Channel: Marine geomorphology http://www.jto.ac.cn EN-US http://www.jto.ac.cn/EN/1009-5470/current.shtml http://www.jto.ac.cn 1009-5470 http://www.jto.ac.cn/EN/10.11978/2021079 Global coastal ecological system is being threatened by climate change and human activities. Based on a typical profile of sandbar-lagoon system and designed wave flume experiments with moving bed, we carried out a quantitative analysis on the effects of submerged vegetation on erosion and deposition under erosion wave conditions. The results show that the submerged vegetation obviously weakens the wave height that increaes in the margin of surf zone on the fore slope of sandbar, and wave attenuation occurs behind the sandbar. Wave reflection and transmission coefficients reduce and wave dissipation coefficient increases due to submerged vegetation. Under the action of erosion waves, the sandbar crest was eroded distinctly, the lagoon showed a deposition trend, and the coastal foredune was eroded in a scarp form. With the influence of vegetation, the maximum erosion thickness of sandbar and foredune zone decreases. In general, vegetation can reduce the net sandbar erosion, the deposition in the lagoon and the offshore sediment transport, which has a good protective effect on the coastal foredune.

]]> http://www.jto.ac.cn/EN/10.11978/2021091 The embayed sandy and gravel beaches are important tourism resources in the coastal zone of islands, with high socio-economic and ecological values and having received study attention for years. This study focuses on five representative embayed beaches on the east coast of the Zhujiajian Island in Zhejiang Province. The beach topography and sediment data were obtained from three field campaigns in the early, middle and late stages of the 2019 typhoon season, and the nearshore hydrodynamic data were also collected and analyzed. Then, the dynamic changes of topography and sediment characteristics of five different embayed beaches were detected. The results show that the five beaches experienced different volume changes during the typhoon season, i.e., 11.93 m³·m^-1(Dashali), -54.41 m³·m^-1 (Dongsha), -19.75 m³·m^-1 (Qiansha), 2.19 m³·m^-1 (Wushitang) and -1.96 m³·m^-1 (Xiaowushitang), respectively. The gravel beaches were more stable than the sandy beaches, and the sandy beach without human activity is less eroded during the typhoon season. The mean grain size of surficial sediments on Dashali, Dongsha, Qiansha, Wushitang and Xiaowushitang during the typhoon season are 2.47 Փ, 2.24 Փ, 2.64 Փ, -5.96 Փ and -6.03 Փ, respectively, with coarsening and onshore transport being the main performances of their changes. The change of sediment grain size on sandy beaches is more significant than that on the gravel beaches. The sediment grain size, the energy level of typhoon, and the angle between the main wave direction during the typhoon and the beach orientation, and coastal engineering can have impacts on beach morphological variation during typhoon seasons. This research can provide reference for the beach management during typhoon seasons.

]]> http://www.jto.ac.cn/EN/10.11978/2021096 The dynamic process of beach profile changes under the actions of waves and tides is the key to understanding coastal evolution, coastal defence design and tourism resource planning. Taking an artificial beach in the Shajing Peninsula of the Qinzhou Bay in Guangxi province as the study area, the study aims to explore the dynamic process of beach profile changes by analyzing the erosion and single-width volume of beach profile, as well as using EOF (Empirical Orthogonal Function) to reveal its model, based on the monthly surveyed profile elevation data from January 2018 to December 2019 using by GPS-RTK, The main results were shown as followed: (1) during the observation period, the artificial beach profile showed a regular variation characterized by accretion in winter and spring and erosion in summer and autumn; (2) due to the transverse sediment transport over the artificial beach, the single-width volumes of beach profile displayed opposite variation trends in adjacent transversal zones, which was manifested by a alternation between erosion and accretion; (3) the variation modes of artificial beach profiles could be divided into three parts that include the main model that beach profile elevations decreased significantly due to heavy rainfall and typhoon, the secondary model that the beach profile recovered gradually after heavy rainfall and typhoon under the influences of tides and normal waves, and the third model that beach profile changes resulted from wave breaking-induced currents under the action of normal waves.

]]> http://www.jto.ac.cn/EN/10.11978/2021141 The Swan Lake in Rongcheng of Shandong province is a typical lagoon-barriers system. Since 1980s, under the influence of human activities, the environmental system of the Swan Lake has changed with strong erosion of the sand dam. Studying the erosion and deposition of underwater bank slopes is of great significance for understanding the evolution of lagoon-sand dam system and protecting the environment of Swan Lake. This paper focus on the underwater bank slope of the sand dam at the east side of the Swan Lake. Based on the combination of chart data from 1960 to 2014 and geographic information technology, we have obtained multi-year water depth of the coastal waters on the east side of the Swan Lake since 1960, and analyzed the contour changes, scouring and silting changes and topographic changes of the set profile in the sand dam area on the east side of Swan Lake. The results show that the overall depth contours moved towards the sea from 1960 to 1980, while the tendency of contours moving towards the sea from 1980 to 2014 slowed down, the 0 m depth contour at the entrance of the tidal channel retreated 126.70 m to the shore from 1960 to 2014. The underwater bank slope of the Swan Lake was at a state of siltation from 1960 to 1980, the annual siltation rate is 3.60 cm·a^-1. The erosion area increased significantly from 1980 to 2014, and the erosion and siltation phases were distributed, the annual siltation rate is 0.79 cm·a^-1. The underwater bank slope of the Swan Lake sand dam is steep and gentle, and the north side of the tidal channel entrance is more likely to be eroded than the south side. Human activities have changed the environmental state of the Swan Lake, leading to partial erosion and retreat of the underwater bank slope of the Swan Lake.

]]> http://www.jto.ac.cn/EN/10.11978/2021116 Coastline dynamic change is a common reflection of global change and human activities. Based on seven Landsat remote sensing images and Google Earth high-resolution images of the Guangdong-Hong Kong-Macao Greater Bay Area, Tokyo Bay and San Francisco Bay from 1980 to 2020, this paper used threshold segmentation combined with water index method and Sobel operator method to extract the coastline of each analysis period. In addition, this study also analyzed the changes of coastline in terms of length, spatial morphology, structure and utilization degree, and analyzed qualitatively and quantitatively the influencing factors with the help of geographic detectors. The results show that: (1) from 1980 to 2020, the intensity of annual average length change in the Tokyo Bay is the largest, which is 0.37%, and the coastline tends to be straight; (2) in the past 40 years, the depth of coastlines in all analyzed bay areas has remained stable, and the structures tend to be complex and the morphology tends to be dispersed. Among them, the difference of fractal dimension of coastlines in the Guangdong-Hong Kong-Macao Greater Bay Area is the smallest and the morphology is the most dispersed; (3) during the past 40 years, the natural coastline of each bay area decreased, while the length of port wharf coastline and other artificial coastline increased significantly, and the biological coastline of the Guangdong-Hong Kong-Macao Greater Bay Area fluctuated, and the coastline utilization index increased the most. The results show that temperature, wave height, tide, area and port throughput are the main factors that affect the coastline change, the interaction of any two factors is greater than the single factor.

]]> http://www.jto.ac.cn/EN/10.11978/2021097 The beach morphodynamic process under the influence of extreme high-energy events is directly related to coastal stability and seawalls safety. Based on the profile elevation and surface sediments of the Yintan Beach collected before and after the 2020 16^th typhoon “Nangka”, this paper explores the mechanism of morphological changes of a meso-tidal beach in response to typhoon actions. The results show that: (1) In post-typhoon period, the beach morphological changes are manifested by the disappearance of a large sand bar and the flattening of the bar-trough system, leading to a more gentle beach slope; (2) Sediment in Yintan Beach is mainly medium sand, fine sand and very fine sand, which account for more than 95%; The sediment grains become finer in the high tide zone and coarser in the low tide zone, while both increase and decrease in sediment grain size can occur near the sand bar; The morphodynamic process of mid-to-high tide zone of the Yintan Beach in response to a typhoon is similar to that of a macro-tidal beach, which is characterized by dissipative condition. However, the morphodynamic process of low tide zone of the Yintan Beach behaves like a micro-tidal beach due to the existence of a large sand bar; (3) During the typhoon, wave is the main dynamic factor affecting the beach, and the topography affects the manifestation of nearshore hydrodynamics, thereby changing the grain size distribution of sediment.

]]> http://www.jto.ac.cn/EN/10.11978/2021150 As a new form of coastal engineering, the artificial island has been receiving increasing attentions from community to the beach erosion and accretion due to its construction. Based on the multi-temporal remote sensing images and four field surveys, the influence of winter waves combined with artificial islands on the erosion and accretion of the Riyue Bay beach has been analyzed. Besides, the mechanism of beach erosion and accretion was explored using the wave data in the adjacent sea area. Our new results show that the average significant wave heights and wave periods in the winter were obviously stronger than those in the summer, and the wave force generally shows an increasing trend year by year, but its variation is not significant around the artificial island before and after the construction of islands. The construction of artificial island has a significant impact on the beach topographic profile, showing the obvious characteristics of accretion in the area sheltered by the artificial island and erosion on two sides of the beach. The beach accreted continuously in the island shadow area, and the shoreline in that area moved up to 200 m towards the sea, which may even be linked with the artificial islands to form a tombolo. The beach was eroded and the maximum erosion distance of shoreline to land is about 50 m on both sides behind the artificial island. The Riyue Bay beach is strongly affected by the winter waves and artificial islands. From north to south, the beach was at a state of stability-erosion-accretion, relative stability, slight erosion or accretion-erosion-accretion.

]]> http://www.jto.ac.cn/EN/10.11978/2021167 Suspended sediment concentration(SSC)observation in vertical profile near the bed of mangrove tidal flat under the waves and tidal currents, is important to understand the mechanisms of energy dissipation and deposition promotion by vegetations, and the ecological restoration in coastal zone. In this study, we measured the hydrological data using HR, ADV, Twave and ASM and the typical plants’ parameters of Aegiceras corniculatum for 3 consecutive days of flood tides in the summer of 2019. Then the process of the near-bed vertical SSC of mangrove tidal flat was analyzed in response to waves, tidal currents and spatial structure of Aegiceras corniculatum. The results showed that: (1) the variations of suspended sediment flux and SSC near bed in Aegiceras corniculatum tidal flat had the characteristic of tidal asymmetry, the SSC of flood tide was significantly greater than that of ebb tide. The vertical high SSC area of the profile changed from 0.1~0.37 m to 0.5-0.67 m from the bottom in the early flood tide to flood maximum, and from the upper to the bottom in ebb maximum to end. (2) In the intra-tide, the suspension and resuspension stages occurred at the early flood and latest ebb that were dominated by waves. The advection and deposition stages occurred at the whole period from flood maximum to ebb maximum dominated by tidal currents. (3) Dense branches and leaves in the canopy of Aegiceras corniculatum intercepted more than 40% of the suspended sediment in the upper canopy during flood tide. The SSC decreased by more than 71% when the flow during ebb tide. This dynamical deposition mechanism of asymmetrical flood and ebb tide was conducive to sediment transportation landward which then led to flat expansion.

]]> http://www.jto.ac.cn/EN/10.11978/2022036 The Haitan Strait in Fujian province is an important channel for the southward transportation of material from the Minjiang River and the Zhejiang-Fujian Coastal Current to the Xinghua Bay and the south-central coast of Fujian. The topographic data of the Haitan Strait in the past 50 years was analyzed using GIS tools. The results indicate that strong siltation occurred in the Haitan Strait due to the influence of several catastrophic floods in the Minjiang River Basin during 1964~1975. The Haitan Strait experienced strong erosion during 1975~1990 which is caused by water and soil conservation and reservoir constructions in the Minjiang River Basin. After that, the erosion rate declined gradually and the strait maintained slight scouring. Large-scale artificial sand mining started since 2007 that caused severe erosion in the northern part of the Strait. Before 2007, sediment discharge from the Minjiang River was the main factor restricting the geomorphological evolution of the Haitan Strait. Flood, sand mining and reservoir construction had a direct impact on the geomorphologic evolution of the strait. However, after 2007, large-scale artificial sand mining activities and reclamation projects in the strait have gradually become the main factors to affect its geomorphologic evolution.

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