Application of PCSWMM to Explore Possible Climate Change Impacts on Surface Flooding and CSOs in a Peri-Urban Area of Pathumthani, Thailand

Southeast Asia is becoming increasingly urbanized and this population is highly vulnerable to potential impacts from climate change because many large cities are located in low lying coastal zones, often have under-designed or aging and poorly maintained drainage systems, and already experience significant surface flooding from the high intensity rainfalls characteristic of a monsoon climate. The record floods experienced in central Thailand during December, 2011, impacted some 2.3 million people, created $25 billion U.S. in damage, and underscored the need to re-examine urban drainage systems in the Bangkok area. Given the potential for even greater rainfall extremes under climate change scenarios it seems prudent to begin investigating possible impacts related to localized urban surface flooding and CSO discharges as a starting point to identify and evaluate adaptation options. This study focused on the peri-urban area of Rattanakosin Village, Pathumthani, a region that now can be considered the suburban fringe of Bangkok. PCSWMM previously has been applied to assess CSO quality and surface flooding in the village, so it was possible to build on the past studies. Analysis of daily rainfall data from the nearby Don Mueang International Airport showed the year 2000 as similar to the 30 year norm, 1980-2011, and therefore was used as the baseline against which climate change scenarios were compared. PCSWMM, run at hourly increments with the rainfall from 2000, suggested 11 of 218 nodes in the village would be flooded for more than 24 hours, with a total annual flood volume of 367,201m³ and total annual CSO volume of 1,522,200m³. Two approaches to assess possible impacts associated with climate change were followed. The first approach looked at a synthetic rainfall time series downscaled through the ECHAM4 regional climate model for IPCC emission scenario B2 and available from the Climate Data Distribution System, Thailand. The synthetic rainfall record was divided into three periods and hourly rainfall data were run through PCSWMM for the years 2021, 2061 and 2091. The second modeling approach simply increased the 2000 precipitation by 10% intervals between 10% and 40%. PCSWMM results using the ECHAM4 rainfall showed the number of nodes flooded for more than 100 hours increased by a total of 3, total annual flood volume progressively increased from 370,543 m³ to 483,059 m³ and total annual CSO volume increased from 1,883,118 m³ to 2,071,355 m³ between 2021 and 2091. Under the second (proportional rainfall increase) approach, flood volumes increased by 19, 39, 60 and 82 percentages from the baseline scenario with 10% rainfall increments between 10% and 40%. The increased annual flood volume in 2091 under the

ECHAM4 approach was similar to that generated by the 30% increase in rainfall scenario. PCSWMM results show that greater surface flooding and CSO discharges to a local drainage/irrigation canal (with attendant impacts to water quality) can be expected under the various climate change scenarios. Adaptation measures could include LID technologies as well as hard engineering options such as increased pumping capacity to clear flooded streets (although under extreme events such as observed in 2011 the pumps likely would have limited impact). Adaptation measures should be evaluated in detail using a dynamic modeling approach.