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.
