Deciphering Complex Morphology and Structural Connectivity of High-Magnitude Deep-Seated Landslides via Airborne Laser Scanning: A Case Study in the Vrancea Seismic Region, Romanian Carpathians
Abstract
:1. Introduction
2. Study Area
3. Materials and Methods
- Data Acquisition: The process involved laser emission and reflection, time-of-flight measurement and recording of position and orientation data. To ensure accuracy, GPS and IMU components were integrated to provide precise location information and capture the sensor’s motion and orientation during the survey. The drone followed the terrain at a height of 80–120 m with an average point density of 80 points/m2, all within an accuracy threshold of less than 5 cm.
- Data Pre-processing: Systematic errors or biases in the sensor’s measurements were addressed through calibration. Noise points and outliers were filtered out to enhance data quality. The incorporation of IMU and GPS data, augmented with RTK corrections, facilitated precise georeferencing, thereby elevating the overall data reliability.
- Point Cloud Generation: The point cloud generation process accounted for multiple returns from a single laser pulse, which occurred due to reflections from different surfaces or objects. Each return was recorded separately in the point cloud, providing multiple elevation measurements for each location. This step resulted in different categories, including ground points, vegetation points, buildings and other objects, which was particularly beneficial in vegetated areas.
- Point Cloud Classification: Point cloud classification was carried out to distinguish ground points from aboveground features such as vegetation, buildings and other objects. The classification was performed using the CSF [46] plugin in Cloud Compare (Table 3), which utilized a 3D computer graphics algorithm based on cloth simulation. This allowed the extraction of ground points from the LiDAR point cloud, effectively separating it into ground and aboveground points.
- 5.
- Digital Terrain Model Generation: The DTM encapsulated the bare earth surface, achieved by filtering out the non-ground points from the previously classified point cloud (Figure 6). This filtration rendered a refined depiction of the natural terrain topography, omitting aboveground attributes. The ultimate gridded representations of the DTM were derived employing a grid-based interpolation technique, specifically Delaunay triangulation, with a designated cell size of 0.25 m.
- 6.
- Data Export: The processed data, the point cloud and the DTM were exported as raster files, facilitating further analysis and utilization in geomorphological evaluation and mapping.
4. Results and Discussion
4.1. UAV LiDAR DTM
4.2. Geomorphologic Interpretation
- The dormant ancient landslide: With a buffered geomorphologic impact and a null superior impact surface, the first landslide (which we assume to be a first-time failure, taking into account the morphological characteristics of such processes in the study area, as described by Ielenicz [38]) remained suspended on the slope, without reaching the Buzău Valley. Occurring in the upper catchment of a small, second-order tributary, it developed as an alternation of translational and rotational (mainly retrogressively, towards the main scarp, as the crown was reducing in size) shallow-to-medium-seated landslide (earth, debris and even small rock slides), outlining a 20 ha depletion area, bordered by a 5–10 m scarp, close to the watershed and presently discontinuous, flattened during a longer evolution, and thus difficult to be sharply recognized. All these considerations suggested an older age, expressed in relative terms by comparing it to the following one. The accumulation of the heterogeneous landslide material is presently noticeable through a hummocky landscape marked by scattered, mostly buried large sandstone blocks. The accumulation deposit (2–5 m thick), presently largely flattened and without obvious rugosity (or cracks and fissures), at its maximum extent, advanced progressively for a short distance along the second-order stream and it stopped, remaining suspended. There is no clear evidence as to how long it advanced, but taking into account local morphological features (especially related to the local lateral extension and longitudinal profile), we assumed that the travel distance was short, of the order of some tens of meters.
- The dormant recent landslide: This occurred during the time period following the first (old) landslide, but without clear definition of its time span. It progressively affected the entire longitudinal slope profile, along the second-order stream (left tributary of the Buzău River). Its younger age (in comparison with the first one) was estimated by comparing the aspect of the scarps (flattened and faded in the first case, very much visible and even largely active, determined through retrogressive development, in the second case), the overall morphology of the depletion zones (with better outlined erosional and gravitational processes in the case of the recent one, deeper incised and active gullies, especially in the middle sector, where the two accumulation deposits are overlapping, and fresh rock fall accumulations) and the better-pronounced micromorphology and increased overall rugosity (including clear cracks and fissures) of the landslide deposit. Both depletion and accumulation sectors are marked by active processes (erosional—sheet wash, gully erosion—as well as gravitational—rotational and translational shallow debris and earth slides). The elongated scarps (reaching even 10–20 m in height), situated in the immediate vicinity of the watershed, present numerous sectors (2–10 m high) marked by recent activity (debris and rock accumulations at their foot), despite being largely forested. The large extent of the depletion area (60 ha), the 600 m long and the 60–160 m relatively narrow runout channel and the very large terminal accumulation fan (35 ha) indicate a (most likely) sudden occurrence: the 2–5 m debris deposit (in the median part) and an area up to more than 60–70 m thick (in the final, accumulation fan part) moved progressively along half of the entire slope’s longitudinal profile, along the second-order stream, ending with an occlusion geomorphologic impact, during which the Buzău River was shifted with some 500 m westwards, as well as an areal impact surface. The lack of older lateral pressure ridges along the present runout channel or in its immediate vicinity and the compact morphology of the accumulation fan strongly suggest a single pulsation (in comparison, several generations of terminal fans, each one deposited according to a certain pulsation, may be easily witnessed to other similar processes in the immediate vicinity, like the Chirlești earth flow [36]). Meanwhile, this sudden accumulation of landslide deposits on top of alluvial formations were documented through geophysical measurements, as detailed below.
- The presently active sectors: Corresponding to shallow- and medium-seated reactivations (mainly in the rotational form, but also as translational debris slides) of the landslide toe, these sectors are witnessing the reactivation role of slope gravitational and fluvial processes (see Figure 9). The reactivations are conditioned either by the lateral erosion of the Buzău River (the NW one), as a response of the river’s return to its normal hydraulic flow parameters, or by the adaptation of the toe’s steep profile (inclinations of up to 15–25°) to the local base level (S and SE ones; the latter is equally conditioned by erosion exerted by the gully which forms the limit between the landslide deposit and the in situ rock). A similar process of slope undercut due to river lateral erosion was witnessed immediately north, where the toe of an even older, but similar (at least in morphology) debris flow was completely cut by the Buzău River, which straightened its course once the landslide accumulation deposit was removed. With a more reduced magnitude (1–2 ha each), these presently active sectors show a riparian geomorphologic impact and a linear impact surface (the NW one), as well as a buffered geomorphologic impact (S and SE ones, showing no clear physical contact between the landslide toe and the river) with a null impact surface (the landslide, with both its sectors, depletion and accumulation, is geomorphically decoupled by the river channel).
- Independent events or those that change each other’s preparing framework (first an earthquake, triggering, in the context of local amplifications, rock falls and shallow landslides in the vicinity of the ridge, resulting in the accumulation of a potentially unstable deposit, with low cohesion and increased permeability, prone to a subsequent rainfall-induced displacement).
- Coupled events (high-magnitude seismic shaking and co-seismic landslide). Clear signs of earthquake-induced landslides are associated with high-magnitude earthquakes, exceeding Mw > 7, in boh dry and humid conditions [35]. Moreover, knowing that for large, deep-seated landslide initiation, extended periods (7–180 days) with precipitation are needed and the return period of heavy precipitation (FR10) and very heavy precipitation (FR25) range from 30 to 100 years [26,27], one should assume that at least 1–3 times per century, the Vrancea seismic region faces a propitious multi-hazard (large earthquakes and heavy precipitation) scenario, resulting in major geomorphic impacts of deep-seated landslides on river networks.
5. Conclusions and Perspectives
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Aspect | Multi-Rotor Drone | Fixed-Wing Drone (Trinity f90+) |
---|---|---|
Flight time | Shorter flight times due to energy demands | Longer flight times due to aerodynamic design |
Maneuverability | Highly maneuverable, can hover and change directions quickly | Less maneuverable, requires space for takeoff and landing |
Coverage | Suitable for smaller, more detailed areas | Suitable for larger area coverage |
Terrain adaptability | Better suited for following terrain in areas with high relief amplitude due to hover and slow-flight capabilities | Challenged by areas with high relief amplitude as it requires space and time for the drone to adjust altitude, especially at a speed of about 18 m/s |
Aspect | Mission 1 | Mission 2 | Mission 3 | Mission 4 | Mission 5 |
---|---|---|---|---|---|
UAV model | Trinity f90+ | ||||
Sensor model | Qube 240 | ||||
Flight height (m) | 105 | 110 | 85 | 85 | 100 |
Number of points in cloud | 231,785,663 | 224,859,106 | 148,990,858 | 304,908,982 | 224,603,685 |
Point cloud resolution (points/m2) | 90 | 85 | 110 | 110 | 95 |
Surface (ha) | 268 | 272 | 257 | 237 | 260 |
DEM resolution | 0.25 | 0.25 | 0.25 | 0.25 | 0.25 |
Parameter Name | Description | Value | Options/Units |
---|---|---|---|
General Parameters | |||
Scenes | Set the scene type for the point clouds to determine the terrain’s rigidness | Steep slope | Steep slope, relief, flat |
Slope Processing | This option fine-tunes the cloth fit to better match ground measurements | On | On/Off |
Advanced Parameters | |||
Cloth Resolution | Refers to the grid size of the cloth used to cover the terrain | 0.6 | Units of point clouds |
Max Iterations | Refers to the maximum iteration times for terrain simulation | 600 | |
Classification threshold | A threshold to classify the point clouds into ground and non-ground parts based on the distances between points and the simulated terrain | 0.5 | Units of point clouds |
N° | Coupling | Geomorphic Impact | Impact Surface | Trigger | Single/Multi-Hazard | Recurrence |
---|---|---|---|---|---|---|
1 | Decoupled/ indirectly coupled | Buffered | Nil |
|
| 1–5 years |
2 | Directly coupled | Occlusion/ blockage/ obliteration | Areal/ linear |
Single (1–3 months antecedent wet intervals or 0–72 h heavy precipitation exceeding 1–2 times the monthly averages); Mw > 7 earthquake (in dry/humid conditions)
Multiple (combination of above) |
Single hazard
| 1–3 per century |
3 | Directly coupled | Riparian, suspended | Linear/ punctual |
Mw > 4.5 earthquake (with site effects) |
| 1–5 years |
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Micu, M.; Vasile, M.; Miron, F.; Onaca, A.; Sîrbu, F.; Skyline Drones Team. Deciphering Complex Morphology and Structural Connectivity of High-Magnitude Deep-Seated Landslides via Airborne Laser Scanning: A Case Study in the Vrancea Seismic Region, Romanian Carpathians. Remote Sens. 2023, 15, 5286. https://doi.org/10.3390/rs15225286
Micu M, Vasile M, Miron F, Onaca A, Sîrbu F, Skyline Drones Team. Deciphering Complex Morphology and Structural Connectivity of High-Magnitude Deep-Seated Landslides via Airborne Laser Scanning: A Case Study in the Vrancea Seismic Region, Romanian Carpathians. Remote Sensing. 2023; 15(22):5286. https://doi.org/10.3390/rs15225286
Chicago/Turabian StyleMicu, Mihai, Mirela Vasile, Florin Miron, Alexandru Onaca, Flavius Sîrbu, and Skyline Drones Team. 2023. "Deciphering Complex Morphology and Structural Connectivity of High-Magnitude Deep-Seated Landslides via Airborne Laser Scanning: A Case Study in the Vrancea Seismic Region, Romanian Carpathians" Remote Sensing 15, no. 22: 5286. https://doi.org/10.3390/rs15225286