Leveraging decentralized rooftop rainwater harvesting system to mitigate Chennai's water challenges using a multi-purpose approach

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Chennai, one of the largest cities in India, has been suffering from ‘’too little, too much and too polluted’’ water. As a response, in 2002, the local government took a step forward at the policy level by mandating the provision of rainwater water harvesting structure for every building. This system contributes to take advantage of the excess water during monsoon and palliating the situation during the dry season, while preventing it from being discharged into the polluted waterways. However, the widespread uptake of rooftop rainwater harvesting systems has been slow partly due to the lack of accurate and reliable information on the benefits of rooftop rainwater harvesting to make more informed decision. This research seeks to bring forward the potential of leveraging decentralized rooftop rainwater harvesting (RRWH) systems to mitigate Chennai’s water challenges by quantifying the hydrological effect of RRWH using a multi-purpose approach. To do so, a RRWH model was developed using daily continuous simulation method with ‘Yield Before Spill’’ as the operational rule to determine the optimum design capacity required to meet the domestic water demand and to provide its associated hydrological benefits on water supply, groundwater recharge and urban flooding. In this research, a closed system of RRWH designed to maximize water supply is applied for the analysis. Two areas of Chennai were investigated : urban area and peri-urban area. The Mambalam area, located in the historical center of Chennai, was selected as the urban area case study. Assuming 380,000 inhabitants are living in an estimated area of 11,690,000m2, approx. four million cubic meters of water can be harvested annually from the existing building’s roofs. From this, 50% of the buildings in the Mambalam is assumed to be residential which can provide 51d/yr/p of the domestic water demand. Thus, other sources of water supply are required to supplement the water demand. Maximizing water supply reduces groundwater recharge to nearly 0m3/yr in the Mambalam area. There is clearly a trade-off between water supply and groundwater recharge. However, when considering the adoption of RRWH for groundwater recharge (also refer as open system of RRWH) for the remaining 50% of the non-residential buildings in the Mambalam, approx. two million m3/aof rainwater can be recharged into the aquifer, balancing out the urban water system. This volume of recharged groundwater can also be considered as available groundwater for water supply because in urban area, groundwater is also used for domestic water supply. Together, the potential of decentralized water supply is increased up to 30% of the annual water demand (equivalent to 105d/yr/p) . Finally, the combined systems of RRWH for water supply and RRWH for groundwater recharge can contribute to reduce a volume of approx. four million m3/yr going into the stormwater drainage network and the polluted waterways in Chennai. According to the results, RRWH can reduce up to 60% of the stormwater runoff during a heavy rain event in the Mambalam. The results show that scaling up RRWH at the macro-scale level can have a significant impact in terms of drought and flood resilience for the Mambalam area. These numbers can serve as inputs for stakeholders’ dialogues to make informed decisions and raise awareness on the benefits of multi-purpose RRWH to transition Chennai toward a water resilient city. In practice, retrofitting existing building with RRWH for water supply in urban areas may become challenging mainly due to political, legal, physical and socio-economic factors. The adoption of a close system for RRWH is found to be more relevant for the periurban areas of Chennai. Indeed, buildings are developed on top of marshland with a high-water table level and saline water. This is the case of many residential apartment complexes located along the IT Corridor in the southern part of Chennai. Groundwater recharge and groundwater abstraction for water supply are not possible. As a
consequence, people need to rely solely on water tankers which is around 20 times more expensive than the cost of water per kiloliters in urban areas. The case of Sabari Terrace residential apartment complex showed that the adoption of RRWH for water supply contributes to 15% of the annual water demand and it saves up to $6/yr/p.