Five Points for Early Detection of Slope Failures at Solar Power Plants Using Drone Surveying

Table of Contents

• Why early detection of slope failures is important at solar power plants

• Item 1: Regularly inspect slopes for deformation and steps.

• Item 2: Don't overlook drainage problems and changes in water flow paths

• Item 3: Confirm changes in slope vegetation and the appearance of bare ground

• Item 4: Check for settlement around mounting racks, walkways, and drainage facilities

• Item 5: Detect small changes by comparing with past data

• Key operational points for leveraging drone surveying in slope management

• Summary: Toward a management system that detects early signs of landslides

Why Early Detection of Slope Failures Is Important at Solar Power Plants

Solar power plants are sometimes installed not only on flat developed land but also in mountainous areas, hilly terrain, sites that include slopes, and sites where development has produced steps in the ground. At such sites, in addition to managing the power generation equipment itself, it is important to continuously monitor the condition of the ground and slopes. In particular, with slope failures it is desirable not to respond only after they occur but to notice small signs of deformation early and promptly follow up with on-site inspections and decisions about repairs.

When a slope failure occurs, soil and debris can flow beneath panels, the foundations around the mounting racks can be scoured, and maintenance access routes can become difficult to pass. If drainage channels become filled with sediment, runoff during the next rainfall may flow in unexpected directions, potentially expanding the area of damage. Slope deformation can sometimes progress significantly at once, but in practice it often appears as small signs such as cracks, steps, loss of surface soil, emergent groundwater, and disturbed vegetation. Whether these changes can be detected regularly determines the quality of operation and maintenance.

On the other hand, solar power plant sites are expansive, with rows of panels and a complex arrangement of mounting racks, fences, cables, and drainage facilities. Walking the entire site and inspecting it with the human eye alone takes time, and what is overlooked can vary depending on the inspector. Steep slopes and areas with poor footing can also be difficult to approach for safety reasons. One effective method is aerial, area-wide inspection using drone surveying.

Drone surveying makes it easier to understand slope conditions over a wide area by using site-wide photographs, orthophotos, terrain models, and point cloud data. It also allows you to check the surroundings of areas that are difficult for personnel to enter without having to approach them directly. Furthermore, regularly surveying and comparing the same area can provide clues to trends in subsidence and sediment movement that are difficult to judge by visual inspection alone.

However, introducing drone surveying does not automatically allow you to detect slope failures. You need to decide where to look, what kinds of changes to treat as candidates for anomalies, and how to compare with past data. Survey accuracy also varies depending on flight conditions, imaging conditions, the placement of ground control points, analysis methods, grass height, and how the ground surface appears. In this article, from a practical standpoint, we outline five items to check with drone surveying to enable early detection of slope failures at solar power plants.

Item 1: Regularly inspect slopes for deformation and unevenness

The first thing to check for the early detection of slope failures is any change in the slope’s shape. Specifically, cracks at the top of the slope, settlement near the slope crest, steps or offsets partway down the slope, bulging of soil and debris, and accumulation of soil near the slope toe. These are clues that soil within the slope or on its surface may be beginning to move.

When inspecting a site on foot, changes at the upper parts of slopes and at slope shoulders tend to be overlooked. This is because if you only check the areas visible from beneath the panel rows or from maintenance walkways, it is difficult to notice small cracks or steps occurring higher up on the slope. In addition, on steep slopes there are areas that cannot be approached for safety reasons. Drone surveying can photograph these locations from above and allow you to detect anomalies while observing the overall continuity.

When inspecting slope deformation, it is important not merely to view photographs, but to preserve data that captures the terrain's undulations. Recording the slope gradient, the positions of the slope crest and toe, locations where steps (scarps) develop, and depressions where sediment tends to accumulate will make comparisons during the next survey easier. In particular, immediately after construction, heavy rain, a typhoon, or an earthquake, checking the difference from the previous survey makes it easier to determine whether deformation is progressing.

One thing to note is that changes on a slope do not necessarily appear as a large collapse. At first they may present as a small step, a thin linear crack, or a slight surface settlement. Such changes can be difficult to assess from a single visit on site. However, if you perform regular drone surveys of the same area under as similar conditions as possible, it becomes easier to identify changes as differences from the previous survey.

In practice, rather than managing slopes within a power plant all at once, it is effective to divide and inspect high-risk locations such as the upper slope, mid-slope, slope toe, areas around drainage facilities, and slopes near racking foundations. If slope deformation is found, do not immediately conclude it is a collapse; conduct an on-site inspection and check for the presence of seepage, the softness of the ground surface, the direction of soil flow, and impacts on nearby structures. Drone surveying serves to quickly detect anomaly candidates as the entry point for on-site judgment.

Item 2: Don't overlook poor drainage and changes in water flow paths

Slope failures and surface soil movement are often related to the flow of water. At solar power plants, rain that falls on panel surfaces can concentrate on the ground below, site development can alter natural water flows, and sediment or fallen leaves can accumulate in drainage channels and catch basins. If such drainage problems persist, the slope surface can be eroded and water can more easily infiltrate the ground, potentially affecting slope stability.

What you want to confirm with drone surveying is the overall picture of where water flows from and to. Immediately after rainfall, puddles, muddy areas, wet streaks, and traces of sediment flow become more visible. Even in clear weather, differences in surface color, fine erosion rills, sediment accumulation, and the way vegetation is flattened can allow you to infer the traces of watercourses. Features that look like isolated puddles from the ground can sometimes be recognized from above as channels running across an entire slope.

Particular attention should be paid to locations where water concentrates from the slope shoulder down the slope. If there are small steps or depressions at the slope shoulder, rainwater can collect there and carve the slope longitudinally. Even if the initial erosion is only thin, streak-like channels, they deepen with each rainfall and can eventually lead to shallow surface failures. The area near the outlet of drainage channels is also important. If drainage concentrates at a single point and flows onto the slope face, it can cause scour at the slope toe and the loss of soil.

In solar power plants, the arrangement of panel rows can cause drip points to line up. When rainwater falling from the edges of panels concentrates along the same line, linear channels can form on the ground. If such a channel aligns with the slope direction, water can flow more easily and soil and sediment are more likely to move. Aerial drone surveys make it easier to identify erosion along panel rows and downslope movement of soil and sediment.

The condition of the drainage facilities themselves must not be overlooked. Clogging of side ditches, sediment accumulation around catch basins, damage to drainage channels, and scouring near outlets are important items to check as precursors to slope failure. Even if drone surveys cannot judge every detail, identifying in advance locations suspected of abnormalities allows you to focus inspections on those areas during site patrols. In large power plants, rather than inspecting all drainage facilities with the same density, it is more efficient to prioritize places where water tends to concentrate.

Also, the appearance of poor drainage changes with the seasons. During the rainy season and typhoon season, deterioration can progress in a short period, while during dry periods it may appear calm at first glance. In addition to regular surveys, combining them with ad hoc surveys after heavy rainfall makes it possible to detect changes in watercourses more quickly. To detect slope failures early, it is important to interpret not only the shape of the ground but also where water is gathering, where it is eroding, and where it is transporting sediment.

Item 3: Check for changes in slope vegetation and bare ground

Changes in vegetation are also an important factor to check when assessing the condition of a slope. A slope with uniformly growing grass, one where only parts are dead, and one where bare soil is exposed in places may differ in slope stability and water flow. While vegetation plays a role in protecting the ground surface, disturbance of vegetation can be a sign of concentrated water flow or soil erosion.

Drone surveying makes it easier to grasp the extent of bare ground and areas where vegetation is thinning by photographing the entire power plant from above. From the ground, rows of panels and mounting racks can obstruct the view, making it difficult to continuously assess vegetation conditions across the whole slope. However, from above it is easier to see which parts of the slope are thinning, the directions in which the thinning is spreading, and how this relates to drainage channels and maintenance paths.

On bare slopes, during rainfall the soil is directly exposed to raindrops, making the surface more susceptible to erosion. Particularly on sloped sections, fine soil particles can be washed away, forming shallow rills on the surface. If noticed at this stage, it is easier to consider surface protection, drainage improvements, and revising inspection frequency. If left unattended, the area of bare ground can expand, sediment runoff can advance, and sediment can accumulate at the toe of the slope and in drainage facilities.

When examining changes in vegetation, focus not only on whether there is more or less grass, but on differences relative to the surrounding area. Areas where only part of the same slope has flattened grass, where grass is thin in streaks, where only the lower part of a slope is covered by soil and debris, or where the color suddenly changes may be traces left by rainwater or debris flow. Regularly placing images taken by drone surveys side by side makes it easier to determine whether bare ground is spreading or whether the change is temporary.

In solar power plants, the effects of weed control operations also need to be taken into account. Immediately after mowing, the ground surface becomes more visible, but at the same time you can also observe areas where protection by vegetation has weakened. If you record slopes after weeding with drone surveying, it becomes easier to find slope cracks, erosion gullies, and traces of soil movement. On the other hand, during periods when the grass has grown too long, small surface deformations tend to be hidden. Therefore, not only fixing the timing of surveys but also recording grass height and the timing of maintenance work makes it easier to maintain the accuracy of comparisons.

Abundant vegetation does not necessarily mean it is safe. In places where springs occur, you may see vegetation that differs from the surroundings. Changes such as a local abundance of plants that prefer wet conditions, areas that always appear denser only in the same spots, or a moist band at the lower part of a slope may indicate differences in moisture conditions. If water is emerging from within the slope, the ground can become more prone to instability, so it is important to record this as something to check during an on-site inspection.

Item 4: Check for settlement around mounting structures, walkways, and drainage facilities

When considering slope failures at solar power plants, it's not sufficient to look only at the slope. Signs of ground change can appear around equipment within the plant—such as mounting racks, access walkways, drainage facilities, fences, and cable routes. In some cases slope deformation affects equipment, while in others drainage around equipment or site grading affects the slope.

What you should check around the mounting structure are scour near the foundations, settlement around the support posts, surface level differences, and sediment inflow and outflow. If the soil around the structure’s posts is scoured out, it may indicate concentrated rainwater. For structures installed on slopes, water flowing from upslope can be diverted around posts and foundations, causing localized erosion. Inspecting a wide area with drone surveys makes it easier to identify trends such as erosion advancing only along certain rows or sediment accumulating downslope.

Maintenance access paths are also important. If cracks, settlement, ruts, uneven steps, or standing water appear on a path, the surrounding ground may be moving. In particular, on paths along slopes you may see settlement on the slope-shoulder side and outward heaving at the slope toe. During ground patrols attention tends to focus on whether the path is passable, but drone surveys make it easier to grasp the overall alignment and changes in elevation of the entire path. Signs such as slight waviness in the path, changes in the drainage gradient, or the road shoulder beginning to fail should be recorded early.

Around drainage facilities, check for settlement of side ditches, displacement of joints, voids in the surrounding ground, sediment accumulation near catch basins, and scour at drainage outlets. Drainage facilities are intended to convey water safely, but blockages, damage, or poor gradients can instead cause water to concentrate. By overlaying drone-surveyed drainage channel positions with mapped slope deformations, it becomes easier to identify which facilities are associated with risk locations.

Changes can also occur around fences. Tilting of fence foundations, displacement of posts, settlement of the surrounding ground, and accumulation of sediment at the toe of slopes can be clues indicating deformation of slopes and outer perimeter areas. While the perimeter fence is often inspected during routine patrols, the slopes outside the fence and the condition of drainage are sometimes overlooked. Using drone surveying makes it easier to confirm the topography near the property boundary and the movement of soil and sediment along the outer perimeter.

When checking for settlement or deformation around equipment, it is important to distinguish between abnormalities in the power generation equipment and abnormalities in the ground. For example, even if a support structure appears to be tilted, you need to determine whether this is due to adjustments made during construction or to ground settlement by comparing past data and on-site measurements. Drone surveying is effective for capturing the relationship between equipment and terrain across an area and for narrowing down the locations that require on-site inspection.

Item 5: Detect small changes by comparing with past data

What matters for the early detection of slope failures is not just looking at the current condition, but comparing it with the past. Small changes on a slope can be difficult to judge from a single photograph or a single inspection. However, by continuously surveying the same location with a drone and comparing the data with the previous survey, the one before that, immediately after construction, after heavy rain, and so on, it becomes easier to grasp trends in change.

The items to check in a comparison are changes in elevation, changes in slope geometry, the extent of sediment deposition, the spread of bare ground, scour around drainage channels, and settlement of maintenance access routes. In particular, if the position of the slope crest or slope toe has shifted, the slope may be moving. If sediment is gradually accumulating at the slope toe, surface soil may be washing down from upslope. If only the terrain near the outlet of a drainage channel has lowered, scour by flowing water may be progressing.

When comparing with past data, it is important to keep survey conditions as consistent as possible. If the flight area, flight altitude, shooting direction, reference points, processing methods, or data resolution differ greatly each time, it becomes difficult to determine whether observed differences are actual changes or simply due to differences in survey conditions. In practice, it is desirable to establish a flight plan for periodic surveys in advance and acquire data under the same conditions as much as possible.

Also, it is important to organize comparison results in a form that can be used on site. Indicate on drawings the locations where changes occurred, and record the photo numbers, survey dates, descriptions of the changes, results of on-site verification, and the status of any responses so they can be more easily tracked at the next inspection. In slope management, it is important to continue monitoring locations once they have been identified as potential anomalies. Even minor changes should be given higher priority if they have expanded after several months. Conversely, confirming that a change has stopped can provide a basis for deciding to avoid excessive measures.

After heavy rain or a typhoon, in addition to the usual periodic comparisons, it is effective to compare the most recent survey data with pre-disaster data. This allows you to confirm which areas were eroded by the rain, where sediment has moved, and whether drainage facilities were functioning. If drone surveys can identify locations with large changes before walking around the entire power plant, it becomes easier to prioritize on-site inspections.

However, you should avoid determining the level of danger based solely on the results of data comparisons. Slope stability is affected by multiple factors, including soil properties, groundwater, construction history, drainage conditions, and surrounding topography. If changes are found by drone surveying, it is important to carry out on-site verification and, if necessary, proceed to professional investigations or consideration of repairs. The role of drone surveying is to efficiently observe a wide area, record changes, and support early decision-making.

Key operational points for leveraging drone surveying in slope management

To make effective use of drone surveying for slope failure prevention at solar power plants, it is important not to treat surveying itself as a one-off task. Even if you capture data only once at the time of implementation, without the ability to compare with past data it will be difficult to achieve early detection. To apply it to slope management, you need to assemble regular surveys, ad hoc surveys, on-site patrols, and record organization into a single operational workflow.

First, decide the surveying frequency. The appropriate frequency varies depending on the power plant’s location, the slope gradient, the site development history, drainage conditions, and whether there have been any past deformations. In addition to routine inspections during normal periods, it is effective to carry out ad hoc checks at times when slopes are likely to be affected—such as after heavy rain, after typhoons, after earthquakes, or after repair work on earthworks. The important thing is not to rush into conducting flights after an anomaly occurs, but to have baseline data from normal conditions.

Next, clarify the inspection scope. In addition to photographing the entire power plant, designate priority inspection areas related to slope failures, such as slopes, slope shoulders, slope toes, drainage channels, racking foundations, maintenance access paths, and perimeter fences. Separating the basic areas to be checked at every survey from those to be checked additionally after rainfall makes operations easier. In particular, locations that have experienced past sediment outflow, places where water tends to concentrate, and areas prone to becoming bare ground should be recorded for continuous monitoring.

Quality control of flight and photography is also important. To compare changes in slopes, data accuracy and reproducibility are required. Weather, wind, sunlight conditions, grass height, and ground moisture affect how photos appear and the results of data generation. During times with strong shadows or seasons when grass has overgrown and the ground is not visible, slope changes can be easily overlooked. It is important to record the survey conditions each time so they can be taken into account during comparisons.

Decide on the procedure after finding anomaly candidates in drone surveys. For example, if you find a line in an image that appears to be a crack, you need to determine whether to verify it on site, monitor it until the next survey, inspect drainage facilities, or share it with the repair team. Organizing decision criteria in advance can shorten the time from detection to response. For early detection of slope failures, it is important not only to find them but also to have a system that prevents leaving them unaddressed after discovery.

Careful thought is needed about how records are kept. Centrally organizing the survey date, flight area, weather, inspector, locations of anomalies, photos, topographic data, on-site inspection results, and response status makes them easier to use as a management history. Even if power plant personnel change, retaining past anomaly histories allows the same locations to be continuously checked. Making anomaly locations viewable on a map, not just in reports, also makes them easier to use during site rounds.

Also, drone surveying should be regarded not as a replacement for human inspections but as a way to streamline on-site verification, which fits practical operations. Anomaly candidates detected from the air should be checked on the ground, and any irregularities noticed on the ground should be incorporated into the next flight plan to improve management accuracy. By combining drone surveying with on-site patrols, it becomes easier to achieve both broad-area awareness and detailed inspections.

Safety management is also essential. Within the power plant there are mounting structures, cables, fences, access roads, workers, vehicles, and so on. Before flight, it is necessary to check takeoff and landing sites, flight routes, access control, the surrounding environment, and weather conditions, and to conduct operations within safe limits. In areas where slope collapse is suspected, it is also effective to first use drone surveying to confirm the overall situation so that ground workers do not approach carelessly.

Summary: Toward a management system that quickly detects early warning signs of slope failure

Slope failures at solar power plants are a significant risk that affects the operation, maintenance, and safety of power generation equipment. Rather than responding after a major collapse occurs, it is important to detect early small slope deformations, changes in drainage channels, disturbances in vegetation, subsidence around equipment, and differences from past data. Drone surveys are an effective means to efficiently confirm these changes over wide areas and to retain them as records.

In particular, the five items—slope deformation and steps, poor drainage and water flow paths, changes in slope vegetation, subsidence around mounting racks and walkways, and comparison with past data—are checkpoints that are likely to be related to early detection of slope failures. By checking these regularly, it becomes easier to notice changes that are easily overlooked by visual patrols alone. The larger the power plant site, the greater the benefit of being able to inspect the entire area from above.

However, drone surveying does not end with just capturing images. It is necessary to establish baseline data during normal conditions, conduct surveys regularly under the same conditions, verify on-site any locations where changes have occurred, and operate in a way that leads to necessary responses. If survey data, photographs, inspection records, and repair histories can be linked and managed together, it becomes easier to continuously track slope conditions. To detect early warning signs of slope failures, not only technology but also systems for recording and decision-making are important.

In maintaining solar power plants, attention tends to focus on power output and equipment inspections, but ground and slope conditions are also important factors that support long-term operation. By using drone surveying, you can grasp the overall situation before approaching hazardous areas, narrow down the locations that need inspection, and more easily and objectively confirm differences from the past. By incorporating early detection of slope failures into routine management, you can help ensure the stable operation of the entire plant.

If you want to manage slopes at solar power plants more efficiently going forward, it is important to organize on-site surveying, inspection, recording, and sharing as a single workflow. By using drone surveying not merely as aerial photography but as management data to detect slope deformations, you can more readily improve the accuracy of early detection and early response. Establishing operational rules tailored to each site’s topographical conditions, drainage plans, and inspection regimes, and continuing them in a sustainable, manageable way, leads to management that does not overlook slope risks.