Since the 1970s, 44% of all disaster events have been flood related (IPCC, Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022). Since antiquity, it is well known that floods can threaten lives, cause physical injuries, inundate properties and businesses, destroy belongings, prevent access to essential public services and damage agricultural fields, as well as public and private infrastructures. Between 2013 and 2022, an average of 5,034 deaths per year were attributed to floods (with data from Statistica, Reference Seneviratne, Zhang, Adnan, Badi, Dereczynski, Di Luca, Ghosh, Iskandar, Kossin, Lewis, Otto, Pinto, Satoh, Vicente-Serrano, Wehner, Zhou, Masson-Delmotte, Zhai, Pirani, Connors, Pean, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekci, Yu and Zhou2023). Although mortality has been reduced worldwide, figures remain at least one order of magnitude higher in low-income countries than in higher-income ones. Flooding is associated with increased water-borne diseases, it contributes to reduced food availability and increased food prices and it is amongst the most common climatic drivers for disasters, especially in poor parts of Asia, Africa and South and Central America (IPCC, Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022).
Unfortunately, this list above of negative impacts is not exhaustive: one missing impact is the effect of floods on water quality. This effect has been frequently overlooked in literature, most of all because the majority of negative effects have an immediate and significant impact on population and assets, but also because the effects on water quality are much more complex and less well understood. This book contributes to shed some light on the subject.
Floods are a natural phenomenon, and they become a disaster only when they occur in areas where population, infrastructure or ecosystems are exposed and damaged. It is to discuss if floods are a natural disaster or simply a disaster, as nowadays many experts concur that the term natural disaster should simply be replaced by the term disaster, since, for it to happen, a combination of a natural hazard or a human-made hazard impacting a vulnerable community, infrastructure or ecosystem must occur.
Floods usually start with an extreme precipitation, but this is not the only cause, nor is it the predetermining one, as the occurrence of a flood also requires a combination of factors related to hydrology, climate and human management of a basin (land use and land cover, urbanisation and economic development). There are many types of floods: pluvial floods, flash floods, river floods, groundwater floods, surge floods and coastal floods (Kovács, Reference Kovács2013; Ranasinghe et al., Reference Ranasinghe, Ruane, Vautard, Arnell, Coppola, Cruz, Dessai, Islam, Rahimi, Ruiz Carrascal, Sillmann, Sylla, Tebaldi, Wang, Zaaboul, Masson-Delmotte, Zhai, Pirani, Connors, Pean, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekci, Yu and Zhou2021; Weber, Reference Weber2023). The IPCC (Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022) opted to regroup them in three main categories: river, pluvial and coastal floods.
A fluvial or river flood occurs when the water level in a river, lake or stream rises and overflows onto the neighbouring land. It is caused by an excessive (amount and/or intensity) precipitation. In addition to precipitation, soil moisture, basin morphology, land-use and land-cover characteristics, hydraulic infrastructure and tidal levels (when available) also play a role. When the flood plain (the land adjacent to a river) is flooded, and houses and infrastructure were built on it, these assets and their inhabitants become vulnerable to floods.
Extreme rain events exceeding the capacity of natural and artificial drainage systems can lead to pluvial floods in cities, roadways, farmlands, industries, metro stations, subway tunnels and building basements. When pluvial floods occur in cities, they are called urban floods. Very intense rainfall, along with a high fraction of impervious surfaces, can result in flash floods in urban areas. Urban flooding can be linked to a major disaster, such as hurricanes, extreme rain events and intense hail precipitation. Frequent sewer blockages, sewer backups or sewer overflows are observed. Even small amounts of rain can overwhelm the urban drainage if it is deteriorated, poorly designed or built, or simply blocked with uncontrolled solid wastes (partly from: Ranasinghe et al., Reference Ranasinghe, Ruane, Vautard, Arnell, Coppola, Cruz, Dessai, Islam, Rahimi, Ruiz Carrascal, Sillmann, Sylla, Tebaldi, Wang, Zaaboul, Masson-Delmotte, Zhai, Pirani, Connors, Pean, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekci, Yu and Zhou2021; Seneviratnet et al., 2021).
Coastal floods occur in cities, infrastructure or land located along the shores. They can affect communities, farmlands, buildings, transportation routes, industries and, eventually, ecosystems. Coastal floods are caused by extreme total water levels which are the combination of the relative sea level, tides, storm surge and high wave setup at the shoreline. Coastal floods can affect port and harbour operations. Frequent inundation with saline water can also have significant impacts on water resources, crops, aquaculture and infrastructure (due to corrosion).
Floods and Climate Change
According to the IPCC (Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022) report, there is high confidence that climate change and projected socioeconomic development would increase exposure in inundation areas, resulting in a large increase in direct flood damages, and an estimated increase of 120% and 400% of population affected by river flooding at 2°C and 4°C warming, respectively. The highest numbers of people affected by river flooding are projected for countries in Southern, Eastern and Southeastern Asia. Under climate change scenarios, many regions are expected to experience more frequent and intense urban and flash floods, up to levels that can exceed current flood protection design standards. For coastal areas, the sea level rise will also bring, more frequent and intense floods. In Europe alone, coastal flood damage is projected to increase at least 10 folds by the end of the 21st century (IPCC, Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022).
Floods and Water Quality
Concerning water quality, heavy rain events mobilise chemical and biological pollutants from the paved ground and urban soil to water sources, endangering its quality. Floods intensify the mixing of floodwater with wastewater and the redistribution of pollutants. For coastal areas, the risks also include a higher salinisation of water sources (IPCC, Reference Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022). Furthermore, floods damage infrastructure, including water and sanitation services, leaving population exposed to water quality problems. Unfortunately, little research on the impacts of floods on water quality has been conducted and, when available, it focuses almost exclusively on Europe, the United States and Canada.
Adaptation and Solutions
This book analyses the state of the art on floods and their impact on water quality. It presents adaptation methods and solutions to the problems caused, varying from the control of floods to the control of its effects on water quality.
Chapter 1, “Urbanisation process and urban water challenges,” describes urban water challenges, setting the scene for the rest of the book. Merging inputs from authors coming from three different regions (Asia, Latin America and Europe) give not only an interesting view on how cities impact water as a resource but also how water resources affect cities with a focus on floods and water quality. Following global economic and political decisions, the urbanisation process is dominating the way humans settle in, such a manner that there was a need to define the “urban water cycle” to better understand the phenomena and processes linking both cities and water. Consequently, the frequently overlooked link between urban floods and water quality is considered.
Chapter 2, “General Approaches on Urban Water challenges,” describes current and future water challenges and presents general approaches on how to face the effects of urban flooding.
It initially provides a brief description of the transition in the management of urban water challenges, from the initial sanitary engineering to the sustainable period. It also provides an evolution in terminology and types of urban systems and a presentation of strategies and examples of flood risk management in selected countries. Chapter 2 also refers to the different approaches used to conceptualise urban drainage, including low impact development (LID), sustainable urban design systems (SUDS), water-sensitive urban design (WSUD), ending with the Sponge Cities concept. These different practices have different priorities, application scopes, and management styles; decision-makers can choose amongst them according to the actual condition of their specific regions.
Chapter 3, “Integrating water quality in urban flood risk management: structural approaches,” demonstrates how the use of structural measures, such as SuDS, provides multiple benefits and flexibility to reduce the effects of storm peaks while supporting cities’ efforts in adapting to climate change effects. SuDS can become an integral part of WSUD and provide the opportunity for reducing water demand in cities where water is considered in planning and is part of the constructed environment.
Chapter 4, “Impacts of urban drainage management on water quality following extreme rainfall,” examines, in particular, the main factors to be considered by a city to reduce the impact of a flooding event and presents the ways to manage the drainage system sustainably. Their presentation includes all management phases, from the project phase to the maintenance and cleaning activities and even the operation of the infrastructure during the flood.
“Online forecast model for urban flooding and induced water contamination” is the title of Chapter 5. The need of early warning systems to predict pluvial flood events in urban environments is discussed in this chapter. However, the nature of the threat during pluvial floods in urban areas is dual, as further to the large amounts of water the city has to receive and evacuate in a short period of time, it needs to deal with the release and transport of potentially hazardous substances. The chapter examines existing numerical models addressing the challenge and presents a fully coupled urban flow model along with a transport model to assess the migration of contaminants during and after a pluvial flood event.
In Chapter 6, the methods to identify the contaminant levels in river floods that occurred in the past are discussed. Riverine floods can result in the dispersal of contaminants throughout inundated areas, including urbanised landscapes and regions downstream of industrialisation. This chapter reflects on a missed opportunity that could support water quality managers and engineers in their efforts to build robust flood mitigation strategies. In particular, it presents how water quality and hydrologic data collected from sedimentary records by palaeoenvironmental scientists can be incorporated into models and into management frameworks.
Chapter 7 is very relevant to protect water supply users’ health. It describes how to cope with water quality problems that occur on the supply of water during floods. This chapter provides insight to the basic principles to be followed during an emergency flood situation. Analysing the planning exercises to take place, the identification, development and maintenance of the water supply to be used and the quality control using available facilities it presents suggestions to manage the response and recovery part.
A case study on the impact of urban flooding on water quality in India, with special reference to Mumbai, is presented in Chapter 8. This situation described in here is common to many of the heavily populated cities of India. In many of these cities, the urban water table has dropped significantly, and raw sewage is discharged into the open stormwater drains. During the monsoon, raw sewage in stormwater drains overflows polluting water in the surroundings and resulting in outbreaks of the diseases such as gastroenteritis, malaria and dengue.
In June 2016, the Seine and the Marne, in the Paris region of France, had substantial flooding that was unusual at that time of the year as flooding usually occurs in the winter. Chapter 9 presents the impact of this flood on the operation of the sanitation system of the Paris urban area, including the subsequent impact on the quality of the receiving environment: the Marne and the Seine. One aspect that was retained from this experience is the relevance of having a coordinated and effective management between all sanitation operators together with a good communication system between them and all the stakeholders. With regard to the sanitation system, it was assessed that the treatment systems were able to process substantial volumes of the pollution received during floods limiting the further impact on the environment.
Chapter 10 discusses about “The Sponge City” initiative in China, where frequent urban flooding is a major concern. Pervious natural lands are gradually subsumed by the dramatically increasing impermeable concrete surface that prevents any water infiltration and results in urban floods. To deal with them, the concept of the Sponge City was developed. The Sponge City was initially defined as “natural storage, natural infiltration, and natural purification.” Its purpose is to mitigate urban flooding and purify stormwater runoff. The Sponge City is also beneficial for ecological restoration and microclimate improvement. It is noted that the Sponge City concept is not purely “green” but an integration of green and grey infrastructures. This chapter describes in detail the concept and its implementation, presenting practical cases in which it has been applied.
The relevance and significance of this book has been evident during the 2024 Olympic Games in France, where the local government decided to use the Seine River, as a location to host some water related competitions. This visionary way to promote the city soon gave place to be an unexpected way to show to society the impact that extreme, or even intense, rain events can have on the quality of water bodies which are used for recreational, sportive and even drinking water purposes. In several occasions the competitions had to be rescheduled until the quality of water attaint an acceptable level for the organisers to allow its use. Similarly, in October 2024 a Depresión Aislada en Niveles Altos (DANA), or isolated depression at high levels, caused torrential rain and flooding in an Eastern and Southern Spain resulting to the loss of more than 200 lives and an unprecedented crisis in access to drinking water, adding stress to the already complicated situation the people found themselves in.
