geosciences
Article Tropical Cyclone-Induced Hazards Caused by Storm Surges and Large Waves on the Coast of China
Zai-Jin You 1,2 1 Institute of Ports and Coastal Disaster Mitigation, Ludong University, Yantai 264025, China; [email protected]; Tel.: +61-490661759 2 School of Civil Engineering, University of Queensland, Brisbane, QLD 4072, Australia
Received: 20 January 2019; Accepted: 13 March 2019; Published: 18 March 2019
Abstract: The mainland coast of China is about 18,000 km long and houses about 70% of China’s largest cities and 50% of its population. For the last few decades, the rapid growth of the Chinese economy has resulted in extensive development of the coastal infrastructure and property, large-scale expansion of coastal ports, excessive reclamation of coastal land, and a significant increase in the coastal population. Previous studies have indicated that tropical cyclones (TCs) have struck the coast of China at a higher frequency and intensity, and TC-induced coastal hazards have resulted in heavy human losses and huge losses to the Chinese coastal economy. In analyzing the long-term and most recent coastal hazard data collected on the coast of China, this study has found that TC-induced storm surges are responsible for 88% of the direct coastal economic losses, while TC-induced large coastal waves have caused heavy loss of human lives, and that the hazard-caused losses are shown to increase spatially from the north to south, peak in the southern coastal sector, and well correlate to storm wave energy flux. The frequency and intensity of coastal hazards on the coast of China are expected to increase in response to future changing TC conditions and rising sea levels. A simple two-parameter conceptual model is also presented for the assessment of coastal inundation and erosion hazards on the coast of China.
Keywords: storm surge; tropical cyclone; coastal erosion; inundation; coastal storm
1. Introduction There is about 50% of the Chinese national population living and working along the 18,000 km mainland coast of China. Although the coastal land area is less than 14% of the territory of China, it creates more than 50% of the national wealth, 45% of the state-owned assets, 64% of the township enterprise assets, and 60% of the national social wealth [1]. For the last few decades, the rapid economic growth in China has resulted in extensive development of the coastal infrastructure and property, excessive reclamation of coastal land, huge expansion of coastal ports, and a significant increase in the coastal population. Furthermore, there are also increasing pressures to further develop Chinese coastal areas for residential, commercial, tourism, and recreational purposes. Tropical cyclones (TCs) strike with higher frequency and intensity in China than any other country in the world [2]. There are about 6–10 landfall TCs in a typical year on the coast of China. TCs that strike East and South-East Asia have intensified by 12–15%, with the proportion of storm categories 4 and 5 having doubled or even tripled [3,4]. The high landfall frequency and intensity of TCs is due to the facts that the coastline of China is oriented in the west of the northwest Pacific Basin, which has a high frequency and intensity of TCs, and furthermore the coastline is quite long and is oriented in a north–south direction thus exposing to the dominant direction of TCs. The Meteorological Administration of China maintains the Tropical Cyclone Data, including typhoon track data, satellite reanalysis data, typhoon landfall data, wind data, and rainfall data. The globally averaged intensity of
Geosciences 2019, 9, 131; doi:10.3390/geosciences9030131 www.mdpi.com/journal/geosciences Geosciences 2019, 9, 131 2 of 11
TCs is projected to continue to shift toward stronger storms with an expected increase of 2% to 11% by 2100 [5]. TC-induced storm surges are a major coastal hazard, especially in the three provincial sectors of Zhejiang, Fujian, and Guangdong all situated on the southern coast of China. The storm surge hazard on the coast of China is a regional paroxysmal one that can cause enormous loss of human lives, property, and infrastructure. The highest rate of low-magnitude surges is observed in the western North Pacific, as the coast of China has 54 surges larger than 1 m per decade [6], and the prominent consequences from the large storm events could remain for many years. In 2014, the China Sea Level Bulletin stated that from 2004 to 2006, more frequent and higher intensity storm surges were responsible for the greatest losses of coastal property in the provincial coastal sectors of Zhejiang and Fujian, and the losses were more serious than those in the historical records. The increasing economic losses on the coast of China are also found to be associated with the rapid socio-economic development of the coastal areas; global climate change and sea level rise may also play an important role [7,8]. The storm surge hazard has resulted in severe damage to the Chinese coastal economy and heavy human losses. For example, on 21 August 1994, TC (No. 9417) hit the coast of Zhejiang Province, resulting in 1248 deaths, the flooding of 189 coastal towns, affecting 22.8 million people, and causing a direct economic loss of US $2.5 billion [9]. In the East Asia and Pacific region, the coast of China is expected to be a high-risk area due to the future storm surge hazard. Even though numerous studies were undertaken to investigate TC-induced storm surges on the coast of China, the great impacts of storm waves and their elevated mean water levels were often not considered in the modeling of coastal inundation [10]. TC-induced coastal erosion is another major hazard on the coast of China. About 46% of the Bohai Sea coastline, 49% of the Yellow Sea coastline, 44% of the East China Sea coastline, and 21% of the South China Sea coastline have severe erosion problems [11]. The shoreline retreat in low-lying areas around the Shandong Peninsula is greatly accelerated, and a maximum shoreline retreat of 300 m/year is found around the mouth of the Yellow River. This has resulted in Chinese national land losses of 14.3 ha and economic losses of US $50 million per year [12]. Even though several types of physical processes-based numerical models, such as X-Beach, have been used to estimate coastal storm erosion rates on the coast of China [13], how to predict long-term shoreline changes especially for some of the muddy coastal regions remains unsolved. On the basis of the long-term and most recent coastal hazard data collected by the State Oceanic Administration of China (SOA) from 1989 to 2017, this study is undertaken to systematically investigate the major coastal hazards and their unique drivers, determine the impact of storm waves and different timescales of mean water levels on the coast of China, and finally present a simple two-parameter conceptual model for the assessment of coastal inundation and erosion hazards on the coast of Yantai, China.
2. Study Area The study area is the mainland coast of China, which consists of eight provincial coastal sectors in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea (see Figure1). In addition, one provincial Island of Hainan (8) is also included in this study. In considering the geomorphic characteristics of the coastline, Hangzhou Bay, which is located on the dashed red line in Figure1, divides the mainland coastline into two geomorphic regions: southern and northern. In the northern region of the coast, some of the high mountains and hills and the Shandong Peninsula extend to the coastal zone, but most of them are very flat and low-lying (e.g., the Yangtze River Delta region) with an average level of 2–5 m above mean sea level (MSL) and are protected by seawalls or embankments. In the southern region of the coast, the coastline is more irregular, rocky and steep, more than half of the southern coastline being predominantly rocky, and most of the remainder is sandy and muddy. The provincial coastal sector of Shandong (3) has more sandy beaches than the other coastal sectors. Geosciences 2019, 9, x FOR PEER REVIEW 3 of 11 diurnal in several coastal regions, while tides in the Yellow Sea and East China Sea are mainly semidiurnal with a large and variable tidal range. In the South China Sea, tides are more complex, varying from diurnal to semidiurnal. The spring tidal ranges (Ω = 2.5–5 m) in the middle coastal sectors of (4)–(6) are higher than those (Ω < 2 m) in the other coastal sectors of (1)–(3) and (7)–(9) [15]. Wave patterns on the coast of China are determined by monsoon wind patterns, wind blowing fetch, and swell penetration. In general, northerly coastal waves prevail in the winter, moving progressively southwards from the Yellow Sea and spanning the entire mainland coast of China; southerly coastal waves are dominant in the summer, moving northward from the South China Sea, and there are no prevailing waves when wind directions are variable during monsoon transitional periods [14]. Significant wave heights and periods (Hs = 1.0–1.2 m, Ts = 5–7 s) in the southern coastal sectors of (5)–(8) are generally larger than those (Hs < 1 m, Ts < 4 s) in the other coastal sectors of (1)– (4) and (9) [16]. TCs on the coast of China generally develop from the north-western Pacific, and there are about 6–10 landfall TCs in a typical year, with one to two additional bypassing storms coming close enough to the coast of China to cause significant damage [17], e.g., three extreme storms in 1956, 1969, and 1994 resulted in more than 7400 human casualties. The historical best-track typhoon Geosciencesdatasets 2019 are, 9 maintained, 131 by the Tropical Cyclone Data Centre in the China Meteorologica3 of 11l Administration, which provides six-hourly typhoon locations and intensities [18].
Figure 1. The study area includes eight mainland provincial coastal sectors (1) (1)–(7)–(7) and (9) and one island provincial coastal sector (8) in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea.
3. MainAstronomic Coastal Hazards tides on theand coast Analysis of China are variable in type and amplitude [14]. Tides in the Bohai Sea are predominantly semidiurnal although they are also irregular semidiurnal or irregular diurnal in 3several.1. Coastal coastal Hazard regions, Datawhile tides in the Yellow Sea and East China Sea are mainly semidiurnal with a largeThe and coastal variable hazard tidal range. data In have the South been collectedChina Sea, on tides the are coast more of complex, China by varying the State from Oceanic diurnal Administrationto semidiurnal. Theof China spring (SOA) tidal rangessince 1989. (Ω = There 2.5–5 m)are in six the types middle of main coastal coastal sectors hazards of (4)–(6) investigated, are higher namelythan those storm (Ω 3. Main Coastal Hazards and Analysis 3.1. CoastalGeosciences Hazard 2019, 9, Data x FOR PEER REVIEW 4 of 11 Thedata coastalare about hazard hazard-caused data have losses been to the collected coastal economy on the and coast the loss of Chinaof human by lives the on State the coast Oceanic Administrationof China from of China1989 to (SOA)2017. since 1989. There are six types of main coastal hazards investigated, namely stormThere surges, are four large sets waves,of the more algal recent blooms, coastal sea hazard ice, oil data spill, used and for coastal this study erosion. collected The by hazard SOA: data are about(1) historical hazard-caused data on annual losses losses to the to coastalthe national economy coastal andeconomy the loss and ofhuman human losses lives (1989 on–2017) the coast; (2) of spatial data on annual losses to the provincial coastal economy and human losses (2008–2016); (3) China from 1989 to 2017. historical data on annual losses to the national coastal economy caused by different types of coastal There are four sets of the more recent coastal hazard data used for this study collected by SOA: hazards (2001–2014); and (4) historical data on annual losses of human lives due to storm surges and (1) historicallarge waves data (2001 on annual–2016). lossesThese datasets to the national are published coastal annually economy in the and Oceanic human Disaster losses (1989–2017); Bulletin (2) spatialAnnual data Report on annualby SOA. losses In the coastal to the provincialhazard data coastalused in this economy study, andthe direct human losses losses to the (2008–2016); coastal (3) historicaleconomy data and on human annual losses losses are tocaused the national only by the coastal first economyfive coastal caused hazards by discussed different above, types while of coastal hazardsthe (2001–2014);direct economic and loss (4)es caused historical by co dataastal onerosion annual can’ lossest be assessed of human in this lives study due as tothere storm are no surges and largelong- wavesterm data (2001–2016). available. These datasets are published annually in the Oceanic Disaster Bulletin Annual Report by SOA. In the coastal hazard data used in this study, the direct losses to the coastal 3.2. Historical Variation of National Economic Losses economy and human losses are caused only by the first five coastal hazards discussed above, while the directThe economic long-term losses variation caused of annual by coastal direct erosion economic can’t losses be assessedfor the entire in this coast study of China, as there which are no long-termwas caused data available. by all five types of coastal hazards, is shown in Figure 2A, where historical inflation has been considered. The inflation rate in China is taken from the National Bureau of Statistics of China, 3.2. Historicaland all hazard Variation-caused of National economic Economic losses in Losses different years of the hazard data are converted to the values relative to the most recent year 2017. The long-term average of annual direct economic losses Theis about long-term US $2.6 variation billion/year of from annual 1989 direct to 2017. economic Four most losses severe for economic the entire losses coast greater of China, than which US $5 was causedbillion by all occurred five types in 1994, of coastal 1996, 1997 hazards,, and 2005, is shown of which in Figurethree of2 A,them where occurred historical in a four inflation-year period has been. considered.Although The the inflation hazard- ratecaused in Chinaeconomic is takenlosses fromhave thebeen National shown to Bureau decrease of since Statistics 2005, of they China, remain and all hazard-causedquite high, economic i.e., above lossesUS $700 in million different every years year. of the hazard data are converted to the values relative to the mostThe recent long- yearterm 2017.variation The of long-term annual human average casualties of annual caused direct by all economic five types losses of coastal is about hazard USs $2.6 billion/yearis also presented from 1989 in toFigure 2017. 2B Four. The mostloss of severe human economic lives, which losses is averaged greater from than 1989 US to $5 2017, billion is about occurred 240 persons per year. The highest number of human casualties, which is more than 500 human in 1994, 1996, 1997, and 2005, of which three of them occurred in a four-year period. Although the deaths, occurs in 1989, 1994, 1996, and 1999. Especially in 1994, when there were 1248 human hazard-caused economic losses have been shown to decrease since 2005, they remain quite high, i.e., casualties caused by TCs. Since 2006, the number of annual human casualties is shown to have abovereduce US $700d greatly, million but every still remains year. quite high, i.e., above 20 deaths per year. 7 1400 (A) (B) (A) 6 1200 (B) 5 1000 4 800 3 600 ic Damage (US$ Billon) 2 400 Losses of Human Lives (Person) 1 200 Direct Econom 0 0 1989 1994 1999 2004 2009 2014 1989 1994 1999 2004 2009 2014 YEAR YEAR FigureFigure 2. Historical 2. Historical coastal coastal hazard hazard data on: data (A on:) annual (A) annual direct direct economic economic losses; losses and; ( Band) human (B) human casualties causedcasualties by all coastal caused hazardsby all coastal on the hazards entire on coast the entire of China coast (1989–2017). of China (1989–2017). The3.3. Spatial long-term Variation variation of Provincial of annual Economic human Losses casualties caused by all five types of coastal hazards is also presentedThe mai in Figurenland 2coastB. The of lossChina of consistshuman lives, of eight which provincial is averaged coastal from sectors 1989 (see to 2017,Figure is 1) about; one 240 personsprovi perncial year. island The of highest Hainan number(8) is also ofconsidered human casualties,in this study. which On the is basis more of than the available 500 human coastal deaths, occurshazard in 1989, data 1994, (2008 1996,–2016) and on 1999. the Especiallyannual provincial in 1994, coastal when economicthere were losses, 1248 humanFigure 3 casualtiesA shows the caused spatial variation of annual provincial coastal economic losses from the first coastal sector of Liaoning Geosciences 2019, 9, x FOR PEER REVIEW 5 of 11 Geosciences(1) in the2019 far ,north9, 131 to the last coastal sector of Guangxi (9) in the far south. The direct economic losses5 of 11 in Figure 3A are seen to increase from the north to south and peak in the southern coastal sector (7). The coastal sector of Guangxi (9) is well protected by Hainan Island (8). The most severely affected bycoastal TCs. provinces Since 2006, are the identified number of and annual ranked human as Guangzhou casualties is (7), shown Fujian to have(6), Zhejiang reduced greatly,(5), Jiangsu but still(4), remainsand Hainan quite (9) high, based i.e., on above the criterion 20 deaths of pereconomic year. losses. Similarly, the spatial variation of annual provincial human casualties from the north to south is 3.3. Spatial Variation of Provincial Economic Losses shown in Figure 3B, which is consistent with the distribution of annual direct economic losses in FigureThe 3A mainland. The coastal coast sector of China of Hainan consists Island of eight(8), which provincial is more coastal exposed sectors to TCs, (see has Figure more1 );human one provincialcasualties, islandbut Hainan of Hainan Island, (8) which is also is considered less developed in this than study. the Oneight the mainland basis of thecoastal available provinces, coastal is hazardexpected data to (2008–2016) have lower on hazard the annual-caused provincial economic coastal losses. economic The coastal losses, sector Figure of3 A Guangxi shows the (9) spatial is well variationprotected ofby annual Hainan provincial Island, resulting coastal economicin much lower losses human from the casualties first coastal in comparison sector of Liaoning with the (1)other in thecoastal far northsectors. to the last coastal sector of Guangxi (9) in the far south. The direct economic losses in FigureOn3A the are basis seen toof increaseboth Figure from 3 theA and north 3B to, the south most and severely peak in theaffected southern coastal coastal provinces sector (7).may The be coastalranked sectoras Guangzhou of Guangxi (7), (9) Fujian is well (6), protected Zhejiang by (5), Hainan Hainan Island (9), (8). and The Jiangsu most severely(4) in terms affected of both coastal the provinceslosses of coastal are identified economy and and ranked human as Guangzhou lives. The (7),hazard Fujian-induced (6), Zhejiang losses(5), on Jiangsuthe coast (4), of and China Hainan are (9)shown based to ongenerally the criterion increase of economic from the north losses. to south and peak in the coastal sector (7). (A) (B) Figure 3. A B Figure 3. SpatialSpatial coastalcoastal hazardhazard datadata on:on: ((A)) annualannual directdirect economiceconomic losses;losses; andand ((B)) annualannual humanhuman casualties caused by all coastal hazards on the coast of China. casualties caused by all coastal hazards on the coast of China. Similarly, the spatial variation of annual provincial human casualties from the north to south 3.4. Analysis of Economic Losses due to Storm Surges is shown in Figure3B, which is consistent with the distribution of annual direct economic losses in FigureThe3A. coastal The coastal hazard sector data ofon Hainaneconomic Island losses (8), have which also is been more collected exposed for to each TCs, type has moreof five human coastal casualties,hazards, namely, but Hainan storm Island, surges, which sea ice, is algal less developedblooms, large than waves, the eight and mainlandoil spills from coastal 2001 provinces, to 2014. isThe expected sea ice tohazard have lowerwas found hazard-caused only in the economic Bohai Sea losses. of the The coastal coastal sectors sector (1) of– Guangxi(2), while (9) the is algal well protectedbloom hazard by Hainan generally Island, occurred resulting off inall much eight lowermainland human coastal casualties sectors. in Figure comparison 4 shows with the the hazard other coastalimpact sectors.intensity (HII) of the five coastal hazards, which is defined as the ratio of economic loss by one hazardOn the to basis total of economic both Figure losses3A,B, by theall the most five severely hazards, affected see Figure coastal 4. provinces may be ranked as Guangzhou (7), Fujian (6), Zhejiang (5), Hainan (9), and Jiangsu (4) in terms of both the losses of coastal economy and human lives. The hazard-induced losses on the coast of China are shown to generally increase from the north to south and peak in the coastal sector (7). 3.4. Analysis of Economic Losses due to Storm Surges The coastal hazard data on economic losses have also been collected for each type of five coastal hazards, namely, storm surges, sea ice, algal blooms, large waves, and oil spills from 2001 to 2014. The sea ice hazard was found only in the Bohai Sea of the coastal sectors (1)–(2), while the algal bloom hazard generally occurred off all eight mainland coastal sectors. Figure4 shows the hazard impact intensity (HII) of the five coastal hazards, which is defined as the ratio of economic loss by one hazard to total economic losses by all the five hazards, see Figure4. Geosciences 2019, 9, 131 6 of 11 Geosciences 2019, 9, x FOR PEER REVIEW 6 of 11 Geosciences 2019, 9, x FOR PEER100 REVIEW 6 of 11 100 80 80 60 60 40 40 20 Hazard Impact Intensity (%) Intensity Impact Hazard 20 Hazard Impact Intensity (%) Intensity Impact Hazard 0 Storm Surges Sea Ice Algal Blooms Large Waves Oil Spill 0 Coastal Hazard Types Storm Surges Sea Ice Algal Blooms Large Waves Oil Spill FigureFigure 4. Hazard 4. Hazard impact impact intensity intensityCoastal of of coastal coastal Hazard hazard-causedhazard Types-caused economic economic los lossesses on China on China’s’s coast. coast. It canItFigure becan seen be 4. Hazardseen from from impact Figure Figure intensity4 that 4 that theof thecoastal storm storm hazard surge surge-caused hazardhazard economic has has the thelos highestses highest on China HII HII ’vas coaslue, value,t. resulting resulting in in 88% of88% the of total the total hazard-caused hazard-caused economic economic losses, losses while, while the the other other four four coastal coastal hazards hazards onlyonly accountaccount for a smallfor portionIt acan small be of seenportion 12%, from i.e., of Figure 12%, sea ice i.e., 4 7.3%,that sea the ice algal storm7.3%, blooms surgealgal 2.7%,bloomhazard larges 2.7%,has the waves large highest 2%,waves HII and 2%, va oillue, and spill resulting oil 0.1%. spill 0.1%. Therefore,in 88% of the total hazard-caused economic losses, while the other four coastal hazards only account the stormTherefore, surge the hazard storm issurge the hazard most dominant is the most one dominant on the one coast on ofthe China. coast of China. for a small portion of 12%, i.e., sea ice 7.3%, algal blooms 2.7%, large waves 2%, and oil spill 0.1%. The directThe direct losses losses to theto the coastal coastal economy economy caused caused by by storm storm surgessurges are also significantly significantly affected affected by Therefore,by astronomic the storm tides. surge A s hazardtorm tide is the most, which dominant is the combination one on the coastof a storm of China. surge and an astronomic astronomic tides. A storm tide R , which is the combination of a storm surge and an astronomic tide, tidThee, is direct expected losses to tohave the a coastal greaterst economy impact on caused the coast by stormof China surges when are a alsostorm significantly surge coincides affected with a is expected to have a greater impact on the coast of China when a storm surge coincides with a high by astronomichigh tide rather tides. than A s torma low tide tide. Figure , which 5 shows is the thecombination spatial variations of a storm of surgehigh spring and an tidal astronomic levels and tidetid ratherespring, is expected thantidal arang to low havees tide., which a greater Figure were impact5 showsaveraged on thethe from spatialcoast a of wide variationsChina range when of of atidal high storm data spring surge that tidalcoincides were levels collected with and a by spring tidalhighnumerous ranges, tide rather which coastal than were tidea low gauges averaged tide. Figure deployed from 5 shows aalong wide the the range spatial coast ofof variations tidalChina data [1 5of]. thathighThe mean werespring spring collected tidal leveltidal by srange and numerous in coastalspringthe tide middletidal gauges rang coastales deployed, which sectors were of along Jiangsu averaged the (4), coast fromZhejiang of a China wide (5), [andra15nge]. Fujian The of tidal mean (6) aredata spring shown that tidalwereto be rangelargercollected in(Ω the=by 2.5 middle– coastalnumerous5 m) sectors than coastal i ofn the Jiangsu tide other gauges (4),coastal deployed Zhejiang sectors along; (5), the andthestorm coast Fujian tide of hazardChina (6) are [1is shown5 ]expected. The mean to beto springcause larger moretidal (Ω = rangedamage 2.5–5 in m) to than inthe thethese middle other coastal coastalcoastal sectors sectors sectors; with of theJiangsuhigher storm tidal (4), tide Zhejiangranges hazard than (5), is thoseand expected Fujian with smaller (6) to are cause showntidal more ranges. to be damage larger ( toΩ = these 2.5– coastal 5 m) than in the other coastal sectors; the storm tide hazard is expected to cause more damage to sectors with higher tidal ranges than those with smaller tidal ranges. these coastal sectors with higher tidal ranges than those with smaller tidal ranges. (A) (B) (A) (B) Figure 5. (A) High spring tidal levels, and (B) spring tidal ranges analyzed from a number of tide gauge station datasets on the coast of China (modified from [15]). FigureFigure 5. 5.( A(A)) HighHigh spring spring tidal tidal level levels,s, and and (B) (Bspring) spring tidal tidal range rangess analyzed analyzed from a fromnumber a number of tide of tide gauge station datasets on the coast of China (modified from [15]). gauge station datasets on the coast of China (modified from [15]). Geosciences 2019, 9, 131 7 of 11 3.5. Analysis of Human Casualties Due to Storm Waves Geosciences 2019, 9, x FOR PEER REVIEW 7 of 11 The number of human casualties caused by storm surges is also compared with those caused by 3.5. Analysis of Human Casualties Due to Storm Waves storm waves in Figure6, based on the hazard data collected from 2001 to 2016. The large wave hazard is shown to beThe much number more of human hazardous casualties than caused the by storm storm surgesurges is hazard also compared in terms with of th theose caused number by of human storm waves in Figure 6, based on the hazard data collected from 2001 to 2016. The large wave casualties,hazard even is though shown to only be much 2% more of the hazardous total economic than the storm losses surge were hazard caused in terms by of the thestorm number wave of hazard (see Figurehuman4). Since casualties, 2009, even the though number only of 2 human% of the total casualties economic caused losses were by the caused storm by the surge storm hazard wave has been shown tohazard have reduced (see Figure significantly, 4). Since 2009, butthe number the human of human casualties casualties caused caused by the large storm waves surge remainhazard high, i.e., above 18~132has been deaths shown per to yearhave beforereduced 2016. significantly, but the human casualties caused by large waves remain high, i.e., above 18~132 deaths per year before 2016. 350 327 Storm Surge Storm Waves 300 265 250 234 200 165 143 150 137 136 132 121 103 96 100 94 91 68 Number of Casualties (Person/yr) Casualties of Number 56 57 59 60 49 50 38 30 25 23 18 18 5 9 6 7 0 0 0 0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 YEARS Figure 6. AnnualFigure 6. humanAnnual human casualties casualties caused caused by stormby storm surges surges compared compared with with those those caused caused by large by large storm waves onstorm the coast waves of on China the coast (modified of China (modified from [19 from]). [19]). As already shown in Figure 3, the losses to the coastal economy and the loss of human lives in As already shown in Figure3, the losses to the coastal economy and the loss of human lives in the coastal sector of Guangdong (7), which has the smaller tidal range of Ω < 1.5 m, are seen to be the coastalmuch sector greater of Guangdongthan those on the (7), adjacent which coastal has sector the smaller of Fujian tidal(6), which range hasof theΩ much< 1.5 larger m, tidal are seen to be much greaterrangethan of 2.5 thosem ≤ Ω on< 4.5 the m. adjacentThis implies coastal that in sectoraddition of to Fujianthe storm (6), tide which hazard, has there the must much exist larger tidal range of 2.5another m ≤ mainΩ < coastal 4.5 m. hazard This responsible implies that for such in addition heavy losses to in the the storm coastal tide sector hazard, of Guangdong. there must exist Wave energy flux P [20] is used to describe wave power and estimated as another main coastal hazard responsible for such heavy losses in the coastal sector of Guangdong. P = (H T ) ≈ H T , (1) Wave energy flux [20] is used to describe wave power and estimated as where P is measured in kw/m, is the significant wave height (m), and is the mean wave ρg2 1 period (s). Equation (1) is also derived analytically2 by [16] with 2a new wave analysis method of wave P = Hs T ≈ Hs T, (1) power and zero-crossing. 64π 2 The seasonal variation of wave power P on the coast of China is shown in Figure 7, where the where P iswave measured data used in are kw/m, the 36-Hyears is ERA the-Interim significant reanalysis wave wave height data (1980 (m),– and2015)T fromis the the meanEuropean wave period (s). EquationCentre (1) for is Medium also derived-Range analyticallyWeather Forecasts by [(ECMWF).16] with aEven new though wave the analysis grid resolution method (0.125 of° wave × power and zero-crossing.0.125°) of the ERA-Interim wave data is coarse, the distribution trend of wave power on the coast of China can be determined [16]. In Figure 7, the wave power is shown to increase spatially from the The seasonal variation of wave power P on the coast of China is shown in Figure7, where the wave north to south and peak in the southern coastal sector of Guangdong (7) in all seasons, but vary data usedsignificantly are the 36-year seasonally, ERA-Interim e.g., the wave reanalysis power in wave winter data and autumn (1980–2015) is generally from higher the European than in Centre for Medium-Rangesummer and Weather spring. The Forecasts wave power (ECMWF). in the coastal Even sector though (7) is the shown grid to resolution be larger than (0.125 in its◦ × 0.125◦) of the ERA-Interimneighboring wave coastal data sector is of coarse, Fujian the (6) distribution with a much trendlarger tidalof wave range power. This indicates on the coast that of China TC-induced high wave power on the coast of Guangdong (7) is another main driver responsible for can be determined [16]. In Figure7, the wave power is shown to increase spatially from the north to the heavier economic and human losses. south and peak in the southern coastal sector of Guangdong (7) in all seasons, but vary significantly seasonally, e.g., the wave power in winter and autumn is generally higher than in summer and spring. The wave power in the coastal sector (7) is shown to be larger than in its neighboring coastal sector of Fujian (6) with a much larger tidal range. This indicates that TC-induced high wave power on the coast of Guangdong (7) is another main driver responsible for the heavier economic and human losses. Geosciences 2019, 9, x FOR PEER REVIEW 8 of 11 Large storm waves can cause significant damage to the coast as they can generate huge wave forces, large oscillatory fluid velocity, and high mean water levels, while storm tides can cause coastal damage only by raising the mean water level. This may explain why storm waves have caused more human casualties than storm surges, as is partially shown in Figure 6. It is noted that the economic and human losses in Figure 5 are also affected by other factors such as provincial coastal assets, population density, coastline length, coastal bathymetry. For example, the coastal sector of Guangdong (7), which is much more developed and populated than those of Hainan Island Geosciences 2019(8) and, 9, 131Guangxi (9), is expected to have more economic losses even under the same coastal hazard 8 of 11 conditions. Figure 7. FigureSpatial 7. andSpatial seasonal and seasonal variations variations of of wave wave power (kw/m) (kw/m) on the on coast the of coast China of averaged China averagedfrom from 36-year ERA-Interim reanalysis wave data (1980–2015). 36-year ERA-Interim reanalysis wave data (1980–2015). 4. Inundation-Erosion Model Large storm waves can cause significant damage to the coast as they can generate huge wave Coastal inundation and erosion hazards often occur at high storm tides during coastal storms. forces, largeStorm oscillatory-induced large fluid breaking velocity, waves and higharrive mean at the waterbeach andlevels, generate while large storm volumes tides canof high cause coastal damage onlykinematic by raising water that the runs mean up wateronto the level. beach Thishigh enough may explain to attack why the beach storm berm waves and dune have toe caused. more human casualtiesThis causes than sand storm to be taken surges, from as the is dune partially toe because shown of inthe Figure down-6rush. It ismechanism noted that in the the swash economic and human losseszone. inWhen Figure the eroding5 are also dune affected slope becomes by other larger factors than the such equilibrium as provincial dune slope coastal, the dune assets, front population collapses and lumps of the dune sand are transported into the surf zone by the high-speed density, coastlinedown-rush length, water. The coastalse physical bathymetry. processes For continue example, to further the erode coastal or sectorinundate of the Guangdong dune–beach (7), which is much moresystem developed until the storm and waves populated become smaller than those and the of storm Hainan tides Islandlower. (8) and Guangxi (9), is expected to have moreA economic wide range losses of coastal even field under data on the beach same–dune coastal profiles hazard and sediment conditions. grain-size distributions of more than 200 sandy beaches in Australia were collected by You et al [21]. They found that the 4. Inundation-Erosionsand dunes of their Model surveyed beaches were generally eroded when the dune toe elevations were less than 3.0 m above MSL. Their finding was further confirmed by the convincing field data that were Coastalcollected inundation in a 500 m and test section erosion of the hazards long and often straight occur Lighthouse at high Beach storm on the tides coast during of New South coastal storms. Storm-induced large breaking waves arrive at the beach and generate large volumes of high kinematic water that runs up onto the beach high enough to attack the beach berm and dune toe. This causes sand to be taken from the dune toe because of the down-rush mechanism in the swash zone. When the eroding dune slope becomes larger than the equilibrium dune slope, the dune front collapses and lumps of the dune sand are transported into the surf zone by the high-speed down-rush water. These physical processes continue to further erode or inundate the dune–beach system until the storm waves become smaller and the storm tides lower. A wide range of coastal field data on beach–dune profiles and sediment grain-size distributions of more than 200 sandy beaches in Australia were collected by You et al. [21]. They found that the sand dunes of their surveyed beaches were generally eroded when the dune toe elevations were less than 3.0 m above MSL. Their finding was further confirmed by the convincing field data that were collected in a 500 m test section of the long and straight Lighthouse Beach on the coast of New South Wales. The high sand dune sat on a rock platform at both the north end and the middle of the test section, while the sand dune sat on sand bed at the south end. It was observed that the dune toe at 3.16 m above MSL at the north end was well protected by dune vegetation and no erosive evidence Geosciences 2019, 9, 131 9 of 11 Geosciences 2019, 9, x FOR PEER REVIEW 9 of 11 was found, but the dune toe at 2.32 m above MSL in the middle section and the dune toe at 2.64 m above MSLWales. at the The south high sand end dune of the sat test on a section rock platform were at all both eroded. the north end and the middle of the test In thissection, study, while the the simple sand conceptualdune sat on sand model bed [at21 the] was south applied end. It was to assessobserved both that coastalthe dune inundation toe at and 3.16 m above MSL at the north end was well protected by dune vegetation and no erosive evidence erosion hazards on the coast of China: was found, but the dune toe at 2.32 m above MSL in the middle section and the dune toe at 2.64 m above MSL at the south end of the test section were all eroded. In this study, the Φsimple= R2 conceptual− (Rt + R models + R [w21+] wasRr +appliedRh) = toR assess2 − R both1, coastal inundation (2) and erosion hazards on the coast of China: where R2 is the dune or seawall toe level for the assessment of coastal erosion hazard or R2 is the dune Φ = R2 − (Rt + Rs + Rw + Rr + Rh) = R2 − R1, (2) or seawall top level for the assessment of coastal inundation hazard, Rt is the tidal level, Rs is the where R2 is the dune or seawall toe level for the assessment of coastal erosion hazard or R2 is the storm surge height, R is the wave setup height, R is the wave run-up height, R is the mean sea level dune or seawallw top level for the assessment of coasr tal inundation hazard, Rt is the tidalh level, Rs is (MSL), andtheR storm1 = ( Rsurget + R height,s + R wRw+ isR ther + waveRh). setup All parametersheight, Rr is the in wave Equation run-up height, (2) are Rh defined is the mean and sea illustrated in Figure8level. Rather (MSL), thanand R1 indirectly = (Rt + Rs + R measuringw +Rr + Rh). All differentparameters mean in Equation water (2) are level defined components and illustrated in Equation in Figure 8. Rather than indirectly measuring different mean water level components in Equation (2) (2) [22,23], R1 and R2 can be directly measured in the field [21,22]. [22,23], R1 and R2 can be directly measured in the field [21,22]. Figure 8. FigureStorm 8. tide Storm and tide wave-elevated and wave-elevated mean mean waterwater levels levels associated associated with coastal with coastalinundation inundation and and erosion hazards (modified from [24]). erosion hazards (modified from [24]). The two-parameter conceptual model in Equation (2) may be used to assess: (1) coastal The two-parameterinundation levels conceptualR1 with different model return in Equation periods, e.g. (2), dune may–beach be used will to be assess: inundated (1) coastalwhen the inundation levels R1 withdune or different seawall top return level R periods,2 is lower e.g.,than the dune–beach wave run-up will elevation be inundated R1; (2) dune– whenbeach stability, the dune e.g. or, seawall dune–beach system will be unstable when R1 > R2; and (3) shoreline setback horizontal distance ∆X top level R2 is lower than the wave run-up elevation R1; (2) dune–beach stability, e.g., dune–beach when R1 > R2, and ∆X can be directly derived from Equation (3) as system will be unstable when R > R ; and (3) shoreline setback horizontal distance ∆X when R > R , 1 2 ∆ 1 2 ∆X = = , (3) and ∆X can be directly derived from Equation (3) as where ∆z= (R1 − R2) and tanβ is the mean beach slope above mean water level. Equation (3) is similar ∆Z R − R to the Buun Rule [25], but it also consider∆X =s small timescales= 1 of mean2 , water level components such as (3) storm tide and wave-elevated mean water tanlevels.β Therefore,tanβ Equation (3) may be considered as a more general model than the Buun Rule for the estimation of shoreline setback distance. where ∆z = (R1As− anR 2 example,) and tan theβ conceptualis the mean model beach in Equation slope above (2) was mean applied water to assess level. the Equation dune–beach (3) is similar to the Buunstability Rule of [25 Yantai], but Beach it also in the considers coastal sector small of Shandong timescales (3). of Yantai mean Beach water is a level7 km long components popular such as swimming beach, with sediment grain size of d50 = 0.2–0.25 mm. Most of Yantai Beach is protected by storm tide and wave-elevated mean water levels. Therefore, Equation (3) may be considered as a more vertical seawalls, one swimming section is artificially nourished to overcome beach erosion general modelproblems, than and the the Buun last beach Rule section for the is estimationgenerally stable of shorelinewith a natural setback sand dune distance.. The nearshore As antides example, are regular the semidiurnal conceptual with model a tidal range in Equation of 1.64 m. (2) Figure was 9 appliedshows the tospatial assess variation the dune–beachof stability ofdune Yantai or sea Beachwall toe in elevation the coastal along the sector 7 km of beach, Shandong which is (3).averaged Yantai from Beach the field is data a 7 collected km long popular swimming beach, with sediment grain size of d50 = 0.2–0.25 mm. Most of Yantai Beach is protected by vertical seawalls, one swimming section is artificially nourished to overcome beach erosion problems, and the last beach section is generally stable with a natural sand dune. The nearshore tides are regular semidiurnal with a tidal range of 1.64 m. Figure9 shows the spatial variation of dune or seawall toe elevation along the 7 km beach, which is averaged from the field data collected monthly from December 2017 to December 2018. The seawall toe elevations are shown to be R2 ≤ 2 m, and some are even lower than the high tide levels resulting in beach inundation. The nourished beach section Geosciences 2019, 9, 131 10 of 11 Geosciences 2019, 9, x FOR PEER REVIEW 10 of 11 monthly from December 2017 to December 2018. The seawall toe elevations are shown to be R2 ≤ 2 m, with dune toe elevations of 1.6 m ≤ R2 < 2.6 m was found to have some erosion problems, while the and some are even lower than the high tide levels resulting in beach inundation. The nourished stable beach section has the dune toe elevations of 2.3 m ≤ R < 2.8 m. The mean dune toe elevation of beach section with dune toe elevations of 1.6 m ≤ R2 < 2.62 m was found to have some erosion the stable beachproblems, section while isthe estimated stable beach to section be R 2has= 2.7the dune m, which toe elevations is averaged of 2.3 m from ≤ R2 < all 2.8 datam. The points mean collected monthly fromdune Decembertoe elevation 2017of the stable to December beach section 2018. is estimated The estimated to be R2 = 2.7 value m, which of R is2 averaged= 2.7 m from in this all study is data points collected monthly from December 2017 to December 2018. The estimated value of R2 = slightly lower than R2 = 3.0–3.5 m proposed [21]. This difference may have resulted from inaccurate 2.7 m in this study is slightly lower than R2 = 3.0–3.5 m proposed [21]. This difference may have MSL datum,result theed limited from inaccurate field data MSL used,datum, or the different limited field dynamical data used, or drivers. different dynamical drivers. 3.0 2.5 2.0 1.5 1.0 Toe Elevation Toe (m, MSL) Seawall 0.5 Nourished Natural 0.0 0 1 2 3 4 5 6 Distance (km) Figure 9. TheFigure spatial 9. The variationspatial variation of dune of dune or or seawall seawall toetoe elevation elevation measured measured from December from December 2017 to 2017 to December 2018December on Yantai2018 on BeachYantai Beach on the on coastalthe coastal sector sector ofof Shandong (3) (3)in China. in China. 5. Conclusions 5. Conclusions Over recent years, the coast of China has been periodically struck by tropical cyclones at a Over recenthigher intensity years, the; thecoast globally of China averaged has intensity been periodically of TCs is projected struck to by continue tropical to shiftcyclones toward at a higher intensity; thestronger globally storms averaged with an expected intensity increase of TCs of is2% projected to 11% by to2100. continue TC-induced to shift coastal toward hazards stronger on the storms with an expectedcoast of China increase have resulted of 2% toin heavy 11% byhuman 2100. losses TC-induced and large losses coastal to the hazardsChinese coastal on the economy. coast of China TC-induced storm surges are found to be responsible for 88% of the total hazard-caused coastal have resultedeconomic in heavy losses human (US $2.6 losses billion/year) and large on the lossesmainland to thecoast Chinese of China,coastal while TC economy.-induced large TC-induced storm surgescoastal are waves found together to be responsiblewith storm surge fors 88%have caused of the heavy total human hazard-caused losses (264 coastaldeaths/year) economic. The losses (US $2.6 billion/year)hazard-caused on losses the to mainland the coastal coast economy of China,and the whilehuman TC-inducedlosses are shown large to spatially coastal increase waves together with stormfrom surges the north have to caused south, peak heavy at the human coastal lossessector of (264 Guangdong deaths/year). (7), and well The correlate hazard-caused to coastal losses to wave power especially in the southern coastal sectors of Zhejiang (5), Fujian (6), and Guangdong (7). the coastalThe economy hazard-caused and thelosses human to the coastal losses economy are shown and the to human spatially casualties increase estimated from in the this north study to south, peak at thewith coastal the most sector recent of Guangdonghazard data are (7), generally and well consistent correlate with tothose coastal of You wave et al power[19]. The especially simple in the southern coastaltwo-paramet sectorser conceptual of Zhejiang model (5), (Equation Fujian (2) (6), together and Guangdongwith Equation (3) (7).), which The hazard-caused has not yet been losses to the coastaltestified economy by You and et theal [19 human] with available casualties field data, estimated generally in agree thiss studywith the with R2 field the data most collected recent hazard on the beach of Yantai in China, and therefore may be considered as a more general model than data are generallyBruun Rule consistent for the assessment with those of of coastal You inundationet al. [19]. and The erosion simple hazards two-parameter with different conceptual return model (Equationperiods. (2) together Further with field studies Equation on the (3)), parameters which R1 has and notR2 are yet urgently been needed. testified by You et al. [19] with available fieldFunding: data, This generally study has been agrees financially with supported the R2 byfield the Major data Research collected Grant on (Ref the No: beach U1806227) of Yantaifrom the in China, and thereforeNatural may Science be Foundation considered of China as a(NSFC) more and general the Provincial model Natural than Science Bruun Foundation Rule of for Shandong. the assessment of coastal inundationConflicts of andInterest: erosion The founding hazards body ha withs no role different in the design return of the periods. study, in the Further collection, field analyses, studies or on the parametersinterpretationR and R ofare the urgentlydata used for needed. this study, in the writing of the manuscript, and in the decision to publish the 1results. The2 author declares no conflict of interest. Funding: ThisReferences study has been financially supported by the Major Research Grant (Ref No: U1806227) from the Natural Science Foundation of China (NSFC) and the Provincial Natural Science Foundation of Shandong. Conflicts of Interest: The founding body has no role in the design of the study, in the collection, analyses, or interpretation of the data used for this study, in the writing of the manuscript, and in the decision to publish the results. The author declares no conflict of interest. References 1. Chen, J.Y. The impact of sea level rise on China’s coastal areas and its disaster hazard evaluation. J. Coast. Res. 1997, 13, 925–930. Geosciences 2019, 9, 131 11 of 11 2. Sousounis, P.J.; He, H.; Healy, M.L.; Jain, V.K.; Jung, G.; Qu, Y.; Shen, T.B. A Typhoon Loss Estimation Model for China. In Proceedings of the 88th Annual American Meteorological Society Meeting, New Orleans, LA, USA, 20–24 January 2008. 3. Elsner, J.B.; Kossin, J.P.; Jagger, T.H. 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Bruun Rule and its application to coastal zone of China. Mar. Geol. Quat. Geol. 2000, 20, 87–91. © 2019 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).