| Benggang erosion of granite red soil region is one of the major environmental problems in the tropical and subtropical regions of southern China,which seriously restricts the safety of local social,economic and ecological development.As an erosion type originally caused by water erosion,the formation of Benggang generally initiates from the slope erosion,experiences the evolution process of sheet erosion-rill erosion at the slope scale,and evolves from the further formation of gully erosion,which is the advanced stage of slope gully erosion development.On the slope scale of granite red soil,the sheet erosion-rill erosion stage is a part of the evolution of regional gully system.It is the key stage of the transformation from slope erosion to gully erosion stage,which directly affects the occurrence and development direction of subsequent erosion gullies.The current studies,however,mostly focus on the geotechnical mechanics,hydraulic characteristics,environmental factors and other fields in the Benggang development stage.Few studies explore slope erosion evolution process and erosion mechanism on the granite red soil slope before the formation of Benggang.Therefore,this study selects the granite red soil in four typical Benggang distribution areas as the experimental materials,deeply explores the slope erosion evolution process through a series of indoor simulated rainfall/runoff experiments,systematically analyzes the characteristics of sediment yield,erosion morphological characteristics and sediment sorting mechanism,and reveals the internal relationship between slope erosion process and Benggang erosion from the level of mechanism.The main results were showed as follows:1.On the basis of clarifying the soil basic properties,the indoor simulated experiments under different erosion conditions(rainfall-runoff and rainfall kinetic energy conditions)were carried out by selecting granite red soil at different horizons(topsoil horizon,illuvial horizon,and parent material horizon)to explore the effect of the differences between soil horizons on erosion process and mechanism.(1)Soil erosion rates and runoff coefficients increased gradually with the evolution of erosion phase from sheet erosion to rill advancement and then to rill maturity.The soil erosion rate increased with the increase of rainfall-runoff intensity and soil horizon(p<0.01),but the topsoil always showed the lowest erosion degree.Rill headward erosion rate and rill density showed similar variation pattern to the soil erosion rate.In addition,the soil surface in the inter-rill area was gradually enriched with quartz coarse grains(>0.25 mm),especially on the CTA slopes.Quartz coarse grain cover had a negative effect on soil erosion and helped to transfer the main erosion source from the inter-rill area to the rills.(2)Under different rainfall kinetic energy(KE)conditions,the runoff rate and soil erosion rate of different soil horizons increased linearly with the increase of rainfall KE.The soil clay content was introduced into the erosion model,and the erosion prediction model of soil erosion rate based on stream power was constructed(R~2=0.93).The proportion of fine particles(clay-and silt-sized particles)in eroded sediment decreased with rainfall duration,whereas the proportion of coarse particles(sand-and gravel-sized particles)increased,indicating that sediment transport was initially dominated by suspension/saltation,and gradually changed to saltation/rolling.In this process,the fine particles in the sediment were enriched(enrichment ratio>1),while the coarse particles were depleted.Based on rainfall KE and stream power,the erosion prediction models for the enrichment ratio of sediment particle were constructed(R~2>0.59).2.On the basis of the erosion characteristics of different horizons,the topsoil horizons with the strongest erosion resistance at four sampling sites(TCA,GXA,CTA,and WHA)were selected as the experimental soils,and the simulated rainfall experiments(rainfall and rainfall-runoff conditions)were carried out.The slope erosion process and mechanism of four topsoils were further studied and compared,and the differences of slope erosion characteristics of different topsoils were analyzed.(1)Under the condition of intermittent rainfall,the four topsoils were in different erosion phases(only the WHA slope developed obvious rills).The order of surface roughness was WHA>TCA>GXA>CTA,which responded to the slope erosion degree of each soil(p<0.01).The order of quartz coarse grained coverage on the surface of each soil was WHA<TCA<GXA<CTA,which was positively correlated with the clay and gravel contents in the original soil(p<0.01).In addition,the mean soil erosion rate of each soil decreased with the increase of soil clay content,which was negatively correlated with the quartz coarse grain coverage on the surface(p<0.01).Quartz coarse grain coverage helped to promote the production of fine particles on the slope,resulting in the enrichment of fine particles in eroded sediment.Combined with soil clay content and stream power,the soil erosion prediction model for predicting different topsoils was constructed(R~2=0.93).(2)Under the condition of rainfall-runoff,the rills in maturity phase developed in the four topsoils.The CTA slope with high clay and gravel contents in the four topsoils showed the lowest soil erosion rate,rill density,headward erosion rate and slope evolution rate.Rill promoted slope erosion and sediment yield,and the maximum soil erosion rate(0.61kg/m~2·min)of WHA slope in rill maturity phase was 1.69 times higher than that in sheet erosion phase.Meanwhile,the rill headward erosion rate of WHA slope with the largest rill depth was also at the highest level.Geomorphologic comentropy could indirectly evaluate the soil erosion rate and degree of different slopes.Among the four soils,rill erosion accounted for about 70%of the total soil erosion.3.Based on the above four experiments,the soil slope with complete soil profile configuration was further selected as the experimental object.The evolution process of slope erosion under the long-term(120 h)simulated rainfall condition was explored through a series of quantitative observations,and the effect of slope in-situ erosion on subsequent erosion was deeply analyzed.There were obvious differences in the dominant erosion forms of slope erosion in sheet erosion phase,rill advance phase and rill maturity phase.Specially,the rill advance phase was dominated by downward erosion and headward erosion,and the rill maturity phase was dominated by lateral erosion.Besides,quartz coarse grain gradually enriched in the inter-rill areas,and its coverage first increased with the rainfall duration,and then tended to the dynamic equilibrium state of erosion-deposition(47.68%-56.74%).Quartz gravel cover had a cumulative effect on subsequent erosion,which changed the slope erosion process,caused erosive settlement in the inter-rill areas,and intensified the lateral erosion intensity of rills.As a result,the whole slope surface was continuously eroded by layers,which promoted the gradual degradation of soil layers and the exposure of erodible subsoils,which promoted and accelerated the occurrence of gully erosion conducive to the formation of Benggang.To summarize,this study found that two dominant factors,the soil stability affected by soil clay content and the slope erosion resistance affected by the quartz gravel coverage,controlled the slope erosion process of different soils.The effect of soil clay content on soil stability had been widely discussed.Noted that the quartz gravel on the slope surface was not only the result of erosion,but also the reason affecting erosion.Quantitative analysis showed that the quartz gravel cover was closely related to the soil gravel content,and its cumulative effect on erosion gradually strengthened with the rainfall duration,and had a long-term nature.This result was indirectly supported by the enrichment of fine particles in eroded sediment.Quartz gravel cover reduced the in-situ erosion degree,while the quartz gravel in the rills forced lateral erosion to intensify,resulting in layered erosion of the slope,which promoted the exposure of erodible subsoils in the catchment area and promoted the occurrence of gully erosion conducive to the formation of Benggang.This work provides a reference for the prediction and control of Benggang erosion in granite red soil area. |
