ABSTRACT
The present paper assesses whether the extremely climate-intensifying rate (i.e., 14%/°C) for maximum rainfall in Bangkok is severely propagating into flooding. The impact evaluation involves the development of a flooding assessment method for a drainage system (DS) representative of the capital and the application of the developed procedure to assess the interested effect of the DS on its flooding. Considering the DS of Thammasat University (DS-TU) as the typical DS for the city, the popular storm water management model (SWMM) is then used for developing the dynamic simulation method against the most critical flooding condition of the DS-TU. Applying this SWMM for impact assessment of the intense rainfall rate has indicated that the rising temperature in excess of 2 °C would cause the gradient to severely propagate into the flooding because of the rapid increase in its associated peak, volume and duration properties. The propagated flooding has been shown to be further escalated, as a result of the clogging in the DS-TU. The clogging height that exceeded the 30% threshold of the full openings would intensify the volume and duration properties increasingly.
HIGHLIGHTS
The rising temperature limit that causes the flooding to be more severe than expected is found.
Considering the common clogging problem in a drainage system is proposed for flooding assessment.
INTRODUCTION
Flooding in Bangkok is one of the serious problems that causes increasingly huge direct and indirect economical losses every year. Management of the total flooding by diversion of partially incoming upstream flood and construction of flood-control polder system (Saito 2014; BMA 2020; Kiguchi et al. 2021) has currently changed the inundation problem to be due mainly to only the maximum storm rainfall of the study capital. This principal rainfall–flooding connection consequently causes the city inundation to be greatly influenced by global warming impact through the climate-intensifying precipitation shown by many previous investigations (Nguyen et al. 2007; Wilby et al. 2009; Trenberth 2011; Arnbjerg-Nielsen et al. 2013; Ashish 2013; Wasko & Sharma 2015; Wibig & Piotrowski 2018; Hamilton et al. 2019; Nguyen & Nguyen 2020; Agarwal et al. 2023a, b).
Recently, the physical-based Clausius–Clapeyron (C–C) relationship between maximum rainfall and associated temperature (Trenberth 2011; Wasko & Sharma 2015; Wibig & Piotrowski 2018) has been used to assess the climate change impact on the precipitation for the city, and the analyzed C–C relation is found to be quite extreme (Worawiwat et al. 2021). The increased scaling of the relation is almost double the normal 7%/°C rate that is usually used in most preceding studies on intensified flooding (Hettiarachchi et al. 2018; Alexander et al. 2019; Maimone et al. 2023). This, hence, raises an interesting question as to whether the extremely climate-intensifying rainfall is severely propagating via a drainage system (DS) into the city flooding.
Moreover, the intense climate impact on the flooding would be even more critical if the service level of the storm sewer system was considered. In particular, the continuous drain of runoff and wastewater flows under poor solid waste disposal in the capital causes a progressive increase in blockages of the DS. This steadily reduces the conveyance capacities of the system network and consequently increases the climate-related flooding. Although the explained clogging problem that additionally increases the climate effect is common (Qian et al. 2022; Quang et al. 2022), none of the preceding impact assessment studies for the city has, however, taken it into consideration.
Hence, the present paper develops a flooding assessment method for a DS representative in Bangkok and applies the developed procedure to evaluate the impact of the extremely increasing rainfall rate (i.e., 14%/°C) on the flooding of the typical DS. Among several different approaches available for forecasting and modeling flood flows (see, e.g., Niazi et al. 2017; Hettiarachchi et al. 2018; Zhou et al. 2018; Agarwal et al. 2021, 2022), the scheme of rainfall–runoff relation as the main cause and effect of the city flooding was then adopted for a modeling base when developing the assessment method in this study. Choosing the DS of Thammasat University, TU (Rangsit) to be a usual DS in the capital, its rainfall–runoff relationship for the most critical flooding condition in the DS-TU was then developed with the commonly used SWMM – i.e., storm water management model (Niazi et al. 2017). Application of the developed SWMM for the above-mentioned impact assessment has demonstrated that the rise in temperatures over 2 °C would bring about the propagation of the intense 14%/°C rate into severe flooding. In addition, it is also shown that the flooding of the extreme scaling would be further increased notably if the clogging in the DS-TU climbed over the 30% level of the fully opened condition.
METHOD
A method that could evaluate the impact of climate-intensifying rainfall on the most critical flooding via a typical DS for Bangkok has been developed in this section. Development of this assessment procedure composed of choosing the DS, building its general rainfall–runoff model and finding the condition for the most serious flooding in the system is briefly presented in the following.
A typical DS for Bangkok
Located 42 km north of Phahonyothin Road in the greater Bangkok area, the DS-TU enclosed by flood-control dykes was selected as a typical DS for the capital in this study. The chosen DS-TU possesses physical characteristics and soil types that are generally similar to those of most drainage networks used. Having been established in 1987, the DS-TU is responsible for the drainage of total wastewater and runoff flows over the whole TU area of 2.26 km2. The DS has been continuously improved to meet the increase in the total flows, as a result of converting open and green spaces into impervious areas during the development of the university. Moreover, following the 2011 Great Thailand flood, it was then surrounded by flood-control dykes with a height from 2.5 to 3.0 m above ground level implemented for preventing the university from inundation in the neighbors.
Schematic diagram of the current flood-control DS-TU consisting of a combination of storm sewers, ditches, canals, detention ponds, dykes, roads and a pump station. Note: The drainage facilities are designed using the maximum flow for 25-year return period. See basic information for these facilities in Haruyama (1993), Eamathicom & Ounsiri (2011) and Kongprasert & Lekpungthiam (2013).
Schematic diagram of the current flood-control DS-TU consisting of a combination of storm sewers, ditches, canals, detention ponds, dykes, roads and a pump station. Note: The drainage facilities are designed using the maximum flow for 25-year return period. See basic information for these facilities in Haruyama (1993), Eamathicom & Ounsiri (2011) and Kongprasert & Lekpungthiam (2013).
Model development
A rainfall–runoff simulation model of the DS-TU has been formulated using the comprehensive SWMM software of the U.S. Environment Protection Agency (Niazi et al. 2017) that is freely downloaded via www.epa.gov/water-research/storm-water-management-model-swmm. Following the schematic DS-TU diagram in Figure 1, the formulation of the SWMM started with delineating the whole university area into 79 sub-areas (see the partitioned blocks in the figure) and then totally assigning 466 computational nodes for the system considered (see the black circle dots in the figure). The bed elevation of the appurtenant drainage structures was then evaluated using the LIDAR dataset in 2-m resolution of the Royal Irrigation Department, Thailand (non-officially published data) together with the necessary information of the DS.
Subsequently, the model establishment adopted the rainfall–runoff relation of the National Resources Conservation Service, previously well-known as the U.S. Department of Agriculture Soil Conservation Service (SCS), for explaining the infiltration loss of the sub-watersheds (Rossman & Huber 2016). Accordingly, using the SCS loss that depended mainly on a combination of land uses and soil types would enable the formulated SWMM to be applicable for the impact assessment of urbanization, one of the most important environmental changes in hydrology (Anuthaman et al. 2023). In this formulation, the rainfall losses expressed in terms of runoff curve numbers (i.e., lowest to highest imperviousness ranges from 0 to 100) for all of the sub-areas were found to be between 79 and 98, provided that the underlying silty clay soil was type D (Haruyama 1993) and the land use information was taken from the appropriate Airbus satellite images in 2020 exported from Google Earth (Image ©2020 Airbus). Note that the level of soil moisture content was assumed to be normal.
Next, the Saint-Venant flow equations in one-dimensional form (Rossman 2017) were specified to simulate the flows in the DS-TU. Moreover, as they might be discharged gravitationally or reversely depending on a condition whether the surroundings were flooded or not, the accurate flow simulation of the SWMM was, hence, performed using the technique of fully dynamic wave routing. In addition, the pumping capacity for the DS-TU outflow was defined at the rate of 2 m3/s, and the 2-m flow depth upstream of the system outlet was also set as the threshold to start pumping.
The first- and second-quartile hyetographs of 25-year current storms for 1-h duration (Bangkok).
The first- and second-quartile hyetographs of 25-year current storms for 1-h duration (Bangkok).
The most critical condition of the model
As described earlier, the SWMM could simulate the university flooding that varies widely with the circumstance of nearby inundation and the type of storms striking. However, if we would like to use the SWMM as an impact assessment method of the interested climate change on flooding, it is, therefore, necessary to define the model for the flow simulation against only a condition that results in the most critical inundation. To find the worst flooding condition, we rationally expected that if the surroundings were flooded, the local inundation would be much more serious, as compared with that of the free-from-flooded neighbors. This is a consequence of the pumping capacity of 2 m3/s that is considerably smaller than the maximum free-outflow rate of 16.5 m3/s.
Following the rationale discussed above, the SWMM of the flooded surroundings was then used to further evaluate the effect of a few different storms. Such storms considered were the fast-increasing and decaying storm mentioned earlier, and the slow-rising and receding one (see their rainfall hyetographs in Figure 2). Results of the simulated inundation cases were later compared with those of the first-quartile storm for the missing nearby flooding condition. Particularly, the comparisons were performed using a total flooding hydrograph and its characteristics such as peak, volume and duration properties.
The total flooding hydrographs of the first- and second-quartile storms for the DS-TU with or without the inundated surroundings.
The total flooding hydrographs of the first- and second-quartile storms for the DS-TU with or without the inundated surroundings.
In addition, the worst flooding circumstance classified is further verified by the referred characteristics of the hydrographs. Table 1 shows the normalized hydrograph peak, volume and duration of the flooded surrounding condition for the first- and second-quartile storms. Notice that the normalization is the ratio of the actual property considered over the corresponding base of the dry condition for the first-quartile storm. Evidently, the table indicates that the condition of the surroundings is still substantiated to be the most serious local flooding case because of the increase in the peak and volume characteristics by, at least, 25% from the conventional bases. Also, the table confirms the small effect of the different storms as the normalized properties of both storms are quite close to each other.
Normalized hydrograph peak, volume and duration of the flooded surrounding condition for the first- and second-quartile storms
Storm . | Peak . | Volume . | Duration . |
---|---|---|---|
First quartile | 1.26 | 1.26 | 1.04 |
Second quartile | 1.29 | 1.26 | 1.03 |
Storm . | Peak . | Volume . | Duration . |
---|---|---|---|
First quartile | 1.26 | 1.26 | 1.04 |
Second quartile | 1.29 | 1.26 | 1.03 |
Note: The normalization is the ratio of the actual property considered over the appropriate base of the dry condition for the first-quartile storm.
RESULTS AND DISCUSSION
Considering the SWMM of the most critical condition, the propagation of the extreme climate-intensifying rate for maximum rainfall in Bangkok (Worawiwat et al. 2021) into the flooding has been investigated. The examination used the super C–C rate of 14%/°C to increase the first-quartile 25-year existing storm (see Figure 2) over a few degree rise in local temperature (IPCC 2007, 2014). The time distribution of the derived storms followed the temporal pattern of the NOAA in which their hyetographs were later modified by building an empirical dynamic relationship between the incremental storm rainfall and associated temperature into them (Trenberth 2011; Wasko & Sharma 2015).
The first-quartile storm of the extraordinary 14%/°C rate for the 1 °C rising temperature, and the net increase of the storms for the higher temperatures from the 1 °C base.
The first-quartile storm of the extraordinary 14%/°C rate for the 1 °C rising temperature, and the net increase of the storms for the higher temperatures from the 1 °C base.
Subsequently, the intensified storms were considered for the investigation of the extreme climate impact. Two types of examination depending on the clogging condition of the DS-TU were performed. First, if the DS was assumed to be fully opened, this investigation would be carried out for the pure climate effect. For the second examination, we have proposed the system to be partially clogged. Being the usual system condition but having been often ignored in most preceding studies, the proposal was, hence, examining the additional climate impact in association with the clogging level.
Fully opened system
Comparison of the total flooding hydrographs propagated by the climate-intensifying rate of (a) 14%/°C and (b) 7%/°C for a few degrees Celsius in rising temperature with that transmitted via the current climate condition.
Comparison of the total flooding hydrographs propagated by the climate-intensifying rate of (a) 14%/°C and (b) 7%/°C for a few degrees Celsius in rising temperature with that transmitted via the current climate condition.
Increase per degree Celsius of the hydrograph peak, volume and duration properties
Rising temperature (°C) . | Peak (m3/s) . | Volume (×103 m3) . | Duration (min) . | |||
---|---|---|---|---|---|---|
7%/°C . | 14%/°C . | 7%/°C . | 14%/°C . | 7%/°C . | 14%/°C . | |
0–1 | 2.02 | 4.69 | 7.36 | 15.82 | 14 | 32 |
1–2 | 3.21 | 6.37 | 9.14 | 21.51 | 18 | 52 |
2–3 | 2.45 | 11.95 | 10.5 | 32.75 | 24 | 64 |
Rising temperature (°C) . | Peak (m3/s) . | Volume (×103 m3) . | Duration (min) . | |||
---|---|---|---|---|---|---|
7%/°C . | 14%/°C . | 7%/°C . | 14%/°C . | 7%/°C . | 14%/°C . | |
0–1 | 2.02 | 4.69 | 7.36 | 15.82 | 14 | 32 |
1–2 | 3.21 | 6.37 | 9.14 | 21.51 | 18 | 52 |
2–3 | 2.45 | 11.95 | 10.5 | 32.75 | 24 | 64 |
Increasing 14 and 7%/°C trends of hydrograph peak, volume and duration characteristics over the rising temperatures. Note: The normalization is the ratio of the characteristic considered over the corresponding base of the present-day climate condition.
Increasing 14 and 7%/°C trends of hydrograph peak, volume and duration characteristics over the rising temperatures. Note: The normalization is the ratio of the characteristic considered over the corresponding base of the present-day climate condition.
Clogged system
The 14%/°C intensified hydrographs of the assumed clogging levels and the fully opened condition for (a) T+1°C, (b) T+2°C and (c) T+3°C in rising temperature.
The 14%/°C intensified hydrographs of the assumed clogging levels and the fully opened condition for (a) T+1°C, (b) T+2°C and (c) T+3°C in rising temperature.
Increase of the characteristics for each interval of successive clogging levels
Clogging level . | Peak (m3/s) . | Volume (×103 m3) . | Duration (min) . | ||||||
---|---|---|---|---|---|---|---|---|---|
T + 1 °C . | T + 2 °C . | T + 3 °C . | T + 1 °C . | T + 2 °C . | T + 3 °C . | T + 1 °C . | T + 2 °C . | T + 3 °C . | |
None to low (30%) level | 2.84 | 3.98 | 4.03 | 11.73 | 11.97 | 7.26 | 58 | 42 | 38 |
Low to moderate (50%) level | 2.87 | 3.40 | 3.33 | 14.70 | 15.26 | 14.45 | 144 | 132 | 104 |
Moderate to high (70%) level | 3.54 | 3.30 | 4.74 | 26.67 | 29.17 | 31.56 | 94 | 114 | 198 |
Clogging level . | Peak (m3/s) . | Volume (×103 m3) . | Duration (min) . | ||||||
---|---|---|---|---|---|---|---|---|---|
T + 1 °C . | T + 2 °C . | T + 3 °C . | T + 1 °C . | T + 2 °C . | T + 3 °C . | T + 1 °C . | T + 2 °C . | T + 3 °C . | |
None to low (30%) level | 2.84 | 3.98 | 4.03 | 11.73 | 11.97 | 7.26 | 58 | 42 | 38 |
Low to moderate (50%) level | 2.87 | 3.40 | 3.33 | 14.70 | 15.26 | 14.45 | 144 | 132 | 104 |
Moderate to high (70%) level | 3.54 | 3.30 | 4.74 | 26.67 | 29.17 | 31.56 | 94 | 114 | 198 |
Increasing tendency of (a) the hydrograph peak property, (b) the volume attribute and (c) the duration characteristic over several different clogging levels and various rising temperatures. Note: The normalization is the ratio of the characteristic considered over the corresponding base of the current climate condition.
Increasing tendency of (a) the hydrograph peak property, (b) the volume attribute and (c) the duration characteristic over several different clogging levels and various rising temperatures. Note: The normalization is the ratio of the characteristic considered over the corresponding base of the current climate condition.
CONCLUSIONS
Currently managed flooding in Bangkok has been known to closely link with local maximum storm rainfall. This resulting linkage mainly causes the city inundation to certainly increase because the precipitation is usually intensified by the impact of global warming. Recent analysis of the climate-intensifying rate of the rainfall over each degree of rising temperature for the capital has found the gradient to be so extremely high (i.e., almost double the normal 7%/°C scaling) that none of the previous flooding investigations have taken it into account but most of the pasted studies have considered only the 7%/°C scaling. Hence, understanding how the intense rainfall would propagate through a typical DS into flooding is necessary for sustainable management of the city inundation in the context of climate change.
Studying the propagation of the increased rainfall into the flooding necessitates the development of a general rainfall–runoff model that simulates the flows of the most critical flooding condition for a chosen DS representative in the capital. In this paper, the DS-TU in the flood-control polder was, at first, adopted and then modeled its rainfall–runoff relation using the traditional SWMM base. Calibration of the formulated SWMM was later performed by matching the simulated flooding hotspots resulting from the flows generated by the first-quartile 25-year current storm to the observed counterparts. Applying this developed SWMM for different storms and nearby flooding circumstances has identified the case of the inundated surroundings to be the most critical condition for the DS-TU. The condition gives the flooding hydrograph peak and volume that is approximately 25% greater than those of the other dry case.
The most critical SWMM has been later applied to assess how the extreme 14%/°C rainfall is propagating into flooding. The first-quartile hyetograph of a 25-year intense dynamic storm for a rise in temperatures ranging from 1 to 3 °C was then constructed for simulation of the flooding situations using the SWMM. Comparing the total flooding hydrograph and appropriate properties extracted (e.g., peak, volume and duration) of the increased storms with those of the normal 7%/°C counterparts have shown that the climb of the rising temperature above the threshold of 2 °C would lead the impact of the extreme rate to severely transmit into the flooding. This is because the intensified flooding characteristics are exponentially greater than those of the 7%/°C rate.
The impact of the 14%/°C intensifying rainfall has been further evaluated by considering a common clogging problem that is often ignored in previous studies. Accordingly, three different clogging levels (i.e., 30, 50 and 70% of the full openings) in the DS-TU were initially assumed, and the flooding situation of each considered level was subsequently simulated using the assessment SWMM. A comparison of these flooding results has demonstrated that taking the clogging problem into account additionally increases the flooding attributes of the extreme rate. The addition would be large and accelerating for the volume and duration properties if the clogging exceeded the low 30% level.
ACKNOWLEDGEMENTS
We would like to thank Thammasat University for supporting data on the study storm sewer system. Also, we appreciate two anonymous reviewers for their valuable comments regarding this paper.
AUTHOR CONTRIBUTIONS
AW contributed to programming software, and performing experimental cases. CC contributed to designing experiment, analyzing results, and writing and editing this paper.
FUNDING
Financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0090/2560) is acknowledged. The authors also acknowledge Thammasat University for additional research funding in fiscal year 2023.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.