# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 1 Introduction  
A river flood occurs when the river flow can no longer be contained within its bed, and over spills its banks. Flooding is a natural and regular reality for many rivers, caused by any pulse of overflowing water (heavy rainfall, peak seasonal rains, or seasonal snow melt) that overwhelms a river channel, which supports the most naturally dynamic ecosystems, modeling soil and re-distributing nutrients inassociated alluvial lands [1]. However, humans often perceive floods negatively due to damage and loss of life. According to the United Nations [2], flooding is the most widespread hydrological hazard worldwide. According to this report, between 1995 and 2015, there were 3,062 flood disasters, representing 56% of all weather-related disasters, which affected 2 to 3 billion people, with a total of 157,000 fatal casualties. A report from the European Environment Agency (EEA) [3] found that from 1980 to 2010, European countries registered 3563 floods in total. The highest number of floods was reported for 2010 (321 floods), with 27 countries affected. The data analyses on these floods indicate significant increases in floods and envisage that flood losses will be multiplied by five as a consequence of climate change, the increase in urban development, and the price of land around floodplains. In Spain, there is an average of 10 serious flood episodes per year. Floods have caused the death of 312 people in the last 20 years and material damages worth about 800 million euros per year [4]. Extreme flooding is a hydrological event that affects water management, conservation efforts, economic activities, hydomorphological alterations on the ecosystem services, and human health. It presents a large number of technical challenges in terms of efficient management and public policy worldwide [5]. The mitigation of the risks associated with flooding requires certain management of flood zones sustained by data and information about the events. The estimation of damages caused by floods and their associated impacts must be supported by data on water levels, discharges, and flooded areas [6], and the integration of this data into a mathematical model for hydrological analysis of the related factors. When major progressive flooding occurs, it is essential to delimit the affected areas, and to be able to assess the damage caused, in order to provide a \"ground truth\" of great interest for better empirical knowledge of the water environment. These accurate mappings of the flooded areas are needed when monitoring and evaluating the status, impact, and effectiveness of efforts [7]. The delimitation of flooded areas through the collection of data in the field used to be complicated because access and mobility through the affected areas is difficult. Aerial flights to take vertical photographs used to generate mosaics of georeferenced images and High Definition (HD) videos are a common alternative. The flights are scheduled to fly over each of the sections at the peak of flooding; however, they have a high economic cost and are limited by weather conditions. Remote sensing techniques are an effective source of information to discriminate bodies of water over large areas, and therefore, they can be used to map flooded surfaces with sufficient temporal and spatial resolution [8]. In the case of multispectral images, water bodies can be easily discriminated because of their spectral behavior in the visible and infrared spectrum because it is different from other land covers [9; 10], but they only operate in natural daylight conditions and cloudless skies. Since severe flooding usually occurs as a result of heavy rainfall, they are not always useful for this purpose. In Spain, due to the physiographic characteristics of the basins and the Mediterranean climatic conditions, most floods are associated with periods of heavy rainfall. The validity of multispectral observations for this purpose is limited by clouds because unclear weather conditions are common during floods events [11].