Effect of urbanization on storm runoff characteristics of Dhaka city

Abstract

An urban or urbanizing watershed can be defined as an area in which all or part of the watershed will be covered by impervious structures, such as roads, sidewalks, parking lots and houses. In Bangladesh, the urban centres are expanding rapidly. During the last 25 years or so rapid urbanization has taken place in Dhaka city. Substantial increase in built-up areas has taken place due to development of residential and commercial areas mostly through private land developers and real estate business. This trend is likely to continue in future. These activities resulted in substantial increase in impervious area, created obstruction to natural drainage pattern, and reduced detention basins, which in tum lead to an increase of the flood peak and shortening of the runoff concentration time. The main objective of this study was to investigate the impact of land use changes due to urbanization on storm runoff characteristics of Dhaka city. To assess the impact of urbanization on runoff characteristics, rational method and rainfall runoff model have been used. Due to nonavailability ofland use data, only four years ofland use data could be gathered. The past and present status of impervious area as well as the trend of impervious area in future was estimated through the analysis of land use data. Runoff coefficient C was also estimated with this land use information. A correlation was developed between runoff coefficient and percent impervious area. The modelling study focuses on the impact of urbanization which causes increase of volume and peak rate of runoff. Urbanization of a watershed changes its response to precipitation. The most common effects are reduced infiltration and decreased travel time, which result in significantly higher peak rates of runoff. The volume of runoff is determined primarily by the amount of precipitation and by infiltration characteristics related to soil type, antecedent rainfall, type of vegetal cover, impervious surfaces, and surface retention. A rainfall runoff model was used for the assessment of impact of urbanization. NAM modelling system developed by the Danish Hydraulic Institute (DHI) and available with the Surface Water Modelling Centre (SWMC) was applied for this study. In this study, 1986 to 1995 rainfall and evaporation data were used. The model results indicate that there would be significant impact on runoff due to urbanization. Future Scenarios with future land use pattern for the year 2010 as estimated by JICA (1992) were considered and the volume and peak rate of runoff under future scenario was computed.