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A numerical-empirical approach for evaluating morphodynamic processes on gravel and mixed sand-gravel beaches

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A numerical-empirical approach for evaluating morphodynamic processes on gravel and mixed sand-gravel beaches
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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/223388464 A numerical–empirical approach for evaluatingmorphodynamic processes on gravel and mixedsand–gravel beaches  Article   in  Marine Geology · June 2007 DOI: 10.1016/j.margeo.2007.02.013 CITATIONS 38 READS 104 4 authors:Some of the authors of this publication are also working on these related projects: Itzï computer model   View projectExtreme ocean waves and climate change in the Gulf of Mexico   View projectAdrián Pedrozo-AcuñaUniversidad Nacional Autónoma de México 83   PUBLICATIONS   325   CITATIONS   SEE PROFILE David John SimmondsUniversity of Plymouth 31   PUBLICATIONS   407   CITATIONS   SEE PROFILE Andrew John ChadwickUniversity of Plymouth 61   PUBLICATIONS   658   CITATIONS   SEE PROFILE Rodolfo SilvaUniversidad Nacional Autónoma de México 128   PUBLICATIONS   624   CITATIONS   SEE PROFILE All content following this page was uploaded by Adrián Pedrozo-Acuña on 10 January 2017. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  A numerical – empirical approach for evaluating morphodynamic processes on gravel and mixed sand – gravel beaches Adrián Pedrozo-Acuña  a, ⁎ , David J. Simmonds  a  , Andrew J. Chadwick   a  , Rodolfo Silva  b a  University of Plymouth, Centre for Coastal Dynamics and Engineering, School of Engineering, Drake Circus, PL4 8AA, Plymouth, United Kingdom  b  Instituto de Ingeniería, Universidad Nacional Autónoma de México, Cd. Universitaria, 04510 D.F., México Received 30 September 2006; received in revised form 19 February 2007; accepted 22 February 2007 Abstract Pedrozo-Acuña et al. [Pedrozo-Acuña, A., Simmonds, D.J., Otta, A.K. and Chadwick, A.J., 2006. On the cross-shore profilechange of gravel beaches. Coastal Engineering, 53(4): 335 – 347] presented a numerical – empirical investigation of the processesthat control sediment transport in the swash zone on steep gravel beaches. This was based on a sensitivity analysis of a sediment transport/profile model driven by a highly non-linear Boussinesq model [Lynett, P., J., Wu, T.-R. and Liu, L.-F., P., 2002.Modelling wave run-up with depth-integrated equations. Coastal Engineering, 46: 89 – 107] which was compared to near full-scalemeasurements performed in the GWK flume in Hanover. In this paper we have extended our analysis to compare these earlier results with those relating to a mixed sediment (gravel and sand) beach. The parametric sensitivity analysis also incorporates adiscussion of the effects of acceleration about which there is much debate. The sensitivity analysis suggests that fluid accelerationcan contribute to the onshore movement of sediment that causes steepening of initially flat beach faces composed of coarsesediment. However acceleration alone cannot be the cause of the observed berm growth during the GWK tests. Instead, a complex balance of processes is responsible for the profile evolution of coarse-grained beaches with no single dominant process.© 2007 Elsevier B.V. All rights reserved.  Keywords:  swash; Boussinesq; gravel beach; mixed beach; sediment transport; friction; acceleration; infiltration 1. The role of swash on steep beaches Gravel and mixed sediment beaches are much ignoredyet comprise important coastal features which, inlocations such as the South Coast of the UK, have great significancefortheprotectionofcoastalcommunitiesandenvironmental and agricultural resources (Mason andCoates, 2001). Sediment composed of gravel or graveland sand mixture forms natural shoreline units that includebarrierbeachesandspitsthatprotectestuariesandhinterland from flooding, and steep terraces that providetoe protection to soft cliffs. Also of importance are com- posite beaches that comprise a gravel berm atop a sandylowerforeshoreandengineeredbeachesthathavebeenre-nourished with mixed sediment. Notwithstanding their  prevalence comparatively little research effort has beenfocussedonstudies ofthese importantcoastal defencesasopposed to that of sandy environments.Coarse beach environments have a gradient of reposethat is typically in the range 1:12 to 1:2. This steepness Marine Geology 241 (2007) 1 – 18www.elsevier.com/locate/margeo ⁎  Corresponding author. Tel.:+441752233686;fax:+441752232638.  E-mail address:  apedrozoacuna@plymouth.ac.uk (A. Pedrozo-Acuña).0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2007.02.013  is a function of both the physical characteristics of the beach material and its porosity which permits significant infiltration and absorption of incoming waves resultingin a comparatively weak backwash. The size of thesediments for the gravel varies between 2 to 64 mm,however sometimes cobbles ranging from 64 to 256 mmcan be found. Thus, sediment movement is only likelyclose to the shoreline inside the narrow energetic surf zone that is itself a function of the steep profile.Indeed, most of the morphological development appears to occur in the swash zone, and results in anupper beachface that is generally steeper comparedwith the mean beach face. This was demonstrated in arecent large scale experiment with coarse sedimentsreported by López de San Roman-Blanco et al. (2006). This investigation took place at the Large Wave Flume(GWK) in Hanover, Germany, and focussed on thestudy of the morphological response of two types of coarse-grained beach, one gravel and the other amixture of gravel and sand. López de San Roman-Blanco (2003) described many aspects of this research.In particular a conceptual model was constructed inorder to summarise the physical processes that mayhave an important role in shaping these beaches.However, although detailed comparisons of processessuch as the drainage characteristics, hydraulic gradientsand setup at the shoreline for both the beaches weredrawn, a discussion of specific swash zone processeswasomitted.Indeed,fromtheprofilemeasurementsitisclear that for all wave conditions the majority of profilechange occurred within the swash region, withsignificant changes occurring above the mean water level tothe point of maximum run-up.It is inthisregionthat understanding of the morphodynamics of coarse-grained beaches needs to be advanced.Swash zone hydrodynamics and sediment transport has been an active topic of research over the past decade.Yet until the recent GWK experiments, most field andlaboratory experiments have been restricted to observa-tions of sand beaches with mild slopes (Masselink andHughes, 1998; Butt and Russell, 2000; Kobayashi andJohnson, 2001; Butt et al., 2002; Puleo et al., 2003;Masselink and Russell, 2006). It might be argued that swash processes on sandy beaches should be less of aconcern to the coastal engineer, given that swash sed-iment mobilisation represents only a fraction of the totalamount mobilised on sandy beaches, but the majority oncoarse-grained beaches (Austin and Masselink, 2006). Recently Van Wellen et al. (2000) presented an empir-ical model for swash transport that predicted well thelongshore transport of sediment within the accuracy of field measurements. More recently Pedrozo-Acuña et al.(2006) have reported on investigations of the sensitivityof profile development to specific swash parameters, inrelation to the GWK gravel beach experiments. It isnotoriously difficult to measure sediment transport  parameters directly, especially in the swash. The ap- proach that was adopted was to carry out a sensitivityanalysis of a suitable numerical modelling framework.This permitted a discussion of the sensitivities of pro-cesses that might be shown to control the cross-shore profile development in terms of those parameters that are widely acknowledged as serving as aggregateddescriptions of the immeasurable micro-scale processes.The modelling framework comprised the coupling of ahighly non-linear Boussinesq model with a sediment transport formulation and a morphology module. In thestudy, special attention was paid to discussing the rolesof the infiltration and bottom friction parameters on theskill (Brady and Sutherland, 2001) with which predic-tions of the GWK gravel profiles could be achieved. Inaddition, Buscombe and Masselink (2006) presented areview which summarises some of the aspects that require further study in order to understand gravel beachdynamics.It should be borne in mind that a universal quanti-fication of sediment transport derived from the funda-mental physics has not been established owing to theenormous complexity of the phenomenon (Van Rijn,1993). This is largely due to the inherent difficulties inmaking accurate but non-invasive measurements of ki-nematics and sediment fluxes, especially in transient,aerated and shallow flows. This has, thus far, proven anintractable problem, despite advances in optical andacoustic measurement technology.Thus a myriad of competing formulae for sediment transport under waves and currents have been developed basedonamacroscopicapproachtooscillatoryandsteadyfluid motion. Many of these are presented as universal intheir application yet have mostly been developed for sandsized material. It is desirable that a bottom – up descriptionofprocessesonthegrain – graininteractionscaleshouldbedeveloped, but this is currently unavailable. Indeed it can be argued that at the micro-scale, the discrimination between or validation of some of the more advancedmodellingapproaches(Deigaardetal.,1986)isbeyondthe capability of current measurement technology.Therefore, in Pedrozo-Acuña et al. (2006), and in theworkpresentedhereamodellingframeworkofappropriatecomplexity has been adopted for comparison with themacro-scale observations of profile change alone. The “ appropriate complexity ” is defined after our discussion of swash transport processes on steep beaches and modellingstate of art. 2  A. Pedrozo-Acuña et al. / Marine Geology 241 (2007) 1  –  18  This present work extends Pedrozo-Acuña et al.(2006) in two directions, again concentrating on profiledevelopment due to swash processes. Firstly, a newanalysis of profile development over the GWK mixed beach is presented and compared with this earlier studyof the gravel beach. Both beaches were subjected to thesame series of wave conditions permitting comparisonsto be drawn regarding the influence of beach material onswash processes.  A priori  it might be expected that mixed sand and gravel, typical of a dredged beachrecharge, would behave more like a sand beach becauseof the improved sediment packing and correspondingreduction in porosity.Secondly, an investigation into the role that fluidacceleration plays in coarse-grained beach profileevolution is presented. The latter is motivated by theconclusions put forward by Nielsen (2002) and Puleo et al. (2003) who suggested that the strong onshoredirected acceleration and backwash deceleration arelikely to affect sediment transport in the swash zone, andthus beach profile evolution. 2. Present understanding of swash transport onsteep beaches One of the most striking features of swash motion onsteep beaches is the asymmetry of flow velocity andwater volume between the uprush and backwash. Thiscan create a corresponding asymmetry in the cross-shoresediment fluxes that depends on the phase of the flowand the steepness of the beachface (Masselink andHughes, 1998). In order to make any progress in the prediction and modelling of swash transport, a carefulassessment of the complex interaction between thecharacteristics of the beach material, groundwater andthe hydrodynamic inputs is required. Amongst the processes that are reported to affect sediment transport in the swash zone are: bottom friction, infiltration andexfiltration through the beach face, acceleration effects,and turbulence generation following bore collapse. 2.1. Friction Pedrozo-Acuña et al. (2006) have reported that thereis considerable debate in the literature concerning valuesof quadratic friction parameters for sediments. Whilst some researchers suggest a phase dependant value for the effective quadratic friction parameter is appropriate,others, including Raubenheimer et al. (2004) suggest that values in both phases are equivalent. Furthermorewhilst  Cox et al. (2000) suggest from laboratory exper-iments that friction is higher in the uprush phase, Puleoand Holland (2001) deduced the opposite from their field experiments. It must be borne in mind that thesediscrepancies can be attributed to measurement andanalysis differences and accuracies. 2.2. Groundwater effects Interaction between the flow above the beachfaceand the groundwater flows, has been a matter of researchin several studies. Its effect on the direction of sediment transport has been highlighted by Turner (1995), Turner  and Nielsen (1997), Turner and Masselink (1998), Butt  et al. (2001).Downward and upward pressure gradients created bythe pressure loading of the swash lens, during run-upand run-down result in infiltration and exfiltration of water through the beach face. The effects of this onsediment transport in the swash zone were summarised by Elfrink and Baldock (2002) as follows: •  Reduction of backwash volume and duration. •  Increase and decrease of the effective weight of sediment particles. •  Increase and decrease of the shear force on sediment  particles.The reduction in the backwash volume, which isdependentonthebeach permeability,leads tobothlower mean and maximum velocities in the offshore direction.This changes the flow asymmetry and therefore thesedimentdepositionpatterns.Theeffectisexpectedtobeof lower magnitude for sandy beaches where permeabil-ity is usually neglected,due to the small verticalflux that is present. However, in beaches with coarser sediment like gravel beaches, this effect may become important.The amount of water infiltrated into the beach willdepend on its permeability, and thus on both the meangrain size and the grain size distribution. In a recent study,analysingtimeseriesofvelocityandconcentrationwith a modified Shields parameter, Butt et al. (2001)found that there is a critical grain size at which theinfluence of infiltration – exfiltration changes the direc-tion of sediment transportfrom offshore toonshore. Thishas since been supported by Karambas (2003) innumerical simulations using a Boussinesq modelcoupled with a porous flow model. He identified thesamecriticalgrainsizethat determinedthedominanceof these effects. This investigation also concluded that relatively small changes in friction factor might changethe direction of the apparent influence of infiltration – exfiltration, also mentioning that the wave conditionshave no influence on this process. 3  A. Pedrozo-Acuña et al. / Marine Geology 241 (2007) 1  –  18  Conley and Inman (1994) analysed the flow and theturbulencecharacteristicsofventilatedboundarylayersinthe laboratory, which is analogous to a porous beachface.They found that turbulence in the case of infiltration isconfined to a compact layer close to the bed, whereas it ismore evenly distributed across the water column duringexfiltration. The thinning of the boundary layer duringinfiltration leads to enhanced flow velocities and shear stress in the boundary layer, whereas the opposite occursin exfiltration.Infiltration and exfiltration effects are also associatedwith additional pressure forces on sediment particles.Turner and Masselink (1998) showed that the criticalshear parameter can vary significantly due to the alteredeffective weight. Nevertheless, they found that the effect of the enhanced bed-shear stress was more important than the altered effective weight and concluded that infiltration/exfiltration processes support onshore sedi-ment transport in the swash zone. These effects areexpected to increase for increasing sediment grain size,since vertical flow velocities and the resulting additional bed-shear stress become larger for coarser sediments,and a relatively larger part of the sediment transport occurs close to the bed. 2.3. Acceleration effects In early investigations, the effect of fluid accelerationwas normally assumed to produce delayed boundarylayer growth and hence higher velocity near the bed thanflows with weaker acceleration after the same durationof boundary layer development ( Nielsen, 1992). Several works have shown that acceleration has a direct effect on sediment transport, mainly as a further suspensionmechanism (King, 1991; Ribberink and Al-Salem,1995). Moreover, in recent studies, fluid accelerationshave also been related to sandbar morphology in the surf zone, showing that the peak in acceleration skewness of surf zone flows was well correlated with onshore bar motion (Elgar et al., 2001; Hoefel and Elgar, 2003). In the swash zone, the acceleration effects are more pronounced with a strong onshore acceleration under the propagating bore and weak acceleration during the backwash. Recent works presented by Nielsen (2002)and Puleo et al. (2003) have concluded that the strongonshore directed acceleration and backwash decelerationare likely to affect sediment transport. However, HughesandBaldock(2004)havesuggestedthattheappearanceof strongonshore accelerations insomefield datamay be anartefact, related to the difficulty of measuring velocitiesaccurately in very shallow water. Moreover, results foundin a recent investigation by Terrile et al. (2006) show that fluid acceleration is indeed important for the initiation of sediment motion. Nevertheless, further studies arenecessary in order to identify and describe the effects of acceleration-dependent transport. 2.4. Bore collapse The leading edge of the swash (collapsing bore)could be expected to entrain or maintain high concen-trations of sediments because of the associated highturbulence levels reaching the bed (Puleo et al., 2000).During the uprush phase Yeh et al. (1989) showed that turbulence is advected with the bore front and can act onthe dry beach face.The level of turbulence depends on the beach slope.Steeper beaches produce plunging breakers with moreconcentrated turbulence than on wide dissipative surf  beaches (Butt et al., 2004).This observation was also put forward by Ting andKirby (1994) in laboratory experiments. Indeed, in theshallower area of the swash zone, turbulent vortices mayreach the seabed and should be expected to entrain or maintain high concentrations of sediments (Puleo et al.,2000). Jackson et al. (2004) have also observed in the field that sediment entrainment during bore collapse(seaward of the base of the swash zone) is an important mechanism for swash transport. It is thus evident that  bore collapse is important for swash transport in theuprush phase (Masselink and Russell, 2006). 2.5. Swash processes  —  conclusion The problem that is now addressed is  “ how is it  possible to simulate swash transport to an accuracythat allows comparison with measured profile devel-opment? ”  First, it is necessary to choose a model of theappropriate complexity to match the problem. Thisshould comprise a coupled hydrodynamic model witha sediment transport formulation and profile evolutionmodel. The model should also include parameters that represent friction, acceleration and groundwater.Presently, an investigation of the role of turbulencefollowing bore collapse is beyond the scope andcomplexity of this current work and we focus on theremaining processes. The sediment transport predic-tions are critically dependent on the prediction of wave-induced velocities. The asymmetry between the on-shore and offshore velocities plays an essential role indetermining the magnitude and direction of the wave-induced sediment transport. Thus a phase-resolvingmodel of swash transport driven from outside the surf zone is required. 4  A. Pedrozo-Acuña et al. / Marine Geology 241 (2007) 1  –  18

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