Rip Current Hazards in South China Headland Beaches

Ocean & Coastal Management 121 (2016) 23e32

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Ocean & Coastal Management

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Rip current hazards in South China headland beaches

Zhiqiang Li

Ocean Engineering Department, Guangdong Ocean University, Zhanjiang, 524088, China article info abstract

Article history: Beaches and coastal regions are commonly primary tourist destinations. Dollars spent by beach visitors Received 1 June 2015 annually provide signiﬁcant input to local and regional economies. Hazards associated with rip currents Received in revised form can inﬂuence the suitably of any given stretch of coast as a recreational resource, and thus impact tourist 2 December 2015 dollars spent in addition to the safety and well-being of beach visitors. Therefore it is crucial to evaluate Accepted 6 December 2015 the degree of rip current risk along recreational beaches in order to protect human lives, maintain tourist Available online 23 December 2015 attraction, and aid beach managers. Accordingly, a new rip current hazard assessment method is proposed based on the beach state model. Using the measured morphodynamic parameters (wave pa- Keywords: Rip current rameters, tidal ranges and sediment grain size) of 51 South China beaches, a preliminary assessment of Morphodynamic rip current hazards was carried out. Results show that 71% of South China beaches may develop rip Beach hazard currents. The beaches in southern Fujian, Shanwei (in eastern Guangdong) and western Guangdong, and Beach drowning accident eastern and southern Hainan coasts have the highest risk of rip current development. The evaluation results are consistent with remote sensing observations, and beach incident (drowning and near- drowning) media reports. This paper indicates that the beach morphodynamic model of U-RTR (dimensionless fall velocity-relative tidal range) is a reasonable method to assess the rip current risk along sandy beaches. This information can aid coastal managers as well as reduce the frequency of rip current related incidents. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction By definition, rip currents are strong, jet-like, seaward-directed currents that originate within the surf zone owing to alongshore Controlled by waves, tides, tidal currents, as well as antecedent gradients in wave-induced radiation stresses and pressure geology, the complex morphology of beaches yield many hidden (Huschke, 1959; Bowen, 1969; Dalrymple, 1975, 1978; Bascom, safety issues and present challenging questions to tourism man- 1980; Haller et al., 2002). Average offshore rip current velocities agement. Rip currents, the subject of this paper, present a signifi- are around 0.3 m/s, and can exceed2 m/sunder certain circum- cant hazard for beach tourists worldwide. In the United States, rip stance (e.g. Lushine, 1991; Huntley et al., 1988; Short and Hogan, currents account for more than 80% of all surf rescues per year 1994; Houser et al., 2011a). These strong currents can quickly based on estimates from United States Lifesaving Association sta- carry dispersing material, such as suspended sediments, nutrients tistics (USLA, 2012; Brewster and Gould, 2014; Lascody, 1998; and other floating material away from the surf zone into deeper Lushine, 1991; Gensini and Ashley, 2010). In Australia, 89% of the water within minutes. Accordingly, rip currents can be considered more than 25,000 annual surf rescues conducted by lifeguards, an important morphodynamic process that transports both water lifesavers, and surfers are caused by rip currents with an estimated and sediment, and consequently, drives changes in beach 40e50 rip-related drownings per year (Short and Hogan, 1994; morphology (Brander, 2005). Herker et al., 2008; SLSA, 2011). Rip currents are responsible for Rip current hazard mitigation requires a variety of management 67% of all individuals rescued by lifeguards on UK beaches, and strategies (Miloshis and Stephenson, 2011; Hatfield et al., 2012; Royal National Lifeboat Institution (RNLI) lifeguards assisted 12,607 Shaw et al., 2014). Accordingly, beach specific rip current hazard people from 5798 rip current incidents between 2006 and 2011 assessments are fundamental to establishing effective management (Woodward et al., 2013). strategies and can provide beach managers with a basis for advising people on how to recognize and avoid rip currents. Beach morphology is an important indicator of rip current risk and has been used in a variety of locations as a beach hazard management E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ocecoaman.2015.12.005 0964-5691/© 2015 Elsevier Ltd. All rights reserved. 24 Z. Li / Ocean & Coastal Management 121 (2016) 23e32 tool (e.g. Australia (Short and Hogan, 1994; Short, 1996), UK (Scott the mountain ranges, rivers, coastal capes, bays, peninsulas, and et al., 2007), USA (Benedet et al., 2004; Houser et al., 2008, 2011a, islands were developed along both structural trends, resulting in 2011b), Costa Rica (Arozarena et al., 2015), Brazil (Klein et al., hundreds of headland beaches forming along the subject coastline. 2003) and Korea (Yang et al., 2014)). Although rip currents are a These beaches have variable orientations, with lengths ranging common physical phenomenon on natural beaches along China's from a few meters to over ten kilometers. coast, rarely have investigations been carried out addressing both Wave and tide conditions are complex. Under the monsoon rip current theory and application. This paper focuses on rip current background, wave directions vary from NE in winter to SE in hazard assessment along south China headland beaches, the most summer. Most incident wave directions are NE-E with the domi- common type of beach in South China, and largely relies on beach nant wave direction NE in winter, and SE-S in summer. Maximum morphodynamic model. wave heights occur during the passage of typhoons during summer and fall; however, winter storms can also generate high waves for 2. Needs for rip current hazard assessment in China durations on the order of 10 days. Influenced by the coastal terrain, the distribution of annual mean wave height and period show a China has hundreds of excellent beaches along its 18,000 km gradually decreasing trend from east to west. The annual mean coastline, which attract large numbers of tourists and local resi- wave height from Southern Fujian to eastern Guangdong and Pear dents. A tourism driven economy is becoming more and more River Estuary is 1.1e1.3 m with wave periods ranging from 3.9 to important for China. Chinese marine economic statistical bulletin 5.1 s. Western Guangdong and eastern Hainan Island have an (2013) shows that the scale of China's coastal tourism economy annual average wave height of 0.48e1.0 m and an annual average continues to increase, and the added value reached RMB 7851 period of 3.5e4.2 s. The annual mean wave height along Beibu Bay billion (about $1282 billion) in 2013, which was the largest is 0.5e0.8 m with an average wave period of 3.0e3.1 s. The tidal component of the marine economy, accounting for 34.6% of the range along the southern Fujian and southern Guangxi coast is the total output value of the marine economy (SOA, 2013). Owing to an largest at 4.0e6.0 m on average. Microtidal conditions exist from overall lack of beach hazard awareness and lack of proper beach eastern Guangdong to Maomin, with an average tidal range of management strategies, a large number of beach drowning acci- 1.0e2.0 m, while the tidal range on the east and west sides of dents occur in China every year. Although the exact number of rip Leizhou Peninsula is about 2.0e4.0 m. current related deaths is difficult to estimate in China because of data gathering limitations, online/internet news can provide some 3.2. Data collection insights into the nature of China's beach related drowning accidents. Table 1 illustrates reported drowning accidents that occurred In order to investigate the rip currents risk along the South at Dadonghai Beach, Sanya, Hainan Province, China between 2004 China coast, 51 headland beaches with lengths exceeding 1 km and 2013. Unfortunately, in China, rip currents are not very well were selected for this study (Fig. 1). Beach morphodynamic pa- understood by most people, including many beach managers. Most rameters are shown in Fig. 2. These data mainly come from CCRBC Chinese people refer to rip currents as undertow, invisible currents, (1993a, b, and c), CEGCZH (1995), Huang and Ye (1995), Su and Yuan or rip tides. Many local residents have described the phenomena as (2004), and Wang et al. (2006). It should be noted that these re- “the seabed is not plain, there are many pits, trenches, and slopes on searchers used different survey data, therefore, wave heights and the seabed, and these pits and trenches easily lead to seawater gath- tidal ranges have some variability along some coastal stretches. ered, and then form the undercurrent.”Thus, a more rigorous un- However, in those cases, their parameters are typically determined derstanding of rip current risk is important for the protection of by taking average values. Because the early nearshore wave data in human life in China. China were gathered using optical wave measurement instruments, wave heights in those data are the highest one-tenth of waves (H1/ 3. Study area and data collection 10). The H1/10 was transformed into mean wave height by linear wave theory. Tidal range was determined using the average and 3.1. Description of the study area maximum tidal range. Some beaches lack wave height measurements and tidal range data. For those coastal stretches, data are A series of Mesozoic to Cenozoic-age NEeSW and NWeSE interpolated from direct measurements made along adjacent and striking fault structures influence the coastal geomorphology of the nearby beaches. Sediment grain sizes come from CCRBC (1993c), South China coast. The trend, morphology, and pattern for many of Wang et al. (2006), Cao et al. (2006), and Li and Zhu (2015).

Table 1 Drowning accidents reported by the online news during 2004e2013.

Date (mm/dd/yy) Accident Reference

07/18/2004 3 deaths, 4 rescues Su (2004) 07/30/2004 12 drownings Gao (2004) 05/01/2005e05/07/2005 3 deaths, 67 rescues Su (2005a) 05/09/2005 2 rescues Zhang and Xu (2005) 08/09/2007 11 rescues Wang (2007) 08/23/2009 1 death Zhang (2009) 09/28/2011 1 death Zhang (2011) 05/07/2012 50 rescues in 5 days Xu (2012) 06/18/2012 2 deaths Cai (2012) 06/21/2012 over 70 rescues and 14 deaths from Jan to Jun,2012 Li (2012) 08/05/2012e08/12/2012 4 deaths, many rescues Li & Wang (2013) 08/01/2013e08/21/2013 Aug 4th, 1 death, 8 rescues. Zhang (2013) Aug 9th, 1 death, 11 rescues Aug 16th,1 death 4 deaths and over 80 rescues from Aug 1st to 21st. Z. Li / Ocean & Coastal Management 121 (2016) 23e32 25

Fig. 1. Locations of 51 beaches in South China, Beach code: 1 Beipinghai Bay, 2 Jingfeng Bay, 3 Jinmen Bay, 4 Huqian Bay, 5 Qianhu Bay, 6 DaaoBay, 7 Wujiao Bay, 8 Dacheng Bay, 9 Chishi Bay, 10 Guangaodong Bay, 11 Guangao Bay, 12 Haimenbei Bay, 13 Haimennan Bay, 14 Jinghai Bay, 15 Shenquan Bay, 16 Jiazi Bay, 17 Hudong Bay, 18 Jieshi Bay, 19 Houjiang Bay, 20 Jianao Bay, 21 Shanwei Bay, 22 Houmen Bay, 23 Xiaomo Bay, 24 Nanpinghai Bay, 25 Gaolan Beach, 26 Nan Bay, 27 Sanya Bay, 28 Shanbai Bay, 29 Dajiao Bay, 30 Heibei Bay, 31 Shuidong Bay, 32 Aonei Bay, 33 Wangcun Bay, 34 Haikou Bay, 35 Puqian Bay, 36 Fengjia Bay, 37 Long Bay, 38 Wuchuang Bay, 39 Lingshui Bay, 40 Tufu Bay, 41 Tufunan Bay, 42 Yalong Bay, 43 Sanya Bay, 44 Yazhou Bay, 45 Basuo Bay, 46 Yangpu Bay, 47 Chengmai Bay, 48 Wushi Bay, 49 Caotan Bay, 50 Beihai Bay, 51 Sanniang Bay. 26 Z. Li / Ocean & Coastal Management 121 (2016) 23e32

Fig. 2. Morphodynamic parameters of 51 studied beaches in South China.

4. Rip current risk estimate methodology 1) Reﬂective group (U < 2)

4.1. Beach morphodynamic model and rip current For U < 2 and RTR <3, beaches are fully reflective (R). The beach face is steep and commonly cuspate. Waves are generally surging or Rip currents can occur due to the existence of gaps between plunging and most of the wave energy is at incident and sub- nearshore reefs. They can also occur where sea water pushes up harmonic (twice the wave period) frequencies (Huntley and against coastal structures resulting in side-currents being forced Bowen, 1975; Wright and Short, 1984). out to sea in one direction. Excluding reefs and coastal structures, For RTR is 3e7 and U < 2, beaches exhibit a low tide terrace rip current phenomena have close relations to the beach state. commonly accompanied by rip currents (LTTR). Additionally, these Previous researchers (e.g. Benedetet al., 2004; Lin et al., 2009; Scott beaches are generally steep and reflective during high tide, with a et al., 2011; Arozarena et al., 2015)commonly estimate rip current relatively flat terraceforming during low tide. hazard by using the beach state morphodynamic model proposed As the RTR approaches and exceeds 7, the very wide and dissi- by Wright and Short (1984, 1985). The model defines a dimen- pative low tide terrace becomes increasingly dominated by un- sionless fall velocity parameter (U)as broken shoaling waves which produce a uniform, featureless low tide terrace commonly absent of rip channels. These beaches are U ¼ = Hb TWs (1) low tide terrace beaches without rip currents (LTT). where Hb is the wave breaking height (m), T is the wave period (s) 2) Intermediate Group (U¼2e5) and Ws is the sediment fall velocity (m/s). Based on the dimensionless fall velocity parameter, the model classifies beach state For the lowest relative tide ranges (RTR <3) the beaches are into 3 basic types: (1) reflective (U < 1), (2) intermediate state barred (B) with several types of bars occurring. Bar-topography (1 6). The intermediate beach may consist of alternating transverse bars with associated rip state is divided FURTHER into four beach types: (1) Longshore Bar currents. and Trough (LBT), (2) Rhythmic Bar and Beach (RBB), (3) Transverse For RTR ¼ 3e7, the beaches are low tide bar/rip beaches (LTBR). Bar and Rip (TBR), and (4) Low Tide Terrace (LTT). These beaches have a relatively steep upper gradient in the mid- In meso to macro tidal environments, tidal range becomes intertidal zone with possible swash bars, and bar and rip important to beach morphodynamics and rip formation. In order to morphology around low tide level. The bar and rip morphology consider the impact of the tidal range, Masselink and Short (1993) only exists on the low tide beach and is only active approaching or added a non-dimensional relative tide range parameter (RTR) to the following low tide. Wright and Short's (1984, 1985) model: 3) Dissipative Group (U > 5) ¼ = RTR TR Hb (2) For U > 5 and RTR <3, these are barred dissipative beaches (BD), where TR is the mean spring tide range (m). characterized by subdued longshore bar-trough morphology. Using the parameters RTR and U, beaches are divided into 4 As RTR increases >3, the beaches tend to be characterized as groups and 8 types. The characteristics of each group and type are non-barred dissipative beaches (NBD), and while maintaining as follows: Z. Li / Ocean & Coastal Management 121 (2016) 23e32 27 similar dimensions, become flatter and featureless with no bars. proposed by Komar and Gaughan (1972): 2=5 4) Ultra-Dissipative Group (U > 2 and RTR > 7) ¼ : 1=5 2 Hb 0 39g TH∞ (3) fl Ultra-dissipative beaches (UD) are generally at and featureless 2 where g is the gravityconstant, g ¼ 9.8 m s . H∞ is the deepwater and have very wide intertidal zones. wave height. The sediment fall velocity was calculated using an equation 4.2. Rip current risk estimating model derived by Ferguson and Church (2004): 0:5 As described above, different types of beaches have different rip 2 3 us ¼ RgD C1v þ 0:75C2RgD (4) current occurrence probabilities. As reviewed by MacMahan et al. (2006), most rip currents occur in association with a shore- connected shoals (transverse bars) or by cutting through along- where R is the submerged specific gravity (1.65 for quartz in water), shore bar forming a rip channel. All laboratory and field measure- D is the sediment size in m,n isthe kinematic viscosity of the fluid 6 1 1 ments (e.g. Brander, 1999; Hamm, 1992; Haller et al., 1997, 2002; (v ¼ 1.0 10 kg m $s for water at 20 C), and C1and C2 are Dronen et al., 2002; Haller et al., 2002; Haas and Svendsen, 2002; constants. For natural sand grains, Ferguson and Church (2004) Kennedy and Thomas, 2004; Houser et al., 2008, 2011a, 2011b; recommend C1 ¼ 18 and C2 ¼ 1. Barrett and Houser, 2012) have confirmed these phenomena with the exception of Hamm (1992). Based on the relationship between 5. Results topography, sandbars in the surf zone, and the development of rip currents (Munk, 1949a,b; Aagaard et al., 1997; Brander and Short, Fig. 4 is the U-RTR scatterplot figure for 51 beaches in South 2000), in conjunction with the research of Wright and Short China. Because the dimensionless fall velocity parameter and (1984, 1985), Scott et al. (2011), Lin et al. (2009), Houser et al. relative tide range parameter are very close in some beaches, some (2011a, b), Barrett and Houser (2012), a frame work to evaluate scatter points overlap. Most of the dimensionless fall velocity the rip current hazards is proposed: (1) LTBR and B types of beaches values are greater than 1 and less than 6, so the beaches in South have maximum probability of rip current occurrence, presenting China would be considered mainly intermediate state beaches. the most hazardous beaches for swimmers. The two types of bea- Some researchers (e.g. Benedet et al., 2004; Lin et al., 2009; Scott ches corresponding to the intermediate beach state in the dimen- et al., 2011; Arozarena et al., 2015) have used the parameter U to sionless fall velocity parameter model (Wright and Short, 1984, distinguish the risk of the beaches, but this method requires 1985) are characterized by rip current morphology. As U de- extensive field observation and subjective judgments making it creases, the energy of the rip current system increases. Houser et al. difficult for beach rip current risk assessment. Conversely, with the (2011b), Barrett and Houser (2012) found that most drownings and Masselink and Short (1993) proposed model, each type of beach rescues take place along stretches of beach hosting transverse bar state has a different probability of rip current formation and is more and rip morphology. (2) Rip currents may occur in the BD and LTTR practical to apply in practice. The morphodynamic state types and types of beaches during certain wave and tide conditions, yielding risk establishment results are showed in Table 2. According to our medium risk beaches. (3) LTT, NBD, UD, and R types of beaches long-term observations, the eastern coast of Leizhou Peninsula, the seldom develop rip currents and are classified as low rip current longest straight beach in South China, has a high rip current risk. A risk beaches. The rip current hazard model is shown in Fig. 3. The zoning map of rip current risk level of the South China coast can be model provides insights into rip current risks, although it does not developed by connecting the studied beaches (Fig. 5), with results discuss the risks associated with bathing beaches. It should also be showing that most beaches in South China have a high rip current noted that based on the Short and Hogan's study (1994), dissipative hazard, especially those along southern Fujian, Shanwei (in eastern beaches, influenced by surf zone width and beach bottom under- Guangdong) and western Guangdong, and eastern and southern tow, can also pose substantial rip current hazards. Hainan. In order to calculate the dimensionless fall velocity parameter U, Due to the difficulty in obtaining complete drowning incident the breaking wave height was calculated using the expression data in China, the largest Chinese search engine, Baidu (www.

Fig. 3. Rip current hazard model. Fig. 4. Scatter plot of the U-RTR model results. 28 Z. Li / Ocean & Coastal Management 121 (2016) 23e32

Table 2 Beach state and rip current hazard of 51 beaches in South China.

Risk Beach Beach Name (beach code) level type

Low UD Beipinghai Bay (1), Jinmen Bay (3) NBD Jingfeng Bay (2), DaaoBay (6), Wujiao Bay (7), Dacheng Bay (8), Chishi Bay (9), Guangaodong Bay (10), Guangao Bay (11), Nanpinghai Bay (24), Sanya Bay (27), Shanbai Bay (28), Dajiao Bay (29), Beihai Bay (50) LTT Sanniang Bay (51) Middle LTTR Haikou Bay(34), Puqian Bay(35), Wushi Bay(48), Caotan Bay(49) BD Haimenbei Bay (12), Haimennan Bay (13), Jinghai Bay (14), Shenquan Bay (15), Jiazi Bay (16), Hudong Bay (17), Jieshi Bay (18), Jianao Bay (20), Houmen Bay (22), Xiaomo Bay (23), Fengjia Bay (36), Long Bay (37), Basuo Bay (45) High LTBR Huqian Bay (4), Qianhu Bay (5), Gaolan Beach (25), Nan Bay (26), Heibei Bay (30), Aonei Bay (31), Shuidong Bay (32), Wangcun Bay (33), Wuchuang Bay (38), Yangpu Bay (46), Chengmai Bay (47) B Houjiang Bay (19), Shanwei Bay (21), Lingshui Bay (39), Tufu Bay (40), Tufunan Bay (41), Yalong Bay (42), Sanya Bay (43), Yazhou Bay (44)

baidu.com) was queried, along with the largest Chinese journal database, Cnki (www.cnki.net) in order to identify drowning accidents occurring over the last decade. Table 3 shows the drowning incidents as a function of location and illustrates the high frequency of drownings that have occurred along the South China coast, in contrast to other coastal stretches which had relatively fewer accidents. To further elaborate the rationality of the above model assessment results, the following discusses the different rip currents risks associated with typical beaches.

5.1. Low rip current risk beaches

Mesotidal and macrotidal beaches of southern Fujian and Guangxi coasts have a low rip current risk. The morphology of these beaches is characterized by a wide beach face and gentle slope. For example, Beihai Bay (in Guangxi) beach (Figs. 5 and 6), a famous tourist beach in China, is a non-barred (Huang et al., 2011; Li and Fig. 5. Rip current risk level along the South China coast. Zhu, 2015) dissipative beach with a slope of about 0.001. Incident wave energy is dissipated by bottom friction during wave propa- gating through the wide surf zone. All these phenomena meet the characteristic of a non-barred dissipative beach (NBD). Although Beihai Bay beach has hundreds of thousands of visitors every year, the drowning accidents are much less than along southern Hainan beaches, and may be related to beach bottom undertow (Short and Hogan, 1994).

5.2. Middle rip current risk beaches

These beaches are mainly located in eastern Guangdong, eastern and northern Hainan, and western Leizhou Peninsula. These beaches typically exhibit wide surf zones and low tide terraces with rip current formation conﬁned to low tide stage similar to that of Haikou Bay beach (in northern Hainan) (Figs. 5 and 7). According to Zhou et al. (2013), Haikou beach has a slope of 115% up to the high Fig. 6. Photograph showing the non-barred dissipative (NBD) morphodynamic state of tidal level, and a slope of 100% within the intertidal zone. The sub- Beihai Bay beach (view towards west). tidal topography is characterized by shore parallel trough-ridge morphologies. The ridge makes it easy for rip currents to develop 5.3. High rip current risk beaches during low tide. Fig. 7 shows the nearshore low tide bar/rip morphology. Haikou Bay beach is considered a low tide bar/rip High rip current risk beaches are mainly located in southern beach (LTBR) (Li and Zhu, 2015).

Table 3 Incomplete statistics of drowning accidents that occurred on the South China coasts in the last 10 years.

Coast section South Fujian Shanwei Yangjiang Maoming Eastern Hainan Southern Hainan

Number of rescued 97 66 47 26 137 >350 Number of drowning deaths 25 14 12 7 30 >50

*Data are collected by using www.baidu.com and www.cnki.net. Z. Li / Ocean & Coastal Management 121 (2016) 23e32 29

seem calm, underwater topography is not the same...... The seabed was swashed into bumps, there are a lot of pits, trenches and slopes, and these pits and trenches easily lead to seawater gathering, thus easy to form the undercurrent...... the seawater looks like it propagates from the shoreline, while it is actually the undercurrent pumping seawater into the sea, swimming is very dangerous in this case “(Su, 2005b). What they referred to as a trench is actually the rip channel while the undercurrent is the rip current in the context of beach morphodynamics. From the beach state classiﬁcation of U-RTR model, southern Hainan beaches generally belong to the sandbar (barred) beach (B). Fig. 8c and d are remote sensing images of Dadonghai and Yalong Bay respectively, and show the well- developed rip channels and sandbars. In the image (Fig. 9), waves breaking on the top of sandbar can be seen along with surf zone sediment being transported outside of the surf zone forming a Fig. 7. Photograph showing the low tide bar/rip (LTBR) morphodynamic state of typical rip current neck and head structures. Haikou Bay beach (view towards east).

6. Discussion

Fujian (Huqian Bay and Qianhu Bay), western Guangdong, and South China has a long coastline with complex hydrodynamic southern Hainan and Shanwei (Houjiang Bay, Shanwei Bay). There, conditions making it difficult to determine morphodynamic fac- of the 19 beaches in total, over 37% are considered high risk rip tors. There are also some uncertainties in selecting such factors. It current beaches, and further illustrates the need for improved rip is worth noting again that due to a paucity of hydrodynamic current risk management strategies along the South China beaches. measurements along the South China coast, several sources of The following uses the Shuidong Bay beach (in western Guang- data were used to determine hydrodynamic parameters by linear dong) and beaches of southern Hainan as examples of the two types interpolation method. These factors will have an influence on the (LTBR and B) of high risk rip current beaches. final estimation results. Similarly, beach sediment distribution and sampling is inconsistent, which can also influence results. In 5.3.1. 1ShuidongBay beach addition, while the beach state model has been widely used, its Shuidong Bay is located in Maoming City, Guangdong Province. validity in a complex coastal environment is still in dispute. All of The beach along Shuidong Bay is one of the longest headland these factors will increase the uncertainty of the estimation beaches in China. Shuidong Bay beach contains low tide bar/rip model. However, by combining observations presented here in beaches (LTBR) according to the U-RTR model. According to Chen conjunction with related research, the beach classification results et al. (1990, 1991, 1996), the accumulative underwater topography and rip current risk levels are basically consistent with the actual (sand bar) disappeared from the west segment to the east segment states, especially along the important tourist beaches of South of the beach. There is clearly a transformation linking surf zone China. topography to beachface topography. When the quasiperiodic While rip currents may develop along localized stretches of subtidal sand bar migrates to the shoreline, a beach berm forms on beach, beaches without rip currents are relatively safe. In this pa- the beachface. The erosive beach cusp and rip current can be per, the average beach morphodynamic state is used as a pre- observed in the surf zone (Fig. 8). In the remote sensing image, liminary estimate of rip current hazard, and can provide reference while characteristic sand bar and rip current topography is evident for the selection and management of bathing beaches. Beach haz- during low tide (Fig. 8a), these morphologic features are largely ards, like morphodynamics, can vary temporally. Scott et al. (2014) absent during higher tide levels (Fig. 8b). All these phenomena described the time scales of rip hazards as: annual, seasonal, meet the characteristics of low tide bar/rip beaches (LTBR). weekly and prevailing. That study focused on the annual long-term Accordingly, this is a hazardous beach for swimmers during low characteristics of rip current hazards, which appear to be largely tide levels. controlled by environmental setting, which links directly with beach types (Scott et al., 2014). Rip current occurrence probability 5.3.2. Southern Hainan beaches and location are strongly related to incident wave direction, wave Hainan Island is the most tropical tourist destination in China. height, infragravity wave, water level, and so on (Short, 2007; Beach drowning accidents have increased significantly as the MacMahan et al., 2004; Munk, 1949a, b; Smith and Latgier, 1995). number of tourists visiting the beach has increased over the last Long-term in-situ observations must be conducted before fore- decade. The beaches of Dadonghai Bay, Sanya Bay, Yalong Bay, and casting rip current hazards for a specific beach can be established. Haitang Bay are famous tourist beaches in southern Hainan and Prevailing rip dynamics and hazards can vary dramatically from have had a high number of drowning accidents. At Dadonghai hours to seconds controlled by variability in water level and even beach on the evening of July 18, 2004, 7 people were caught in a rip wave conditions and wind (Scott et al., 2014). This model provides a current and 3 swimmers died (Su, 2004). During the golden week qualitative assessment of average rip current hazard levels, while holiday of 2005, Dadonghai beach had more than 70 incidents the development of rip channels is beyond the scope of this model. resulting in 3 drowning deaths, and 67 rescues (Su, 2005a). On Real time monitoring is needed for the management of high risk August 9, 2013, 11 tourists were rescued with one fatality (Li, 2012). bathing beaches. The beach safety rating model proposed by Short According to local residents: and Hogan (1994), and Short (1996) can be used for daily rip risk Although these beaches are covered by the deep blue sea water and management, however, the parameter values need to be modified 30 Z. Li / Ocean & Coastal Management 121 (2016) 23e32

Fig. 8. Remote sensing images of Shuidong Bay (a, b), Yalong Bay (c) and Dadonghai (d) beaches. Arrows indicate rip currents.

for beaches with different hydrodynamic and morphological et al., 2015). As previously mentioned, there is still a great deal of characteristics. misunderstanding regarding rip currents made by the Chinese Based on this study, 36 beaches or 71% of the beaches along the public. For example, many bathing beaches in China as well as in South China may form rip currents (including high and middle rip other counties have warning signals, but the content of such current risk beaches), indicating a more exercisable and effective warnings is simple, usually in the form of warnings reminding beach safety strategy must be established in China. The safety visitors that it is dangerous or forbidden to swimming in that strategy should be comprehensive, including rip current and beach particular sea zone (Fig. 10). Therefore, efforts should be imple- safety education, risk forecasting and monitoring, rip current mented in the future to more thoroughly educate beach visitors on warning signal design, and beach bathing layout as well as in-situ the hazards of rip currents, as well as how to minimize the con- management and emergency services (Hartmann, 2006; sequences of active rip currentsalong recreational/bathing beaches. Fletemeyer and Leatherman, 2010; Brander et al., 2011; Cervantes The results of this paper provide a basis through which beach managers can estimate rip current risk along South China's recreational beaches.

7. Conclusions

While beaches are the most desirable tourist destinations for people world-wide, the hazards associated with rip currents present some of the greatest challenges facing beach managers. Based on the U-RTR model, a model was proposed to preliminarily estimate the rip current hazards of the South China beaches. Results indicate 71% of South China beaches can develop rip currents. Beaches in southern Fujian, Shanwei (in eastern Guangdong) and western Guangdong, and eastern and southern Hainan have the highest rip current risk. These results are consistent with ﬁeld observations as well as drowning incidents reported by the media. The rip current estimation model may be used for bathing beach selection and management; however, systematic long term obser- Fig. 9. Rip currents in Dadonghai beach (indicated by arrows, view towards south vations must be carried out before more accurate forecastingof rip east). currents can be achieved. Z. Li / Ocean & Coastal Management 121 (2016) 23e32 31

Fig. 10. Some warning signals at China beaches. Meanings of the Chinese texts: (a) Dangerous undercurrent and no swimming. (b) High waves and no swimming. (c) Swimming is forbidden because of turbulences, undertows and high waves. (d) Complex bathymetry. Do not swim in the sea.

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