Impact of urbanization on hydrological processes: a case study in the eastern part of Dhaka city

Abstract

In Dhaka City urbanization and land use change have increasingly become among the major challenges for sustaining the hydrological processes. The rate of urbanization is increasing alarmingly due to high urban growth rate and increased migration from both the rural areas and other cities and towns. To accommodate the overburden pressure of population, Dhaka city is expanding, especially to the eastern parts affecting the current hydrological processes. The overall objective of this study was to assess the impact of urbanization on hydrological processes particularly on infiltration and storm runoff for the natural, present and future land use conditions. The study was carried out in a selected area in the eastern part of Dhaka City. The area covers about 5 km2 with the dominant land uses of agriculture, fallow, water body, etc. The area is proposed to be urbanized as per layout plan of the Eastern Housing Ltd. Intake characteristics of major soil types in the study area were determined from infiltration of soils measured in the field. Both SCS Curve Number and Water Balance methods were applied to estimate runoff at the natural, present and future conditions. Annual infiltrations were estimated as 2034 mm, 1546 mm and 1074 mm in the years of 2007, 2008 and 2010, respectively, under the present land use condition. These values were found to be 1860 mm, 1440 mm and 1000 mm in the years of 2007, 2008 and 2010, respectively, under a natural land use condition, which prevailed earlier. The infiltrations were estimated to be 961 mm, 766 mm and 560 mm in the years of 2007, 2008 and 2010, respectively, under the future land use condition with built up areas. The analysis reveals that infiltration will be reduced by about 50% when the study area will be fully urbanized. At the present land use condition, annual runoffs were found to be 948 mm, 373 mm and 177 mm in the years of 2007, 2008 and 2010, respectively, estimated from the SCS Curve Number method. These values were 806 mm, 269 mm and 203 mm from the water balance method. The future annual runoffs were estimated as 1674 mm, 802 mm and 447 mm from the SCS Curve Number method, which were 1870 mm, 1050 mm and 720 mm from the water balance method in the years of 2007, 2008 and 2010, respectively. When the area will be urbanized, most of the lands will be paved and compacted leaving a nominal amount of land in natural condition. This will alter the runoff process by increasing the future runoff to more than two times its present runoff.