Natural hazard and hydrological risk assessment includes the analysis of the so-called hazard processes based on spatial and temporal occurrence in terms of probability and susceptibility. Then, that approach is combined with the foreseeable consequences, mainly on society, the environment and the territory. According to the United Nations Disaster Relief Organization (UNDRO) [1], that is termed a vulnerability, i.e. the degree of loss to a given element at risk or set of such elements resulting from a natural phenomenon of a given magnitude. UNDRO [1] also highlights two still key concepts: (1) natural hazard as the probability of occurrence within a specific period of time in a given area of a potentially damaging natural phenomenon, and (2) risk as the expected number of lives lost, persons injured, damage to property and disruption of economic activity due to a particular natural phenomenon, and consequently the product of specific risk and elements at risk.

Nowadays, it is common to study sensitive regions with a multi-hazard approach. According to the United Nations Office for Disaster Risk Reduction (UNDRR) [2], the multi-hazard concept is seen as “(1) the selection of multiple major hazards that the country faces, and (2) the specific contexts where hazardous events may occur simultaneously, cascadingly or cumulatively over time, and taking into account the potential interrelated effects”. As a result, the relationships between hazards are diverse and complex because new concepts emerged, termed multi-hazard risk or multi-risk (e.g., [3, 4]).

Hazard and risk mapping are a key aspect of a comprehensive process aiming to identify elements and resources available to reduce individual levels of hazard or risk that potentially cause a disaster. For example, geologic or geotechnical instability processes are the primary threats correlated with extreme precipitation and/or seismic activity events (e.g., [5, 6]). In fact, risk mapping is based on a set of thematic maps that spatialize the various risks (natural, mixed or technological) by applying specific methodologies based on data collection in fieldwork and historical records search, data analysis, numerical modelling, and geovisualization mapping techniques.

The natural hazards and disasters are various, such as seismic, volcanic, hydrological, and geotechnical. In addition, anthropogenic hazards result from anthropic interactions with nature. Finally, technological hazards happen due to exposure to hazardous substances [7]. Furthermore, the right to a safer and better quality environment is a rising expectation of society. Thus, in hazard or risk areas, it is essential to have a comprehensive knowledge of the functioning of dangerous phenomena and/or processes. Additionally, assessing their potential consequences is vital to minimize the damage to social and economic activities. Consequently, implementing mitigation measures and proper territory management concerning future human interventions is crucial.

Therefore, studying natural hazards is fundamental for implementing risk mitigation measures connected with potentially hazardous phenomena. Furthermore, geomatic techniques, fieldwork, monitoring, data analysis, modelling and mapping are the topical significance to delineate the degree of exposure to the different elements’ risks and thus suggest sustainable measures for mitigating and preventing disasters. Lastly, changing Earth systems shall be designed, developed and implemented sustainable and resilient solutions with nature, society and geoethics in a multi-hazard risk and disasters approach [3, 6, 8,9,10].

The topical collection (TC) on “natural hazards and hydrological risks: climate change-water-sustainable society nexus” is mainly grounded in the 2nd International Workshop on Natural Hazards (NATHAZ’19), held in Lajes do Pico municipality, Pico Island, Azores, on the 9th and 10th of May 2019 (see [11] for details). In fact, the Azores could be seen as a natural laboratory of geohazards, namely earthquakes, volcanic eruptions, landslides, floods, coastal erosion, and damages to engineering works (e.g., [12, 13]). Therefore, the workshop underlined all facets of natural hazards, particularly hydrological risks. That was the common ground for the development of this TC.

This TC gathered original papers focused on natural hazards and disasters, particularly hydrological risks, climate change and sustainable society nexus. The set of published papers aims to enhance the knowledge and valuable information on natural hazards focusing on hydrological risks, hydrologic services, water resources impacts, coastal hazards, and climate change issues on hydrosystems and interconnected natural compartments (soil, air, and biota). The TC comprises over 21 contributions in several model regions, mainly in Asia (China, India, Bangladesh), the Middle East (Iran), Africa (Burkina Faso, Ethiopia, Ghana, Tunisia), America (Brazil, Canada, United States of America), and Europe (Italy, Portugal, including Azores islands, Balearic islands—Spain). Therefore, the TC is valuable to scientists and practitioners in applied geosciences, hydrology, groundwater science, hydrological engineering, military engineering, environmental sciences and natural hazards.

The TC presents topical studies on natural hazards and hydrological risks. The published articles highlight a diverse range of studies and applications in diverse model regions, such as: (1) papers focus on studies in natural hazards and meteorological issues: adaptation on climate change framework (e.g., assessment of spatio-temporal variability on rainfall, air and temperature temporal analysis, drought analysis using several approaches, modelling studies on the effect of climate change and maximum temperature or inconsistent variation of return periods of temperature, numerical studies about climate-induced flood inundations and analysis of empirical flash flood vulnerability in urban areas); (2) several contributions on climate change issues on hydrosystems and interconnected natural compartments (e.g., assessment of radiological hazards on soil samples in sensitive regions, hazard evaluation of trace elements natural enrichment in topsoils, dissolved radon in groundwater monitoring as tool for the seismic activity evaluation, the role of hydrological services as potential of peatlands in a climate variability framework, and factors influencing climate change adaptation strategies based on farmer perceptions); (3) other studies focus on natural hazards that impact land planning and management, watershed management, coastal hydrology, and hydrogeotechnical issues (e.g., land use and cover change analysis related to potential hazards, historical hydrological studies related to a regional basin, studies on embayed beach for coastal risk management, numerical modelling hazards studies for unsteady hydraulics of sediments in rivers and apparent roughness coefficient in overbank flows, post-wildfire slope stability effects studies and the role of the water on risk assessment on landslides).

This TC develops invaluable knowledge of natural hazards in environmental and earth sciences, hydrology, groundwater science, and engineering. This approach comprises the climate change issues on hydrosystems and interrelated natural compartments (water, air, soil and biota) within a sustainable, geoethical and eco-responsible engineering design with natural hazards.