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.

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.

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.

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.

Sequence stratigraphy and depositional sequence interpretation:A case study of“George”Field,offshore Niger Delta,Nigeria

The seismic sequence stratigraphic analysis revealed four depositional sequences(DS-1,DS-2,DS-3 and DS-4).The accompanying systems tracts were interpreted and mapped in the study area based on the log motifs of the reference well and the spatial distribution of the recognized constrained surfaces:maximum flooding surfaces(MFSs),sequence boundaries(SBs)and transgressive surfaces(TSs)on the seismic data.Depositional systems in the study area comprise lowstand systems tracts(LSTs),transgressive systems tracts(TSTs)and highstand systems tracts(HSTs).The LSTs are represented by coeval facies dominated by deposition basinward of the shelf-edge during maximum regression and are characterized by shallow-water deposition from gravity flows and/or traction processes within the shelf-edge or canyon-head delta.The sediments associated with lowstand systems tracts recognized in the study area are the fluvial channel sands and slope fans(SF).The transgressive sand units were interpreted as shoreface sands deposited in the shelf region during rising sea levels.Highstand systems tracts are characterized by intervals of coarsening and shallowing upwards,with both fluvial and deltaic sands prograding laterally into neritic shales.In the study area,the units are very thick.The highstand and lowstand system tracts exhibit blocky log patterns and are associated with the reservoirs while the transgressive system tracts serve as seals to the reservoirs.The environment of sediments deposition in this area is delta plain,shelf,slope to toe of slope.

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.

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.