Deposited sediment settlement and consolidation mechanisms

In order to study deposited sediment settlement and consolidation mechanisms, sediment settlement experiments were conducted using a settlement column. Based on the experimental results, sediment settlement stage definition, excessive pore pressure （EPP） dissipation, and consolidation constitutive equations are discussed. Three stages, including the free settlement, hindered settlement, and self-weight consolidation settlement stages, are defined. The results of this study show that sediment settlement is mainly affected by the initial sediment concentration and initial settlement height, and the interface settlement rate is linearly attenuated with time on bilogarithmic scales during the hindered settlement and self-weight consolidation settlement stages. Moreover, the deposited sediment layer in the self-weight consolidation settlement stage experiences large strains, and the settlement amount in this stage is about 32% to 59% of the initial height of deposited sediment. EPP is nonlinearly distributed in the settlement direction, and consolidation settlement is faster than EPP dissipation in the self-weight consolidation settlement stage. Consolidation constitutive equations for the hydraulic conductivity and effective stress, applicable to large-strain consolidation calculation, were also determined and fitted in the power function form.

The buffering effects of aquatic sediments against acidic deposition

The cation exchange and the chemical weathering are two important processes for sedi-ments to buffer the acidification of surface water. In this paper, for sediments these two processeshave been studied. The relationship between BFC of sediments and ANC of the surface waters andthe weathering characteristics of sediments were discussed. The buffering effect and mechanism of sedi-ments against acidic deposition have also been studied. The results show that BFC of sediments arepositively correlative to ANC of the surface waters. the chemical weathering rate of sediments can beexpressed as R=k×[H￣ +]￣m.

Influencing factors of sediment deposition and their spatial variability in riparian zone of the Three Gorges Reservoir,China

Many factors can affect the sediment deposition and soil erosion process in riparian zone, including terrain, sediment transport and water level fluctuations. Clarifying the factors influencing sediment deposition process in the riparian zone of the Three Gorges Reservoirs is an important problem to determine the key area of sediment deposition and its trend of development in the study area. In order to reveal the influence of these environmental factors on the sediment deposition in riparian zone of the Three Gorges Reservoir, this study investigated 1） the amount of deposited sediment in different environmental conditions, 2） the potential factors affecting sediment deposition in riparian zone of the Three Gorges Reservoir, 3） the relationship between the deposited sediment amount and these factors previously mentioned using correlation analysis, and 4） the influence of human activities considered as an additional factor. This study found that 1） slope gradient, elevation, inundating duration and human activities were the main factors influencing sedimentation in riparian zone of the Three Gorges Reservoir, and 2） the impact of each factor varied with spaces. Specifically, in the upper reach from Jiangjin to Fuling, human activities such as gravel dredging, bank revetment and ports and wharfs constructing disturbed considerable amounts of deposited sediment, as a result, there was no natural law to dictate the distribution. In the middle reach from Fuling to Fengjie, slope gradient and inundating duration were the controlling factors, and the sediment deposition amount was greater in the areas with a gentler slope or lower elevation. Water flow on gentler slopes generally had lower velocity, resulting in more sediment to deposit. Sites with lower elevations would be drowned by sediment-laden flow with a longer duration resulting from hydrologic regime controlled by the operating strategy of the Three Gorges Reservoir, leading to a larger amount of sediment deposition. In the lower reach from Fengjie to Zigni, slope gradient was similar to the middle reach, performing a primary factor, while other factors showed little relationship with sediment amount.

Comparative Analysis of Sequence Characteristics among Different Superimposed Stages of the Chelif Basin,Algeria

Superimposed basins were investigated with respect to tectonic evolution, sediment deposition and petroleum characteristics within a single superposition stage generally. The comparative study was seldom seen. Sequence characteristics were compared for two different superimposed stages - an expanding rifting stage and a depression-foreland transition stage - in the Chelif Basin during the Miocene in this paper. A model and mechanism for sequence evolution of superimposed basins in different dynamic situations are discussed with respect to sequence similarities and differences. The compared characters include sequence thickness, sequence boundaries and system tracts, as well as sediment deposition within sequences and sequence development patterns. Finally, some typical features of sequence development concomitant with changes of superimposed stages in the Chelif Basin are discussed.

Analysis on Suspended Sediment Deposition Rate for Muddy Coast of Reclaimed Land

A new expression for calculating suspended fine-sediment deposition rate is developed based on theoretic analysis and experiments. The resulting equation is applied to simulation of fine sediment deposition in the reclaimed land in the Hangzhou Bay, China. The hydrodynamic environment in this area is solved by use of a long wave model, which gives the 2D-velocity field and considers bathymetric changes due to fine sediment deposition. The expression is proved convenient to use in engineering practice, and the predicted deposition rate agrees with the annual data available from field measurements from the first year to the third year after the construction of the long groin as a reclaiming method.

Silting in the Lower Courses of Tidal Sluices in China

Serious sediment deposition often occurs after the construction of tidal sluices in small or medium-sized tidal muddy estuaries, so desilting or dredging is needed to meet the demands of flood discharge, saltwater retaining, and navigation in those areas. In this paper, the problem of sediment deposition induced by construction of tidal sluices is analyzed. Different problems of silting near tidal sluices for different types of estuaries are summed up, at the same time, corresponding methods are given to solve these problems, and a few successful examples are also given. The idea of comprehensive regulation and utilization of estuaries is put forward, and some proposals for solution of sediment deposition in this kind of estuaries are made.

Analysis of deposition and erosion of Dongting Lake by GIS

The sediments of the Dongting Lake come from four channels (one of them was closed in 1959), connected with the Yangtze River, four tributaries (Lishui, Yuanjiang, Zishui and Xiangjiang) and local area, and some of them are transported into the Yangtze River in Chenglingji, which is located at the exit of the Dongting Lake, some of them deposit into drainage system in the lake region and the rest deposit into the lake. The annual mean sediment is 166,555x104 t, of which 80% come from the four channels, 18% from the four tributaries and 2% from local area, whereas 26% of the total sediments are transported into the Yangtze River and 74% deposited into the lake and the lake drainage system. Based on topographic maps of 1974, 1988 and 1998, and the spatial analysis method with geographic information system (GIS), changes in sediment deposition and erosion are studied in this paper. By overlay analysis of 1974 and 1988, 1988 and 1998, erosion and sediments deposition areas are defined. The main conclusions are: (1) sediment rate in the lake is larger than erosion rate from 1974 to 1998. The mean deposition in the lake is 0.43 m; (2) annual sediment deposition is the same between 1974-1988 and 1988-1998, but the annual volume of deposition and erosion of 1988-1998 is bigger than that in 1974-1988; (3) before the completion of the Three Gorges Reservoir, there will be 7.82x108 m3 of sediments deposited in the lake, which would make the lake silted up by 0.33 m; (4) in the lake, the deposition area is found in the north of the east Dongting Lake, the south-west of the south Dongting Lake, and the east of the west Dongting Lake; while the eroded area is in the south of the east Dongting Lake, the middle of the south Dongting Lake, the west of the west Dongting Lake, as well as Xiangjiang and Lishui river flood channels.

Late Quaternary sedimentary environmental evolution offshore of the Hangzhou Bay,East China——implications for sea level change and formation of Changjiang alongshore current

This study focuses on sedimentary environmental changes offshore of Hangzhou Bay, East China, since the Late Quaternary. AMS ^14C ages from core CJK10, lithologies, distribution of foraminifera, heavy minerals, and S and C1 elements show a fluvial terrace environment during -23.2-11.0 cal ka BP; a littoral to tidal-flat environment during 11.0-10.2 cal ka BP; and a shallow marine environment with a relatively low sedimentation rate （0.1-0.22 cm/a） since 4.3 cal ka BP. High depositional rates （-1.6 cm/a） from 10.9 to 10.2 cal ka BP resulted from sufficient accommodation space created by rapid sea level rise from -44 m to -33 m, from high sediment delivery by local rivers, and effective trapping of sediments by tidal-flat vegetation. The rate of sea level rise was variable; relatively high from 10.9 to 10.6 cal ka BP （2.1 cm/a）, and lower since 10.6 cal ka BP （1.2 cm/a）. The Changjiang alongshore current crossed the Hangzhou Bay to form the mud wedge on the inner shelf of the East China Sea later than 9.4 cal ka BP. The CJK10 site was a tide-dominated shelf environment and experienced erosion from approximately 9.4-9.2 cal ka BP to 4.3 cal ka BP. The depositional hiatus was caused by the Changjiang alongshore current, which was relatively weak during 9.4-7.5 cal ka BP and increased in strength during -7.5-4 cal ka BP. From -4.3 cal ka BP, a large amount of sediment from the Changjiang River was partly deposited on the continental shelf of Hangzhou Bay with some transported southward. Therefore, this study clarifies the history of Changjiang-derived sediment dispersal and deposition, although a detailed record of the changes in the Chang3iang alongshore current since 4.3 cal ka BP is difficult to obtain because of the scarcity of evidence.

Numerical simulation of sediment deposition thickness at Beidaihe International Yacht Club

The finite element method （FEM） was used to simulate sediment hydrodynamics at the Beidaihe International Yacht Club, and a two-dimensional model was established. The sediment movement and deposition were analyzed under many tidal conditions in conjunction with the hydrological regime of the Daihe River. The peak value of the sediment deposition thickness appears in the main channel and around the estuary. The sediment deposition thickness is essentially constant and relatively small in the project area. The sediment deposition thickness in the main channel, in the yachting area, and around the hotel is greater than the other areas in the project. Regular excavation and dredging of the channel is the best measure for mitigating the sedimentation.

The analysis on reservoir sediment deposition and downstream river channel scouring after impoundment and operation of TGP

According to the measured data after impoundment and operation of the Three Gorges Reservoir,the reservoir sediment deposition and downstream river channel scouring are described briefly and compared with the research results achieved in the demonstration stage.It is indicated through analysis that the reservoir sediment deposition and downstream river channel scouring during 8-year impoundment and operation are still within the original forecast,so the original forecasting results are feasible.The further observation and comparison should be conducted because the comparison between the observed data and the original forecast is not so sufficient in time and the prototype observation and related research work should be strengthened in the future.

Three-dimensional CFD simulation of inlet structure flow in pumping station based on Eulerian solid- liquid two-phase flow model

Sediment deposition in the pumping station has a huge negative impact on unit operation.The three-dimensional CFD method has been used to simulate inlet structure flow in pumping station based on the Eulerian solid- liquid two-phase flow model. The numerical results of the preliminary scheme show that sediment deposition occurs in the forebay of pumping station because of poor flow pattern therein. In order to improve hydraulic configuration in the forebay,one modified measure reconstructs water diversion weir shape,and another measure sets a water retaining sill in the approach channel. The simulation results of the modified scheme prove that back flow in the forebay has been eliminated and the sediment deposition region has also been reduced.

Effects of an oasis protective system on aeolian sediment deposition:a case study from Gelintan oasis,southeastern edge of the Tengger Desert,China

Desert-oasis ecotones are boundary areas between oases and desert ecosystems.Large efforts to control sediment and stabilize these boundaries depend on understanding sedimentary processes,especially aeolian transport and deposition.Previous studies on aeolian sediment deposition have focused primarily on a single land surface type or a single engineering approach.Few studies have considered deposition in a multi-layer oasis protective system.A complete oasis protective system consists of an outer bare sand area,a sand barrier zone,a shrub and herbaceous plant zone,and a farmland shelter zone.This study used sedimentary analysis to quantify grain-size characteristics in samples from the four land surfaces under different types of weather conditions in the Gelintan oasis of the Tengger Desert,the fourth largest desert in China.The results showed that aeolian sediment deposition decreased from the outer bare sand area through the oasis protective system and into the interior.The four land surface types showed significant differences in deposition volume(P<0.05).Deposited sediment showed gradual decrease in dominant grain-size from sand to silt,but sediment deposited during dust weather contained a larger coarse-grained fraction.From the outer desert to the inner oasis,transport mechanisms shifted from saltation(sand)to suspension(silt and smaller)in non-dust weather.During dust weather,deposition primarily occurs from near-surface aeolian sand transport with saltation.Sediment sorting decreased from exterior to interior zones of the protective system while skewness and kurtosis showed no significant change(P<0.05).These results can help inform strategies for stabilizing and protecting desert-oasis ecotones in this region and other localities.

Experimental Analysis of Sediment Deposition Due to the Effect of an Upstream Reservoir Backwater

The phenomenon of aggradation due to sediment accumulation upstream reservoirs had been studied in this research. For this purpose, groups of experiments were conducted in a laboratory with 25 m long, 0.80 m wide and 0.70 m deep channel. A block was built at the end of the channel to work as a dam to impound water. The channel was supplied with drainage pipes on both sides to release water out in a manner similar to what happens in reservoirs. The bed of the channel was filled with sand of 0.80 mm median sieve diameter and 0.72 geometric standard deviation. The slope was 0.0093 for all experiments. Two sizes of sand were used representing the sediment. The median diameter and geometric standard deviation of the first were 0.365 mm and 0.46 mm, respectively. The second sample had 0.65 mm median diameter and 0.67 standard deviation. A total of 70 experiments were conducted in two groups to examine effects of sediment transport rate, particle size of sediment and flow velocity on aggradation characteristics. The results showed that there was a strong linear direct relationship between aggradation elements （length and depth） with the rate of sediment transport. Groups of dimensionless parameters affecting the aggradation characteristics were used to develop empirical equations to predict the length, maximum depth of aggradation and predict transient bed profile. The results of empirical approach were compared with the measurement data and previous numerical method. The results indicated that the percentage error was 19% to 31% for length of aggradation and -21% to 26% for maximum depth of aggradation. The results also showed that the sediment materials were deposited closer to the body of the dam when the released water from the dam is higher than the inflow.

梯级水库蓄水对三峡水库泥沙淤积滞后响应规律的影响

Delayed response behaviour commonly occurs in conjunction with changes in riverbed scouring and sediment deposition and is a key component in understanding the intrinsic behaviour of reservoir siltation.Due to the complexity of the riverbed siltation process,the variability in the factors that influence siltation and the limitations of available research methods,the understanding of the delayed response behaviour of the sedimentation process in the Three Gorges Reservoir(TGR)is currently merely qualitative,and there is a lack of quantitative in-depth understanding.In addition,the effects of changes in water and sediment conditions on sedimentation in the TGR before and after cascade reservoir impoundment have not been quantified,so further studies are needed to provide a reference for better understanding the intrinsic behaviour of sedimentation in the TGR and the implications for the long-term use of the reservoir.Based on measured water and sediment data from 2003 to 2020 and topographic data from 2003 to 2018,a delayed response model for sedimentation in the TGR is constructed and combined with theoretical derivation to analyse the changes in the delayed response behaviour of the TGR before and after the impoundment of the cascade reservoirs and the associated causes.Then,the influence of changes in water and sediment conditions in previous years on sedimentation in the reservoir area is determined.The results show that(1)the improved delayed response model of sedimentation,which considers variations in external water and sediment conditions,reservoir scheduling,and riverbed adjustment rates,can effectively reflect the sedimentation process in the TGR,especially after the impoundment of the cascade reservoirs.Additionally,the typical section elevation delayed response model can simulate the section elevation adjustment process.(2)After the impoundment of the cascade reservoirs,the decreased variation in incoming water and sediment and more concentrated incoming sediment in the flood season increased the adjustment rate of the riverbed,and the delayed response time of TGR sedimentation was shortened from the previous 5 years to the previous 3 years.The impact of the previous water and sediment conditions is not negligible for the sedimentation process in the TGR,and the cumulative proportion of the previous influence reaches more than 60%.(3)The influence of incoming sediment on the sedimentation process and typical section adjustment process in the reservoir area increased after the impoundment of the cascade reservoirs,and the influence of the water level in front of the dam on sedimentation remained the largest.

Local hydraulic jump effects on sediment deposition in open-channel flume experiments

When a river channel is narrow,bifurcated,or intersected,or when extreme weather or geological disasters cause shed rock masses to occupy a river flood channel,local hydraulic jumps may develop in the channel.Natural disasters such as landslides,floods,and debris flows occur upstream,will result in large transport rate of large-sized gravel particles.Those particles may be blocked in hydraulic jump areas,causing river channel water depth to rise.In this study,the effect of local hydraulic jumps on the sediment deposition rate was investigated in flume experiments.The ratio of upstream and downstream Froude numbers,particle size,Sediment supply intensity,and flow discharge all affected the sediment deposition rate.With increases in the ratio of upstream and downstream Froude numbers,particle size,and sediment supply intensity,the sediment deposition rate increased.The sediment deposition rate decreased with an increase in flow discharge.Approach hydraulic conditions and particle properties jointly determined the sediment deposition rate in a hydraulic jump section,and an empirical formula was developed using those parameters to calculate the sediment deposition rate.Thus,to identify risks and prevent disasters in mountain rivers,local changes in hydraulic conditions and particle properties need to be jointly evaluated.

Application of a ^(137)Cs fingerprinting technique for interpreting responses of sediment deposition of a karst depression to deforestation in the Guizhou Plateau,China

Deforestation to reclaim land often triggers severe soil erosion in the Guizhou Karst Plateau. 137Cs dating of the deposited sediments in the karst depression bottom was used to estimate soil losses by surface erosion since deforestation started in 1979 on hillslopes in the Shirenzhai catchment, Puding County, Guizhou Province. The catchment has a drainage area of 0.054 km2. The average 137Cs contents of the top and peak layers in five cores of the depression bottom (with an area of 2652 m2), collected in 2009, were 2.35 and 7.25 Bq kg-1, respectively. The medium depths in the depression (which ranged between 84 cm and 113.5 cm with a mean value of 92.1 cm) showed the presence of sediments deposited in 1979. The total volume and weight of the deposited sediments since 1979 were estimated to be 1965 m3 and 2496 t, respectively. The depression bottom can be treated as a temporary impoundment and its sediment trapping efficiency was estimated to be 0.7. The relevant average soil erosion rate on the hillslopes was 2315 t km-2 yr-1 since 1979. The mean 137Cs inventory of the five cores was 7693 Bq m-2, which was ~10 times the local reference inventory of 782 Bq m-2. The total 137Cs activity of the sediments in the bottom was 20.4×106 Bq, and the relevant 137Cs inventory loss from the hillslopes was 358 Bq m-2 (since 1954), which accounted for 45.8% of the reference inventory. As soil erosion was not severe before and after the period of deforestation and following cultivation in 1979-1990, the erosion rates on the hillslopes could potentially reach 7000 t km-2 yr-1.

Sediment storage and morphology of the Yalu Tsangpo valley due to uneven uplift of the Himalaya

The fluvial process of the Yalu Tsangpo River occurs concurrently with the uplift of the Qinghai-Tibet Plateau.Therefore,the river exhibits unique features in morphology and sediment deposition.Field investigations were performed from 2009–2011and the depth of the interface between the sediment deposits and bed rock was detected with an electromagnetic imaging system(EH4)at 29 cross sections.Sediment deposits were sampled along the Yalu Tsangpo valley from Xietongmen to the Yalu Tsangpo Canyon.The results show that a huge amount of sediment has been deposited in four wide valley sections because the uplift rate in these sections was lower than that in the downstream gorge sections over the past million years.About 518 billion m3of gravel and sand have been stored in the high mountain river valleys,which has changed the V-shaped mountain river valley into a U-shaped wide river valley in the four sections.In the sections with high uplift rates the river bed is incised and has formed gorges and the Yalu Tsangpo Canyon.

Deposition of eroded soil on terraced croplands in Minchet catchment,Ethiopian Highlands

In the Ethiopian Highlands,soil and water conservation practices are of utmost importance to conserve eroded soil and combat soil loss.This study provides detailed results on on-site sediment deposition and net soil loss in terraced croplands in a catchment in the sub-humid Ethiopian Highlands.Sediment deposition was measured on horse bean and maize fields during the crop growing seasons of 2014 and 2015.Measurements took place on observation plots on terraced cropland with varying spacing between terraces and varying slope gradients.Net soil loss,in this case the amount leaving the terraced cropland,was calculated by modelling the Universal Soil Loss Equation(USLE)for the whole observation field and subtracting the measured sediment deposition.The study result showed about 8–11 t ha^(−1) sediment was deposited in the deposition zone of the terraced cropland,with greater sediment deposition on terraces with narrow spacing and steeper slope gradients.Sediment deposition was highest in July and August,and relatively low in September.Annual soil loss ranged from 32 to 37 t ha^(−1) in the terraced cropland of the study area.From the total soil loss in the crop growing season,about 54–74%sediment was deposited on the deposition zone of terraced crop fields.Implementation of soil and water conservation with narrow spacing,especially on the steep slopes of the sub-humid Ethiopian Highlands or other similar area,are thus highly recommended as they enable conservation of the eroded soil in the cropland.

A MATHEMATICAL MODEL FOR UNSTEADY SEDIMENT TRANSPORT IN NINGXIA REACH OF THE YELLOW RIVER

A one-dimensional mathematical model for unsteady sediment transport in the Ningxia reach of the Yellow River was developed. In the model, the formulas for the sediment carrying capacity and the manning roughness coefficient, which reflect the features of the Yellow River, were adopted. A coefficient of sediment distribution was defined to represent the ratio of the bottom to the average concentration under the equilibrium conditions, which is not constant and is evaluated by using an empirical expression obtained by integrating the sediment concentration along water depth. The concentration distributions and the mean diameter distributions of suspended sediment in the transversal direction were also estimated in this model. A four-point (Preismann type) finite difference scheme and TDMA were employed in the numerical simulation. The amount of sediment deposition during the period of 1993～1999 in the Ningxia reach of the Yellow River from Xiaheyan to Shizuishan with a length of 197.43km were numerically simulated with the model. The computed results, such as the amount of sediment deposition and water stage agree well with the field data. Finally the validated model was used to predict the riverbed deformation during the period of 1999～2019 in the Ningxia reach of the Yellow River.

Estimating landscape susceptibility to soil erosion using a GIS-based approach in Northern Ethiopia

Soil erosion is a very critical form of land degradation resulting in the loss of soil nutrients and downstream sedimentation of water storages in the highlands of Ethiopia.As it is technically and financially impossible to conserve all landscapes affected by erosion,identification of priority areas of intervention is necessary.Spatially distributed erosion models can help map landscape susceptibility to erosion and identify high erosion risk areas.Integration of erosion models with geographic information systems(GIS)enables assessing evaluate the spatial variability of soil erosion and plan implementing conservation measures at landscape levels.In this study,the Revised Universal Soil Loss Equation adjusted for sediment delivery ratio was used in a GIS system to assess landscape sensitivity to erosion and identify hotspots.The approach was applied in three catchments with size being 10–20 km^(2) and results were compared against quantitative and semi-quantitative data.The model estimated mean soil loss rates of about 45 t ha^(−1) y^(−1) with an average variability of 30%between catchments.The estimated soil loss rate is above the tolerable limit of 10 t ha^(−1) y^(−1).The model predicted high soil loss rates at steep slopes and shoulder positions as well as along gullies.The results of the study demonstrate that knowledge of spatial patterns of high soil loss risk areas can help deploy site-specific conservation measures.