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Coastal and Nearshore Morphology, Bedforms and Sediment Transport Pathways at Teignmouth (UK)

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Coastal and Nearshore Morphology, Bedforms and Sediment Transport Pathways at Teignmouth (UK) Available onlinewww.sciencedin at t.com SCIENCE ^DIRECT9 CONTINENTAL SHELF RESEARCH ELSEVIER Continental Shelf Research 24 (2004) 1 ] -1202 www. el sey ier. com/locate/csr Coastal and nearshore morphology, bedforms and sediment transport pathways at Teignmouth (UK) V. van Lanckera,b’*, J. Lanckneusa, S. Hearnc, P. Hoekstrad, F. Levoy6, J. Milesf, G. Moerkerkea, O. Monfort6, R. Whitehouse6 a Marine Geological Assistance (Magelas), Violier straat 24, B-9820 Merelbeke, Belgium b University o f Gent, Renard Centre o f Marine Geology, Krijgslaan 281, S-8, B-9000 Gent, Belgium c HR Wallingford, Howbery Park, Wallingford, Oxon 0X10 8BA, UK d University of Utrecht, Institute for Marine and Atmospheric Research, Heidelberg laan 2, 3508 TC Utrecht, Nederland UK e University o f Caen, Unité Morphodynamique Continentale et Côtière, Station Marine, 54 Rue Du Dr Charcot, F-14530 Luc-Sur-Mer, France f Institute o f Marine Studies, University o f Plymouth, Drake Circus, Plymouth PL4 8AA, UK Received 28 February 2001; accepted 9 March 2004 Abstract The beach and nearshore area of Teignmouth (UK) has been the object of a multidisciplinary research project, COAST3D, carried out by 11 European institutions. This paper emphasises the sedimentological and morphological investigations of the intertidal and nearshore area using a variety of techniques. From the results presented, it is clear that the Teignmouth site is very dynamic. Sediment mobility is high during fair-weather conditions owing to the strong jet-like current flows associated with the presence of the narrow estuary mouth. The beach at Teignmouth can be categorised as composite with a dissipative low-tide terrace and a reflective upper beach. From surficial sediment sampling, selective transport processes were identified, imposed by the high current velocities at the estuary mouth. This process also leads to the formation of characteristic depositional lobes with poorly sorted sediment. From digital side- scan sonar mosaicing, cross-shore and longshore-oriented bedforms could be observed, with their characteristics being dependent on the pre-survey hydro-meteo conditions. Although the nearshore system is dominated by the jet-outflow, littoral drift processes can also be important. They proved to be important for beach morphological changes leading to a rythmic alongshore beach behaviour which was attributed to a reversal of the littoral drift imposed by a period of larger waves. Moreover, the influence of waves and associated longshore currents cannot be neglected as they form the link between different sedimentary environments. From combined side-scan sonar imagery and sediment trend analysis, residual sediment transport directions were identified. These showed that fair-weather and storm-dominated processes are needed to explain the textural and morphological differentiation at this site. Further analysis will be required to identify the different time-scales involved in producing these changes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Teignmouth estuary; Nearshore and beach sediment and morphodynamics; Sand ripple profiler; RoxAnn; Digital side-scan sonar; Sediment trend analysis; COAST3D * Corresponding author. University of Gent, Renard Centre of Marine Geology, Krijgslaan 281, S-8, B-9000 Gent, Belgium. E-mail addresses: [email protected], [email protected] (V. van Lancker), [email protected] (S. Hearn), [email protected] (P. Hoekstra), [email protected] (F. Levoy), [email protected] (J. Miles), [email protected] (O. Monfort), [email protected] (R. Whitehouse). 0278-4343/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.03.003 1172 V. van Lancker et al / Continental Shelf Research 24 (2004) 1171-1202 1. Introduction ment (Hydraulics Research Station, 1966), and a model to examine how improvements to the The beach and nearshore area of Teignmouth harbour entrance would influence neighbouring (UK) has been the object of a multidisciplinary beaches (Hydraulics Research Station, 1970). research project, COAST3D,1 carried out by 11 More recently, Wimpol (1989) deployed two European institutions (Soulsby, 1998). The moored current meters outside the estuary, and COAST3D project was intended as a combined tide gauges in and out of the Teign estuary, in field and modelling study of the hydrodynamical, relation to works for a new outfall. They measured morphological and sedimentological behaviour of tidal currents and established the harmonic con­ a beach and nearshore area. Teignmouth was stituents for Teignmouth. Beach processes have chosen because of the irregular and three-dimen­ also been studied in the framework of the British sional (3D) nature of the coastline, with a rocky Beach and Nearshore Dynamics Programme headland, nearshore banks, and an estuary mouth (Davidson et al., 1993). In addition, the effect of all adjacent to the beach with its sea defences. a seawall on beach processes has been studied at Although, the data collected at Teignmouth are Teignmouth by Miles et al. (1996). The estuary only valid on a temporal scale of 1 month, the itself has been surveyed on several occasions results provide insight into the functioning of the (Hydraulics Research Station, 1970; Sea Sedi­ coastal system and coastal process geomorphology ments, 1979). in particular. As the site has many similar features The main investigations at Teignmouth took to coastlines found in western and southern place between 18 October and 26 November 1999 Britain, Ireland, northern and western France, under a range of prevailing tide and wave northern Spain, Portugal, and Norway, the data conditions (Hearn et al., 2000; Whitehouse and will improve our understanding of the way in Waters, 2000). This period included frequent which coastal processes operate at these types of and persistent storm wave conditions likely to coastal sites. Moreover, the collected data set produce a significant amount of morphological provides a good test for computer models both change. now and in the future. Similar investigations were Apart from the larger-scale geo-acoustical mea­ carried out during the first phase of the project surements and the sediment samplings in the where research activities were focussed on the nearshore, the COAST3D experiment was set up sandy beach and nearshore area of Egmond-aan- on the basis of three main transects to examine the Zee, the Netherlands (Soulsby, 2000). cross-shore and longshore processes, and gradients Teignmouth’s beach and estuary system have in processes, and to provide data for model been the subject of considerable interest since as validation. The two cross-shore transects (north early as the mid-19th century. One such early and south) form the basis of two contrasting linear hydrographic survey of the area was made by arrays, one in the relatively open coast to the north Spratt (1856), who documented cyclic movements and the other in the area where the 3D effects are of bars in the estuary mouth. This theme was later expected to be greatest. The shore-parallel east taken up by Craig-Smith (1970) and Robinson transect was provided to analyse variations in the (1975), who monitored bar movements at the alongshore direction. Various fixed instruments entrance to the harbour for 10 years, in order to for measuring conditions close to the boundaries establish the cyclic nature of the process. Hydrau­ of the area and two continuous monitoring frames lics research have made several studies on the area, (depth, waves, currents) were installed (White­ including an examination of the harbour and the house et al., 2000). stabilisation of the entrance channels (Hydraulics This paper gives an introduction to the environ­ Research Station, 1958), a sediment tracer experi- mental setting of Teignmouth. In particular, it presents results on the morphological and sedi­ 'COASTSD is the acronym for Coastal Study of Three- mentological characteristics of the beach and dimensional Sand Transport Processes and Morphodynamics. the nearshore area obtained using a variety of V. van Lancker et al / Continental Shelf Research 24 (2004) 1171-1202 1173 techniques and methodologies. A single-beam 2. Environmental setting of the teignmouth site echo sounder was used in the nearshore to determine the bathymetry and earlier recordings 2.1. Large-scale morphology from a ' RoxAnn' system were used to determine parameters for the acoustical classification of the The study area is situated at Teignmouth in the seafloor. A full coverage very-high-resolution county of Devon, in the southwestern part of digital side-scan sonar survey was carried out as England. The town of Teignmouth is located on a basis for a detailed morphological and sedimen- the north side of the mouth of the Teign estuary tological investigation. Grab samples were taken (Fig. 1) Teignmouth has a busy commercial port on a grid basis over the entire nearshore area, and local fishing fleet and is a popular location for both to provide sediment size distribution as beach users and watersports users (Whitehouse well as calibration of the RoxAnn data by et al., 2000). Teignmouth is a good example of a visual inspection. The intertidal area was char­ coastal site with a multiplicity of potential Coastal acterised through repetitive beach profile surveys, Zone Management problems, i.e.—flood defence, sediment sampling and measurements of the navigation, sewerage disposal, tourism, environ­ incident wave climate to quantify the morphody- ment—for
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