Quantitative Monitoring Generation And Development Process Of Ephemeral Gully Erosion On Hillslope Using Ree

The research on the generation and development process of ephemeral gully erosion on hillslope is seriously lacking.In this thesis,the rare earth element tracer method is used to study process of ephemeral gully on loessial hillslope by performing the indoor simulated rainfall experiment.The process of runoff and sediment yield on the hillslope,the change process of the erosion type of the slope,the influence of the soil type and rainfall kinetic energy on the erosion stage and the erosion process of Wabei type landform ephemeral gully were investigate in this thesis.The results of the study provide a scientific reference for it may help the planners and managers to take proper decision for the conservation of ephemeral gully erosion area,especially in the Loess Plateau.The main conclusions are as follows:(1)The process of runoff and sediment yield under different rainfall intensity,different texture soil and different rainfall kinetic energy conditions were studied.Both runoff rate and erosion rate increased at first and be stable then decreased or increased at first and be stable with the increase in the cumulative rainfall.Rainfall intensity,soil type and rainfall kinetic energy have a great impact on the runoff rate and erosion rate.(2)The change of erosion type on hillslope were quantified.The Eu tracer layer on the surface began erode at the beginning of the rainfall,the erosion of other trace layer occurred under the action of raindrops and runoff effect.All of ten trace layer(50 cm depth)have obvious erosion at the process of rainfall of 405 mm,while there have no significant erosion occurred in the bottom of three layers(18 cm depth with 6 cm depth per layer)for the Yangling Lou soil and the Changwu Heilu soil.The sheet erosion increased at first then decreased with fluctuation with the increase of cumulative rainfall,while the contribution rate of sheet erosion showed complex change.The sheet erosion contribution rate was high and provided the main contribution rate at the early stage of rainfall,it then reduced gradually to a stable stage with the development of rill and further decreased after the emergence of ephemeral gully erosion.Rill erosion increased at first and be stable with fluctuation then showed a bit decrease when ephemeral gully erosion generated and increases again and be stable with fluctuation then decreased with the increase of the cumulative rainfall.The rill contribution rate increases rapidly and be stable with fluctuation and gradually decreases after the emergence of ephemeral gully erosion then be stable with fluctuation again.Ephemeral gully erosion increased at first and be stable or increasedat first and be stable then decreased with the increase of the cumulative rainfall.The ephemeral gully erosion contribution rate increased gradually and be stable with fluctuation.According to the change law for contribution rate of three erosion type,the slope erosion process can be divided into the five stages: sheet erosion stage(sheet erosion contribution rate larger than 80%),rill erosion development stage,rill erosion stabilization stage(rill erosion contribution rate larger than 70% and sheet erosion contribution rate less than 20%),ephemeral gully erosion development stage and ephemeral gully erosion stabilization stage(ephemeral gully erosion contribution rate in range of 50%-70%,rill erosion contribution rate in range of 20%-40% and sheet erosion contribution rate was less than 5%).(3)The influence of soil type and rainfall kinetic energy on the erosion type was analyzed.Soil type and rainfall kinetic energy had great influence on the change of erosion type.The duration of each erosion stage increased with the increase of the stability of the aggregates except for the rill erosion development stage.With the increase of the rainfall kinetic energy,it promoted the rill erosion development stage and rill erosion stabilization stage while its influence on ephemeral gully erosion development stage was more complicated and promoted the gully development in generally.The <0.002 mm and 0.002-0.02 mm particles enriched obviously during the erosion process,more <0.002 mm and 0.002-0.02 mm particles are transported in the form of >0.05 mm aggregates with the increase of soil aggregates stability.The rainfall kinetic energy has a great influence on sediment sorting at the sheet erosion stage,while the influence at other erosion stage is rare.Soil type and rainfall kinetic energy have a greate impact on the rill/gully width.The rill/gully depth for Huangmian soil was deeper than those of Lou soil and Heilu soil.The effect of rainfall on the rill/gully depth was small during the whole rainfall process.(4)The process of Wabei type landform ephemeral gully was studied.The runoff rate of Wabei type landform ephemeral gully increased initially and be stable with fluctuation while the sediment rate increased at first and be stable with fluctuation then decreased with the increase of the cumulative rainfall.The erosion from gully slope position increased slowly initially and increased rapidly to the maximum then decreased with the increase the cumulative rainfall.The contribution of erosion from gully slope position was stable with fluctuation at first,and gradually increased to a maximum value then decreased with fluctuation to a small value and be stable with fluctuation with slightly decreasing trend.The erosion from gully bottom position increased rapidly initially and increased slowly to a large value and be stable with fluctuation then decreased with the increase of the cumulative rainfall.The contribution of erosion from gully bottom position was stable with fluctuation at first,and gradually decreased with fluctuation to the minimum value then increased with fluctuation to a large value and be stable with fluctuation with slightly increasing trend.The contribution rate for gully bottom position is higher than that for gully slope position during the rainfall process.There exist three obvious turning points at the contribution rate for both gully bottom position and gully slope position.Based on the three turning points,the erosion process of Wabei type landform ephemeral gully can be divided into four stages: the undercutting of gully bottom position stage,the rapidly collapse of gully slope position,the slowly collapse of gully slope position and gully stabilization stage.