A Study on Coastal Flooding and Risk Assessment under Climate Change in the Mid-Western Coast of Taiwan

Tai-Wen Hsu, Dong-Sin Shih, Chi-Yu Li, Yuan-Jyh Lan, Yu-Chen Lin
2017 Water  
This study integrated coastal watershed models and combined them with a risk assessment method to develop a methodology to investigate the impact resulting from coastal disasters under climate change. The mid-western coast of Taiwan suffering from land subsidence was selected as the demonstrative area for the vulnerability analysis based on the prediction of sea level rise (SLR), wave run-up, overtopping, and coastal flooding under the scenarios of the years from 2020 to 2039. Databases from
more » ... al gauges and satellite images were used to analyze SLR using Ensemble Empirical Mode Decomposition (EEMD). Extreme wave condition and storm surge were estimated by numerical simulation using the Wind Wave Model (WWM) and the Princeton Ocean Model (POM). Coastal inundation was then simulated via the WASH123D watershed model. The risk map of study areas based on the analyses of vulnerability and disaster were established using the Analytic Hierarchy Process (AHP) technique. Predictions of sea level rise, the maximum wave condition, and storm surge under the scenarios of 2020 to 2039 are presented. The results indicate that the sea level at the mid-western coast of Taiwan will rise by an average of 5.8 cm, equivalent to a rising velocity of 2.8 mm/year. The analysis indicates that the Wuqi, Lukang, Mailiao, and Taixi townships are susceptive, low resistant and low resilient and reach the high-risk level. This assessment provides important information for creating an adaption policy for the mid-western coast of Taiwan. which can be seen in both the observational historical records, from the late nineteenth century, and with paleoclimatic records spanning the last millennia. These changes are manifested by the warming of the atmosphere and oceans, by a decrease in the mass of the cryosphere, and by an increase in the concentrations of atmospheric greenhouse gases, among other types of processes [3] . The estimated average global flood losses are approximately US$6 billion per year, increasing to US$52 billion by 2050 with projected socio-economic change alone [4] . Recent studies concluded that climate scenarios from General Circulation Models (GCMs) or Regional Climate Models (RCMs) are the largest uncertainty for the estimation of future flows [5] [6] [7] [8] . More studies have furthermore pointed out that climate change has been shown to affect local rainfall amounts, surface runoffs, and distribution of water resources [9] [10] [11] [12] . This is typically characterized by shifts in temperature and precipitation, in response to the specifics of the regions, e.g., alteration of extremes, intensities, frequencies, and spatial and temporal patterns [13] [14] [15] . Climate change alters the risk of hydrological extremes on regional scales, and the hydrological response of a catchment can vary substantially, not only due to its location but also depending on the characteristics of the catchments [16] [17] [18] [19] . During the past few decades, the upward historical trends of sea level rise (SLR) quantified from a small set of California tide gauges to a value of approximately 20 cm/century. This is very similar to the estimated value of the global mean sea level [20] . It is attributed primarily to global climate change and associated with the land situations [20] [21] [22] [23] . So, using a dynamic modeling framework to examine the effects of global climate change, and SLR in particular, on tropical cyclone-driven storm surge, inundation is implemented [15, [24] [25] [26] [27] . The number of people estimated to be affected by flooding in 2100 is 16-388 million people/year for the mid (55-cm) global-mean SLR scenarios and up to 510 million people/year for the high (96-cm) scenario [28] . On the northern Gulf of Mexico, the total inundated land area increases by 87% and peak surge increases by as much as 1 m above the applied SLR in some areas, and other regions were subject to a reduction in peak surge with respect to the applied SLR, indicating a nonlinear response [25] . Passeri et al. [27] addressed that tidal amplitudes within the bays increased by 67% of 10.0 cm under the highest SLR scenario, and the ratio of the maximum flood to maximum ebb velocity decreased in the future scenarios by 26% and 39%. Bilskie et al. [24] showed that the storm surge response to SLR is dynamic and sensitive to changes in the landscape. Hovenga et al. [15] presented the effects of climate change on overland processes, river inflow, and sediment loading for the Apalachicola region. For the same region, Chen et al. [29] reported that the seasonal responses of runoff and sediment loads are slight, with contrasting behaviors from different models. However, flow increased from the baseline by 8% using model simulation. Previous researches have shown that tidal flows and storm surge dynamics really depend on future SLR scenarios [24, 26, 27] . Taiwan is located at the hub of the typhoon route in the West Pacific Ocean and is frequently struck by typhoons and tropical depressions with raging storms and pouring rains during the summer and autumn seasons. According to past century typhoon records from the Central Weather Bureau (CWB) in Taiwan, there are on average 4.0 typhoons per year in Taiwan. It is also noted that the strength and the scale of a typhoon has increased with the decreased center pressure in the last 14 years. This brings about heavy rains, big waves, storm surges, and floods in the coastal areas. The related long-term observations indicate that the SLR increases at a speed of 3.32 mm/year [24, 30, 31] . Statistics of historical data from different stations also confirm the increasing rising trend. A global rate of 3.36 ± 0.41 mm/year over the 14 years from 1993 to 2007 is investigated, but the regional sea level trend comparisons for the time periods of 1993-1999 and 1999-2005 reveal strong basin-scale polarities and pronounced inter-decadal variability, with a relative increase in the global mean SLR trend of 1.5 ± 0.7 mm/year in the latter seven years [30] . Water level changes due to storm surges and high tides resulting from typhoons often cause severe disaster to the coastal areas of Taiwan [32] . The storm surges and high tides directly striking the coasts bring about serious beach erosion and waves overtopping the seawall, causing seawater intrusion and floods in the coastal areas. These damage the crops, the fishing farms, life, and properties. They also have a colossal negative impact on the development of marine
doi:10.3390/w9060390 fatcat:rd5p6s3grjf25pzyk426n562bq