A new paradigm of rainwater management

1.1 RAINWATER: DRAIN OR COLLECT?

Recently, there have been several global water problems, such as flooding, drought, water shortage, water pollution, and groundwater depletion. All of these issues are related to rainwater: flooding occurs because of excessive rainwater; drought and water shortage are results of a reduced rainfall; non-point source pollution occurs during rainfall; and groundwater depletion results from reduced rainwater infiltration in areas where the number of impermeable surfaces has increased. Therefore, proper rainwater management (RWM) is required to mitigate most of these global water issues, thus ensuring sustainability and enhancing the resilience of the existing water infrastructure. The fundamental questions regarding RWM are: 'Rainwater — Drain or Collect?' and 'How do we make everybody happy?'

1.1.1 Cities draining rainwater

Traditionally, most cities have been designed to drain rainwater as rapidly as possible. This can be easily recognized by observing old and new buildings in Europe, the Americas, or Asia. Rainwater from the rooftop is drained through downpipes and discharged to the road surface or sewer system. Rainwater from the streets is also discharged to the sewers. Flooding occurs when the flow rate increases beyond the sewer capacity. Rainwater will mix with the contaminants on roads, resulting in the pollution of surface water.

A drained city can experience heat wave problems due to the loss of the evaporative cooling effect of water. All of the present cities in which none of the citizens recognizes the importance of rainwater while all participate in draining it are defined as Drain Cities. This stems from several scientific misunderstandings about rainwater.

Q1: Is acid rain safe?

Most people believe that rainwater is acidic and therefore, dangerous. The pH of pure and uncontaminated rainwater is 5.6 due to the chemical equilibrium with CO2 in the atmosphere. In the past, air pollution has caused severe acid rain. However, due to the development of treatment technology and regulations to inhibit air pollution emission, such harmful acid rain events seldom occur nowadays under ordinary conditions, although rain is naturally in the weakly acidic range.

A simple pH measurement can prove that while rainwater is acidic during precipitation, the rain collected following contact with rooftops covered with some dust is alkaline. Following one day of storage, it becomes neutral (Figure 1.1). Therefore, rain is not dangerous in an uncontaminated or slightly contaminated area. It is interesting to know that the pH of healthy human skin is approximately 5.6, which is close to the pH of (acid) rain.

Q2: Is rainwater clean?

Figure 1.2 shows the natural water cycle: rainwater that has fallen on mountains flows through cities and rivers, eventually reaching the sea. The originally-clean rainwater becomes contaminated as it travels. The degree of contamination depends on the distance it has moved after touching down on the ground and the duration of contact with the surface or contaminants. The numbers in the figure show the trend of the total dissolved solids (TDS) concentration of the water: the higher the mileage of the water, the more contaminated it is. Based on the concept of 'mileage of water', rainwater from the building rooftop has zero mileage, thus making it the cleanest.

Q3: Is rainwater safe to drink?

Many people ask me 'Is it safe to drink rainwater?' Before I answer, I ask them back, 'Is it safe to drink river water?' The answer is no. We must treat the river water before drinking it. It is the same with rainwater. We must treat it to make it drinkable. Therefore, a more appropriate question might be: 'Is river water or rainwater more economical to treat?' The answer is certainly rainwater.

1.1.2 A new paradigm of rainwater management

We must think about rainwater differently from the traditional concept. Once we clarify the scientific misunderstandings about rainwater, we need to ensure that rainwater is collected where it falls, before it gets contaminated, and use it subsequently for multiple purposes. Our cities should be changed or developed according to the new paradigm of rainwater management.

1.2 CHALLENGES OF DRAIN CITY

The traditional and currently accepted concept of managing rainwater in cities is to drain it as quickly as possible, considering it a menace, rather than an asset. People ignore the importance of rainwater, draining it thoughtlessly. Architects design buildings to drain rainwater. Land planners plan cities while considering rainwater a nuisance, draining it while constructing infrastructure such as pumping stations, and storm water retention basins. The design codes developed by experts are based on the concept of draining rainwater. The regulations and laws are also made to drain rainwater as quickly as possible. Therefore, the only purpose of managing rainwater in a city with a sewer system or a 'Drain City' is to drain it, so as to prevent flooding in 'MY' area. It is interesting to note that the rainwater drained from upstream will eventually cause damage downstream and lead to the requirement for further investment in water infrastructure downstream.

This can be shown conceptually, as in Figure 1.3, using a palm. Here, the hand and lines represent the city or country and sewer or river, respectively. While rain falls at any location within the given area, people drain the rainwater to a sewer or river (the line), attempting to manage it along the line. This is called 'management by line'. The runoff flow increases downstream. It is not possible to design and maintain the sewer and river systems to ensure safety because the rainfall may be heavier than expected due to climate change compared to the designed capacity.

The inherent design concepts of a Drain City are as follows:

(1) Rainwater is a waste: Rainwater is considered to be contaminated and a nuisance. Therefore, all the rainwater should be drained assuming that it is a waste, instead of collecting it as a resource.

(2) Combine the rain flow, increasing its momentum: In the design of the sewer system, all the rainwater is combined downstream. This causes the quantity to gradually increase, requiring an increase in the capacity of the sewer system. When the volume of flow is higher than the sewer capacity, flooding occurs.

According to elementary physics, the momentum of a body is proportional to the product of mass and velocity. If the rainwater from each street is combined while moving downstream, the momentum increases. Therefore, the sewer system or river bank system needs to be strengthened to resist the higher momentum.

(3) Collect rain downstream on a large scale: Rainwater has always been collected downstream, such as near the river, where the potential energy of the water is lost. In other words, energy is required to send the water back to the upper region. In a traditional storm water management system, since rainwater is collected downstream from a large catchment area, the system should be constructed on a large scale. Since the damage may be significant in case of an adverse event, a storm water basin should be built with a higher safety factor, requiring extra investment. Although it is expensive, it is used for only a short period in a year, if at all. Furthermore, since it is intended for flood prevention, it must remain empty for most of the time to store rainwater in the event of an unexpected heavy rainfall.

(4) Do it for a single purpose: Since the sole purpose of rainwater management in line is flood prevention, all the rainwater needs be drained. There is no reason to collect rainwater from the roof or road surfaces. Instead, the rainwater is expected to be removed as soon as possible.

(5) Social responsibility of rainwater management: Most architects or land developers assume that they can design a building or landscaping, while disregarding adequate rainwater management. Their only concern regarding rainwater is to drain it downstream. They do not seem to consider it their responsibility to manage the rainwater in the design area. Moreover, they do not seem to be concerned about the consequences of draining rainwater on the people or environment downstream.

Increased runoff from buildings and cities damages the area downstream, requiring extra investment in water infrastructure. With decreased infiltration due to paved surfaces, the groundwater level will be depleted, gradually destroying the environment.

1.3 RAIN CITY AS A SOLUTION

A 'Rain City' can solve most challenges faced in a Drain City. This can be shown conceptually (Figure 1.4), as an example of rainwater management by area. Since rainfall covers the entire area, if all the buildings and small sites were designed to collect rainwater at site on a small scale by storing or infiltrating it, then the amount of runoff would be reduced. This solution can cope with even major rainfall events without increasing the current sewer capacity. Additional benefits can be expected by using the collected rainwater for water supply, emergency storage, or groundwater recharge.

The inherent design concepts of a Rain City are as follows:

(1) Rain is money: In our building at Seoul National University (Seoul, Korea), a 250 m3 rainwater tank was included from the design stage. When it rains, the tank is filled with rainwater, which is used for flushing toilets, thus saving the money otherwise used to pay for tap water. Therefore, we believe that 'rainwater is money'. If people realize that rainwater is like money, they will collect it. Rainwater is the origin of all water sources, such as surface water and groundwater. Rainwater harvesting and utilization can not only save the money and energy required for water treatment and transportation, but also enhance resilience against damage due to flooding, water shortage, pollution, or fire.

(2) Distribute and manage rainwater: By distributing rainwater using small-scale rainwater storage and infiltration, the flow rate can be reduced, along with the investment in building and operating water infrastructures. If we distribute the rainwater runoff by collecting it on site, its mass will decrease, along with its momentum. Soil erosion will be reduced as well. By distributing rainwater, we can control its mass and momentum, thus managing it, instead of being managed by it.

(3) Collect rainwater upstream on a small scale: If we collect rainwater where it falls, we can obtain good water quality with relatively less treatment. An additional benefit of reducing the volume of runoff by direct storage or ground infiltration is the reduced threat of flooding at a local level. After collection, the stored water can be used for various purposes. Moreover, if we collect upstream, we can make use of the potential energy: it can be transported downstream by gravity. Instead of a large-scale centralized system, several decentralized systems, coupled with proper management, will not only improve the runoff reduction effect, but also reduce the cost and energy requirements.

(4) Make it multipurpose: The value of a rainwater management system increases if it is designed as a multipurpose system. Considering that many water-related problems, such as flooding, drought, water pollution, heat island effect, groundwater depletion, and mountain fires are related to rainwater, several problems can be solved simultaneously. Multipurpose rainwater management can be a good option, solving these existing problems more efficiently.

(5) Social responsibility of rainwater management: It is the responsibility of architects and land developers to manage the extra rainwater that is discharged from the properties they develop.

Some countries, like Germany, charge a rainwater tax based on the property area. Alternatively, the tax is reduced when a rainwater management system is installed at the site. In ancient Korea and Vietnam, traditionally, when a large building or palace was constructed, ponds were built on or near the site as a sign of social responsibility of rainwater management, while enjoying the multiple benefits of these ponds. There remain several good examples, including the ponds at Gyeongbokgung Palace (Figure 1.5), which was built in the 14th century at the beginning of the Chosun Dynasty (a kingdom before the Republic of Korea).

1.4 FROM DRAIN CITY TO RAIN CITY

A Rain City is a city in which all the citizens understand that rainwater is valuable, introducing regulations to promote rainwater by offering subsidies or incentives for people implementing a rainwater system.

There is a good example of this in the Republic of Korea. Through continuous promotions in the mass media and appropriate education, by October 2016, 59 cities declared themselves Rain Cities. These cities focused on two objectives: (1) increasing their water independency ratios, and (2) trying to maintain their original water status. Achieving these objectives involves the cooperation of all stakeholders, which is gained by offering financial incentives, while imposing compulsory requirements and technical guidelines for rainwater management facilities.

The success of a Rain City is founded on reminding residents of what the word 'village' ([TEXT NOT REPRODUCIBLE IN ASCII], pronounced 'Dong') means in the Korean language. It is worth mentioning that the names of all local districts in Korea end with the word 'Dong', such as Myoung Dong, Insa Dong, and Pyungchang Dong, etc. The literal meaning is water ([TEXT NOT REPRODUCIBLE IN ASCII]) + same ([TEXT NOT REPRODUCIBLE IN ASCII]) (Figure 1.6). This implies that the first thing to be considered in a city or village should be water because it is the most important. The second meaning is that it reminds all villagers that they depend on the same water, thus encouraging them to conserve water and reduce pollution. The third definition is that the water status should be kept in the same condition after development (no impact), which is a more reasonable and advanced philosophy than Low Impact Development (LID). The fourth meaning is that rainwater that falls at each site should be managed as the major source of water. This means that a high water independency ratio can be maintained for each village. The philosophy embedded in 'Dong' provides important insights for sustainable water management.

Climate change adaptation to protect against flooding and drought, while providing safe drinking water, will be a significant challenge to water experts and politicians. Rainwater management can partly solve these problems. Rainwater that is collected on a building or a site is a relatively clean water source that can reduce the energy used to treat and transport the water, as well as reducing the flood risk in local sewers. A decentralized, multiple rainwater tank system, with the associated IT-based decision-making tools, will be the only solution to localized heavy rainfall in congested cities, where increasing the sewer capacity is technically and financially not feasible.

Innovative job opportunities in designing, operating, maintaining, and promoting Rain Cities can be offered. Modeling tools can be developed to design a system of decentralized rainwater tanks, while predicting the effects of flooding, water conservation, and energy conservation. Products for collecting, treating, and storing rainwater on various scales, in accordance with the existing urban planning and water infrastructure, will be made available. Based on new types of rainwater management, the water industry is expected to grow.

The techniques and the engineering of small-scale decentralized rainwater systems are more straightforward than the large-scale centralized storm water systems. People working in water management may be reluctant to adopt the concept as they might be concerned about losing their jobs or being unsuccessful in their first attempt. The most challenging thing is changing people's mindset and misconceptions regarding rainwater management through education. The most important thing is to make the approach multipurpose by adding more value, while eliminating the sources of conflict. The approach should be both top-down and bottom-up. By changing the voters' mindsets and influencing decision makers with engineering data, appropriate regulations can be introduced.

Since current climate-change-related water issues such as floods, drought, and water shortages are related to rainwater, proper rainwater management is essential. On-site, decentralized rainwater management has significant potential for water conservation, energy conservation, and flood mitigation. It can be an excellent supplement to the existing centralized water systems. Both technical data and public education are required to change the mindset of ordinary citizens and the regulations of a city. The primary slogan of this movement is:

'Revolution of Rainwater: From Drain City to Rain City by Training Brain Citizens'.

1.5 SUMMARY

In this chapter, the fundamental question about rainwater ('Drain or Collect?') has been raised and addressed. Traditionally, rainwater has been managed using the Drain City concept, which promotes draining rainwater as rapidly as possible, while considering it a nuisance. It is managed by the sewer and river systems, using some significant measures for the sole purpose of flood prevention. The philosophy of a Drain City is self-comfort, without considering others' needs.