| Soil erosion is one of the global ecological and environmental problems.Mulch(such as vegetation,litter,gravel)is often used as an important measure to control slope erosion.However,most of the current studies focus on the impact of mulch on slope erosion,and have not yet distinguished the impacts of different parts of mulch on slope erosion,especially on the sorting of eroded sediment particles.Thus,in this paper,three kinds of surface mulch(grass,litter and gravel)were selected as the research objects.To study the influence of surface mulch on the soil erosion processes and the sorting of eroded sediment particles in hillslopes,the control experiment of surface mulch(grass:canopy and root,litter:undecomposed litter layer and semi-decomposed litter layer,gravel)on water erosion was carried out through simulated rainfall experiment and indoor scouring experiment.Furthermore,the mechanism of soil erosion and eroded sediment particle sorting was revealed from the perspective of hydrodynamics.The research results are helpful to construct soil erosion model and provide theoretical basis for soil and water conservation ecological environment construction.The main conclusions are as follows:(1)Mulch could delay runoff velocity.The runoff velocity of grass planting slope,root retaining slope,the undecomposed litter slope,the semi-decomposed litter slope and gravel slope decreased by 45.64%-81.77%,25.97%-51.49%,23.47%-45.95%,39.63%-64.41%and 8.95%-27.47%,respectively.The reduction effect of grass root on runoff velocity was higher than that of grass canopy.The reduction effect of the semi-decomposed litter on runoff velocity was higher than that of the undecomposed litter.The existence of mulch played a role in increasing the runoff depth and reducing the Reynolds number and Froude number.The increase of rainfall intensity and slope gradient,as well as the decrease of coverage degree,lead to the increase of Reynolds number and Froude number.However,the increase of rainfall intensity and coverage degree and the decrease of slope gradient lead to the increase of runoff depth.Mulch enhanced the blocking effect of topsoil on runoff.The resistance coefficient of grass planting slope,root retaining slope,the undecomposed litter slope,the semi-decomposed litter slope and gravel slope increased by 4.90-70.81,1.43-5.98,1.03-3.70,3.11-13.60 and 0.24-1.17 times,respectively.The increase effect of grass canopy on resistance was higher than that of grass root.The increase effect of the semi-decomposed litter on resistance was higher than that of the undecomposed litter.In addition,in the gravel fixed bed scouring experiment,the flow phenomenon of overland flow was obvious.The variation trend of water depth around the gravel was water depth in front of the gravel>water depth on both sides of the gravel>water depth behind the gravel.At a small slope gradient(2° and 4°),the difference between water depth in front of the gravel and water depth behind the gravel first increased and then tended to be stable with the increase of flow discharge.At a larger slope gradient(6°,8° and 10°),the difference between water depth in front of the gravel and water depth behind the gravel first increased and then decreased with the increase of flow discharge.(2)Mulch could reduce runoff rate.Compared with the bare slope,the reduction rates of grass planting slope,root retaining slope,the undecomposed litter slope,the semi-decomposed litter slope and gravel slope to runoff rate were 24.83%-56.98%,12.31%-21.71%,10.91%-27.04%,12.91%-36.05%and 6.73%-19.18%,respectively.The runoff rate of each covered slope increased first and then tended to be stable with the extension of rainfall duration.The increase of rainfall intensity and slope gradient and the decrease of coverage degree increased the runoff rate of each covered slope.The reduction effect of grass canopy on runoff rate was higher than that of grass root.The reduction effect of the semi-decomposed litter on runoff rate was 1.18-1.74 times that of the undecomposed litter.Furthermore,mulch could reduce the erosion rate.Compared with the bare slope,the reduction rates of grass planting slope,root retaining slope,the undecomposed litter slope,the semi-decomposed litter slope and gravel slope to erosion rate were 48.87%-79.19%,30.46%-57.77%,13.35%-40.98%,17.16%-59.46%and 7.95%-34.57%,respectively.The erosion rate of each covered slope decreased first and then tended to be stable with the extension of rainfall duration.The increase of rainfall intensity and slope gradient and the decrease of coverage degree increased the erosion rate of each covered slope.The reduction effect of grass canopy on erosion rate was lower than that of grass root.The reduction effect of the semi-decomposed litter on erosion rate was 1.13-1.45 times that of the undecomposed litter.(3)Mulch affected particle content of eroded sediment.With the increase of rainfall intensity and slope gradient and the decrease of coverage degree,the content of smaller particles(clay and fine silt particles)increased and the content of larger particles(coarse silt and sand particles)decreased in the erosion sediment from grass planting slope.However,smaller particles decreased and larger particles increased in the erosion sediment from other slopes(root retaining slope,the undecomposed litter slope,the semi-decomposed litter slope and gravel slope).With the extension of rainfall duration,the content of smaller particles increased and the content of larger particles decreased in the erosion sediment from grass planting slope,while the other slopes were opposite.The clay and sand particles in the erosion sediment from each covered slope were transported by runoff in the form of aggregates,while most fine silt particles and coarse silt particles were transported by runoff in the form of original particles.Compared with the test soil,the erosion sediment from grass planting slope mainly concentrated in larger particles,while the erosion sediment from the other slopes mainly concentrated in smaller particles.(4)The content of eroded sediment particles on grass planting slope and root retaining slope was power function with resistance coefficient,quadratic function with runoff shear force and linear function with other hydrodynamic parameters.The content of eroded sediment particles on the semi-decomposed litter slope and the undecomposed litter slope and gravel slope was linear function with all the runoff hydrodynamic parameters.When one of the runoff hydrodynamic parameters changes as follows(the increase of u,Re,Fr,w and P,the decrease of h and f),the content of smaller particles increased and the content of larger particles decreased under the condition of grass planting slope.Nevertheless,the content of smaller particles decreased and the content of larger particles increased under the condition of other slopes.In addition,when the runoff shear force increased,the smaller particles first increased and then decreased,while the larger particles showed the opposite trend under the condition of grass planting slope;the smaller particles first decreased and then increased,while the larger particles showed the opposite trend under the condition of root retaining slope;the smaller particles increased and the larger particles decreased under the condition of the undecomposed litter slope and the semi-decomposed litter slope;the content of eroded sediment particles had no obvious change under the condition of gravel slope.In addition,the best hydrodynamic parameter affecting the erosion rate of grass planting slope and root retaining slope was unit runoff power and flow velocity,respectively,while the best hydrodynamic parameter affecting the erosion rate of the undecomposed litter slope,the semi-decomposed litter slope and gravel slope was all Reynolds number.(5)The radial basis function neural network based on grey relational analysis was used to simulate the erosion sediment particles on each covered slope.The simulation effect of time series model was better than linear regression model to simulate the dynamic change of eroded sediment particles on each covered slope.In addition,except for the undecomposed litter slope,the erosion rate of the other slopes(grass planting slope,root retaining slope,the semi-decomposed litter slope and gravel slope)was affected by the eroded sediment particles content.Stepwise regression analysis and path analysis were used to obtain the dominant influencing factors and their total contribution rates of erosion rate based on eroded sediment particles.On grass planting slope,the dominant factors and total contribution rate of erosion rate were clay particles(42.57%),runoff power(41.10%)and rainfall intensity(16.32%).On root retaining slope,the dominant factors and total contribution rate of erosion rate were runoff velocity(37.94%),clay particles(-31.34%)and runoff power(30.72%).On the undecomposed litter slope,the dominant factors and total contribution rate of erosion rate were Reynolds number(54.89%),resistance coefficient(-40.41%)and slope gradient(4.70%).On the semi-decomposed litter slope,the dominant factors and total contribution rate of erosion rate were Reynolds number(33.27%),clay particles(-32.29%),water depth(-22.93%),rainfall intensity(10.09%)and slope gradient(-1.43%).On gravel slopes,the dominant factors and total contribution rate of erosion rate were Reynolds number(39.99%),clay particles(-34.11%)and slope gradient(25.90%). |
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