Assessment of present status and conveyance capacity of drainage system in selected areas of Dhaka city

Abstract

Water logging has been a constant hazard for Dhaka city for quite a long time. The situation is severely aggravated during the monsoon. In many times, key portions of the city are inundated in rainwater several hours or even days after the torrent have been passed. The blame seldom goes to the unusual rainfall pattern and the drastic and unplanned urbanization of the city which have heavily contributed to this intolerable situation. However, the age old and poorly planned drainage network is mostly held liable for such a mishap according to the opinion of majority of the city dwellers as well as experts. To make things worse, the inadequate, existing network is very poorly maintained and as a result, the performance of this network itself has diminished drastically. The rivers and khals in and around the city of Dhaka is severely in the grasp of abuse, pollution and encroachment and losing its ground and capacity on a daily basis. Now a days, very few khals runs through the city and only a handful of lakes are available to be used as critical water reservoir. A study area of 15.6 sq. km. within the heart of the city was selected to assess the said aspects of the drainage network. The study area is highly developed urban region consist of residential, commercial and business zone of national importance. After critically assessing the existing drainage network of study area, it was found that many of it are left unattended and uncared for years and as a result have lost most of its working capacity. In some places, drainage system elements like manholes, catchpits or conduits are found totally shut off and causing water logging even in the dry season. The present efficiency of the entire drainage network looks merely half of its design capacity. Calculations show that existing network have a mare capacity of discharging 324 cum per sec which is way below it’s required limit. This below capacity network cannot handle this excess rainwater and hence results in severe waterlogging. A probable supplementary network of conduits in addition to the existing network was proposed and theoretically implemented. A theoretical simulation shows that the proposed drainage network combined with the existing network can handle suggested historically probable high rainfall with considerable efficiency. However, it still fails to meet the needs if the rainfall goes excessively high. Designing a drainage network for such high probable intensity of rainfall is unrealistic and non-economic.