Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay,Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.     

Sand bodies and incised valleys within the Late Quaternary Sanaga–Nyong delta complex on the middle continental shelf of Cameroon

Seismic data and sediment cores collected offshore from the Sanaga River and Nyong River mouths were used to analyse a loose mantle of yellow to reddish sandy gravel with a range of fluvial and deltaic characteristics cropping out in the middle part (25–65 m water depth) of the continental shelf of Cameroon. Contrary to most of the Atlantic shelves, where the lowstand systems tract was destroyed by erosion, we found 80–120 ms (60–90 m) of sediment mainly in the middle part of the shelf, which correspond credibly to MIS 2–4. Fluvial paleodrainage systems are preserved beneath the shelf and individual filled channels with planar infillings were mapped that cross the shelf along two surfaces of erosion. These incisions suggested westerly and northwesterly drainage shifts during presumed lowered base level. The presence of closely spaced channel fills suggests repeated avulsion of a single stream during a long-lasting sub-aerial erosion period. The seismic facies of these Pleistocene deposits distinguish themselves clearly from well-stratified older strata showing deformation (Pliocene) or intense folding (Miocene). The orientation of the paleovalleys appears strongly controlled by the N60°E trending cross faults within Mesozoic–Cenozoic strata.     

杭州湾淤泥质海岸岸线变化及其动态模拟

通过对杭州湾北岸南竹港-龙泉岸段实测岸滩断面资料的统计分析,进而对淤泥质海岸岸线变化、影响因素及其动态模拟预报进行探讨。结果表明:因长江来沙减少以及侵蚀/淤积波自东向西移动,导致近10a来该岸段岸滩处于侵蚀状态,其中岸线整体后退;因季节性的波侯作用,岸滩具有冬淤夏冲的特征,岸线则表现为冬涨夏退,因局部工程作用影响的岸线则具有冬退夏涨的特征。此外,基于-3m和-8m等深线构建的径向基函数神经网络模型预报岸线的变化是可行的。     

Determination of wave–shoreline dynamics on a macrotidal gravel beach using Canonical Correlation Analysis

This paper describes the application of Canonical Correlation Analysis (CCA) to derive forcing–response relations between the wave climate and shoreline position on a macrotidal gravel barrier located in the southwest of the U.K., and to develop a tool to determine shoreline positions from wave records. The data sequences comprise wave climate recorded by a nearshore directional wave buoy and video-derived shorelines over a time span of one year and a half. The hydrodynamic conditions are used to determine the probability density function of wave heights and alongshore energy fluxes. These are then related to shoreline change through a CCA analysis. The CCA analysis identifies patterns of behaviour of the wave conditions and the shoreline position, and the relation between both patterns is found to provide useful information about the beach response to wave action. The analysis shows that the movement of sediment is greater at the southern end of the study area and that there is an immediate shoreline response to the wave action. In the case of coastal management it is more often the case that wave forecasts are available on a routine basis. The ability of the CCA to provide useful estimates of shorelines from wave conditions was tested by using measured waves to calculate the corresponding shoreline position from additional data at the end of the sequences. Shoreline positions determined with the CCA agreed well with the measured ones. Thus, the CCA is found to be a useful tool to determine unknown shoreline positions and support effective coastal management if good quality hydrodynamic and morphological data are available to input into the initial set-up of the technique.     

Distribution and thickness of sedimentary facies in the coastal dune, beach and nearshore sedimentary system at Maspalomas, Canary Islands

Numerous studies have shown that most beaches and coastal dune systems of the world are currently eroding but very few have investigated the combined sediment budgets of subaerial and nearshore submarine systems. In the case of the dune field of the Maspalomas Natural Special Reserve (in the south of Gran Canaria), the adjacent Maspalomas and El Inglés beaches and the adjacent submarine platform, the sediment budgets have been severely affected by erosion over the past few decades. The objectives of this study were to investigate the availability of sand within the modern sedimentary system, including the coastal dunes, the beaches and the submerged shelf, but also to assess local sediment sinks. An isopach map generated on the basis of topo-bathymetric data and seismic-reflection profiles revealed that sediment thickness varies from 0–22 m in the study area. Expanses of relatively low sediment thickness were identified in the south-western sector of the coastal dune field along Maspalomas beach, and in the nearshore region to the south of this beach. These localized sediment-deficit areas earmark Maspalomas beach as the most vulnerable shore strip threatened by erosion. The shallow seismic data also revealed that the submarine platform south of Maspalomas represents a marine terrace cut into an ancient alluvial fan, thus documenting an influence of the geomorphological heritage on the present-day morphodynamics. A side-scan sonar mosaic of this nearshore platform enabled the delimitation of areas covered by rock, boulders and gravel, vegetated sand patches and a mobile sand facies, the latter including ripple and megaripple fields. The megaripple field in a valley close to the talus of the marine terrace has been identified as a major sediment sink of the Maspalomas sedimentary system. It is fed by south-westerly storm-wave events. The sediment deficit in the coastal dune field and along Maspalomas beach can therefore only be explained by a currently faster loss of sediment to an offshore sink than can be compensated by the supply of sand from outside the system.     

Beach and nearshore facies: Southeast Australia

The morphology, texture and facies sequence on seven sand beaches, located in low, moderate and high wave energy, microtidal environments in southern Australia were investigated using box coring and Scuba observations. Systematic variation in facies occur both within and between the beaches. Low-energy reflective beaches are limited in lateral and vertical extent and in facies to beach laminations separated by coarse step deposits from finer nearshore cross-lamination facies. Moderate-energy intermediate beaches characterised by rip circulation possess increasingly wider surfzones with ridge and runnel and bar-trough facies separating the beach and step facies from the more extensive nearshore sequence. High-energy dissipative beaches may have 500 m wide surfzones containing multiple bar-trough topography. Fine beach laminations with backwash structures grade into 4–5 m thick bar-trough sequences then the extensive nearshore facies. As wave energy increases from low (H_b < 1 m) to high (H_b > 2.5 m) the vertical extent of the beach to nearshore sequence increases from <10 m to approximately 30 m, and the width from 100 m to several kilometres. Consequently one would expect higher-energy paleo-beach sequences to be represented more by diagonal than vertical facies sequences.     

Horizontal spatial and temporal distribution patterns of nearshore larval fish assemblages at a temperate rocky shore

There have been no previous studies of the composition of nearshore larval fish assemblages along the coast of Portugal. We aimed to describe the composition and horizontal distribution patterns of larval fish assemblages and their temporal dynamics near a rocky reef at depths shallower than 13 m (inshore) and at two miles (3.70 km) from shore (offshore), as well as along transects perpendicular to the shoreline, from the reef to 10 miles offshore (18.52 km). Samples were taken using 5 min sub-surface trawls at the rocky shore of the Arrábida Marine Park (W Portugal). A total of 1021 larvae were collected, belonging to 61 taxa inshore and to 29 taxa offshore. Along transects, 626 larvae of 52 taxa were collected. Most larvae belonged to coastal species associated with rocky reefs. Total larval abundance and diversity were higher from May to July, which is consistent with the spawning activity of adults. Diversity and total larval abundance decreased significantly with increasing distance from shore, both in the inshore/offshore comparison and in the transects, where this decrease was evident at a very small spatial scale (within the first mile from the reef). Species assemblages differed in the pattern of distribution, with most species clearly associated to the extreme nearshore. The distribution patterns obtained were independent of the spawning mode of species. Results are discussed in the light of the possible physical mechanisms that can potentially act at the Arrábida Marine Park to facilitate larvae retention and the role of larval behaviour.     

人工岛对沙质海岸动力泥沙环境及岸滩冲淤演变的影响研究

基于历史海图、历史时期卫星遥感影像、2019年实测水下地形和潮流、波浪数值模拟成果,研究该海域岸滩的发育演变背景、海床和岸线自然冲淤动态、动力泥沙环境及岸滩冲淤动力机制。采用LITLINE岸线演变数学模型,模拟计算了不同离岸距离和不同平面形态人工岛布置型式对近岸岸线变形的影响,岸滩泥沙动力机制和岸滩演变趋势表明,人工岛实施后近岸最大潮流流速小于近岸泥沙起动流速,不具备起动近岸岸滩泥沙的基本条件; SSE向波浪作用时,人工岛西北侧波高明显减弱,波向西偏; S向波浪作用时,人工岛北侧波高明显减弱; SSW向波浪作用时,人工岛东北侧波高明显减弱,波浪传播方向东偏;波浪场改变后人工岛掩护区东西两侧岸滩附近泥沙分别具有向东和向西运移至人工岛掩护区的趋势,形成掩护区的淤积和东西两侧岸线冲刷。     

Shoreline change detection using DSAS technique: Case of North Sinai coast,Egypt

This study focuses on the shoreline change detection along the North Sinai coast in Egypt using geographic information system and digital shoreline analysis system (DSAS) during the elapsed period from 1989 to 2016. The measurement of shoreline variation is mainly described for three zones: zone I, El-Tinah plain bay; zone II, El-Bardawil Lake; zone III, El-Arish valley. The rates of shoreline changes in the form of erosion and accretion patterns are automatically quantified by four statistical parameters functioned in DSAS namely endpoint rate, net shoreline movement, linear regression rate (LRR), and least median of squares. LRR results elucidate that the western seaside of El-Tinah plain bay has experienced an extremely dynamic feature with an average erosion rate of ?8.17?m/year. The littoral drifts have been driven by eastward alongshore currents toward the east side of the bay to be accreted with an average rate of +4.28?m/year. Moreover, the shoreline has progressed west of El-Bardawil inlet (1), El-Bardawil inlet (2), and El-Arish harbor. Subsequently, the corresponding average beach growth rates are found to be +2.7, +8.5, and +6.5?m/year, respectively. In contrast, the shoreline on the down-drift side to the east has negatively retreated, and the corresponding beaches have regressed at rates of ?4.5, ?8.65, and ?2.9?m/year, respectively.     

莺歌海三莺村岸段人工沙滩工程岸线数值模拟以及模型的验证方

以乐东莺歌海三莺村岸段人工沙滩工程为例,其完工1年后的岸线监测结果,表明需要进一步对人工沙滩后的海滩岸线演变进行研究分析。通过GENESIS岸线演变模型模拟人工沙滩后的岸线变化,并尝试采用3种方法对模型进行验证。经过验证后,对人工沙滩工程后1年、3年、5年岸线变化进行了模拟分析。研究结果表明:GENESIS岸线演变模型对于包围型(拦沙堤和离岸堤结合方式)人工沙滩工程是适用的。工程完工1年后,补沙岸线形成了侵淤变化的凹凸弯曲岸线,侵淤幅度不大,5年后侵淤幅度变缓,开始逐渐趋于平衡。据此,也为人工沙滩后期的养护、补沙防护工作起到重要的参考作用。     

海岸冲淤动态的理论分析：物质收支、剖面形态、岸线进退

海岸线动态经常被作为海岸冲淤的判据,然而,由于未能涵盖物质收支和海岸剖面形态的双重因素影响,此判据具有局限性。基于沉积物收支方程性质和海滩-潮滩剖面形态的理论分析,认为将物质收支与岸线进退速率或海岸剖面形态相结合,才能准确判别海岸冲淤状态。沉积物收支方程含有沉积体系规模、冲淤强度、系统生长极限等信息;海滩剖面形态决定于物质粒径、波能大小,波能耗散最小原理决定了海滩均衡剖面的存在性,而潮滩剖面形态决定于沉积物供给、粒径组成和潮汐动力。根据沉积物收支方程和海岸剖面理论,融合极端事件（风暴等）和海面变化因素,可获取砂质海岸（以海滩为代表）、泥质海岸（以潮滩为代表）各种侵蚀现象的发生机制、速率和时间尺度信息,海岸线变化速率从低（＜100 m/a）到高（101～102 m/a）有数量级的差异,冲淤过程的时间尺度包括10−2 a（风暴事件）到103 a（海面变化）的范围。根据沉积物收支和海岸线进退的不同组合,可将海滩、潮滩海岸冲淤动态分为4类,其中第一类为堆积海岸,其余三类为侵蚀海岸,与不同的地貌演化方向和时间尺度相联系。高强度、长时间持续侵蚀主要与物质供给中断和海面上升相关,同时也有人为因素影响。     

Shoreline migration and beach-nearshore sand balance over the last 200 years in Haifa Bay (SE Mediterranean)

Several researchers have investigated morphological changes on the south-eastern Mediterranean coast during the late Holocene. However, very few of these studies include quantitative data covering the last 200 years. In this study, topographical maps, nautical charts and aerial photographs are used to estimate the shoreline migrations and beach–nearshore sand balance over the last 200 years in Haifa Bay, Israel, the northernmost final depositional sink of the Nile littoral cell. The findings reflect two main periods. During the first period, between 1799 and 1928, human intervention along the bay’s coast was negligible, a significant coastal expansion of ∼50 to 150 m (averages of 0.4–1.2 m/year) was measured, and sand accumulation was estimated at ∼70,000 m³ annually in the beach–nearshore area. A dramatic change in the sedimentological pattern was observed during the second period, between 1928 and 2006, following the completion of Haifa Port’s main breakwater (1929–1933). During this period, most of the bay’s coast was in a steady state, with seasonal fluctuations of less than about ±20 m, and slight erosion of ∼7,000 m³ annually. These findings are consistent with previous studies which conclude that from approximately 4,000 years ago until the construction of Haifa Port, sea level remained relatively stable, and a continuous accumulation of Nile-derived sand dried up the Zevulun Plain and shifted the Haifa Bay shoreline westwards to its present location. This long-term trend ceased after completion of the Haifa Port main breakwater.     

The impact of various methods of wave transfers from deep water to nearshore when determining extreme beach erosion

Several levels of increasing complexity of transferring wave information from offshore to nearshore have been studied to quantify their influence on extreme beach erosion estimates. Beach profiles which have been monitored since 1976 were used to estimate extreme beach erosion and compared to predictions. Examination of the wave propagation assumptions revolves around two types of offshore to nearshore transfer: excluding or including wave breaking and bottom friction. A second complication is whether still water level variations (ocean tide plus storm surge) are included.The inclusion of various combinations of wave propagation processes other than shoaling and refraction in the wave transfer function changes on the extreme erosion distribution tail through lowering estimates above one year return period. This brings the predicted tails closer to the observations, but does not capture the upper limit of storm demand implied by the extensive beach profile data set. Including wave breaking has a marked effect on probabilistic estimates of beach erosion. The inclusion of bottom friction is less significant. The inclusion of still water level variability in the wave transfer calculation had minimal impact on results for the case study site, where waves were transferred from offshore to water at 20 m depth. These changes were put into perspective by comparing them to changes resulting from limiting beach erosion by adjusting the statistical distributions of peak wave height and storm duration to have maximum limits. We conclude that the proposed improvements on wave transformation methods are as significant as limiting wave erosion potential and worth including.     

Sediment transport and morphology at the surf zone of Presque Isle, Lake Erie, Pennsylvania

A four-year investigation of surf zone sedimentation at Presque Isle, Pennsylvania, was undertaken in preparation for the design of a segmented breakwater system. Sediment transport calculations were based on hind-cast annual wave power statistics and “calibrated” by known accretion rates at the downdrift spit terminus. 30,000 m³ of sediment reaches the peninsula annually from updrift beaches. The transport volume increases downdrift due to shoreface erosion and retreat of the peninsular neck. At the most exposed point on Presque Isle (the lighthouse) the annual transport is 209,000 m³. East of the lighthouse is a zone of net shoreface accretion as the longshore transport rate progressively decreases.

The downdrift variation in sediment supply, combined with increasing refraction and attenuation of the dominant westerly storm waves produce a systematic change in prevailing surf zone morphology. Storms produce a major longshore bar and trough along the exposed peninsular neck. The wave energy during non-storm periods is too low to significantly alter the bar which consequently becomes a permanent feature. The broad shoreface and reduced wave energy level east of the lighthouse produce a morphology characterized by large crescentic outer bars, transverse bars, and megacusps along the beach. At the sheltered and rapidly prograding eastern spit terminus the prevalent beach morphology is that of a ridge and runnel system in front of a megacuspate shore.

The morphodynamic surf zone model developed for oceanic beaches in Australia is used as a basis for interpretation of shoreface morphologic variability at Presque Isle. In spite of interference by major shoreline stabilization structures, and differences between oceanic and lake wave spectra, the nearshore bar field at Presque Isle does closely correspond to the Australian model.     

海南省万宁东部砂质海岸侵蚀特征分析

2011—2014年于万宁东部海岸建立了海岸动态监测系统, 包括120个岸线监测站位和30个岸滩剖面监测站位, 收集了4年间岸线和岸滩的实时变化信息。以这些实测资料为基础, 结合收集的海岸资料和实地调查, 对万宁东部海岸的侵蚀规律与特征进行了探讨。在季节变化中, 海岸呈现夏秋侵蚀冬春淤积的规律; 在年际变化中, 海岸总体呈侵蚀趋势, 但相邻年份间又呈现侵蚀和淤积相间的现象。万宁部分海岸受前滨采砂影响严重, 加上频繁的台风天气, 致使侵蚀加剧, 在两年时间内后滨侵蚀发育8m高的软质陡崖, 造成难以恢复的地形改变。     

Classification of the Alaskan Beaufort Sea Coast and estimation of carbon and sediment inputs from coastal erosion

A regional classification of shoreline segments along the Alaskan Beaufort Sea Coast was developed as the basis for quantifying coastal morphology, lithology, and carbon and mineral sediment fluxes. We delineated 48 mainland segments totaling 1,957 km, as well as 1,334 km of spits and islands. Mainland coasts were grouped into five broad classes: exposed bluffs (313 km), bays and inlets (235 km), lagoons with barrier islands (546 km), tapped basins (171 km) and deltas (691 km). Sediments are mostly silts and sands, with occasional gravel, and bank heights generally are low (2–4 m), especially for deltas (<1 m). Mean annual erosion rates (MAER) by coastline type vary from 0.7 m/year (maximum 10.4 m/year) for lagoons to 2.4 m/year for exposed bluffs (maximum 16.7 m/year). MAERs are much higher in silty soils (3.2 m/year) than in sandy (1.2 m/year) to gravelly (–0.3 m/year) soils. Soil organic carbon along eroding shorelines (deltas excluded) range from 12 to 153 kg/m² of bank surface down to the water line. We assume carbon flux out from depositional delta sediments is negligible. Across the entire Alaskan Beaufort Sea Coast, estimated annual carbon input from eroding shorelines ranges from –47 to 818 Mg/km/year (Metric tones/km/year) across the 48 segments, average 149 Mg/km/year (for 34 nondeltaic segments), and total 1.8×10⁵ Mg/year. Annual mineral input from eroding shorelines ranges from –1,863 (accreting) to 15,752 Mg/km/year, average 2,743 Mg/km/year, and totals 3.3 ×10⁶ Mg/year.     

Two-dimensional wave-current interaction with a locally enriched quadtree grid system

Wave-induced currents may drive nearshore mixing and transport processes, including coastal pollutant dispersion, littoral drift, and long-term morphological changes through beach erosion and accretion. In this study, a numerical model is newly developed to simulate wave climate and localized currents in complicated coastal environments. The model developed is based on a quadtree grid system. The two-dimensional hydrodynamic governing equations are solved by using an explicit Adams-Bashforth finite difference scheme. Effects of wave breaking, shoaling, refraction, diffraction, wave-current interaction, set-up and set-down, turbulent mixing, bed friction, and shoreline movement are incorporated in the model. Results are presented for set-up at a beach in a flume due to normally incident waves, and longshore currents generated by oblique waves on a plane beach.     

Application of remote sensing video systems to coastline management problems

This contribution evaluates the application of coastal video systems to monitoring and management of coastal stability problems on sandy coastlines. Specifically, video-derived parameters (coastal state indicators or CSIs) are developed which facilitate the measurement of the shoreline evolution (erosion/accretion) and response to storms, seasonal cycles and anthropogenic interventions like beach/shoreface nourishment and dredging. The primary variable which forms the basis for all the CSIs discussed in this contribution is the shoreline position derived from time-averaged video images. These waterlines are used to generate secondary products including shoreline contours at a constant pre-defined level, (intertidal) beach volumes, and momentary shoreline positions which reflect the sand volume in a meter wide section of the intertidal coast. Video-derived coastal state indicators were verified via comparisons with traditional topographical/bathymetric surveying techniques and a good agreement was found in all cases. CSIs were computed for three contrasting sandy coastal environments including an unprotected natural beach, a protected beach and a spit. Firstly, results are presented which demonstrate the advantages of coastal video systems over and above infrequent traditional topographic/bathymetric surveying methods. Namely, the ability of video-derived CSIs to quantify the magnitude, accurate location, precise timing and rates of change associated with individual extreme events and seasonal variability in the wave climate. Secondly, video-derived coastal state indicators were used to monitor two different types of human intervention, including beach nourishments and a dredged pit in a navigation channel. The video-derived datasets of coastal state indicators offered significant improvement to current CZM practices, facilitating better timing of management interventions as well as more effective monitoring of the spatial impact and longevity of these actions.     

莺歌海三莺村岸段人工沙滩工程岸线数值模拟以及模型的验证方法

以乐东莺歌海三莺村岸段人工沙滩工程为例,其完工1年后的岸线监测结果,表明需要进一步对人工沙滩后的海滩岸线演变进行研究分析。通过GENESIS岸线演变模型模拟人工沙滩后的岸线变化,并尝试采用3种方法对模型进行验证。经过验证后,对人工沙滩工程后1年、3年、5年岸线变化进行了模拟分析。研究结果表明:GENESIS岸线演变模型对于包围型(拦沙堤和离岸堤结合方式)人工沙滩工程是适用的。工程完工1年后,补沙岸线形成了侵淤变化的凹凸弯曲岸线,侵淤幅度不大,5年后侵淤幅度变缓,开始逐渐趋于平衡。据此,也为人工沙滩后期的养护、补沙防护工作起到重要的参考作用。     

近60年莱州湾东部砂质海岸地貌的时空动态

在野外考察的基础上,以不同时期测量和成像的海图、地形图和高分辨率遥感影像为数据源,综合使用数字岸线分析、遥感、地理信息系统等方法,对近60年来莱州湾东部砂质海岸（界河口-刁龙嘴）地貌演变进行研究。结果表明,研究期内莱州湾东部岸线迁移和水下岸坡冲淤存在强烈的时空差异。岸线迁移表现为冲淤进退交替发生,其在时间过程上具有非线性的显著特点。1959-2013年,侵蚀岸线所占比例波动起伏,具有阶段性。侵蚀岸段年均演变速率以1985年为界分成两个阶段,前期由极大值4.95 m/a（1959-1969年）锐减至极小值1.97 m/a（1969-1985年）,后期由1.97 m/a增大到4.43 m/a（1985-1998年）,之后均处于高强度侵蚀。岸线迁移空间分布的差异性主要表现在4个岬湾的岸线变迁特征与莱州湾东岸岸线整体变迁并不一致。同时,水下岸坡冲淤条带大体相间分布,同一岸段1959-1985年和1985-2008年的冲淤演变趋势基本相反。进一步分析表明最近60年来,莱州湾东部砂岸在水库拦沙、潮上带工厂化水产养殖、大型海岸工程等人类活动的地貌效应共同控制下,发生了5次岸线淤积前进与侵蚀后退以及冲淤速率大小的交替。