Comment on esurf-2021-97

The transportation of bank-collapsed materials is a key issue among river evolution processes. In this study, a series of flume experiments were conducted to monitor riverbank collapse processes and to explore the regularity of transportation for cohesive collapsed materials. The collapsed materials, both the bed and suspended loads, that transformed from collapsed materials were intensively evaluated under experimental conditions. The results showed that the collapsed materials contributed to 12~20 % sedimentation in situ, 8~14 % suspended loads and 70~80 % bed loads. In addition, the bed load motion efficiency coefficient (eb), suspended load motion efficiency coefficient (es) and sediment carrying capacity factor (U3/gRω) were introduced to describe the transportation of collapsed materials in terms of energy dissipation. This research provides theoretical and practical benefits for predicting channel evolution processes.

A high-accuracy numerical solution, coupling one-dimensional shallow water and bed-evolution equations, with, for the first time, a suspended sediment advection equation, thereby including bed and/or suspended load, is used to examine two swash events on an initially plane erodible beach: the event of Peregrine & Williams (J. Fluid Mech., vol. 440, 2001, pp. 391–399) and that of a solitary wave approaching the beach. Equations are solved by the method of characteristics, and the numerical model is verified. Full coupling of suspended load to beach change for Peregrine & Williams (J. Fluid Mech., vol. 440, 2001, pp. 391–399) yields only slightly altered swash flows, depending on beach mobility and sediment response time; a series of similar final beach change patterns results for different beach mobilities. Suspended- and bed-load transport have distinct morphodynamical signatures. For the solitary wave a backwash bore is created (Hibberd & Peregrine, J. Fluid Mech., vol. 95, 1979, pp. 323–345). This morphodynamical bore propagates offshore initially, and leads to the creation of a beach bed step (Larson & Sunamura, J. Sedimentary Petrology, vol. 63, 1993, pp. 495–500), primarily due to bed-load transport. Its height is directly related to bed-load mobility, and also depends strongly on the bed friction coefficient. The shock dynamics of this bed step is explained and illustrated. Bed- and suspended-load mobilities are quantified using field data, and an attempt is made to relate predictions to measurements of single swash events on a natural beach. Average predicted bed change magnitudes across the swash are of the order of 2 mm, with maximum bed changes of up to approximately 10 cm at the bed step.

Fan, F.; Zhu, Z.; Lu, Y.; Yang, B., and Zhao, C., 2020. Numerical investigation of the effect of suspended load on the morphology of a beach under the action of tsunamis. In: Zheng, C.W.; Wang, Q.; Zhan, C., and Yang, S.B. (eds.), Air-Sea Interaction and Coastal Environments of the Maritime and Polar Silk Roads. Journal of Coastal Research, Special Issue No. 99, pp. 174–180. Coconut Creek (Florida), ISSN 0749-0208.To study the morphology of a beach under the action of a tsunami, a beach evolution numerical model based on the open-source calculated fluid dynamics (CFD) code OpenFOAM is constructed. The bed load and suspended load models are fully coupled with the hydrodynamics model in the beach evolution model, and the moving mesh method is used to capture the change in bed morphology. Then, a laboratory experiment on the scour induced by solitary waves is adopted to validate the constructed numerical model, and the surface elevation, bed shear stress and bed profile of the model fit very well with the experimental observations. Finally, the effect of the suspended load on the evolution of beach morphology and the process of beach evolution under the action of a solitary wave are studied with a numerical model. The study results show that the beach evolution mainly occurred at the time the solitary wave ran down and that the suspended load played a major role in the beach evolution, contributing approximately 85% to the scour profile.

Bed material load has been classified into a bed load and a suspended load based on their pattern of motion. And then the theoretical and expreimental investigations were carried out on each motion separately. But the moving characteristics of each motion should be connected smoothly, because both types of sediment are supplied into the flow from the bed due to the action of water flow. Considering such a concept, the first unified simulation model for bed material load was constructed in this study by extending the saltation model proposed previously. This model enables us to investigate the mechanism of transition between a bed load and a suspended load, and to analyze a series of particle trajectory in the condition where both type of motion coexist.

Lateglacial climatic oscillations exerted profound impacts on the Meuse fluvial system. In the Lower Meuse (southern Netherlands), geomorphological studies in the last decades mainly centred on Lateglacial vegetation evolution, channel pattern changes and river terrace formation. Little information has been reported about the paleohydrology and its relation with the regional climate conditions. This study investigates a sediment core that contains flood sediments deposited from the early Allerød up to the middle Holocene. We conducted grain size analysis, thermogravimetric analysis (organic matter and calcium carbonate content), pollen and macrofossil analysis, and determined the oxygen and carbon stable isotope ratios of biogenic carbonate (opercula of the freshwater gastropod Bithynia tentaculata). The chronology of the core is based on AMS 14C dating and pollen biostratigraphical correlation. The pollen and macrofossil studies reveal that the core site was a lake environment during the Allerød and Younger Dryas periods. The oxygen isotope record in conjunction with organic matter and carbonate content are believed to have captured the intra-Allerød Cold Period (IACP). The synchronous variation of oxygen and carbon isotopes with calcium carbonate content indicates a dominant evaporation effect on carbonate chemistry of the lake environments during the warm Allerød interstadial. End-member modelling decomposes the grain-size distributions into two sandy end members (bed load) and two silty-clayey end members (suspended load). In order to highlight the flood signal, we constructed two flood energy indexes (FEI-1 and FEI-2) that reflect the coarseness of the suspended load and bed load, respectively. Both indexes show a relatively high flood condition during the IACP, followed by a low flood phase in the Late Allerød and quickly intensified flood conditions at the onset of the Younger Dryas. In the second phase of the Younger Dryas, deposition of sandy aeolian sediments to the core site complicates the paleoflood identification using FEI-2 (bed load). This study shows a high sensitivity of the hydrological process and sedimentary environment at the Lower Meuse to the regional climate system.

Vertically-averaged and moment equations for flow and sediment transport

<p><strong>Trace metal and nutrient fluxes into Arctic ocean by largest Siberian rivers (ArcticFlux)</strong></p><p>Precise estimates of river runoff are one the most challenging fields of river hydrology. Quantitative assessment of the fluxes of suspended and, especially, bed load, as well as their correlation with the flow dissolved loads remains weekly studied with a crucial need of in-situ observations, especially in large rivers.</p><p>The project is focused on semi-empirical and modeling study of flows, concentrations, modes and loads of trace metals and nutrients fluxes of the major rivers of Arctic. Monitoring stations were organized at the outlets of largest Siberian rivers: Ob, Yenisei, Lena, Kolyma, which transport more than 60 % of the water flow from the Russian Arctic. Observations were made for high and low water regime periods on the regular basis, and the total number of samples today exceeds 210. For each sample analyses were made for trace metals (68 elements), nutrients and dissolved and suspended organic carbon matter content both in dissolved and particulate (suspended and bed loads) forms. These samples can determine annual and seasonal distribution to 70% of the chemical elements and substances, carried by large rivers of the Russian Arctic into the Arctic Ocean.</p><p>For more accurate flux assessment, a new sampling technique was used. It allows to determine all components of the dissolved, suspended and, especially, bed load along the river section and includes sampling at 3-5 verticals on different depth. As a result, it is possible to determine the variability of the fluxes along the width of the section. As an example, concentrations of suspended sediments on the left and right banks of the Kolyma River differ in 6-7 times (up to70 mg / dm<sup>3</sup>) and there are significant differences in Ni, Fe, Al, Cu, and Pb fluxes. Heterogeneity in the distribution of sediment and chemical flow across the width of the rivers arise due to the inflow of tributaries and as a result of permafrost melting and wave erosion of the banks. The study of the intensity of bank erosion and sedimentation at the outlets of Arctic rivers both in the field and according to remote sensing data is a significant part of the project. Based on the modeling techniques and application of erosion models for all four Arctic catchments it will also focus on the novel quantitative assessment of bank and catchment erosion contribution into chemical and sediment loads.</p><p>The project concept is considered as a part of Marine component of Pan-Eurasian program (PEEX) and builds a bridge to integrate PEEX marine components with the existing terrestrial/atmospheric PEEX</p><p>The reported study was funded by RFBR according to the research project 18-05-60219</p>

Bed load transport under different streambed conditions – a field experimental study in a mountain stream

A depth-averaged two-dimensional (2D) numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered, curvilinear grid. The 2D shallow water equations are solved by the SIMPLE(C) algorithms with the Rhie and Chow’s momentum interpolation technique. The proposed sediment transport model adopts a nonequilibrium approach for nonuniform total-load sediment transport. The bed load and suspended load are calculated separately or jointly according to sediment transport mode. The sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes. An empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed-load movement direction in channels with steep slopes. Flow and sediment transport are simulated in a decoupled manner, but the sediment module adopts a coupling procedure for the computations of sediment transport, bed change, and bed material sorting. The model has been tested against several experimental and field cases, showing good agreement between the simulated results and measured data.

Can detailed sampling and taphonomical analysis of foraminiferal assemblages offer new data for paleoecological interpretations?

The total amount of sediment transported through a given section of a river for the given flow and sediment bed conditions is called total-load transport. Based on the mode of sediment transport, the total load is the sum of the bed, suspended, and wash load. The total load is also informally called the bed-material load as it contains only those sediment particles that come from the sediment bed excluding the wash load. There are two general approaches to determine the total load. They are indirect approach and direct approach. In indirect approach, bed and suspended loads are estimated separately and then added together to quantify the total load. In direct approach, the total-load function is directly determined without dividing it into bed and suspended loads. The method of computation of total-load transport is illustrated through worked out examples.

Contrasting modes of sediment infilling and geomorphic change within a sand dominated dredge pit on the inner Louisiana shelf

Prediction of total sediment load has been a challenge to river engineers for decades. Two approaches are typically used: One is to directly calculate the total sediment load from measured flow and sediment properties, and the other is to separate total sediment load into bed load and suspended load and calculate them independently. Because the criteria that separate bed load and suspended load is still a debatable subject, practical engineers prefer to use the total load equation for estimating sediment load. However, there are more than 31 equations for calculating total sediment load, and the discrepancies of those equations are in the orders of magnitude. To obtain a general equation, this study analyzed more than 4,000 sets of laboratory experimental and 3,000 sets of field measurements of total sediment load. Based on the dimensional analysis, eight new dimensionless parameters are formulated to quantify total sediment load. Correlations of dimensionless total sediment load with those new and other conventional parameters are calculated using the observed data. The highest correlation, 0.94, was found between the dimensionless total sediment load and a new dimensionless parameter, τ* (u* - u*c) / ω , in which u* is shear velocity, ω is settling velocity, and τ* is dimensionless shear stress. A simplified power-law relation is formulated from fitting the measured data. This new relation is compared with the commonly used total sediment load relation, such as Engelund-Hansen (1967), Ackers-White (1972), Yang (1973, 1979). Results showed the new simplified equation yielded the best matches of this set of total sediment load data.

Coarse sediment transport in Himalayan rivers exerts a first-order control on river morphology, flood hazard, and the longevity of hydropower infrastructure. Variations in gravel or sand export from the mountain front govern downstream channel incision and aggradation rates, directly modifying channel conveyance capacity and flood risk on the Gangetic Plains. These highly mobile gravel-bed rivers draining the Himalayas are prone to abrupt channel switching driven by the mobility of coarse bedload, exacerbating flood hazard. Despite their importance, current flood models and hydropower development plans remain limited by uncertainties in bedload flux estimates. To understand the controls on sediment yield from the mountain front, we monitor both suspended sediment load through direct sampling, and bedload through acoustic and seismic monitoring at the mountain front of the Karnali and West Rapti Rivers in Nepal. Two Aquarian hydrophones connected to AudioMoth data loggers are installed at each site to monitor bedload transport for the duration of an entire monsoon season. This acoustic dataset is combined with seismic data from a DiGOS geophone at each site, water level data from the Department of Hydrology and Meteorology in Nepal, and manual suspended sediment concentration sampling. These complementary approaches independently approximate the sand and gravel fractions respectively, allowing us to evaluate the accuracy of current approximation methods for bedload in Himalayan rivers. The results indicate that suspended sediment loads are primarily supply-limited, showing a pronounced seasonal hysteresis in suspended sediment concentration with peaks that record local events such as landslides and storms in the catchment. In contrast, the bedload flux appears to be transport-limited, closely tracking the river discharge. Acoustic data show initial bedload movement at lower discharges, with an abrupt increase in bedload transport signal at a threshold discharge. At higher discharges there is a saturation of the signal, which may reflect boundary conditions of bedload transport or data clipping. These contrasting signals indicate independent transport mechanisms for suspended load and bedload, with a high degree of variability in suspended load relative to a much more predictable bedload flux. Seasonal fluctuations in this bedload-to-suspended-load ratio demonstrate that bedload estimates based on suspended sediment measurements can substantially misrepresent total sediment fluxes, with major implications for flood early warning systems and urban planning.

Surrogate technologies to monitor bed load discharge have been developed to supplement and ultimately take over traditional direct methods. Our research deals with passive acoustic monitoring of bed load flux using a hydrophone continuously deployed near a river bed. This passive acoustic technology senses any acoustic waves propagated in the river environment and particularly the sound due to interparticle collisions emitted during bed load movement. A data set has been acquired in the large Alpine gravel‐bedded Drau River. Analysis of the short‐term frequency response of acoustic signals allows us to determine the origin of recorded noises and to consider their frequency variations. Results are compared with ancillary field data of water depth and bed load transport inferred from the signals of a geophone array. Hydrophone and geophone signals are well correlated. Thanks to the large network of deployed geophones, analysis of the spatial resolution of hydrophone measurements shows that the sensor is sensitive to bed load motion not only locally but over distances of 5–10 m (10–20% of river width). Our results are promising in terms of the potential use of hydrophones for monitoring bed load transport in large gravel bed rivers: acoustic signals represent a large river bed area, rather than being local; hydrophones can be installed in large floods; they can be deployed at a low cost and provide continuous monitoring at high temporal resolution.