Influence Of Permeability On The Performance Of Shingle And Mixed Beaches
PB11207-CVR.qxd1/9/0511:42 AMPage 1Joint Defra/EA Flood and Coastal ErosionRisk Management R&D ProgrammeInfluence of permeability on theperformance of shingle and mixedbeachesR&D Technical Report FD1923/TR
Joint Defra/EA Flood and Coastal Erosion RiskManagement R&D ProgrammeInfluence of permeability on theperformance of shingle and mixedbeachesResearch Scoping StudyR&D Technical Report FD1923/TRProduced: December 2007Authors: Kaiming She, University of BrightonDiane Horn, University of LondonPaul Canning, WS Atkinsi
Statement of useThis document provides information for the Defra and Environment AgencyStaff about the influence of permeability on the performance of gravel andmixed sand-gravel beaches and constitutes an R&D output from the Joint Defra/ Environment Agency Flood and Coastal Defence R&D programme. This reportdescribes work commissioned by Defra under Project FD1923 Influence ofPermeability on the Performance of Shingle and Mixed Beaches, within theFluvial, Estuarine, and Coastal Processes Theme.Dissemination statusInternal: released internallyExternal: released to public domainKeywords:Coastal defence, mixed beaches, permeability, cliffingResearch contractor:Kaiming She,University of BrightonEmail: k.m.she@bton.ac.ukDefra project officer:Bill SymonsEmail: bill.symons@DEFRA.GSI.GOV.UKPublishing organisationDepartment for Environment, Food and Rural AffairsFlood Management Division,Ergon House,Horseferry RoadLondon SW1P 2ALTel: 020 7238 3000Fax: 020 7238 6187www.defra.gov.uk/environ/fcd Crown copyright (Defra);(2008)Copyright in the typographical arrangement and design rests with the Crown.This publication (excluding the logo) may be reproduced free of charge in anyformat or medium provided that it is reproduced accurately and not used in amisleading context. The material must be acknowledged as Crown copyrightwith the title and source of the publication specified. The views expressed in thisdocument are not necessarily those of Defra or the Environment Agency. Itsofficers, servants or agents accept no liability whatsoever for any loss ordamage arising from the interpretation or use of the information, or reliance onviews contained herein.ii
Published by the Department for Environment, Food and Rural Affairs. Printedin the UK, (March, 2008) on recycled material containing 80% postconsumer waste and 20% chlorine-free virgin pulp.PB No. 12527/25iii
Executive SummaryMany of the beaches on the UK coast that constitute the main defence againsterosion and flooding are composed of highly permeable sediments, usually amixture of sand and gravel. Recharge material dredged from offshore isincreasingly used to replenish these mixed sand-gravel beaches. Becausebeach recharge materials may contain a larger proportion of fine sediment thanthe natural beach, sediment size distributions, sorting and hydraulic conductivitycan be significantly altered, as is beach profile response. Even when the sizedistributions of the natural sediment and the recharge sediment are quitesimilar, the standard recovery technique produces an increased proportion ofsand on the upper foreshore, which is normally composed of coarse sediment.The higher amount of fine sediment leads to the development of cliffing aroundthe high water mark, which results in enhanced loss of recharge material due toundercutting by wave action. In addition, such cliffs can be extremely hazardousdue to their natural instability and for public safety may require removal at thefirst opportunity.This study aims to address issues such as cliffing, the influence of permeabilityon the performance of recharged beaches, sediment resources and theirmanagement, efficiency of sediment placement techniques, and costeffectiveness of frequent and focussed recycling operations. The specificobjectives are as follows: to produce a review of existing knowledge of the impacts of permeability onthe performance of gravel and mixed sand-gravel beaches; to examine possible methodology by which sediment properties on mixedsand-gravel beaches can be characterised; to examine the effects of the sand fraction on the permeability and porosityof mixed sand-gravel sediment, and the ways forward in alleviating theproblem of cliffing; to carry out numerical modelling to improve understanding of the effect ofpermeability on beach profile response on mixed sand-gravel beaches,including the relative importance of parameters such as hydraulicconductivity, wave friction factor, sediment grading, and groundwater flow; to propose recommendations for a framework of field and laboratory studiesto advance knowledge of the influence of permeability on beachperformance.The investigation takes the form of an extended literature review, theoreticaldevelopment coupled with laboratory experiments, numerical modelling with thesupport of laboratory and field data, and case studies of three current/recentbeach recharge programmes.The literature review covers a wide range of topics associated with gravel andmixed sand-gravel beaches. Fundamental research questions and challengesidentified by this review relate toivExecutive Summary
1) understanding the difference between coastal dynamics on mixed sand andgravel beaches and beaches dominated by relatively uniform sediment sizes(either sand or gravel)2) developing methodology to quantify and/or classify complex, spatially andtemporally variable sediment characteristics effectively on mixed sand andgravel beaches3) developing methodology to parameterise the effects of bimodal sedimentsand mixed sand and gravel in hydrodynamic and morphodynamic models4) adapting existing numerical models to predict the processes andmorphological evolution of mixed sand and gravel beachesNo standard method is yet available for characterising the sediments of a mixedsand-gravel beach. A primary difficulty is that mixed sand-gravel beachesexhibit a high degree of variability, both spatially and temporally, in terms ofsediment size and density, sediment shape, sorting, hydraulic conductivity,porosity, specific yield and moisture content. However, research indicates thatthe percentage of sand and its size relative to the gravel are among the mostimportant parameters associated with the performance of a mixed sand-gravelbeach, and thus may be used as key parameters characterising mixed sandgravel beaches. There is also an indication that sediment transport is affectedby the relative proportions of sand and gravel, and that adding sand to mixedsediments can increase gravel transport as well as the total transport rate.A re-analysis of some existing laboratory data was undertaken to assessperformance of mixed sand-gravel beaches in contrast to that of gravelbeaches. The experiments were carried out using a model gravel beach and amodel mixed sand-gravel beach, both being subjected to a range of identicalwave conditions. The comparison shows that under the same wave conditions,mixed sand-gravel beaches have reduced volumetric changes, less onshoretransport, and more offshore transport than gravel beaches. This may bedirectly related to the fact that the presence of sand in a mixed sand-gravelbeach significantly reduces the permeability of the beach, impairing the waterflow within the sediment media.Laboratory experiments and numerical modelling also show that altering beachgroundwater levels affects profile response on fine and coarse beaches, with agreater amount of change on coarse beaches. A lower groundwater level leadsto increased onshore transport and a higher groundwater level to increasedoffshore transport for both accretionary and erosional conditions.The presence of sand in a mixed sediment has been known to affect theporosity and hydraulic conductivity of the sediment. In this study, a theoreticalapproach was taken using a bimodal sediment model. The constituent sand andgravel each has a known grain size, porosity and hydraulic conductivity. Theanalytical work led to some simple equations relating the porosity, hydraulicconductivity and bulk density of the mixed sediment to the percentage of sand.These equations were successfully validated by a series of specially designedlaboratory experiments. The hydraulic conductivity of the mixed sediment isshown to be greatly influenced by the presence of sand. As the sandpercentage increases, the hydraulic conductivity of the sediment mix reducesrapidly until the sand fraction reaches about 30 40%, beyond which thehydraulic conductivity remains close to but below that of pure sand. TheExecutive summaryv
minimum hydraulic conductivity occurs at a sand percentage in the region of30 40%, and has a value of approximately 55% less than that of pure sand.The sand percentage corresponding to the minimum hydraulic conductivity is ofcritical significance and is referred to as the critical point. Additional loadingexperiments using “sand castle” models showed that cliffing would occur whenthe sand percentage exceeds the critical value, and that the load bearingcapacity appeared to be greater than at higher sand percentages. The criticalpoint thus represents the worst scenario in terms of the likelihood of cliffing.The theoretical analysis and laboratory tests also showed that compaction ofthe sediment due to heavy plant operations on the beach can greatly reduce thehydraulic conductivity and lower the critical sand percentage, thus enhancingthe likelihood of cliffing of a recharged sand-gravel beach.The theory suggests that the cliffing problem may be significantly alleviated bycontrolling the sand percentage, which should not exceed a critical value of30 40%. It is also noted that the control of the sand percentage is only requiredfor the upper beach, or just the beach crest, and it is achievable throughmanaged use of sediment sources and improved sediment placementtechniques.Limited numerical modelling shows that the hydraulic conductivity of thesediment and the groundwater level both have significant effects on theevolution of the beach surface. Model simulations suggest that accretion on theupper beach face increases with increasing hydraulic conductivity. There is aclear need for improved numerical models specifically designed to deal withmixed sand-gravel beaches.The case studies included three sites: Pevensey Bay in East Sussex, Tankertonin Kent and Hayling Island in Hampshire. The analysis of the data collectedfrom the three sites highlights the importance of frequent and focused recyclingoperation and the widespread problem of cliffing. At the Pevensey site, thevolume of annual recycled material is of the same order as the annualmaintenance recharged material, leading to significantly reduced operationalcost while improving the efficiency of use of limited sediment resources. Thefield data also show that the high sand percentage coupled with an unnaturallysteep beach slope seems to be the predominant cause of the cliffing problem.Laboratory and field data indicate that a natural slope of a mixed sand-gravelbeach is around 1:9, but recharged beaches tend to have a design slope of 1:7. The experiences from the three sites indicated that reducing the sandpercentage in the upper beach had the positive effect of alleviating the cliffingproblem. Reduction of the sand percentage in the upper beach or beach crestmay be achieved in two ways: improved recovery technique at the point ofdelivery, and managed use of the sediment resources. The modified rainbowingtechnique experimented with at Pevensey is an example of the former, while thelatter needs additional regulations by the government.viExecutive Summary
ContentsExecutive Summary . iv1.Introduction . 11.1Background .11.2Objectives . 21.3Methodology . .21.4Outline of report . . 32.Literature Review .42.1Introduction .42.2Characterisation of sediment properties of mixed beaches . 42.3Sediment processes .52.4Profile Evolution 62.5Infiltration and Exfiltration .62.6Internal Flow and Hydraulic Gradient .72.7Wave Reflection .72.8Numerical Models .72.9Recharge Material .83.Theoretical Study of Mixed Sand-Gravel Sediment . .93.1Introduction .93.2Porosity Analysis .93.3Hydraulic Conductivity . 103.4Bulk Density 113.5Comparison with Experimental Results . 113.6Loading Experiment and Implications on Cliffing . 153.7Effects of Compaction .3.8Summary . 214.Performance of Gravel and Mixed Beaches . 224.1Introduction . 224.2The Beach Model . 224.3Test Conditions . 234.4Results and Discussions . 24Contents, List of Figures and Tables19vii
4.5Summary255.Influence of Groundwater Level on Beach Evolution . 325.1Introduction . 325.2Model (BeachWin) tests against field data . 335.3Model (BeachWin) tests against laboratory data . .335.4Summary .356.Case Studies . 446.1Introduction . 446.2Performance Issues of Recharged Mixed Beaches . 456.3Methods of Alleviating the Cliffing Problem 486.4Recycling of Material .6.5Summary 537.Review of Aggregate Production, Placement and Mixing . 557.1Introduction 557.2Overview of Aggregate Dredging Techniques . 557.3Aggregate Placement and Mixing Techniques 557.4Existing and Potential Aggregate Resource 578.Conclusions and Recommendations . .659.References and Bibliography 6910.Acknowledgements . .87viii53Contents, List of Figures and Tables
FiguresFigure 1.1Figure 3.1Cliffing photographed at Hayling Island (left) andPevensey Bay (right)2Sediment Grading of sand and gravel used forpermeability tests12Comparison between analytical prediction and measuredporosity (present study)12Comparison between analytical prediction and measuredporosity of13Comparison between analytical prediction and measuredpermeability (present study)13Comparison between analytical prediction & measuredpermeability of Mason (1997)14Relative hydraulic conductivity as a function of sandpercentage14Figure 3.7Collapsed “sand castle” (l 20%)16Figure 3.8Collapsed “sand castle” (l 30%)16Figure 3.9Different stages of “sand castle” test (l 40%)17Figure 3.10Collapsed “sand castle” (l 36%)17Figure 3.11Collapsed “sand castle” (l 40%)18Figure 3.12Collapsed “sand castle” (l 50%)18Figure 3.13Collapsed “sand castle” (l 100%)19Figure 3.14Effects of compaction on porosity20Figure 3.15Effects of compaction on hydraulic conductivity21Figure 4.1Schematic of beach model set-up22Figure 4.2Sediment characteristics of beach models23Figure 4.3Profiles recorded under monochromatic wave conditions(Condition B: f 0.53Hz, Hs 0.04m) with a mixed beachmodel25Profiles recorded under random wave conditions(Condition I: f 0.53Hz, Hs 0.052m) with a mixed beachmodel25Comparison of gravel (uniform grain) and mixed beachprofiles (Monochromatic wave conditions A: f 0.53Hz,Hs 0.02m)26Comparison of gravel (uniform grain) and mixed beachprofiles (Monochromatic wave condition B: f 0.53Hz,Hs 0.04m)26Figure 3.2Figure 3.3Figure 3.4Figure 3.5Figure 3.6Figure 4.4Figure 4.5Figure 4.6Contents, List of Figures and Tablesix
Figure 4.7Comparison of gravel (uniform grain) and mixed beachprofiles (Monochromatic wave condition C: f 1.05Hz,Hs 0.02m)27Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition I: f 0.53Hz, Hs 0.052m)27Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition K: f 1.05Hz,Hs 0.033m)28Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition L: f 1.05Hz, Hs 0.053m)28Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition E: f 1.05Hz, Hs 0.06m)29Comparison of gravel (uniform grain) and mixed beachprofiles (Monochromatic wave condition G: f 1.58Hz,Hs 0.04m)29Comparison of gravel (uniform grain) and mixed beachprofiles (Monochromatic wave condition H: f 1.58Hz,Hs 0.06m)30Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition J: f 0.53Hz, Hs 0.084m)30Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition P: f 1.58Hz,Hs 0.052m)31Comparison of gravel (uniform grain) and mixed beachprofiles (Random wave condition M: f 1.05Hz,Hs 0.069m)31Figure 5.1Wave flume, beach layout and instrumentation.34Figure 5.2Comparison of fine and coarse sand profiles for swellconditions (T 2.5s, H 0.15m), inland groundwater levelbelow SWL37Comparison of fine and coarse sand profiles for swellconditions (T 2.5s, H 0.15m), inland groundwater levelthe same as SWL37Comparison of fine and coarse sand profiles for swellconditions (T 2.5s, H 0.15m), inland groundwater levelabove SWL38Comparison of fine and coarse sand profiles for stormconditions (T 1s, H 0.1m), inland groundwater levelbelow SWL38Comparison of fine and coarse sand profiles for stormconditions (T 1s, H 0.1m), inland groundwater level thesame as SWL38Figure 4.8Figure 4.9Figure 4.10Figure 4.11Figure 4.12Figure 4.13Figure 4.14Figure 4.15Figure 4.16Figure 5.3Figure 5.4Figure 5.5Figure 5.6xContents, List of Figures and Tables
Figure 5.7Comparison of fine and coarse sand profiles for stormconditions (T 1s, H 0.1m), inland groundwater levelabove SWL39Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the coarse sand beach,inland groundwater level below SWL39Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the fine sand beach,inland groundwater level below SWL39Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the coarse sand beach,inland groundwater level the same as SWL40Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the fine sand beach,inland groundwater level the same as SWL40Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the coarse sand beach,inland groundwater level below SWL40Measured versus modelled groundwater levels for swellconditions (T 2.5s, H 0.15m) on the fine sand beach,inland groundwater level below SWL41Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the coarse sand beach,inland groundwater level below SWL41Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the fine sand beach, inlandgroundwater level below SWL41Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the coarse sand beach,inland groundwater level the same as SWL42Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the fine sand beach, inlandgroundwater level the same as SWL42Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the coarse sand beach,inland groundwater level below SWL42Measured versus modelled groundwater levels for stormconditions (T 1s, H 0.1m) on the fine sand beach, inlandgroundwater level below SWL43Figure 6.1Location of selected case studies44Figure 6.2Cliffing at Eastoke Hayling Island, photographed on24/03/200445Figure 5.8Figure 5.9Figure 5.10Figure 5.11Figure 5.12Figure 5.13Figure 5.14Figure 5.15Figure 5.16Figure 5.17Figure 5.18Figure 5.19Contents, List of Figures and Tablesxi
Figure 6.3Cliffing at Pevensey Bay, photographed on 02/07/200246Figure 6.4Cliffing at Pevensey Bay, photographed on 20/10/200546Figure 6.5Profile evolution due to consecutive swell and stormaction (Hayling Island)47Profile evolution due to consecutive swell and stormaction (Hayling Island)48Figure 6.7Cliffing recorded at Tankerton site (45% Sand)48Figure 6.8The state of the “Capped” bay in contrast to that of Figure6.749Figure 6.9Sediment mound and sample positions50Figure 6.10Photographs of sediment at different positions of themound51Figure 6.11Cross-shore size variation (a: mound 1; b: mound 2)52Figure 6.12Longshore size variation in (a) mound 1 and (b) mound 2.52Figure 6.13Cumulative new and recycled material53Figure 7.1Map showing usage of marine dredged sand and gravel59Figure 7.2Licensed dredge areas in UK waters62Figure 7.3Variation in use of dredged sand and gravel for beachreplenishment62Figure 6.6xiiContents, List of Figures and Tables
TablesTable 3.1Collapsing load of “sand castles”19Table 4.1Experimental wave conditions23Table 5.1Experimental runs for each grain size35Table 5.2Model and prototype parameters35Table 7.1Summary of primary aggregate (sand and gravel) sales58Table 7.2Use of permitted reserve (1973 to 2003)60Table 7.3Summary of permitted reserves of sand and gravel61Table 7.4Regional volumes for dredged sand and gravel (2004)63Table 7.5Relative volumes of land won and marine dredged sandand gravel64Matrix of recommendations67Table 8.1AppendicesAppendix 1 Literature ReviewAppendix 2 Case Studies and Review of Aggregate Production, Placementand MixingContents, List of Figures and Tablesxiii
1.Introduction1.1BackgroundAlthough interest in gravel and mixed sand-gravel beaches has increased inrecent years, processes on coarse-grained beaches are less well understoodthan on sand beaches. Most sediment transport models concentrate on sandsized sediment and surf zone processes and little field data are available toindicate the temporal and spatial variability of gravel and mixed beaches.The sediment processes on gravel and mixed sand-gravel beaches show twodistinct zones of activities, the surf zone and the swash zone. In the surf zone,the sediment particles are under the influence of the wave motion andturbulence created by wave breaking. The sediment movement is in the form ofbedload and suspended load. The swash zone on steep coarse-grainedbeaches is characterised by a violent breaking wave impact directly on thesediment particles followed by uprush and backwash. The beach in the swashzone is partially saturated and the sediment movement is primarily bedloadand/or sheet flow. Sediment transport in the swash zone is likely to be moresignificant on gravel beaches than on sand beaches (Van Wellen et al. 2000).The high percolation flow allowed within the sediment media makes a gravelbeach an extremely efficient system for absorbing the incident wave energy. Itforms an ideal system of coastal defence against storm attacks. In practice,pure gravel beaches are rare. In cases of beach renourishment schemes, therecharged material inevitably contains a varying amount of sand. At low sandpercentage, the mixed sand-gravel beach may be expected to function like agravel beach. As the sand fraction increases, the beach permeability is likely toreduce significantly and hydraulic performance of the beach may be greatlyimpaired. The question is then how the presence of sand affects theperformance of a mixed sand-gravel beach due to the reduced permeability ofthe sediment media.Recharged mixed sand-gravel beaches are a common means of sea defence inthe UK. Field experiences have revealed a number of significant problems inrelation to such schemes, including, critically, safety concerns as a result ofcliffing. Cliffs of up to two metre height are common among newly rechargedmixed beaches (Figure 1.1). These cliffs can be extremely hazardous due totheir natural instability and for health and safety of the public may requireremoval at the first opportunity. The question is what causes cliffing and howthe problem may be resolved or alleviated.Section 1. Introduction1
Figure 1.1 Cliffing photographed at Hayling Island (left) and PevenseyBay (right)1.2ObjectivesThis study aims to address issues such as cliffing, the influence of permeabilityon the performance of recharged beaches, sediment resources and theirmanagement, efficiency of sediment placement techniques, and costeffectiveness of frequent and focussed recycling operations. The detailedobjectives of the project are to produce a review of existing knowledge of the impacts of permeability onmixed and gravel beach performance; to investigate the cliffing problem of recharged mixed sand-gravel beaches; to examine possible methodology by which sediment properties on mixedsand/gravel beaches can be characterised; to carry out numerical modelling to improve understanding of the effect ofpermeability on beach profile response on mixed beaches, including therelative importance of parameters such as hydraulic conductivity, wavefriction factor, sediment grading, and groundwater flow; to propose recommendations for a framework of field and laboratory studiesto advance knowledge of the influence of permeability on beachperformance.1.3MethodologyFive distinct approaches were employed in this study:1) Literature reviewThis provided an overall view of the current state of knowledge andunderstanding in relation to gravel and mixed sand-gravel beaches.2) Theoretical analysisThis involved firstly the development of theoretical equations describing therelationship between the porosity and permeability of a mixed sediment andthe sand percentage of the sediment mix. The theory was then validated by2Section 1. Introduction
laboratory experiments. Additional series of experiments were carried out toestablish the relationship between the sand percentage and cliffing problem.3) Re-analysis of existing gravel and mixed beach experimentsThis examined the performance of a mixed sand-gravel model beach incontrast to a gravel beach model results. This provided an indication of theinfluence of permeability on the performance of the beach.4) Numerical modellingThis involved the use of a numerical model to show the influence of thegroundwater level on the beach profile evolution.5) Case studiesBy looking at three current/recent beach recharge programmes, significantissues in relation to beach recharge operations were identified and possiblesolutions proposed.1.4Outline of ReportThe report consists of 10 chapters as follows:1. Introduction2. Literature review3. Theory of bimodal mixed sand-gravel sediment4. Performance of mixed beaches versus gravel beaches5. Groundwater level and beach evolution6. Case studies7. Review of aggregate production, placement and mixing8. Conclusions and recommendations9. References10. AcknowledgmentsSection 1. Introduction3
2.Literature Review2.1IntroductionThe literature review was based on a collection of 275 publications, includingjournal and conference papers and government reports. The reviewsummarises the current state of knowledge on mixed beaches and, in particular,the effects of permeability on gravel and mixed beach behaviour. Typicalproblems faced by those responsible for managing mixed beaches include the inability to determine the sensitivity of the beach profile and crosssectional area to variations in sediment distributions, poor predictive capacity for cross-shore response of mixed sedimentbeaches to storms and their recovery after storms, uncertainty in predicting longshore or offshore losses of recharge sedimentover time, inability to predict beach response in the vicinity of coastal structures, and inability to predict the importance of seepage through barriers.Some of the fundamental research questions and challenges identified by thisreview relate to understanding the difference between coastal dynamics on mixed sand andgravel beaches and beaches dominated by relatively uniform sediment sizes(either sand or gravel) developing methodology to quantify and/or classify complex, spatially andtemporally variable sediment characteristics effectively on mixed sand andgravel beaches developing methodology to parameterise the effects of bimodal sedimentsand mixed sand and gravel in hydrodynamic and morphodynamic models adapting existing numerical models to predict the processes andmorphological evolution of mixed sand and gravel beachesA detailed report of the literature review is included in Appendix 1 (Review ofMixed Sand and Gravel Beaches), and the following summarises the mainfindings of the review.2.2Characterisation of Sediment Properties of MixedBeachesMixed beaches show a high degree of variability, both spatially and temporally,in terms of key parameters such as sediment size and shape, sorting, hydraulicconductivity, permeability, porosity, specific yield and moisture content. In4Section 2. Literature Review
particular, the amount of air contained in beach sediments is likely to varyacross the beach profile and also temporally, at both tidal and wavefrequencies, and can significantly reduce hydraulic conductivity. The degree ofcompaction, and hence porosity, of sediment is also highly variable, particularlyin the swash zone. However, very few field measurements of these parametershave been reported in the literature.No standard method is available for characterising bimodal sediments. Thedegree of bimodality and the nature of the mixture has been shown to beimportant in the initiation of motion, sediment transport, and beach profileevolution, and should be included in sediment parameterisation.The percentage of sand in a mixture has been suggested as a simple indicatorof the performance of a mixed beach. However, the percentage of sand on ahighly mixed beach is not easy to determine and is probably not constant overtime.Properties such as sediment sorting, particle shape and packing have a majoreffect on porosity and sediment transport of mixed sediments; these are alsohighly variable on mixed beaches, but hard to reproduce in laboratoryexperiments.2.3Sediment ProcessesSediment transport is affected by the relative proportion of sand
consumer waste and 20% chlorine-free virgin pulp. PB No. 12527/25. iv Executive Summary Executive Summary Many of the beaches on the UK coast that constitute the main defence against . morphological evolution of mixed sand and gravel beaches No standard method is yet available for characterising the sediments of a mixed
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Constant Head Falling Head Permeability Cell 38-T0184/C11 Constant Head permeability cell, 75 mm internal dia., 3 take-off points. 38-T0185/C12A Falling Head permeability cell, 100 mm internal dia. complete with 75-micron gauze and 2 m of tubing conforming to BS EN ISO 17892-11. 38-T0184/C2 Constant Head permeability cell, 114 mm internal dia.,
Most in situ permeability tests are interpreted by assuming isotropy of permeability (3, 8). The permeability of a soil may be calculated from the results of variable-head permeability tests from the standard expression A k - F T (1) From the results of constant-head tests more commonly used in the United Kingdom,
