General morphology and sedimentology of coarse clastic shorelines Professor Simon K. Haslett Centre for Excellence in Learning and Teaching Simon.email@example.com 10th June 2010
Introduction Coarse clastic shorelines are characterised by the dominance of large-size material (between 2-2000 mm diameter), and a steep shore-face, which usually means the shoreline is of a reflective type or domain.
Unlike sandy (fine-grained) beaches, the study of coarse clastic beaches has been neglected, mainly because it is difficult to deploy sensitive instruments in what are quite often high-energy and destructive environments.
No consensus about the use of terms to describe coarse clastic beaches.
This presentation aims to describe the morphology and sedimentology of coarse clastic shorelines.
Coarse clastic shorelines 1 Coarse clastic shorelines are found mainly in areas that are either:
in formerly glaciated areas (a);
on tectonic coasts where high-gradient streams deliver bedload to the shore (b); or
in wave-dominated areas subject to rock cliff erosion (c).
(a) Glacial till providing a rich source of coarse sediment for gravel beach development along the southern shores of Galway Bay, Ireland. (b) Gravel beaches along the Queensland coast, Australia which are supplied with coarse sediment by high-gradient streams flowing off the uplifting coastal mountains. (c) Gravel barrier at Porlock, Somerset (UK) supplied by the updrift wave erosion of cliffs.
Coarse clastic shorelines 2 They may be classed as:
barriers that are free-standing or fringing, with a well-defined back-slope and back-barrier depression, enclosing lagoons and wetlands and capable of migrating inland.
Coarse clastic shorelines 3 Or beaches that have no well-developed wave-formed landward facing slope, and are often confined between headlands, as pocket beaches and abut cliffs with no means of retreat, that can be further sub-divided into swash-aligned or drift aligned. Gravel pocket beach. Anse du L’och, Brittany (France).
Morphology and process 1 The form of the shoreline is crucial to its morphodynamic status. There are two basic forms:
the first comprises a single rectilinear slope from crest to wave base (ignoring small ridges), which remains reflective under almost all wave conditions; and
the second comprises a concave-up form, which may exhibit a marked break in slope at or around the mid- to low-tide position, which alters the morphodynamic status of the shoreline as wave height to depth and depth to wavelength ratios vary, as tide levels change. The break in slope is often mirrored by a change in sediment character.
Morphology and process 2 Concave-up form (curved slope). Rectilinear slope (steady gradient).
Morphology and process 3 A consequence of wave reflection is the development of edge waves, which on coarse clastic shorelines are developed by: waves reflected off the shoreline and trapped by refraction (see Carter, 1988, pp. 71-74); and waves resonating between headlands, usually associated with pocket and fringing beaches. On the shoreline, the main manifestation of edge waves is the appearance of swash cusps, and the spacing of the cusps is related to edge wavelengths.
Morphology and process 4 Cusp morphology may exert a strong control over sediment movement and sorting. Furthermore, cusp development may dictate the pattern and position of barrier breaches and overwashing events during storms. (a) a gravel barrier overwash fan at Ru Vein in the Baied’Audierne, Brittany. (b) a breach in the gravel barrier at Porlock, Somerset (UK) formed by severe storms on 28 October 1996.
Sedimentology 1 Permeability of the shore is important in sediment transport. If clasts are large enough, it may be that all swash sinks into the beach and returns to the sea. This minimises or eliminates backwash, so that net coarse sediment transport is landward. However, with large pore spaces, sediment decoupling may occur whereby fine sediment can be washed back through the beach to re-emerge seaward, usually at a break of slope in concave-up beaches. Here, sand may be stored in the form of a sand terrace, and this may be added to by material introduced by longshore drift.
Sedimentology 2 Sediment transport is concentrated within a narrow zone between breakers and the beach face and is dominated by bedload transport. Transport can involve either the movement of individual clasts or clast populations. Individual clasts on a flat shore are likely to move landward rapidly. Attachment of algal fronds may aid this movement due to increasing clast buoyancy.
Sedimentology 3 Individual clasts tend to aggregate and, as accumulations of coarse particles develop, group-imposed transport controls are introduced e.g. position, contact stresses. These controls influence sediment sorting, and there is a transition between an initial unsorted population to sorted sub-populations in terms of spatial distribution and size/shape characteristics.
Sedimentology 4 The net result is that coarse clastic shorelines tend to become organised, in that they develop distinct cross-shore and along-shore facies which act to limit further transport. For example, more spherical clasts accumulate in the lower foreshore, while disc-shaped clasts tend to accumulate at or near the beach crest. Varying facies of gravel beaches in Brittany, France highlighting zonation of different grain sizes.
Sedimentology 5 Sorting such as this reflects wave energy, with the discs (and blades) being sorted from the more spherical clasts during high energy storms, and deposited at the landward limit of storm wave activity. Sorting amongst the spherical clasts, into spheres and rollers, takes place in lower wave energy conditions in the lower foreshore zone. Backwash preferentially transports the more mobile spherical clasts back down the beach face.
Summary Coarse clastic shorelines are characterised by the dominance of large-size material and a steep shore-face. Different settings. May be classed as barriers or beaches. The slope of the shoreline is crucial to its morphodynamic status – reflective or dissipative. Edge waves can manifest themselves as swash cusps, comprising an upstanding horn and a depressed embayment. Cusps may exert a strong control over sediment movement and sorting and may dictate the pattern and position of barrier breaches and overwashing. Permeability of the shore is important in sediment transport. Dominated by bedload transport. Coarse clastic shorelines tend to become sorted, in terms of clast size and shape.
References Carter, R.W.G. 1988. Coastal Environments: An Introduction to the Physical, Ecological and Cultural Systems of Coastlines. Academic Press, 617pp. 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. Haslett, S.K. 2008. Coastal Systems (2nd ed.). Routledge, 240pp.
This resource was created by the University of Wales, Newport and released as an open educational resource through the 'C-change in GEES' project exploring the open licensing of climate change and sustainability resources in the Geography, Earth and Environmental Sciences. The C-change in GEES project was funded by HEFCE as part of the JISC/HE Academy UKOER programme and coordinated by the GEES Subject Centre. This resource is licensed under the terms of the Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales license (http://creativecommons.org/licenses/by-nc-sa/2.0/uk/). All images courtesy of Professor Simon Haslett. However the resource, where specified below, contains other 3rd party materials under their own licenses. The licenses and attributions are outlined below: The name of the University of Wales, Newport and its logos are unregistered trade marks of the University. The University reserves all rights to these items beyond their inclusion in these CC resources. The JISC logo, the C-change logo and the logo of the Higher Education Academy Subject Centre for the Geography, Earth and Environmental Sciences are licensed under the terms of the Creative Commons Attribution -non-commercial-No Derivative Works 2.0 UK England & Wales license. All reproductions must comply with the terms of that license.