Design considerations for rainwater harvesting in residential plots of Dhaka city

Abstract

Like many other cities, urban flood and draught has been major problem of Dhaka city. Expansion of the city built up area and reduction in rainwater infiltration is considered to be the main cause of the problem. Proper rainwater management even in the individual plot level might bring noteworthy transformation in this regard. As residential use pattern occupies a significant portion of the city surface, the scope of rainwater harvesting and design decisions in the residential buildings of Dhaka city can be investigated. So far, this has been considered as an engineer’s task, though the basic decisions like building operation, interaction, site planning and site layout are the concern of architects in the current context. This research aims to explore the architectural design considerations in the residential plots of Dhaka city for rain water harvesting. In this regard, this research has studied basics of rainwater harvesting (RWH), contextual information i.e. Hydro-geological, meteorological data of Dhaka city and the rules and legislations for residential building constructions. The study also has explored contemporary RWH practices in the local and international context. On the basis of theoretical understanding, 21 sample case studies have been taken from seven different categories of plot size from different residential part of Dhaka city. Case studies were in different construction phases. Primary data of all 21 case study buildings have been collected from physical survey, design drawing and questionnaire survey. All necessary secondary data have been collected. Collected primary and secondary data have been calculated and analyzed for each case study, based on the theoretical understanding. Main findings have been compiled and compared to observe residential development trend with building Rainwater Harvesting (RWH) system. The study has found that, in ‘Multifamily Apartment Building’ of Dhaka city, storage and recharge structure size for rainwater harvesting, increases along with occupants’ number, with the increasing size of the plot. Hence, demand vs supply ratio remains almost the same for different sizes of plots, and consequently the size of the storage structure increases significantly in large scale plots. It is found that, Rainwater can meet only 12%-30% of the total restricted water demand of the residential buildings of Dhaka city. Therefore, Recharge is more suitable than Storage. It has been apparent from the study that, prevailing residential development trend offers various scopes to design RWH system within the plot. Among the 21 Case Studies, 86% have parking area at the ground floor, which provides scope to incorporate RWH system, even in a built project. 85% of the Case Study buildings have Front Garden Area (FGA) that can be converted into rain garden. Moreover, Space Over the Toilet (OTS) might be converted as rainwater storage space, instead of making single large Storage structure. While designing plots more than 804 m², Architects can design Reflective pools, considering Rainwater as a source of water. Study also found that, while underground tank is preferable for already constructed buildings, design innovation is open for the building under design phase. In this research, Architectural Design Considerations and scopes have been discussed focusing on current Residential Building Development Trends in Dhaka to incorporate RWH system within the plot, and it is clear that the Architect has ample scope to design the system efficiently and aesthetically in order to improve the urban flood and draught situation of Dhaka city.