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Coastal and Shelf Sediment Transport: an Introduction

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Coastal and Shelf Sediment Transport: an Introduction Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Coastal and shelf sediment transport: an introduction MICHAEL B. COLLINS 1'3 & PETER S. BALSON 2 1School of Ocean & Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton S014 3ZH, UK (e-mail." mbc@noc, soton, ac. uk) 2Marine Research Division, AZTI Tecnalia, Herrera Kaia, Portu aldea z/g, Pasaia 20110, Gipuzkoa, Spain 3British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK. Interest in sediment dynamics is generated by the (a) no single method for the determination of need to understand and predict: (i) morphody- sediment transport pathways provides the namic and morphological changes, e.g. beach complete picture; erosion, shifts in navigation channels, changes (b) observational evidence needs to be gathered associated with resource development; (ii) the in a particular study area, in which contem- fate of contaminants in estuarine, coastal and porary and historical data, supported by shelf environment (sediments may act as sources broad-based measurements, is interpreted and sinks for toxic contaminants, depending by an experienced practitioner (Soulsby upon the surrounding physico-chemical condi- 1997); tions); (iii) interactions with biota; and (iv) of (c) the form and internal structure of sedimen- particular relevance to the present Volume, inter- tary sinks can reveal long-term trends in pretations of the stratigraphic record. Within this transport directions, rates and magnitude; context of the latter interest, coastal and shelf (d) complementary short-term measurements sediment may be regarded as a non-renewable and modelling are required, to (b) (above) -- resource; as such, their dynamics are of extreme any model of regional sediment transport importance. Over the years, various approaches must account for the size, location and and techniques have been applied to the determi- composition of sedimentary sinks. nation of sediment transport pathways and the derivation of erosion, transport, and deposition On the basis of the above summary, it is evident rates. Such wide-ranging approaches include the that it is timely to review a representative selec- refinement and application of numerical model- tion of the different approaches, by reference to ling; and the development of new and more effi- recently undertaken coastal and shelf investiga- cient field equipment, e.g. video systems (coastal/ tions. A number of such studies (13) are included inshore) and multibeam. within this Special Publication, operating at a In general, sediment transport can be defined variety of temporal and spatial scales, within dif- on the basis of direct observations, indirect obser- ferent regions of the UK/European continental vations and by modelling. Direct observation shelf, and elsewhere. methods include: acoustic backscatter; optical The concept of different scales, in relation backscatter; sediment traps; artificial tracers, for to sediment dynamics has been proposed sand and pebbles; natural tracers or labelled sedi- (Horikawa 1970) for classifying coastal phenom- ments, for silts and clays; and the determination ena into three (temporal and spatial) categories: of water movements, using drifters, SPM (sus- macroscale (year/kilometre); mesoscale (day- pended particulate matter) and remote sensing. hour/metre); and microscale (second/millimetre). Indirect observational methods include: sediment Subsequently, the following observations have characteristics, including GSTA (grain size been made (Horikawa 1981): trend analysis) and mineralogy; geomorphology, (a) to treat the macroscale phenomena, the including coastal landforms, estuarine volumes approach of the geologist and geomorph- and asymmetric bedforms (ripples, sandwaves ologist is helpful for understanding the and sandbanks); and, finally, the internal struc- general tendencies of the coastal processes; ture of the sediment bodies (cross-bedding and (b) changes in shoreline and sea-bottom topog- accretionary sequences). On the basis of these raphy, bar and cusp formation, together various approaches and techniques, it may be with nearshore currents, all fall into the concluded that: category of mesoscale phenomena; From: BALSON,P. S. & COLLINS, M. B. 2007. Coastaland Shelf Sediment Transport. Geological Society of London, Special Publications, 274, 1-5.0305-8719107l$15.00 9 The Geological Societyof London 2007. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 2 M.B. COLLINS & P. S. BALSON (c) within the context of a microscale approach, phenomena, the macroscale phenomena. At the extensive research needs were identified time of the publication (Horikawa 1981), such such as, in particular, various aspects of connections could not be made. wave-current interaction. The above concept has been developed further by Larson & Kraus (1995), in relation to Interestingly, the observation is made that, spatial and temporal scales for investigating theoretically, the complete superposition of sediment transport and morphological processes. microscale phenomena should compose the In Figure 1, microscale is seen to refer to changes mesoscale phenomena and that of mesoscale from sub-wave period to several periods, over Fig. 1. Relationships of contributions to the Special Publication, in terms of their spatial and temporal scales (based upon Larson & Kraus 1995). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 INTRODUCTION 3 lengths of millimetres to centimetres. At mesos- suspended sediment component close to the bed, cale, net transport rates over many wave periods together with the bedload itself. The labelling are evaluated for distances of metres to a kilo- of pure clays and estuarine sediments, with metre. Macroscale involves seasonal changes lanthanide (La) is described by Spencer et al. and a space scale of kilometres, whilst megascale Here, it is concluded that further investigation describes decade to century changes over coastal is required, of the use of alternative lanthanide sub-reaches and reaches, e.g. over a littoral cell. group elements, for such studies. Optical and The concepts applied here are applicable, acoustic backscatter sensors are described then equally, to the inner continental shelf (< 60 m (Bass et al.), within the context of the mea- water depth) - at the very least. The main conclu- surements of the mud and sand component in sions reached (Larsen & Kraus 1995) are that transit, at a site located to seawards of the Wash calculations at different scales can be related embayment, southern North Sea. and reconciled, if limitations in the predictions The problems of field measurements and of initial and boundary conditions and in the quantification of longshore sediment transport fluid flow, are recognised. Against this back- (LST) is considered by Cooper & Pilkey, in terms ground the contribution of the present pub- of mechanisms and present approaches. It is lication are superimposed; these range from pointed out, by these authors, that the inability micro- to mega-scales, on the basis of the to measure the total LST has important implica- generalized classification. tions for coastal zone management; this is Interestingly, Dronkers (2005) has adopted a because so many coastal management initiatives similar approach, based upon the original synthe- sis of Holman (2001). The former investigator rely upon quantified volumes of LST. In terms of makes the following pertinent observations: coastal and shelf seas, in general, a relatively simple analytic (algebraic) approach is described (a) at small spatial scales, seabed morphology (Aldridge), to complement full-scale numerical and water motion adapt to each other, with calculations and assist in the interpretation of a short delay, but at a large spatial scale, the the numerical results. However, the results adaptation period can be very long; obtained rely upon the implicit assumption that (b) if erosion and sedimentation are balanced, the supply of material available for transport is averaged over large temporal and spatial scales, it may happen that these is an imbal- not exhausted, over the tidal cycle. ance at smaller scales or vice versa - in fact, The repeated survey of banner tidal sand- the phenomena of erosion, sedimentation banks, using multibeam, is described by Schmidt and sediment transport always have to be et al. Interestingly, dunes connect over the crest defined with respect to particular spatial and of the bank despite opposing sediment transport temporal scales; directions on the flanks. A new numerical model, (c) the physics of sedimentary coastal environ- that identifies the paths taken by a large number ments is related to temporal and spatial of identified ('tagged') sand grains in coastal scales - the physical processes that deter- areas in response to waves and currents, is mine coastal morphology span a range of described by Soulsby et al. Within this context, temporal scales, covering more than ten a validation exercise is applied simulating the orders of magnitude. dispersal of radioactive sand tracers. Particle tracking is considered, in terms of a somewhat For large temporal, but small spatial scale pro- cesses, time-series are restricted; sometimes, they different approach, by Black et al. Used in are not of sufficient high quality to overcome conjunction with a range of more traditional any uncertainties, i.e. separating processes from methods, particle tracking (particle
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