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.

The report starts in Chapter 1 with an introduction to the Sponge City Programme (SCP) in China and the project area which is the ErQi International Business District in Wuhan. In this chapter, the problem statement, our collaboration with Arcadis and our project goals are also introduced. Chapter 2 delves into our methodology to tackle the brief. Starting from how we shaped our interdisciplinary approach, we explain our approach towards the project and our decision to include resiliency with the Sponge City concept as an objective. We continue by providing background information on ErQi area in Chapter 3 to get an overall understanding of the planned urban design and potential urban flooding. To provide a thorough analysis and recommendations for the selection process of adaptation measures to mitigate excess rainfall as part of the SCP in the context of ErQi area, an assessment of the Wuhan Sponge City criteria, a stakeholder analysis complemented by a spatial assessment was performed and described in Chapter 4. Setting the context allows understanding the complexity of the system and its constraints in the implementation of the SCP. Thus, we decided to first focus on the implementation of low-impact development (LID) measures using a multi-criteria analysis (MCA) presented in Chapter 4 and then developed an integrated and resilient system design later in the report. As the Sponge City is not sufficient to cope with high precipitation events (Arcadis, 2017), the project combines sponge city and resiliency principles in an integrated system approach.
The guiding resilient design principles of the Sponge City are further described and explained in Chapter 5 and applied in the opportunistic design process in Chapter 6, bridging the research with the designs. Here the designs of the MengQiao Bridge and the Water Road are presented along with their proposed effects on the urban flooding. Chapter 7 serves to assess the designs through the criteria of the integrated sponge city to improve flood resilience. The following chapter serves to share our conclusions on the challenges for implementing a functioning of the SCP that includes the concept of resilience. It also touches upon the difficulty of implementing the value-based design in a profit-based context. The final chapter is composed of five parts, all of which is our recommendations. It starts with our recommendations to improve the Sponge City criteria to make them more effective in reaching the goals of the programme. Then we give our recommendations for the selection process of LID followed by what we have learned of this interdisciplinary approach. That includes what we consider to be crucial to achieving a genuinely interdisciplinary process resulting in an integrated design. The final part of the chapter is dedicated to what we believe should be researched further. We believe a more in-depth assessment of the designs with the Sponge City criteria and input of the stakeholders is required for a final design. Further, the working definitions and approach of the Wuhan city government need to be considered, and an approach that assesses the necessary maintenance protocols is necessary.