IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 13, Issue 6 Ver. VI (Nov. - Dec. 2016), PP 68-73 www.iosrjournals.org DOI: 10.9790/1684-1306066873 www.iosrjournals.org 68 | Page Roof-Top Rainwater Harvesting-An Alternative Option to Urban Flood Mitigation and Groundwater Recharge for the City of Dhaka, Bangladesh S M Mostafizur Rahman 1 and Md Mobassarul Hasan 2 1 Engineer in Chief’s Branch, Army Headquarters, Bangladesh Army, Dhaka, Bangladesh, 2 Research Associate, Tropical Marine Science Institute, National University of Singapore, Singapore, Abstract: Dhaka, the capital city of Bangladesh, is one of the quickest growing mega cities in the world. Due to large number of population and increasing trend of urbanization, recently it is facing severe crisis in supply of clean and pathogen free water. Around 87% of the present total city water supply comes from groundwater with a huge pressure on groundwater table. In addition the groundwater system is endangered due to rapid industrialization for last two decades. Some factories have installed deep tube wells privately of various capacities in the city running without any monitoring, making the situation even worse. One of the major reasons for the depleted condition of groundwater is abstracting water indiscriminately without assessing the situation. According to Bangladesh Water Development Board, the groundwater table in upper aquifer goes down by 3.5 meters every year. However in 1990 the rate was only 00.33 meter/year. Since the underground water level is regularly going down in the city, many pumps can no longer lift water. Cost of abstracting water is also increasing every year because the pumps need to go further down. In excess of exploitation of groundwater is causing high depletion rate. The water table is continuously going down in Dhaka city posing environmental threat. Moreover, in recent years Dhaka city is affected by urban flash flood caused by high intensity raindrop in small area due to the impact of climate change and global warming.Owing to rapidly developed urbanization most of surface areas are impervious so that water can infiltrate into ground. This paper focuses rainwater harvesting from roof of building by underground reservoir with pervious floor which can recharge ground aquifer. In addition the storing rain water from roof-top in underground reservoir can mitigate flash flood induced by high intensity rain in Dhaka city. During monsoon(May to September) around 315 mm monthly average rainfall occurs in Dhaka city which is situated on around 370 sq km of land with a roof area of 75 sq km as there are around 675,000 concrete houses. It is estimated that 120 million of cubic meter water can be harvested during monsoon with the current rainfall. This can reduce the dependency on groundwater at least for six months of monsoon and decrease the pressure on existing drainage system in Dhaka city. It will reduce around 20% of volume of water which induces inundation of some area for several days in Dhaka city. Keywords: rainwater harvesting, urban flood mitigation, groundwater recharge, roof-top of building, global warming, climate change, Dhaka city. I. Introduction Rainwater harvesting and storing is not a new technology in Bangladesh. It has also been used for domestic, agricultural, runoff control, air-conditioning etc. for a long time in different part of the world especially in arid and semi-arid regions. Around 36% of households are using the rainwater as drinking water source during the monsoon season in coastal areas having high salinity problems [3]. However, rainwater harvesting is not a common practice in Dhaka, capital city of Bangladesh. Dhaka is one of the quickest growing mega cities in the world and facing severe crisis in supply of clean and pathogen free water. The situation is worsening further owing to the increasing population and the association expansion of urbanization and industrialization, all of which require more water and thus impose a tremendous pressure on the limited and fragile groundwater. In excess of exploitation of groundwater is causing high depletion rate. The water table is continuously going down in Dhaka city posing environmental threat. Around 87 percent of the present total city water supply comes from groundwater with a huge pressure on groundwater table. Total demand of domestic water supply has been calculated approximately 2000 MLD by DWASA in 2008 and estimated to increase to nearly 4200 MLD in 2025 [1]. Around 87% of the present total city water supply comes from groundwater abstracting through 520 DTWs operated by DWASA; rest of 13% of water is supplied from four water treatment plants situated at Saidabad and Chandnighat in Dhaka and GodnailinNarayangonj. This is a huge pressure on groundwater table.