The Viability Of Independent Underground Floodwater Vaults As A Sustainable Mitigation Technique For Urban Flooding
1)???? INTRODUCTION
Urban flooding has been a severe threat to life and property in modern times. Flood in urban areas is more severe than in rural areas because of the intensively built-up surface, dense population, and the damage to more expensive properties. Although a permanent solution to this problem is challenging, multiple innovative projects are implemented to reduce fatalities and damages. For example, the Japanese Metropolitan Outer Area Underground Discharge Channel Channel[1] in Tokyo is an engineering marvel and a wonder to water departments worldwide, which helps reduce flooding in the city. However, this costly and enormous interconnected tunnel system for the whole city is not viable for developing countries. This is where a new idea of constructing individual underground floodwater storage vaults systemfor each commercial and apartment building and other structures in collaboration with the developers becomes significant and feasible. The program can create a shared responsibility among the stakeholders and people to curb urban flooding.
Although urban flooding is a worldwide issue, this project focuses mainly on cities in developing countries because the concept is more apt for developing countries. However, this paper discusses global urban flooding events, causes, impacts, and solutions from a global perspective. It also explains the construction procedures, benefits, and challenges of the underground floodwater storage vault. Moreover, the study gathered popular opinions on the concept from primary sources using an online survey and other information from a global literature review. The author expects this paper to be a good base for further discussions of the concept.
2)???? METHODOLOGY
Referring to research methodology, the author used quantitative data collection methods to gather people’s opinions and support the literature review. Although the research mainly used secondary sources, it conducted an online survey using Google Forms. The online survey helped people understand the concept and express their opinions. The survey respondents were mostly educated people chosen from multiple cities, and an introduction to the program was given at the beginning of the survey form. Five questions were asked, and the responses received were supportive, reasonable, and logical. The survey was open for a week, collecting twenty-eight responses from people living in different flood-affected cities. Therefore, the study gathered popular opinion on the concept from primary sources and other information from a literature review.
The survey question about the main reason for urban flooding brought logical responses reflecting the respondents’ awareness of the reasons for urban flooding. Twelve people (42.8%) responded that built-up land was the main reason for urban flooding. However, the blocked sewage system was also supported by the same number of respondents. The question regarding the expected efficiency of the underground storage system brought fifteen (53.6%) positive responses.
Moreover, most respondents (89.3%) pointed out that urban flooding is a serious threat. Also, half of them (50%) were willing to spend their resources on a sustainable solution. The tables show the percentage of the responses received for each survey question. The responses were generally positive and reflected the need and support for sustainable urban flood mitigation measures.[2]
3)???? URBAN FLOODING
Urban flooding?(pluvial flooding) is a major problem in many parts of the world and is one of the most destructive natural disasters that occur yearly. Urban flooding cannot be avoided, but sustainable mitigation methods can curb its impacts. The most common causes of urban flooding include high-intensity rainfall, impermeable land surface, inadequate drainage system, and clogged sewage channels. In urban areas, the likelihood of flooding is high due to rapid development. Also, the conversion of farmland, vegetation cover, and bare soil into built-up areas results in increased runoff.[3] Runoff is the water that flows above the surface without percolating down into soil during rainfalls. Anna Weber refers to urban flooding as excessive runoff in developed areas where the water does not have anywhere else to go.[4] Flooding in urban areas caused by prolonged rainfall overwhelming the drainage system's capacity is one of the serious hazards in modern world cities. For example, during the 2015 Chennai floods, the city received about 25% of its average annual rainfall on a single day.[5] It often leads to devastating economic, social, and environmental impacts.? Urban flooding is quick and often happens with little warning in areas that are not even prone to flooding, making it hard to predict.[6] Some of the worldwide urban flooding examples are discussed below.
Despite their development and geographical location, urban flooding occurs in all countries and regions. However, the impacts are severe in less developed areas as they lack advanced technology and skills. Also, climate change in recent years has led to a larger percentage of precipitation in intense, short spells. For example, parts of southern China received their heaviest downpours of sixty years in June 2022, which caused floods and landslides, damaging homes, crops, and roads, and evacuating more than 300,000 people with more than $400 million in damages.[7] Also, in a 2021 flood event, Zhengzhou went underwater when it received its entire year's rainfall quota in four days.[8] These disasters are not restricted to China but all over the world.
According to a 2005 report, the world cities at flood risk are Guangzhou, Miami, New York, New Orleans, Mumbai, Nagoya, and Tampa-St. Petersburg, Boston, Shenzen, Osaka-Kobe, and Vancouver.[9] Regarding urban flooding and its impacts in the US, a 2019 Consensus Study Report by the National Academies of Sciences, Engineering, and Medicine wrote:
Flooding is the natural hazard with the greatest economic and social impact in the United States, and these impacts are becoming more severe over time. Catastrophic flooding from recent hurricanes, including Superstorm Sandy in New York (2012) and Hurricane Harvey in Houston (2017), caused billions of dollars in property damage, affected millions of people, and damaged the economic well-being of major metropolitan areas.[10]
The report reflects the severity of urban flooding in US cities. Also, Germany was seriously affected by urban flooding in Europe, and Chancellor Angela Merkel called for a determined battle against climate change.[11] The Netherlands, Luxembourg, and Switzerland are also flood-affected countries. The Indonesian capital city of Jakarta is another frequently flood-affected area. For example, the city experienced eight significant floods in the last twenty-five years.[12] According to Deden Rukmana's report, the 2007 floods inundated about seventy percent of the city, killing at least fifty-seven people and causing about 340,000 to flee their homes.[13] Another devastating flood hit the Indonesian megacity in 2014, causing twenty-three deaths and displacing more than 60,000 people.[14] These disasters forced the government to dredge rivers and release floodwater into the sea via canals as quickly as possible. In India, the most damaging urban floods were in Hyderabad in 2000, Ahmedabad in 2001, Delhi in 2002, Mumbai in 2005, and Chennai in 2004 and 2015.[15] Also, several areas in the south Indian city of Kochi got submerged, and the city witnessed traffic disruptions due to the flooding.[16]
The Indian city of Mumbai is often referred to as a prime example of urban flooding in India. For instance, a United Nations habitat?paper?on monsoon floods in Mumbai mentioned that even after ten days of rainfall, the low-lying areas remain waterlogged.[17] Furthermore, Mumbai ranks fifth among the world’s cities prone to flooding, recording annual losses amounting to USD 284 million.[18] According to the National Disaster Management Authority (NDMA) of India, the average rainfall for July in Mumbai is 868 mm, while its annual rainfall is 2401 mm,[19] which shows the intensity of heavy rainfall in a short time.
4)???? CAUSES AND IMPACTS
Urban flooding has little to do with water bodies such as rivers and lakes, but anthropogenic reasons dominate it. However, intensive rainfall is the immediate cause of every urban flooding, which refers to heavy downpours in a short duration. Urban flooding is generally an under-recognized problem. It causes widespread and massive destruction but remains a lesser-recognized threat because of its chronic nature. There are multiple reasons for urban flooding, but the most significant is the impermeable built-up surface in urbanized areas. For example, in its report, the Federal Emergency Management Agency (FEMA) defined urban flooding as the inundation of property caused by increased rainfall amounts on impervious surfaces and overwhelming the capacity of drainage systems.[20] Therefore, the report suggests that urban flooding is mainly caused by the combined effects of intensive rainfall, impermeable super-built-up areas, and reduced drainage capacity. Although many factors contribute to floods, a warming atmosphere caused by climate change makes extreme rainfall more likely,[21] which leads to urban flooding.
a)???? Causes: Built-up Areas
Built-up area refers to the land surface covered by concrete and tarred roads. These areas have little water percolation capacity. In uninhabited areas, rainfall integrates into the natural water cycle in different ways, such as percolating in the ground and becoming groundwater, joining water bodies as runoff, being absorbed by plants, and being evaporated into the atmosphere. However, as cities develop, impervious pavement is laid over permeable land, encroaching drainage systems and blocking underground streams. Therefore, in urban areas with paved roads, highways, and parking lots, water has only one way to recede: evaporation, which increases water accumulation during intensive rainfalls leading to urban flooding. Twelve (42.8%) survey participants supported this reason for urban flooding (Table 1).
Although there are rules and regulations to protect environmentally sensitive land areas, they become inactive in most construction activities. For example, Delhi’s Akshardham Temple Complex and Commonwealth Games Village were built on the river Yamuna's floodplain.[22] Another example is the Brihanmumbai Storm Water Disposal System project, proposed in 1993, intended to solve the city's vulnerability to flooding. However, no action was taken until the major flooding of 2005.[23] Moreover, as a result of inefficient regulations and poor planning, and mismanagement of environmental resources, the number of water bodies and green covers has vanished over the years worldwide.
b)??? Climate change and storms
According to the Intergovernmental Panel on Climate Change (IPCC), “Climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity.”[24] However, the changes in weather events over time and their reasons are unknown, nonetheless referred to as climate change. While rainfall is the most important reason for flooding, scientists suggest that rainfall varies due to changing weather patterns resulting from global warming, which is climate change. However, a flood occurs not always because the total annual rainfall amount is high but because the precipitation intensity is high. Arek Sinanian’s work supports this point:
“It is important to note that increases in heavy precipitation may not always lead to an increase in total precipitation over a season or over the year. Some climate models project a decrease in moderate rainfall and an increase in the length of dry periods, which offsets the increased precipitation falling during heavy events.”[25]
Therefore, floods can occur even when the total precipitation is lower than the usual annual amount, depending on the distribution of rainfall during the season. Also, only four (14.4%) survey participants supported this reason for urban flooding (Table 1). However, the effect of changes in precipitation results in disastrous flood events destroying life, property, and landscape.
Globally, the number of studies investigating climate change impacts on urban flooding is increasing as the importance of this topic is recognized. For example, one study by Akeh and Mshelia suggested that global warming, the type, frequency, and intensity of extreme weather events such as tropical cyclones, floods, droughts, and heavy precipitation, are expected to rise even with a relatively small increase in temperature.[26] Besides, Mark Lynas explains in the book, Six Degrees: Our Future on a Hotter Planet how temperature-increase per Celcius affects weather changes and impacts the future.[27] However, research focusing on climate change impacts on precipitation patterns and flooding is limited.
c)???? Blocked Drainages and Sewages
Although intense rainfall remains the root cause of flooding, clogged sewage systems in cities have significant roles to play. Twelve (42.8%) survey participants supported this reason for urban flooding (Table 1). Most cities have been built with little regard for natural topography. With the rapid urban expansion, builders have been constructing increasingly on reclaimed wetlands, floodplains, and lowlands of the city as these areas have a cheaper land rate. Although the?provisions of rainwater harvesting, sustainable urban drainage?systems, and building bylaws exist,?their implementation at the user and enforcement agencies levels remains weak. For example, the primary factor of recurrent urban flooding in Mumbai is the city's old drainage system, which is heavily?silted and damaged.[28] Public bodies focus largely on flood adaptation methods after disastrous events, and mitigation efforts are ignored.? Therefore, appropriate sewage maintenance and cleaning of water bodies can always reduce urban flood impacts.
d)??? Urban Flood Impacts
The impacts of urban floods are not limited to loss of property and life. They also create damage, such as loss of hourly wages for those unable to reach their job, hours lost in traffic, rerouting of traffic, disruptions in supply chains, mental agony, and disruptions in education and healthcare. For example, one study?by the Center for Neighborhood Technology found that 84% of people affected by urban flooding in the Chicago area suffered stress; 13% reported effects on their health or the health of someone in their household.[29] Moreover, these effects are more impactful to lower-income people who are likely to live in flood-prone areas?and?are not covered by flood insurance. Therefore, it is clear that urban flooding threatens people more who are already socially and economically vulnerable.
Moreover, urban areas are centers of economic activities with vital infrastructure and business communications. For instance, recent flooding in the Indian city of Bangalore made the IT companies suffer a loss of ?225 crores on a single day as the employees were stuck in traffic for hours and the companies aired their grievances before the government.[30] Therefore, flood damage to vital infrastructure can have local, national, and global implications. Although severe floods fall on the rich and poor alike, the capacity to respond to and recover from flooding is much lower in socially vulnerable populations. The flood losses can be broadly categorized as tangible, which includes physical and economic losses, and intangible, which includes loss of life, health, and environmental imbalance.[31] However, the social dimensions of urban flooding are far less studied and understood than the physical ones.[32] Therefore, upcoming research should focus more on the social dimensions of urban flood impacts.
Since floods can potentially increase the transmission of water- and vector-borne diseases, such as typhoid, cholera, malaria, and yellow fever, it is essential to protect the water sources.[33] The most common risks associated with flooding are drinking-water contamination and breeding mosquitoes. Studies identified unhygienic living conditions in slums and water accumulation during and after monsoons leading to diseases. For instance, the World Bank reported a 217% increase in malaria incidence during the last decade.[34] Cities being inundated for hours to several days can create widespread impacts, including temporary relocation of people, damage to civic amenities, deterioration of water quality, and spread of diseases. However, problems associated with urban floods mostly range from local to national.
5)???? MITIGATION MEASURES
Flooding risk in cities has been exacerbated by increased urbanization and climate change, with catastrophic casualties and economic losses. While finding solutions to urban flooding, consideration of flood factors and their inter-relationships is needed to ensure for successful design and implementation of mitigation methods.[35] Academic studies focused on communities affected by urban floods and regulatory mechanisms like the Environmental Impact Assessment can help effective capacity building against flood hazards. Flood mitigation measures should begin by assessing the city’s flood risk, which enables to identify and understand current and future flood impacts.[36] Some of the widely used solutions to urban flooding are discussed in this section, which is mainly categorized into two grey infrastructure and green infrastructure.
a)???? Grey infrastructure
The standard flood mitigation measures of a single purpose can be classified into this group. For example, the grey infrastructure includes traditional flood control measures such as dams, seawalls, water channels, sewages, and pipes, which are largescale, expensive, and publicly funded projects.[37] Although some have multi-purposes, they are built for the need of the hour without foresight and are not sustainable. Therefore, most of these measures are incapable of carrying out their purposes and also not feasible to repair as the cities have grown a lot. Such as, a sewage channel built thirty years ago is inefficient in discharging the heavy amount of waste produced today in the area, and repairing it will cost equal to or more than building a new one. This is where green infrastructure becomes significant.
b)????? Green infrastructure
Green infrastructure refers to methods such as constructing gardens that can absorb more water and has other purposes simultaneously. It also includes planting trees, constructing parks, creating floodplains, planter boxes, green roofs, green medians, green pavements, cisterns, rainwater harvesting, and underground stormwater vaults that help slow down, soak up, filter, and store rainwater where it falls.[38] The role of green infrastructure solutions in urban flood risk management is significant. It represents an approach that uses natural processes to manage flood impacts while delivering environmental, social, and economic benefits.[39] Although green infrastructure measures are recognized as the most effective mitigation measures, the performances of these systems have been investigated only at small scales.[40] Therefore, these measures should be constructed on a large scale, and their efficacy should be studied.
c)???? Underground Discharge Channel
The Japanese Metropolitan Outer Area Underground Discharge Channel in Tokyo is an engineering marvel and a wonder to water departments in the world.[41] It is an underground tunnel and storage system to collect and pump out floodwater from the city. Fifty meters under Tokyo lies a subterranean world, with more than six kilometers of tunnels, water tanks, and massive pillars constructed to withstand 200 tonnes of water per second.[42] Thus, the system collects the runoff from the city’s surface and transfers it through the underground tunnels into a river flowing away.
The Japanese underground discharge channel works successfully in Tokyo; however, its enormous cost and large-scale construction procedure are not feasible for developing countries. Moreover, the project is centralized for the whole city and financed and built by the city administration. This is where multiple independent projects with private participation gain significance.
d)??? Sponge Cities
The sponge city concept of China is another suitable mitigation technique for urban flooding. According to a study report by Hui Li et al., “A sponge city refers to sustainable urban development including flood control, water conservation, water quality improvement, and natural eco-system protection.”[44] Referring to the Chinese idea of creating spongy cities, Poornima Apte wrote,
One way to make cities spongier is to use permeable pavements, such as porous asphalt made with a lot of large stones rather than fine aggregates such as sand and with added cellulose fibers to hold the porous asphalt together. This creates more pores and increases the air void up to 15 or 20 percent, allowing more rainwater to seep through. Permeable pavements are typically laid on top of stabilizing material and a gravel layer, which functions as a reservoir to hold and eventually disperse the water into the soil underneath.[45]
Therefore, the Chinese took up the idea and implemented it by planning to build Sponge cities. China has already built sponge cities as model cities, which are under observation. ?A video by a news agency, The Hindu, reported, “Hebi City is one of the country’s model sponge cities. In 2015, the city designed nearly thirty square kilometers as a pilot area for sponge city construction.”[46] The idea envisions cities with permeable surfaces to absorb, store, infiltrate, and purify rainwater and release it for reuse when needed.
Furthermore, a construction material called travertine, available in volcano areas, can also be used to make permeable surfaces. According to one study, only forty percent of travertine is considered usable for its current purpose in modern homes, and the other sixty percent is wasted, which can be used to make permeable city surfaces.[47] The concept indicates that a city could function as a sponge with excellent resilience to environmental changes and natural disasters. However, the idea needs high technological skills, knowledge, and continuous maintenance.
6)???? INDEPENDENT UNDERGROUND FLOODWATER STORAGE
The independent underground floodwater storage and discharge system is a new concept inspired by the Japanese model. However, like the Japanese project, this is not an extensive single storage system for the whole city built by the authority, but it should work independently with the contribution and cooperation of private parties. It is a sustainable urban flooding mitigation technique. This section discusses the procedure of the program and its benefits and drawbacks.
a)????? The Concept
According to the concept, every structure in a city must have an underground floodwater storage vault proportional to its area so that the rainwater can be stored underground and reduce the surface runoff into the open area in the city during rainfalls. When each building and other structures collect and store the water falling into its area, the amount of runoff will significantly decrease. However, this is not only personal responsibility but should operate under public-private partnership (PPP), a program that has systematically rolled out to deliver high-priority public utilities and infrastructure.[48] Following the idea of public-private partnership, the city authority should build canals or drainage channels for the water stored in individual vaults to be pumped into after rainfalls subside. Also, nearby rivers can be used to discharge the collected water if available.
However, this flood mitigation capacity building is applicable for new constructions. Therefore, a stormwater fee could be levied by fixing a specific area limit from the existing buildings. The collected amount should be used for building new drainage channels to support the private sector. These storage vaults could be built under bus stations, impermeable sports grounds, schools, hospitals, factories, indoor stadia, auditoriums, houses, and all other commercial and residential buildings considering environmental factors. For example, the picture below shows a similar mitigation measure in Jakarta, the capital city of Indonesia. Although not purposely built for storing floodwater, the under pass and its wider area cater as a flood water storage vault during heavy downpours. It also facilitates the street vendors and entertainers during dry seasons. The underpass acts like a bowl sucking in floodwater around saving the groundlevel roads and residences from inundation.
This is a sustainable flood mitigation measure, as the storage system works as long as the structure exists. Furthermore, the stored water and the dry area could be used for multi-purposes after the rainy season. The concept is most suitable for newly developing cities. For example, the Indonesian government’s shifting of its capital from Jakarta to Kalimantan[49] shows an excellent opportunity to implement this concept as everything is newly built for the future capital city. Although public-private collaboration is mandatory in developing countries, developed countries’ governments can offer applicable rebates based on the available underground vault area or permeable surfaces to encourage people to follow the mitigation measures.
b)????? Benefits
Every project has benefits and drawbacks. From a positive perspective, the independent underground floodwater storage and discharge project has a green infrastructure approach because the system provides multiple benefits, such as water storage, flood control, and safety and security for metropolitan cities. It allows small-scale and decentralized interventions encouraging the participation of homeowners and businesses. Participation of the local business and people is significant for the success of any public project. For example, FEMA reports that locally supported steps should be taken to incentivize individual homeowners in flood mitigation efforts.[50] The underground project solves the challenging land acquisition procedure and leaves no chance for conflicts. It also helps share flood impacts mitigation responsibility between private businesses and public authorities. Moreover, roads in rainy areas are vulnerable to frequent potholes, and they need yearly maintenance. Concrete vaults built under roads can enhance the stability of the roads and save maintenance costs. The project can also decrease other indirect flood impacts and prevent an economic disaster.
c)??? Challenges
Every project has challenges; however, creating proper awareness and active engagement of the society in a project is the biggest challenge the stakeholders face. Although renovating existing channels may be peaceful, land acquisition for new drainage channels may create significant conflicts. Also, the cost of building new drainage channels cannot be avoided. Educating the people explaining the negative consequences of not having a sustainable mitigation measure is another primary task. Moreover, it may need amendments in the building rules and regulations, which should be approved and passed in the legislature. Above all, some extra costs may be needed while construction, which can burden the builders. However, comparing the yearly flood damages and economic losses incurred, the challenges posed against underground floodwater storage are less severe. The independent underground floodwater storage and discharge system, along with permeable pavements, is an efficient, sustainable floodwater mitigation measure for cities.
Indicating the efficacy of the green infrastructure, a news reported that permeable pavements have demonstrated up to 90% reduction in runoff volume, while underground stormwater vaults constructed under parks, playgrounds, or parking lots store runoff and reduce peak flows.[51] The Indian city of Mumbai has already started building underground vaults;[52] however, the high cost incurred will overburden the municipal corporation. In contrast, creating and imposing rules to build individual underground storage vaults can significantly reduce this excessive financial burden on the authority and help share the responsibility among multiple stakeholders as well.
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7)???? CONCLUSION
In conclusion, the individual underground floodwater vault system plays an innovative role that is feasible and viable due to private participation in it. As it has everyone’s contribution, the principle of little drops of water make the mighty ocean is applied in this sustainable floodwater mitigation technique. Individual constructions can store rainwater underground, and the government can facilitate it by constructing canals to flow away the stored water. Therefore, the expenses for the construction process are split into multiple hands without burdening the government or people alone. However, further scientific studies are needed to decide and formulate the technical specification of the vaults suitable for each location and structure.
Moreover, green infrastructure such as permeable pavements and green roofs can reduce runoff and decrease urban flood impacts. However, porous pavements have limited capacity as they may soak and fail during prolonged rainfalls. In contrast, underground floodwater storage can back up the capacity of permeable pavements. Therefore, a combined effort of permeable pavements and underground floodwater storage can save a city from inundation. Furthermore, a sustainable mitigation effort for urban flooding is mandatory as the severity of the loss increases yearly. Therefore, this method will be viable where the reason for flooding is intensive rainfall, and it is less challenging for developing countries' governments to deal with severe urban flooding impacts.
Declaration of Conflict of Interests: The author has no conflicts of interest to declare. There are no co-authors, and there is no financial interest to report. I certify that the submission is original work and is not under review at any other publication.
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