Bed Load Transport Rate in Scouring and Armoring Process of Non-uniform Sediment River Bed

Flume experiments were carried out to study bed load transport rate during rive bed scouring and ar- moring.A theoretical differential equation linking the transport rate to the probability of incipient motion of non-uniform sediment is solved.The transport rate is shown to decrease exponentially with time,according to the theory,which is in good agreement with the experiment data.

Sediment generation and soil mound denudation in areas of high-density tree throw along a river valley in the Jura Mountains, Switzerland

A preliminary field-based investigation was undertaken in a small(<10 km^(2))river valley located in the mountainous Jura region of northwest Switzerland.The aims of the work were to assess sediment generation and annual sediment transport rates by tree throw on forested hillslopes,and to document surface hydrology characteristics on four fresh soil mounds associated with recent tree throws over a 24-day monitoring period.For the soil mounds,average sediment recovery ranged from 7.7-28.2 g(dry weight),equivalent to a suspended sediment concentration of 145.2-327.8 g L^(-1),and runoff coefficients ranged from 1.0%-4.2%.Based on a soil bulk density value of 1,044 kg m^(-3),upslope runoff generation areas were denuded by an average 0.14 mm by the end of the 24-day monitoring period,representing an erosion rate equivalent to 2.1 mm yr^(-1).A ca.50 cm high soil mound could therefore feasibly persist for around 200-250 years.For tree throw work,the dimensions of 215 individual tree throws were measured and their locations mapped in 12 separate locations along the river valley representing a cumulative area equivalent to 5.3 ha(av.density,43 per ha).Tree throws generated a total of 20.1 m^(3) of fine-sediment(<2 mm diameter),or the equivalent of 3.8×10^(-4) m^(3) m^(-2).The process of tree throw was originally attributed to two extreme weather events that occurred in west and central Europe in late December 1999.Taking the 18-year period since both storms,this represents an annual sediment transport rate of 2.7×10^(-5) m^(3) m^(-1) yr^(-1).Exploring the relationship with wind on fall direction,65.5%of tree throws(143)generally fell in a downslope direction irrespective of hillslope aspect on which they were located.This infers that individual storms may not have been responsible for the majority of tree throws,but instead,could be associated with root failure.Given the high density of tree throws and their relative maturity(average age 41 years),we hypothesise that once trees attain a certain age in this river valley,their physiognomy(i.e.height,mass and centre of gravity)compromises their ability to remain securely anchored.We tentatively attribute this possibility to the presence of bedrock close to the surface,and to the shallow soil profile overlaying steep hillslopes.

Aeolian sediment transport over sandy gobi:Field studies in the Nanhu gobi along the Hami-Lop Nor Railway

Wind-blown sand over sandy gobi with an abundant sediment supply can cause severe sand hazards.However,compared with the study of aeolian transport over gravel gobi with a limited sediment supply,less attention has been devoted to sandy gobi,and thus,our understanding of wind-blown sand movement on sandy gobi is still poor.Here,we report the results of observations of three transport events on a sandy gobi along the Hami-Lop Nor Railway based on high-frequency saltation particle count and horizontal sediment flux measurements coupled with instantaneous wind velocity measurements.The results reveal that,unlike the notably intermittent aeolian saltation over gravel gobi,continuous transport occurred on the sandy gobi.The mean saltation layer height was 0.23±0.07 m,and it was positively related to both the grain size of surface particles and the wind velocity regardless of the gobi type.The sediment transport rates could be expressed as the power function Q=ap/g[u^(*)(u^(*2)-u^(*)t^(2))]b,and the scaling parameter(b)reached to 2.5,which is much larger than that of other gobi areas(b=1).Our findings suggest that the wind-blown sand over sandy gobi is much more severe than that over gravel gobi,and the Nanhu sandy gobi is the major sand source for sand hazards of the Hami-Lop Nor Railway.Sand-fixation measures such as checkerboard sand barriers with enhanced checkerboard size and barrier height should be the main subject of sand control systems for the Hami-Lop Nor Railway in sandy gobi.

Sediment transport in pure acceleration-skewed oscillatory sheet flow

In the present study, an analytical concept model is built, using a two-phase model for the sediment transport in a pure acceleration-skewed oscillatory sheet flow. The analytical model is based on the asymmetric wave theory, the irregular boundary layer theory and the exponential concentration distribution theory, to be used for analyzing the phase lag and the boundary layer development related to the acceleration skewness. The two-phase model is applied for the calculations of the instantaneous erosion depth, the sediment flux, the boundary layer thickness and the sediment transport rate, as well as the differences between the positive acceleration stage and the negative acceleration stage caused by the acceleration skewness, as very important in the net current and sediment transport. The effects of the sediment diameter and the phase lag are explained by a comparison with the instantaneous type empirical formula, as is closely related to the acceleration skewness. With the analytical concept model and the two-phase model, the generation of the net sediment transport in the pure acceleration-skewed flows is clearly explained. The phase lag effect is important for the instantaneous sediment transport in the pure acceleration-skewed flow, whereas the boundary layer development difference between the positive acceleration stage and the negative acceleration stage plays a major role in the determination of the net sediment transport.

Instantaneous sediment transport formula for sheet flow beneath asymmetric wave and current

An instantaneous formula is obtained for the sheet flow transport beneath the wave and the current by a product integration of the concentration and velocity profiles over the mobile seabed. The formula involves: (1) The product of the erosion depth and the free stream velocity, which can be reduced to the Shields parameter of power 3/2 in accordance with the classical formulae. (2) The ratio of the wave boundary layer thickness to the erosion depth. The formula incorporates the effects of the acceleration and the phase lag on the erosion depth, the asymmetric wave boundary layer and the wave-current interaction. The validation of the formula is made by the data obtained from the oscillatory tunnels covering a wide range of wave-current and sediment conditions. The instantaneous sediment transport rate is compared with the power function of the velocity with different exponents. The formula consists of a wave force part and a current force part and their relative importance depends on the wave shapes. The present formula gives very good results for the net sediment transport as compared to several existing steady and unsteady formulae. The net sediment transport rates are affected by not only the acceleration, the phase lag and the wave-current interaction, but also the asymmetric boundary layer development.

An Investigation of Stochastic Nature of Bed Load Motion in Chuanjiang River

Bed Load Motion and its transport rate is one of the basic issues in river dynamics.In this paper, the authors discussed the stochastic nature of bed load motion in Chuanjiang River in details.Chungjiang lies in the upstream reach of Yangtze River.Its stochastic nature is shown in the following four aspects.Firstly, even though all the conditions are the same,due to the fluctuation of the flow,the bed load discharge and the location and width of sediment transport belts are different.Secondly,during the ...

Comparative Analysis of the Erosion Mechanism of Different Profiles in the Arcuate Foreshore under Typhoon Action

The comparison results of three beach profile data repeatedly measured before and after the typhoon in Shuidong Bay,west Guangdong province which show that there are significant differences in beach profile erosion and response process.And the changes of beach profile can be divided into:strong downward overall low shoreline regressive type and overall slight erosion shoreline regressive type.Application of the modified mildslope equation along three beach profile are simulated wave high reflection to the sea side,to the section vertical shore pressure gradient and including water roll force and radiation stress,the vertical shore forces one dimensional profile along the momentum conservation equation(radiation stress and water roll force)bottom friction and lateral mixing reaction between numerical solution,the momentum conservation equations of the wave increases the water flow velocity and section along the profile distribution of wave height and related forces.The analysis shows that the extent and difference of coastal erosion depend on the shoreline erosion mode stimulated by the maximum surge water of the coastal current and the maximum velocity of the coastal current and the dynamic state of the profile topography under the action of the profile location,morphology and incident wave elements.

Development of All-Weather and Real-Time Bottom-Mounted Monitor of Bed Load Quantity

Quantity of bed load is an important physical parameter in sediment transport research. Aiming at the difficulties in the bed load measurement, this paper develops a bottom-mounted monitor to measure the bed load transport rate by adopting the sedimentation pit method and resolving such key problems as weighing and desilting, which can achieve long-time, all-weather and real-time telemeasurement of the bed load transport rate of plain rivers, estuaries and coasts. Both laboratory and field tests show that this monitor is reasonable in design, stable in properties and convenient in measurement, and it can be used to monitor the bed load transport rate in practical projects.

室内模拟戈壁地表降雨对风蚀过程的影响

In the present study,indoor experiments were carried out to investigate the effects of rainfall on subsequent wind erosion processes on the simulated Gobi surface,with soils and gravels collected from the Alax Gobi in northwestern China.The results showed that the wind erosion rate(WR)ranged from 0.4 to 1931.2 g m^((-2)min^((-1)and that the sediment transportation rate(STR)ranged from 0.00 to 51.47 g m^((-2)s^((-1)under different gravel coverage conditions(0%,20%,40%and 60%)when the wind velocity changed from 6 m s^((-1)to 18 m s^((-1).Both WR and STR increased with increasing wind velocity as a power function and decreased with increasing gravel coverage.The rainfall event had significant inhibitory effects on WR and STR,and the complex effects of soil crust formation and the changing soil surface roughness(SSR)by rainfall event played significant roles in reducing these rates during subsequent wind erosion.In this study,a valuable exploration of the effects of rainfall events on subsequent wind erosion processes in the Gobi area was conducted.The findings are of great significance for a better understanding and effective prediction of dust emissions in this region.