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The contribution of Unmanned Aircraft Systems (UAS, UAV or RPAS) is increasingly acknowledged, researched, tested and applied in a vast variety of fields. Earth Sciences are among the greatest beneficiaries of this development, in terms of having more than an eye in the sky during field work, approaching inaccessible locations and applying Structure from Motion (SfM) methodologies for research in geomorphology, geological mapping, hydrological modeling, mass movement mapping etc. Detailed mapping and evaluation of findings is a critical element of syn- and post-disaster period, for assessing the impacts and their spatial distribution, helping recovery by facilitating aid allocation, coordinating relief efforts, monitoring lifelines status, monitoring of clean-up, and assessing secondary effects or induced hazards. In this paper, the research team summarizes the experience obtained during their involvement in natural disaster mapping of recent (2017-2018) significant catastrophic events in Greece. A survey team equipped with cost effective drones was deployed during and/or after disastrous earthquake, mass movement, flash flood and wildfire events. This resulted in a variety of product types in terms of contribution to post disaster research and emergency management, resulting also in a number of pros and cons in terms of prerequisites and risks of the process. Mapping with drones is essentially field work, and is designed and executed as such. Moreover, it is aviation, so application of strict safety guidelines is a prerequisite. The combination of the two at disaster sites requires technical efficiency, expertise according to the disaster type, team coordination, pilot skills, experience and training, along with a flexible mission risk assessment at all times.Herewith, results of disaster mapping are presented for the earthquake (2017) and mass movements (2017-2018) in Lesvos, the deadly flash floods in West Attica (Mandra, 2017, 2018) and the deadly wildfires in East Attica (Rafina, Mati, 2018). Pros, such as capturing of phenomena dynamics and intact disaster scenery in explicit detail and cons, such as risks, equipment limitations and safety issues are discussed, and some elementary guidelines for the overall process are proposed. Following the cycle of disaster management, the process involves steps to be taken during the pre-disaster phase, the warning/alarm phase, the impact and emergency response phase etc. so that a drone equipped team be readily operational and effective at any given time a disaster occurs. [ABSTRACT FROM AUTHOR]

The spatial and temporal scale of flash flood occurrence provides limited opportunities for measurements and observations using of conventional monitoring networks. These observational difficulties, often accompanied by a lack of instrumental data have turned the focus to event-based, post-disaster studies. Post-flood surveys are particularly useful, as they provide the opportunity to observe aspects of hydrological behaviour of catchments under rare runoff conditions and extreme meteorological forcing, by capitalizing on field evidence.Recently, unmanned aerial vehicles (UAVs) and the Structure for Motion (SfM) technique have been used to enhance field surveys and monitoring related to different aspects of disasters. The simple consumer-grade equipment required, its enhanced observation capabilities and certain conveniences they offer in field surveys indicate a strong potential of these technologies in many aspects of flash floods, especially given the opportunistic nature of their study. This work explores further this potential, aiming to demonstrate the application of UAVs in post-flood peak flow estimation. The research team surveyed two selected cross sections after the catastrophic 2017 flood of Mandra, developing a high resolution (2.7cm) digital surface model (DSM) using UAV imagery and the Structure from Motion technique (SfM). The detailed DSM was used to study channel geometry and flow obstructions, extract cross sections and calculate the cross sectional area and wetted perimeter. Water energy slope was defined with the use of high water marks that were placed on the DSM on the appropriate elevation.Peak discharge in the two ungauged sections were estimated at 170m3/s and 140m3/s (with a range to account for the uncertainty inherent in the dynamic nature of Manning coefficient), using the slope-conveyance method, indicating a unit peak discharge of approximately 9-10m3/s/km2. Following recommendations described in relevant literature on the integration of survey observations by means of hydrological modelling, we applied a spatially distributed hydrologic model to simulate discharge at the surveyed cross-sections (the Kinematic Local Excess Model (KLEM)) and other checks, all of which were in agreement, with the peak flow estimates provided by the UAV-aided survey analysis.The UAV allowed the collection of aerial imagery in a rapid way from an extensive area, despite that a large portion of it was inaccessible due to road closures and safety issues. In addition, detailed DSMs created using aerial imagery and the SfM technique was found to be particularly useful for studying channel geometry, obstructions and other characteristics of flow as well as for measuring cross sectional areas on demand, even at a later time, when the channel had changed due to human intervention (e.g. cleanup using heavy machinery) and water flow. These capabilities fit to the opportunistic context of studying flash flood events in the sense that UAVs can rapidly collect information within the short time frame that it is available. Nevertheless, it has to be noted that combination of ground with aerial observations is preferable. [ABSTRACT FROM AUTHOR]