Study On The Change Of Sediment Transport Rate Under Gravel Cover

Desertification is one of the major global environmental problems that human beings are facing at present.Soil wind erosion caused by wind-sand movement is one of the main causes of desertification.Appropriate wind erosion control measures can effectively reduce the negative impact of soil wind erosion.Gravel cover is a common surface cover in arid and semi-arid areas.It is often used as a roughness element to change the interaction between wind and soil surface,thereby inhibiting soil wind erosion and reducing sand transport rate.Because the influence of gravel cover on soil wind erosion process is very complex,the comprehensive effect of gravel cover on soil sediment transport rate under different wind speed conditions cannot be accurately evaluated,and it is not clear whether the applicability of different soil types is the same.In this paper,through the large wind tunnel experiment platform,the soil wind erosion experiment of 25 kinds of gravel covered surface（five diameter gravel model,five gravel coverage）was carried out under five wind speed conditions for three different soil types（aeolian sandy soil,light chestnut soil and chestnut soil）.The sand flow of gravel covered surface was measured and the sediment transport rate（q）was calculated.Based on the experimental data,the effects of wind speed（u）,gravel size（D）and gravel coverage（GC）on soil wind erosion were revealed.The protective ability of gravel coverage on different soil types was compared,and the sediment transport rate equation including u,D and GC was established.The following new understandings were obtained:1.The q of aeolian sandy soil,light chestnut soil and chestnut soil increases with the increase of u,and basically presents the relationship of chestnut soil>aeolian sandy soil>light chestnut soil.However,in the case of low GC,such as GC=5%（D=1.6～4.0cm）,GC=10%（D=2.0～3.15cm）,the rate of q increasing with u is sandy soil>chestnut soil>light chestnut soil.2.The q of aeolian sandy soil,light chestnut soil and chestnut soil increased with the increase of D,showing the relationship of chestnut soil>aeolian sandy soil>light chestnut soil.In general,when D increases,the wind erosion process of chestnut soil increases most obviously,followed by sandy soil,and the wind erosion process of light chestnut soil changes least.Under the same conditions of u and GC,gravel with smaller D has better wind erosion control effect than gravel with larger D.3.The q of sandy soil,light chestnut soil and chestnut soil decreased with the increase of GC,and the relationship was basically sandy soil>chestnut soil>light chestnut soil.In general,under the same condition of D,the increase of GC leads to the increase of wind erosion protection rate,which is most obvious on the surface of aeolian sandy soil type,followed by light chestnut soil type surface,and finally chestnut soil type surface.Aeolian sandy soil is more prone to wind erosion when GC is low,and increasing GC can effectively inhibit the wind erosion process of such soil.When u is higher,the protective effect of gravel coverage of the three soils is worse.4.The differences in soil properties such as particle size（d）,organic matter（SOM）content and calcium carbonate（Ca CO₃）content have important effects on the sediment transport rate（q₀）of bare soil.d is positively correlated with q₀;SOM content was negatively correlated with q₀.The effect of Ca CO₃ content on q₀ is more complex.When the Ca CO₃ content is high,the increase of Ca CO₃ content will aggravate the wind erosion process.When the Ca CO₃ content is low,the increase of Ca CO₃ will inhibit the wind erosion.The complexity of soil properties leads to different effects of gravel mulching on different types of soil.The difference of soil properties should be considered when applying gravel mulching measures to different types of soil.5.By comparing the gravel coverage case q and no gravel coverage case q₀ found that gravel coverage for aeolian sandy soil,chestnut soil and chestnut soil inhibition of wind erosion effect is different,on the whole,the same conditions of gravel coverage on aeolian sandy soil inhibition of wind erosion effect is best,chestnut soil,chestnut soil is the worst.In the case of large D and small GC,gravel coverage will lead to the aggravation of soil wind erosion process,and the chestnut soil surface is the most obvious.6.For aeolian sandy soil,light chestnut soil and chestnut soil,q increases with u in accordance with the equation q=a₁exp（b₁u）;The increase of q with D conforms to the equation q=a₂exp（b₂D）;The increase of q with GC conforms to the equation q=a₃exp（-b₃GC）。Since q is affected by the three factors of u,GC and D and the function forms of each single factor are similar,the comprehensive expression of q and u,GC and D can be expressed as q=（aexp（b₄u+b₅D-b₆GC））^1/3.The function has a good fitting effect on the three soil types,R² is higher than 0.85,and has been tested by the leave-one-out test.The equation has simple parameters and is convenient to use.It can be used to express the variation of sediment transport rate of different gravel covered surface under different u conditions.This paper systematically analyzes the influence of gravel coverage on q,which is helpful to quantitatively analyze the influence of u,D and GC on wind erosion.It can provide new ideas for improving soil wind erosion model and provide theoretical support for constructing soil wind erosion model with wind erosion dynamics.