Coarse clastic shorelines and climate<br />Professor Simon K. Haslett<br />Centre for Excellence in Learning and Teaching<br />Simon.firstname.lastname@example.org<br />10th June 2010<br />
Introduction<br />Coarse clastic shorelines possess a morphosedimentary and dynamic distinctiveness which sets them apart from sandy fine-grained shorelines.<br />They may be classed as barriers or beaches.<br />The slope of the shoreline is crucial to its morphodynamic status – reflective or dissipative.<br />A consequence of wave reflection is the development of edge waves which manifest themselves as swash cusps on the shoreline.<br />Coarse clastic shorelines tend to become sorted, in terms of clast size and shape.<br />Coarse clastic material has a high preservation potential and often mark former sea-level positions (e.g.Eronen, 1983).<br />This presentation describes the response of coarse clastic shorelines to changes in wave condition and sea-level.<br />
Effect of changes in wave condition and sea-level 1<br />Changes in wave condition can influence the morphodynamics of coarse clastic shorelines.<br />Orford et al. (1988) document a case where sediment supply to a shoreline was artificially stopped, eventually removing the seaward low-angle slope component of a barrier due to longshore currents.<br />The disappearance of this dissipative element led to an increase in wave energy reaching the reflective barrier.<br />Cusp formation was initiated which provided a template for overwashing, breach formation, and general barrier degradation.<br />Sediment was redistributed landward as washover fans.<br />
Effect of changes in wave condition and sea-level 2<br />Current research on the effect of sea-level rise (e.g. Orfordet al. 1995) has suggested that clastic shorelines may respond as follows:<br /><ul><li>The crest may build up in height by a transfer of material to the crest from the mid-tide zone as sea-level rises. A barrier may start to migrate landward through the process of rollover, which is the general transfer of sediment to the landward side of a barrier.</li></ul>Barrier rollover at Porlock, Somerset (UK). Pebbles can clearly be seen encroaching onto agricultural land behind the barrier.<br />
Effect of changes in wave condition and sea-level 3<br />Continued:<br /><ul><li>In the absence of adequate sediment supply, the beach will steepen leading to increased reflectivity.
Crest build-up cannot continue indefinitely, as crest construction requires spilling waves for sediment transport, which in turn requires a measure of dissipative character, so as reflectivity increases the rate of crest build-up will slow, and this shift in morphodynamic domain from dissipative to reflective will induce cusp formation.</li></ul>B<br />A<br />Spilling waves in a) New South Wales, Australia; and b) Brittany, France.<br />
Effect of changes in wave condition and sea-level 4<br />Cusps reaching the crest may then act as a template <br /> for overwash and breaching, and the eventual <br /> breakdown of a barrier.<br />In this way barriers will tend to migrate landward under <br /> a sea-level rise regime.<br />Beaches confined by backing cliffs may develop swash <br /> ramps extending several metres above the high water <br /> mark, and with cusp development, ramps may become <br /> fragmented and separated by intra-beach erosional<br /> bays.<br />Right: Beach cusps formed in a gravel storm beach east of Nash Point, Glamorgan Heritage Coast, Wales.<br />
Effect of changes in wave condition and sea-level 5<br />A characteristic feature of a landward-migrating barrier is the presence of a terrace or terraces in the seaward to low- to mid-tide zone.<br />These terraces are low-angled and dissipative.<br />Erosional slopes are often formed as a barrier migrates landward.<br />Low-gradient beaches often possess an extensive sand terrace around the low-tide level. Brittany, France.<br />
Effect of changes in wave condition and sea-level 6<br />Where a rock platform is exposed, it may supply the barrier with clasts thus slowing down migration, so the areal ratio of barrier to terrace can increase as well as decrease.<br />There is also a type of depositional terrace resulting from sea-level rise and landward barrier migration, which is known as a boulder frame.<br />This frame results from particle sorting, providing a residue or lag of larger and relatively immobile clasts at the foot of the beach.<br />If this residue of clasts remains immobile, then as the barrier migrates landward, the frame will expand in width.<br />
Effect of changes in wave condition and sea-level 7<br />With the expansion of low-angled terraces within the barrier morphodynamic system, reflectivity will change from being the dominant domain, to co-dominant to non-dominant through time, giving rise to an overall dissipative domain, which may lead to barrier abandonment. <br />Boulder terrace. Porlock, Somerset.<br />
Effect of changes in wave condition and sea-level 8<br />Furthermore, where a barrier progressively loses material to its frame as it migrates, the barrier’s migration rate may increase, which may ultimately overstretch the barrier, leading to breaching.<br />In order to maintain stability, therefore, the rate of volumetric loss needs to be balanced by the increasingly dissipative role of the frame (Carter and Orford, 1993).<br />The breach in the gravel barrier at Porlock, Somerset (UK) formed by severe storms on 28 October 1996. The base map shows the approximate location of the breach.<br />
Assessment: virtual fieldtrip to the Vale of Porlock, Somerset (UK) 1<br />The purpose of this field visit it to undertake a detailed geomorphological reconnaissance of the coarse <br />clastic shoreline in the Vale of Porlock. Using these and the notes provided in the accompanying <br />presentation ‘Morphodynamics and sedimentology of coarse clastic shorelines’, use Google Earth to <br />study the shoreline at Porlock (c. 4.5km long), and make detailed field notes and sketch maps that<br />address the following points:<br />General<br /><ul><li>Is this shoreline a typical coarse clastic shoreline with large-size material and a steep shore-face?
Coarse clastic shorelines are typically found only in certain areas (e.g. formerly glaciated areas, coasts where high-gradient streams deliver bedload to the shore, wave-dominated coasts with rocky cliffs). Assess whether this shoreline occurs in a typical area.
What sort of coarse clastic shoreline is this? Is it a barrier or beach or both? If it is both, draw sketch maps showing the location of the different types.
If it is a barrier, what is the character of the back-barrier depression? Is it lagoonal, wetland, reclaimed land, or a combination of these?
If it is a beach, is it a pocket beach?</li></li></ul><li>Assessment: virtual fieldtrip to the Vale of Porlock, Somerset (UK) 2<br />Morphology and Process<br /><ul><li>What is the form of the shoreline? Is it rectilinear or concave-up or both? Again, if it is both draw sketch maps showing their locations.
How is the shoreline related to the orthogonal fetch of incident waves? Is it swash-aligned, drift-aligned, or both? If both, draw sketch maps showing locations.
What is the morphodynamic domain of the shoreline? Is it reflective, dissipative, or both? If both, indicate locations on a sketch map.
Is there any evidence of wave reflection here (e.g. edge wave development)? If so, how are the edge waves formed (i.e. trapped by refraction or reflected off headlands or both)?
What principle rock-types (lithology) are the clasts derived from?
What is the range in clast size? Is there a distinct distribution pattern of different size categories? Draw maps/profiles to indicate the distribution.
Is the clast size such that sizeable interstitial pores are formed? Is there any evidence of sediment decoupling?</li></li></ul><li>Assessment: virtual fieldtrip to the Vale of Porlock, Somerset (UK) 3<br />Sedimentology continued:<br /><ul><li>What is the range of clast shapes (e.g. according to Zingg)? Is there a distinct distribution pattern of different shape categories? Draw sketch maps/profiles of the distribution.
Generally, is the shoreline poorly sorted or well sorted?
Is the shoreline in equilibrium, or is it under attack from changes in wave condition or sea-level (i.e. does it appear to be migrating inland)? Things to look out for are the formation of cusps, breaches, overwash fans, and terraces (depositional, erosional, residual, or a combination). If it is under attack, how is it responding, and what is the final outcome likely to be?
By the end of the day you should be able to make a reasonable attempt at establishing the past, present and future development of this coastline.</li></li></ul><li>Summary<br />Changes in wave condition and sea-level can influence the morphodynamics of coarse clastic shorelines.<br />Without adequate sediment supply, a beach will become more reflective as its low-angled, low-tide slope is removed by longshore currents.<br />A shift in morphodynamic domain from dissipative to reflective will induce cusp formation.<br />Cusp development can influence where barrier breaches and overwashing events occur during storms.<br />Barriers may start to migrate landwards as a result of sea-level rise.<br />Sand terraces or boulder frames around the low-tide level often signify a landward-migrating barrier.<br />Terraces decrease reflectivity.<br />A barrier can become overstretched, and ultimately breach.<br />
References<br />Carter, R.W.G. 1988. Coastal Environments: An Introduction to the Physical, Ecological and Cultural Systems of Coastlines. Academic Press, 617pp.<br />Carter, R.W.G. and Orford, J.D. 1993. The morphodynamics of coarse clastic beaches and barriers: a short- and long-term perspective. Journal of Coastal Research, 15(Special issue): 158-179.<br />Eronen, M. 1983. Late-Weichselian and Holocene shore displacement in Finland. In: Smith, D.R. and Dawson, A.G. eds. Shorelines and Isostasy. Academic Press, pp. 183-207.<br />Haslett, S.K. 2008. Coastal Systems (2nd ed.). Routledge, 240pp.<br />Orford, J.D., Carter, R.W.G., Forbes, D.L. and Taylor, R.B. 1988. Overwash occurrence consequent on morphological change following lagoon inlet closure on a coarse clastic barrier. Earth Surface Processes and Landforms, 13: 27-36.<br />Orford, J.D., Carter, R.W.G., Jennings, S.C. and Hinton, A.C. 1995. Processes and timescales by which a coastal gravel-dominated barrier responds geomorphologically to sea-level rise: Story Head barrier, Nova Scotia. Earth Surface Processes and Landforms, 20: 21-37.<br />
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