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Coastal and shelf : 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 , 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. complete picture; , 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 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 and ; natural tracers or labelled sedi- (Horikawa 1970) for classifying coastal phenom- ments, for 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 . hour/metre); and microscale (second/millimetre). Indirect observational methods include: sediment Subsequently, the following observations have characteristics, including GSTA ( been made (Horikawa 1981): trend analysis) and mineralogy; , (a) to treat the macroscale phenomena, the including coastal landforms, estuarine volumes approach of the and geomorph- and asymmetric (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, 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 27, 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 27, 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 (< 60 m (Bass et al.), within the context of the mea- water depth) - at the very least. The main conclu- surements of the 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 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, 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 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, 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 or sediment background noise. From an engineering perspec- tracing, including the deliberate marking of tive, on the basis of the scientific limitations natural or synthetic sediment with an identifiable in understanding, the best available method to signature) is an additional tool, which provides predict sediment transport rates in the marine further lines of evidence. environment may not be able to achieve much Changes in shoreline morphology along the better than a factor of 5; cf. 2, in the case of Dutch are investigated by Hinton & (Soulsby 1997). Nieholls; this is a wave-dominated uniform coast- Initially, in this Special Publication errors line, uninterrupted by tidal inlets. The analysis and uncertainties are examined, in relation to the undertaken has shown that the upper, middle measurement of SSC (suspended sand concentra- and lower shoreface are coupled; this has tion) using Acoustic Backscatter (Vincent). A widespread significance in the understanding of major uncertainty is identified, in terms of the long-term coastal-evolution. Surficial nearshore Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

4 M.B. COLLINS & P. S. BALSON

sediments are described then, in terms of their Sea - reveals a different pattern. Using a high distribution and spatial patterns (McDowell quality data set of waves and currents, from et aL). Temporal changes in substrate and a particular site, the contribution of different bedforms suggest development and combinations to long-term transport, has been boundary migration, between winter and assessed (Soulsby 1987). Under such conditions, summer seasons. waves act as a stirring agent to move sediment, The application of grain size trend analysis to whilst it is transported by the current. The con- Carmathen Bay, Bristol , is described by ditions analysed ranged from calm seas and Cooper & McLaren. Complex patterns of move- neap tides, to major storms coupled with ment are interpreted, incorporating a number of tides. Interestingly, the following conclusions tidally-induced gyres. This approach is included, were reached: within the context of seabed sediment transport (a) waves enhance transport, by up to a factor investigations, by Velegrakis et al. An integrated of 10, compared with transport in the approach is outlined, in relation to case studies absence of waves: and from the southern UK inner (< 60 m water depth) (b) in terms of long-term (sediment) transport, continental shelf. Conceptual sediment transport the largest contributions were provided models are presented, associated with different by 'fairly large', but not infrequent waves, levels of confidence in their interpretation. superimposed upon currents lying approxi- Finally, at the scale of the NW European con- mately between the peak speeds of mean tinental shelf, SPM is modelled (Souza et aL). neap and mean spring tides. Tidal signals and seasonal variations are identified within the spatial patterns. Nonetheless, because the sediment transport The various investigations incorporated rate depends non-linearly on the current speed, within this Volume, as outlined above, represent also because the effect of wave-stirring is impor- a wide range of temporal and spatial scales; tant, the direction of the long-term transport may these are, in turn, associated with appropriate be very different from the residual current direc- instrumentation and analyses. Consequently, it is tion (Soulsby 1997). The very strong currents appropriate to incorporate each of the studies and very large waves were found not to make here into an 'overview'. significant contributions to long-term transport. In parallel with this approach/concept lies As such, the transition between storm-induced the importance of extreme (storm) events which, processes at the coastline, compared with the interestingly, appears to vary according to the influence of various non-linear wave/current location of a particular environment, within the interactions offshore, is an important area of overall sediment dynamics system. For example, sediment dynamics research. the 'episodicity' of the transport of sediment, Overall, the presentations made at the meet- within the coastal zone, has been described ing (transposed, mainly on the basis of a peer- (Seymour & Castel 1985). On the basis of 1 to review process, into the contributions in this 3 years of nearshore directional wave measure- Issue), incorporate the concept and approaches ments from seven US west coast , time- reviewed above: direct/indirect observations and/ series of daily net longshore transport rates were or modelling; different temporal and spatial derived. Transport was found to be very episodic, scales, in relation to sediment dynamics; the with approximately only 10% of the time importance of wave/current interactions; and the required to move half of the sediment trans- impact of episodic events. As such, it is hoped ported during a year. Elsewhere, measurements that 'state-of-the-art' science and instrumenta- of large-scale coastal response to multiple storms tion is incorporated into this unique publication. on three coastal beaches have revealed a hetero- However, it should be remembered that sediment geneous response, with isolated hotspots of transport is still an inexact science on the basis erosion (List et al. 2006). Within a few days, these of: biological effects; the presence of (mixed) hotspots of erosion are reversed rapidly by post- sediments, containing a wide range of grain storm accretion. Such observations provide a size components; time-history effects; and wave- new view on the coastal response to storms, at current interactions. Finally, it is speculated that scales much larger than site-specific experiments strong non-linear processes, such as sediment (List et al. 2006). morphodynamics, may exhibit chaotic behaviour In contrast to the importance of storms in (in a mathematical sense), in the same way as the controlling the morphology of the coastline, weather (Soulsby 1997) the effect of wave/current interaction on sedi- The Editors acknowledge the contribution of ment transport from the inner continental shelf the reviewers and the patience of the authors, ( < 60 m water depth) area - the southern North during the production of this volume. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

INTRODUCTION 5

The authors are grateful for discussion of some of the HORIKAWA, K. 1981. Coastal Sediment Processes. concepts here, with Adonis Velegrakis (University of Annual Review of Fluid Mechanics, 13, 9-32. the Aegean, Greece). Likewise, Dr Haris Plomaritis and LARSON, M. & KRAUS, N. C. 1995. Prediction of Kate Davis are acknowledged for their assistance Cross-Shore Sediment Transport at Different in preparing/commenting upon the manuscript and Spatial and Temporal Scales. Marine , 126, preparing the figures, respectively. 111-127. LIST, J. H., FARRIS, A. S. & SULLIVAN,C. 2006. Revers- References ing Storm Hotspots on Sandy Beaches: Spatial and Temporal Characteristics. Marine Geology, 226, DRONKERS, J. 2005. Dynamics of coastal systems. 261-279. Advanced Series on Ocean Engineering, 25, World SEYMOUR, R. J. & CASTEL, D. 1985. Episodicity Scientific Publishing Co. Pte. Ltd., London. in Longshore Sediment Transport. Journal of HOLMAN, R. A. 2001. Pattern formation in the Waterway Port Coastal and Ocean Engineering, 111, nearshore. In: SEMINARA, G. & BLONDEAUX, P. 542-551. (eds) , Coastal and Estuarine Morphodynamics. SOULSBY, R. L. 1987. The relative contributions of Springer-Verlag, Berlin, 141-162. waves and tidal currents to transport. HORIKAWA, K. 1970. Advanced treatise on coastal Hydraulic Research Ltd., Report SR 125. sedimentation. In." Summer Seminar on Hydraulic SOULSBY, R. L. 1997. Dynamics of Marine : A Engineering, Japanese Society of Civil Engennrs., Manual for Practical Applications. Thomas Telford, 501-534 [in Jananese]. London.