Mathematical modelling for climate change impact on stormwater runoff of Hatirjheel catchment

Abstract

Dhaka has faced major floods in 1954, 1955, 1974, 1987, 1988, 1998, 2004, 2007 and 2009. Among these flood scenarios recent events were particularly severe. Both Climate Change and land use / cover changes are playing key roll behind this increasing trend. Moreover due to insufficient capacity of local drainage system in urban areas situations are getting worse. As a result urban floods are occurring very often even after normal rainfall events in the monsoon. In future urban flooding situation is likely to worsen because of increases in monsoon rainfall, including higher intensity of extreme events due to Climate change. The objective of this hydrologic study is devised to assess the impacts of climate change on Central Dhaka’s water logging for various scenarios. The Central part of Dhaka which drains to Hatirjheel Lake is mostly affected by excess rainfall. The Central part of Dhaka is protected by the western Dhaka flood protection embankment on the west and road cum embankment named “Pragati Sharani” on the East. The region is demarcated by “New Airprot Road” in North, Old Dhaka in South, Mirpur Road in West and “Pragati Sharani” in East. Most of this area is already developed and densely populated. All the areas are protected by flood protection embankments. As a result this part of Dhaka city does not suffer from river flooding, although in 1998 the river water entered the area due to delay of closing the gates of regulators of the embankments and areas remained inundated for 30 days. The country-specific changes in climate variables as per GFDL 01 based projections for average temperature and precipitation in Bangladesh are for the years 2030 temperature will increase by 0.7oC and rainfall will increase by 11% in monsoon and for the years 2050 temperature will increase by 1.1oC and rainfall will increase by 28% in monsoon. Therefore, excessive rainfall in the monsoon would cause flooding and drainage congestion, while there would be very little to no rainfall in the winter. The increase in the magnitude of the rainfall event will also affect the intensity and return period of design storms. This is evident as the rainfall events at near past of 2004, 2007 and 2009 all were 100 year rainfall event. Again the drainage system of Dhaka city is designed for 1day 5 year event and the pumps are designed for 2 day 5 year event. Therefore they cannot cope with these storms and causes urban flooding. As the extreme rainfall events are occurring more frequently, this study will analyze 100 year event and also the events for which the drainage system is designed for with additional effect of climate change. To assess the effect of climate change on the drainage system several scenarios were considered. The flood event of September 2004 was used to calibrate the drainage model. For future drainage system, drainage system with planned improvement was simulated for 1 in 10 year, 30 year and 100 year event with the effect of climate change was simulated and analyzed. To reduce the effect of climate change of drainage system, various adaptation measures was considered and the drainage system was simulated and analyzed for 1 in 10 year, 30 year and 100 year event with the effect of climate change on rainfall. This was done to assess the effectiveness of adaptation measures. As the A1FI is the worst scenario for climate change, A1FI was used as climate change scenario. Results of the scenarios shows that the area of greater flood depth as well as average duration of the flood increases for the increased return period of the rainfall event. The flood depth and duration for 1 in 10 year and 1 in 30 year events are less significant. Therefore, the adaptation measures are more effective for the 1 in 100year event. The drainage system with adaptation measures shows significant improvement for flood free areas and duration of flood. The flood maps for different scenarios are presented in the Figure A-8, Figure A-10 and Figure A-11. Proper maintenance of the drainage system is essential as it is considered as an drainage improve measure. Some advance drainage improvement criteria such as pervious concrete, rainwater harvesting etc. are considered as soft measures. But again the pervious roads as well as dry detention ponds could not be considered for various reasons which is explained in chapter 4.10.2. The temporary pumps as well as permanent pumps are considered in case of heavy rainfall event. The use of early warning system can be used to reduce damage which is more effective and least costly approach. In a densely populated city like Dhaka, the government should encourage for green buildings which will increase pervious areas and subsequently the runoff induce by excess rainfall. The co-operation between the concern government agencies are vital and can be essential in case of disasters. Adaptation measures which are considered will reduce the effect of flood. For a ever growing city like Dhaka reducing flooded area and duration can reduce damage by millions.