Fig 4 - uploaded by Paolo Billi
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The typical arrangement of pebbles in transverse ribs (TR) on the dry streambed of the Dandero R., Danakil, Eritrea.
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A first aim of this paper is to investigate the existence, if any, and the parameters defining the occurrence conditions of a continuum of rhythmic, transverse roughness elements of gravel-bed rivers such as transverse ribs, step-pools and riffle and pool sequences. The investigation
is carried out by the analysis of a large data set consisting of...
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Citations
... A full understanding of AHG variations will require reduction of these uncertainties, and more generally to understand the key catchment-and reach-scale processes that govern channel shape and friction. In particular, the observed relationship between Fr 50 and AHG exponents suggests to further investigate the hydromorphological processes that drive the distribution of geomorphic units (riffle, run, and pool; Thompson, 2001) and the various forms of friction in reaches (Billi et al., 2014;Yochum et al., 2012). Such investigations would help to better predict hydraulic geometry and therefore aid river management at unvisited locations or data poor locations. ...
Reach‐scale at‐a‐station hydraulic geometry (AHG) relationships are power laws that describe variations of reach‐averaged water depth, wetted width, and current velocity in stream reaches when discharge varies. Modeling AHG exponents is important, because the variations of hydraulics with discharge in stream networks influence physical habitats of aquatic species, biodiversity, water temperature, nutrient fluxes, and sediment transport. Theoretical approaches indicated that AHG exponents should depend on topographic descriptors of cross sections and roughness elements. Empirical approaches suggested that AHG exponents partly depend on hydraulic characteristics measured at a single discharge. We used a unique data set of AHG observed in 812 stream reaches (obtained from measurements at several discharge rates or from hydrodynamic models) to (1) test the consistency of theoretical and empirical predictions of AHG exponents and (2) test the generality of AHG predictions across rivers of different countries with variable landscape settings. We found that observed AHG depended on topographic predictors (describing cross‐section shape and substrate size) as expected from theory. However, AHG exponents were better predicted by empirical hydraulic characteristics of reaches: the ratio of wetted width to bankfull width and the reach Froude number. The effects of hydraulic variables were consistent with those of topographic predictors. In addition, relations between AHG and hydraulic predictors were significant and with similar direction in different data sets. Despite limited explanatory power, our results help identifying general drivers of AHG exponents. Their application still requires measurements at a single discharge rate but open perspectives of generalized AHG prediction by remote sensing.
... A full understanding of AHG variations will require reduction of these uncertainties, and more generally to understand the key catchment-and reach-scale processes that govern channel shape and friction. In particular, the observed relationship between Fr50 and AHG exponents suggests to further investigate the hydromorphological processes that drive the distribution of geomorphic units (riffle, run, pool; Thompson, 2001) and the various forms of friction in reaches (Yochum et al., 2012;Billi et al., 2014). Such investigations would help to better predict hydraulic geometry and therefore aid river management at unvisited locations or data poor locations. ...
Les variations temporelles et spatiales des caractéristiques hydrauliques des cours d’eau (c.-à-d. largeur mouillée, hauteur d’eau, vitesse du courant) à travers les réseaux hydrographiques influencent les habitats physiques et la biodiversité, la température de l'eau, les flux de nutriments et de polluants, le transport des sédiments et les services écosystémiques associés. Par conséquent, de meilleures descriptions hydrauliques des bassins versants sont essentielles pour de nombreuses applications écologiques, y compris la définition des débits écologiques, la bonne gestion des pollutions et la restauration des habitats. Les relations de géométrie hydraulique « stationnelle » décrivent les changements temporels de la hauteur d’eau, de la largeur mouillée et de la vitesse avec le débit dans un tronçon de cours d’eau donné. Les relations de géométrie hydraulique « longitudinale » décrivent les changements spatiaux des caractéristiques hydrauliques le long d'un cours d’eau ou entre des tronçons, à un débit de référence donné (par exemple, le débit médian ou le débit moyen annuel de crue). Bien que les variations des deux relations aient été largement documentées, elles sont encore peu comprises. Ces travaux de thèse visent à améliorer les connaissances sur les relations de la géométrie hydraulique et à développer des modèles qui prédisent les caractéristiques hydrauliques à travers les réseaux hydrographiques en utilisant un important jeu de données intercontinentales de mesures hydrauliques à l'échelle du tronçon. Pour prédire les paramètres de la géométrie hydraulique, de nombreuses variables mesurées sur le terrain, cartographiées ou issues de la télédétection ont été analysées dans cette étude. Ces travaux montrent que la géométrie hydraulique est bien expliquée par des modèles intercontinentaux, suggérant une grande généralité de nos modèles. Les modèles de géométrie hydraulique qui en résultent présentent un potentiel considérable pour améliorer les outils de gestion à l'échelle nationale et/ou du bassin versant. À titre d'exemple d'application écohydraulique, les modèles sont utilisés pour estimer les impacts du changement climatique sur les habitats hydrauliques des poissons dans un grand bassin versant
... The friction factor was obtained by the equations of Leopold and Wolman (1957) and Limerinos (1970), suitable for sandy gravel river, Bray (1982), which was developed particularly for sandbed rivers and Knighton (1998) which was developed for gravelly sand rivers. These roughness equations were used as they are considered the most suitable for the study river (for more details about their application; see Billi, 2008Billi, , 2011Billi, Preciso, & Salemi, 2014). The average value of the flow velocities, computed by these equations, was taken for a better approximation to the actual ones in the discharge calculation as the data obtained do not show any significant difference according to the analysis of variance (ANOVA) test performed (F = 0.12, F crit = 2.7, p > .01). ...
The magnitudes of flash floods and their effect on channel width changes were investigated for Hara river in the Raya graben (northern Ethiopia). Precipitation was measured using rain gauges evenly distributed over the study catchment. Event peak discharges were measured with crest‐stage gauges at a reference cross‐section. Changes in channel width were measured at 6 monitoring sites along a 1.5 km long reach. Decadal channel width change was analyzed over the period 1965‐2014. The average rainfall depth of the rainy days was 14.2 ± 9.4 mm whereas the highest was 41.9 mm and the lowest, capable to generate some flow in the river, was only 1.8 mm. The maximum calculated discharge peaked at 408 m3 s‐1 whereas the lowest flow was 19.5 m3 s‐1. An average widening of 0.11 ± 0.19 m per flood (for a total of 3.74 m) was measured. The cumulative precipitation and cumulative channel widening suggest that larger and longer lasting rains have substantial influence on channel width change. A marked increase in channel width commonly accompanies two‐three days of flood duration; it is less marked where there is riparian vegetation on the banks. The long term (1965, 1986, 2005, and 2010) measurements indicate that channel width increased rapidly over the last 50 years; nearly 25 m of widening. Generally, channel changes are mainly induced by the duration of floods rather than by their intensity and the data clearly show that ephemeral streams channel dynamics can be triggered also by floods much smaller than bankfull.
Rivers play a vital role in our ecosystems, providing fresh water, supporting rich biodiversity, and contributing to human well-being. However, in the face of climate change and intensive human activities, the sediment load in rivers can reach critical levels, presenting a complex set of challenges that require immediate action. The increased sediment load can alter aquatic habitats, clog channels, reduce reservoir storage capacity, and increase the risk of flooding. These direct threats entail high costs in terms of material and ecological damage, loss of life, and expenditure on rebuilding damaged infrastructure. The quantification of bedload in watercourses is therefore crucial for maintaining water and soil resources, safeguarding riparian communities, and preserving ecological balance. The study reports the findings of a three-year monitoring of the bed load of Skhirate Wadi, a river that drains a part of the western Moroccan Meseta. The study used the colorimetric monitoring method, which quantifies the volumes of coarse sediment that were transported by monitoring topographic variations in the riverbed and measuring the distances covered by the sediment. The study showed the sediment was found to move around seven times annually on average. However, the frequency and magnitude of floods and the size of particles affect the variation in this displacement. It also showed sediments travel an average distance ranging from 649 to 883 meters per year, and that the average specific bedload at the watershed scale is 30 m ³ / Km ² / year. Relationships between flood peaks mobilized sediment volumes, and average particle distances are established and discussed. These results are fundamental to understanding of coarse sediment transfer processes in the small rivers of the central plateau. They are also essential for assessing the impact on the aquatic ecosystem, on downstream dams, and on the various existing road and hydro-agricultural infrastructures. This assessment will enable the implementation of appropriate management strategies to anticipate changes and plan the planning of the river and its watershed.
Theoretical analysis and experimental testing of a debris-flow drainage channel with a step-pool configuration are conducted in this paper. The design conditions of the drainage channel with step-pool configuration were obtained by theoretical analysis, including the design shape of the step-pool configuration, design slope of the step section, and total length of a single step-pool configuration. In addition, the correction coefficients of the debris-flow jet length were obtained with different debris-flow densities by comparing the experimental and calculation results. Moreover, the debris flow repeats an acceleration-deceleration motion in the drainage channel to achieve a balanced discharge without scouring or deposition. At the downstream of the step-pool configuration, the average debris-flow velocity decreased by approximately 28 and 40% with debris-flow densities of Ρ = 1,700 and 2,000 kg=m³. Furthermore, the energy-dissipation ratio varied between debris-flow densities of Ρ = 1,700 and 2,000 kg=m³. Finally, the authors discuss different types of drainage channels in practical debris-flow hazard-mitigation engineering and parameter variations of these drainage channels.
