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