Proceedings, International Conference on Disaster Risk Management, Dhaka, Bangladesh, January 12-14, 2019 INCLUSION OF RETENTION POND TO RETAIN RUNOFF BASED ON IDF RELATIONSHIPS IN RAJUK’S MGC BASED DHAKA CITY AREAS Mahmud Al Islam 1 & Md. Mafizur Rahman 2 ABSTRACT Dhaka is a populous city with 9.254 million people living in an area 300.97 square kilometers and having an astounding density of 30,748 per square kilometer. Uncontrolled construction of building and infrastructure due to this burden of population has encroached the uncovered area. Urban flooding has become a common phenomenon in the last few decades. The present drainage network of Dhaka is unable to manage the peak runoff volume after a heavy rainfall. So to minimize the water logging and urban flooding, retention pond can be designed in between the uncovered space of multiple buildings. In this study, last 30 years (1988- 2017) rainfall data was analyzed to find the runoff volume and retention pond depth in different parts of the city. Then the relationship between the uncovered area and the retention pond depth was prepared to find the feasibility of retention ponds in RAJUK’s FAR and MGC based areas. Also the land use pattern of different wards of Dhaka city and catchment basins of Dhaka city using the DEM was prepared with ArcGIS as a part of the endeavor. Introduction Dhaka is subjected to annual average rainfall of 2,076 mm. The tropical monsoon climate comprises four hydrological seasons: pre-monsoon (April–May), monsoon (June–September), post-monsoon (October– December), and dry season (January–March). At the start of the monsoon and post-monsoon seasons, cyclones with strong winds hit Bangladesh, sometimes causing heavy rainfall in Dhaka city with subsequent urban flooding. About 56 percent of Dhaka was inundated by the 1998 flood, which was unprecedented in terms of duration and damage. Most of the city’s eastern portion suffered from river flooding, and 23 percent of its western part was affected by urban flooding (Hasnat,2006). Urban flooding is one of the reason that make city life hideous. The drainage capacity is not enough to capture the storm water after a heavy rainfall in Dhaka city. So as a result most parts of the city goes under water. This system has proven unsatisfactory and it leads severe flooding in low-lying areas. Two separate drainage systems are operating in Dhaka City: one is for managing storm water and the other one is for domestic waste water (Haq, 2006). Although the city had an excellent natural drainage system consisting of 24 natural canals and a large retention wetland pond before 1940 (Haq, 2006), with the rapid and unplanned urbanization, most of the canals have been illegally occupied by real estate companies and this has resulted reduced carrying capacity of storm water of the city. About 85 % of the city is now drained through 40 (lined) channels to the surrounding rivers (Tawhid, 2004) which is not enough. Methodology Urbanization changes the way water flows through a catchment. The volume of runoff on urbanized areas are greater. So the uncovered spaces in between some buildings have been calculated in the selected areas through ArcGIS soft-wares. The areas are divided into several blocks including 4 to 8 no of buildings. The runoff was calculated using the design rainfall. The uncovered spaces in between closely spaced structures have been calculated in the selected areas through ArcGIS and Google Earth Pro. The minimum uncovered space among several blocks was used as the retention pond area. The runoff was calculated using the design rainfall. Then the required depth of the Retention pond was calculated from the runoff volume. 1 Department of Civil Engineering, Bangladesh University of Engineering and Technology. 2 Professor, Department of Civil Engineering, Bangladesh University of Engineering and Technology. E-mail of Corresponding Author- omemahmud@gmail.com