Introduction
Above a small village in a mountainous area, rehabilitation work was undertaken to cut back and bench a ravelling natural slope. Beneath the rehabilitated area, based on visual assessments and after a number of rainfall events, continued sloughing of the mountainside face necessitated specialist monitoring utilising Synthetic Aperture Radar (SAR), MSR Halo. Portions of the mountainside contained backscarps and failed material from localised instabilities, which is the focus of this case study.
The MSR Halo was deployed to collect data in order to identify areas of interest which showed displacement, to classify the rate of movement accumulation and to apply site calibrated alarm thresholds. An image of the mountainside and the live view synthetic map displacement data is illustrated in Figure 1.
*An observational scale is one that is outside of the alarm threshold application and shows the displacement and average velocity at increments defined by the user to show as much detail as is required.
Identification of the Areas of Interest: 1 and 2
Two principal areas of interest were identified, for which detailed polygons were drawn (Area 1 and Area 2), with a defined area covering the concentrated movement signature (Defined Area 1 and Defined Area 2). A single point/pixel was also allocated at the centre of the concentration of movement to track the highest level of movement being measured by MSR Halo (P1 and P2).
Note that the movement convention is positive, that is, above the zero line on the trend plot which indicates movement towards the radar. For a 72 hour review period of the collected data (Figure 2 and Figure 3 for Area 1 and Area 2 respectively), a steady state accumulation profile was identified with a fluctuating average velocity (one hour time window).
Alarm Threshold Application and Trend Plot Observations
Alarm threshold values were selected based on the observed accumulation rate of displacement for the two Areas (1 and 2) which was also applied to the Defined Areas (1 and 2) and the Points (P1 and P2).
The alarm thresholds were applicable for both trend plot alarm exceedance and an area threshold (10 m x 10 m = 100 m) criterion (Figure 4). Note that the alarm thresholds are set for a 24 hour period for the displacement. The alarm exceedance instructions are an example and may be configured as per the user’s requirements and operational processes and procedures.
Table 1 (Area 1) and Table 2 (Area 2) provides the movement type, values and alert level ratings based on the alarm threshold application in Figure 4, which is detailed for a 24 hour period.
Based on the assessment of the synthetic map, trend plots, and the alarm exceedance warnings, an Alert Level Rating Synthetic Map was generated for this specific dataset and period of observational time (Figure 5). A time period of 24 hours is applied to the displacement with a one hour time window utilised for the average velocity.
Note that Level 4 is not illustrated on the synthetic map as the displacement and average velocity values had not reached that threshold for the period for which the data is shown.
Conclusion
In summary:
- The displacement and average velocity values provided a baseline of anticipated displacement accumulation and the associated speed for a 24 hour monitoring period.
- Steady state linear movement trends were observed for Area 1 and Area 2 indicating continued displacement, but without significant acceleration.
- The alarm threshold values provided the foundation for an Alert Level Rating scale to be applied to the synthetic map, outside of the observational scale used for visual assessment of the synthetic maps.
- Should the linear movement trend change, the alarm threshold values will need to be updated to represent a higher level of accumulation for both the displacement and the average velocity.
- The MSR Halo provided accurate and valuable deformation data of an active landslide with backscarps from localised instability, making it a highly effective sensor for monitoring natural hazards.
