By 2050, it is projected that more than 70% of the human race will be in urban settings, with many of these urban settings located in mega-cities [1
]. Challenges facing these large urban cities include two particular water-related dimensions, namely (i) urban flooding, in part due to the transformation of the landscape from pervious to significant levels of impervious land uses, plus the more intensive, heavy storms that are expected to occur as a result of climate change (e.g., References [2
]), and (ii) in the search for sufficient supplies of water to satiate the burgeoning urban populations, large groundwater withdrawals may be resulting in land subsidence (e.g., see Reference [5
]). Cities in northern China experience arid and semi-arid climates, with an average annual rainfall of less than 600 mm, with the precipitation concentrated within the rainy season from May to October. Severe water shortages are a major problem in these northern areas of China but, as noted above, the potential of land subsidence is also an important issue as well as the need to protect the integrity of water quality conditions in aquifers.
Another part of the response to these challenges facing large urban cities must be to move away from historical approaches which primarily focused on decreasing urban flooding by encouraging rapid stormwater runoff; an alternative approach in the appropriate circumstances stipulates the merits of efforts to encourage the recharge of groundwater and, at the same time, avoid problems of flooding, while also avoiding water quality impacts.
China has been focusing on widespread urban construction for the last 10 years but frequent inland flooding has shocked citizens. For example, more than 180 cities in China have annually incurred inland flooding caused by stormwater over a period of just three years from 2012 to 2014. In July 2012, Beijing faced the heaviest rainstorm in six decades, resulting in the deaths of 79 people [6
]. This type of massive event has led to the country favoring ‘green’ rather than ‘grey’ and unordered, urbanization. This was shown in China’s 12th
five-year plan, which proposed to pay more attention to the issues of stormwater management [6
Flooding has become a regular occurrence in China’s cities; 62% of Chinese cities surveyed experienced floods and direct economic losses of up to $
100 billion between 2011 and 2014 [7
]. Over 200 people were killed and $
22 billion in losses were incurred across China in 2016, affecting more than 60 million people. It is, thus, key that urban infrastructure is more resilient in response to the impacts of future climate change. It is clear that with the current design scenarios, the frequency of urban floods will increase over time. A survey conducted by the Ministry of Housing and Urban-Rural Development (MOHURD) [7
] reported that 641 out of 654 Chinese cities have incurred frequent floods. In 2008–2010, 62% of 351 cities surveyed suffered from urban flooding and 39% experienced flooding on three or more occasions. Since 2008, the number of Chinese cities affected by floods has more than doubled, and at least 130 cities have experienced flooding nearly every year [8
Severe weather is predicted to be one of the greatest reasons for higher costs in the future delivery of water services and managing infrastructure [9
]. Intensifying concerns also occur in relation to subsidence, with consequences that exacerbate urban flooding problems, particularly in coastal zone cities such as Shanghai, Guangzhou, Zhangjiang, and Tianjin, to name a few [5
]. One of the areas with the most severe land subsidence problems in China is Hangzhou-Jiaxing-Huzhou; by the end of 2010, the land subsidence area reached 4.2 × 103
. Additionally, 66% of the Beijing plain has been affected by land subsidence (>50 mm) with a maximum subsidence of 1.23 m [10
]. It is now known from the records of groundwater extraction, hydraulic head, and land subsidence, that land subsidence is the result of continual and excessive extraction of groundwater from deep confined aquifers (e.g., see Reference [11
As apparent from the above, China’s cities are being forced to simultaneously consider issues of subsidence (as a result of groundwater over-extraction) as well as the problems of urban flooding while protecting against the potential deterioration of groundwater quality. The results of these sometimes-conflicting issues require effective environmental governance, necessarily entailing a holistic and sustained effort [13
]. In December 2013, China’s president first proposed to develop “sponge cities”. From this moment on, there have been many policies and guidelines with the goal to help alleviate the adverse effects of urban construction and recycle 70–90% of stormwater in-situ by combining permeation, retention, storage, purification, and reuse before discharge by applying the green infrastructure concept [14
]. As Wang et al. [15
] reported, China has developed 30 pilot sponge cities thus far, replacing the approach of rapid drainage by using sponge city strategies. The Sponge City Project advocates for various forms of utilization of rainwater as water sources for cities.
As Xia et al. [6
] pointed out, the sponge city construction will be a “hot topic in the future of China” and Li et al. [14
] indicated that the Chinese government has been searching for viable options. In response, the focus of this paper is to demonstrate a potential technology option wherein emphasis in the design is to increase exfiltration to the vadose zone from the stormwater system itself and, hence, pursue two objectives, both to decrease flooding during storms and to increase contributions of stormwater to groundwater (while reflecting the need to assess the potential for deteriorating water quality in the groundwater). This technology, dubbed herein as the ‘Etobicoke Exfiltration System’ or EES, was created, designed and constructed (2.5 km in length) in the former city of Etobicoke (now part of Toronto) in 1993. The primary objective of EES was to restore elements of the natural hydrologic cycle in a built-up area of the City (e.g., see Reference [16
]) while not conflicting with the desired surface land uses and providing the functionality of recharge to groundwater over all four seasons of the year. After 20 years of operation, the majority of the EES is still providing runoff control as well as recharge [17
The EES has been reported as being able to exfiltrate 90% of rainfall events and this groundwater preservation is important to the maintenance of uptake by vegetation that depends on groundwater during periods of drought [16
]. Where groundwater provides stream recharge during low flow periods, this can also be very helpful via the augmentation of the baseflow in a river which will improve the surface water quality, including cooler temperatures of the receiving waterbody and, hence, help in maintaining terrestrial bio-diversity ([16
], Harvey et al. 2017 [20
As a consequence of these outcomes in Ontario, Canada, EES types of systems are included as one of the low impact development practices for stormwater management as per TRCA/CVC [21
], where it is stated that perforated pipe systems can be thought of as long infiltration trenches or linear soakaways [21
] that are designed for both conveyance and infiltration of stormwater runoff. They are underground stormwater conveyance systems designed to attenuate runoff volume and, thereby, also reduce contaminant loads to the receiving waters. Perforated pipe systems are referred to under many terms including pervious pipe systems, exfiltration systems, clean water collector systems and percolation drainage systems. It is noted that there are conditions under which EES types of systems are undesirable due to the potential to deteriorate groundwater quality, as listed in Section 3.8
below and, hence, caution must always be reflected, particularly with regard to the quality of the stormwater, as well situations where the hydraulic conductivity of the ambient soils (e.g., exfiltration from the system may be limited due to ambient soil conditions not being sufficiently transmissive).
As apparent from the above, in the context of improving exfiltration to groundwater as a means of decreasing subsidence, while also decreasing surface flooding, this exfiltration option has several important dimensions. Examples of some of the beneficial impacts for this option are described using a mathematical modeling characterization applied to a hypothetical, small catchment in Beijing, China to illustrate some of the potential opportunities.
There is widespread evidence of major flooding events in urban cities in China and, in many cases, there is groundwater extraction for water supply, resulting in land subsidence. An option that warrants consideration is the use of an exfiltration pipe system where substantial portions of the stormwater can be transferred to a lower, perforated pipe(s), and then exfiltrated to, at least in part, replenish groundwater, after ensuring that the infiltrating water will not result in damages to the water quality of the groundwater. The exfiltration type of the system decreases both flooding and subsidence. The exfiltration pipe system intentionally moves water into the environment to facilitate exfiltration from the lower pipe by ensuring large amounts of water are available for exfiltration.
There are clearly a number of benefits from the exfiltration pipe technology, (a) significant amounts of stormwater are exfiltrated to the vadose zone, (b) decreased overland flows occur, and (c) increasing recharge to a location experiencing declining groundwater aquifer.
Overall, from the computer modeling described herein, the net result for the case study area of Beijing using the exfiltration pipe design indicates the following:
The exfiltration system substantially decreases overland flow and the lower (perforated) pipe would be able to capture, store and, subsequently exfiltrate ~53% of stormwater entering the storm sewer for the small catchment utilized for runoff from a 2-h duration, 5-year recurrence interval storm. Overall, the potential exists to exfiltrate water from the lower pipe, at levels of ~71% of the total annual rainfall for the small catchment utilized. This demonstrably increases vadose zone recharge and ultimately creates additional groundwater recharge which, in turn, will decrease subsidence in areas where this is a major concern (particularly related to coastal zone cities in China);
Decreasing flooding from heavy storms can be accomplished (the so-called heavy storms that occur several times a year); and,
The costs of placement of the second pipe increases the overall cost of the installation of this system (about 15%) which indicates, given the magnitudes of flooding and the resulting damages, this is a technology that warrants consideration.
Nevertheless, while there are many possible benefits of the exfiltration type of system, the utility of this technology for a particular location may be precluded due to a variety of conditions as outlined in the paper, including the potential to deteriorate the quality of the groundwater.