Study On Gully Head Erosion Models And Mechanism On The Loess Plateau

Loess tableland is the flat basin on the Loess Plateau,which is a terrain with steep edges and flat middle formed by wind and sand accumulation over the past 2.6 million years.The typical representative area is Dongzhiyuan.Loess tableland accounts for more than 1/10 of the total area of the Loess Plateau.It has superior land resources and is suitable for farming and living.It is an important area for industrial and agricultural development and population concentration.At present,the most serious problem facing loess tableland is the fragmentation and disintegration of tableland caused by severe gully erosion,especially at the gully head.Due to the large of topographic relief and concentrated runoff,the gully head develops and expands,and the tableland area shrinks continuously,which seriously threatens the development of villages and the safety of people’s life.In recent years,in order to solve the severe problems of tableland breakage and gully head erosion,the government has introduced the project of Gully Consolidation and Highland Protection aiming at the issues of tableland surface fragmentation and headward erosion.However,due to the large scale,wide range,various types of gully head erosion in the tableland area,and the lack of international precedent for reference,it is extremely difficult to control it.Therefore,it is urgent to carry out a large-scale monitoring of gully head erosion morphology and basic research on gully head erosion mechanism in the tableland,so as to provide reference for comprehensive management and planning of the loess plateau.The existing research on gully head erosion mainly focuses on small-scale gully head volume measurement,yet few focused on regional prediction and control of gully head erosion.In view of the above research status,the gully head of the loess tableland in Dongzhiyuan,Gansu Province,is used as a case study in this paper.On the basis of field investigation,data collection and sampling analysis,the multi-temporal synthetic aperture radar（SAR）image data are comprehensively used to carry out high-precision,large-scale gully head erosion deformation monitoring based on time series synthetic aperture radar interferometry（In SAR）technology.The statistical and machine learning methods are used to construct the regional gully head erosion.Combined with the field simulation test of“rainfall and scour”,the morphology and evolution process of loess gully head erosion based on three-dimensional laser scanning technology are studied.The hydrodynamic characteristics,flow and sediment yield characteristics was interpreted,and the hydrodynamic mechanism of gully head erosion were revealed.Furthermore,the starting conditions of gully head erosion in the loess plateau were determined.The main innovative results of this paper are as follows.（1）From the perspective of geomorphology,the position of gully head is determined,and the characteristics and development mode of gully head are analyzed.The gully head area is defined as a catchment area composed of negative terrain below the shoulder line within a certain length of the gully bottom line,starting from the intersection position between the beginning of the gully bottom line and the shoulder line.Based on this,with the help of geographic information system（GIS）and high-precision DEM data,415 gully heads in the study area were extracted,among which 60 gully heads in six sampling areas were randomly selected for validation.Field investigation and verification using both visual method and total station showed that the extraction accuracy was 85.5%.The characteristics of gully head in the study area were systematically analyzed,and four basic attributes of gully head were summarized,including morphological uncertainty,species diversity,spatial distribution complexity and erosion process correlation.Three patterns of gully head development were expounded:gully head advancing,rill cutting down at gully head and gully expansion along gully.It was found that the gully head development degree was high and the erosion was severe in the study area based on calculation of development degree and density of gully.（2）The long-time and high-precision erosion deformation monitoring in gully head area was carried out using In SAR technology,and the gully head erosion rate model was constructed combined with terrain factors.Based on the 22 SAR images data from 2007 to 2011,the gully head erosion rate was obtained using the small baseline subset（SBAS-In SAR）technology.By analyzing the correlation between topographic factors and the erosion rate,the main topographic factors affecting the erosion of the loess gully head were determined,which are slope(Sl,m m^-1),catchment area（Ca,m²）,topographic humidity index（TWI）and slope length（LS,m）.Based on the geomorphic critical relationship and fitting between gully head erosion and correlation factor,the erosion rate model of gully head was established.The expression is.The model predicts gully head erosion accurately in study area（R²=0.889）.In addition,through field investigation and analysis of deformation monitoring results,it is found that the slope of gully head and catchment area have the most significant influence on erosion.In order to effectively reduce gully head erosion,measures such as gully head backfilling,gully mouth damming,flood discharge canal and slope care can be taken to reduce catchment area and increase drainage channel at gully head.（3）Based on GIS platform,using two advanced data mining techniques,certainty factor analysis method and random forest model,the evaluation method and index system of gully head erosion at the small watershed scale were established.The sensitivity of gully head erosion was analyzed and the reliability of the two models was evaluated.Both models can accurately evaluate the sensitivity of loess gully head erosion,and the random forest model（AUC=88.8%）has better prediction effect than the certainty factor（AUC=84.1%）.The erosion sensitivity was divided into four grades of very high,high,medium and low sensitivity.The position of gully head and the proportion of very high and high sensitivity grades in the whole loess tableland were compared and analyzed,and it is confirmed that gully head was the main erosion site.Furthermore,the importance of topographic factors affecting the sensitivity of gully head erosion was evaluated based on the random forest model,and it was determined that slope,aspect,topographic humidity index and slope length were important factors leading to gully head erosion.（4）The loess gully system with simple geomorphological structure was selected as the research object.The morphological evolution characteristics of loess gully head erosion under different slope gradients and water erosion intensities were studied using the field“rainfall and drainage”test and three-dimensional laser scanning technology.The experimental conditions for the most severe gully head erosion were determined as 60°,8 L min^-1 drainage flow and 90mm h^-1 rainfall intensity.By analyzing the changing process of gully head erosion in different periods,four types of erosion evolution were put forward:“surface erosion–sinkhole–gully erosion-collapse”.Based on 3D laser scanning point cloud data,a 3D characterization method of gully head erosion considering terrain change information is proposed.The accuracy of the method is verified by comparing the calculation results with the actual measured erosion.In order to analyze the morphological evolution process of gully head more clearly,the erosion development process at gully head is divided into horizontal and vertical directions.The horizontal development is mainly caused by water erosion and gravity collapse.The vertical development is mainly the mutual feedback between the collapse of the gully head and the sinkhole,which jointly promotes the development of the gully head.The collapse and statistical results show that the collapse occurs mostly at 0-60 cm of the upper slope,and the collapse caves are concentrated at the top and 0-65 cm in the middle of the slope.（5）The evolution characteristics of sediment yield and hydrodynamic parameters of gully head erosion under different rainfall intensities and slopes were quantitatively analyzed.From the perspective of morphological evolution of gully head erosion,hydrodynamics,runoff and sediment yield characteristics,the hydrodynamics,runoff and sediment yield characteristics of loess gully head were revealed.It is determined that the fluctuation of flow velocity is related to gully erosion on gully slope,and the magnitude of flow velocity has no obvious correlation with the slope.The flow pattern on the gully slope is turbulent.The flow power is determined to be the optimal hydrodynamic parameters of gully head erosion.The initial runoff time is controlled by rainfall and slope,and is inversely proportional to them.Soil infiltration rate was negatively correlated with rainfall intensity.Soil runoff is affected by slope erosion and is also strongly correlated with rainfall intensity.Under the condition of 90 mm h^-1 rainfall,the runoff is 9.4–17.5 times that of 35 mm h^-1.