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Subjects:
Geography, Physical
Island territories and their coastal regions are subject to a wide variety of stresses, both natural and anthropogenic. With increasing pressures on these vulnerable environments, the need to improve our knowledge of these ecosystems increases as well. Unmanned Aerial Vehicles (UAVs) have shown their worth as a tool for data acquisition in coastal zones.
- Unmanned Aerial Vehicles (UAVs)
- coastal monitoring
- island territories
- resilience assessment
- spatial decision support system
1. Introduction
Coastal environments serve as the transition zone connecting marine and terrestrial ecosystems. With approximately 40 percent of the human population living within 100 km of a coast, these regions are some of the planet’s most productive and valued ecosystems [1]. Today, population densities in coastal regions are more than three times higher than the global average [2]. Unfortunately, coastal environments are subject to a wide variety of stresses, both natural and anthropogenic. While natural factors such as sea-level rise, erosion, and flooding are exacerbated by the encroaching threat of climate change, anthropogenic factors fuel this change in climate, creating a disastrous feedback loop. The increasing vulnerability of these ecosystems highlights the need to identify the limits and equilibrium of these environments in order to increase their resilience. Island territories naturally host these environments and are rendered even more vulnerable because of them. In most of these island territories, the inhabitants, agriculture, recreational activities, infrastructure, and tourism are condensed along coastal areas, which generates a linear urbanism that is particularly vulnerable to coastal risks [3].
These coastal environments can undergo rapid morphological changes, which can have important socio-economic implications, especially along coasts with high commercial, recreational, and ecological value [7]. These changes can be both naturally occurring such as receding shorelines due to storm erosion [8] or induced/accelerated by anthropogenic processes such as intensive coastal development from port expansion [7]. With increasing anthropogenic and natural pressures on island territories as well as their coastal regions, the need to increase our understanding of these specific ecosystems increases as well. A first step in doing so is to perform repetitive surveys [6]. These efforts allow for effective and efficient monitoring of these environments, which can help inform management decisions [9]. Monitoring these environments comes with challenges, and methods vary extensively based on cost, duration, and accuracy [10]. Aerial approaches are the most common today, and the most generally used methods come in the form of aerial photography (by aircraft), satellite images, airborne light detection and ranging technology (LiDAR) by aircraft, and most recently, by Unmanned Aerial Vehicles (UAVs). Each source provides unique information accompanied by both benefits and limitations. For instance, satellite images can outline shoreline changes temporally and provide remote sensing approaches; however, these images require suitable climatic conditions and have relatively low resolutions. Airborne aircraft LiDAR, on the other hand, does not require a specific weather condition in order to provide accurate Digital Surface Models (DSM), but is relatively costly compared to other sources [6].
While these monitoring methods do have their advantages, when it comes to coastal management and risk assessment, their limitations do not allow for repetitive monitoring at high temporal frequencies. This frequency is a necessity in order to obtain up-to-date information to be able to make decisions, especially when dealing with an environment that undergoes fast morphological changes [6]. Numerous studies have recently shown the effectiveness of using Unmanned Aerial Vehicles (UAVs) for coastal monitoring, [6,8,9,10,11,12,13,14,15,16,17,18], as it allows for an increase in the frequency of monitoring campaigns at a lower cost while obtaining comparable accuracy to LiDAR data, which is the most utilized and available method today for aerial data [6]. Creating said digital models with high spatial resolution levels is important for obtaining predictive data that can adequately provide useful information for decision-makers. The use of UAVs in the context of coastal monitoring on island territories is relatively recent in the scientific literature and not extensively documented; thus, the topic would benefit from a review allowing for an agglomeration of contextualized studies to be analyzed.
2. UAVs for Coastal Zone Monitoring in Island Territories
2.1. Benefits and Advantages
Recent developments and improvements in technology have allowed for UAVs to become important tools in the fields of environmental monitoring and conservation [9]. The range of types and sizes of UAVs differs widely, from light handheld UAVs to large industrial platforms capable of carrying dozens of pounds, as well as different types of platforms such as flying wings similar to planes and multi-copters closer to helicopters. This variability in platform type and size allows for UAV research to adapt to the specific object and study site. The latest technology of UAVs additionally allows for specific sensors, cameras, or even test tubes to attach on to the UAV, adding to the adaptability of this tool [41]. The accessibility, price, simplicity, and high-resolution data help show that these spatial tools are capable of complementing or even replacing other aerial data sources such as satellites and aircraft [43]. The affordability of UAVs both in time and price for coastal monitoring and research has been documented in recent scientific literature [22,27,28,29,30,33].
UAVs have been commercialized over the past decade due to the increase in demand for this technology, which has lowered the price of these devices all the while improving their quality. This commercialization has allowed for user-friendly UAVs and software that have the capacity for pre-flight planning and autonomous flights. Another advantage of using UAVs for research is their ability to fly at lower altitudes and their maneuverability [21]. Flying at low altitudes allows for finer spatial resolution output in addition to not being affected by cloud cover and thus not being as constrained by weather conditions. Subsequently, the high levels of maneuverability provided by using UAVs increase spatial coverage and provide access to areas previously unattainable by airplanes and helicopters [37]. UAVs have, with very clear water, allowed ordinary RGB cameras to obtain information on underwater domains such as coral reef conditions, bathymetric surveys, or species identification, refs. [12,44,45,46], a possibility unavailable to other aerial data sources.
A factor to consider when observing elements along the coast is the altitude of the UAV. A study was conducted on the Turneffe Atoll in Belize, which analyzed the influence of altitude on tropical marine classification using imagery from UAVs [47]. This study determines which altitude is best suited for images taken in five different classes of environmental settings. When observing mangroves, the ideal altitude is 75 m; for sand, an altitude of 85 m was the best of the three altitudes compared. Seagrass is best observed at an altitude of 75 m, while coral images are more effective at 85 m. Finally, when observing the sea, 85 m was the best of the three altitudes tested [47]. Using UAVs that have the capacity to fly at these precise low altitudes allows for these spatial tools to adapt to the study site being observed.
The reviewed studies also suggest that the monitoring of coastal zones prone to disasters by UAVs has noteworthy advantages due to its rapid deployment (low time mission planning), high level of automation, and the possibility of inspecting the images on the terrain in the case of RGB camera use. These factors can allow users to catch errors early on and repeat the survey if necessary [24,30,37]. Considering these benefits, this aerial data source allows for frequent repetitive surveys, which, in the case of island territories, is an essential factor due to the high vulnerability, limited accessibility (rough terrain), and rapid morphological changes of the territory [48].
Additional advantages are low security risks and costs in case of accidents, and risk awareness capacities [6]. UAV imagery and footage can be used as a communicative tool for raising awareness in a community by highlighting environmental hazards and engaging stakeholders at various levels. High-resolution data collected from UAV platforms also has the capacity to provide a rapid overview of the disaster area [43].
Due to the technical advancements made over time, UAVs are helping with the complex task of coastal environmental disaster monitoring and have the potential to greatly increase the availability of data for spatial modeling, specifically in vulnerable and complicated to access/maneuver through territories. The technical advantages, the possibility for frequent surveys, and the capacity for community awareness and communication are advantages that allow UAVs to serve as efficient tools and have the potential to present local authorities and decision-making bodies with a more global picture of the environmental impacts [43].
2.2. Limitations and Challenges
Much like all other aerial data sources, there are also some shortcomings to using UAVs as a tool for coastal monitoring. Both types of platforms mentioned above have their respective limitations. Multi-rotor UAVs are advantageous due to their capacity to operate at low speeds, their maneuverability, and their vertical takeoff and landing abilities. Due to these benefits, multi-rotor UAVs can be used for closer data capture, such as 3D coastal mapping [49]. For these reasons, in the context of coastal monitoring in island territories, these platforms tend to be prioritized over fixed-wing aircraft. However, multi-rotor UAVs require a substantial amount of battery life and are generally limited to about 30 min of flight time (depending on meteorological conditions) [6]. Fixed-wings, on the other hand, are more aerodynamic and have the added benefit of longer flight durations, thus covering more ground, but they are less maneuverable and require extended stretches of dry, flat, and unobstructed land to take off and land. Despite a fixed-wing UAV’s capacity to cover more ground than a multi-rotor, these UAVs cannot compete with manned aircraft. A comparison between the aerial coverage areas of UAVs and those of manned aircraft reveals that UAV aerial surveys are more suitable for covering comparatively smaller areas (0.01–1 km2), while manned aircraft are a useful tool for capturing larger-scale dynamics (10–1000 km2) [29]. Furthermore, the operational distance is limited by the radio link range with the ground control station, which is usually around 5 km [39].
In addition, there are many laws and regulations that limit where and how far you can fly the UAV, depending on the country. France’s airspace, when dealing with UAVs, is relatively controlled compared to other countries, requiring a certain amount of preparation and paperwork, especially in French Polynesia because of the proximity of the airports. Thus, specific permits and licenses are required. A variety of studies listed respecting these regulations and acquiring licenses and authorizations as a challenge to overcome [24,38,45].
Environmental conditions are a non-negligible factor that light or commercial UAVs can encounter when monitoring coastal areas compared to other environments. The first is the weather, more specifically, wind and rain. High winds cause platform instability, which reduces image quality and puts more strain on the battery life, reducing the surface coverage of a flyover [39]. Some drones have the capacity to be waterproof and highly stabilized, thus withstanding high winds and precipitation; however, the UAV platforms reviewed herein did not have these advantages. This problem is decreasing as operational developments of UAVs increase at a rapid pace. Today, lightweight UAVs can easily operate when wind gusts are lower than 25 km/h. Another weather condition limitation mentioned by several studies is the reflection of the sun, specifically in the intertidal and mangrove studies, as well as the issue of turbidity to be able to see through the water [28,35,38].
The second issue is posed by crashing waves or large bodies of water, which can prevent the application of matching techniques. In these cases, masking techniques are used to avoid these areas from being used for point matching, essentially treating these areas independently in terms of ground control [6]. Other studies mentioned difficulties with the postprocessing software. One study [22] encountered complications with mosaicking as well as having to manually contour water bodies because of the limitations in the photogrammetry software. Studies also mentioned complications such as a negative effect of vegetation on the point cloud [24] or limitations in the spatial analysis software for rock fall analysis [21]. The consequences of distortion within the images captured affected a variety of studies as well [21,35,38].
The majority of small commercial UAVs are not suitable for lifting, so attaching LiDAR sensors or higher-capacity cameras is impossible. Attaching a sensor requires a larger UAV, which increases the price of the platform. One of the studies [23] mentioned that the sensor capacity was limited due to the UAV’s inability to lift a heavier LiDAR sensor. Subsequently, despite the large number of sensors (GPS, IMU, etc.) on board a UAV, the produced sensor data does not have the precision required for georeferencing applications, hence the need for GCPs [18]. GCPs in general posed problems to several studies [21,25,31,32,37]. For instance, GCPs in long-term beach studies were identified as a challenge since the beach was constantly changing. Additionally, GCPs cannot be placed on the water and can be moved by incoming tides. Setting out evenly distributed GCPs for image capture due to the inaccessibility of some coastal areas proved to be challenging [25]. The entire GCP process, from laying out the targets to processing the GPS points, was listed as a time-consuming challenge [21,25,37].
A solution to these issues is using a Real Time Kinematic (RTK)-equipped UAV [42]. The RTK system is a precise positioning technique that uses a carrier phase processing GPS signal [50] on board the UAV to provide high-performance positioning accuracy of a few centimeters. Studies have shown that an RTK UAV is capable of using zero to one GCP to properly georeference the data [31]. Using an RTK-equipped UAV thus removes the need for GCP, which has three distinct advantages. The first is the decreased time taken to complete the surveys; the second is the accessibility that it offers; for instance, less accessible coastal areas like coastal cliffs or wetlands are now as simple to survey as an open beach; and the third is being able to do all this while obtaining similar or even higher resolution data [51].
A variety of studies have analyzed the advantages of using RTK-GNSS methods. The authors of [52], for instance, compared the precision of data acquired by RTK-UAV with and without GCPs and found little to no difference in horizontal precision with a slight decrease in vertical precision (+/− 4 cm). The study concludes that both the mean values and the standard deviations show that the lack of GCPs did not significantly affect the final reconstruction of the 3D model of the coastal section [51,52]. Thus, in the context of coastal monitoring in island territories, the RTK method would be most beneficial due to the limited amount of GCPs required, thus allowing for larger surface coverage over a shorter time period and access to previously complicated terrains.
In conclusion, when choosing a UAV platform, the type of sensor/camera, different software, etc., depends on the objectives and parameters of the study; however, when analyzing a coastal island territory, there are several options that will facilitate the operationalization of such a tool. Based on the articles reviewed and the limitations identified, multi-rotor UAVs are more adapted to the specific context of coastal island territories. The researchers recommend using multi-rotor UAVs over those with flying wings because of the facilitated maneuverability that multi-rotors provide as well as their stability during strong winds and more extreme weather. This is especially necessary during takeoffs and landings. A lot of coasts on island territories are narrow or lack a suitable landing strip due to the presence of water, tides, or rocky shores. Additionally, the use of UAVs equipped with RGB cameras for photogrammetric applications is well adapted to this type of territory and allows for a relatively fast data acquisition method that can be repeated over a period of time. Other factors that have shown to be effective are the use of pipeline user-friendly software such as Pix4D or Agisoft Metashape when conducting photogrammetric analyses. RTK-GNSS is another solution to a limitation that appears often in the literature. Placing GCPs on the littoral is time-consuming and sometimes impossible due to the tides. This method (as described previously) removes the need for GCPs and is thus recommended when operating on coastal island territories.
This entry is adapted from the peer-reviewed paper 10.3390/drones7030206
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