3.1. Factors Effecting Disaster Damage in Urban Area
The Tobit model is used to investigate the determinants of disaster damage in urban areas. The significance of the estimated model can be verified by the Likelihood Ratio (LR) Chi-Sqaure test. The test statistics results shown in
Table 2 indicates that the value of LR Chi-Square statistics is 151.38, and the null hypothesis that “all the estimation coefficients are 0” is rejected at the 1% significance level. Therefore, estimated Tobit model is statistically significant. Among the 370 observations, 202 observations are censored. The variance inflation factor (VIF) and the tolerance can be used to test the multicollinearities. The values of VIF and tolerance of the variables presented in
Table 3 are less than 3 and higher than 0.5, respectively, indicating that there is no multicollinearity problem in this model.
The estimated results of the Tobit model are shown in
Table 3. The estimated coefficient for urban green infrastructure (UG) ratio is negative and statistically significant at the 10% level. This results indicate that the change in UG ratio will have a positive effect on mitigating the disaster damage. The marginal effect also shows that 1% increase of UG ratio reduces the disaster damage by 3.71% at an average condition. This result is consistent with the results in previous studies of Kim et al. [
19], Farrugia et al. [
24], Mentens et al. [
15], and Gill et al. [
36]. They suggest that urban green infrastructures and spaces, such as street trees, rooftop greenery, parks, etc., can reduce the stormwater runoff and can be used as an alternative strategy to mitigate the disaster damage in urban area.
The estimated coefficient of all three variables (ATP, HHP, and WIND) of climatic factors are positive and statistically significant at the 1% level. The HHP has a greater influence than ATP on the increase of disaster damage. However the standard deviation from the baseline of ATP is 273.01 mm and of HHP is 13.40 mm that the variation of ATP is larger and the degree of its effect to increase the disaster damage is greater. On the other hand, the WIND showed an 8.67% increase in disaster damage with an increase of 1% in the ratio of strong windy days with respect to the total number of categorized windy days in a year. The characteristics of disaster in Korea are often accompanied by strong winds and heavy rainfall. Therefore, it is expected that the effects of increasing in both the rainfall and the wind speed simultaneously will have the greater effect on total disaster damages in Korea.
Two variables corresponding to the vulnerability factors, IMPA is statistically significant with a negative coefficient, and the coefficient value of ln(RIVER) is negative and not significant. The extent to which disaster damage changes with a unit change can be large by IMPA. This is unexpected result that the disaster damage is increased by 9.82% for 1% increase in the proportion of impervious area with respect to the total city area. This result is not consistent with previous studies of Choi [
37] and Choi and Seo [
29] that the change of urban land use with increasing impervious area leads to the greater risks of disaster damage. River areas are generally classified as disaster prevention facilities in urban areas. However, in the case of heavy storms and rainfall, the damage can increase with the high levels of river that cause flooding in the vicinity near the rivers. This may perhaps explain the insignificance of ln(RIVER) variable in the estimation. The result in the study of Choi [
37] derived the river area is a factor to increase the disaster damage.
All preventive factors, except ln(BASIN) and FINANCE, are statistically significant at 5% level. The retention basins are conventional disaster prevention facilities in urban areas [
38]. The results show that 1% increase in ratio of the length of sewer lines with respect to the total area of the city can reduce the damage by 0.01%. Insignificance of estimated ln(BASIN) coefficient can be explained with insufficient capacity design of the retention basin. Consistent urbanization and heavy rainfall cause runoffs exceeding the previously established retention basin capacity that results flooding damage near the retention basin area. Kim [
39] and Jeong et al. [
34] explained that the cause of degraded disaster prevention function of retention basin is due to insufficient capacity design that does not reflect the consistent progress of urbanization. For these reasons, it is necessary to design an effective disaster prevention facility in preparation for extreme rainfall due to climate change.
If the SEWER and ln(BASINS) are physical factors of prevention, then the FINANCE and BUDGET are economic factors. Only the BUDGET is statistically significant at a 1% level and has a positive effect on reducing the disaster damage. Insignificance of estimated FINANCE coefficient can be explained with an inconsistent relationship of financial independence with the investment in disaster mitigation. The results show that 1 billion won (US
$909,090) increase in annual budget can reduce damage by 0.55%. This results can be interpreted that the size of financial resource allocation for disaster prevention programs and facilities is closely relate to the capacity of disaster prevention. Kahn [
35] study also suggest that the cities with strong financial position can respond to the disaster more effectively, thereby reducing risks with more investments in facilities and activities for disaster mitigation.
3.2. Prediction of Damage under Climate Change Scenarios
According to the Korea Climate Change Report, the average annual precipitations in Korea are increased by 162.84 mm in the past 30 years [
40]. This increase in precipitation level is nearly four times compare to the global average, and it is expected that this trend will continue because of persistent climate change. Therefore, it is important to understand and predict the range of precipitation levels with the possible climate change scenarios.
IPCC addressed in their fifth comprehensive report of climate change that the extreme climate phenomenon with heavy storms and typhoons are likely to occur in coming years. They predicted that extreme precipitation will occur more frequently in most of the mid-latitude continent where Korea is located, and its intensity is likely to increase based on Representative Concentration Pathway (RCP) scenarios [
2]. Among the four scenarios (RCP 2.6, RCP 4.5, RCP 6.0, RCP 8.5) adopted by IPCC, RCP 4.5 and RCP 8.5 scenarios show a 4.1% and 5.9% increase of precipitation in the second half of 21st century compared to the precipitation level in the last 30 years.
This study pursues to estimate the disaster damage by applying the projected precipitation to the estimated model in the previous section. RCP 8.5 scenario is the most extreme scenario among the four RCP scenarios, assuming that current greenhouse gas emissions trends continue without any reductions. Under the RCP 8.5 scenario, the precipitation of the metropolitan areas in Korea is expected to increase by an average of 532.5 mm through 2050.
Table 4 shows the results of precipitation and disaster damage change in each metropolitan area using the estimated marginal effects in
Table 3 and the projected precipitation change under the RCP 8.5 scenario.
According to the results, the largest increase in damage occurs in Busan when annual precipitation increases under the RCP 8.5 scenario, followed by Ulsan, Gwangju, Seoul, Daejeon, Daegu, and Incheon, respectively. The expected damage in Busan is estimated to increase by 1098% compared to the current level of damage. This result is due to a relatively small amount of current precipitation in Busan, but, under the RCP 8.5 scenario, their precipitation is expected to be the largest in 2050 compared to other major metropolitan areas. In addition, according to the status of the metropolitan areas shown in
Table 5, the extreme wind speed and lower financial independence rate in Busan can also be considered to cause the greater damage in this city.
Table 6 presents expected regional economic damage in 2050 under RCP 8.5 scenarios. The results of seven metropolitan areas show that large increase in precipitation will cause greater economic damage. Busan is expected to have the economic damage with about 317 billion won (US
$288 million). Ulsan, where second largest damage is expected with 1000% increase, is estimated to have about 250 billion won (US
$227 million) in economic damage by 2050. Although Ulsan has a relatively small amount of current precipitation (1135.18 mm), their having the largest share of impervious areas (26.52 km
2) makes the city highly vulnerable to the disaster risks.
Meanwhile, Incheon is expected to have the smallest rainfall increase (about 293 mm) among the seven metropolitan areas. The damages are predicted to increase at the lowest level, up 295% from the current amount of damage. However, the expected economic damage in Incheon is about 5.6 billion won (US$5.1 million), which is larger than that of Daejeon and Daegu, where the expected increase in precipitation is greater. This result is due to larger current economic damage in Incheon (about 1.4 billion won (US$1.3 million)) compared to Daejeon and Daegu (about 1 billion won (US$909,090) and 0.02 billion won (US$18,200), respectively).
The change of urban green space ratio scenarios is also employed to observe its effect on regional damage and economic benefit changes under the RCP 8.5 scenario. Current urban green space ratios in seven metropolitan areas are in the range from 6.13 to 13.65%. Relevant urban green space ratio scenarios are necessary for each metropolitan areas to adopt in their urban green space planning strategies. Since most of the seven metropolitan areas are pursuing more green space to provide better amenity to their citizens, an ideal application is to present the scenarios from less to more aggressive strategies.
The first scenario employed in the analysis is 2% increase in urban green space ratio. From 2005 to 2013, the urban green space ratio is changed by about 2%, on average, in seven metropolitan areas. Based on these changes, more aggressive scenarios are applied with 5% and 10% increase in urban green space ratios. The results from these scenarios are presented in
Table 4 and
Table 6. All seven metropolitan areas showed decline in expected damages and increase in economic benefit with higher green space ratios. The effects are in the order of Busan, Ulsan, Gwangju, Seoul, Daejeon, Incheon, and Daegu. The damage in Busan is expected to decline from 1097 to 1013% with the 10% change in their urban green space. This effect leads to about 19.5 billion won (US
$17.7 million) in predicted economic benefit. Daegu has least effect from the scenarios with expected damage declines from 262 to 217% and an approximate 9 million won (US
$8200) increase in expected economic benefit.
Based on the results from the RCP 8.5 scenario and urban green space ratio scenarios, it is suggested that managing vulnerable factors with proper preparation strategies are important in the regions where the increase of precipitation is expected to be large under the climate change. Also countermeasures of the climate change in the regions with a large amount of current damage will be necessary because the damage caused by disasters in these regions can be significant.