Surfzones and processes.pdf

G1466 Coastal Engineering Basic shore processes 2 1. Coastal zone. Definitions. In terms of this course, the coast represents that region of the Earth’s surface affected by coastal processes, i.e. waves and tides. The coastal zone thus defined includes the coastal plain, the estuarine, dune and b...

G1466 Coastal Engineering Basic shore processes 2 1. Coastal zone. Definitions. In terms of this course, the coast represents that region of the Earth’s surface affected by coastal processes, i.e. waves and tides. The coastal zone thus defined includes the coastal plain, the estuarine, dune and beach area, the shoreface (the underwater part of the beach), and part of the continental shelf, Figure 1. Fig. 1. Spatial extent of the coastal zone, including the coastal plain shoreface and continental shelf. The widths of these zones are globally highly variable. (Source: Masselink at atl 2011) 2. Coastal morphodynamic systems Coastal systems (e.g. salt marsh, beach, tidal basin) comprise three linked elements (morphology, processes and sediment transport) that exhibit a certain degree of autonomy in their behavior, but are ultimately driven and controlled by environmental factors, Figure 2. These environmental factors are referred to as ‘boundary conditions’, and include the solid boundary (geology and sediments), climate and external forcings (wind, waves, storms, tides and tsunami), with sea level serving as a meta-control by determining where coastal processes operate. Fig. 2. Conceptual diagram illustrating the morphodynamics approach, showing the coastal morphodynamics systems and the environmental boundary conditions (sea level, climate, external forcing and the static boundary conditions) (Source: Masselink 2012). Human activity should also be taken into account as, in fact, along many of our coastlines human activities, such as beach nourishment, construction of coastal defenses, dredging and land reclamation, are more important in driving and controlling coastal dynamics than the natural boundary conditions and can therefore not be ignored. G1466 Coastal Engineering Basic shore processes 3 Moreover, through climate change, humans are altering the boundary conditions themselves (sealevel rise and changes to the wave climate). Unless long-term coastal change (centuries to millennia) is considered, the boundary conditions can be viewed as given and constant, although it should be borne in mind that external forcing is stochastic (random), and the dynamics of coastal systems arise from the interactions between the three linked elements: (1) Processes: This component includes all processes occurring in coastal environments that generate and affect the movement of sediment, resulting ultimately in morphological change. The most important of these are hydrodynamic (waves, tides and currents) and aerodynamic (wind) processes. Along rocky coasts, weathering is an additional process that contributes significantly to sediment transport, either directly through solution of minerals, or indirectly by weakening the rock surface to facilitate mobilization by hydrodynamic processes. In addition, biological, biophysical and biochemical processes are important in salt marsh, mangrove and coral reef environments. River outflow processes are important in deltas. (2) Sediment transport: A moving fluid imparts a stress on the bed, referred to as ‘bed shear stress’, and if the bed is mobile this may result in the entrainment and subsequent transport of sediment. The ensuing pattern of erosion and deposition can be assessed using the sediment budget. If the sediment balance is positive (i.e. more sediment is entering a coastal region than exiting), deposition will occur and the coastline may advance, while a negative sediment balance (i.e. more sediment is exiting a coastal region than entering) results in erosion and possibly coastline retreat. This makes quantifying the sediment budget a fundamental means for understanding coastal dynamics, as well as providing a tool for assessing and predicting future coastal change. (3) Morphology: The three-dimensional surface of a landform or assemblage of landforms (e.g. coastal dunes, deltas, estuaries, beaches, coral reefs, shore platforms) is referred to as the morphology. Changes in the morphology are brought about by erosion and deposition. It is worth emphasizing that the morphodynamic approach can be applied regardless of the spatial scale of the coastal feature under investigation. For example, at the smallest scale, the approach can be applied to wave and tidal bed forms; at the largest scale, to tidal basins or entire delta systems. Importantly, the spatial and temporal scales of coastal morphodynamic systems are related (Fig. 3): the larger the spatial scale of the coastal system, the longer the timescale associated with the dominant process(es) and the associated coastal morphodynamics. The spatio-temporal relationship is, however, not linear: some coastal systems respond faster than one would expect on the basis of their size (labile systems; e.g. sandy barriers without dunes), whereas other coastal systems exhibit a relatively slow response (sluggish systems; e.g. rocky coasts). The timescale of the response of a coastal system also depends, of course, on the magnitude of the forcing. G1466 Coastal Engineering Basic shore processes 4 Fig. 3. Relationship between spatial and temporal scales at coastal systems. Sluggish and labile systems are those that respond relatively slow and fast, respectively. (Source: Masselink) 3. Beaches 3.1. Introduction and definitions Beaches can be defined as accumulations of sand or gravel found along marine, lacustrine and estuarine shorelines, and deposited by waves and wave-induced flows. Sandy beaches account for about 20% of the world’s coasts. Gravel beaches occupy approximately 10% of the global coastline and are found particularly in high-latitude areas. The beach constitutes the upper part of the shoreface and can be subdivided into three subsystems, Figure 4: (1) Subtidal zone: This zone extends from the low-water mark to typical water depths of 5–10 m. It is here that waves incident from deep water shoal and break. The breaking-wave zone is also called the surf zone, and in its outer part the breaking waves change rapidly in appearance (shape). Further onshore, in the inner region, all breaking waves have transformed in sawtooth-shaped bores that propagate to the shore without any major change in shape. During wave breaking, the motion of the incident waves is transformed into motions of different types and scales. These include small-scale turbulence, about 20–200 s long infragravity waves that are related to the grouped structure of the wind-generated waves, and steady flows, including alongshore, rip and bed return (undertow) flow currents. The topography of subtidal sandy beaches may be simple, but may also contain one or more alongshore ridges known as sandbars. As already explained, the wave shoaling zone is part of the subtidal zone. The wave shoaling zone also known as the nearshore, Fig. 5, lies between the modal or average wave base and the breakpoint. At its seaward limit, waves begin to interact with the seabed sufficiently to entrain sediment and move it shoreward. This limit is also called the closure depth, and was defined as the depth below which seabed change is note detectable. The depth of closure increases with increasing wave height and period and on high wave energy beaches the depth may reach a maximum of 30 m and lie 2 or 3 km offshore, Fig. 5. G1466 Coastal Engineering Basic shore processes 5 (2) Intertidal zone: This zone is located between the low and high-tide marks. Here, the inner surf-zone bores finally expand their energy and run up and down the beachface as swash and backwash, Fig.6. When the tide range is sufficiently large and/or the incident waves are rather low, even shoaling waves can be active in the intertidal zone. With a decrease in bed gradient and an increase in offshore wave height, infragravity waves become increasingly important in swash and backwash action. These low-frequency waves do not suffer from wave breaking as much as the incoming sea and swell waves, and can, therefore, dominate the swash during storms. The intertidal zone may also display sandbar topography. (3) Supratidal zone: This zone is the part of the cross-shore profile between the high-tide level and some physiographic change such as a dune or a cliff, or permanent vegetation. It is normally above the level of the highest swash, except during storms, and is dominated by wind-driven processes. During prolonged periods of low-wave activity, sand may accumulate at the intersupratidal boundary to form a berm (see Figures 4 and 5). It protects the coastal dunes and cliffs from erosion during the early phase of higher-wave conditions. When berm erosion is pronounced, the beach profile may become scarped. In particular, gravel beaches can have multiple berms at different locations, even quite high on the profile (storm berms). Fig 4. Typical cross-shore profile for: (a) a gently sloping sandy beach; and (b) a steep gravel beach, showing characteristic morphological features and hydrodynamical zones. (Source: Adapted from Davidson-Arnott 2010) G1466 Coastal Engineering Basic shore processes 6 Fig. 5. Definition sketch of a high energy beach system including the zone of wave shoaling across the nearshore zone, wave breaking across the surf zone and final wave dissipation in the swash zone. Low energy beaches are smaller in scale and have a small to non-existent surf zone. G1466 Coastal Engineering Basic shore processes 7 Fig. 6. Avoca Beach, NSW, Australia, a reflective beach. Photo shows the inner surf zone and a bore reaching the swash zone in the background and a swash uprush reaching the top of a beach berm in the foreground. (Source: Coastal Wiki) Fig.7. A turbulent bore containing entrained suspended sediment just prior to reaching the swash zone. (Source: Coastal Wiki.) Beaches can be long and uninterrupted for several tens of kilometers (‘open beaches’), can be part of the seaward side of barrier islands, or can be located between headlands (‘pocket beaches’). G1466 Coastal Engineering Basic shore processes 8 Fig. 8. Wave breaking at Torimbia pocket beach (Asturias). Source: Google Earth Fig. 9. Open beach at Islantilla (Huelva). Source: Google Earth 3.2. Scales of nearshore morphology Morphological patterns abound on natural beaches from very small to large spatial scales. Wave ripples, Fig. 10, are the smallest-scale patterns commonly observed on beaches, with typical wavelengths of 10–100 cm and height-over-length ratios (ripple steepness) of 0.2 or less. Wave ripples are roughness elements that reduce the energy of waves and currents, and they also locally modify sediment transport rate and direction. G1466 Coastal Engineering Basic shore processes 9 Fig. 10. Wave ripples (Source: Istock) Beach cusps, Fig. 11, represent alongshore rhythmic patterns in the swash zone of steep, coarsesand or gravel beaches. An individual beach cusp comprises two topographic highs, or horns, and a single topographic low, or bay. The alongshore spacing between consecutive horns is in the order of several tens of meters and is proportional to the horizontal extent of the swash motion, with a constant of proportionality of about 1.5. The wave uprush decelerates over the horns and is deflected from the horns into the adjacent bay, where it piles up and accelerates to return seaward as a narrow jet, or ‘mini-rip’. The patterns of deceleration and acceleration result in sediment deposition on the horns and erosion from the bays. Thus, the beach cusps modify the swash in such a way that their height increases. Fig. 11. Beach cusps in Trafalgar Beach (Cadiz). (Source: Centro Andaluz de Medio Ambiente) G1466 Coastal Engineering Basic shore processes 10 Sandbars are the largest of the patterns found in the nearshore and have been observed both on the subtidal and intertidal beach. Intertidal bars can be found in tidal ranges as large as 6 m, while subtidal sandbars are mostly limited to tideless to mesotidal settings. Subtidal sandbar morphology is located in water depths of less than 10 m and can assume a large variety of configurations, including shore obliquely oriented transverse bars, crescentic bars, and longshore uniform bars, Fig. 12. All types can extend for up to several tens of kilometers along the shore. Crescentic and alongshore bars can occur singularly or in multiples of up to four or five bars, Fig. 13. When the bar morphology is characterized by a dominant alongshore wavelength (i.e. spacing of rips), this is referred to as rhythmic morphology. In multiple-bar settings, the dominant wavelength in outer bars is usually larger than in inner bars. Also, rips tend to be less regularly spaced in inner than in outer bars. The dynamics of intertidal and subtidal sandbars is predominantly governed by breaking waves and associated flows. Fig. 12. The high intensity bands in each image are due to persistent wave breaking on the inner and outer bars of the beach.(Source. Pape et al. ) Fig. 13. Sketch explaining the processes observed in the previous figure. Dimensions do not necessarily correspond to the beach in the previous figure. Sandbars are highly important morphological features from a societal point of view. Firstly, subtidal sandbars are shallow parts in a cross-shore profile and thus force wave breaking to occur away from the shoreline, providing a means of coastal protection. Secondly, subtidal sandbars are rather voluminous and are thus significant in coastal budget studies. Thirdly, sandbars continuously modify their position in response to changes in the wave forcing. The change in sandbar position, and also the presence and location of rip channels, constitute the major control on nearshore bathymetric variability on the timescales of weeks to years. This is precisely the timescale most relevant to coastal zone managers. Accordingly, many measures to mitigate coastal erosion nowadays involve modifying sandbar characteristics, such as increasing their height by artificial sand dumping.