The coast between Cabo de Santa Maria (Portugal) and Rabat (Morocco): a mega-size headland-bay shoreline under control of the North Atlantic swell?

Equilibrium headland-bay beach systems have been mathematically described by logarithmic, parabolic and hyperbolic curve functions. The largest system of this type reported to date has a shoreline length of about 62 km. In the present study, an apparent headland-bay system is presented which has a shoreline length of about 500 km. It was discovered on satellite images, and is located between Cabo de Santa Maria in Portugal and the coastal city of Rabat in Morocco. It appears to be controlled by long-period North Atlantic swells diffracting around Cabo São Vicente at the south-western tip of Portugal, in combination with SW–SE wind wave climates impinging on the northern shoreline of Cádiz Bay. The coast shows two marked departures from the equilibrium shoreline along its central section north and south of the Strait of Gibraltar, which are easily explained. Thus, the promontories to the north of the strait still exist because there has not been sufficient time to erode these back to the equilibrium shoreline since postglacial sea-level recovery. The coastal indentation to the south is explained by an insufficient sediment supply from terrestrial sources to facilitate the required beach accretion. Perfectly adjusted planimetric headland-bay shoreline shapes represent situations where wave orthogonals approach the coast at right angles everywhere, i.e. there is no longer any alongshore sediment transport. Equilibrium shorelines form independently of the grain size of the beach sediment, whereas morphodynamic beach states are indirectly affected by the shoreline shapes because the latter are modulated by wave period and breaker height which also control the morphodynamic response of the beach in combination with the local grain size.     

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.     

Short-term shoreline changes due to cross-shore structures: a one-line numerical model

The proposed numerical model simulates the short-term temporal changes in shoreline position due to a structure interrupting the longshore sediment flux. The impacts of both the groin-type construction and underwater trench of arbitrary orientation relative to the shore are discussed. In order to estimate the sediment mass trapped by the structure, a submodel of the longshore sediment transport induced by a random wave field is developed. The contribution of the surface roller in momentum balance as well as in sediment suspension is included. The shoreline changes are computed from the equation deduced from the mass conservation. The perturbations in the longshore sediment discharge caused by a structure are assumed to concentrate within some boundary area of which the spatial scale is proportional to the structure's length until the latter is exceeded by the width of the sediment flux. It is shown in particular that the total effect of a long trench (channel) and a pier in its nearshore part results in general shoreline recession except for the vicinity of a pier. The model is tested against the laboratory data of Baidei et al. (1994) and applied to the Baidara Bay coast (Kara Sea) where a pipeline would be designed.     

Rip currents, mega-cusps, and eroding dunes

Dune erosion is shown to occur at the embayment of beach mega-cusps O(200 m alongshore) that are associated with rip currents. The beach is the narrowest at the embayment of the mega-cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune, to undercut the dune and to cause dune erosion. Field measurements of dune, beach, and rip current morphology are acquired along an 18 km shoreline in southern Monterey Bay, California. This section of the bay consists of a sandy shoreline backed by extensive dunes, rising to heights exceeding 40 m. There is a large increase in wave height going from small wave heights in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over the Monterey Bay submarine canyon results in larger wave heights. The large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale. The strongly refracted waves and narrow bay aperture result in near normal wave incidence, resulting in well-developed, persistent rip currents along the entire shoreline.

The alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95% confidence. The alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95% confidence. Therefore, it is concluded the mega-cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega-cusp.     

Alongshore Sediment Transport Near Tidal Inlets of Chilika Lagoon; East Coast of India

ABSTRACT

Chilika, a lagoon along the east coast of India, is undergoing transformation due to frequent shoreline change near inlet(s). Shoreline change near inlet includes change in position and shape of inlet, inlet channel length, and spit growth/erosion. These variable features of lagoon inlet(s) critically depend on alongshore sediment transport (LST) and discharge (water and sediment) from the lagoon to the sea. The LST and the processes responsible for sand spit growth/erosion, considered as important attributes of inlet stability, are the subject matter of the present investigation and hence the study assumes importance. The study includes integration of observational and modeling framework. Observations include nearshore wave, bathymetry, beach profile, shoreline and sediment grain size of spits while numerical modeling includes simulation of the wave using MIKE 21 Spectral Wave model and LST simulation using LITtoral DRIFT. The results indicate that the predominant wave directions as S and SSE, which induces round the year south to north alongshore transport with significant seasonal variation in magnitude. The estimated LST closely matches with previous studies near Chilika inlet and for other locations along the Odisha coast. Besides temporal variability, the study reveals spatial variability in alongshore transport near Chilika inlet and considers it as one of the important attributes along with northward spit growth for inlet migration/closure/opening.     

江苏海岸中部近岸冬季潮汐和潮流特征

南黄海西侧的江苏海岸近岸区域,素以地形复杂、潮流强劲、悬沙输运剧烈著称,但是较长期的同步潮位和潮流观测数据仍然缺乏,尤其是在近岸(20 km)浅水(20 m)区域。2014年1月在大丰港附近开展了连续潮位和潮流观测,获得的数据揭示了一系列特征。此地潮汐潮流为正规半日潮,浅水分潮显著。平均潮差为3.05 m,最显著的两个分潮为M2和S2分潮,振幅分别为1.45 m和0.52 m。潮流最显著的半日分潮M2分潮和最显著的浅水分潮M4分潮在沿岸方向上振幅分别为0.84m/s和0.12m/s,在跨岸方向上振幅分别为0.24 m/s和0.01 m/s,沿岸方向占绝对优势。潮波的沿岸传播介于前进波和驻波之间,驻波的特征稍强。M2分潮潮流椭圆最大流(长轴)方向为南偏东7.4°。存在冬季沿岸向北的余流,垂向平均值的大小为2.2 cm/s。以上潮汐潮流特征为该区域海洋物质输运研究提供了基础资料。     

Long-term shoreline response of a nontidal,barred coast

Long-term variations of shoreline positions along the southern Baltic coast were investigated using multichannel singular spectrum analysis (MSSA) to determine the most dominant long-term response patterns. The investigated beach is located at Lubiatowo on the Polish Coast and is mildly sloping with multiple bars. Data on coastal morphology have been collected at Lubiatowo including (1) bathymetric surveys since 1987 twice a year, and (2) beach topography surveys since 1983 every 4 weeks on the average, extending from the shoreline to the dune foot. Furthermore, several dedicated field campaigns have been carried out at Lubiatowo, as well as measurements of deep-water wave properties since 1998. MSSA was employed to the whole data set of shoreline position from all survey lines. In summary, three patterns emerged reproducing alongshore standing waves with different periods 7 to 8, 20+ and several decades. They represent long-term shoreline response, such that at some locations the longest wave is most predominant, at other locations the medium cycle predominates, whereas the shortest is the most prominent at yet other locations. However, all three can be detected at every location monitored, eliminating the confusion resulting from ordinary singular spectrum analysis (SSA) analysis, done previously for the same data set.     

Assessment and prediction of shoreline change using multi-temporal satellite images and statistics: Case study of Damietta coast,Egypt

The shoreline trajectory of Damietta city, locates at the Northern coast of Egypt, is dramatically subjected to kinematic changes. These variations mainly occur based on the incessant duel hydrodynamic impacts of both wave action and coastal currents. Several types of coastal measures have been applied substantially along the coastal stretch of Damietta to protect shoreline such as detached breakwaters, Jetties, groins, and seawalls. This study is essentially focused on the assessment of shoreline kinematics response due to the existence of these structures during the period from 1990 to 2015. In addition, the future changes of the shoreline at 2020, 2025 and 2035 are predicted using satellite images, Geo-spatial tools and Digital Shoreline Analysis System (DSAS) by the meaning of End Point Rate (EPR) and Linear Regression Rate (LRR) methods. Four Landsat images at different periods; TM1990, TM 1999, ETM 2003 and ETM 2015 are used to detect shoreline changes. Three semi-automatic extraction techniques are initially tempted for Landsat ETM 2003 imagery namely; Iso cluster technique, threshold method, and onscreen digitizing method to select the optimal one. Iso cluster technique is used as the optimal technique which achieves the least errors with the corresponding field data in 2003 by value of 0.34. Furthermore, the extraction shoreline change for Damietta coast is extensively measured for three zones: zone (1) the western sector encompassing Damietta port with two jetties; zone (2) the central sector including detached breakwaters; zone (3) the eastern portion of Damietta estuary passing through a seawall. Verification analysis shows that the EPR is the optimum method for shoreline detection with a value of RMSE by 0.27. The results show that, for zone (1), the western shoreline of Damietta port is progressed by a rate of +10.0 m/year. On the other hand, the shoreline on the down drift side at zone (2) has retreated by a rate of -5.0 m/year. While the shoreline behind the detached breakwaters in the central sector has advanced by +12.0 m/year from 1999 to 2003, then decreased gradually until become stable in 2015. For zone (3), alongshore currents have derived the disassembled sandy soil from west to east leaving a highly eroded area by average rate of -78m/year. The results of this study give indication to shoreline trend of near future which should be under consideration in planning of Damietta coastal zone.     

Modeling beach profile evolution at centennial to millennial scales

The proposed model allows the satisfactory reproduction of the changes in the profile geometry in each time step depending on the sediment budgets in a given morphodynamic system. The applied modification to the general Bruun rule governing the conservation of mass must account for the effect of the sediment transport, which is described in terms of the erosion and accretion rates (Er and and Ac, respectively). The scale of the erosion is a function of the total annual wave energy flux reaching the beach. The accretion is governed by the Er, on the one hand, and by the sediment budget in the morphodynamic system, on the other hand. The equilibrium profile obtained for the case of a balanced sediment budget (Er = Ac) shows good agreement with the observed profiles. A deficit or surplus in the sediment budget results in the shoreline??s retreat or advance accompanied by either a decrease or increase in the slope of the bottom profile. The model accounts for different types of shoreline responses to changes in the sea level (the Bruun rule, the development of a coastal barrier, and abrasion). Sediment budget imbalances can be a factor in the profile??s evolution due to changes in the sea level, while the combination of both factors will produce a variety of behaviors of the shoreline, as was shown by our calculations. The model was verified using historical data on the behavior of the Central Holland coast and the Abkhazian coast during the Late Holocene. It was shown that the model satisfactory reproduces the progradation of coastal barriers. An example of a relatively short-term forecast (over a 100-year period) is given.     

A morphological stability analysis for a long straight barred coast

A morphological stability analysis is carried out for a long straight coast with a longshore bar. The situation with oblique wave incidence and a wave-driven longshore current is considered. The flow and sediment transport are described by a numerical modelling system. The models comprise: (i) a wave model with depth refraction, shoaling and wave breaking, (ii) a depth integrated model for wave driven currents and (iii) a sediment transport model for the bed load transport and the suspended load transport in combined waves and current. The direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity, neglecting the effects of a bed slope and secondary currents. An instability is found to develop around the bar crest. The instability is periodic in the alongshore direction, and tends to form rip channels and to steepen the offshore face of the bar between the rip channels. The alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile.     

Morphodynamic response of nearshore bars to a shoreface nourishment

The autonomous nearshore bar behaviour along the barrier island of Terschelling, The Netherlands, is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline. In 1993, a perturbation of the cyclic bar system was introduced by the implementation of a 2 Mm³ shoreface nourishment supplied to the nearshore bar zone, filling up the trough between the middle and outer bar. The morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years. Bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth, height, width and volume. Along with a pronounced development of a three-dimensional bar system unseen in the autonomous behaviour, the nearshore bars exhibited a 6–7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre-nourishment morphometric values. At the subsequent onset of bar movement, bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends. It is shown that the observed onshore transport of nourished sediment in the 6–7 year arrest results from a gradual deepening of troughs. Cross-shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore (short-wave nonlinearity) and offshore (undertow) sediment transport rates. The gradual reappearance of the pre-nourishment bar-trough morphology is shown to engender a normalisation in the cross-shore distribution of sediment transport rates to pre-nourishment rates.     

人工岛影响下的海滩修复对策研究——以海口西海岸为例

人工岛建设在缓解滨海人地矛盾的同时带来新的环境问题。本研究聚焦人工岛建设引起的岸滩侵蚀与泥沙分配失衡问题,以海口西海岸为例,建立GENESIS岸线演变模型,探讨人工岛影响下的海滩地貌演化机制,并在此基础上提出“循环养护”的海滩修复对策。主要研究结论为:①海口湾南海明珠人工岛对邻近海滩的影响以“岛后方淤积、两侧侵蚀”为显著特征,且两侧侵蚀热点区的响应幅度存在明显差异。五源河口至新国宾馆岸段侵蚀更为强烈的主导因素为物源匮乏与岸线形态外凸。②通过人工补沙与循环养护相结合的生态修复措施,可实现岸滩泥沙的合理分配与地貌的动态平衡,有效缓解人工岛引起的海岸侵蚀问题。本研究成果可为国内相似类型海岸的岸滩修复提供良好的借鉴意义。     

Digital Tide-Coordinated Shoreline

The shoreline is one of the most important features on earth's surface. It is valuable to a diverse user community. But the dynamic nature of the shoreline makes it difficult to be represented in a naturally dynamic style and to be utilized in applications. The officially used shoreline, for example in nautical charts, is the so-called tide-coordinated shoreline. It is also the shoreline that makes the computation of shoreline changes and associated environmental changes meaningful. Mapping of the tide-coordinated shoreline has been very costly. On the other hand, instantaneous shorelines extracted from different data sources may be available. Also, high-resolution satellite and airborne imagery have the capacity of stereo imaging and can be used to extract instantaneous shorelines at a high accuracy and low cost. This article proposes an approach to derivation of digital tidecoordinated shorelines from (a) those instantaneous shorelines and (b) digital coastal surface models and a digital water surface model. Some preliminary study results, analysis, and the potential of the approach are discussed.     

Wave driven alongshore sediment transport and stability of the Dutch coastline

The wave-driven alongshore sediment transport is commonly supposed to smooth out the irregularities on the coastline. However, it has been shown that waves approaching the coast with a high angle with respect the shore-normal can reverse that tendency and cause the rectilinear coast to be unstable [Ashton, A., Murray, A.B., Arnault, O., 2001. Formation of coastline features by large-scale instabilities induced by high-angle waves. Nature 414, 296–300; Falqués, A., Calvete, D., 2005. Large scale dynamics of sandy coastlines. Diffusivity and instability. J. Geophys. Res. 110, doi:10.1029/2004JC002587]. The extended one-line coastline model presented in the latter paper is here applied to investigate the stability of the Dutch coast. The main aim is testing the hypothesis that the shoreline sand waves observed along this coast could be generated by such an instability. It is found that the Dutch coast has potential for instability. This is most prominent on the Holland coast, followed by the Delta coast and is very weak on the Wadden coast. Whether the instability actually occurs or not depends on the cross-shore bathymetric profile of the shoreline waves. Under the sensible assumption that the bathymetric perturbation is just a shift of the equilibrium beach profile, the Dutch coast is stable. In this case, the mean annual coastline diffusivity is evaluated and it is found to be typically about 0.010–0.015 m² s^− 1, that is, roughly smaller by a factor 2 than that predicted by the traditional one-line model. However, the Dutch coast may be unstable with respect to coastline waves with a maximum bathymetric signal at a few hundred meters from the coast. This is shown in one case where the shoals associated with the sand wave are inside the surf zone during moderate storm waves. Thus the sand waves could result from the cross-shore redistribution of the sand associated with an alongshore series of shoals and bed depressions generated by the alongshore transport in the surf zone. While the generation or not of such shoreline waves by this instability strongly depends on their profile, its propagation once they have been created is less sensitive and is well reproduced by the present model. It is explained why the propagation is to the NE along the Delta and Wadden coasts, why it is faster on the latter and why on the Holland coast there is no clear propagation direction.     

Digital Tide-Coordinated Shoreline

The shoreline is one of the most important features on earth's surface. It is valuable to a diverse user community. But the dynamic nature of the shoreline makes it difficult to be represented in a naturally dynamic style and to be utilized in applications. The officially used shoreline, for example in nautical charts, is the so-called tide-coordinated shoreline. It is also the shoreline that makes the computation of shoreline changes and associated environmental changes meaningful. Mapping of the tide-coordinated shoreline has been very costly. On the other hand, instantaneous shorelines extracted from different data sources may be available. Also, high-resolution satellite and airborne imagery have the capacity of stereo imaging and can be used to extract instantaneous shorelines at a high accuracy and low cost. This article proposes an approach to derivation of digital tidecoordinated shorelines from (a) those instantaneous shorelines and (b) digital coastal surface models and a digital water surface model. Some preliminary study results, analysis, and the potential of the approach are discussed.     

Storm-induced readjustment of an embayed beach after modification by protection works

This paper examines storm-induced morphological and hydrodynamic changes after a submerged and a detached breakwater were constructed at La Barceloneta beach (Barcelona, NW Mediterranean) in 2006–2007. The shoreline configurations before and after beach nourishment and the construction of the protective structures were compared using a video dataset comprising 29 storm events spanning the pre- (2001 to 2005, n?=?17) and the post-breakwater situation (2006 to 2011, n?=?12), and hydrodynamic modelling based on the SMC coastal modelling system. As a result of the protection works, La Barceloneta was subdivided into two beaches separated by an artificial salient. The analysis of shoreline response to storms has been improved by using the shoreline hyperbolic tangent fit to represent the beach planform. Comparing the pre- and post-breakwater situations on the basis of these shoreline fits facilitated the identification of beach rotation processes because interference by smaller-scale morphological features was eliminated (e.g. the formation, changes in shape or migration of mega-cusps). In the current post-breakwater situation, there is evidence for a change in the behaviour of the north-eastern beach triggered by the submerged breakwater built in 2007. Furthermore, a counter-clockwise beach rotation has occurred at the north-eastern beach, whereas the south-western beach has experienced a clockwise beach rotation. This morphodynamic behaviour is caused by a new, complex wave-induced circulation system comprising two dominant alongshore currents flowing in opposite directions. In contrast to the pre-breakwater situation, the alongshore component of the radiation stress does not accomplish beach rotation in the post-breakwater situation.     

Determination and control of longshore sediment transport: A case study

The fishery harbor of Karaburun coastal village is located at the south west coast of the Black Sea. The significant waves coming from north eastern direction cause considerable rate of sediment transport along 4 km sandy beach towards the fishery harbor in the region. The resulting sediment deposition near and inside the harbor entrance prevents the boat traffic and cause a vital problem for the harbor operations. In order to determine the level and reasons of the sediment transport, the long-term observations of shoreline changes, the long-term statistical analysis of wind and wave characteristics in the region, and sediment properties have been performed. The data obtained from observations, measurements and analysis were discussed. The long-term statistics of deep water significant wave heights for each direction was discussed by comparing the results obtained from different data sources and methods. For shoreline evolution, the numerical study using one-line model was applied to describe the shoreline changes with respect to probable wave conditions. Initial shoreline was obtained from the digitized image in 1996 since there was no previous shoreline measurement of the site. The results were compared using the techniques of remote sensing obtained from sequent images using IKONOS and IRS1C/D satellites.     

Seasonal-scale nearshore morphological evolution: Field observations and numerical modeling

A coupled waves–currents-bathymetric evolution model (DELFT-3D) is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales. A 2001 field experiment, along the beaches adjacent to Grays Harbor, Washington, USA, captured the transition between the high-energy erosive conditions of winter and the low-energy beach-building conditions typical of summer. The experiment documented shoreline progradation on the order of 10–20 m and on average approximately 70 m of onshore sandbar migration during a four-month period. Significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration, the largest seasonal-scale onshore migration event observed in a natural setting. Both observations and model results suggest that, in the case investigated here, alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes. Alongshore varying incident hydrodynamic forcing, occasionally significant in this region due to a tidal inlet and associated ebb-tidal delta, was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer-scale. The role of fully three-dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor, at least in the case investigated here.     

Sedimentation and Boundary Changes of Virginia Salt Marshes

Sediment collections on a mainland fringing marsh, a lagoon marsh and a barrier island fringing marsh were conducted in conjunction with Geographic Information Systems (GIS) analysis of aerial photographs to relate changes in marsh area to sedimentary processes. The island marsh lost 7·2% of its area in 8 years by overwash. The lagoon marsh lost 10·6% of its area over 41 years by recession of marsh edges. The mainland marsh area increased by 8·2% over 50 years, primarily by upland encroachment. Surface sediment was collected monthly at the mainland and lagoon marshes for 1 year to identify changes in the mean grain size, organic content and mass of sediment deposited on sampling plates. Short-term variability in these characteristics obscured seasonal differences. Grain size and monthly sediment accumulation decreased towards the interior of both marshes, while the organic matter content increased. On the lagoon marsh, coarse sediment at creekside stations, as well as grain size contrasts between surface and subsurface sediment, are consistent with erosion evident in GIS analysis. On the mainland marsh, sediment does not vary as much with location or depth, and topography appears stable. Tidal currents appear to be competent to resuspend most particle sizes represented in surface sediment samples, including aggregates, except at the edge of the lagoon site. Decreasing rates of marsh edge change reflect moderation of oceanic processes, while the interior of both lagoon and mainland marshes remained stable throughout the study. Both lagoon marsh losses to recession and mainland marsh gains from upland reflect the submergence which this shoreline experiences.