4.1 Identification by optical radar images
In order to comprehend the varying patterns of geological hazards within the interior of the Cuojiu Valley area under distinct triggering factors, as well as to understand the dynamic processes of risk evolution, a comprehensive and iterative procedure encompassing field reconnaissance, preliminary interpretation, ground verification, and detailed interpretation was undertaken. The undertaken endeavors comprised a 5 km linear geological survey, drone-based oblique photography covering 16 km2, remote sensing interpretation over 36 km2. This systematic process facilitated the identification of potential hazardous deformations, demarcation of latent material source zones within Cuojiu Valley, and the quantitative assessment of geological hazard susceptibility.
In the initial stages of the research, careful consideration was given to the geological context of the hazard-prone setting and the frequent instances of geological hazards within the study area. This led to the establishment of distinct categories of remote sensing interpretation markers. This categorization was achieved through comprehensive field reconnaissance and the compilation of relevant data from various sources. Employing the established remote sensing interpretation markers for geological hazards within the study region, geological hazards and their contextual geological settings were accurately identified on remote sensing imagery, culminating in the creation of a preliminary remote sensing interpretation map (Fig. 5).
4.2 Verification by drone-based oblique photogrammetry
Based on the preliminary remote sensing interpretation map, this paper subsequently conducted verification through drone-based oblique photogrammetry. The area designated for drone-based aerial photography presents challenges such as high elevation, inadequate transportation infrastructure, and limited signal coverage, which elevate the risks and complexities associated with drone-based aerial photography operations. Consequently, leveraging the outcomes of optical remote sensing interpretation, drone-based photography surveys were predominantly carried out in the source region of Cuojiu Valley, characterized by a heightened concentration of hazard distribution. The results obtained from drone-based oblique photogrammetry serve a dual function: validating optical remote sensing interpretation and furnishing comprehensive route data for subsequent on-site surveys.
The area covered by drone-based oblique photogrammetry in the source region of the Cuojiu Valley is depicted in Fig. 6. The approach covered an aerial area of approximately 20 km2 with a precision range of 5 to 8 cm. The initiative comprised 24 drone flights conducted at altitudes between 350 and 500 m above ground level. The drones were flown at a consistent speed of 12 m/s, with an 80% overlap in flight paths to ensure comprehensive data capture. A total of approximately 30,000 original aerial images were obtained, with minimal impact from cloud and snow cover, accounting for less than 1% of the acquired images.
Following the multi-perspective bundle adjustment and dense matching processing of drone-based oblique photography data, a three-dimensional model was successfully constructed. Drawing from the data acquired through drone-based oblique photography surveys, a model of the Cuojiu Valley's source area was derived (Fig. 7). The three-dimensional geological images obtained through drone photography provide comprehensive terrain information, facilitating the identification of alterations in topography and distinctive landform features.
Subsequently, based on the drone-based oblique photography model of the Cuojiu Valley's source area, the remote sensing interpretation results were verified and further analyzed. This process also aided in determining the optimal route for on-site field investigation. Consequently, these images enable a more precise analysis of potential geological hazard risks, which is essential for disaster prevention and mitigation efforts in geologically vulnerable areas.
4.3 Verification by onsite field investigation
Based on the outcomes of the initial interpretation phase, categories that raised questions during the interpretation process were subjected to on-site verification (Fig. 8) to further validate the accuracy of remote sensing interpretation markers and interpretation results. In the study area, Location a is characterized by proximal alluvial deposits, Location b represents a landslide-prone area in the high-elevation middle of the valley, Location c exhibits glacial deposits at the valley margin, and Location d features a glacial lake at the top of the valley.
The outcomes of the field geological survey hold substantial significance as they serve as a primary foundation for identifying the sources of geological hazards and conducting analyses of disaster occurrences. Through comprehensive field survey, it has been observed that there are three principal origins of quaternary alluvial sediments, colluvial sediments, and glacial moraines, extending from the historical debris flow accumulation fan at the mouth of the gully to the glacial lake and the terminal moraine dam. The presence of dense vegetation cover in the gully's vicinity does not exhibit distinct indications of geological hazards.
The field investigation revealed the development of nearly 20 tributaries on both sides of the main channel of the gully. Within the watershed, various sources of material, such as collapses, landslides, debris flow, glacial deposits, and glacial lakes, are prevalent, alongside diverse hydraulic conditions. The terrain predominantly consists of forests, shrubs, grasslands, steep rock faces, and eroded mountainous areas. There is a potential risk of large-scale debris flow disasters occurring in this area.
After concluding the phase of ground verification, precise adjustments and refinements were meticulously incorporated into the remote sensing interpretation indicators, which were originally devised to evaluate geological hazards in the region. This crucial stage not only facilitated a comprehensive understanding of various categories of loose materials, their spatial dispersion, quantities, and distinct deformation patterns, but also laid the groundwork for an exhaustive and holistic interpretation of geological hazards (Fig. 9). The total provenance reserves amount to approximately 3.75 × 107 m3, comprising 31 adverse geological bodies. These include 14 collapses, 12 glacial lakes, 3 moraines, 1 landslide, and 1 debris flow accumulation body. The collapse source area covers an approximate area of 0.77 km2, while the collapse accumulation body occupies around 0.37 km2, primarily distributed in the high and steep slopes of the source area. The glacial lakes span an area of approximately 0.10 km2, predominantly found in the source area. The three moraines cover areas of 0.14 m2, 0.09 m2, and 0.03 m2, respectively, with a combined total area of approximately 0.26 m2, also concentrated in the source area. The landslide area measures approximately 0.30 km2, located in the primary ditch of the Valley, near the source of the ditch. Lastly, the debris flow accumulation body spans an area of roughly 0.65 km2, taking on a fan-shaped appearance extending directly to the mouth of the ditch.
Drawing upon an analysis of temperature, precipitation, and source characteristics, three distinct categories of high-elevation and long-runout hazards have been identified within the Cuojiu Valley: glacial lake outburst flows, freeze-thaw flows/debris flow, and rainstorm debris flow (Fig. 10). Among these potential hazards, rainstorm-induced debris flow exhibits a relatively heightened probability, whereas the occurrence of freeze-thaw flows, or debris flow is deemed to be exceptionally unlikely.