# A Review of Cutting-Edge Sensor Technologies for Improved Flood Monitoring and Damage Assessment
## 1 Introduction  
Flood hazards include flash floods, river overflow floods, storm surge floods, glacial lake outburst floods, and urban waterlogging. In recent years, the frequency and severity of floods have been increasing globally due to the impact of climate change. According to the Global Natural Disaster Assessment Report (2022) [1], flooding has been the most frequent event in the past 30 years, accounting for 38.87% of the total frequency of all disasters where 4321 large or small flood hazards affected a population of 1927.99 million. Geographically, most flood events occur in Asia, Africa, and North America, but more than 50% of these flood losses happened in developing countries around the Himalayas [2]. A recent report published by the Centre for Research on the Epidemiology of Disasters (CRED) shows that flood events always bring huge economic losses, and a large number of people are affected, as was the case when Typhoon Doksuri hit the Chinese coast causing the second most expensive disaster in the world in 2023. South Asian countries are being severely affected by major river systems, such as the Indus, Ganges, Yarlung Tsangpo, and Meghna rivers, which have brought about civilization but caused many severe floods throughout history [3]. Some coastal countries, such as the United States, Japan, the United Kingdom, and Australia, suffer from floods caused by typhoons and heavy rains. Even by late September of this year, the Sahara Desert region was struck by unprecedented torrential rain and flood events, and Lake Iriqui, which had been dry for 50 years, was filled. Climatechange is making the global hydrological cycle increasingly unstable and unpredictable. According to the World Resources Institute, floods have caused more than $1 trillion in damages globally between 1980 and 2020 [4]. Previous studies show that the vast majority of the world's population, those living in low- and middle-income countries, are exposed to flood risk (89%) [5]. It is clear that people in poverty are far more vulnerable to floods than the rich because low-income households often find it difficult to withstand devastating losses from floods [6]. Most flood-prone countries have unique landscapes and climates where different kinds of flood events occur. The rapid, unplanned, and haphazard urban growth increases flash floods or pluvial flooding in many countries [7]. Floods mainly cause casualties and damage to farmland, livestock, and infrastructure, which creates challenges for developing countries. It is very challenging to control the impact of flooding [8], but many efforts have been made globally for flood governance and management, using both structural and non-structural measures [9].  
Different flood sensors are used for flood detection and early warning systems around the globe [10]. In recent years, flood sensor technology has made significant progress in terms of hardware, communication, and multi-technology linkages, especially with the emergence of radar remote sensing and social media (Figure 1). [PERSON] [11] delineated six milestones in flood monitoring from the technical and policy perspectives by sorting out the breakthroughs and advances in satellite remote sensing of floods over the past 50 years. The majority of this research is concentrated on specific regions and particular technologies, with comprehensive studies on flood sensors being notably scarce in the literature. Therefore, this study aims to review the development of global flood sensors and proposes a new classification of flood sensors by selecting representative flood events. This study further analyzes the difference between developing and developed countries in the development and application of flood sensors, focusing on the strengths, limitations, and future directions of different types of sensors in different types of flood scenarios.  
Figure 1: Annual flood sensor research literature publication.