Graded and uniform bed load sediment transport in a degrading channel

Twenty runs of experiments are carried out to investigate non-equilibrium transport of graded and uniform bed load sediment in a degrading channel. Well-sorted gravel and sand are employed to compose four kinds of sediment beds with different gravel/sand contents, i.e., uniform 100%gravel bed, uniform 100% sand bed, and two graded sediment beds respectively with 53% gravel and 47% sand as well as 22%gravel and 78%sand. For different sediment beds, the experiments are conducted under the same discharges, thereby allowing for the role of sediment composition in dictating the bed load transport rate to be identified. A new observed dataset is generated concerning the flow, sediment transport and evolution of bed elevation and composition, which can be exploited to underpin devel-opments of mathematical river models. The data shows that in a degrading channel, the sand greatly promotes the transport of gravel, whilst the gravel considerably hinders the transport of sand. The promoting and hindering effects are evaluated by means of impact factors defined based on sediment transport rates. The impact factors are shown to vary with flow discharge by orders of magnitude, being most pronounced at the lowest discharge. It is characterized that variations in sand or gravel inputs as a result of human activities and climate change may lead to severe morphological changes in degrading channels.     

Spatial scale impact on daily surface water and sediment fluxes in Thukela river,South Africa

The on- and off-site effects of soil erosion in many environments are well known, but there is still limited understanding of the soil loss fluxes in downstream direction due, among other factors, to scarce and poor quality. A four year study to (i) evaluate water and sediment fluxes at different spatio-temporal scales and (ii) interpret the results in terms of processes involved and the controlling factors, was conducted in Thukela basin, South Africa. Five hierarchically nested catchments; namely microcatchment (0.23 km²), subcatchment (1.20 km²), catchment (9.75 km²), sub-basin (253 km²) and basin (29,038 km²), were used in addition to fifteen (1 m²) microplots and ten (10 m²) plots on five locations within the microcatchment. The results showed 19% decrease of unit-area runoff (q) from 3.1 L m⁻² day⁻¹ at microplot to 2.5 L m⁻² day⁻¹ at plot scale followed by steeper (56%) decrease at microcatchment scale. The q decreased in downstream direction to very low level (q ≤ 0.26 L m⁻² day⁻¹). The changes in q were accompanied by initial 1% increase of soil loss (SL) from 18.8 g m⁻² day⁻¹ at microplot to 19.1 g m⁻² day⁻¹ at plot scale. The SL also decreased sharply (by 39 fold) to 0.50 g m⁻² day⁻¹ at microcatchment scale, followed by further decrease in downstream direction. The decrease of q with spatial scale was attributed to infiltration losses, while initial increase of SL signified greater competence of sheet than splash erosion. The decrease of SL beyond the plot scale was attributed to redistribution of the soil on the hillslope and deposition on the stream channel upstream of the microcatchment outlet. Therefore, erosion control strategies focussing on the recovery of vegetation on the slope and stabilisation of gullies are recommended.     

Resource depletion affects the structure of an experimental benthic food web

In this study, we investigated the long-term (2-year) response of a detritus-based food web to resource depletion simulated in microcosms filled with lake sediment. The effects of resource depletion on the food web were estimated from the biomass responses of a simplified, size-based trophic network comprising three trophic levels: (1) bacteria, (2) bacterivorous meiofauna, and (3) omnivore/predacious nematodes and oligochaetes. The results showed that sediment bacteria and organic carbon stock were not affected by resource depletion, whereas oligochaetes, large nematodes and some bacterivores, such as rotifers and ostracods, decreased dramatically. At the same time, resource depletion had no negative effect on minute bacterivorous nematodes and harpacticoid copepods, suggesting their release from competition with other bacterivores and/or their release from potential predation by large nematodes and oligochaetes. Our results globally confirmed classical trophic dynamics models in which a shortage in resources dampens food-chain length. Yet, low resources did not completely eliminate all large invertebrates, and did not affect the standing stock of detritus and bacteria, suggesting that detritus-based food webs could show a remarkable resilience to the effects of resource depletion.     

Fluid–particle interaction in turbulent open channel flow with fully-resolved mobile beds

The paper presents Direct Numerical Simulations of an open channel flow laden with spherical particles at a bulk Reynolds number of 2941. The transport of thousands of mobile particles is simulated propagating over a rough bed which consists of immobile particles of the same size in hexagonal ordering. An Immersed Boundary Method is used for the numerical representation of the particles. With 22 points per diameter even the viscous scales of the flow are resolved at this Reynolds number. The reference run contains just as many fixed as mobile particles with a relative density slightly above the nominal threshold of incipient motion. Further runs were conducted with decreased mass loading and decreased Shields number together with a simulation containing only immobile particles. The variation of the parameters defining the mobile sediment yields a strong modification of particle–fluid as well as particle–particle interactions yielding different structures in space and time. This is assessed by means of appropriate statistical quantities addressing the continuous and the disperse phase. The results are in qualitative agreement with experimental observations at higher Reynolds number.     

The LHLLC scheme for Two-Layer and Two-Phase transcritical flows over a mobile bed with avalanching,wetting and drying

A numerical method of the Godunov type is presented for solving either Two-Phase or Two-Layer forms of Debris Flow Models (DFMs) describing shallow-water flow and sediment dynamics. DFMs explicitly link sediment concentrations to the momentum balance, and thus can be applied to cases involving high sediment concentrations, as in debris flows, in addition to low concentration test cases typically found in surface waters. In this paper, Two-Phase and Two-Layer DFMs are presented in a common mathematical framework to illuminate key similarities and differences and lay a foundation for a general purpose DFM solver. The proposed solver termed LHLLC is shown to achieve good accuracy over a wide range of test cases. Importantly, numerical diffusion of sediment profiles is minimized, particularly on steep slopes, the scheme is shown to preserve stationary solutions involving wet/dry interfaces, and the scheme accounts for gravity-driven slumping (avalanching) which cannot be resolved by classical DFMs.     

Assessment of reservoir sedimentation effect on coastal erosion in the case of Nestos River,Greece

Coastal erosion that is generated by the reduction of the annual sediment yield at river outlets, due to the construction of reservoirs, constitutes one of the main environmental problems in many parts of the world. Nestos is one of the most important boundary rivers, flowing through Bulgaria and Greece, characterized by its great biodiversity. In the Greek part of the river, two reservoirs, the Thisavros Reservoir and the Platanovrysi Reservoir, have already been constructed and started operating in 1997 and 1999, respectively. The present paper constitutes the first attempt where the assessment of reservoir sedimentation effect on the coastal erosion for the case of the Nestos River delta and the adjacent shorelines is addressed in detail, through mathematical modeling, modem remote sensing techniques and field surveying. It is found that the construction and operation of the considered reservoirs have caused a dramatic decrease （about 83%） in the sediments supplied directly to the basin outlet and indirectly to the neighbouring coast and that this fact has almost inversed the erosion/accretion balance in the deltaic as well as the adjacent shorelines. Before the construction of the reservoirs, accretion predominated erosion by 25.36%, while just within five years after the construction of the reservoirs, erosion predominates accretion by 21.26%.     

Geoaccumulation and distribution of heavy metals in the urban river sediment

Current study presents the application of chemometric techniques to comprehend the interrelations among sediment variables whilst identifying the possible pollution source at Langat River,Malaysia.Surface sediment samples（0-10 cm）were collected at 22 sampling stations and analyzed for total metals（~（48）Cd,~（29）Cu,~（30）Zn,~（82）Pb）,pH,redox potential（Eh）,salinity,electrical conductivity（EC）,loss on ignition（LOI）and cation exchange capacity（CEC）.The principal component analysis（PCA）scrutinized the origin of environmental pollution by various anthropogenic and natural activities：four principal components were obtained with 86.34%（5 cm）and88.34%（10 cm）.Standard,forward and backward stepwise discriminant analysis effectively discriminate 2variables（84.06%）indicating high variation of heavy metals accumulation at both depth.The cluster analysis accounted for high input of Zn and Pb at LA8,LA 10,LA 11 and LA 12 that mergers three（5 cm）and four（10cm）into clusters.This is consistent with the contamination factor（C_1）that shows high Cd（LA 1）and Pb（LA 7,LA 8,LA 10,LA 11 and LA 12）contaminations at 5cm.These indicate that Pb and Zn are the most bioavailable metals in the sediment with significant positive linear relationship at both sediment depths.Therefore,this approach is a good indication of environmental pollution status that transfers new findings on the assessment of heavy metals by interpreting large complex datasets and predicting the fate of heavy metals in the sediment.     

Modelling of meander migration in an incised channel

An updated linear computer model for meandering rivers with incision has been developed. The model simulates the bed topography, flow field, and bank erosion rate in an incised meandering channel. In a scenario where the upstream sediment load decreases （e.g., after dam closure or soil conservation）, alluvial river experiences cross section deepening and slope flattening. The channel migration rate might be affected in two ways： decreased channel slope and steeped bank height. The proposed numerical model combines the traditional one-dimensional （1D） sediment transport model in simulating the channel erosion and the linear model for channel meandering. A non-equilibrium sediment transport model is used to update the channel bed elevation and gradations. A linear meandering model was used to calculate the channel alignment and bank erosion/accretion, which in turn was used by the 1D sediment transport model. In the 1D sediment transport model, the channel bed elevation and gradations are represented in each channel cross section. In the meandering model, the bed elevation and gradations are stored in two dimensional （2D） cells to represent the channel and terrain properties （elevation and gradation）. A new method is proposed to exchange information regarding bed elevations and bed material fractions between 1D river geometry and 2D channel and terrain. The ability of the model is demonstrated using the simulation of the laboratory channel migration of Friedkin in which channel incision occurs at the upstream end.     

Numerical analysis of synthetic granulate deposition in a physical model study

The current study focuses on the application of a three-dimensional numerical model for the prediction of morphological bed changes. The sediment deposition in a reservoir during a 10-year-flood was investigated and the results of the simulation were validated with data derived from a physical model study. Because of the small grain sizes in the prototype, synthetic granulate was used in the physical model. The numerical computation domain was a reproduction of the physical model, including the grain sizes and the density of the particles, in order to ensure comparability. The CFD code SSIIM, which solves the RANS-equations in three-dimensions, was used for the simulations. The sediment transport in SSIIM is divided into suspended sediment transport, computed by solving the convection-diffusion equation, and bed-load transport, calculated by an empirical formula. The results of the numerical simulation correspond well to the results of the physical model study. The simulated location and the pattern of the sediment deposition in the reservoir are an accurate representation of the observed distribution in the physical model.     

Long-chain n-alkenes in recent sediment of Lake Lugu (SW China) and their ecological implications

Long-chain n-alkenes showing a predominance of n-C_25:1 and n-C_27:1 were detected in recent sediment of Lake Lugu, an oligotrophic alpine lake in Yunnan Province, SW China. The abundances of n-C_25:1 and n-C_27:1 alkenes varied considerably in the sediment core, most obviously in showing higher values during the period of 1821–1982, followed by a significant decrease between 1982 and 2012. Such variations were similar to those of long-chain 1,15-alkyl diols, biomarkers of eustigmatophytes, and a significant correlation was observed between n-C_25:1 (or n-C_27:1) alkene and C₃₂ 1,15-alkyl diol in the sediment core. This correlation and the fact that long-chain n-alkenes in some eustigmatophytes (e.g., Nannochloropsis sp.) were dominated by C₂₅ and C₂₇ compounds indicates that eustigmatophytes were the most likely contributor of the long-chain n-alkenes (especially the n-C_25:1 and n-C_27:1 alkenes) in Lake Lugu sediment. Chlorophytes, on the other hand, cannot be excluded as a possible contributor of the long-chain n-alkenes because these algae are common in Lake Lugu and they are known to biosynthesize n-C_27:1 alkene. Productivities for some species of eustigmatophytes and chlorophytes considerably decreased when the lake water was heavily influenced by catchment soil erosion and anthropogenic sewage inputs.