Chennai, an Indian rainwater harvesting city

Manuel Marin, 2013

The city of Chennai began the experiment of rainwater harvesting 10 years ago. As water from underground sources was increasingly used and, conversely, rainwater absorption by the soil began to decrease, due to the expansion of the city and its pavement, intervention to restore the natural aquifer recharge cycle became urgent and the authorities reacted accordingly. Since 2001, rainwater harvesting systems began to be installed in existing buildings and required in new constructions. The recovered rainwater is injected into the subsoil through various mechanisms, from the simplest to the most complex, compensating for the extraction and thus restoring the level of the aquifers. Since the implementation of this strategy, the volumes of water have recovered and today the situation is relatively stable. The water that can be extracted is, in any case, slightly salty, due to the intrusion of sea water during the period when levels are falling. The need to reduce the presence of saline components in the aquifers is therefore one of the objectives that remains to be achieved in the strategy put in place. Most of the water collected is used to feed the underground deposits ; the remaining part is destined for other applications, industrial and domestic, which allows us to speak of process integration in the context of industrial ecology.

The depletion of underground sources had begun to be felt since the 1990s in the Besant Nagar area, in the state of Tamil Nadu in India, whose capital is Chennai ; the strong migration of rural population to the city and rapid urbanization being the main causes. On the one hand, the increase in demand for water, both domestic (a person in the city needs twice as much water as in the countryside) and industrial, particularly in the construction sector, has accelerated the rate of extraction of water from underground deposits. On the other hand, the development of the city reduced the extension of the unpaved surface, preventing the filtering of rainwater through the soil and thus disturbing the recharge of aquifers. In addition, rainfall in the region falls in short bursts of high intensity, making absorption more difficult. In the early 2000s, the situation became critical, with 65% of rainwater flowing to the sea or evaporating before being absorbed by the soil. In order to maintain the growth rate and even avoid a hydrological disaster, the authorities decided to set up an ‘artificial’ aquifer recharge system. However, this artificial system is not far from the cycle of nature.

Sixty years after its independence, India is a country facing enormous problems of resource management and land use planning. In Chennai, the rainwater harvesting initiative started slowly, driven by a group of activists at the height of the supply crisis. The local government had set up water trucks to distribute water around the city and, in the summer months, water was brought in by train from the city of Neyveli, 200 kilometers away. The first rainwater harvesting systems in Chennai were installed in buildings owned by private individuals who were sympathetic to the initiative, motivated by the relatively low cost and effectiveness of the solution. The government followed this first experience and rainwater harvesting systems started to appear in public buildings, schools and hospitals. The next step was the requirement to integrate this type of system into residential buildings, old and new. This initiative was unprecedented in India and perhaps in the world: connection to the water and sewerage system was made conditional on the installation of rainwater harvesting systems in every building.

The  Rain Centre , a model building located in the Santhome area of Chennai, offers visitors the opportunity to experience the operation of a multi-component rainwater harvesting system free of charge and on site. The center’s staff provides information on how to install these components and offers two methods for installation in a standard building. The first is a ground-level grate at the foot of the main door that captures water before it flows to the sidewalk, while allowing pedestrian and vehicle traffic in and out of the building. A speed bump can be installed to increase water accumulation and capture. Then, an underground pipe carries the collected water to a small well 4 to 6 meters deep (to reach the sand layer of the soil) through which the water is integrated into the deep layers.

The second method consists in the collection of water from the roof. The collected water passes through closed pipes to a filter at ground level. The filter must be sized to accommodate the volumes of water that arrive. After the filter, this second method allows the water to be stored in existing tanks or sumps on the property (if they are in good condition), or to be injected into the subsoil through percolation wells. Only the surplus is injected together. The purpose of this second method is to recover as much water as possible for domestic use, such as drinking, flushing, irrigation of the vegetable garden, etc.

In order to evaluate the impact of the implementation of these systems, the GROWNET (Grown Water Network) project, a joint initiative of the IUGS (International Union of Geological Sciences) and UNESCO, commissioned a study from the Anna University of Technology. The results show that, indeed, the recovery and injection of rainwater on the lower layers of the subsoil between the years 2001 and 2005 had a positive effect in the recharge of aquifers. To reach this conclusion, the research analyzed a series of data of water demand, levels of 15 wells in the region and millimeters of rainfall in the years before and after the implementation of water harvesting systems. The research related all of these variables according to the following rule : the increase in water level in a well from one year to the next, minus the consumption, must be explained by the recharge of the well in such and such a year. Using this equation, the aquifer recharge rate was determined and it was observed that from 0.36 cubic meters per m2 in 1995, it had increased to 0.65 in 2005, four years after the first collection systems were put in place.

At the same time, a qualitative water analysis was conducted to see if the recharge of the aquifers by the rainwater injection method was favorably affecting the composition of the water in the springs. Water samples were taken regularly from 11 distributed wells in the city of Chennai between November 2004 and April 2005, during which time the rainwater harvesting systems were in operation. These samples were submitted to the wet chemistry laboratory of Anna University, where the measurement of quality parameters was performed. Comparison of the measurements obtained according to the ranks accepted by IS 10500-1991, led to the conclusion that seawater intrusion had caused considerable damage during the time when freshwater levels in the aquifers were at their lowest ; the addition of rainwater since 2001 had not helped to improve this situation. Yet, even if the quality of the water does not make it potable, its quantity allows for its sustainable use for other purposes.

The strategy of setting up rainwater harvesting systems in Chennai can be described as a success story. The problem of depleting sources has been solved favorably and the solution found is within the framework of sustainability and respect for the environment. What is remarkable in this case is that the developed system restores a cycle of nature that had been interrupted by urban growth. It is a process that is halfway between the natural and the artificial, a kind of « assisted recharging » of aquifers, for which very little material is needed and which allows a profound change in the city and in the way of life of its inhabitants.

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