COASTAL VULNERABILITY AND HAZARD ASSESSMENT
There is an increasing interest in coastal vulnerability assessment, as it represents the preliminary stage of investigation into coastal defence and a tool in the management of the economy of beach- related tourism, which is a significant resource for many communities during the summer season. Very often, the coastal vulnerability and hazard assessment of coastal processes refer to monitoring of the impact of sea storms on coastal areas. Low-lying coasts are particularly prone to erosion and flooding processes. The modelling of these processes can be much improved by the introduction of accurate topographic data as reference surfaces.
Casella et al. (2014) introduced high-resolution DSMs from UAV-SfM (SFM) technology into a wave run-up model and validated the data by comparing the observed and the modelled run-up. They collected aerial photographs (Mikrokopter Okto XL vehicle) after two different swells in a study area in the municipality of Borghetto Santo Spirito (Liguria Region, Italy). Figure 6.4 shows a representation of the studied area, and the data provided by the run-up model.
Since 1944, the study area of this coastal stretch has been the subject of engineering works that have been aimed at its protection from marine erosion. More recently, several interventions of beach nourishment were performed for this shoreline. To obtain a complete beach topography, the point cloud obtained through a UAV survey and the multibeam data were merged to represent the emerged and submerged sectors. Finally, the elevation data were referred to a geoid model and horizontal positions to a cartographic reference system. The use of the elevation referred to a geoid model (above sea level) is fundamental for studies that are focused on coastal vulnerability to storm surges, and the accuracy of the selected geoid model must satisfy the requirements of the model. In Mancini et al. (2015), the DSM of a coastal dune system was referenced to horizontal coordinates in UTM Zone 33N (datum ETRF00), while the vertical values were referred to the mean sea level using the geoid model ITALGE02005 provided by the Italian Geographic Military Institute. Indeed, the use of a (more or less) accurate GNSS device for GCP positioning produced elevations that referred to an ellipsoidal system. Thus, a geoid model was required.
A report from Casella et al. (2014) highlighted the potential of methodologies that incorporate results from UAV surveys into global information systems to provide reliable information on
FIGURE 6.4 Wave run-up based on high-resolution beach topography from UAV images acquired at Borghetto Santo Spirito (Liguria Region, Italy), (a) Aerial view, with location of the investigated area, (b) DEM obtained by combination of UAV-based point cloud and multibeam data, (c) Comparison of observed and modelled maximum run-up for an event that occurred in December 2013 over the orthophotograph from UAV images. (Modified from Casella et al. 2014.) beach topography and geomorphology compared with traditional techniques, without losing accuracy. Researchers and coastal managers might benefit from the use of new elevation datasets in the assessment of the impact of extreme wave events after a storm. However, timely mission planning is fundamental to carry out the aerial survey at significant times. For instance, the net rate of erosion after a sea storm can be more accurately assessed by timely aerial surveys. Again, the extent of flooded areas can be mapped during the event or in the immediate aftermath, before the waters drain out from the flooded areas (Maguire 2014). In addition, the use of high-resolution beach surfaces allows the prediction to some extent of the areas that are more vulnerable to the run-up of extreme swells. Conversely, the deployment of an aircraft at each occurrence with such a short warning period makes the whole mission virtually impossible.
This chapter has introduced the potential for UASs to effectively fill current observation gaps in coastal environment remote sensing, for applications that include coastal change research, wetland mapping, ecosystem monitoring, vulnerability assessment, natural hazard prediction, and coastal engineering. UAVs can replace many conventional survey methodologies, with considerable gains in spatial resolution, vertical accuracy, and cost of data acquisition. Surveys can be conducted in logistically challenging areas, to investigate unstable or inaccessible terrain, where traditional survey methods are not feasible. Thus, the possibility to survey and monitor dynamic landscapes and morpho-sedimentary processes is now widening.
Some of the weather dependency that affects airborne surveys (e.g., cloud cover) can be circumvented with UAVs. Moreover, the possibility of the reconstruction of the coastal morphology at unprecedented resolution opens the way to micro-topographic investigations and accurate sediment budgeting. However, the benefits of the SfM (SFM) technique might be partially reduced due to the presence of water, reflective surfaces, or very smooth textures on the ground. Careful mission planning is required. Despite the potentiality of UASs for coastal applications, there remain technical and legal hurdles that prevent wider use. Although with some differences across different countries, these difficulties include the strict limits imposed by national regulations that allow UAV flights in visual line of sight at moderate distance from the ground control station and within segregated airspace. Drones would not be allowed to fly at night or over densely populated areas. Fixed-wing flights can cover longer ranges. Thus, for such vehicles, the existing regulations are even more restrictive.
Improvements can be expected from even more accurate direct georeferencing of UAVs; for instance, by integrating low-cost dual-frequency GNSS receivers and lighter inertial sensors into the payload. This should provide better altitude determination with very little effort placed on field surveys for GCP collection. Rotary-wing UAVs are susceptible to failure under windy conditions, and their endurance is still limited to 10-20 minutes of flight. For non-tethered UAVs, failure during overflights of coastal areas might result in the loss of the vehicle. These major factors limit their use in applications that are related to coastal environments. In addition, professionals and researchers involved in the use of UAV data still face several difficulties, from the need for metadata and UAV- specific data formats, to the standards for dissemination, standardised calibration of sensors, and effective data validation procedures.
However, the general consensus is that UASs will have increasingly important roles in the investigation of natural resources and land-monitoring needs at reduced costs. In coastal surveys, UAVs will potentially revolutionise the methodologies of data collection, as these can provide timely investigations of logistically challenging areas and inaccessible terrain using an increasing variety of sensors.
We would like to thank Professor Giovanni Gabbianelli for sharing unpublished data from some repeated UAV surveys over the coastal stretch represented in Figure 6.3.
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