Junli Guo

Lianqiang Shi

Shenliang Chen
email: slchen@sklec.ecnu.edu.cn

Bruno Castelle

Yang Chang

Wufeng Cheng

Sand-mud transition dynamics at embayed beaches during a typhoon

Keywords: Embayed beaches, sand-mud transition (SMT), human intervention, typhoon season, beach stability

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Introduction

The concept of sand-mud transition (SMT) was first proposed by [START_REF] Mccave | Transport and escape of fine-grained sediment from shelf areas[END_REF] and [START_REF] Stanley | The "mud-line": An erosion-Deposition boundary on the upper continental slope[END_REF] based on changes in the grain size of surficial sediments, defined where the mean grain size is 63 μm (4Փ) or the mud content is 25%. The differences in the biological, physical, geological, and chemical processes between its two sides make the SMT a particularly important boundary [START_REF] George | Flocculation, heavy metals (Cu, Pb, Zn) and the sand-mud transition on the Adriatic continental shelf, Italy[END_REF].

Generally, all coasts have one or more SMTs while progressively moving offshore, but their offshore distance and water depth vary considerably in different seas [START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF]. The location of the SMT and its water depth are closely related to the hydrodynamic forcing [START_REF] Dunbar | Estimating palaeobathymetry of wave-graded continental shelves from sediment texture[END_REF][START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF][START_REF] Stanley | The mudline: variability of its position relative to shelfbreak[END_REF], sediment supply [START_REF] Chang | Age and morphodynamics of a sandy beach fronted by a macrotidal mud flat along the west coast of Korea: a lateral headland bypass model for beach-dune formation[END_REF][START_REF] Edwards | Variations in sediment texture on the northern Monterey Bay National Marine Sanctuary continental shelf[END_REF][START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF]Zhao et al., 2020a), and beach morphological changes (Zhao et al., 2020b).

Sandy coasts account for approximately 31% of the global ice-free coasts and have high socio-economic and ecological values [START_REF] Luijendijk | The State of the World's Beaches[END_REF]. Previous studies on beach morphodynamics mainly focused on storm-driven beach erosion [START_REF] Ge | LIDARbased detection of the post-typhoon recovery of a meso-macro-tidal beach in the Beibu Gulf, China[END_REF][START_REF] Qi | The response of three main beach types to tropical storms in South China[END_REF], beach equilibrium planform [START_REF] Castelle | Modelling of embayed beach equilibrium planform and rotation signal[END_REF][START_REF] Dai | The configuration of equilibrium beach profile in South China[END_REF][START_REF] Li | Barred beach profile equilibrium investigated with a process-based numerical model[END_REF][START_REF] López | Determination of the study period necessary for calculating the equilibrium beach profile and the depth of closure[END_REF], beach responses to human interventions [START_REF] Cai | Beach nourishment in China: Status and prospects[END_REF][START_REF] Castelle | Beach nourishments at Coolangatta Bay over the period 1987-2005: Impacts and lessons[END_REF][START_REF] Chiva | The effects of sediment used in beach nourishment: Study case El Portet de Moraira beach[END_REF][START_REF] Cooke | Nourishment practices on Australian sandy beaches: A review[END_REF][START_REF] Hamm | A summary of European experience with shore nourishment[END_REF][START_REF] Hanson | Beach nourishment projects, practices, and objectives-a European overview[END_REF][START_REF] Luo | A guide to coastal management: Benefits and lessons learned of beach nourishment practices in China over the past two decades[END_REF] or beach stability [START_REF] Hudson | Delivering sustainable coasts: Monitoring the long-term stability of a breached barrier beach, Porlock Bay, Somerset, United Kingdom[END_REF][START_REF] Liu | A method to nourished beach stability assessment: The case of China[END_REF]. The SMT is closely related to beach morphodynamics, and its landward migration (Zhao et al., 2020a) has gradually attracted the attention of coastal managers and researchers.

Earlier SMT research focused on the location and depth of SMT and their influencing factors. SMT locations detected at the Monterey Bay in the United States showed the contrasting effects of storm-versus fluvial-dominated conditions on sedimentation [START_REF] Edwards | Variations in sediment texture on the northern Monterey Bay National Marine Sanctuary continental shelf[END_REF]. Then, a more comprehensive study about SMT water depth and hydrodynamic conditions in different regions showed that change in SMT depth is mainly controlled by wave-induced bottom shear stress and a functional equation for estimating the SMT water depth using wave energy [START_REF] Dunbar | Estimating palaeobathymetry of wave-graded continental shelves from sediment texture[END_REF]. Controls on SMT depth were hypothesized using a large number of wave data on 14 sites around the world [START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF]. The dynamic changes of sediments and topography of Dongsha beach at Zhujiajian Island, Zhejiang Province, China suggested that the SMT migrates seasonally, which was the first research on SMT migration in China [START_REF] Cheng | Sedimentation and recent morphological changes at Dongsha beach, Zhujiajian Island , Zhejiang Province[END_REF]. Meanwhile, there are a large number of studies focusing on the SMT of sandy-muddy beaches. Different geomorphological stages of the sandy-muddy beaches with various SMT migrations are closely related to the enrichment of fluvial sediment and nearshore mud near the Amazon River estuary [START_REF] Anthony | Short-term changes in the plan shape of a sandy beach in response to sheltering by a nearshore mud bank, Cayenne, French Guiana[END_REF][START_REF] Anthony | The influence of Amazon-derived mud banks on the morphology of sandy headland-bound beaches in Cayenne, French Guiana: a short-to long-term perspective[END_REF], which was also confirmed on the west coast of North Korea [START_REF] Chang | Age and morphodynamics of a sandy beach fronted by a macrotidal mud flat along the west coast of Korea: a lateral headland bypass model for beach-dune formation[END_REF]. Changes in SMT depth of sandy-muddy beaches are mainly controlled by nearshore waves [START_REF] Xu | Study on the sandy-muddy hybrid beach and sand-silt demarcation point: survey on the type of Xiamen Bay Beach[END_REF] and sediment supply [START_REF] Zhou | Comparative study of sand-mud transition on two sandy-muddy transitional beaches along west coast of the Taiwan Straits[END_REF]. The distribution and migration of SMT on sandy-muddy beaches in different estuaries and bays in southeastern China were further summarized by Zhao et al. (2020aZhao et al. ( , 2020b)).

In the past few decades, approximately 50% of China's sandy coasts have been degraded due to erosion (Third Institute of Oceanography, 2010), and numerous protection practices have been implemented on beaches [START_REF] Cai | Beach nourishment in China: Status and prospects[END_REF][START_REF] Kuang | Morphological processes of two artificial submerged shore-parallel sandbars for beach nourishment in a nearshore zone[END_REF][START_REF] Luo | A guide to coastal management: Benefits and lessons learned of beach nourishment practices in China over the past two decades[END_REF]. The Zhoushan Archipelago is a popular tourist destination of Zhejiang Province, China, with more than 30 embayed beaches, of which the beaches on the east coast of Zhujiajian Island, are particularly famous [START_REF] Xia | China's Islands: Zhejiang[END_REF]. Typhoon is a common coastal disaster that affects the Zhoushan Archipelago, with an average of 6 typhoons per year [START_REF] Lu | Research Report on the Regional Climate of 908 Special Island Survey in Zhejiang Province[END_REF], which has a significant impact on beach morphology and even threatens the tourism industry. The period from July to September each year is called the typhoon season, accounting for about 93% of the total number of yearly typhoon landfalls [START_REF] Wang | Analysis on climatic characteristics of typhoon over the past 50 years at Zhoushan[END_REF]. However, the distribution and migration of SMTs

Regional setting

The Zhujiajian Island is the fifth largest island in the Zhoushan Archipelago, Zhejiang Province, China (Fig. 1a), with a land area of 62.2 km 2 and a 79.2-km long coastline [START_REF] Xia | China's Islands: Zhejiang[END_REF]. The embayed beaches on the east coast of Zhujiajian Island are famous tourist destinations in China, of which Dashali, Dongsha, and Qiansha are representative beaches with different degrees of wave exposure and human interventions.

Dashali beach is located in the north of Zhujiajian Island, with a length of about 900 m and headlands at both ends extending approximately 600 m offshore (Fig. 1c).

The Dashali beach slope ranges from 2.2% to 2.8%, with an average slope of 2.4%.

Although there is a seawall backing Dashali beach, this beach still has well-preserved vegetated dunes. The median sediment grain size (D50) is between 2.12 Փ to 2.74 Փ, and the sediment is mainly composed of fine sand. Dongsha beach is a 1500-m long embayed sandy beach, with two headlands extending approximately 1350 m offshore (Fig. 1d). Beach slope ranges along the coast from 2.9% to 3.5%, and surficial sediment D50 is between 1.4 Փ to 2.74 Փ. There were sand dunes at the back of the beach before 2012. The completion of the seawall in 2012 broke the balance of cross-shore sediment [START_REF] Cheng | Sedimentation and recent morphological changes at Dongsha beach, Zhujiajian Island , Zhejiang Province[END_REF], which further led to the disappearance of the dunes. The construction of seawalls and frequent storms resulted in dominant beach erosion for years [START_REF] Guo | The response to tropical storm Nakri and the restoration of Dongsha Beach in Zhujiajian Island, Zhejiang Province[END_REF]. In order to fight against stormdriven erosion and increase the dry beach area, beach nourishments have been performed with a total of 52,000 m 3 placed on Dongsha beach between 2016 and 2017, with large impact on beach processes [START_REF] Guo | Monitoring and evaluation of sand nourishments on an embayed beach exposed to frequent storms in eastern China[END_REF].

Qiansha beach is adjacent to the south side of Dongsha, with a total length of about 1200 m, with headlands extending approximately 800 m offshore (Fig. 1e). Beach slope varies alongshore from 1.6% to 2.9%, gradually decreasing southwards, with an average of 2.2%. The beach is backed by cliff, with a few vegetated sand dunes. The surficial sediment D50 is between 2.18 Փ to 3.18 Փ. Amongst the three studied beaches Qiansha beach is the only natural beach without any coastal engineering work.

Materials and methods

Hydrodynamic data

The typhoon information was obtained from the National Meteorological Center of China (http://typhoon.nmc.cn). From 1949 to 2019, a total of 46 typhoons reached Zhejiang Province, and 236 typhoons affected the coast of Zhejiang. The frequency of typhoons that have affected Zhejiang Province increased during the past decades [START_REF] Guo | Response of Dongsha beach in Zhoushan to continuous storms based on Argus images[END_REF]. The main typhoon types are Severe Tropical Storms and Typhoons, with a few Super Typhoons. From July to September (typhoon season) each year, the typhoons that affect Zhejiang correspond to approximately 93% of the total [START_REF] Wang | Analysis on climatic characteristics of typhoon over the past 50 years at Zhoushan[END_REF].

The tidal data provided by the Shenjiamen Marine Station (29.93°N, 122.3°E, Fig. 1b) shows that the tides in the Zhujiajian Island area are mainly semi-diurnal, with an average tidal range of 2.6 m [START_REF] Xia | China's Islands: Zhejiang[END_REF]. The wave data is obtained hourly from a wave buoy (29.8°N, 122.5°E, Fig. 1b) located in approximately 20 m depth. The dominant wave direction in the study area is from the east, and the waves with larger significant wave heights mainly come from the directions between northeast and southeast [START_REF] Guo | Monitoring and evaluation of sand nourishments on an embayed beach exposed to frequent storms in eastern China[END_REF]. The maximum significant wave height observed by the wave buoy during the 2019 typhoon season was 6.77 m.

Topographic data

To obtain the characteristics of topography and surficial sediment changes of Dashali, Dongsha, and Qiansha beaches and their nearshore area during typhoon season, we carried out three field campaigns on July 18, 2019, August 31, 2019, and September 24, 2019, representing the early, middle and late stages of the typhoon season, respectively. According to the individual characteristics of the three beaches, 10, 30, and 17 beach profiles are set up on Dashali, Dongsha and Qiansha (Fig. 1, Table 1), respectively. In addition, Dashali, Dongsha and Qiansha have five (YA02, YA04, YA06, YA08, YA10, see in Fig. 1c), six (DS05, DS11, DS17, DS22, DS26, DS30, Fig. 1d) and five (QS01, QS05, QS09, QS13, QS17, Fig. 1e) typical profiles, which include beach and nearshore surficial sediments. The beach topography was monitored at low tide using RTK GPS from the network of Continuously Operating Reference Stations (CORS, with plane and vertical precisions of ±8 mm and ±15 mm, respectively) with fixed-point measurements starting from the base (here, the base refers to a boundary between the sandy beach and cliffs/artificial seawall), and the elevation data of 171 profiles were all corrected to the Yellow Sea Datum 1985.

Beach sand volume per unit meter width, Vprofile in m 3 /m, was calculated for every typical cross-shore profile based on the shortest profile over the study period according to [START_REF] Burvingt | Climate forcing of regionally-coherent extreme storm impact and recovery on embayed beaches[END_REF]:

𝑉 𝑝𝑟𝑜𝑓𝑖𝑙𝑒 = ∫ 𝑧𝑑𝑧 𝑧 𝑚𝑎𝑥 𝑧 𝑚𝑖𝑛 (Equation 1)
where z corresponds to the topographic values interpolated every meter, and zmin and zmax are the lowest and the fixed backshore (seawall/cliff base) topographic points (Fig. 2), respectively. The zmin on each beach is slightly different in this study due to the various geological settings, but they are all near the 0 m elevation in Yellow Sea Datum 1985. Profile volume changes per unit meter width are calculated for every survey relative to the first survey during the 2019 typhoon season. Meanwhile, we used the Inverse Distance Weight Interpolation method [START_REF] Shepard | A two-dimensional interpolation function for irregularly-spaced data[END_REF] to generate digital elevation models from all the beach profiles of each survey, and the interpolation was carried out on a regular grid with an alongshore and cross-shore mesh size of 5 m and 1 m, respectively. Elevation difference in m and the total beach volume change in m 3 can be subsequently calculated, based on which we analyzed the erosion/accretion pattern of the three beaches. Vprofile was calculated according to [START_REF] Burvingt | Climate forcing of regionally-coherent extreme storm impact and recovery on embayed beaches[END_REF] between zmin and zmax, and the SMT was calculated according to [START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF] using grain size characteristics. Green rectangles indicate the surficial sediment sampling sites on the beach and in the nearshore.

Surficial sediments

A total of 1097 surficial sediment samples on 171 profiles were collected during the 2019 typhoon season (Table 1). Sampling sites were approximately 50-m spaced along the beach profile starting from the seawall or cliff, while spacing was increased to approximately 200 m in the nearshore part of the profile (Fig. 2). The sediment samples were processed according to standard laboratory procedures [START_REF] Carver | Procedures in sedimentary petrology[END_REF].

Sediment grain-size analyses were carried out by SFY-D sonic vibration type automatic sieving grain size analyzer and Malvern Mastersizer 2000 laser grain size analyzer for sand and mud samples, respectively, after desalination and separation. The graphical method [START_REF] Folk | Brazos River Bar: a Study in the Significance of Grain Size Parameters[END_REF] was then used to calculate the grain size parameters (Mean grain size, Skewness, Sorting coefficient and Kurtosis) of the surficial sediment.

The SMT was determined according to the sediment grain size distribution characteristics, which is a boundary of sand and mud, with the mud content of 25% or the grain size of 63 μm (4Փ) [START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF]. SMT distance was defined as the cross-shore distance of the SMT from the landward base (seawall/cliff, the beginning of each profile). The SMT migration was calculated by comparing SMT offshore distance with that measured during the first survey, with negative (positive) SMT migration corresponding to onshore (offshore) movement.

Results

Nearshore hydrodynamics during the typhoon season

There were four typhoons of different classifications in the study area during the 2019 typhoon season: Tropical Storm Danas, Super Typhoon Lekima, Super Typhoon Lingling, and Typhoon Tabah. The annual average significant wave height and the mean wave period in the study area are only 0.82 m and 3.6 s [START_REF] Guo | Monitoring and evaluation of sand nourishments on an embayed beach exposed to frequent storms in eastern China[END_REF], while the average significant wave height was 1.16 m during the typhoon season of 2019, with a mean wave period of 5.5 s (Fig. 3a &3b). The maximum significant wave height and wave period were 6.77 m and 9.9 s, respectively. Throughout the 2019 typhoon season, waves in the study area were mainly from the southeast, with the maximum significant wave heights mainly from the directions between northeast and southeast (Fig. 3c).

The hourly measured tidal level, astronomical tide and storm surge level with respect to Yellow Sea Datum 1985 in the study area from July 1 to September 30 in 2019 are illustrated in Fig. 3d, covering approximately six lunar tidal cycles. It can be seen from Fig. 3d that the minimum and maximum water levels, which occurred during the spring tide period, reached -1.75 m and 2.49 m, respectively. The four typhoons all caused storm surges (red line in Fig. 3d) in the nearshore of Zhujiajian Island, with values of 0.6 m, 0.96 m, 0.57 m, and 0.98 m, respectively.

Fig. 4 shows the wave rose of the four typhoons during the 2019 typhoon season.

It can be seen from the figure that the dominant wave directions of the first three typhoons were from southeast, while it was from east during the last typhoon. These four typhoons all caused strong storm waves, with the maximum significant wave heights of 4.68 m, 6.77 m, 3.71 m, and 4.03 m (Fig. 3&4), respectively. The corresponding storm peak wave directions were 135°, 40°, 100°, and 84°, respectively.

Beach morphological changes

The

Beach accretion/erosion distribution patterns

The

Variations of beach sediment characteristics

Sediment grain size analysis shows that the sediment of the three beaches is primarily composed of fine sand, and that Qiansha beach had the finest grain size. Grain size varied throughout the typhoon season, with sediment generally coarsening and characterized by increasing sorting.

Dashali beach sediment is mostly composed of fine sand, with a small amount of very-fine sand and coarse sand. At the early, middle and late stages of the typhoon season, the mean sediment grain size was 2.49 Փ (ranging between 1.86 Փ and 2.68 Փ, Fig. 7a), 2.5 Փ (ranging between 1.71 Փ and 2.67 Փ, Fig. 7b) and 2.42 Փ (ranging between 1.5 Փ and 2.69 Փ, Fig. 7c), respectively. The grain size distribution was relatively uniform with a slight change during typhoon season seemingly related to beach morphological changes. The average sediment sorting coefficients of Dashali beach increased throughout the typhoon season, that is, 0.33, 0.36, and 0.49 in the three subsequent stages of the typhoon season (Fig. 8a-c).

In addition to the fine sand components, there was a small amount of very-fine sand, coarse sand and gravel components in the surficial sediments on Dongsha beach, with almost no mud components. The sediment in the southern part of the beach was the coarsest and contained gravel. At the early stage of the 2019 typhoon season, the mean beach sediment grain size was 2.29 Փ (ranging between -1.17 Փ and 3.16 Փ, Fig. 7d). After two typhoons, the mean grain size was 2.37 Փ (ranging between -0.12 Փ and 3.05 Փ, Fig. 7e), and there was still a small amount of fine-grained gravel in southern beach. At the end of the typhoon season, the mean sediment grain size was 2.07 Փ (range between -1.08 Փ and 2.96 Փ, Fig. 7f), with some coarse-grained gravels appearing in the southern part of the beach, where grain size characteristics were the most variable throughout the typhoon season. The sorting coefficient became worse at the end of typhoon season (Fig. 8d-f). Among the three beaches, sediment sorting was the worst at Dongsha beach.

Qiansha beach sediments were primarily fine sand, with a small amount of veryfine sand and coarse sand. At the early stage of the typhoon season, the mean grain size of the surficial sediments on the beach was 2.61 Փ (ranging between 1.63 Փ and 3.1 Փ, Fig. 7g). By the middle stage, the mean grain size slightly decreased to 2.64 Փ (ranging between 2.25 Փ and 2.74 Փ, Fig. 7h). At the end of the typhoon season, the mean grain size was 2.67 Փ (ranging between 2.39 Փ and 2.78 Փ, Fig. 7i). The sediment grain size at Qiansha beach was the most uniform spatially and the finest. The sediment characteristics showed the smallest change of the three beaches during the typhoon season, and the sorting was the best (sorting coefficient ranging between 0.26 and 0.3, Fig. 8g-i).

Location and migration of SMTs

Most of the surficial sediments in the nearshore of Dashali, Dongsha, and Qiansha were muddy sediments, with a dominant seaward fining trend. The nearshore surficial sediments of Dongsha showed the coarsest grain size among the three beaches, while that of Qiansha showed the finest grain size (Table 2). The nearshore surficial sediments all coarsened during the 2019 typhoon season, among which the nearshore sediments at Qiansha were the most stable. Compared with the beach sediments (Table S1), the nearshore sediments showed little changes during typhoon season, with small variations in sorting coefficient(σ), skewness (Sk) and kurtosis (Kg). By calculating the mean grain size (Mz) and mud content of the beach and nearshore surficial sediments, the results show that the average SMT offshore distances from base of Dashali beach, Dongsha beach, and Qiansha beach during the 2019 typhoon season were 201 m, 442 m, and 326 m (Fig. 9), respectively. Their headland extents are 600 m, 1350 m, and 800 m, respectively, suggesting that the longer the headland extent, the farther offshore the SMT, which will be discussed in the next section. Dongsha beach, with the coarsest sediment grain size, has the farthest SMT distance from base. Under the high-energy typhoon season, the SMTs at the three beaches were stable or migrated seaward.

The SMT of Dashali beach showed an average offshore migration in the early stage of the typhoon season by 7.6 m (Fig. 9a). Then, the SMT migrated onshore by approximately 7 m, resulting in a near-zero net migration. The SMT migration was uniform alongshore. In contrast, the Dongsha beach SMT showed continuous seaward migration at all profiles and together with coarsening sediment. Among the three In all panels the red dashed line indicates the average SMT offshore distance during the typhoon season.

Discussion

Factors affecting SMT location

SMT is an important morphological feature of beaches, and its location depends on hydrodynamic settings (mainly waves and tides), sediment supply and potential human interventions. Previous works indicate that the stronger the wave forcing, the farther SMT extends offshore [START_REF] Dunbar | Estimating palaeobathymetry of wave-graded continental shelves from sediment texture[END_REF][START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF], and the coarser beach sediment grain size [START_REF] Friedman | Address of the retiring President of the International Association of Sedimentologists: Differences in size distributions of populations of particles among sands of various origins[END_REF]. This is confirmed here, with Dongsha beach showing the most offshore SMT of the three beaches in Zhujiajian Island (Fig. 9). We also compared the SMT results of the three beaches in this study with 12 other embayed beaches along the east coast of China, for a total of 15 embayed beaches, showing the same relationship (Fig. 10a, Table 3). Tides affect coastal currents and, in turn, sediment transport, beach morphology and SMT location. This is further illustrated when plotting SMT against tidal range, by including the same 12 embayed beaches to our dataset (Fig. 10b and Table 3). In line with Zhao et al. (2020b), the SMT offshore distance at embayed beaches decreases with increasing tidal range. The three embayed beaches in this study have the same mean spring tidal range (2.6 m) and similar nearshore significant wave height, but show different SMT locations (Fig. 10),

indicating that tide and significant wave height are not the only influencing factors.

Headland configuration (shape, distance, offshore extent) at embayed beaches can also remarkably affect nearshore currents and sediment transport [START_REF] Short | Embayed and structurally controlled embayed beaches[END_REF] as well as shoreline response and embayment plane-shape [START_REF] Castelle | Modelling of embayed beach equilibrium planform and rotation signal[END_REF].

Intense rip currents flowing against the headlands can also occur under storm waves and potentially transport large amount of beach sediment seawards [START_REF] Mouragues | High-Energy Surf Zone Currents and Headland Rips at a Geologically Constrained Mesotidal Beach[END_REF][START_REF] Valiente | Role of waves and tides on depth of closure and potential for headland bypassing[END_REF]. Fig. 10c and Table 3 show that SMT offshore distance increases with increasing headland extent at embayed beaches along the east coast of China. Larger headland may promote offshore sand transport and, therefore, more offshore SMT. Wave exposure (embayment orientation) is another important parameter affecting nearshore currents and sediment pathways. Besides, small differences in the inner shelf slope can transform waves differently [START_REF] Dillenburg | Influence of Antecedent Topography on Coastal Evolution as Tested by the Shoreface Translation-Barrier Model (STM)[END_REF][START_REF] Schwab | Influence of inner-continental shelf geologic framework on the evolution and behavior of the barrier-island system between Fire Island Inlet and Shinnecock Inlet, Long Island, New York[END_REF], which may further contribute to the differences of SMT position between the three beaches. Further work, including more data and process-based modelling, will be required to verify these relationships and further explain the influence of tide, wave height, headland extent, wave exposure and inner shelf slope on SMT. Note: SMT Dist. is the SMT distance from the base on each beach. Hsig is the annual average significant wave height of the nearshore.

SMT migration response during typhoon season

Just like beach response, it is critical to better understand SMT response to typhoons. Our results show that SMT migration during the typhoon season is correlated with beach response (Fig. 11). Generally, SMT offshore migration increases with increasing beach erosion. The correlation between SMT migration and beach volumetric change is observed and statistically significant, but mostly during the early period of the typhoon season. An explanation can be that the disequilibrium between SMT/beach volume and the incident wave energy is maximized during the start of the Human activities such as the construction of seawalls and beach nourishments may also affect the migration of SMT. Dongsha beach has seawalls causing continuous erosion on the beach [START_REF] Cheng | Sedimentation and recent morphological changes at Dongsha beach, Zhujiajian Island , Zhejiang Province[END_REF][START_REF] Guo | The response to tropical storm Nakri and the restoration of Dongsha Beach in Zhujiajian Island, Zhejiang Province[END_REF], thus such promoted offshore sediment transport may further lead to a seaward SMT migration. Besides, nourishment projects were implemented on Dongsha beach from 2016 to 2017, with the borrowed sediments mainly placed in the southern end of the beach [START_REF] Guo | Monitoring and evaluation of sand nourishments on an embayed beach exposed to frequent storms in eastern China[END_REF]. The borrowed sediments are not "native" beach sediments and can be easily transported away from the nourishment area [START_REF] Seymour | Rapid erosion of a small southern California beach fill[END_REF]. This may have modified SMT in the south of Dongsha beach. Compared with Dongsha beach, the SMTs of Dashali and Qiansha were relatively stable during typhoon season. These two beaches are more natural than Dongsha, suggesting that SMTs of beaches with less human intervention are more stable. In addition, SMTs of embayed beaches are known to be more stable than estuarine beaches (Zhao et al., 2020b), which is confirmed in this study with overall small SMT migrations.

Embayed beach exposure to incident storm wave conditions is important to beach response during typhoon seasons [START_REF] Qi | The response of three main beach types to tropical storms in South China[END_REF], which may also affect the migration of SMTs. The beach orientations of Dashali, Dongsha and Qiansha are 3°, 19° and 40° (Fig. 1), respectively, while the main storm wave directions of the four typhoons are from southeast and east (Fig. 4). The Super typhoon Lekima with maximum Hs approaching 6.77 m in the early stage of the typhoon season may result in the biggest impact, and the angle between the main storm direction and the orientations of Dongsha and Qiansha are close to shore normal. Therefore, Dongsha and Qiansha eroded significantly during the early-middle stage of typhoon season, which corresponds to the offshore migration of SMTs on these two beaches.

Sediment supply is also an important factor affecting the SMT distribution and migration [START_REF] Anthony | Short-term changes in the plan shape of a sandy beach in response to sheltering by a nearshore mud bank, Cayenne, French Guiana[END_REF][START_REF] Anthony | The influence of Amazon-derived mud banks on the morphology of sandy headland-bound beaches in Cayenne, French Guiana: a short-to long-term perspective[END_REF][START_REF] Chang | Age and morphodynamics of a sandy beach fronted by a macrotidal mud flat along the west coast of Korea: a lateral headland bypass model for beach-dune formation[END_REF][START_REF] George | Wave climate, sediment supply and the depth of the sand-mud transition: A global survey[END_REF]Zhao et al., 2020a). The southward transport of fine-grained sediments from the Yangtze River Estuary driven by coastal currents makes the adjacent sea waters of the Zhoushan Archipelago rich in fine-grained sediments [START_REF] Hu | Characteristics of sediment transport in the Zhoushan Archipelago sea area[END_REF][START_REF] Li | Sediment flux from the Zhoushan Archipelago, eastern China[END_REF]. [START_REF] Cheng | Sedimentation and recent morphological changes at Dongsha beach, Zhujiajian Island , Zhejiang Province[END_REF] found that there is a SMT in the nearshore of This research can provide a reference for beach management and sustainable beach development on such embayed beaches in the world.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  on embayed sandy beaches during the typhoon season and their response to changes in beach morphology are still unclear, especially when they are also potentially affected by human activities. In this study, three embayed sandy beaches (Dashali, Dongsha, and Qiansha) on the east coast of Zhujiajian Island, which are exposed to frequent storms and different human interventions, are studied. Changes in beach morphology and sediment characteristics during the 2019 typhoon season are analyzed. The temporal and spatial evolution of SMTs of embayed sandy beaches are explored and their driving factors are discussed.

Fig. 1

 1 Fig.1 Sketch of study area: geographical locations of Zhuajiajian Island (a), tidal gauge

Fig. 2

 2 Fig. 2 Example of beach profile and sediment sampling sites at Dongsha beach. The

Fig. 3

 3 Fig. 3 Significant wave height (a), mean wave period (b), wave direction (c), and tidal

  beach profiles of Dashali show a slight accretion during the typhoon season. The alongshore-averaged Vprofile changes in the early-middle stage, middle-late stage and the entire typhoon season were 4.57 m 3 /m, 7.35 m 3 /m, 11.92 m 3 /m, respectively. At the early stage of the typhoon season, Dashali beach exhibited a berm located 40-60 m from the seawall. Then, the beach berm was smoothed out by the end of typhoon season.Overall, the profiles became smoother and more gently (Fig.5a-e).The alongshore-averaged Vprofile changes of Dongsha beach during the entire typhoon season is -54.34 m 3 /m, with the beach suffering more erosion during the early stage of the typhoon season(-35.68 m 3 /m). The southern profiles (Fig.5f-h) show the most dramatic change, with erosion during the entire typhoon season reaching -124.17

  Fig. 5 Variations in typical profiles of Dashali beach (a-e), Dongsha beach (f-k) and

  Fig. 6 Topographic difference plot of Dashali beach (a-c), Dongsha beach (d-f) and

Fig. 7

 7 Fig. 7 Maps of mean surficial sediment grain sizes on Dashali (a-c), Dongsha (d-f) and

  beaches, the SMT of Dongsha beach had the largest migration during the typhoon season with also substantial alongshore variability. The SMT of the southern Dongsha beach (DS05, DS11, DS17) migrated seawards by the largest amount (170 m, Fig. 9b), while the SMT in the northern part was relatively stable, with SMT migration ranging between -5 m and 16 m. The SMT of Qiansha migrated offshore by 17.6 m during the early stage of the season before migrating shoreward by 13.6 m (Fig. 9c). During the entire typhoon season, the SMT of Qiansha moved by only 4 m.

Fig. 9

 9 Fig. 9 SMT offshore distances at Dashali (a), Dongsha (b), and Qiansha (c) at the early

Fig. 10

 10 Fig. 10 SMT distance from base against significant wave height (a), tide range (b) and

  Fig. 11 Volumetric change on each profile (Vprofile) against SMT migration at each

  Dongsha beach in approximately 5 m depth (about 430 m from the seawall) with seasonal changes in flood and dry seasons. Dashali and Qiansha have a similar sedimentary environment with Dongsha affected by the fluvial sediments from Yangtze River Estuary, thus the SMTs on these two beaches may also exhibit seasonal changes. 6 Conclusions This study focuses on the location, migration and influencing factors of the SMTs on the embayed sandy beaches of Dashali, Dongsha and Qiansha, east coast of Zhujiajian Island. Beach topographies and surficial sediment characteristics were acquired during the early, middle and late stages of the 2019 typhoon season. During this typhoon season, the beaches showed large and variable morphological changes driven by the high-energy wave forcing. Only Dashali beach showed no net volume change, while Donghsa and Qiansha severely and moderately eroded, respectively.Despite different volumetric changes, all three beaches showed similar profile evolution with berm smoothing and more gently sloping shape, reflecting net offshore sediment transport. Beach sediments coarsened at the three sites with worse sorting performance during the typhoon season, with the nearshore surficial sediments showing similar changes. The SMTs of the three beaches were stable or migrated offshore during the typhoon season. Including data from 12 other embayed beaches in eastern China, the SMT distance offshore was found to positively correlate with significant wave height and headland offshore extent, and negatively with beach volume change and tidal range.

Table 1

 1 Number of profiles and surficial sediment samples (beach and nearshore) collected for each embayed beach during the early, middle and late stages of the 2019 typhoon season.

	Beach	Profiles	Beach (nearshore) sediment samples

Acknowledgements

This study was supported by the Public Science and Technology Research Funds Projects of Ocean, China (NO. 201405037) and the joint PhD program of the China Scholarship Council for Overseas Studies (NO. 202006140106). The authors thank Zuyin Jiang and Min Zhang for their help in field work, the National Meteorological Center of China for the open-source typhoon data and Google Earth for the open-source remote sensing data. We also thank the editor and reviewers for their insightful and constructive comments.

Data Availability

More typhoon data can be obtained from the National Meteorological Center of China (http://typhoon.nmc.cn). Tidal data are provided by Shenjiamen Marine Station. Wave data, topographic data and sediment data are available upon request by contacting the corresponding author (slchen@sklec.ecnu.edu.cn).

Declaration of interest