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Flow conditions (flow discharge, flow depth, and flow velocity) in natural streams are mainly determined via the flow resistance formula such as Manning’s equation. Evaluating the accurate Manning’s roughness coefficient (n), especially in rivers with bed form during floods, to obtain more reliable results has always been of interest to scholars. The interaction between the flow and bed form is very complex since the flow conditions control bed forms, and vice versa. The main goal of the present study is to predict n in rivers with bed forms, using soft computing models, including multilayer perceptron artificial neural network (MLPNN), group method of data handling (GMDH), support vector machine (SVM) model, and genetic programming model (GP). To this end, the energy grade line (Sf), flow Froude number (Fr), the relative submergence (y/d50; y = flow depth and d50 = bed sediment size), and the bed form dimensionless parameters (Δ/d50, Δ/λ, and Δ/y; ∆ = bed form height and λ = bed form length) were used as the input variables, and n was used as the output variable. The results showed that all the test models have acceptable accuracy, while the SVM model showed the highest level of accuracy with the coefficient of determination R2=0.99 in the verification stage. The sensitivity analysis of SVM and MLPNN models and the structural analysis of GMDH and GP models indicated that the most important parameters affecting n are Fr, Sf, and Δ/λ.

Hyporheic flows, which stem from the interaction between stream flow and bedform, transport solute‐laden surface waters into the streambed sediments, where reactive solutes undergo biogeochemical transformations. Despite the importance of hyporheic exchange on riverine ecosystem and biogeochemical cycles, research is limited on the effects of hyporheic fluxes on the fate of reactive solutes within the hyporheic zone. Consequently, we investigate the controls of hyporheic flowpaths, which we link to stream morphology and streamflow, on prevailing hyporheic redox conditions and on biogeochemical transformations occurring within streambeds. We focus on the dissolved inorganic reactive forms of nitrogen, ammonium and nitrate, because nitrogen is one of the most common reactive solutes and an essential nutrient found in stream waters. Our objectives are to explore the influence of stream morphology, hyporheic water temperature and relative abundance of ammonium and nitrate, on transformation of ammonium, removal of nitrates and production of nitrous oxide, a potent greenhouse gas. We address our objectives with analytical solutions of the Multispecies Reactive Advection‐Dispersion Equation coupled with linearized Monod's kinetics and analytical solutions of the hyporheic flow for alternate‐bar morphology. We introduce a new Damköhler number,Da, defined as the ratio between the median hyporheic residence time and the time scale of oxygen consumption, which we prove to be a good indicator of where aerobic or anaerobic conditions prevail. In addition, Dais a key index to quantify hyporheic nitrification and denitrification efficiencies and defines a new theoretical framework for scaling results at both the morphological‐unit and stream‐reach scales. Key Points Modeling dissolved inorganic nitrogen cycle within the hyporheic zone Investigates the effects of bar morphology on hyporheic nitrogen cycle Compares the model results with measured data of nitrous oxide emissions

A set of laboratory experiments were conducted to study the impact of vegetation on bed form resistance and bed load transport in a mobile bed channel. Vegetation stems were simulated by using arrays of emergent polyvinyl chloride (PVC) rods in several staggered configurations. The total flow resistance was divided into bed, sidewall, and vegetation resistances. Bed resistance was further separated into grain and bed form (i.e., ripples and dunes) resistances. By analyzing experimental data using the downhill simplex method (DSM), we derived new empirical relations for predicting bed form resistance and the bed load transport rate in a vegetated channel. Bed form resistance increases with vegetation concentration, and the bed load transport rate reduces with vegetation concentration. However, these conclusions are obtained by using experimental data from this study as well as others available in the literature for a vegetated channel at low concentration.

The paper presents the results of an experimental study of mean fluid flow and turbulence over bed forms in a unidirectional flow with superimposed surface waves. Experiments were performed with only current and with combined wave‐current flows over a series of bed forms under different surface wave frequencies for two different Reynolds numbers. Three‐dimensional velocity was measured using a 3‐D micro acoustic Doppler velocimeter. The superposition of surface waves with increasing frequency leads to an increase in the apparent bottom roughness due to a vortex in the lee, which causes the resistance to the flow. The effect of surface waves is to increase the flow stability, consequently reducing flow separation and enhanced mixing in the lee side of the bed form. A stronger circulation pattern in the lee side of the bed forms is observed at a higher Reynolds number.

One of the most relevant features of alluvial rivers concerns flow resistance, which depends on many factors including, mainly grain resistance and form drag. For natural sand-bed rivers, dunes furnish the most significant contribution and this paper provides an insight on it. To achieve this aim, momentum balance equations and energy balance equations are applied to free flow in alluvial channels, assuming hydrostatic pressure distribution over the cross sections confining the control volume, which includes a reference bed form pattern. The resulting equation in terms of energy grade accounts for an empirical bed form drag coefficient resulting from the actual flow pattern and bed form geometry. The model has been validated using a large selection of field data and it seems somewhat sensitive to the dune geometry and to the Nikuradse equivalent roughness, whereas it is shows greater sensitivity to the adopted grain surface resistance formula (e.g., Manning–Strickler formula).

We report an experimental investigation of the motion of bed load particles under steady and spatially uniform turbulent flow above a flat sediment bed of uniform grain size. Using a high‐speed video imaging system, we recorded the trajectories of the moving particles and measured their velocity and the length and duration of their flights, as well as the surface density of the moving particles. Our observations show that entrained particles exhibit intermittent motion composed of the succession of periods of “flight” and periods of rest. During one flight, a particle may go through phases of reptation, during which it moves in nearly persistent contact with the rough bed, and phases of saltation, during which it travels sufficiently high above the bed to reach high velocities. The distributions of longitudinal and transverse particle velocities obey a decreasing exponential and a Gaussian law, respectively. Interestingly, these observations are similar to those previously reported for viscous flows. The experimental results presented here support the erosion‐deposition model of Charru (2006) and allow the calibration of the involved coefficients. In particular, noting τ*, the Shields number, and τ*c, the threshold Shields number, we find that (1) the surface density of moving particles increases linearly with τ* − τ*c; (2) the average particle velocity increases linearly with τ*1/2 − τ*c1/2, with a finite nonzero value at the threshold; (3) the flight duration scales with a characteristic settling time with no significant dependence on either τ* or the settling Reynolds number; and (4) the flight length increases linearly with τ*1/2 − τ*c1/2. The results presented in this paper should provide a valuable physical framework to describe bed form development in turbulent flows.

The development of sustainable river management strategies requires knowledge of the effect of vegetation on hydrodynamics and sediment transport. To date, the complex physical processes involving the combined effects of leafy flexible vegetation and mobile bedforms are not completely understood. Most sediment transport models have been developed for bare bed conditions so that their performance in the presence of leafy flexible vegetation remains unclear. On the other hand, recently developed models consider vegetated conditions but they typically account only for the presence of rigid cylinders and in some cases scour at their base. For this purpose, laboratory experiments were conducted with mobile dune bed conditions and artificial flexible plants with varying Leaf Area Index to investigate the effect of flexible vegetation on bed morphology and sediment transport. Sediment transport rates and bedform characteristics such as height, wavelength and celerity, were measured in specifically designed experimental runs. The collected data show that the presence of leafy vegetation alters bed morphology, tending to reduce the average dune wavelength and leading to the formation of complex 3D geometries. Bed‐shear‐stress‐based models for predicting sediment transport were verified to be valid under conditions of low vegetation roughness density. On the contrary, the collected data emphasize that the measured bed‐load transport rate increased in the presence of leafy flexible vegetation with higher frontal area. Recent bed‐load models for vegetated channels provide a better interpretation for dense leafy vegetation but are less effective when predominant effects related to dunes are present. Key Points Near‐bed leaves decrease the average dune wavelength and influence their tridimensionality, challenging common bedform stability diagrams Classical bed‐load transport models proved effective in scenarios with limited frontal vegetation area and in the absence of foliage The presence of leafy vegetation leads to higher bed‐load transport than predicted by classical sediment transport models

In rivers worldwide, multiple scales of dunes coexist. It is unknown how the larger, primary dunes interact with secondary bedforms that are superimposed. We test the hypothesis that streamwise variability in the sediment flux inferred from the downstream migration of secondary bedforms explains migration of the host dune, based on bathymetric data from a lowland, sand‐bedded river. Results indicate that transport estimated from secondary bedform migration increases along the host dune stoss, eroding the stoss slope. When the superimposed bedforms disintegrate at the primary lee slopes, results indicate that all sediment transport associated to secondary bedform migration is arrested in the lee of the host dune, explaining migration of the host dune. When secondary dunes persist however, only part of the sediments transport linked to secondary dunes contributes to the migration of the host dune. This study gives novel insight into the fundamental mechanisms controlling the kinematics of compound dunes. Plain Language Summary Dunes are undulating features that can develop on a sandy river bed. They migrate downstream as a result of sediments moving from the stoss, the upstream facing slope of the dune, to the lee, the downstream slope of the dune. Sometimes, multiple scales of dunes coexist, where trains of small dunes travel over larger dunes. In this study we investigate how two dune scales interact and how they contribute to the downstream transport of bed sediments. This is done based on a series of field campaigns in the River Waal. The results indicate that migrating secondary dunes contribute to the displacement of the host dune, the dune over which they migrating. In some cases, secondary dunes travel over the host dune stoss and disintegrate at the host dune lee, depositing sediment there. In other cases, secondary dunes travel over the full length of the host dune toward the next, downstream dune. In this case, part of the sediments transport linked to the secondary dunes contributes to the downstream displacement of the host dune, and part of the sediments are transported to the next primary dune. Key Points Secondary bedforms are omnipresent and are key to understanding primary dune behavior Sediment transport rates linked to migration of superimposed river bedforms increase over the host dune stoss and decrease over the lee side Secondary bedforms control migration of the host dune, both when they persist over the host dune and when they disintegrate at the lee side

Microbes are known to shape topographies; however, mechanisms of biofilm‐sediment interactions and the dynamic evolution of biofilm‐covered bedforms remain poorly understood. Here, we explore the effects of synthetic biofilms on the geometry and temporal evolution of underwater bedforms through flume experiments. Our results demonstrate that synthetic biofilms can produce sedimentary structures similar to those formed by natural microbes, including wrinkles, pits, flip‐overs, roll‐ups, mat chips, and erosional edges. We observed the formation of wrinkles, a common geological feature, due to the accumulation of sand grains on the biofilms. Furthermore, we demonstrated that biofilms can reduce bed roughness by an order of magnitude in the low flow regime. However, the subsequent biofilm‐sediment interactions can increase local bedform size, forming multi‐scale geometries of bedforms. Our study improves the fundamental understanding of the landscape dynamics of bedforms covered by natural biofilms. Plain Language Summary Microbes, such as algae and bacteria, are known to modify landscapes through sediment stabilization, nutrient cycling, and the formation of sedimentary structures. Sediments covered by microbial layers, referred to as biofilms, have been found in various environments, including streams, coastal zones, and other shallow water ecosystems. However, our understanding of the role of biofilms in shaping landscapes remains scarce due to a lack of systematically controlled experiments. To address this research gap, we conducted experiments in a water channel and monitored the evolution of a bed covered by synthetic biofilms. We observed the formation of sedimentary structures that are similar to microbially induced sedimentary structures found in geological records and those observed on sediments covered by natural biofilms. Our experiments suggest that while the initial biofilms can reduce bed roughness by up to an order of magnitude, biofilm fragments can contribute to the formation of multi‐scale geometries of bedforms by locally increasing bed height due to their interaction and aggregation with sediments. Our results provide direct observational data on how biofilms impact the time evolution and shape of bed topographies, offering a foundation for predictions of bedforms and landscapes in the presence of microbial layers. Key Points Typical microbial sedimentary structures were observed on the sediment bed covered by synthetic biofilms Biofilms can reduce bed roughness by an order of magnitude Biofilm‐sediment interactions lead to the formation of multi‐scale geometries of bedforms with locally elevated bedform features