Journal of Marine Science and Engineering

Article Sediment Characteristics and Intertidal Beach Slopes along the Jiangsu Coast, China

Yu Kuai 1,* , Jianfeng Tao 2,* , Zaiyang Zhou 1,3 , Stefan Aarninkhof 1 and Zheng Bing Wang 1,4

1 Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2628 CN Delft, The Netherlands; [email protected] (Z.Z.); [email protected] (S.A.); [email protected] (Z.B.W.) 2 College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China 3 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China 4 Marine and Coastal Systems Department, Deltares, 2600 MH Delft, The Netherlands * Correspondence: [email protected] (Y.K.); [email protected] (J.T.)

Abstract: Tidal flats play an important role in promoting coastal biodiversity, defense against flood- ing, land reclamation and recreation. Many coastal tidal flats, especially the tide-dominant ones, are muddy. However, the number of studies on the profile shape and surficial sediment distribu- tion of muddy tidal flats is small compared to sandy beaches. Based on high spatial-resolution measurements along the tide-dominant Jiangsu Coast, China, we analyzed the morphology and sediment characteristics of the unvegetated intertidal flats along the Jiangsu Coast. The Jiangsu Coast can be divided into an eroding northern part (north coast) and an accreting southern part (south coast). The beach slope of the north coast shows a southward flattening trend, apart from some outliers related to rocky parts of the coastline. We found alternating very fine and coarse sediment



 (depending on the local clay content) for different locations along the north coast, which can be explained from consolidation and armoring-induced erosion resistance. In the south coast, we found Citation: Kuai, Y.; Tao, J.; Zhou, Z.; gradual coarsening of bed surface sediment and gradual flattening of beach slopes to the south. Aarninkhof, S.; Wang, Z.B. Sediment Characteristics and Intertidal Beach This seemingly unexpected pattern is explained by the flood-dominant current causing landward Slopes along the Jiangsu Coast, China. sediment transport, larger tidal range in the south part, sheltering effect of the Radial Sand Ridges, J. Mar. Sci. Eng. 2021, 9, 347. and contribution of different sediment sources, viz. the Abandoned Yellow River Delta and the https://doi.org/10.3390/jmse9030347 Radial Sand Ridges. In the cross-shore direction, the sediment grain size decreases landward. Waves are only of secondary importance for the sediment dynamics at the unvegetated tidal flats along the Academic Editor: Matteo Vacchi Jiangsu Coast.

Received: 8 March 2021 Keywords: intertidal beach; beach slope; surficial sediment grain-size; human intervention; Jiangsu Coast Accepted: 19 March 2021 Published: 22 March 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Located at the boundary between land and sea, tidal flats are under the joint control of published maps and institutional affil- iations. both terrestrial and marine processes, while they play a vitally important role in promoting coastal biodiversity, protecting coastal regions from flooding, and providing potential land resources and recreation. Nowadays, in the progress of coastal resource development, many human activities (e.g., ports construction, reclamation) are conducted on tidal flats, like the Tongzhou Bay port on the Jiangsu Coast, China [1] and the Maasvlakte reclamation Copyright: © 2021 by the authors. in the Netherlands [2]. Therefore, tidal flat evolution has been a key research topic in the Licensee MDPI, Basel, Switzerland. field of coastal engineering. This article is an open access article Many studies have been carried out on the characteristics of sandy, wave-dominated distributed under the terms and beach profiles [3–6]. Sandy beaches with coarse materials tend to be steeper [7,8]. Coarser conditions of the Creative Commons Attribution (CC BY) license (https:// sands have larger angle of repose than finer ones. In addition, due to their higher perme- creativecommons.org/licenses/by/ ability and roughness, coarser materials tend to be more stable in dynamic conditions [8]. 4.0/). However, because of difficulties in field observation, the number of studies on the profile

J. Mar. Sci. Eng. 2021, 9, 347. https://doi.org/10.3390/jmse9030347 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 2 of 21

J. Mar. Sci. Eng. 2021, 9, 347 2 of 20

However, because of difficulties in field observation, the number of studies on the profile shape and sediment distribution of the muddy and silt-muddy tidal flats is far less than forshape sandy and beaches. sediment It distribution is found that of theaccretin muddyg tidal and silt-muddyflats tend to tidal have flats a convex-up is far less than shore- for normalsandy beaches. profile, while It is found eroding that tidal accreting flats tidaltend flatsto have tend a toconcave-up have a convex-up one [9,10]. shore-normal The inter- tidalprofile, flats while in muddy eroding environments tidal flats tend are to more have convex-up a concave-up than one those [9,10 in]. sandy The intertidal environments flats in [11].muddy environments are more convex-up than those in sandy environments [11]. Silt-muddySilt-muddy tidal tidal flats flats are are shallow shallow areas areas characterized characterized by byfine fine cohesive cohesive sediment sediment in- including clay (<0.004 mm), silt (0.004–0.062 mm) and very fine sand (0.062–0.125 mm) cluding clay (<0.004 mm), silt (0.004–0.062 mm) and very fine sand (0.062–0.125 mm) sup- supplied from adjacent rivers, estuaries and coasts. They are found worldwide under plied from adjacent rivers, estuaries and coasts. They are found worldwide under a vari- a variety of climatic, hydrodynamic and sedimentological conditions, such as the East ety of climatic, hydrodynamic and sedimentological conditions, such as the East Coast of Coast of China [12], the Northwest Coast of America [13], the Severn Estuary of the China [12], the Northwest Coast of America [13], the Severn Estuary of the UK [14] and UK [14] and the Wadden Sea in the Netherlands [15]. Based on the different dynamic the Wadden Sea in the Netherlands [15]. Based on the different dynamic conditions and conditions and terrains, silt-muddy tidal flats can be divided into open coast, embayment terrains, silt-muddy tidal flats can be divided into open coast, embayment and estuarine and estuarine tidal flats, among which the open coast tidal flat can be well developed tidal flats, among which the open coast tidal flat can be well developed because of com- because of complex dynamics and less restrictions for sediments transport routes. Figure1 plexshows dynamics a characteristic and less restrictions cross-shore for sediment sediments zonation transport pattern routes. for Figure a well-developed 1 shows a char- silt- acteristicmuddy tidal cross-shore flat. sediment zonation pattern for a well-developed silt-muddy tidal flat.

FigureFigure 1. 1. AA profile profile view view of of sediment sediment zonation zonation (modified (modified from from [11]). [11]).

AlthoughAlthough silt-muddy silt-muddy tidal tidal flats flats exist exist worldw worldwideide broadly, broadly, they they are are of of different different char- char- acteristicsacteristics for for several several reasons. reasons. First First of of all, all, the the sediment sediment transport transport itself itself is is a a complex and multi-dimensionalmulti-dimensional process thatthat closelyclosely relatesrelates to to the the interactions interactions involving involving various various external exter- nalforcing forcing agents agents (e.g., (e.g., tide, tide, wave, wave, wind wind and and storm storm surge) surge) [16 ,[16,17].17]. In addition,In addition, the the difference differ- encein sediment in sediment compositions compositions (e.g., (e.g., clay, clay, silt and silt veryand very fine sand)fine sand) and biologicaland biological activities activities have haveapparent apparent impacts impacts on the on processes the processes like sedimentslike sediments erosion, erosion, mixing, mixing, deposition deposition and and bed bedconsolidation consolidation [18 ].[18]. Furthermore, Furthermore, changing changing environments environments (e.g., (e.g., sea sea levellevel rise,rise, sediment supply)supply) and and human human activities activities in in different different region regionss can can also also lead lead to to different different tidal tidal flat flat mor- mor- phologiesphologies [19]. [19]. Thorough understandingunderstanding ofof nearshore nearshore morphodynamic morphodynamic processes processes including includ- ingthe the link link between between hydrodynamic hydrodynamic forcing, forcing, surficial surficial sediment sediment distribution distribution and beach and profilebeach profilechange change is of crucial is of importancecrucial importance for the assessment for the assessment of coastal of safety coastal and safety natural and values, natural as values,well as as the well impact as the of impact human of interventions human interventions in the coastal in the zone. coastal zone. JiangsuJiangsu Coast, China,China, isis such such a a typical typical open open coast coast with with extended extended silty-muddy silty-muddy tidal tidal flats. flats.It represents It represents great economicgreat economic and environmental and environmental value for value the country;for the country; it not only it not provides only providesshelters for shelters the littoral for the flora littoral and flora fauna, and land fauna, resources land resources for agriculture for agriculture and aquaculture, and aqua- but culture,also protects but also the protects safety of the coastal safety cities. of coastal The goalcities. of The this goal study of isthis to increasestudy is ourto increase insight ourin the insight morphological in the morphological characteristics characteristics of the unvegetated of the unvegetated intertidal flats intertidal along theflats Jiangsu along theCoast. Jiangsu On theCoast. basis On of the high-resolution basis of high-res bedolution surface bed sediment surface grain sediment size and grain cross-shore size and cross-shoreprofile elevation profile data elevation from a large-scaledata from a coastal large-scale investigation, coastal investigation, we analyzed we the analyzed beach profile the beachevolution profile and evolution sediment and distribution sediment patterns distribution along patterns this coast. along this coast.

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2. Area Description 2. Area Description The Jiangsu Coast (Figure 2c) is situated between the Yangtze River and the Xiuzhen RiverThe Estuary Jiangsu [20]. Coast Tidal (Figure flats 2alongc) is situated the Jiangs betweenu Coastline the Yangtze are characterized River and theby (1) Xiuzhen its large Riverwidth Estuary (with [a20 mean]. Tidal width flats of along 8 km), the Jiangsu(2) abun Coastlinedant sediment are characterized supply from by the (1) two its large large widthrivers (with (from a the mean Yangtze width River of 8 during km), (2) the abundant end of the sediment late Pleistocene supply fromand from the twothe Yellow large riversRiver (from from the1128–1855 Yangtze AD), River and during (3) silt the domina end ofnt the sediments late Pleistocene [21]. It has and been from regarded the Yellow gen- Rivererally from as a 1128–1855typical example AD), andof open (3) siltcoast dominant tidal flats sediments [22]. From [21 the]. Itembankment has been regarded to mean generallylow water, as afour typical distinctive example zones of open can coast normally tidal flatsbe found, [22]. Fromwhich the are embankment grass flat (freshwater to mean lowor brackish water, four water distinctive wetland), zones Suaeda can normally salsa flat be(saltmarsh) found, which and aremud grass flat flatand (freshwater silt or sand orflat, brackish respectively water wetland),[21]. Suaeda salsa flat (saltmarsh) and mud flat and silt or sand flat, respectively [21].

Figure 2. Map of the Bohai Sea, Yellow Sea and East China Sea (a). Subplot (b) shows the co-tidal chartsFigure of 2. the Map M2 of constituent the Bohai forSea, the Yellow southern Sea and Yellow East Sea. China Amplitude Sea (a). Subplot is shown (b) in shows cm, and the phase co-tidal in degreescharts (fromof the [M223]). constituent Subplot (c )for indicates the southern the Jiangsu Yellow Coastline Sea. Amplitude evolution is from shown 1855 in to cm, 2007, and transport phase in patterndegrees of (from the sediment [23]). Subplot eroded ( fromc) indicates the Abandoned the Jiangsu Yellow Coastline River evolution Delta and from the location 1855 to of 2007, measured profiles.transport The pattern locations of the ofthe sediment Abandoned eroded Yellow from Riverthe Abandoned Delta and theYellow Radial River Sand Delta Ridges and are the labelled location inof (a measured), and the profiles. historical The shoreline locations location of the inAban (c)doned is collected Yellow from River [24 ],Delta in which and the there Radial is a detailedSand Ridges are labelled in (a), and the historical shoreline location in (c) is collected from [24], in which description of the shoreline data. there is a detailed description of the shoreline data. Two distinct geomorphological units can be recognized in the north and south parts of theTwo Jiangsu distinct Coast, geomorphological respectively, i.e., units the Abandonedcan be recognized Yellow in River the north Delta and (AYRD) south andparts theof the fan-shaped Jiangsu Coast, Radial respectively, Sand Ridges i.e., (RSRs) the Abandoned [25–27]. During Yellow 1128–1855 River Delta AD, (AYRD) the Yellow and the Riverfan-shaped flowed Radial into the Sand South Ridges Yellow (RSRs) Sea [25–27]. at the north During part 1128–1855 of the Jiangsu AD, the Coast Yellow causing River abundantflowed into fine the sediment South Yellow supply Sea into at thethe coastalnorth part areas, of the and Jiangsu gradually Coast forming causing the abundant (now) Abandonedfine sediment Yellow supply River into Delta the [coastal25]. The areas, paleo-Yangtze and gradually River forming also brought the (now) a large Abandoned amount ofYellow sandy sedimentRiver Delta to the[25]. RSRs, The paleo-Yangtze during the end River of the also late Pleistocenebrought a large [28]. amount These two of largesandy sedimentsediment sources to the RSRs, led to during a rapid the tidal-flat end of the formation. late Pleistocene After 1855, [28]. The These Yellow two large River sediment shifted itssources lower reachled to toa rapid the Bohai tidal-flat Sea [ 29formation.]. As the After Yangtze 1855, River The Estuary Yellow movedRiver shifted southward its lower as wellreach [25 to]. Thethe Bohai suspended Sea [29]. sediments As the Yangtze in the coastal River areasEstuary are moved since then southward mainly as generated well [25]. by bed erosion, instead of fluvial supply, and the whole coastal areas can be treated as

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3. Material and Method 3.1. Data Source Data of cross-shore beach profile bathymetry and grain size distribution of sediment samples are from a comprehensive field survey on coastal zone of Jiangsu province, which has been reported in [36]. To the authors’ knowledge, this is the latest field survey dataset with such a large spatial scale on the Jiangsu Coast. The large-scale investigations were performed between 2006 and 2008. Fifty cross-shore profiles starting from Xiuzhen estu- ary to Lvsi (Figure 2c) were defined along the Jiangsu Coast. The bed elevation was meas- ured along these 50 profiles and 173 bed surface sediment samples were taken along only 36 of these profiles. For the bed elevation, the intertidal part was measured in 2007 by Real J. Mar. Sci. Eng. 2021, 9, 347 Time Kinematic (RTK) instrument (with a cross-shore interval of ~70 m), and subtidal4 ofpart 20 was measured in 2008 by vessel measurement (with a cross-shore interval of ~10 m). The samples were collected from the same layer (approximately the top ~10 cm of the bottom) in 2006 and 2007, and only the top (1–2 cm) of the samples are adopted for sediment gra- a quasi-enclosed sediment system [30,31]. As a result, severe coastal erosion took place dation analysis. For the sediment samples on the same profile, the distance between adja- around the AYRD due to the cutoff of sediment supply (Figure2c). cent sample points is around 500 m. Because the south part of the Jiangsu Coast has wider After the 1970s, some shoreline protection projects were built at the AYRD, which intertidal flats than the north part, more sample points were taken on the southern pro- decreased the shoreline regression rate. While the shallow part of the coastal profile files. In addition, the Landsat 7 ETM satellite images were used to check for detailed in- remained in place, erosion of the deeper part continued resulting in a steepening of the formation, such as locations of the tidal creeks and human interventions in the past 20 cross-shore slope. Sheltered by enormous offshore ridges and fed by sediments supply years. The satellite images are downloaded from the United States Geographical Survey derived from the eroding AYRD and these ridges, the coast between Sheyang and Lvsi iswebsite still accreting, (USGS, http://www.usgs.gov most notably at the (accessed supratidal on flats. 14 January The mudflats 2020)). from Sheyang to Jianggang are the widest and fastest accretionary mudflats in China [32]. Meanwhile, due 3.2. Data Processing to decreasing sediment supply from the AYRD and erosion of the outer edges of the radial sand ridgesIn Figure2c, (thus two decreasing dash line the rectangles length of were sheltered marked coastline), to divide the the eroding whole coastline section near into Sheyangtwo parts, is graduallybased on the expanding shoreline southwards evolution co [27ndition]. (erosion or accretion). The blue one (JD1-25,The hereafter semi-diurnal referred tide to is as of north major part/nor importanceth coast) for contains the tidal the flats Haizhou along theBay Jiangsu and the Coast.AYRD. The Haizhou tidal waveBay is first the entersonly zone the of southern the Jiangsu Yellow Coast Sea having and part rock of coast it is reflectedwith typical by thesandy Shandong sediment. Peninsula, The coastline forming around an anticlockwisethe AYRD is under rotational severe tidal erosion wave since system. 1855 when The rotationalthe Yellow wave River and switched the progressive its course. waveDuring from the thelarge-scale southern investigation Yellow Sea (2006–2008), converge near the Jianggangshoreline (Figureerosion2 b).in this The part convergence was already of these at atwo much tidal slower waves rate. leads The to red the formationone (JD26-50, of anhereafter approximately referred standing to as south tidal part/south wave and coast) the radial is under tidal currentcontinuous field accretion. in this area This [23,33 part,34 ].is Thethe meanmost typical tidal range silty alongbeach thealong Jiangsu the Jiangsu Coast isCoast. about The 2–4 alongshore m (see Figure distance3,[ 22 is]). measured The tide therethe between is flood the dominant, landward which ends is of of the great adjacent importance profiles. to net sediment transport [32,35].

FigureFigure 3.3. InterpolatedInterpolated mean high water water (MHW) (MHW) and and mean mean low low water water (MLW) (MLW) at at each each profile profile (the (the verticalvertical dash dash lines lines indicate indicate thethe locationlocation ofof everyevery fifthfifth profile;profile; N N denotes denotes North North andand SS denotesdenotes SouthSouth andand these these are are the the same same for for the the following following figures). figures).

TheIn order waves to alongcompare the the Jiangsu profile coastline shapes, the are intertidal influenced flat primarily slope was by determined. the monsoon We climate,estimate characterized the intertidal by flat a mixtureslope (slope of swells = tan andα, see locally Figure generated 4) from the wind width waves of withthe inter- the lattertidal beingbeach, dominant. as determined Waves from from the the locations north prevail of the inmean this high region water over (MHW) the year and and mean the probability of wave height smaller than 1 m is about 85% [22]. Due to the wave energy attenuation on the wide and shallow tidal flats, the effect of wave action on changing the coastal morphology is much weaker than the tidal action [22,35]. Classified by the mean grain size (MZ) and the sand (−1~4Φ, Φ = −log2d, d is grain size in mm), silt (4~8Φ), and clay (>8Φ) proportion, the tidal flat sediments at the Jiangsu Coast are mainly composed of five types of sediment, named fine sand (MZ 2–4Φ, sand 70–90%), sandy silt (MZ 3–5Φ, sand 30–40%, silt >50%, clay <10%), silt (MZ 5–6Φ, sand 15–20%, silt 65–75%, clay <10%), clayed silt (MZ 5.5–6.5Φ, sand 5–10%, silt >60%, clay 20–25%) and clay (MZ 7.5–8Φ, sand <17%, silt 20–25%, clay 45–70%). The distribution of surface sediment on intertidal flats shows a seaward coarsening trend [21].

3. Material and Method 3.1. Data Source Data of cross-shore beach profile bathymetry and grain size distribution of sediment samples are from a comprehensive field survey on coastal zone of Jiangsu province, which J. Mar. Sci. Eng. 2021, 9, 347 5 of 20

has been reported in [36]. To the authors’ knowledge, this is the latest field survey dataset with such a large spatial scale on the Jiangsu Coast. The large-scale investigations were performed between 2006 and 2008. Fifty cross-shore profiles starting from Xiuzhen estuary to Lvsi (Figure2c) were defined along the Jiangsu Coast. The bed elevation was measured along these 50 profiles and 173 bed surface sediment samples were taken along only 36 of these profiles. For the bed elevation, the intertidal part was measured in 2007 by Real Time Kinematic (RTK) instrument (with a cross-shore interval of ~70 m), and subtidal part was measured in 2008 by vessel measurement (with a cross-shore interval of ~10 m). The samples were collected from the same layer (approximately the top ~10 cm of the bottom) in 2006 and 2007, and only the top (1–2 cm) of the samples are adopted for sediment gradation analysis. For the sediment samples on the same profile, the distance between adjacent sample points is around 500 m. Because the south part of the Jiangsu Coast has wider intertidal flats than the north part, more sample points were taken on the southern profiles. In addition, the Landsat 7 ETM satellite images were used to check for detailed information, such as locations of the tidal creeks and human interventions in the past 20 years. The satellite images are downloaded from the United States Geographical Survey website (USGS, http://www.usgs.gov (accessed on 14 January 2020)).

3.2. Data Processing In Figure2c, two dash line rectangles were marked to divide the whole coastline into two parts, based on the shoreline evolution condition (erosion or accretion). The blue one (JD1-25, hereafter referred to as north part/north coast) contains the Haizhou Bay and the AYRD. Haizhou Bay is the only zone of the Jiangsu Coast having rock coast with typical sandy sediment. The coastline around the AYRD is under severe erosion since 1855 when the Yellow River switched its course. During the large-scale investigation (2006–2008), the shoreline erosion in this part was already at a much slower rate. The red one (JD26-50, hereafter referred to as south part/south coast) is under continuous accretion. This part is the most typical silty beach along the Jiangsu Coast. The alongshore distance is measured the between the landward ends of the adjacent profiles. In order to compare the profile shapes, the intertidal flat slope was determined. We estimate the intertidal flat slope (slope = tan α, see Figure4) from the width of the intertidal beach, as determined from the locations of the mean high water (MHW) and mean low water (MLW) beach contours. The MHW and MLW levels are determined from the historical Yellow Sea mean tidal range distribution data after [22], assuming Mean Sea Level (MSL) is half-way between MHW and MLW (see Figure3). The elevations thus obtained can be used to determine the MHW and MLW locations at each profile. This approach is obscured by potential presence of old/new dykes at the shoreward end of the profile and presence of tidal channels at the seaward end of the profile, which hamper consistent identification of the MHW and MLW contour locations. A systematic approach was followed to overcome these difficulties at either end of the beach profile, in order to enable quantitative beach slope estimates along the Jiangsu Coast. This approach (Baseline Method) is explained here: At the shoreward end of the profile, old dykes and newly built dykes are occasionally located below MHW. Instead of calculating the intertidal beach slope from the MHW level, at influenced profiles (e.g., 17, 33, and 35) we choose the most seaward dyke toe and associated beach elevation to be the start point (see Figure4). At the seaward end of the profile, as we link the satellite images (Figure5) to the measured profile data (Figure6), we can find that in the region of the RSRs, the intertidal flat is cut through by tidal creeks of different sizes at many places, and these tidal creeks change their locations continuously. Here, we took profile JD 38 (see Figure2c) for example. As we see in Figure5, the profile JD38 shows strong fluctuations. It is not easy to define the seaward end point for intertidal beach slope calculation on this profile, because there are several positions of which the elevation is equal to MLW. For the previous approach, we simply took the most seaward one as the end point, as the blue arrow shows in Figure5. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 6 of 21

low water (MLW) beach contours. The MHW and MLW levels are determined from the historical Yellow Sea mean tidal range distribution data after [22], assuming Mean Sea Level (MSL) is half-way between MHW and MLW (see Figure 3). The elevations thus ob- tained can be used to determine the MHW and MLW locations at each profile. J. Mar. Sci. Eng. 2021, 9, 347 This approach is obscured by potential presence of old/new dykes at the shoreward6 of 20 end of the profile and presence of tidal channels at the seaward end of the profile, which hamper consistent identification of the MHW and MLW contour locations. A systematic approach was followed to overcome these difficulties at either end of the beach profile, in However,order to enable as some quantitative tidal creeks beach could slope be rather estimates wide along and deep, the Jiangsu we cannot Coast. take This the approach whole profile(Baseline as a Method) continuous is explained one, because here: this could make the beach slope definition criterion differ amongAt the shoreward profiles. Hence, end of for the these profile, profiles old dy (e.g.,kes and 38, 39,newly 43~50) built cut dykes through are occasionally by a large (widerlocated than below 100 m MHW. or deeper Instead than 1of m) calculating tidal creek, the we intertidal defined the beach end pointslope as from the landwardthe MHW banklevel, (the at influenced point where profiles cross-shore (e.g., 17, slope 33, an changesd 35) we abruptly) choose the of themost tidal seaward creek, dyke as the toe red and arrow shows in Figure5. associated beach elevation to be the start point (see Figure 4).

J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEWFigure 4. Scheme of calculating the intertidal flat slope. 7 of 21 Figure 4. Scheme of calculating the intertidal flat slope.

At the seaward end of the profile, as we link the satellite images (Figure 5) to the measured profile data (Figure 6), we can find that in the region of the RSRs, the intertidal flat is cut through by tidal creeks of different sizes at many places, and these tidal creeks change their locations continuously. Here, we took profile JD 38 (see Figure 2c) for exam- ple. As we see in Figure 5, the profile JD38 shows strong fluctuations. It is not easy to define the seaward end point for intertidal beach slope calculation on this profile, because there are several positions of which the elevation is equal to MLW. For the previous ap- proach, we simply took the most seaward one as the end point, as the blue arrow shows in Figure 5. However, as some tidal creeks could be rather wide and deep, we cannot take the whole profile as a continuous one, because this could make the beach slope definition criterion differ among profiles. Hence, for these profiles (e.g., 38, 39, 43~50) cut through by a large (wider than 100 m or deeper than 1 m) tidal creek, we defined the end point as the landward bank (the point where cross-shore slope changes abruptly) of the tidal creek, as the red arrow shows in Figure 5. Thus, in this Baseline Method, for the profiles not affected (Figure 7 black dots) by the dykes or large tidal creeks, we calculate the beach slope from MHW to MLW beach contours; for the affected ones (Figure 7 black circles), we calculate their slopes using the approach introduced above. In this study, we used the Baseline Method to calculate each

intertidal beach slope. FigureFigure 5.5.Satellite Satellite imageimage ofof thethe JDJD 3636 toto 3939 fromfrom LandsatLandsat 77 ETMETM onon 2424 JanuaryJanuary 20072007 (left(left panel panel)) and and 55 JulyJuly 20082008 ((rightright panelpanel).). TheThe yellowyellow boxbox indicatesindicates the human interventions taking taking place place between between thesethese two two image image times. times. The The dash dash black black box box indicates indicates the the measured measured profile profile JD38 JD38 in in October October 2008, 2008, and and the red and blue lines in the dash box are associated with two beach slope calculation meth- the red and blue lines in the dash box are associated with two beach slope calculation methods. ods.

To ensure the consistency of the approach among different profiles, all sediment sam- ples are included in the analysis, although same of them are located behind the new dykes. At each sample point, we take the mean sediment grain size (MZ according to the [37] definitions as the representative grain size (see Equation (1)). Then we plotted the mean sediment grain size with reference to the cross-shore location to have a clear view of sed- iment grain size distribution pattern. Sediment types are classified according to the scheme proposed by [38], in which sand, silt and clay are defined as sediment with grain size of −1~4𝛷, 4~8𝛷 and >8𝛷, respectively. In order to further seek into the alongshore distribution pattern of sediment charac- teristics, more sediment characteristic parameters (sorting (𝜎), and skewness (𝑆𝑘)) were calculated based on [37] definitions (see Equations (2) and (3)):

𝛷 𝛷 𝛷 𝑀 = (1) 3

𝛷 −𝛷 𝛷 −𝛷 𝜎 = (2) 4 6.6

𝛷 𝛷 −2𝛷 𝛷 𝛷 −2𝛷 𝑆𝑘 = (3) 2𝛷 −𝛷 2𝛷 −𝛷

where 𝛷, 𝛷, 𝛷,𝛷, and 𝛷 represent the phi values at 84, 16, 50, 95, 75, 25 and 5 percentiles in a cumulative frequency curve (i.e., horizontal axis: Sediment grain size in phi; vertical axis: Percentage of sediment coarser than a certain size by weight). The verbal classification scales for sorting and Skewness are shown in Table 1 and Table 2, respec- tively.

Table 1. Classification scale for sorting.

𝜎 Values Sorting Verbal Scale <0.350 very well sorted 0.35 ~ 0.500 well sorted 0.5 ~ 0.710 moderately well sorted

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0.71 ~ 1.00 moderately sorted 1.00 ~ 2.00 poorly sorted 2.00 ~ 4.00 very poorly sorted >4.00 extremely poorly sorted

Table 2. Classification scale for skewness.

𝑺𝒌𝑰 Values Skewness Verbal Scale Graphically Skewed to −1.00 ~ −0.30 strongly negative skewed strong coarse tail −0.30 ~ −0.10 negative skewed coarse tail −0.10 ~ 0.10 near symmetrical symmetrical 0.10 ~ 0.30 positive skewed fine tail 0.30 ~ 1.00 strongly positive skewed Strong fine tail

Because of continuous reclamations along the Jiangsu Coast, some dykes were newly built, which made it impossible to carry out the analysis of bed slope and sediment grain size from the same horizontal reference. Using Landsat 7 ETM satellite images, we checked the human activities processes on each profile from 1988 to 2008 and derived the time between measurement and nearest reclamation at each profile. The bed levels in front of and behind the newly built dykes were compared to find out whether these dykes in- fluence intertidal beach evolution.

4. Results 4.1. Cross-Shore Intertidal Beach Shape The measured 50 cross-shore profiles are shown in Figure 6 (note that the horizontal axis scales are different between the first three and the other two panels). As the measure- ments started from the top of old dykes, a sharp decrease in bed elevation in the seaward direction can be observed at the beginning of each profile. Some abnormal peaks can also be noticed on the upper part of the profiles; according to the investigation report [36], the large peaks are newly built dykes and small ones are abandoned stones and fences of small fishponds. Generally, the south part of Jiangsu Coast has wider and flatter tidal flats with a width that may exceed 10 km. However, the southern profiles also tend to be more fluctuating, and that is because of the existence of shore-parallel tidal channels and creeks, J. Mar. Sci. Eng. 2021, 9, 347 especially in the RSRs region (see Figure 5). 7 of 20

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Figure 6. Cont.

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FigureFigure 6. 6.Measured Measured bed bed elevation elevation of of the the 50 50 profiles. profiles.

Thus, in this Baseline Method, for the profiles not affected (Figure7 black dots) by the dykes or large tidal creeks, we calculate the beach slope from MHW to MLW beach contours; for the affected ones (Figure7 black circles), we calculate their slopes using the approach introduced above. In this study, we used the Baseline Method to calculate each Figureintertidal 6. Measured beach slope. bed elevation of the 50 profiles.

Figure 7. Calculated intertidal flat slopes of the 50 profiles based on Baseline Method (b) is a zoom in of the 0~5‰ part on (a). Black dots represent profiles not affected by dykes or large tidal creeks, and black circles are associated with profiles which are affected.

Figure 7 depicts the intertidal flat slopes of the 50 cross-shore sections. Generally, cross-shore slope decreases from north to south along the Jiangsu Coast, see Figure 7b for Figure 7. Calculated intertidal flat slopes of the 50 profiles based on Baseline Method (b) is a zoom in Figurethe 0–5 7. Calculated‰ part. At intertidal the eroding flat slopes north of coast, the 50 intertidal profiles based flat slopeon Baseline is several Method times (b) largeris a zoom than of the 0~5‰ part on (a). Black dots represent profiles not affected by dykes or large tidal creeks, and inon of thethe 0~5‰ accretionary part on ( asouth). Black part. dots Furthermore, represent profiles the notslope affe distributioncted by dykes fluctuates or large tidal a lot creeks, in the andblack black circles circles are associatedare associated with with profiles profiles which which are affected.are affected. north part. JD9 has very small value, because the measurement took place near small es- tuary, where riverine supplied sediments can prevent severe erosion. The very large val- FigureTo ensure 7 depicts the consistency the intertidal of theflat approachslopes of the among 50 cross-shore different profiles, sections. all Generally, sediment ues (>15‰) between JD10 and JD15 are because of the existence of rocky coastline. Apart cross-shoresamples are slope included decreases in the from analysis, north although to south along same ofthe them Jiangsu are Coast, located see behind Figure the 7b new for dykes.from these At each local sample outliers, point, the slope we takedecreases the mean from sedimentnorth to south grain in size the (erodingM according north part. to the 0–5‰ part. At the eroding north coast, intertidal flat slope is several timesZ larger than theAt [the37] south definitions part, the as the slope representative is basically below grain size1‰ (see and Equation the southward (1)). Then flattening we plotted trend the can on the accretionary south part. Furthermore, the slope distribution fluctuates a lot in the meanbe more sediment apparent grain than size it in with the reference north part. to the cross-shore location to have a clear view north part. JD9 has very small value, because the measurement took place near small es- of sediment grain size distribution pattern. Sediment types are classified according to the tuary, where riverine supplied sediments can prevent severe erosion. The very large val- ues (>15‰) between JD10 and JD15 are because of the existence of rocky coastline. Apart from these local outliers, the slope decreases from north to south in the eroding north part. At the south part, the slope is basically below 1‰ and the southward flattening trend can be more apparent than it in the north part.

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scheme proposed by [38], in which sand, silt and clay are defined as sediment with grain size of −1~4Φ, 4~8Φ and >8Φ, respectively. In order to further seek into the alongshore distribution pattern of sediment charac- teristics, more sediment characteristic parameters (sorting (σI), and skewness (SkI)) were calculated based on [37] definitions (see Equations (2) and (3)):

Φ + Φ + Φ M = 16 50 84 (1) Z 3 Φ − Φ Φ − Φ σ = 84 16 + 95 5 (2) I 4 6.6

Φ84 + Φ16 − 2Φ50 Φ95 + Φ5 − 2Φ50 SkI = + (3) 2(Φ84 − Φ16) 2(Φ95 − Φ5)

where Φ84, Φ16, Φ50,Φ95, and Φ5 represent the phi values at 84, 16, 50, 95, 75, 25 and 5 percentiles in a cumulative frequency curve (i.e., horizontal axis: Sediment grain size in phi; vertical axis: Percentage of sediment coarser than a certain size by weight). The verbal classification scales for sorting and Skewness are shown in Tables1 and2, respectively.

Table 1. Classification scale for sorting.

σI Values Sorting Verbal Scale <0.350 very well sorted 0.35 ~ 0.500 well sorted 0.5 ~ 0.710 moderately well sorted 0.71 ~ 1.00 moderately sorted 1.00 ~ 2.00 poorly sorted 2.00 ~ 4.00 very poorly sorted >4.00 extremely poorly sorted

Table 2. Classification scale for skewness.

SkI Values Skewness Verbal Scale Graphically Skewed to −1.00 ~ −0.30 strongly negative skewed strong coarse tail −0.30 ~ −0.10 negative skewed coarse tail −0.10 ~ 0.10 near symmetrical symmetrical 0.10 ~ 0.30 positive skewed fine tail 0.30 ~ 1.00 strongly positive skewed Strong fine tail

Because of continuous reclamations along the Jiangsu Coast, some dykes were newly built, which made it impossible to carry out the analysis of bed slope and sediment grain size from the same horizontal reference. Using Landsat 7 ETM satellite images, we checked the human activities processes on each profile from 1988 to 2008 and derived the time between measurement and nearest reclamation at each profile. The bed levels in front of and behind the newly built dykes were compared to find out whether these dykes influence intertidal beach evolution.

4. Results 4.1. Cross-Shore Intertidal Beach Shape The measured 50 cross-shore profiles are shown in Figure6 (note that the horizontal axis scales are different between the first three and the other two panels). As the measure- ments started from the top of old dykes, a sharp decrease in bed elevation in the seaward direction can be observed at the beginning of each profile. Some abnormal peaks can also be noticed on the upper part of the profiles; according to the investigation report [36], the large peaks are newly built dykes and small ones are abandoned stones and fences of small fishponds. Generally, the south part of Jiangsu Coast has wider and flatter tidal flats J. Mar. Sci. Eng. 2021, 9, 347 10 of 20

with a width that may exceed 10 km. However, the southern profiles also tend to be more fluctuating, and that is because of the existence of shore-parallel tidal channels and creeks, especially in the RSRs region (see Figure5). Figure7 depicts the intertidal flat slopes of the 50 cross-shore sections. Generally, cross-shore slope decreases from north to south along the Jiangsu Coast, see Figure7b for the 0–5‰ part. At the eroding north coast, intertidal flat slope is several times larger than on the accretionary south part. Furthermore, the slope distribution fluctuates a lot in the north part. JD9 has very small value, because the measurement took place near small estuary, where riverine supplied sediments can prevent severe erosion. The very large values (>15‰) between JD10 and JD15 are because of the existence of rocky coastline. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEWApart from these local outliers, the slope decreases from north to south in the eroding11 north of 21

part. At the south part, the slope is basically below 1‰ and the southward flattening trend can be more apparent than it in the north part.

4.2.4.2. Sediment Sediment Grain Grain Size Size Distribution Distribution Mean grain size (MZ) averaged over sample points on the intertidal part of each pro- Mean grain size (MZ) averaged over sample points on the intertidal part of each profile fileare are shown shown in in Figure Figure8. 8. The The sediment sediment grain grain size size shows shows a a notablynotably southwardsouthward coarsening patternpattern (𝛷 (Φ valuevalue decreases) decreases) in in the the south south part. part. In In th thee north north part, part, it itshows shows much much variability. variability. AA southward southward fining fining pattern pattern can can be be clearly clearly observed observed only only in in the the small small part part from from profile profile 20 20 toto profile profile 25. 25. Compared Compared to to the the surrounding zone,zone, thethe bedbed surface surface sediment sediment on on profile profile 20, 20, at atthe the top top of of AYRD, AYRD, is quiteis quite coarse coarse (Figure (Figure8). This 8). This is understandable, is understandable, as the as top the of top the AYRDof the AYRDis under is under heaviest heaviest erosion. erosion.

FigureFigure 8. 8. AveragedAveraged (through (through profile) profile) bed bed surface surface mean mean sediment sediment grain grain size size of of each each profile. profile.

FigureFigure 9 shows the spatial (alongshore and cross-shore) variation of the sediment characteristiccharacteristic parameters. parameters. As As can can be be seen seen in in the the figure, figure, most most sample sample points points are are located located on on thethe intertidal intertidal flats flats except except only only very very few few points points in in the the south south part part (Figure (Figure 9 firstfirst panel). InIn the the cross-shore cross-shore direction, direction, the the grain grain size size generally generally decreases decreases in the in thelandwards landwards di- rection.direction. This This is a common is a common pattern pattern for tide for dominated tide dominated coasts coasts[21]. However, [21]. However, in the along- in the shorealongshore direction, direction, we cannot we cannot directly directly tell if tell no ifrth north part part of Jiangsu of Jiangsu Co Coastast has has finer finer sediment sediment thanthan the the south south part, part, because because both both dark dark red red do dotsts (very (very fine fine sediment) sediment) and and dark dark blue blue dots dots (very(very coarse coarse sediment) sediment) can can be be found. found. The The sedi sedimentment type type in the in the north north Jiangsu Jiangsu Coast Coast can canbe sometimesbe sometimes sand sand dominant dominant or clay or clay dominant, dominant, wh whileile in inthe the south south part, part, it itis is basically basically silt silt dominant.dominant. As As the the Jiangsu Jiangsu coastal coastal areas areas can can be be treated treated as as a aquasi-enclosed quasi-enclosed sediment sediment system system nowadays, the sedimentation in the south Jiangsu Coast partly comes from the eroded fine nowadays, the sedimentation in the south Jiangsu Coast partly comes from the eroded sediments in the north part. However, it is noted the extremely fine sediments are hardly fine sediments in the north part. However, it is noted the extremely fine sediments are found in the south part. hardly found in the south part. For most profiles, the bed surface sediment becomes from sand dominant to silt For most profiles, the bed surface sediment becomes from sand dominant to silt dom- dominant landward (Figure9 panel 4~6). Only in the very north zone (blue zone) the clay inant landward (Figure 9 panel 4~6). Only in the very north zone (blue zone) the clay content can reach a relatively high proportion (note that the color bar limitation for clay content can reach a relatively high proportion (note that the color bar limitation for clay proportion is from 0% to 40%, not 100%). In the south part, the sand proportion increases proportion is from 0% to 40%, not 100%). In the south part, the sand proportion increases southward around MLW, and the clay proportion decreases southward around MHW. The silt content remains almost uniform.

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southward around MLW, and the clay proportion decreases southward around MHW. The silt content remains almost uniform.

FigureFigure 9. 9. BedBed surface surface sediment sediment characteri characteristicstic parameters parameters distribution distribution related related to to distance distance from from old old dykedyke (location (location of of MHW, MHW, MLW, and new dykes are markedmarked inin thethe firstfirst panelpanel).).

TheThe sorting sorting is gettinggetting betterbetter seaward, seaward, which which means means the the sediments sediments in thein the sand sand dominant domi- nantpart part are betterare better sorted, sorted, while while they they are are quite quite mixed mixed in in the the shallower shallower andand silt dominant part.part. This This sorting sorting distribution distribution is also is also consistent consistent with with the observation the observation that in that this in tide-dom- this tide- inateddominated environment, environment, the upper the upper part of part the ofinte thertidal intertidal beach beachis less isdynamic less dynamic than the than lower the lower part [39]. The Skewness distribution shows that the sediment is fine-skewed for most part [39]. The Skewness distribution shows that the sediment is fine-skewed for most of of the samples, and it becomes more fine-skewed landward. This indicates that the upper the samples, and it becomes more fine-skewed landward. This indicates that the upper part of the intertidal flat tends to have excess fine materials, consistent with the landward part of the intertidal flat tends to have excess fine materials, consistent with the landward fining pattern. fining pattern. In the cross-shore direction of the Jiangsu Coast, sediment tends to be finer landward. In the cross-shore direction of the Jiangsu Coast, sediment tends to be finer landward. This is opposite to wave-dominated sandy beaches, where the coarsest sediment is near This is opposite to wave-dominated sandy beaches, where the coarsest sediment is near shoreline. Comparing the alongshore intertidal beach slope variation and sediment grain shoreline. Comparing the alongshore intertidal beach slope variation and sediment grain size variation, an interesting phenomenon has been observed at the accretionary south part of the Jiangsu Coast. While tidal flat slopes are becoming milder towards the south,

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J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 13 of 21 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 13 of 21 size variation, an interesting phenomenon has been observed at the accretionary south part of the Jiangsu Coast. While tidal flat slopes are becoming milder towards the south, the corresponding bed surface sediment grain size is becoming coarser southward. This the corresponding bed surface sediment grain size is becoming coarser southward. This seemsthe corresponding to be opposite bed to surface wave-dominated sediment grain sandy size beaches, is becoming where coarser milder southward. wave conditions This seems to be opposite to wave-dominated sandy beaches, where milder wave conditions tendseems to to be be associated opposite withto wave-dominated finer sediments. sandy Another beaches, seemingly where milder unexpected wave phenomenonconditions tend to be associated with finer sediments. Another seemingly unexpected phenomenon istend that to sediments be associated at some with offiner the sediment profiless. in Another the eroding seemingly north unexpected part are much phenomenon finer than in is that sediments at some of the profiles in the eroding north part are much finer than in theis that accreting sediments south. at some These of seemingly the profiles unexpected in the eroding phenomena north part are are further much discussed finer than inin the the accreting south. These seemingly unexpected phenomena are further discussed in the discussionthe accreting section. south. These seemingly unexpected phenomena are further discussed in the discussion section. discussion section. 4.3. Human Interventions 4.3. Human Interventions 4.3. HumanThe time Interventions difference between the profile measurement and the most recent reclamation The time difference between the profile measurement and the most recent reclama- at eachThe location time difference is shown between in Figure the 10 profile. Recent measurement reclamations and mostly the most took recent place inreclama- the south tion at each location is shown in Figure 10. Recent reclamations mostly took place in the tion at each location is shown in Figure 10. Recent reclamations mostly took place in the part,south wherepart, where the construction the construction time istime very is closevery toclose the to measurement the measurement time. time. Earlier Earlier reclama- south part, where the construction time is very close to the measurement time. Earlier reclamationstions usually usually took place took place above above the MHW,the MHW, whereas whereas more more recent recent reclamations reclamations graduallygrad- reclamations usually took place above the MHW, whereas more recent reclamations grad- uallyextend extend to deeper to deeper areas areas (especially (especially at theat th tidale tidal flats flats shielded shielded byby thethe RSRs, see see profile profile 33 ually extend to deeper areas (especially at the tidal flats shielded by the RSRs, see profile 33and and 35 35 in in Figure Figure9 first9 first panel panel and and FigureFigure 1111).). We We need need to to compare compare the the bed bed elevation elevation in in 33 and 35 in Figure 9 first panel and Figure 11). We need to compare the bed elevation in front of of and and behind behind these these dykes dykes to to find find out out how how dyke dyke construction construction has hasaffected affected the bed the bed front of and behind these dykes to find out how dyke construction has affected the bed level changes. changes. level changes.

Figure 10. 10. TimeTime between between measurement measurement and and nearest nearest reclamat reclamationion at each at each profile profile (circles (circles means means time time Figure 10. Time between measurement and nearest reclamation at each profile (circles means time larger than than 20 20 years). years). larger than 20 years).

Figure 11. Satellite images of the JD 33 and 35 from Landsat 7 ETM on 5 July 2008 (arrows indicate Figure 11. Satellite images of the JD 33 and 35 from Landsat 7 ETM on 5 July 2008 (arrows indicate theFigure location 11. Satellite of new dykes images on of each the pr JDofile 33 andand 35distance from Landsatto the old 7 dykes). ETM on 5 July 2008 (arrows indicate the location of new dykes on each profile and distance to the old dykes). the location of new dykes on each profile and distance to the old dykes).

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Figure 9 (first panel) shows that only for profiles JD17 (at the eroding part), JD33 and 35 (atFigure the accreting9 (first panel) part), shows dykes that are onlylocated for on profiles the intertidal JD17 (at thezone. eroding Zooming part), in JD33around and the 35new (at thedykes accreting of these part), three dykes profiles are located(Figure on6) thewe intertidalcan easily zone. compare Zooming the bed in aroundelevation the in newfront dykes of and of behind these three these profiles dykes (Figure6 12).) we The can bed easily elevations compare remain the bed almost elevation the same in frontin front of and of and behind behind these the dykes dykes (Figure in JD33 12 and). The 35. bedAt JD17, elevations the dyke remain was almostbuilt 20 theyears same ago, inso front we can of andtreat behind it as an the old dykesdyke, and in JD33 the lower and 35. bed At elevation JD17, the in dykefront wasof it builtsuggests 20 yearsstrong ago, so we can treat it as an old dyke, and the lower bed elevation in front of it suggests erosion. strong erosion.

FigureFigure 12. 12.Zoom Zoom in onin on the the bed bed elevation elevation in front in front of and of behindand behind the newly the newly built dykes built (notedykes that (note the that scales the of scales the three of the panels three arepanels not uniform). are not uniform).

InIn summary, summary, we we conclude conclude that: that: 1.1. IntertidalIntertidal beach beach slopes slopes are are larger larger in in the the eroding eroding north north part part of of the the Jiangsu Jiangsu Coast Coast than than inin the the accreting accreting south south part. part. Apart Apart from from some some outliers outliers that that relate relate to to rocky rocky parts parts of of the the coastline,coastline, beach beach slopes slopes show show a a southward southward flattening flattening trend trend in in both both the the north north and and south south partpart of of the the coastline, coastline, albeit albeit more more obvious obvious in in the the accreting accreting south south part. part. 2.2. InIn the the cross-shore cross-shore direction, direction, the the bed bed surface surface sediment sediment grain grain size size decreases decreases landward, landward, andand the the sorting sorting is is getting getting better better seaward. seaward. In In the the alongshore alongshore direction, direction, the the sediment sediment graingrain size size distributiondistribution pattern is is more more complex, complex, with with explicitly explicitly the the following following two two fea- features:tures: In Inthe the eroding eroding north north part, part, both both extr extremelyemely fine fine sediment sediment do dominantminant profiles profiles and andcoarse coarse sands sands dominant dominant profil profileses can can be be found. found. In In the accreting southsouth part, part, sediment sediment graingrain size size shows shows a a southward southward increasing increasing pattern. pattern. 3.3. HumanHuman interventions interventions continuouslycontinuously took place along the the Jiangsu Jiangsu Coast. Coast. The The most most re- recentcent dykes dykes in in the the northern northern Jiangsu Coast werewere builtbuilt moremore thanthan 20 20 years years before before the the timetime of of measurement. measurement. While While in in the the south south part, part, new new dykes dykes were were built built more more recently, recently, mostmost within within one one year year time time before before the the measurement. measurement.

5.5. Discussion Discussion 5.1. Reliability of the Results 5.1. Reliability of the Results The way we defined the beach slope cannot guarantee all results are calculated under The way we defined the beach slope cannot guarantee all results are calculated under the same reference, because at some profiles MHW is above the elevation of the dyke toe, the same reference, because at some profiles MHW is above the elevation of the dyke toe, while at some other profiles large alongshore direction tidal creeks could cut the profiles while at some other profiles large alongshore direction tidal creeks could cut the profiles into separate parts. This was resolved through application of the Baseline Method; however, thisinto could separate influence parts. the This alongshore was resolved intertidal through beach application slope distribution of the Baseline pattern Method; we found. how- Inever, order this to checkcould ifinfluence slope definition the alongshore boundaries intertidal influence beach the slope observed distribution variation pattern pattern, we intertidalfound. In beach order slopes to check were if recalculatedslope definition on theboundaries basis of different influence contour the observed levels, namelyvariation frompattern,−1 mintertidal to 1 m (Sensitivitybeach slopes scenario were recalcul 1) andated−0.5 on m the to basis 0.5 m of (Sensitivity different contour scenario levels, 2), respectively,namely from as − the1 m meanto 1 m sea (Sensitivity level is around scenario 0 m.1) and Sensitivity −0.5 m to scenario 0.5 m (Sensitivity 2 thus provides scenario a zoom2), respectively, in with respect as the to scenariomean sea 1. level These is twoaround scenarios 0 m. Sensitivity with narrowed scenario vertical 2 thus ranges provides can avoida zoom the in dykes with and respect some to of scenario the tidal 1. creeks These influences.two scenarios with narrowed vertical ranges canFigure avoid the13 showsdykes and the resultssome of of the alongshore tidal creeks beach influences. slope distribution calculated with differentFigure upper 13 andshows lower the boundaries. results of alongshore For most profiles, beach slope results distribution are similar calculated to each other, with withdifferent only upper minor and difference lower boundaries. (less than 1 ‰For) most in magnitude. profiles, results They are all similar show a to southward each other, slopewith decreasingonly minor trenddifference and this(less trendthan 1‰) is more in magnitude. notable in They the south all show part. a southward With all these slope boundaries, the beach slope along the north part is quite fluctuating, because the beach

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J. Mar. Sci. Eng. 2021, 9, 347 14 of 20 decreasingdecreasing trend trend and and this this trend trend is is more more notable notable in in the the south south part. part. With With all all these these bounda- bounda- ries,ries, thethe beachbeach slopeslope alongalong thethe northnorth partpart isis quitequite fluctuating,fluctuating, becausebecause thethe beachbeach typetype isis variablevariabletype is variablein in this this part. part. in this However, However, part. However, there there are are there still still some aresome still profiles profiles some having profileshaving large large having variations variations large variations among among thesetheseamong scenarios. scenarios. these scenarios. Figure Figure 14 14 Figureindicates indicates 14 the indicatesthe standa standard therd deviation deviation standard of of deviation the the beach beach of slopes slopes the beach calculated calculated slopes withwithcalculated threethree different withdifferent three boundaries.boundaries. different boundaries. ItIt isis basedbased It onon is based threethree ondatadata three pointspoints data onlyonly points andand only servesserves and as servesas anan indicationindicationas an indication to to highlight highlight to highlight variability variability variability in in results. results. in Apparently, results.Apparently, Apparently, for for the the north north for thepart part north more more part variabil- variabil- more ityityvariability can can be be noticed. noticed. can be While While noticed. in in the the While south south in part, thepart,south th thee variations variations part, the are are variations relatively relatively are small. small. relatively Therefore, Therefore, small. ininTherefore, the the north north inpart, part, the the norththe estimated estimated part, the beach beach estimated slopes slopes beach are are more more slopes sensitive sensitive are more to to sensitivethethe chosen chosen to upper upper the chosen and and lowerlowerupper boundariesboundaries and lower of boundariesof thethe intertidal intertidal of the profileprofile intertidal asas used used profile inin thethe as analysis.analysis. used in the TheTheanalysis. aboveabove comparisoncomparison The above resultresultcomparison meansmeans theresultthe beachbeach means slopeslope the definitiondefinition beach slope boundabounda definitionriesries dodo boundaries havehave anan impactimpact do have onon an thethe impact detaileddetailed on slopeslopethe detailed distribution, distribution, slope but but distribution, do do not not change change but the dothe notkey-key- changefindingsfindings the on on key-findingsthe the beach beach slope slope on southward thesouthward beach slopeflat- flat- teringteringsouthward pattern. pattern. flattering pattern.

FigureFigureFigure 13. 13. 13. Intertidal IntertidalIntertidal beach beach beach slopes slopes slopes of of of the the the 50 50 50 profile profile profilesss calculated calculated calculated with with with different different different boundaries. boundaries. boundaries.

Figure 14. Standard deviation of three intertidal beach slopes calculated with different boundaries. FigureFigure 14. 14. Standard Standard deviation deviation of of three three intertidal intertidal beach beach slopes slopes calculated calculated with with different different boundaries. boundaries. In Figure 15, the beach profiles were divided into two parts (divided at 0 m, about theIn meanIn FigureFigure sea 15, level)15, thethe in beachbeach order profilesprofiles to see the werewere profile dividividedded shape intointo (concave-up twotwo partsparts (divided(divided or convex-up). atat 00 m,m, JD20 aboutabout is thethelocated meanmean on seasea the level)level) top of inin AYRD orderorder (see toto seesee Figure thethe 1 profileprofilec), which shapeshape is under (concave-up(concave-up the greatest oror convex-up).convex-up). erosion. From JD20JD20 JD20 isis to JD50, the beach profiles changes from concave-up to convex-up, which is consistent with shoreline condition (eroding profiles tend to be concave-up while accretionary ones tend to

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J. Mar. Sci. Eng. 2021, 9, 347 15 of 20 located on the top of AYRD (see Figure 1c), which is under the greatest erosion. From JD20 to JD50, the beach profiles changes from concave-up to convex-up, which is consistent with shoreline condition (eroding profiles tend to be concave-up while accretionary ones betend convex-up). to be convex-up). As the tidal As the range tidal increases range in southward,creases southward, the beach the profile beach tends profile to betends more to convex-upbe more convex-up towards thetowards south. the This south. pattern This was pa alsotternfound was also at the found tidal at flats thein tidal South flats San in FranciscoSouth San Bay Francisco [40]. Bay [40].

FigureFigure 15. UpperUpper and and lower lower intertidal intertidal beach beach slope slope differe differencence of the of the50 profiles 50 profiles calculated calculated with withdif- differentferent boundaries. boundaries.

FromFrom thisthis sensitivitysensitivity analysis,analysis, wewe concludeconclude thatthat thethe southwardsouthward flatteningflattening beachbeach slopeslope patternpattern foundfound inin thisthis studystudy isis notnot affectedaffected byby thethe specificspecific definitionsdefinitions usedused toto derivederive beachbeach slopesslopes fromfrom thethe fieldfield data,data, nornor fromfrom thethe qualityquality ofof thethe datadata itself.itself. 5.2. Extremely Fine or Coarse Sediment in the North Coast 5.2. Extremely Fine or Coarse Sediment in the North Coast In the north part, extremely fine (clay component dominant) or extremely coarse (sand In the north part, extremely fine (clay component dominant) or extremely coarse component dominant) sediment are present (Figure9). This fluctuating pattern can be (sand component dominant) sediment are present (Figure 9). This fluctuating pattern can explained by the occurrence of two erosion resistance mechanisms, namely self-weight consolidationbe explained by and the armoring occurrence of the of two beach erosio surface.n resistance mechanisms, namely self-weight consolidationCohesive and fine armoring sediment of is anthe importantbeach surface. component of the silt-muddy tidal flat and its self-weightCohesive fine consolidation sediment is processes an important [18] play component a significant of the role silt-muddy on the tidal tidal flat flat mor- and phologicalits self-weight evolution consolidation [41]. Consolidation processes [18] makes play a the significant bed material role on less the erodible. tidal flat Figuremorpho-9 showslogical thatevolution the grain [41]. size Consolidation of fine sediment makes in the the bed north material Jiangsu less Coast erodible. can be Figure less than 9 shows 8Φ, whichthat the is grain in the size range of fine of clay. sediment During in the the long-time north Jiangsu formation Coast ofcan the be AYRD, less than the 8 cohesive𝛷, which sedimentsis in the range settled of onclay. the During delta gotthe enoughlong-tim timee formation to get well of the consolidated AYRD, the and cohesive formed sedi- an erosionments settled resistant on layer.the delta Thus, got although enough time the northto get Jiangsuwell consolidated Coast is under and formed erosion, an the erosion erod- ingresistant system layer. ends Thus, up at although the erosion the resistant north Jian layergsu and Coast the is upper under layer erosion, silts the are eroding transported sys- southward.tem ends up This at the erosion erosion resistance resistant due layer to consolidation and the upper effects layer hassilts resulted are transported in the presence south- ofward. fine-sediment This erosion hotspots resistance along due the to northconsolidation Jiangsu Coast.effects has This resulted mechanism in the explains presence the of seeminglyfine-sediment unexpected hotspots phenomenon along the north that Jiangs sedimentsu Coast. at This some mechanism profiles in explains the eroding the northseem- partingly can unexpected be finer than phenomenon in the accreting that sediments part of the at coast. some profiles in the eroding north part can beWhen finer the than profiles in the accreting have sufficient part of coarse the coast. sediment available, armoring of the bed surfaceWhen by suspensionthe profiles ofhave the sufficient fines also coarse becomes sediment an important available, factor armo limitingring of the erodibility. bed sur- Asface it by can suspension be seen from of the Figure fines 9also, the becomes bed surface an important sediment factor can alsolimiting be coarseerodibility. at some As it profiles,can be seen which from is theFigure contribution 9, the bed of surface the sand sediment armoring can effect. also be When coarse the at clay some content profiles, is quitewhich little, is the the contribution finer components of the sand are armoring easier eroded effect. than When the the coarse clay ones.content Under is quite normal little, hydrodynamicthe finer components conditions, are easier the silt eroded components than the were coarse taken ones. away, Under and normal the sand hydrodynamic components stillconditions, remained the on silt the components bed, which were make taken the bedaway, surface and the sediment sand components be coarser—hence still remained more resistanton the bed, against which erosion. make the bed surface sediment be coarser—hence more resistant against erosion.These two erosion resistance mechanisms can explain why the sediments in the erodingThese north two part erosion can beresistance extremely mechanisms fine or coarse, can explain depending why the on thesediments local clay in the content. erod- Additionally,ing north part it can indicates be extremely that silt fine is the or maincoarse components, depending transported on the local southward.clay content. Addi- tionally, it indicates that silt is the main components transported southward.

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5.3. Alongshore Variation of Intertidal Beach Slope and Sediment Grain Size The development of the tidal flat along the Jiangsu Coast is facilitated by three most essential conditions, namely a low-energy environment, sufficient sediment supply and a medium to large tidal range [42,43]. In addition to the hydrodynamics and sediment sources, human interventions can also influence the beach evolution. These three factors are discussed in the following in order to explain the seemingly strange relation between the grain size and beach slope variations along the coast: southward coarsening and flattening.

5.3.1. Hydrodynamics Tide force is the main driver influencing the sediment erosion and transport processes along the Jiangsu Coast. The tidal range is much larger in the south part than in the north (Figures2b and3). Larger tidal range favors wider flats [ 44]. Tidal motion, especially the alongshore tidal current, is considered to be the main force causing sediment erosion and transport along the Jiangsu Coast. Ref. [45] found that in a wave-absence systems, bed profile and mud content on the upper flat are independent of the alongshore tidal current magnitude. In contrast, the strong alongshore currents can erode mud on the lower flat and promote landward sand transport from the subtidal area to the lower flat, forming a sandy flat. In-situ measurements near Jianggang showed that the mean flood current speed was about 1.4 times of the ebb current speed, and the mean suspended sediment concentration during floods was 1.25 times of that during ebbs [20,46]. Thus, in the south part, coarser sediments in the subtidal area provided by RSRs tends to be easier transported to the shoreline, and the landward transport process made the sediment on the whole profile coarser. This tide-induced mechanism explains why the southward coarsening pattern is more apparent in the accreting part (i.e., JD25 to JD50). The wave height is generally lower than 1 m in the RSRs area [47], because the RSRs form a natural barrier for the shoreline, dissipating a large amount of wave energy. This provides the sheltered flats in the south part favorable conditions to develop to flatter beach profiles. On wave-dominated beaches, as waves are further dissipated towards the shoreline, the maximum dynamics are found near the shoreline and decrease to deeper water. Therefore, the coarsest sediments are found near the shoreline and the sorting is better near the shoreline as well [48,49]. At the Jiangsu Coast, these three patterns were observed to be opposite, which further indicates that this is a tide-dominated coastline and waves are of secondary importance.

5.3.2. Sediment Sources Both in-situ measurements and model simulations proved that the major sediment source for the modern Jiangsu mudflat are the RSRs and AYRD [20,50,51]. The width of intertidal zone at equilibrium is positively related to sediment supply, which means higher sediment supply leads to wider and flatter tidal flats [52]. The southward beach flattening tendency coincides with the shoreline evolution state that the north part of the Jiangsu Coast is eroding while the south part is accreting (Figure2c). This means that the north part is losing sediment forming a sediment source for the south part. The south part can receive sediment supply not only from the eroded north part but also from the outside RSRs. The large-scale sediment budget the Jiangsu Coast according to model simulations [34] shows that tidal flats south to Jianggang receive more sediment from the RSRs than from AYRD. Towards the south, the intertidal flat has thus more potential sediment supply and can become wider and flatter. For the southward coarsening pattern, we consider different sediment sources would also be a main reason contributing to this. In order to further analyze sediment composition in different zones, we plotted the sand-silt-clay triangle (Figure 16). Usually, sediment from the same source tends to show a straight band in this triangle (with constant silt-clay ratio) [53]. For our case, samples at JD26–50 have a turning point around the silt content between 60~80%. This indicates that the sediments in the south part come from different sources. The alongshore sediment mineralogical composition difference further proves that J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 18 of 21

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between 60~80%. This indicates that the sediments in the south part come from different sources. The alongshore sediment mineralogical composition difference further proves thethat sediments the sediments on the on Jiangsu the Jiangsu Coast Coast are from are differentfrom different sources sources [54,55 ].[5 The4,55]. sediments The sediments in the AYRDin the AYRD mainly mainly camefrom came the from Yellow the Yellow River, andRiver, sediments and sediments from the from Paleo-Yangtze the Paleo-Yangtze River contributedRiver contributed to the to formation the formation of the of RSRs. the RS NowadaysRs. Nowadays both both the the AYRD AYRD and and RSRs RSRs supply sup- sedimentsply sediments to the to souththe south Jiangsu Jiangsu tidal tidal flat. Theflat. RSRsThe RSRs provided provided sediments sediments are coarser are coarser than thethan AYRD the AYRD provided provided ones. ones. As the As south the south part tends part tends to receive to receive more sedimentmore sediment from the from RSRs, the theRSRs, bed the surface bed surface sediment sediment tends totends be coarser. to be coarser.

Figure 16. Sand-Silt-Clay triangle triangle with with two two different different co colorlor scatters, scatters, and and colors colors representing representing differ- differ- ent zones.zones.

Studies on thethe beachbeach profileprofile andand sedimentsediment graingrain sizesize relationshiprelationship forfor sandysandy beachesbeaches are allall underunder thethe premisepremise ofof equilibriumequilibrium statestate [[3–6,8],3–6,8], althoughalthough equilibriumequilibrium conceptconcept inin coastal environment can be questioned [56]. [56]. Besi Besides,des, this this conclusion conclusion is is driven driven based based on on a alarge large dataset dataset from from different different beaches, beaches, and and we we do do not not know know if if it also works forfor differentdifferent profilesprofiles inin thethe samesame beach.beach. As the century-lo century-longng erosion of AYRD andand RSRs,RSRs, theirtheir sedimentsediment supply isis reducingreducing [[30],30], whichwhich meansmeans thethe evolutionevolution ofof thethe JiangsuJiangsu tidaltidal flatsflats hashas notnot beenbeen at an equilibrium equilibrium state. state. As As we we only only have have on onee time time large large scale scale data, data, we cannot we cannot tell the tell tem- the temporalporal change change of the of relationship. the relationship. Will this Will relationship this relationship change change as the Jiangsu as the Jiangsu Coast further Coast furtherevolves evolvesand will and sediment will sediment from different from different resources resources further redistribute? further redistribute? We still need We stillnu- needmerical numerical models to models help us to explore help us the explore change the of change this relationship of this relationship as sediment as supply sediment re- supplyduces. reduces.

5.3.3. Human Interventions Apart from naturalnatural processes,processes, thethe beachbeach slopeslope andand sedimentsediment graingrain sizesize patternspatterns wewe foundfound couldcould alsoalso bebe causedcaused byby humanhuman interventions.interventions. According to previous studiesstudies (e.g., [[57]57]),), inin anan accretionaryaccretionary systemsystem affectedaffected byby landland reclamations, the intertidal flat will become narrower and steeper, and the surficial sediment reclamations, the intertidal flat will become narrower and steeper, and the surficial sedi- tended to become finer on the mid-upper intertidal flat but coarser on the lower intertidal ment tended to become finer on the mid-upper intertidal flat but coarser on the lower flat. However, the reclamations taking place in intertidal beaches are mostly located in intertidal flat. However, the reclamations taking place in intertidal beaches are mostly lo- the south part of the coast (see Figure9 first panel). The observed southward flattening cated in the south part of the coast (see Figure 9 first panel). The observed southward pattern is thus not in accordance with earlier findings by [57]. Meanwhile, we cannot find flattening pattern is thus not in accordance with earlier findings by [57]. Meanwhile, we the accretion (see Figure 12) and finer sediment (Figure9 first panel) near the dyke toe cannot find the accretion (see Figure 12) and finer sediment (Figure 9 first panel) near the on the reclamation influenced profiles. The bed elevation in front of and behind the new dyke toe on the reclamation influenced profiles. The bed elevation in front of and behind dykes are almost the same (see Figure 12, JD 33 and 35). This is mainly because the time of thisthe new reclamation dykes are are almost so close the to same the measurement (see Figure 12, (Figure JD 33 10 and), so 35). that This morphology is mainly have because not respondedthe time of tothis the reclamation human interventions are so close yet.to the From measurement Figure5 we (Figure can also 10), see so itthat clearly morphol- that theogy humanhave not interventions responded to took the placehuman between interventions 2007 and yet. 2008, From only Figure several 5 we months can also before see it theclearly measurement, that the human which interventions means there is took limited place time between for the tidal2007 flatand to 2008, respond. only It several partly

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explains why we find that there is no obvious difference in bed elevation and sediment grain size between in front of and behind the dykes. In conclusion, the influence of human interventions on the morphological and sedi- mentological characteristics of the Jiangsu Coast cannot be determined from this dataset, as it was collected too shortly after the implementation of large-scale land reclamation schemes. In order to check the influence of human interventions, more field survey needs to be carried on the temporal variation of tidal flat morphology.

6. Conclusions Jiangsu Coast is a typical tide-dominant open coast with extended intertidal flats. This study investigates the intertidal beach slopes and surficial sediment grain size distribution pattern, using the high spatial-resolution field data from a large-scale coastal zone investi- gation. We analyzed the morphology and sediment characteristics of the Jiangsu Coast and found the following features: 1. Intertidal beach slopes are larger in the northern eroding part than in the accreting south part. A clear southward flattening pattern can be observed in the accreting south part. 2. The bed surface sediment grain size decreases landward in the cross-shore direction. In the alongshore direction, sediment grain size shows a southward increasing pattern in the south part. 3. Both extremely fine sediment dominant profiles and coarse sands dominant profiles can be found in the north part. 4. Human intervention continuously took place along the Jiangsu Coast. However, its influence on the morphological and sedimentological characteristics of the Jiangsu Coast cannot be determined from this one-time investigation dataset, as it was col- lected too shortly after the implementation of large-scale land reclamation schemes. More field surveys are needed to further study the tidal flat response to human interventions. The extremely fine or coarse sediments in the north coast can be explained by two ero- sion resistance mechanisms, viz. consolidation effect and armoring effect. The southward flattening and coarsening pattern is due to the following factors: Flood-dominant current causing landward sediment transport, larger tidal range in the south part, sheltering effect of the RSRs and contribution of different sediment sources namely AYRD and RSRs. Waves play a minor role in this behavior. Whether the relationship between intertidal beach slope and bed surface sediment size will change when Jiangsu Coast evolution reaches an equilibrium state still remains to be solved.

Author Contributions: Conceptualization, research plan, Y.K., J.T., S.A., and Z.B.W.; supervision, J.T., S.A., and Z.B.W.; data collection and analysis, Y.K. and Z.Z.; writing—original draft preparation, Y.K.; writing—reviewing and editing, J.T., Z.Z., S.A., and Z.B.W. All authors discussed the results and contributed to the final manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The first author thanks the China Scholarship Council for providing the research grant. Jianfeng Tao is grateful to the funding from the National Natural Science Foundation of China (NO. 51620105005 and NO. 52071129) and the National Key Research and Development Program of China (NO. 2018YFC0407501). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: Dineng Zhao from the Second Institute of Oceanography, China, is thanked for the kind assistance and help with the satellite images analysis. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. J. Mar. Sci. Eng. 2021, 9, 347 19 of 20

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