Variable rainfall due to climate change, and increasing population pressures has made urban water management a hot topic in national and international policy discussions. In cities, increasing impervious surface reduces infiltration of water and increases surface runoff. This one physical change creates so many multifaceted problems that greatly impacts the quality of life in a city. Of these problems, stormwater runoff and urban floods have been garnering attention in recent years, particularly in India.
Drowning cities of India
Increasingly variable rainfall is skewed towards large amounts of precipitation in a short period of time. This means that the capacity of urban stormwater management systems need to be much higher than the capacity calculated during the city’s planning and construction. However, urban floods have been increasing because of this discrepancy between stormwater volume and drain capacity for a given period of time.
United Nations Office for Disaster Risk Reduction (UNDRR) declaring floods as the single most widespread disaster risk to urban settlements globally. India’s climate, river landscape, and fast urbanization makes it the second-most flood prone country in the world, after Bangladesh.
Since the disastrous Mumbai Floods in 2005, floods have continuously occurred in Indian cities. Parts of New Delhi face urban flooding every year. The Surat Floods in 2006, the Kolkata Floods in 2012 and the Chennai Floods in 2015 are some other notable examples. Notably, occurrence of such floods have increased even in growing cities like Gorakhpur, Jamshedpur and Rohtak. The causes attributed to these floods are the same; inadequate drainage, uncontrolled development and land-use changes.
Technological measures to solve these problems, like constructing artificial recharge pits, stormwater drains, have proven to be either costly, inadequate or lacking incentives for citizens. Land-use zoning have largely been unheeded to, as evidenced by constructions on floodplains in New Delhi or smaller cities like Sangli.
This article seeks to make a case for a possibility to integrate nature-based solutions (NbS) into city planning as a way to manage stormwater. I am sure that this is an option well suited under the Smart Cities Mission or Atal Mission for Rejuvenation and Urban Transformation, both of which talk about improving stormwater management and disaster resilience in cities. With the recently adopted Jal Shakti Mission in India, there is a further focus on water management.
Urban forests and trees for stormwater interception and control
Lack of tree cover causes rain to directly hit the ground surface, creating rainsplash erosion (Korhnak, 2001). However, where trees are present, canopy drip, stemflow and evapotranspiration slow the movement of water towards the ground, delaying the contact between rainwater and the ground. This delays the stormwater release and reduces stormwater volume in the flow. Tree litter also prevents rainsplash erosion that exposes soil pores and underlying bedrock. Thus, the physical aspects of trees, such as leaves, branches and trunks, play an important role in rainfall interception.
Observing the impact of urban forest canopy cover on rainfall interception is difficult, but has been studied using simulations. In the the municipal urban forest of Santa Monica, California (Xiao & McPherson, 2003), it was found that Annual rainfall interception of the street and park trees of the city amounted to 1.6% of the total precipitation, resulting in avoided stormwater treatment and flood control costs of up to USD 110,890 (USD 3.60 per tree). Another study in Dayton, Ohio, observed that canopy cover can reduce potential runoff by 7-12% (Sanders, 1986).
What kinds of trees work best? Studies showed that tall trees with large canopy covers and wide leaves did a better job at rainfall interception. The canopy size and leaf width of trees play a major role in low to moderately intense rainfall events (Nytch, Meléndez-Ackerman, Pérez, & et. al., 2019). In India, such trees are often the older trees of a city, and usually found in parks or street trees, as tall trees interfere with electric and cable wires in densely populated areas of a city (Nagendra, 2016). Some of these species include Samanea saman (Rain tree), Syzygium cumini (Jamun tree), Azadirachta indica (Neem tree), Mangifera indica (Mango tree), Ficus benghalensis (Banyan tree), Ficus benjamina (Java Fig), Pongamia pinnata (Indian beech) Artocarpus heterophyllus (Jackfruit tree), Ficus religiosa (Peepal tree).
Using green roofs to regulate stormwater runoff from roofs
A popular concept in North America and Europe, green roofs are yet to become a part of the Indian urban landscape. Roofs form 40-50% of the impervious layer of urban centers (Mentens, Raes, & Hermy, 2006), therefore it makes sense to consider these areas as part of the stormwater management plan. Green roofs play a similar role to large trees in reducing peak discharge volume and delaying the peak flow of the stormwater.
A comprehensive study by Fassman-Beck, Voyde, Simcock, & Hong, 2013 in Aukland, New Zealand indicated that in the long-term, peak flows from the living roofs were 62-90% less than a corresponding conventional roof’s runoff, while the green roofs retained up to 56% more runoff than a conventional roof. Shafique, Kim, & Kyung-Ho, 2018 noted that a green roof in Seoul, South Korea reduced runoff from storms by 10-60%, depending on the intensity of the rainfall event. Fioretti, Palla, Lanza, & Principi, 2010 found that the same effects of green roofs on stormwater management are found regardless of the climate of the region, when they tested green roofs in various climatic zones of Italy. However, it is important to consider the material on the green roofs, depth of the substrate, and roof slope, along with the plant species being used. For example, succulent, drought-resistant plants do well in retaining stormwater (Mentens, Raes, & Hermy, 2006).
How can this be implemented?
Urban forestry planning and financing for ecosystem services have been extensively discussed here and here on Eco-intelligent. The money invested in expensive construction and reconstruction post-floods can be used for the development and maintenance of urban forests and street tree networks. Additional financing from carbon trading, for example, can be mobilized if required. Here, we take a closer look at policy initiatives for implementing green roof programs in cities.
The well documented benefits of stormwater control (and urban temperature regulation) have led to various policy initiatives to incentivize green roof development. For example, Auckland has seen a proliferation of green roofs since it was included in The Auckland Plan 2050 to “deliver greater resilience, long-term cost savings and quality environmental outcomes”. This was a community-driven, awareness generation program that aimed to sensitize the benefits of green roofs to citizens.
Chicago looked for a more business-oriented approach for incentivization. The Green Permit Program consists a list of “green options” for builders to choose from while applying for a permit. More the green options in the application, faster is the processing of the building permit. The program also offers a permit fee-waiver for buildings with exceptionally low environmental impact. This move spurred various entrepreneurial ventures in Chicago dedicated to green roof construction and maintenance.
USA has developed a Property Assessed Clean Energy (PACE) Financing or PACE Loan to retrofit existing buildings with disaster resistant structures, including green roofs and water conservation structures. The loan is a long-term, property attached loan that provides 100% upfront financing for the retrofitting works. The loan is financed by municipal bonds. As of February 2019, PACE loans have financed over USD 6 billion worth of retrofitting projects in residential and commercial buildings. Such Special Purpose Vehicles (SPVs) can be explored, particularly within the Smart Cities Mission which already has vehicles designed for financing activities.
The losses from urban floods can be staggering. For example, the Mumbai Floods affected close to 20 million people, including killing 1200 people and 26,000 cattle (Gupta & Nair 2011). The Surat Floods inundated 75% of the city for several days, causing losses in billions of dollars. With UNDRR declaring floods as the single most widespread disaster risk to urban settlements globally (ICLEI-South Asia, 2015), its mitigation is of prime importance in future urbanization decisions. In the long-run, green spaces need to be strongly considered as a cost-effective approach to mitigating these disasters. Urban forests can prevent urban floods.