The starting mechanism study on rainfall-induced sloop loose source in strong earthquake area

The 5.12 Wenchuan earthquake in 2008 led to a large number of loose deposits on the slope on both sides of the gully in the formation area. Under the triggering action of heavy rainfall, the previously unrecognized slope loose source in formation areas easily failed and started earlier than the overall start of the gully debris flow, which has practical significance for realizing the early warning of the gully debris flow starting. The special loose source conditions and postearthquake geological environment change the hydraulic mechanism of traditional debris flow start-up, which brings great difficulties to the monitoring and early warning of postearthquake debris flows. Therefore, based on hydraulics, on the premise of building a hydraulic model of groundwater level change of loose slope accumulation body, the characteristics and rules of groundwater level change are analyzed with the aid of hydraulic seepage theories, and the action characteristics of hydrodynamic pressure and hydrostatic pressure on the accumulation body are quantitatively studied. Based on field exploration data and average slope gradient, we divided the accumulation body in a reasonable manner; completed the sliding force, anti-sliding force and residual-sliding force mechanical expression and establishment; and performed a slope stability analysis. The research results show that the slope starting mode is divided into the thrust-type landslide and the retrogressive landslide mode of the subsection disintegration. The hydraulic deposit mechanism analytical results can also reverse the critical rainfall condition of slope failure. Finally, taking the Yindongzi gully in Dujiangyan as an example, the hydraulic mechanism of typical slope deposits in the formation area was analyzed and combined with the historical debris flow events in the study area to verify the physical simulation test, which was consistent with the field investigation results after the disaster.


Introduction
The Ms 8.0 the Wenchuan earthquake (Sichuan, China) occurred on May 12, 2008, resulting in 28 × 10 8 m 3 loose deposits. These gully sources and bank slope collapse accumulation bodies caused incessant debris flow disasters in the southwestern mountainous areas of China under the excitation of postearthquake heavy rainfall (Tang et al. 2012). On August 13, 2010, the torrential rain in the Longchi area of Dujiangyan led to the simultaneous outbreak of 48 debris flow gullies. On August 20, 2019, the Dujiangyan and Wenchuan areas were affected by subsequent rainstorms, resulting in group-occurring mountain torrents and debris flows (Jin et al. 2019). The sustained and combined effects of postearthquake secondary disasters are obvious. From 5 to 15 years after the earthquake, debris flow activities evolved into low-frequency, medium-and large-scale groups of sticky and transitional debris flows accompanied by mountain floods. Regarding risks from debris flows, previously unrecognized debris flow formation area loose sources on both sides of the gully bank in the upper basin constitute an especially significant threat (Zhao et al. 2020). On the one hand, because the forming region is located in a high valley, there is an abundant quantity of loose sources and a large longitudinal slope ratio; therefore, slope surface sources become the supply sources after slope failure. At the same time, the slope sources transport materials that further induce gully debris flows with high velocity, and slope source failure is highly invisible before outbreak and large chain destructiveness (Fan et al. 2018;Ouyang et al. 2019;Allaire et al. 2009;Gabet et al. 2006;Lourenço et al. 2015). On the other hand, heavy rainfall often occurs in southwestern mountainous areas during the flood season, and it takes time for the rain collection area above the formation area to form flow confluences and enter the debris flow gullies. Therefore, the failure of slope sources in formation areas is 10 to 15 min earlier than the start of the gully debris flows (Wang et al. 2016b;Ouyang et al. 2017;Lin et al. 2018). This short ten minutes is enough for common people to evacuate from the hazard range (Zhang et al. 2012;Zhang et al. 2013;Hürlimann et al. 2019). Therefore, it is of great realistic significance to understand the hydrodynamic conditions and mechanical mechanisms of gully bank slope sources to realize early warning and prediction of gully debris flow initiation. Soil (void ratio, particle size distribution, permeability coefficient, fine particle content, etc.), hydraulic (rainfall intensity, rainfall pattern), topographic and geomorphic conditions all affect the initiation of slope source materials (Shieh et al. 2009;Johnson et al. 1990;Marchi et al. 2002;Cannon et al. 2010). Under the sustained action of rainfall, the pore-water pressure increases, the soil shear strength correspondingly decreases, and the residual downward slippery force of the soil above the slope increases and accumulates continuously. When the down-sliding force is more than the anti-sliding force, the whole slope becomes unstable. When the slope body encounters heavy rainfall, the tip of the tensile crack in the slip band expands and deforms, and the pore-water pressure at the foot of the slope increases sharply, resulting in excess pore-water pressure formation, and the shallow landslide turns into a debris flow (Wen et al. 2005).
The sink flow produced by rainfall has the effects of bottom tearing and scouring, coercion, scraping and transporting of loose materials on the ditch bed replenishing the debris flow. Moreover, lateral erosion, devolution and dam breaks are also involved in the dynamic process of hydraulic debris flows (Zhuang et al. 2013;Gao et al. 2011). Cui (1992 carried out 48 groups of flow-scouring experiments in 1992, and the results showed that soil saturation, gully bed gradient and fine particle content jointly controlled the critical conditions for the initiation of hydraulic gully debris flow. Previous studies have been performed on soil-mechanical and hydraulic debris flows, and preliminary research on the mechanism of rock-soil masses with loose structural characteristics transforms into debris flows. However, these studies are not involved in the initiation problem of slope debris flows under special postearthquake conditions (Wu et al. 2012), and the relevant results could not meet the requirements of disaster prevention techniques for postearthquake reconstruction. The mechanical properties and particle composition of the loose postearthquake deposits have changed greatly compared to those before the earthquake. The postearthquake loose deposits on the gully bank slope have a wide range of sizes, from clay (particle size < 0.001 mm) to boulder (particle size > 100 mm), and are collectively called wide-grading loose soils (WGLS). Wide-grading loose soils (WGLS) are a special slope deposit that often serve as a source for debris flows in the western mountainous area of China and are mainly composed of gravel, soil, sand and block stone. WGLSs have a disorganized attitude, poor separation, loose structure, large void ratio and high permeability coefficient, and the corresponding hydraulic properties are very different from those of current water accumulation in regard to microstructure (Guo and Cui 2020;Cui et al. 2017), which makes the traditional studies on debris flows starting difficult to interpret in regard to the problem of postearthquake debris flow initiation (Cui et al. 2019). Under the triggering action of heavy rainfall, the instability and movement of skeleton particles are mainly affected by seepage and flow scouring, and the movement of accumulated soil particles occurs as sliding with a small amplitude and then they become stable, transitional type sliding or rapid slip phenomena (Yang et al. 2014). The hydraulic mechanism of postearthquake loose deposit initiation is complex. The establishment of the hydraulic model of instability and the identification of the starting critical condition (rainfall threshold) can provide a theoretical basis for the early warning and prediction of postearthquake debris flows (Yang et al. 2019).

Hydraulic mechanism analysis of postearthquake loose slope source materials
The continuous priming action of steady-state heavy rainfall reduced the stability of the postearthquake slope in the formation area and eventually led to slope failure and further transformed into slope debris flows with a solid-liquid-vapor triphase. The initiation of slope failure is the result of water and soil coupling; therefore, it is necessary to combine the theories of soil mechanics, hydraulics and permeation fluid mechanics to reveal the mechanical mechanism of postearthquake slope material failure, which then further transforms into a debris flow.

Underground water level of slope materials formed by heavy rainfall
We set the steady-state rainfall intensity as I , the surface area of the slope or water-collecting area as S and is the slope inclination angle; then, the rainfall flow that infiltrates the whole slope is: The slope deposits in the southwestern mountainous area are mainly composed of gravelly soil, with a loose structure and a high permeability coefficient. When rainstorms occur in the flood season, heavy rainfall continuously infiltrates the slope, and the soil around the bedrock surface is saturated and generates a certain height H is the depth of the diving level. The accumulation body becomes unstable under hydraulic action and may eventually transform into a debris flow.
According to the Darcy law that can be applied in groundwater laminar flow, the seepage flow amount that rainfall infiltrates into the slope is directly proportional to the water level difference of the updown stream (elevation difference from the geoidal surface) and the wetted cross-section and inversely proportional to the length of the seepage path: where Q is the seepage quantity, ( H 2 − H 1 ) is water level difference of the updown stream, A is the cross-section area, which is perpendicular to the water flow direction, namely, the wetted area, L is the length of the seepage path, K is the permeability coefficient. Figure 1 shows the hydrological model of loose slope deposits. We can infer that: where W is the slope width and H is the depth of the diving level ( Fig. 1). We bring Eq. (3) into Eq. (2) and we can obtain that: By integrating Eq. (1) and Eq. (4), we can obtain: Theoretically, with continuous rainfall, the height of the groundwater level can be continuously raised until the slope is completely saturated (i.e., H = Z , where Z is the height of the slope), and eventually, surface runoff or excess infiltration surface flow is formed on the slope surface.
According to the hydraulics expression of runoff flux in saturated soil (Bishop et al. 1961;Lei et al. 2017), we can infer that: where T is the transmissibility coefficient.
Then, by integrating Eq. (4) and Eq. (6), we can obtain: We bring Eq. (7) into Eq. (5) and the formula can be simplified to obtain the groundwater level height generated in the slope after rainfall infiltration: According to Eq. (8), the multivariate function relationship between groundwater depth and geometric parameters such as rainfall intensity, slope inclination angle and slope area is established. Groundwater is deeply affected by the above factors: the depth of the groundwater level (i.e., the height of the diving level) is positively proportional to rainfall intensity and slope height and inversely proportional to the transmissibility coefficient, slope width and slope inclination angle, and the depth of the groundwater level is mainly controlled by rainfall intensity.

Hydrostatic pressure action
2.2.1.1 Hydrostatic pressure According to hydraulics, the height of the free water head at the upper entrance of the strip accumulation body is set as h 2 , the free water head at the inferior outlet is set as h 1 , and a coordinate system with the X direction along the gully bed and the Y direction perpendicular to the gully bed is established. Figure 2 is a schematic diagram of the hydraulic function of the loose slope accumulation body.
According to the hydrostatic pressure at a location in the accumulation body (Montgomery and Dietrich 1994; Wilkinson et al. 2002;Li et al. 2018) P = w ⋅ h (where w is the unit weight of water in the accumulation body and h is the depth of underground water as well as the free water depth of this location), the osmotic hydrostatic pressure at a location along the X direction can be obtained as follows: By integrating P m along the X-axis, the hydrostatic pressure of the strip accumulation body per unit width can be obtained: The hydraulic function analysis of loose slope deposits at groundwater level By setting H m = h 1 +h 2 2 , H m is the average water depth of the strip accumulation body m and in the case calculation, the value of H m is also the calculation result of Eq. (8). The buoyancy of the strip accumulation is: Hydrostatic pressure is the spherical stress, which increases linearly with the increase of groundwater level depth ( P = w ⋅ h ). For the micro-unit, the pressure difference between the upper and lower surfaces (i.e. pore pressure gradient) leads to the generation of buoyancy (Liang 2019, Liang 2021. Considering the slope as a whole, hydrostatic pressure per unit width at the sliding surface is P m , namely buoyancy. The hydrostatic pressure of phreatic flow on the bedrock surface of the accumulation body is:

Dynamic water pressure action
Postearthquake slope deposits with loose structures, large pores and grain compositions are given priority with gravel soil. When the rainfall intensity is large enough and the duration is long enough in the flood season, the groundwater level at a certain height is produced in the internal deposits. In the meantime, laminar flow occurs at the bottom of the deposit, which generates seepage dynamic water pressure. The effect of hydrodynamic pressure on the soil skeleton is presented in the form of a "drag".
According to the continuous media principle, the seepage dynamic water pressure at any point in the seepage zone is: where → G d and → I are the dynamic water pressure vector and hydraulic gradient vector, respectively.
Then, the seepage dynamic water pressure of laminar flow in the bedrock bottom to the slope accumulation body of unit width is: where is the void ratio of the loose deposits and I m is the underground water hydraulic slope of the unit width loose slope deposits: The drag force of underground water on the slope accumulation body under seepage action, namely dynamic water pressure is: When slope underground water flows through a loose accumulation body, the main internal and external causes of water head loss are the viscous action of rainwater and the friction resistance of the soil granular medium to slow waterflow, respectively. At this time, if the physical and permeability properties of the slope accumulation body are determined, the head loss per unit flow distance of bottom laminar flow at low speed is determined or can be obtained by a response model test. Equation (14) can be simplified as: Of course, for the whole loose slope accumulation, if the underground water head disappears before it flows out of the accumulation body, the drag effect of seepage water pressure of the hydraulic action mentioned above cannot be reflected. Assuming that the free water head at the top of the loose accumulation body is h m1 and the whole accumulation body is divided into m strips, the conditions for the bottom laminar flow on the bedrock surface (interface between bedrock and overburden) are as follows:

Stress and stability analysis of slope accumulation bodies in formation areas under heavy rainfall conditions
By considering that the slope gradient of the postearthquake loose slope accumulation body is not uniform, the loose slope accumulation body can be further finely divided according to the slope change of the bedrock surface during the stress analysis to form multisegment unit strips, as shown in Fig. 3. Without considering shear and dislocation between each unit strip, the external forces that control the stability of the m section of the strip are gravity, the residual-sliding force between the strips, hydrostatic pressure, dynamic water pressure, supporting force of the bedrock surface and basal anti-sliding force. If the sliding force is greater than the anti-sliding force, the force system is balanced, and the accumulation body is stable. where S m is the sliding force of the strip m , G m is the gravity of the strip m , and T m−1 is the residual-sliding force of the previous strip m − 1 . In particular, when m = 1 , the initial strip has no residual-sliding force T m−1 from the previous strip; similarly, when m = n , the unit strip is at the bottom of the accumulation body, and there is no reaction force T m of the next strip m + 1 , namely: The anti-sliding force acting on the strip m is: where T m is the reaction force of the next strip and f m is the basal anti-sliding force of the strip m . In 1773, Coulomb proposed the Moore-Coulomb yield criterion: where c is the cohesion of the loose accumulation soil, is the internal friction angle of the loose accumulation soil, and w and w are the shear stress and normal stress on the slip surface, respectively. By combining the hydrostatic pressure Eq. (11), we can obtain the following: After simplification, we can obtain: According to the force analysis of the accumulation body, the condition for the stability of the loose accumulation body unit strip m is F m ≥ S m ; in theory, strip 1 to strip m are all stable. Especially when the strip n of the loose accumulation body still meets the condition F n ≥ S n , the whole accumulation body is in the overall stable state. In contrast, when the unit strip accumulation body m is stable while the unit strip accumulation body n is unstable, the loose accumulation body on the slope surface will partially start. In the meantime, the unit strip accumulation body n must also meet the condition: T n + f n ≤ S n .

Profile of the study area
To verify the theory of hydraulic mechanism analysis in this paper, the Yindongzi gully in Dujiangyan city is taken as an example. The Yindongzi debris flow gully is located in Longchi town, Dujiangyan city, which is a serious disaster area of the Wenchuan earthquake. The geographic location coordinates of the gully mouth are 103°40 ′19′′ E and 31°9′ 46′′ N, and the elevation is 1070~2 2050 m. It belongs to the tectonic erosion landform of Zhongshan Canyon. The mountain trend is mostly toward the northeast and southwest, and the crest is narrow. The general terrain slope gradient is between 30° and 55°, and the valley cutting is deep, steep above and slow at the bottom, and is mostly a "V"-type valley.
The exposed strata in the gully consist of bedrock and Quaternary loose deposits, among which bedrock contains granite, andesite, diorite, tuff and some metamorphic rocks of the Sinian lower series volcanic rock group (Za). The Quaternary strata consist of Holocene residual-slope accumulation (Q4el+ pl), diluvial-slope deposits (Q4pl+dl) and colluvial-slope deposits (Q4col+dl), which are dominated by loose wide-graded gravel soils with a thickness of approximately 1~20 m. The thickness of deposits varies greatly, generally having thin ridges and thick gullies, and there are a total of 5 deposit bodies on both sides of the gullies, which can provide abundant loose materials for debris flow initiation. The area of Yindongzi gully is approximately 2.2 km2, the overall length of the main gully is 2.5 km, the average longitudinal slope declination of the main gully is 310‰, the total material source volume of the gully is 83.55×104 m 3 , and the dynamic reserve volume on the slope that can participate in debris flow activity is 23.02 × 104 m 3 (Fig. 4).
The clear water area of the Yindongzi debris flow gully is a funnel-shaped terrain surrounded by mountains on three sides and an exit on one side. The formation area elevation is 1560 m-1330 m, the gully is 813 m long, the rainwater collecting area is 0.35 km2, the gully is deep, the terrain is steep, the valley slope is 45°-75°, and the average longitudinal slope decline is 283‰. This terrain condition enables the rapid and straight downward flow of debris flows; moreover, the formation area materials start earlier than the gully debris flow. Therefore, the four loose source deposits in the formation area are the key research objects of this study, which can realize the early warning of gully debris flows. The circulation area is the coseismic landslide of the Wenchuan earthquake. The accumulation area has a 7° slope and is located at the mouth of Yindongzi gully, with a total volume of 2.8 × 104 m 3 consisting of a loose accumulation body. On the right side of the accumulation fan at the gully mouth of the debris flow, the postdisaster reconstruction resettlement site of Lianhe Village of Hongkou Township was built. Fifty-six families and 228 residents are planned to be settled in the resettlement site, which is adjacent to the Baisha River watershed (Fig.5).
The study area has abundant rainfall but uneven spatial and temporal distributions. The annual mean rainfall in recent years was approximately 1100 mm, and the maximum rainfall in one day was 183.2 mm (2010.8.13). There are frequent rainstorms and concentrated rainfall in the flood season, which provides hydrodynamic conditions for the start of slope

Case study on the hydrodynamic mechanism of postearthquake slope source materials
Before starting the mechanism study of postearthquake slope source materials, a detailed field investigation and geological exploration work of the Dujiangyan Yindongzi debris flow gully were carried out, and precise mapping and exploratory trench excavation were performed on the whole basin. In addition, the geological and geomorphic elements of slope sources in the formation area were calculated and counted in detail on a topographic map at a scale of 1:500 (Fig. 4) to explore the distribution, reserves, morphology and other characteristic elements of loose source deposits. To study the starting pattern and conditions of the postearthquake loose accumulation body under the rainfall excitation action, the typical slope H was selected as the study area and where Section Plane 3-3' is located are shown in Fig. 4. According to the field geological survey, the slope body is 23.07-115.96 m wide and 326.7 m long, with an area of approximately 5000 m2 and a slope of 8.16°-45.34°. The slope is mostly a loose landslide accumulation body, which is thin on the top and thick on the bottom and is unstable at present (Fig. 6). Grain-size distribution curve of loose wide-graded gravel soils was shown in Fig. 7. Based on the geological sectional drawing of the accumulation body, it can be divided into AB, BC, CD, DE and EF 5 sections from top to bottom according to the average width, slope gradient and accumulation thickness, as shown in Fig. 8 and Fig. 9, and relevant calculation parameters are shown in Table 1 and Table 2.

Variation law of groundwater level height and hydraulic power of slope accumulation body under the rainfall effect
According to the geometric and mechanical parameters of the loose slope accumulation body in the study area (Table 1 and Table 2), combined with Eq. (8), the groundwater It is necessary to further explain that Slope 3-3' in the study area is divided into five sections according to the average slope gradient. The first section accumulation body is located at the top of the slope; therefore, the rainfall collecting area is the section rainfall receiving area as well as the slope surface area. Classification grades based on rainfall intensity include light rain (<10 mm/d), moderate rain (10-25 mm/d), heavy rain (25-50 mm/d), rainstorm (50-100 mm/d), severe rainstorm (100-250 mm/d) and severe torrential rain (>250 mm/d). Figure 10 shows the groundwater level depth of accumulation bodies in each section under different steadystate rainfall intensities.  Theoretically, with the increase in rainfall intensity, the height of groundwater level H in the accumulation body will increase until it exceeds the height of the accumulation body to form surface runoff. However, due to the large thickness of the slope accumulation body in the study area, there will be no groundwater exposure under the condition that the severe torrential rainfall intensity is 600 mm/d. When the slope gradient is smaller, the slope surface is narrower and the deposit is thicker, the groundwater level will be higher, and the influence of the catchment area will be largest, resulting in Sect. 2 producing the highest groundwater level due to the dip length and catchment area of deposit BC being the largest. Meanwhile, deposit section DE with smaller catchment area produced the lowest groundwater level. Second, the groundwater level height is also highly sensitive to the slope gradient, which results in the accumulation body in the third Section CD and fifth Section EF having the largest slope gradient, producing a higher groundwater level height.
If the groundwater level height of deposits under different rainfall conditions varies, then hydraulic function also changes. By Eq. (16), we can determine that the groundwater of the accumulation body will experience laminar flow and seepage effects of dynamic hydraulic pressure in the form of a drag effect, and the entire deposit dynamic hydraulic pressure is far less than the hydrostatic pressure (Fig. 11). This indicates that the instability of postearthquake loose slope deposits is the main result of hydrostatic pressure change and groundwater level raising. As the rain intensity continues to increase, theoretically, hydrostatic pressure, dynamic hydraulic pressure and other hydraulic characteristics will remain constant only when groundwater flows out.

The starting force analysis of postearthquake loose slope source materials
Combined with the stress stability analysis of the accumulation body in 2.3, the critical conditions, start-up pattern and stability of the loose accumulation body in Dujiangyan Yindongzi gully are analyzed and calculated. If the remaining sliding force is negative, it indicates that the accumulation body in this section is self-stable and that there is anti-sliding reserve. In this case, if the accumulation body is stable, the force on the next section of the accumulation body will be written as 0. The calculation results are   Table 3 and Table 4, the unit of rainfall intensity I is mm/d, Sm is the sliding force, fm is the basement anti-sliding force, Tm is the residual-sliding force, and the units of the above three factors are 106 N. When Tm is negative, it indicates that the accumulation body can be self-stable and has no force effect on the next section of the accumulation body 1 3 shown in Table 3 and Table 4. Under the rainfall condition of severe torrential rain, the residual-sliding force of the Sect. 1 and Sect. 2 accumulation body is still negative, indicating that this accumulation body can be self-stable, will not start, and cannot be used as a debris flow source to start and supply the dynamic reserves of the gully source. From the longitudinal perspective of Table 3 and Table 4, rainfall intensity and slope gradient have the greatest influence on slope stability. For the same section of accumulation body. When the rainfall intensity increases, the sliding force increases, and the anti-sliding force continues to decrease, leading to a gradual increase in the residualsliding force. Table 3 and Table 4 show that slope is mostly sensitive to slope stability. According to the slope gradient of the deposits in Table 1, the accumulation body slope gradient of Sect. 3 and Sect. 5 are both greater than 40°, and it is difficult to achieve self-stability even under the very small rainfall intensity condition. The slope of the Sect. 4 accumulation body is again gentle (22.49°), resulting in the remaining sliding force decrease; furthermore, these results demonstrate that Sect. 4 resisted part of the pushing force from the previous section of the accumulation body. Figure 12 shows the variation regulation of the residual-sliding force under different rainfall conditions, and Δ is the difference value between the residual-sliding force in this section and the minimum absolute values under different rainfall conditions. Section 1and Sect. 2 of the deposit will realize a self-stable state under any rainfall condition under different rainfall intensities. Deposits 3 to 5 are particularly unstable, and they will lose anti-sliding reserves under light rainfall intensity conditions. As a result, the consecutive and integral instability of Sect. 3 to Sect. 5 of accumulation bodies and this type of starting pattern represents typical thrust-type instability. Theoretically, if Section m of the accumulation body is stable and Section m+1 of the deposit starts, then the loose accumulation body starts in sections by disintegration and evolves into a debris flow, which is a typical overall thrust-type failure.

Comparison and verification of historical debris flow events in the study area and hydraulic mechanism analytical results
According to the above stress analytical results, the Sect. 1 and Sect. 2 of the accumulation body is stable, while Sects. 3 to 5 are in an extremely unstable state. The Yindongzi debris flow gully in Dujiangyan started 15 times after the earthquake, including the 7.17 debris flow in 2009, 8.13 debris flow in 2010, and 7.9 debris flow in 2013 (Table 5).
Although the lowest rainfall in 24 h was 39 mm/d (2011.9.6), the gully debris flow could be started at a full-scale. A topographic map (Fig. 4) was explored in July 2016, and monitoring instruments, such as rain gauges, pore pressure and moisture content sensors, surface tiltmeters and video monitors, were installed in the study area. From August 18 to 19, 2017, and August 24 to 25, 2017, the city of Dujiangyan had two obvious regional rainstorm weather processes (the former referred to as the "8.18" rainstorm, the latter referred to as the "8.24" rainstorm) (Fig. 13). The rainfall of the 8.18 rainstorm was up to 156 mm/d in 24 h, and the accumulated rainfall of the 8.24 rainstorm was up to 178 mm. According to the field investigation after the disaster, except for a small part of the upper accumulation body of slope accumulation body H in the formation area that was "self-stable" (Sect. 1 and Sect. 2), the rest of the slope accumulation body (Sect. 3 to Sect. 5) were all started. Under such rainfall conditions, the starting and instability failure state of accumulation body H was completely consistent with the calculation results in Sect. 3.2.2. In addition, because the rainfall over 24 h was heavy, water gushed out at the bottom of the slope (Fig. 14) and converged into a highspeed water flow with a width of approximately 1 m in the gully (Fig. 15). Field investigation results after the disaster showed that rainfall led to the formation of a groundwater level with a certain height in the accumulation body. According to the height of the mud trace on both sides of the gully, the overflow height was approximately 6 m, and the maximum flood flow velocity was approximately 100 m/s. After the debris flow, the total amount of accumulation below the ditch mouth reached 6 × 10 4 m 3 , and the deposit amount in the gully above the ditch mouth reached 9 × 10 4 m 3 , which represents a largescale debris flow. The debris flow did not destroy the houses of the settlement site, the road through the village was buried, the rainfall early warning signal was issued in time,

Discussion
This paper is based on hydrology, on the premise of constructing a hydraulic model of groundwater level variation in a loose accumulation body, and with the aid of hydraulic theory, the change in hydraulic characteristics and the variation in hydraulic power with increasing groundwater level were analyzed. Furthermore, the mechanical expression of rainfall infiltration on the accumulation body was constructed, the stability of the slope was analyzed, and the critical condition of source instability and the mechanical prediction model were obtained. The research object is the postearthquake loose slope source materials in the formation area, and the application scope of the hydraulic model is postearthquake wide-grading loose soils (WGLS) with a large void ratio and high permeability coefficient in the mountainous area of Southwest China. The physical and mechanical properties of this kind of crushed soil are quite different from those before the earthquake, and the interior of the slope body forms a groundwater level of a certain height under the excitation action of steady heavy rainfall. Different from the shallow surface landslides of the rainfall type, the postearthquake slope source starting is the result of the combined action of hydrostatic pressure and hydrodynamic pressure, and before the slope starts, the hydraulic condition continuously deteriorates, which belongs to the hydraulic class starting form.
Studies of hydraulic-type gully debris flows have become more mature, and the failure time of slope source materials on both sides of gully banks in watershed formations was earlier than the comprehensive start of gully debris flows. The hydraulic analyses of postearthquake slope source materials not only prove the hydraulic mechanism of slope source initiation but also invert the critical steady rainfall condition of its initiation, which has practical and feasible significance for realizing the early warning of gully debris flow initiation. Li et al. (2018) used more than 20 groups of artificial rainfall physical test methods to set rainfall intensity and slope gradient as control variables and studied four slope types (32°, 34°, 37° and 42°) under five rainfall intensity conditions (60 mm/h, 90 mm/h, 120 mm/h, 150 mm/h, and 180 mm/h), the instability mechanism and failure mode of loose source materials on slope surfaces (Fig.16). According to the critical condition of slope start-up, regression analysis of test results was conducted by using mathematical statistical methods. The multiparameter early warning model and rainfall threshold of 24 h cumulative rainfall were proposed as 180.2 mm/d. The experiments showed that the slope gradient was positively correlated with the slope starting rainfall intensity. Under a slope condition of 32°-37° medium degree, the slope source materials were fully activated, and above 37 degrees slope, an integral failure mode dominated the slope starting, while a certain height of groundwater level was generated. When the 8.18 and 8.24 rainstorms occurred, each section failure state of accumulation body H and the height of the groundwater level generated in the slope were consistent on the one hand with the calculation results of the hydraulic mechanism (2.2.1) and on the other hand verified the field investigation results after the disaster (2.2.3).
The results of the theoretical analysis, historical debris flow events in the study area, physical model test results and real-time field monitoring data of the postearthquake slope source materials in the formation area were compared and analyzed. The results are basically consistent, and the results of hydraulic mechanism analysis are relatively reliable. The study area of the Yindongzi debris flow gully has detailed geological exploration content and rich historical debris flow data; at the same time, field monitoring work has lasted for ten years, the typical geological disaster site field monitoring work is ongoing and will continue for a long time, and primary field monitoring data will constantly improve and effectively modify slope source starting hydraulic models.

Conclusions
(1) The postearthquake slope source materials are mainly gravel soil, which has the characteristics of wide-grain grading and a large permeability coefficient. The instability of loose slope accumulation bodies is the result of the continuous increase in groundwater level and deterioration of hydraulic conditions under the triggering action of heavy rainfall.
(2) According to the theoretical derivation of hydraulics and soil mechanics, the height of the groundwater level generated in the accumulation body is a comprehensive function of parameters such as slope rainfall area S, steady-state rainfall intensity I, slope geometric parameters (W, H, Z) and transmissibility coefficient T. For the physical and geometric properties of the slope accumulation body, the larger S and I are, the higher the height of groundwater level H; conversely, the larger T, W and Z are, the lower the height of groundwater generated in slope body Z. Generally, the narrower the slope is, the deeper the accumulation body thickness, the larger the slope surface area, and the greater the loose accumulation body instability, which further translates into debris flows.
(3) In terms of the two hydraulic effects caused by the uplift of the groundwater level, on the one hand, the increase in hydrostatic pressure P in the slope reduces the anti-sliding force of the base of the accumulation body; on the other hand, the increase in hydrodynamic pressure Gd increases the sliding force that causes slope failure. However, the effect of hydrodynamic pressure on the loose accumulation body instability after rainfall is relatively small. (4) According to the existence form of the remaining sliding force, the starting mode of the postearthquake loose slope accumulation body can be divided into two types: the sliding mode of the thrust-type landslide and the retrogressive landslide mode of the subsection disintegration. Taking the case of the Yingdongzi gully as an example, except for Sect. 1 and Sect. 2 of the accumulation body, the residual-sliding force of the accumulation body of Sect. 3 to Sect. 5 continuously accumulated downward, presenting a starting mode of overall thrust-type failure.