Acta Oceanol. Sin., 2016, Vol. 35, No. 7, P. 87–95 DOI: 10.1007/s13131-016-0907-9 http://www.hyxb.org.cn E-mail: [email protected]

Wave-dominated, mesotidal headland-bay beach morphodynamic classsfications of the Shuidong Bay in South China YU Jitao1, 2, DING Yuanting1, 2*, CHENG Huangxin3, CAI Lailiang1, CHEN Zishen4 1 School of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China 2 School of Environmental Studies, China University of Geosciences, Wuhan 430074, China 3 Department of Landscape Architecture, China University of Geosciences, Wuhan 430074, China 4 Geography and Planning School, Sun Yat-sen University, Guangzhou 510275, China

Received 8 July 2015; accepted 15 October 2015 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016

Abstract

Beach morphodynamic classifications have achieved extensive acceptance in foreign coastal geomorphological studies. Three beaches located in different zones of a headland-bay coast are classified according to a dimensionless fall parameter, a relative tide range parameter and a dimensionless embayment scaling parameter. Synchronous data, including wave, tide, sediment and beach morphology, are respectively collected from the tangential beach, the transitional beach and the shadow beach of the Shuidong Bay during each spring tide for 16 successive months. The research results indicate that (1) the beach in the tangential zone falls between two major categories which are low tide terrace beaches with rips and barred beaches; the beach in the transitional zone exhibits two main types which are low tide bar/rip beaches and barred dissipative beaches; and the beach in the shadow zone mainly mirrors dissipative states with presence or absence of bars; and (2) the sequential changes and differences of beach states in different coastal zones reflect spatial and temporal variabilities of the headlandbay coast, totally meeting the actual measured beach morphology changes, showing that studies on wavedominated, meso-macrotidal beaches need to consider the influences of the tides. Meanwhile, the research mainly provides a framework about beach state studies, due to different beach states with different erosion patterns, which requires the need to strengthen the researches in this respect, in order to further enrich theoretical basis for a beach topography evolution, beach morphodynamic processes and beach erosion prevention in China. Key words: headland-bay beach, morphodynamic classifications, South China Citation: Yu Jitao, Ding Yuanting, Cheng Huangxin, Cai Lailiang, Chen Zishen. 2016. Wave-dominated, mesotidal headland-bay beach morphodynamic classsfications of the Shuidong Bay in South China. Acta Oceanologica Sinica, 35(7): 87–95, doi: 10.1007/s13131-0160907-9

1  Introduction A beach morphodynamic classification refers to the morphodynamic characteristics of a beach system, including its beach and surf zone morphology and associated wave shoaling, surf and swash characteristics (Short, 1999), which is available in supplying a conceptual framework within which beach and surf zone environments can be identified and studied (Scott et al., 2011). Since the 1970s researchers have been concerned about beach morphodynamic classifications and their responses to changing wave conditions resulting in the development of several morphological or morphodynamic beach models, which include microtidal sandy beaches of open coasts (e.g., Sonu, 1973; Guza and Inman, 1975; Short, 1979a, 1979b; Short, 1987; Wright and Short, 1984; Wright et al., 1985), as well as all wave-dominated beaches in all tidal ranges (Short, 1991; Short, 1996; Masselink and Short, 1993) and beach types of headland-bay coasts (Short, 1996; Klein and Menezes, 2001). Wright and Short (1984, hereafter referred to as “WS84 model”) used the dimensionless fall velocity (Ω) to quantify single

bar, microtidal beaches and to classify three distinctive beach types, = H b=(W s ¢ T);

(1)

where Hb is the breaker height (m), Ws is the sediment fall velocity (m/s), and T is the wave period (s). Value when the dimensionless fall velocity is less than 1 is related to a reflective state, the value of the dimensionless fall velocity between 1 and 6 with an intermediate state and the value when the dimensionless fall velocity is greater than 6 is associated with a dissipative state. Four different intermediate beach states are defined as low tide terrace (LTT), transverse bar and rip (TBR), rhythmic bar and beach (RBB) and longshore bar trough (LBT) with increasing dimensionless fall velocity. However, WS84 model is restricted the application to environments with larger tide ranges, because tides play a passive or indirect role including a retarding sediment transport rate and changes of beach morphology and shifting the relative ranges of the swash zone, surf zone and shoaling wave zone (Davis Jr, 1985;

   

Foundation item: The National Natural Science Foundation of China under contract No. 41301005; the Postdoctoral Science Foundation of China under contract No. 2014M552118. *Corresponding author, E-mail: [email protected]

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Masselink and Short, 1993), in order to incorporate increasing tide effect, Masselink and Short (1993, hereafter referred to as “MS93 model”) introduced the relative tide range parameter, RTR, which is defined as R TR = MS R =H b ;

(2)

where MSR is mean spring tide range (m), and Hb is the breaker height (m). By using Eqs (1) and (2), sandy beaches could be grouped into eight types that are illustrated in Fig. 1. When the RTR is less than 3 and Ω less than 2, the microtidal beach types dominate. When the RTR is between 3 and 15, tides increasing occupy the wide intertidal beach, with cusps confined to a high tide level, and bars and rips, if present, to a low tide level. When the RTR is greater than 15, the transition to tidal flats is reached. Both WS84 and MS93 models above apply to long beaches with no boundary effects. However, end effects (e.g., headlands, rocks, reefs, and structures) will influence the beach and the surf zone by inducing a wave refraction and attenuation, and by limiting the development of longshore currents, rips and rip feeder currents. Short and Masselink (1999, hereafter referred to as “SM99 model”) presented that the degree of end effects can be predicted using the dimensionless embayment scaling parameter ± 0 : ± 0 = S 12 =kC 1H b ;

(3)

where S1 is the embayment shoreline length (m), C1 is the chord length (m) between headlands, k is the surf zone slope (°), and Hb is the breaker height. A normal circulation occurs when the dimensionless embayment scaling is greater than 20, transitional circulation for the dimensionless embayment scaling parameter between 8 and 20, and cellular circulation for the dimensionless

 

embayment scaling being less than 8. In summary, these studies aforementioned encompassed a wide range of temporarily and spatially varying environmental conditions, sediment types, and morphodynamic states, which had an important theoretical significance in beach erosion processes and beach development, utilization and protection. By contrast, domestic studies on the beach classifications are very sparse (e.g., Chen and Li, 1990; Chen et al., 1991, 2000; Wang and Yang, 1996; Cai et al., 2007; Qi et al., 2010; Li and Zhu, 2015). Previously Chen and Li (1990), Chen et al. (1991, 2000), and Wang and Yang (1996) respectively employed the surf scaling parameter index (Guza and Inman, 1975) to study the beach state types of some sites along the coasts of South China. These studies may have achieved some valuable conclusions for the microtidal beaches, but possibly ignored tide effect for meso-macrotidal beaches leading to some deviations of morphodynamic classifications. Cai et al. (2007) proposed a grouping of sandy beaches into cape-bay coast, barrier-lagoon coast, and straightened coast. Recently, based on the MS93 model, Qi et al. (2010) classified three main beach types under the effect of tropical storms in South China. Similarly, Li and Zhu (2015) also used the MS93 Model and divided 51 headland-bay beaches in South China into seven groups with the help of hierarchical cluster analysis. The former studied the characteristics of storm-effects on beaches with different morphodynamic types through one profile located in each beach, and the latter viewed each headland-bay beach as a whole, without considering morphodynamic differences of different coastal zones within a headland-bay coast. The headlandbay beach is a dominant coastal landform in South China, which spreads widely and exceeds 100 locations (Yu and Chen, 2011). The lack of associated studies and the little attention on the morphodynamic classifications of a single headland-bay beach, to some extent, restricted beach evolution and morphodynamics

  Fig. 1.  Eight beach states determined by dimensionless fall velocity and relative tide range (Masselink and Short, 1993).

 

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in China. Therefore, the objectives of this paper are: (1) to review three types of beach classification models and preliminarily elucidate morphodynamic sequences and classifications for a wavedominated, mesotidal headland-bay coast; and (2) to put forward a framework and some perspectives on the studies for headland-bay sandy beach states in South China. 2  Study area and data set 2.1  Study sites The Shuidong Bay is located in Maoming City of Guangdong Province, which is comprised of a bar, a lagoon and a tidal channel. About 9 km long bar, spreading from NE to SW, separates the northern lagoon from the southern sea. Because a huge ebbing delta with a small water depth plays an energy dissipation effect on incident waves and ebbing tidal currents with jet flows characteristics produces resistance to incident waves, the ebbing delta has a property similar to a depositional headland within the bay. Under the shelter of this “depositional headland” and the control of Yanjing headland downcoast, the coastline planform in the west of the bay mouth is characterized by a headland-bay sandy coast with a curved shadow zone, a gently curved transitional zone, and a relatively straight tangential zone (Fig. 2). The different morphodynamic characteristics of beaches in the shadow zone, the transitional zone and the tangential zone are as follows, respectively.

  Fig. 2.  Position of the study sites.

(1) The tangential beach of this bay begins at the foredune which is composed of a mixture of medium and fine sand. The foredune with a maximal height of about 6 m decreases gradually from the west to the east. This beach consists of a relatively steep, swash dominated beach face, which is linear with the average slope in excess of 7°. But the slope under the beach face significantly becomes gentle with the average slope of 0.03°. The sediment of this zone is mainly coarse sand, and a low wave energy and long wave periods dominate this zone. And the beach in the tangential zone is characterized by the persistent berm almost all the year round and ridge and runnel migrating shoreward in a quasiperiodic way. (2) The shadow zone extends eastward to the west side of the tidal inlet and the sediment of the beach is made up of fine sand or very fine sand. The beach is characterized by a wide, low gradient swash zone, a wide surf zone, driven by spilling breakers which dissipate their energy across the wide surf zone. The beach, the gentle ridge plain, and connecting low sea-cut scarp

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are the main topographic features. Sometimes, some small transverse or oblique sandbars can be observed under the low water line. The beach profile is a concave with an average slope of 1.5° (3) The transitional zone located between the tangential and shadow zones, whose backshore links with the discontinuous gentle foredune. The sediment composition, topographic characteristics and profile morphology vary largely. Form the west to the east of the transitional zone, the subaerial accumulated topography including berm and cusp gradually disappears. And along the longshore direction, the slope of this zone changes from steep to gentle, simultaneously with obvious transformation between the subaerial and subaqueous topographies. When the subaqueous bar moves shoreward in a quasiperiodic way, the berm will be rebuilt on the beach, and the low tide terrace will be formed around the low tide level. Meanwhile, the type of waves within this bay is mainly wind waves (or sea), with an average wave height of 0.66 m and an average period of 4 s. A SSE wave prevails over the course of 1 a, but during the summer months the SE or SEE prevailing waves dominate this bay. The tide pattern in this marine area can be classified as the irregular semidiurnal tide with an average spring tidal range at 2.6 m. In term of the wave conditions and tide regimes, this bay can be grouped as the lower wave-dominated, mesotidal coast (Chen and Li, 1993). The waves, superimposed on the periodic tidal levels, are the most important forcing to the beach evolution, while tides play a passive or indirect role in a sediment transport and changes in beach morphology (Davis, 1985). 2.2  Data set In this paper data were from the study of Chen et al. (2000) . Three profiles designated No. 1, No. 2, and No. 3 were arranged along the Shuidong Bay coast, representing the tangential, transitional, and shadow zones, respectively (Fig. 2). Each profile began at the conjoint site landward between the bottom of foredune and backshore that storm waves are difficult to reach, and ended at around low tide level shoreward. According to Chen and Li (1993) , these three profiles were measured monthly during spring and neap tides for 16 months, obtaining 68 beach morphological data on No. 1 profile and 67 beach morphological data on No. 2 profile or No. 3 profile. At the same time, wave, tide and wind data were collected by a wave and tide gauges. Additionally, the sediment samples were gathered monthly from the high-tide, mid-tide, and low-tide zone respectively, in order to observe the sediment changes from alongshore and onshore-offshore directions. In this paper, 32 profile data and associated wave, tide, sediment data were used to study for each zone during the observation of 16 months. Under the normal wave conditions due to the influence of this depositional headland around the tidal inlet, the wave refraction and diffraction would occur to make the wave directions and heights change. Thus, Yu (2011) used the modified mild-slope equation for a combined refraction-diffraction of waves proposed by Ebersole (1985) to simulate the average wave field of Shuidong Bay with 2 m wave height input of the normal conditions, and found that under the impact of this headland the wave directions in the shadow zone changed and the wave heights slightly decreased relative to the wave heights in the tangential or transitional zone, but the changes of the wave directions and heights in the tangential and transitional zones were not obvious. Thus the deep-water wave data measured at Yanjing wave gauge could be used to extrapolate the breaker heights and further analyze the wave characteristics in different coastal zones. It must be

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noted that because of the influence of submarine topography in the shadow zone, the deep-water wave heights were a bit lower leading to the extrapolated breaker heights a bit higher than the actual heights. The breaker height was solved by the deep-water wave height and period according to the formula proposed by Komar and Gaughan (1972), which is defined as H b = 0:39g 1=5 (TH 02 )2=5 :

(4)

According to Masselink and Short (1993), a high tide beach sediment size, a modal wave height and period were used to compute the dimensionless fall velocity Ω, and the mean spring tide range together with the modal wave height was used for obtaining the relative tide range. Two tidal constituents were considered by a tidal harmonic analysis, the principal lunar (M2) and the principal solar (S2), whereby the mean spring tide range is 2 ( HM2 + HS2 ), equal to 2.02 m in this paper. The relative tide range was obtained by the mean spring tide range and the breaker height in term of Eq. (2). Additionally following SM99 model, the value of the dimensionless embayment scaling parameter by Eq. (3) was far more than 19, reflecting that the normal beach circulation occurred and the influence of the headland on the beach circulation was

 

very small, thus WS84 model and MS93 model were available within this bay. Table 1 lists the wave periods, the breaking wave heights, the relative tide ranges, and the dimensionless fall velocities and the types of three different coastal zones. 3  Results and analyses 3.1  Beach state types and characteristics of the tangential beach The beach states in the tangential zone of the Shuidong Bay during the 32 times spring tide observations for 16 months are shown in Fig. 3. A type Ⅱ appears 19 times with a frequency of 59.4%, and Type Ⅳ occurs 9 times with a frequency of 28.1%. A total of frequency of these two beach types reaches 90.7%. Additionally, the type Ⅰ happens twice and Type Ⅴ or Ⅵ comes up once respectively. A total of frequency of the latter three beach types accounts for 12.5%. The morphodynamic processes associated with these five beach types are as follows. (1) Type Ⅱ corresponds to the low tide terrace beach with rips in the reflective group (Ω<2 and 3
Table 1.  Wave period, extrapolated breaking wave height, dimensionless fall velocity, relative tide range and beach types of the Shuidong Bay Profile sequence

T/s

Hb/m

RTR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

5.45 3.50 3.76 4.38 3.68 3.70 4.03 4.10 4.13 4.61 3.53 2.24 3.86 3.57 4.31 4.61 3.96 3.12 3.28 3.80 4.36 3.78 3.67 3.83 3.73 3.63 3.70 3.65 4.15 4.22 4.05 4.14

3.15 0.46 0.51 0.72 0.56 0.83 1.01 0.95 0.48 1.18 0.88 0.45 0.62 0.54 0.89 0.71 0.66 0.48 0.39 0.63 0.89 0.41 0.56 0.44 0.56 0.48 0.41 0.56 0.57 0.89 0.57 0.87

0.6 4.4 3.9 2.8 3.6 2.4 2.0 2.1 4.2 1.7 2.3 4.5 3.3 3.7 2.3 2.8 3.1 4.2 5.2 3.3 2.3 4.9 3.6 4.5 3.6 4.2 4.9 3.6 3.5 2.3 3.6 2.3

Tangential beach Ω Type 6.7 Ⅵ 1.5 Ⅱ 1.6 Ⅱ 1.9 Ⅰ 1.8 Ⅱ 2.6 Ⅳ 2.9 Ⅳ 2.7 Ⅳ 1.3 Ⅱ 3.0 Ⅳ 2.9 Ⅳ 2.3 Ⅳ 1.9 Ⅱ 1.8 Ⅱ 2.4 Ⅳ 1.8 Ⅰ 1.9 Ⅱ 1.8 Ⅱ 1.4 Ⅱ 1.9 Ⅱ 2.4 Ⅳ 1.3 Ⅱ 1.8 Ⅱ 1.3 Ⅱ 1.7 Ⅱ 1.5 Ⅱ 1.3 Ⅱ 1.8 Ⅱ 1.6 Ⅱ 2.4 Ⅳ 1.6 Ⅱ 2.4 Ⅳ

Transitional beach Ω Type 17.6 Ⅵ 4.0 Ⅴ 4.1 Ⅴ 5.0 Ⅳ 4.6 Ⅴ 6.8 Ⅵ 7.6 Ⅵ 7.0 Ⅵ 3.5 Ⅴ 7.8 Ⅵ 7.6 Ⅵ 6.1 Ⅶ 4.9 Ⅴ 4.6 Ⅴ 6.3 Ⅵ 4.7 Ⅳ 5.1 Ⅶ 4.7 Ⅴ 3.6 Ⅴ 5.0 Ⅴ 6.2 Ⅵ 3.3 Ⅴ 4.6 Ⅴ 3.5 Ⅴ 4.6 Ⅴ 4.0 Ⅴ 3.4 Ⅴ 4.7 Ⅴ 4.2 Ⅴ 6.4 Ⅵ 4.3 Ⅴ 6.4 Ⅵ

Shadow beach Ω Type 24.3 Ⅵ 5.5 Ⅶ 5.7 Ⅶ 6.9 Ⅵ 6.4 Ⅵ 9.4 Ⅵ 10.5 Ⅵ 9.7 Ⅵ 4.9 Ⅴ 10.8 Ⅵ 10.5 Ⅵ 8.4 Ⅶ 6.7 Ⅶ 6.4 Ⅶ 8.7 Ⅵ 6.5 Ⅵ 7.0 Ⅶ 6.5 Ⅵ 5.0 Ⅶ 7.0 Ⅶ 8.6 Ⅵ 4.6 Ⅴ 6.4 Ⅶ 4.8 Ⅴ 6.3 Ⅶ 5.6 Ⅶ 4.7 Ⅴ 6.4 Ⅶ 5.8 Ⅶ 8.9 Ⅵ 5.9 Ⅶ 8.8 Ⅵ

 

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  Fig. 3.  Beach states in the tangential zone of the Shuidong Bay determined by the dimensionless fall velocity and the relative tide range.

1993). During the on-site observations, the average beach face slope with 19 beach profiles belonging to this type (Fig. 4b) is 7.36° (actually 5.45°–9.01°), which shows that the beach face is steep. The high-tide beach cusps remain all the year round (Chen et al., 1991; Chen and Li, 1993). The median sand size D50 is 0.73 mm within the high-tide zone and 0.63 mm within the low-tide zone. In addition, the low tide terrace beach is also known as “ridge and runnel”. The ridge and runnel system within the tangential beach has been confirmed by Chen and Li (1993) , at the same time Fig. 4b also indicates the existence of the low tide terrace. At mid-tide waves usually break right across the shallow bar and may induce a weak, ephemeral rip circulation. These characteristics above conform to those descriptions of Masselink and Short (1993) to this kind of beach.

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(2) Type Ⅳ refers to the barred beach in the intermediate group (2<Ω<5 and RTR<3). Chen et al. (1991) used a factor analysis to study beach profile change processes in the tangential zone of the Shuidong Bay and presented that the first principal factor embodies the maximum fluctuation in the surf zone as the leading characteristics, that is to say, the subaqueous sediment moves landward by means of the ridge and runnel system, which constitutes the matter source and the principal feature resulting in the formation and variation of the entire profile. Figure 4c indicates the presence of the subaqueous bars. At this time waves break by plunging breaker heavily on the outer edge of the bar and dissipate across the ridge and runnel or the terrace. (3) Type Ⅰ is the reflective beach in the reflective group (Ω<2 and RTR<3). The reflective beach is characterized by the steep beach face and commonly cusped morphology. Wave are usually surging or plunging on the beach and most of the wave energy are at incident and subharmonic (twice the wave period) frequencies (Huntley and Bowen, 1975; Wright and Short, 1984). The fluid dynamic characteristics may better explain the lower cusps around the mean high-tide level during the filed measurement (Chen et al., 1991) (Fig. 4a). (4) Type Ⅴ points to the low bar/bar in the intermediate group (2<Ω<5 and 3
  Fig. 4.  Measured different types of beach profile shapes in the tangential zone of the Shuidong Bay. a. Type Ⅰ is the reflective beach, b. Type Ⅱ is the low tide terrace beach with rips, c. Type Ⅳ is the barred beach, d. Type Ⅴ is the low tide bar/rip beach, and e. Type Ⅵ is the barred dissipative beach.

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sipative group (Ω>5 and RTR<3). The dissipative beach profile is characterized by a subdued longshore bar-trough morphology. Waves are of the spilling type and water motion in the inner surf zone would be dominated by infragravity waves. An onshore mass transport is by the spilling waves and bores while a strong offshore-directed bottom flow dominates the return current pattern (Wright et al., 1982; Greenwood and Osborne, 1990). This type of the beach occurred one time throughout the entire observation period, which resulted from the typhoon as Number 8616. During the typhoon, the infragravity edge waves motivated by the high energy conditions strongly disturbed the beach face to move sediment offshore (Chen, 1996), which led to an obviously gentle beach face and a decreasing slope and the formation of the subaqueous bars. The bars and wave set-up in the nearshore zone caused the highly dissipative beach. Above all, the main beach states in the tangential zone of the Shuidong Bay are the low tide terrace beach with rips in the reflective group and the barred beach in the intermediate group. The beach in this area is markedly characterized by the steep beach face, which embodies the pronounced reflectivity. 3.2  Beach state types and characteristics of the transitional beach Figure 5 indicates the beach state types in the transitional zone of the Shuidong Bay during the 32 times spring tide observations for 16 months. Type Ⅴ occurs 18 times with a frequency of 56.3%, and Type Ⅵ appears 10 times with a frequency of 31.3%, with a total of frequency of these two beach types reaching 87.6%. Furthermore, Type Ⅳ or Ⅶ happens twice, respectively, with a total of frequency of these two beach types accounting for 12.5%. The morphodynamic processes associated with these four beach types are as follows. (1) Type Ⅴ refers to the low tide bar/rip beach in the intermediate group (2<Ω<5 and 35 and RTR<3). The beach is characterized by the subdued longshore bar-trough morphology (Masselink and Short, 1993) (Fig. 6c) and the low gradient beach face which

 

  Fig. 5.  Beach states in the transitional zone of the Shuidong Bay determined by the dimensionless fall velocity and the relative tide range.

consists of fine or very fine sand (the average sediment size at 0.16 mm measured in the low-tide zone). The type of wave breaking is the spilling breaker and water motion in the inner surf zone would be dominated by the infragravity waves. The spilling breakers and bores dominate an onshore mass transport, however the return current pattern is controlled by a strong offshore-directed bottom flow (Wright et al., 1982; Greenwood and Osborne, 1990). (3) Type Ⅳ is the barred beach in the intermediate group (2<Ω<5 and RTR<3; Fig. 6a). And Type Ⅶ indicates the nonbarred dissipative beach in the dissipative group (Ω>5 and 3< RTR<7). At this stage, the beach becomes flatter and more featureless with no bars (Fig. 6d). Thus to sum up, the beach in the transitional zone of the Shuidong Bay mainly vibrates between the low tide bar/rip beach in the intermediate group and the barred dissipative beach in the dissipative group. The existence of the low-tide or submarine bar is the predominant feature in this zone. 3.3  Beach state types and characteristics of the shadow beach Figure 7 illustrates the beach state types in the shadow zones of the Shuidong Bay during the 32 times spring tide observations for 16 months. Type Ⅵ or Ⅶ with a frequency of 43.8% respectively happens 14 times. In addition, Type Ⅴ occurs four times

  Fig. 6.  Measured different types of beach profile shapes in the transitional zone of the Shuidong Bay. a. Type Ⅳ is the barred beach, b. Type Ⅴ is the low tide bar/rip beach, c. Type Ⅵ is the barred dissipative beach, and d. Type Ⅶ is the non-barred dissipative beach.

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with a frequency of occurrence arriving at 12.4%. The morphodynamic processes associated with these three beach types are as follows.

  Fig. 7.  Beach states in the shadow zone of the Shuidong Bay determined by by the dimensionless fall velocity and the relative tide range.

Both Type Ⅵ and Ⅶ can be grouped into the dissipative group (Ω>5). As the relative tide ranges are between 0 and 3, Type Ⅵ indicates the barred dissipative beach which is represented by the subdued longshore bar-trough morphology (Fig. 8b). However as the relative tide ranges increase (3
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and none regularly, go through a entire sequence, such as reflective-intermediate-dissipative or dissipative-intermediate-reflective; rather most oscillate between two or three beach states. As noted above, the beach states vibrate between the low terrace beach with rips and the barred beach in the tangential zone, between the low tide bar/rip beach and the barred dissipative beach in the transitional zone, and between the barred and nonbarred dissipative beach in the shadow zone, which embodies the temporal and spatial variability of the beach states in different zones of a headland-bay coast. Figure 9 shows the sequential changes of the beach states in the tangential, transitional, and shadow zone during the 32 times spring tide observations for 16 months, which implies that the studies on the beach states should need synchronous data (including wave, tide, sediment, etc.) for a long time to adequately acquire the main state types of a beach. Because the beach morphology depends on several variables such as wave, tide, sediment and so on (Anthony, 1998; Jackson et al., 2005), the beach state classifications with historical data maybe produce much deviation (e.g., Li and Zhu, 2015; Li, 2015). Furthermore, the differences of the main beach states among three coastal zones reflect the spatial and temporal variabilities of the beach states, basically conforming to the field beach morphology changes. That also implies that studies on the meso-macrotidal beach morphodynamics should consider the effect of tides, in accordance with the study results of Yu et al. (2015) . 4.2  Different wave-dominated beach types with erosion patterns Beach type changes are an indicator of temporal and spatial variability of beach morhpodynamics, and different types of the beaches have diverse erosion patterns or beach stability. (1) The beach in the shadow zone of the Shuidong Bay is mainly characterized by the remarkable dissipativity. The dissipative beaches usually remain a relatively stable morphology, and have minimal shoreline change (Short and Hesp, 1982; Short, 1983). However, Chen (1996) observed a dune erosion and the formation of parallel scarping at the bachshore. Previous studies points out that on a dissipative beach, the beach erosion is resulting from higher waves, which induce the increases of the infragravity wave energy, the dimension of the sanding waves and associated wave set-up and set-down, and the intension and seaward limit of the bed-return flow (Short, 1999). The higher swash waves and infragravity bores, superposing on an increasing level of wave set-up, would lead to the backshore be seriously scoured (Wright, 1980). Thus, the most observable and remarkable erosion on a dissipative beach occurs at the backshore. (2) The highest occurrence frequency appears to be the low tide bar/rip beach in the intermediate group during the observations for 16 months in the transitional zone of the Shuidong Bay.

  Fig. 8.  Measured different types of beach profile shapes in the shadow zone of the Shuidong Bay. a. Type Ⅴ is the low tide bar/rip beach, b. Type Ⅵ is the barred dissipative beach, and c. Type Ⅶ is the non-barred dissipative beach.

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  Fig. 9.  Sequential beach state changes of different zones of the Shuidong Bay for 16 months. a. The tangential beach, b. the transitional beach, and c. the shadow beach.

The intermediate beaches are not only the most common beach type, but the most dynamic beach type in the temporal and spatial scales. The associated studies indicate that the erosion mode of intermediate beaches is driven by the rip currents which move seaward. Wright (1980) proposed that the most severe erosion appears in or in lee of arrested rips, which Short and Hesp (1982) named as a rip embayment erosion. On an intermediate beach, the size and intensity of rip would increase while the wave height increased (Short, 1985), resulting in discontinuous erosion maxima in lee of rips. Thus, the rips may be the primary agent of beach erosion for the transitional beach of the Shuidong Bay. (3) The beach in the tangential zone of the Shuidong Bay is predominantly characterized by the pronounced reflectivity. In this paper the presence of the low tide terrace is identified and the low terrace beach with rips is the overwhelmingbeach state in this zone. Short (1999) pointed out that reflective beaches were relatively stable. The erosion of the reflective beaches is attributed to the growth of both incident wave swash and subharmonic edge waves. First, the increase of a swash volume makes the beach saturated, eventually difficultly building a beach face and an erosion scarp during the periods of short waves and/or neap or low tide. But during long waves and/or high or spring tide, it would result in overwashing and thus slumping of the beach face (Short, 1979b). The final result is the subaerial beach erosion, along with the accretion commonly as an attached bar or the terrace at the base of the beach. Second, while the erosion is following by the growth of the subharmonic edge waves, that would produce the edge wave-dominated maximum erosion with erosion length scales reaching tens of meters, as a result of the formation of “erosion” cusps (Short, 1979a, 1979b). Chen (1996) observed that the edge waves excited by storm waves could result in beach cut located at the toe of the foredune. Therefore, the erosion of a reflective beach usually occurs during a high-energy or potentially destructive event. 5  Conclusions and prospective This paper reviews three representative beach morphody-

namic classification models about wave-dominated beaches with the microtidal ranges, as well as all tidal ranges, and the headland-bay beaches. In the context of considering the headland effect, this paper focuses on a wave-dominated, mesotidal headland-bay coast—the Shuidong Bay in west Guangdong Province and uses the model proposed by Masselink and Short (1993) to study the beach state types and variability in the tangential, transitional, and shadow zones of this bay. Some interesting and important conclusions were obtained based on the aforementioned analysis. (1) The main beach states in the tangential zone are the low tide terrace beach with rips in the reflective group or the barred beach in the intermediate group, and a beach pronounced reflectivity is the predominant characteristic in this zone. The beach states in the transitional zone mainly change between the low tide bar/rip beach in the intermediate group and the barred dissipative beach in the dissipative group, and the existence of the low-tide or submarine bar is the major feature in this zone. And the beach states in the shadow primarily are the barred or non-barred dissipative beach in the dissipative group, and the beach remarkable dissipativity is the prime signature. (2) The sequential changes and differences of the beach states in different zones of the Shuidong Bay reflect the beach spatial and temporal variability. Because the studies on the beach state classifications involve several factors such as wave, tide, sediment etc., which would require concurrent measurements for topography, fluid dynamics and sediment for a long time in the future research. In addition, previous research carried out by Wright et al. (1985) based on field observations made over a period of 6.5 a revealed that the instantaneous dimensionless fall velocity value was a poor indicator of the day-to-day beach state. And the morphodynamic response to waves is not instantaneous, thus the prediction could be improved by using a weighted average value for a given period. (3) The beach states are a three-dimensional morphodynamic problem relating beach morphology with fluid dynamics. Different beach states would accompany with different erosion pat-

 

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