Development of an urban drainage system for a small township: a case study

Abstract

Due to urbanization and increase in population, urban regions of Bangladesh require immediate improvement of drainage system. Nowadays, climate change has become a global issue and Bangladesh is in a high probability of being heavily affected by climate change. The effect will be severe in Bangladesh’s urban areas, where drainage is already a serious problem. Realizing the importance of this issue, Government of Bangladesh (GOB) has emphasized on this matter; and has already taken initiative to develop the drainage system of the small Township/Municipal (Urban regions of a District) areas. Dhamrai Municipality, located in the North Central Region of Dhamrai upzila of Dhaka District under Dhaka Division is the object of this study. This study provides a framework to water management policy within Municipal areas. Despite of its location in the North Central Region, significant part of its area is affected by external flood. The parts of north-west and south-east are flood affected. Two earthen borrow pit exists along both sides of the Dhaka- Aricha highway. But these borrow pits are encroached at reaches by local inhabitants which at present scenario is hampering the natural drainage route to the desired outfall. Moreover, many of the drains fall into relatively low lying areas in a haphazard ways, thus causing drainage congestion and water logging problems in some places after heavy rainfall. The proposed drainage system within the study area is an open drainage system. Total seven major drains have been proposed with storm drains. Among them, five major drains has been recommended as priority needs while two major drains are proposed in view of future needs for the study area. Rests of the zones are planned with their outfalls for future drainage details. Three different scenarios for two different case studies were formulated and simulated to test the drainage capacity. The results of the simulations for each scenario were analyzed individually in order to evaluate the possible risks of future inundations in the study area. Two cases were conducted by considering a proposed drainage system without re-excavated borrow pits and another considering proposed drains with re-excavated borrow pits. The scenarios are based upon the analyses such as design year rainfall events and various outfall water level stages. In order to simulate the scenarios, the DHI computer program MIKE11 has been used to create a rainfall-runoff model which consists of a hydrological and a hydrodynamic model. The necessary information to create the model was taken from IWM, BMD, BWDB, SRDI and several literature reviews. The calibration optimizes the hydrodynamic model so when the historical rainfall data is put into the model and is simulated the out coming graph of the water level is made as equal or nearly equal to the graph of the recorded water level as possible. The results are presented in the form of flood map and damage map, so the area of inundations under different rainfall events and water level stages along with its pattern of damages could easily be assessed out. The historical rainfall was utilized in the simulations which showed, Case Study 2 provided better results and improvements in flooded areas compared to Case Study 1. Around 99% and 80% of land will be above flood level under scenario S1 for Case Study 2, which means a small impact for the design year rainfall events on the drainage system. Backwater effects from the river Bangshi, has large impacts on the water level in the low-lying parts of the drainage network. Only 50% and 46% land will be above flood levels under scenario S3 for Case Study 2, which shows 5% and 6% improvements in flooded areas compared to Case Study 1. Due to flooding, most of the damages will occur in homesteads/ residential areas but majority of the damage remains within first category of damages. Moreover, industrial sectors and commercial enterprises also undergo low to moderate type of damages. None of the scenarios indicate any additional areas in risk of flooding and damages in the future compared with today’s situation. It is recommended that, construction of some tertiary drains connecting the proposed secondary drains or raising of the land levels, will improve rainfall flooding conditions within the places which are facing water logging problems at present scenarios, due to the presence of pocket depressions.