Impact of climate change on urban drainage system

The Experience of flooding in Bangkok riparian city tends to increase with warming temperatures due to the impact of climate change. Hence, this study aims (i) to propose a hydrologic approach for assessing the impact of climate change on extreme precipitation and flooding under the short record available, high variability and changes in urban conditions under stationary catchment flooding characteristics, (ii) to demonstrate the feasibility of these proposed approaches by applying them to quantify the impact of global warming on extreme precipitation and flooding in Bangkok, (iii) to investigate the hydraulic performance of a typical storm sewer system in an experimental urban polder using the global warming obtained from the previous objective and (iv) to propose appropriate low-impact development solutions for mitigating local flooding intensified by warming temperatures. In this study, proposed hydrologic linkages consist of two parts: the linkage of global warming to extreme precipitation and to the probability of flood occurrence. For the first, the Clausius-Clapeyron (CC) relationship was provided as a linkage between dominant temperature and extreme precipitation. This linkage can perform under the short record with high variability by applying a robustness quantile regression estimation through regional pooled data. Seven years (2011-2017) a high-resolution series of 23 rainfall-climate data in Bangkok was used to examine the proposed linkage. Results show that extreme rainfall will increase with rising local temperatures by approximately 12%/°C and 4%/°C for the east and west of Bangkok, respectively, due to warmer climates and urban activities. The second part of the method attempted to demonstrate whether warming temperatures were a result of rising urban flooding. Here, the Bayes theorem was applied to ascertain the conditional probability of flooding given the wet-day temperature. This proposed linkage is based on the assumption that catchment flooding characteristics are stationary and inlet capacity is a controlled flooding threshold. Six years (2012–2017) of flooding depth data from ten stations in Bangkok were matched to those of the climate-rainfall data at the proximity station. This study found that warming temperatures result in rising flood occurrence processes with an average rate of increase of 1.6%/°C. Moreover, this flooding rate was applied to five global circulation models to project the flood occurrence in Bangkok. The analysis of multiple lines of evidence indicates that flooding in Bangkok tends to increase with the relative increase in flood occurrence from the current temperature of 10%, 25% and 50% for the near-, mid- and long-period (2030s, 2060s and 2090s) respectively. To quantify the drainage performance, an experimental greater Bangkok polder, Thammasat University Drainage System (TUDS) was selected to ascertain the impact of intensification in rainfall rates by the warming temperature via the SWMM program. The peak, volume and duration of the flood were measured to criticize the worsening flood scenario by varying the drainage conditions (i.e., free flow and pumping conditions) and storm patterns. The critical scenario was then applied to quantify the impact of global warming on an examined catchment. Results indicate that the 2nd quartile storm fell on pumping conditions made the catchment at the highest risk of flooding. Moreover, the storms intensified by warming temperatures with rates of 7%/°C and 14%/°C were applied to such critical catchment conditions. Analysis results indicate that the flooding characteristics exponentially exacerbate with warming temperatures and the doubled rate can intensify the flooding characteristics by doubling. At the expected relative increase in temperature of 3°C, the peak and volume of flooding were exacerbated by approximately 2 and 4 times (compared to the current temperature) for rates of 7%/°C and 14%/°C respectively. To mitigate this impact, enhanced pumping capacity, combined green roof-rain barrel and rain garden were applied to the system to identify the performance of each strategy. From among them, the result indicates that the combined green roof-rain barrel approach better performs in reducing the exacerbation of overall flooding characteristics by warming temperature with the relative increase in mitigation degree up to 1.5°C under the intensifying rate of 14%/°C. Furthermore, when all mitigation strategies are applied to the catchment, the result indicates that the compensation degree for the relatively rising temperature is improved up to 2°C