guidelines for a more sustainable approach to river management based on hydrogeomorphology (HGM) concepts applied to three contrasted rivers in Quebec (Canada). Mobility and flooding spaces are determined for the three rivers and three levels of "freedom space" are subsequently defined based on the combination of the two spaces. The first level of freedom space includes very frequently flooded and highly mobile zones over the next 50 years, as well as riparian wetlands. It provides the minimum space for both fluvial and ecological functionality of the river system and corresponds to a highly variable width, approximately 1.7 times the channel width on average, for the three studied sites. The second level includes space for floods of larger magnitude and provides for meanders to migrate freely over a longer time period. The last level of freedom space represents exceptional flood zones. We propose the freedom space concept to be implemented in current river management legislation because it promotes a sustainable way to manage river systems and it increases their resilience to climate and land use changes in comparison with traditional river management approaches which are based on frequent and spatially restricted interventions. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems CorporationPlease find attached our revised manuscript. As you will see, we have made substantial change to the original manuscript based on the very thorough and stimulating comments received from the three reviewers. The detailed changes are described in the attached 27-page long letter. The most important change is that we have decided not to include the cost-benefit analysis in order to focus on better explaining the concepts and methodological issues of the freedom space approach. This is why the title of the paper has been revised to "Freedom space for rivers: a sustainable management approach to enhance river resilience".We would like to take this opportunity to thank the reviewers for their thoroughness in assessing the initial version of this paper. Their comments and suggestions prompted us to clarify our approach so that it can hopefully be applied in other regions of the world to improve resilience of river systems. We hope you will find this revised version suitable for publication in your journal. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 Abstract 8 River systems are increasingly under stress and pressure from agriculture and urbanization 9 in riparian zones, resulting in frequent engineering interventions such as bank stabilization or 10 flood protection. This s...

[1] The spatial heterogeneity of fully turbulent, near-bed flows above gravel river beds is examined using a realistic replica of a natural gravel patch in a large flume. Threedimensional velocity time series were obtained at four heights (0.008-0.1 m) above the local bed in each of 99 closely spaced verticals for three flows of increasing intensity. The spatial heterogeneity of time-averaged velocities and root-mean-square velocity fluctuations increases under stronger flows and closer to the bed. However, streamwise velocity becomes spatially homogeneous at a distance from the bed of between 2-4 times the median bed elevation. Heterogeneity in the direction of the velocity vector is independent of mean flow velocity, but in all cases it decreases approximately linearly with distance above the surface. Skewness of the instantaneous velocity distributions suggests that slowly moving fluid emanating from the near-bed region impinges upon higher levels with greater frequency and greater spatial coverage as the average flow velocity increases. Spatial heterogeneity in turbulent kinetic energy increases with flow velocity and maxima occurr at positions that intercept layers of intense vortex shedding in the lee of obstacle crests. The spatial organization of the flow properties is nonrandom and consistent across the three flows. Simple regression models are developed to provide a basis for investigating the heterogeneity of near-bed flow at the patch scale ($2 m 2 ) in gravel bed rivers.

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The investigation of the flow field with a pulse-coherent acoustic Doppler profiler has led to new high resolution observations of the secondary flow pattern occurring in a natural ice-covered meander reach. Surveys were conducted during two successive winter periods with different ice conditions. Massive frazil ice accumulation was present during one of the survey and its influence on the flow pattern could be assessed. Results show that the primary flow is clearly deflected towards the outer bend. Secondary flows are one order of magnitude less than the primary flow and they display two stacked counter-rotating helical cells pattern occurring at the entrance of the bend. This pattern is associated with the parabolic shape of the velocity profiles entering the bend. The pattern rapidly evolves downstream, reducing to one helical cell rotating in an opposite direction than what is observed in open channel flows. Flow mixing and morphological non-uniformity are potential factors governing the development of the helical cells throughout the bend. Our observations show that a similar coherent flow pattern rapidly forms downstream of a massive frazil ice obstruction in the bend. Frazil ice does not constrain the formation of helical flow pattern in river bends.

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