Impact of climate change on drainage cost in selected zone of Dhaka

Abstract

The change of climate, e.g. the significant increment of rainfall intensity is important for drainage system in a city. Specially in densely populated Dhaka city the impact is huge. Climate change has played a vital role in changing the climatic behavior of Bangladesh, consequently increasing the extreme rainfall events of the Dhaka city. Dhaka, the capital city of Bangladesh with rapid and unplanned urbanization, is subjected to annual average rainfall of 2,000 mm. Most of the drainage systems disposing storm water of Dhaka city are often unable to manage peak runoff volume and hence urban flooding is common due to improper and inadequate sewer size for excess increment of runoff.

This study aims to assess the Storm sewer network construction cost in Purbachal New Town Area with significant change of climate in near future.

A study area of 401.44 Hectares of Purbachal New town, situated at the northeastern side of Dhaka was selected to assess the storm sewer system construction cost with different Return Periods and areas. This is a newly growing area where no drainage system has been existed for storm water disposal. Application of EPA Storm Water Management Model was used in the Study area to size the storm sewer system. Costs were estimated for 10, 15, 20 and 25 years return period with different size of catchment areas. From the cost assessment it was found that construction cost will be getting higher in future with consequently increasing rainfall events. Calculation has shown that for a constant area 201.03 Hectares, the lowest cost for construction of storm sewer system comes 20.8 crore for 10 Years Return Period, whereas the highest cost is 24.3 crore in 25 Years Return Period for that same amount of area.

EPA SWMM was calibrated with an additional zone, Dhanmondi Residential Area for 10 Years return period’s rainfall event. From Land use pattern, it was analyzed that the selected catchment area is highly developed with 60 % imperviousness. From Drainage capacity assessment by EPA SWMM, it was seen that for 35 Hectares area, 2 meter RCC pipe would be needed for final discharge from the total area whereas the existing outfall pipe diameter was 1.8 meter. So the present capacity of the entire drainage network is a little bit low of its required limit. This present storm sewer network might not handle the full disposal of storm water from the selected area and will unable to manage the excess rainwater with increasing rainfall intensity for higher return period in future and hence will result in severe waterlogging.