# A Survey on Unmanned Surface Vehicles for Disaster Robotics: Main Challenges and Directions
## 3 USVs for Natural Disasters
### Floods  
Floods are natural events caused by an overflowing of a large amount of water that submerges or inundates a piece of land that is usually dry. Floods are common after-effects of different extreme events, such as hurricanes, dam ruptures, massive storms, and tsunamis. They affect several million people each year, being one the most significant natural hazards our society is currently subject to ([PERSON] et al., 2018). In fact, the total population located in zones prone to flooding have increased dramatically over recent decades and are expected to increase further ([PERSON] et al., 2018). As indicated by [PERSON] et al. (2018), the most affected people inhabit underdeveloped countries, which lack early warning systems, flood control, and emergency response infrastructure. [PERSON] et al. (2018) affirm that USVs are ideal to effectively address this problem since they are simple, robust, and reliable, being ideal for flood mitigation and response. Current applications of USVs for disasters caused by floods are described below. [PERSON] et al. (2018) present a technical description of the problem from the perspective of multi-agent systems, where some cooperative boats might be deployed to provide situational awareness over a small area. In this sense, a set of autonomous boats should provide situational awareness, damage assessment, and deliver supplies before more traditional emergency response assets can access affected areas. They suggest the use of airboats ([PERSON] et al., 2018) since they are flat-bottomed boats with an above-water fan to propel themselves forward safely and effectively through shallow or debris-filled water. Airboats may also collect water, checking for diseases carried by the flood. [PERSON] et al. (2018) use a combination of USV ([PERSON] et al., 2018) and UAV to map a river/estuary since it is fundamental to aid the monitoring during critical events as heavy rains that could produce flooding. While the UAV generates ultra-high-resolution imagery from a bird's-eye view, the USV maps the riverbanks with more details, collecting images from different points of view enhanced by an RGB + depth (RGBD) sensor (i.e., a camera that provides both color and dense depth images). Using a USV is important mainly in areas that require maintenance due to a significant presence of canopy that occludes the identification of the riverbanks. The authors choose a small USV since small river/basin require vehicles with the capability to navigate in the presence of shallow water or presence of canopy or algae. Tests are performed in the Province of Ancona, Italy, to detect short-term changes, i.e., identify the changes occurred in the river basin during a strong rainy event. [PERSON] et al. [73] use a combination of a USV and a UAV in an aerial-surface system for rescue operations in flooded areas. In this system, the USV carries a UAV into the complex stricken area to acquire information and rescue survivors. While the USV navigates in the area, the UAV takes off from the USV and sends global information about the environment, such as photos and videos. When the mission is finished, the UAV should be capable of landing back on the USV automatically. In this configuration, the USV is responsible for collecting local environmental information with laser and camera to generate a map for navigation planning, and for releasing rescue equipment to survivors (e.g., a rescue rope throwing equipment to throw the rope to the survivors). The combination of USV with UAV allows the USV to obtain global information for trajectory planning while creating a local path with its local information. [PERSON] et al. [74] provides a complete description of this flooding disaster-oriented USV. [PERSON] et al. [75] deal with the problem of measuring the vertical profiles of horizontal flow velocity for estimations of flushing time, fluxes of water, salt, suspended sediments, and other waterborne materials.