Longshore Drift

The occurrence of a longshore drift in Whitsand Bay has been noted past Vincent and Osborne (1993), who estimated a drift speed of ∼0.1ms−1 during their observations.

From: Marine Protected Areas , 2020

On sediment dispersal in the Whitsand Bay Marine Conservation Zone

R.J. Uncles , ... R. Torres , in Marine Protected Areas, 2020

Longshore drift currents

Waves breaking at the shoreline may also preferentially transport sediment due to longshore drift ( Komar, 1998; Masselink and Hughes, 2003). Considering the dominant wave directions are from the west (270°, 57% occurrence) and the southwest (225°, 24% occurrence) and the shoreline normal (the line perpendicular to the coastline) at the coast is oriented between approximately 175° at PW increasing to approximately 215° at PC, wave refraction and wave breaking at a non-normal angle to the coastline is likely to occur, together with the formation of a longshore drift (Komar, 1998; Masselink and Hughes, 2003). The occurrence of a longshore drift in Whitsand Bay has been noted by Vincent and Osborne (1993), who estimated a drift speed of ∼0.1   m   s−1 during their observations.

The speed of the longshore drift depends on the square root of the root-hateful-square breaker summit and the moving ridge bending at its interruption bespeak (Komar, 1998). Every bit an example (given in Masselink and Hughes, 2003), a breaker top of only 1   m and a wave-direction to shoreline-normal angle of 10° produces a current of 0.63   m   s−1 at the mid-surf zone position, which in the MCZ would convey sediments and other materials along the coast from Prisoner of war toward PC and Rame Head (Fig. 31.1B).

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Global Modify

J.L. Stauber , ... S. Apte , in Marine Ecotoxicology, 2016

ten.3.seven Embankment Restoration

Beach rebuilding is the process of repairing beaches using materials such as sand or mud from inland or offshore. This can be used to build up beaches suffering from beach starvation or erosion from longshore drift ( Nordstrom, 2000; Hamm and Stive, 2002). Although information technology is not a long-lasting solution, it is cheap compared to other types of coastal defenses. Beach nourishment has long been seen as a necessity for littoral protection just is also a form of extending living and recreational possibilities. These amend the quality of life for millions of people. For case, Australia'southward coastline and sandy beaches are an essential recreational and tourist resource. Currumbin-Tugin Beach on the Gold Declension of Australia was severely eroded before reclamation took place. The same applies to Spain'southward Mediterranean and Atlantic coasts and many other coastal areas. The east and w coasts of the United States, Netherlands, and Belgium are also replenished annually.

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Geomorphology

C. Woodroffe , in Encyclopedia of Sea Sciences, 2001

Muddy Coasts

Muddy coasts are associated with the everyman energy environments. Mud banks may occur in high-energy, moving ridge-exposed settings where big volumes of mud are supplied to the oral fissure of large rivers. Thus longshore drift north west of the Amazon, and effectually Bohai Bay downdrift from the Yellow River, enables mud-shoal deposition in open-water settings. Elsewhere mud flats are typical of sheltered settings, inside delta interdistributary bays, around coastal lagoons, etc. These dingy environments are likely to be colonized by halophytic vegetation. Mangrove forests occur in tropical settings, whereas salt marsh occurs in higher latitudes, frequently extending into tropical areas also.

These coastal wetlands farther promote retention of fine-grained sediment. The dirty environments are areas of complex hydrodynamics and sedimentation. Sedimentation is likely to occur with a negative feedback such that as the tidal wetlands accrete sediment and equally the substrate is elevated, they are flooded less frequently and therefore sedimentation decelerates. Boundary conditions, particularly body of water level, are likely to vary at rates like to the rate of sedimentation and prograded littoral plains contain complex sedimentary records of changes in ecological and geomorphological state.

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Coastal Zone Management

A. McLachlan , A.C. Brown , in The Ecology of Sandy Shores (Second Edition), 2006

Bear upon of Hard Structures on Longshore Sand Ship

Coastal engineering structures built out into the water from the shore (such as groins) cake the natural littoral migrate of sand prevailing along most coasts. This deprives beaches of sand and initiates erosion on the downdrift side of the structure, while sand deposits updrift where the beach advances seaward. Littoral drift is not a constant phenomenon at whatever given site. It varies enormously with wave activity and the direction of wave attack, and in that location is normally even a reversal of drift direction nether different conditions (notably during storms). The summation of all individual sediment send events over a year is the net longshore transport, and it is this value that is important in determining the furnishings of coastal structures on erosion and deposition, rather than any unmarried transport event.

Amid the several examples cited by Komar (1998) , the development of the Port of Madras in India is of particular involvement because it was constructed on the open shore in an area of stiff longshore drift and considering resulting changes have been documented since its construction was sanctioned in 1875 ( Figure 15.8). Due to stiff northward drift, sand deposited rapidly on the beach to the south of the breakwater while it was even so under construction, and erosion to the northward dictated the placement of groins. The original harbor entrance faced due east (out to sea), but accumulation of sand on the southward side congenital the shoreline seaward until sand began to drift around the eastern terminate, causing shoaling of the harbor entrance. This became then serious that past 1920 the entrance had to be moved to the north side of the harbor and an outer quay constructed in an attempt to deflect sand away from the new archway. The entire shoreline has changed for many kilometers on either side of the harbor — severe erosion having taken place to the north (despite attempts to halt it) and dramatic deposition to the south, the shoreline advancing out to sea.

Figure fifteen.viii. Coastline changes associated with the evolution of Madras Harbor, Bharat, from 1876 to 1950 (subsequently Komar 1998).

In another example, jetties constructed in 1935 (with the object of stabilizing the inlet of Bounding main City, Maryland, USA) blocked the due north-to-south transport of sand, resulting in a seaward advance of the shoreline to the n. This was not considered a disadvantage. However, the barrier island Assateague, to the south, suffered such severe erosion that the shore retreated some 450 chiliad over a period of 20 years. By 1961, the south beach had separated from the inner terminate of the jetty (leaving a gap of almost 240 m of open water), and the post-obit yr a storm opened a breach over ane km wide. Attempts to restore the beach past dredging and filling take apparently only been successful in the short term.

Literally hundreds of other well-documented examples could be given from all over the world. I remedy is, of course, nourishment by transferring sand from the updrift side of the construction to the downdrift side. Bypassing the obstacle in this manner can be done past transporting the sand past truck or dredging and pumping the sand, mixed with h2o as a sludge, through a pipe to the downdrift area.

It might be thought from what has been said previously that structures congenital on coasts with no meaning net longshore sand transport would cause neither erosion nor deposition of sand. This is by no means always the instance, yet, and there are several examples of quite unpredicted erosion and loss of property resulting from construction in such areas. It is not that erosion fails to occur on a coast with zero net migrate simply that following structure of a bulwark a new equilibrium is established, an equilibrium that may in fact involve drastic changes to the shoreline. On the other manus, where there is a internet coastal drift no balanced sediment upkeep can e'er exist established without bypassing, and the processes of erosion and deposition continue indefinitely.

Clearly, the power to predict the effects of marine constructions on shorelines is of paramount importance in planning and management. Computer-based modeling of nearshore processes and shoreline changes have considerable potential for such prediction. The details of computer-simulation models lie outside the telescopic of this book and the reader is referred to Komar (1998) for an introduction to the techniques and applications involved.

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Fine Sediment Systems

Edward J. Anthony , ... Guillaume Brunier , in Reference Module in Earth Systems and Environmental Sciences, 2021

vii Longshore mud transport

Wells and Coleman (1978, 1981) initiated efforts on the understanding of mud send over the shoreface that have been complemented by a number of theoretical studies and a few field investigations. These accept suggested a leading office for wind-generated waves in alongshore mud mobility (Jiang and Mehta, 1996; Rodriguez and Mehta, 1998; Tatavarti and Narayana, 2006; Gratiot et al., 2007; Chevalier et al., 2008). Wave liquefaction of mud comprises an alongshore transport component that is fundamental to mud streaming. This streaming may exist modulated by seasonal changes in Monsoon and merchandise winds. Eisma et al. (1991) used the angle of shore incidence of synoptic winds as a surrogate for assessing temporal variations in the intensity of longshore drift, and, hence, mud-banking company migration rates on the Guianas coast. This approach was further used by Augustinus (2004) to explain both changes in rates of mud-depository financial institution migration and in the lengthening of mud banks. The Amazon longshore mud arrangement extends, in both the highly-concentrated form of mud banks and highly turbid suspensions, over 1500   km to the mouth of the Orinoco.

A 44-year record (1960–2004) of the ERA-twoscore wave dataset generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) was used past Gratiot et al. (2007), together with complementary field investigations in French Guiana, to define both event-scale and longer-term patterns of mud mobilization induced by waves. From analyzes of dirty bed profiles, fluid mud layer thickness and mud loading and their relationship with the wave data, these workers highlighted a close relationship between wave energy and fluid mud mobilization, and singled out the ratio H 0 3 /T 2 , combining wave height H and menstruum T, and the angle of wave incidence, as the most relevant parameters for describing wave forcing. Gratiot et al. (2007) showed that significant phases of increased wave energy are attended past college long-term (annual) rates of longshore mud-bank migration, just the correlation was rather poor between the moving ridge forcing parameter H0 iii/T2 and migration rates because stronger moving ridge forcing is mostly associated with low angles of wave incidence. This suggests a complementary role of other hydrodynamic mechanisms, such equally geostrophic and tidal currents, in longshore mud bank migration (Chevalier et al., 2008).

Mud-depository financial institution migration rates can vary significantly. Forth the South American declension, they exhibit depression multi-annually averaged rates (0.two–1.eight   km   year−   1) in the early on 1980s and high rates (i.viii–3.0   km/year) from the mid-1990s to 2005 (Gardel and Gratiot, 2005). The mean mud bank migration rate of 1995–2000 was twice higher than that of 1979–84, for instance, while the moving ridge forcing parameter was only 4/three higher. A first source of divergence is in variations in the wave characteristics themselves, moving ridge incidence angle and moving ridge energy, related, for instance, to NAO/ENSO oscillations (eastward.g., Walcker et al., 2015). A second fix of factors involves local irregularities such as nearshore bedrock outcrops and rocky headlands, river mouths and fluvial discharge (Anthony et al., 2013; Gardel et al., 2021), all of which are expected to affect significantly the migration or stabilization of mud banks. Closely related to this is the big-scale plan shape of the coast itself, which, in many areas, comprises alternations of balmy capes and embayments that should affect wave drift gradients alongshore (Augustinus, 2004). A third source of difference is the rheology of the mud banks. The rheological beliefs of the mud shows a strongly non-linear and thixotropic response to stress (Fiot and Gratiot, 2006). Beyond a threshold forcing, the apparent mud viscosity decreases considerably, and this would, in turn, induce an increase in mud-bank migration rate, due to the increase of wave forcing. Finally, mud-bank migration must too be conditioned by a combination of other lower-lodge forcing mechanisms mentioned above, notably geostrophic currents, wind-induced currents and upwelling and downwelling, tidal currents, and density currents.

The importance of tidal currents has been highlighted in a number of studies, peculiarly in regional ocean contexts in Asia associated with loftier-belch rivers. This is the case of the Ganges-Brahmaputra connexion with the Sundarbans, where mud streaming alongshore from the active loftier mud-discharge mouths of these ii mega-rivers assure fine-grained deposition over the Sundarbans, thus maintaining mangrove growth in an abandoned function of the deltaic apparently (Bomer et al., 2020). The suspended sediment connection between the mouths of the Ayeyarwady (west) and the Salween (due east) in the Gulf of Motama (Fig. 4), Andaman Sea has been considered equally tide-driven (Ramaswamy et al., 2004; Rao et al., 2005), although wind stress associated with the southwest (Indian) Monsoon besides likely contributes to this eastward transport (Damodararao et al., 2016; Anthony et al., 2019a). End-member analysis of sediment dispersal on the 50   m-thick Heuksan mud chugalug on the southwest Korean shelf suggests that, these sediments, previously thought to be partly supplied past Chinese rivers, on the footing of geochemical and clay mineral studies, are derived from distant Korean rivers and probably redistributed by tidal currents (Ha et al., 2021).

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Swash-Zone Processes

1000. Brocchini , in Continental Shelf Research, 2006

Still, a much more than complicated task is still to be undertaken, i.e. the analysis of longshore flows which are essential for waves non-orthogonal to the shore. In this case the main difficulty is the definition of a mean (wave-averaged) longshore velocity which accounts for the entire (integrated over the SZ width) longshore drift of the SZ water mass. It is clear that the problem is like to that of representing the migrate of h2o particles of a progressive wave (Stokes' drift) in terms of a hateful drift velocity associated to the through location. This idea has been exploited by Brocchini (1997) to discuss the longshore mass send due to finite-aamplitude waves. The longshore drift velocity, which is non the longshore velocity of the boundary x l , rather the mean longshore velocity of the SZ h2o mass, has been computed for specific periodic flows. Such analysis is currently beingness exploited and extended to define a SBC suitable to stand for longshore flows. This condition is profoundly influenced past the intensity of the seabed friction so that a detailed analysis on the office of friction in very shallow water is needed.

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Ecological responses to ecology modify in marine systems

Eliza C. Heery , ... Katherine A. Dafforn , in Journal of Experimental Marine Biology and Ecology, 2017

3 Factors influencing the direction and magnitude of impacts

The way in which artificial structures modify sedimentary communities depends on their design and spatial configuration, the characteristics of the abiotic and biotic environment in which they are placed, and the scale of the impact, including area afflicted and elapsing (Airoldi et al., 2005; Martin et al., 2005). Unfortunately, many scientific-based assessments oftentimes neglect these complex interactions and scaling issues, which limits our current capability to predict the impacts of future developments (Loke et al., 2015). Studies to appointment accept found tremendous variation in the patterns and trends they have observed in sedimentary habitats where bogus structures take been placed. This is likely due at least in office to inherent variation in the direction and magnitude of impacts from bogus structures, both over infinite and fourth dimension, and across multiple spatial scales. For each of the effects documented above, there are a number of factors that likely influence variation in observed patterns in the field and are worth considering when seeking to identify generalizable trends.

Placement loss (Griggs, 2005; Department 2.1), by definition, increases with the aeriform extent of foundations constructed in sedimentary habitat, and may be especially large in coastal areas where construction of structures is accompanied past backfill to reclaim state. In littoral environments, losses tin be amplified by passive erosion, which results from structures inhibiting natural cycles of shoreline retreat (Griggs, 2005). The extent of such passive erosion tin can depend on the tidal elevation at which a defense force structure is built, as well as whether a shoreline is presently in an accretive or erosive country (Archetti and Romagnoli, 2011; Lin and Wu, 2014). Active erosion of sediment adjacent to structures, through moving ridge reflection, scouring, and 'end effects' (Griggs, 2005) tin also bear on the magnitude of habitat loss. The effects are greatest where sand input is low and wave energy high (Lin and Wu, 2014; Miles et al., 2001). They are also dependent on the extent to which structures are designed to blot versus reverberate wave free energy (due east.m. hollow seabed versus solid concrete seawall designs) (Hettiarachchi and Mirihagalla, 1998; das Neves et al., 2015; Zanuttigh et al., 2005).

Impacts of artificial structures on sediment communities also vary spatially according to the extent to which they modify the abiotic and biotic conditions and local processes that control soft-sediment customs assembly (Airoldi et al., 2005; Martin et al., 2005 ). The position (i.e. onshore vs offshore), orientation (i.e. perpendicular or parallel to shorelines), permeability (solid versus rock wall), dimensions and spacing of structures are all factors that could influence the extent to which structures intercept longshore migrate, tidal and other currents, which in turn shape sedimentary communities by determining sediment, larval and resource (e.g. wrack and organic thing) send and deposition ( Martin et al., 2005; Bishop et al., 2017-inthisissue). For case, Shyue and Yang (2002) constitute that the area of scour surrounding subtidal artificial reefs was heavily influenced by the structure's height, although differences in ambient flow between locations were also important (Shyue and Yang, 2002). The placement of seawalls with respect to tidal height and local wave free energy are important factors determining the extent of scour and sediment coarsening in intertidal environments (Weigel, 2002). Equally some other example, the impacts of oil rigs on adjacent sediment communities could exist mitigated at deeper waters considering of higher environmental stability and greater potential of dilution and dispersion of pollutants (Burns et al., 1999; Ellis et al., 1996). Terlizzi et al. (2008), however, reported an opposite trend, maybe because platforms at deeper sites are taller, therefore leaching greater amounts of contaminants or providing more than expanse for growth of fouling invertebrates which slough off to influence sedimentary communities (Goddard and Love, 2010; Love et al., 1999; Terlizzi et al., 2008).

The spatial arrangement and isolation of bogus structures could affect sedimentary environments both direct, past affecting patterns of sediment deposition, and indirectly, by affecting the capability of artificial reefs to attract grazing and predatory fish communities. For example, on a Brazilian artificial reef, the proximity of reef assurance to one another influenced their effect on organic and fine sediment inputs to adjacent habitat (Zalmon et al., 2014), with inputs greatest at a larger spacing. Overall, the big-scale effects of multiple structures (such every bit offshore structures) may differ from their local effects. For example, parks of offshore wind farms tin can human action as a partial blockage of the overall electric current field: the blocked water volume is forced effectually the park, which leads to a decrease in the flow inside the park and an increase in flow velocities on the sides of the park (Airoldi et al., 2016). These blockages depend on the distance between piles (typically 600 to 1200   thousand), the diameter of the piles (half dozen–10   g), the overall number of wind turbines in the park and the lay-out of the farm.

The sediment grain size and hydrodynamic regime can also make up one's mind the extension and severity of some of the impacts. For case, the effects of crab-tiles used to attract crabs for harvest depend on the grain size of the sediments where they are placed (Sheehan et al., 2010a). Similarly, the impacts from the sediment spills due to dredging for foundation and cable trenches of offshore activities will primarily exist of local nature in low-current environments, while in high-electric current environments far-field impacts of lower intensity will prevail, due to advection and dilution (Airoldi et al., 2016). Farther, the impacts of structures on sediments may vary spatially co-ordinate to the processes occurring at the fourth dimension of their construction, for example fouling community colonization (Underwood and Anderson, 1994) which in turn determines resource subsidies to adjacent sedimentary habitats (Airoldi et al., 2010; Goddard and Beloved, 2010; Love et al., 1999).

The effect of structures on sediment communities may also be expected to vary according to the diversity and identity of soft sediment communities at disturbed sites (Martin et al., 2005). For example, the diverse communities of dissipative beaches are more than susceptible to the effect of structures than the more depauperate assemblages of exposed sandy beaches (Martin et al., 2005). Oil and gas rigs or artificial reefs that exclude angling vessels may have large positive furnishings on biodiversity past removing or alleviating dredge or trawling disturbance to ecosystem engineers such as clams, tube worms, or seagrasses (González-Correa et al., 2005; Pearce et al., 2014). Conversely, if bogus structures accept a negative outcome on ecosystem engineers (e.g. Lemasson et al., 2017-inthisissue; Teagle et al., 2017-inthisissue).

Not merely practice the effects of structures vary spatially according to their abiotic and biotic context, but they may also vary temporally. Effects of bogus structures on sediment communities may strengthen or weaken with time since their construction. For case, because the evolution of fouling communities on structures takes time (Underwood and Anderson, 1994), indirect effects on sediment communities resulting from sloughing of algae or beat (Airoldi et al., 2010; Goddard and Love, 2010; Love et al., 1999) or fouling communities depositing feces (Maar et al., 2009), may increment with time since construction. Conversely, pulse impacts associated with the construction phase, such equally those resulting from turbidity plumes or structure noise deterring benthic predators (Slabbekoorn et al., 2010) may weaken over fourth dimension (Jaramillo, 2012). The upshot of structures on sediment communities may too vary temporally according to natural variation in the strength of the abiotic and biotic processes they disrupt. For example, artificial reefs in Brazil reduce electric current velocities predominantly during months of high flow from the Paraiba do Sul River (Machado et al., 2013) and, feasibly, enhancement of predator foraging patterns effectually artificial structures may vary seasonally according to the biological science of species.

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