| The Black soil region in Northeast China is the largeset commodity grain production base.which is also affected by soil and water loss seriously.The problem of soil and water loss threatens the sustainable agricultural production and food security of China.Gully erosion occurs in the region is the main process and most serious type of land degradation,which ranks only second to the Loess Plateau.Now,the problem has caught the attention of Chinese Government.However,the history of erosion gully research in the Black soil region of Northeast China is short,and it mainly focused on distributions and hazard assessments of gully erosion based on remote sensing interpretations.The study on the characteristics of erosion gully formation and development and erosion forces driving mechanism is weak and cannot provide scientific proof the gully erosion combating.Funded by the project of National Key Research and Development Program of China "The northeast black soil area erosion gully ecological restoration key technology research and demonstration promotion",a typical active erosion gully in plain and hill of Black soil region was chosed as the study goal.The Structure from Motion(SFM)-Multi View Stereo(MVS)using Unmanned Aerial Vehicle(UAV)was applied to investigate the gully morphology characteristics,define the active parts and quantify the projected area and volume of erosion gully.We also analysis the effects of different erosion forces like rainfall,snowmelt,freeze thawing,gravitational force etc.on the development of erosion gully in Black soil Region.The main results are as follows:(1)Built a "integration of heaven and earth",accurate and quick measuring technique apply to the Black soil region.Based on the "integration of heaven and earth"| technique,through a compared experiment with UAV pixel,flight height,shooting angle,image overlap,amount of ground control points and control site,we found that the observation errors of X,Y,Z direction were 0.3 cm,2.3 cm and 1.6 cm,respectively under the conditions of 20 million pixels,10 m height,70%image overlap rate,and five ground control points.The experiment obtained the complete property information of monitoring gully and catchment areas,the data met the requirements of 1:1000 mapping,and the accuracy of comparative analysis results met the research needs.Considering the efficiency,accuracy,difficulty in operation and data processing,cost and other factors,the experiment adopted Inspire2 UAV combined with RTK to monitor the target gully.(2)Rainfall is the main driving force of gully development in the typical Mollisols area of northeast China and has a significant impact on gully development.In the typical Mollisols area of northeast China,the majority of the erosive rainfall occurred between June and August.During the rainfall period of 2019,the head advanced 2.6m,the average gully depth increased by 0.21m,the average width of gully bank on both sides was 1.61m,the projected area of the gully increased by 46.8m2,and the volume increased by 56.4m3.The variation was mainly concentrated at the near head of the gully,and the length was about 1/3 of the total length of the active section of the gully.In rainy season,the soil erosion modulus in the gully catchment area is 1063t·km-2 a-1,which is 5 times more than the allowable soil loss in the black soil area of northeast China.The gully develops rapidly,and nearly 80%of the sediment is mainly from the gully itself,indicating that gully erosion is the main source of regional sediment production.The rainy season soil erosion modulus on the slope where the ditch is located exceeds the allowable soil loss in the northeast black soil area(3)Snowmelt erosion has a great influence on the development of gully in the typical Mollisols area of northeast China,but has no obvious influence on slope erosion.There is spatial heterogeneity in the distribution of snow in the catchment area.Snowmelt lasts about 6 days and is relatively concentrated.In 2017,the projected area of erosion gully increased by 56.96m2 during snowmelt,accounting for 4.85%of the projected area of original ditches.Gully head advanced 2.3m,accounting for 2.98%of the length of the original gully.The runoff at the initial stage of thawing accounted for 92.2%of the total runoff during thawing,and the regional soil erosion modulus was 278t·km-2 a-1.Most of the runoff and sediment transport occurred during the early stages of snowmelt,and more than 98%of the sediment came from gully erosion.During thawing,the change trend of discharge runoff and sediment content at different site in the gully was similar,and the discharge runoff was correlated with air temperature,sediment content and ground temperature,but the discharge runoff was not correlated with sediment content.(4)The occurrence of gravity and freezing-thawing coupling erosion depends on the interaction and transformation relationship among various factors.In the black soil area of northeast China,the main form of slope instability is gully bank crack:compared with October 2017,the total length of the crack increased by 2.5 times,the average width was 8.0cm,the contrast increased by 60%,and the average depth was 26.3cm,the contrast increased by 58.9%.Melt-slide of gully bank:taking the observation data after snowmelt in April 2019 as an example,there are 7 melting and sliding places on both sides of the ditch slope,with a total area of 254,9 m2 and an average drop of 29 cm.Melt flow of gully slope:mainly occurred on slopes with a gradient greater than 60°and a vegetation coverage less than 50%.In 2019,the typical melting zone was observed to be 14.11m3(11.2m long,4.2m wide,0.3m thick)in volumeand 18.35t in weight.Melt-collapse of gully wall:slope of gully wall where melting collapse occurs is mostly close to or up to 90°,and even some of the gully wall is in concave shape.Gravity is the driving factor for melting collapse of gully wall,but freezing-thawing plays a crucial role in promoting it.The slope of gully is the controlling factor of gravity and freezing-thawing coupling erosion,which determines the characteristics and intensity of erosion.The change of soil water content caused by snowmelt is an important trigger factor for erosion caused by the coupling effect of small gravity and freezing-thawing.Vegetation development has a good inhibitory effect on the melt flow of gully slopes,but has little effect on the cracks and the melting of gully banks.(5)The results of soil physical and mechanical properties test showed that with the increase of soil depth,soil clay content and bulk density increased,which led to the deterioration of soil permeability,which was conducive to the formation of stagnant water layer and intensified the driving effect of soil flow on the development of gully.At the same time,the ability of soil to resist erosion decreases with the increase of soil depth,indicating that once the topsoil stripping caused by erosion agent reaches or exceeds the plough bottom depth in northeast black soil area,soil erosion will accelerate and gully erosion is easy to form.(6)The distribution of vegetation and roots can improve soil corrosion resistance.Compared with bare land,grassland and clipped grassland can effectively improve the anti-erosion performance of soil from the beginning to the end of sediment production.Compared with the large scour flow,the sediment yield with low scour flow tends to be stable earlier with the process of time.Vegetation cover enables sediment yield to reach a stable state earlier and at a lower level.Forest and grass measures can slow down the development of gully for soil and water conservation.As above,the gully erosion in plain and hill of Black soil region was serious.The gully erosion is more serious than slope erosion.The gully erosion increased the soil and water loss,significantly.The major performance of gully developing was the gully headward erosion and gully-wall collapsing.The gully headward erosion increased the length and projected area of erosion gully,which was mainly affected by the surface runoff induced by rainfall and snowmelt.The gully-wall collapsing increased the width and projected area of erosion gully,which was mainly affected by the interaction of gully-wall cracks induced by freezing and thawing,surface runoff induced by rainfall and snowmelt,and gravitational force.It has the characteristics of coupling,superposition and interaction.Based on the characteristics of erosion gully formation and development and erosion forces driving mechanism,the gully erosion combating measures should be allocated in gully head and gully wall according to that the most severe period of gully development in this article is from June to August(rainfall period)and mid-late March(the concentrated snow melting period),and the main erosion processes are gully head retreat and gully expansion within one third of total gully length near the gully head.The gully headward erosion combating should consided both guide runoff and eliminate power.The gully-wall collapsing combating should cut slope and reduce cracks and gravitational force.Vegetation should be planted to fix the slope.Some soil and water conservation practices conducted in the catcment area of gully is also necessary. |
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