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Properties of DredgedMaterial

Final Report

Report SR 517January 2000

Properties of Dredged Material

Final Report


��Address and Registered Office: HR Wallingford Ltd. Howbery Park, Wallingford, OXON OX10 8BATel: +44 (0) 1491 835381 Fax: +44 (0) 1491 832233

Registered in England No. 2562099. HR Wallingford is a wholly owned subsidiary of HR Wallingford Group Ltd.

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Contract

This report describes work funded by the Ministry of Agriculture, Fisheries andFood under Contract CSA 3112. Publication implies no endorsement by theMinistry of Agriculture, Fisheries and Food of the report’s conclusions.

The MAFF project code was AE0228. The HR Wallingford Job Number wasDDS 0006. The work was carried out by Mr Nigel Feates, Ms Helen Mitchenerand Dr Jeremy Spearman. The Project Manager was Dr Mike Dearnaley. TheMinistry’s Nominated Project Officers were Dr Lindsay Murray and Dr PaulGurbutt. HR’s Nominated Project Officer was Dr Stephen Huntington.

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© Ministry of Agriculture, Fisheries and Food 2002

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Executive Summary

Properties of Dredged Material

Final Report


HR Wallingford was commissioned by the Ministry of Agriculture, Fisheries andFood (MAFF) to carry out a study aimed at investigating the physical properties ofdredged material and the importance of these properties with regards to disposal atoffshore locations.

Since the winter of 1995/1996 minipods have been deployed at various locationsaround the periphery of disposal sites offshore of Harwich and the River Tees onthe east coast of England. A minipod is a multi-parameter bottom landerdeveloped by CEFAS (Centre for Environment, Fisheries and AquacultureScience), an agency of MAFF. The purpose of the minipod deployments wasprimarily to measure the hydrodynamic conditions that exist at the sites as an aidto predicting the likely level of dispersion of the material being placed at thedisposal site. The duration of the minipod deployments was typically five to sixweeks.

The physical properties of the dredged material at the two sites was measured foreach of the three phases of the dredging cycle i.e. pre-dredging, dredging and post-dredging. Measurements of the physical properties were made both in the fieldand in the laboratory. In the field, specially designed instruments were used tomeasure the critical erosion threshold in-situ. The instruments used to make thesemeasurements were ISIS (Instrument for Shear Stress In-Situ) and its developmentSedErode (Sediment Erosion device). Laboratory measurements were made onbulk samples obtained from each of the sites.

The results of the field studies at the Harwich disposal site were considered ofsufficient interest and value to be worthy of comparing predictive modellingtechniques for the dispersion of placed material with the field observations.

This Final Study Report brings together an extensive programme of researchundertaken under this contract. Supporting information is provided in a series ofTechnical Reports which give details of the various studies carried out.

For further information on this study please contact Dr Mike Dearnaley orMr Nigel Feates in the Dredging and Sedimentation Group at HR Wallingford.

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ContentsTitle page iContract iiiExecutive Summary vContents vii

1. Introduction ................................................................................................ 11.1 Background.................................................................................... 11.2 Objectives ...................................................................................... 11.3 Approach ....................................................................................... 11.4 Technical Report Source List ........................................................ 21.5 Report structure ............................................................................. 2

2. Knowledge Enhancement ........................................................................... 32.1 Literature review............................................................................ 32.2 Issues associated with dispersion from offshore disposal sites ..... 32.3 Consolidation of dredged material................................................. 42.4 Beneficial use of dredged material ................................................ 4

3. Previous information on properties of dredged material ............................ 43.1 Processes of erosion and deposition of sediment........................... 43.2 Dispersion of dredged material...................................................... 53.3 Laboratory tests on mud ................................................................ 63.4 Laboratory tests on sand ................................................................ 63.5 Laboratory tests on mud/sand mixtures ......................................... 63.6 Erodability tests on mudflats ......................................................... 73.7 In-situ erosion flumes .................................................................... 73.8 Anecdotal evidence........................................................................ 7

4. Field Measurements.................................................................................... 74.1 Minipod deployments .................................................................... 8

4.1.1 Instrumentation................................................................. 84.1.2 Data logging configuration............................................... 9

4.2 In-situ erosion threshold measurements ........................................ 94.2.1 NIOZ corer sampling methodology.................................. 94.2.2 Hopper sampling methodology....................................... 104.2.3 Critical shear stress measurement procedure.................. 114.2.4 Other measurements and observations ........................... 11

4.3 Summary of field measurements ................................................. 11

5. Measurements at the River Tees............................................................... 125.1 Tees physical properties .............................................................. 13

5.1.1 Summary of data............................................................. 135.1.2 Analysis of data .............................................................. 14

5.2 Tees hydraulic environment ........................................................ 155.2.1 Summary of data............................................................. 155.2.2 Analysis of data .............................................................. 16

6. Measurements at Harwich ........................................................................ 176.1 Harwich physical properties ........................................................ 17

6.1.1 Summary of data............................................................. 18

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Contents continued

6.1.2 Analysis of data .............................................................. 206.2 Harwich hydraulic environment .................................................. 20

6.2.1 Summary of data ............................................................ 216.2.2 Analysis of data .............................................................. 22

6.3 Coring at the WD Fairway placement site .................................. 23

7. Findings and their Application ................................................................. 237.1 Sediment variability .................................................................... 247.2 Probability of erosion .................................................................. 247.3 Application in numerical modelling............................................ 25

7.3.1 Introduction .................................................................... 257.3.2 Description of the placement.......................................... 267.3.3 Methodology .................................................................. 267.3.4 Results ............................................................................ 267.3.5 Discussion and conclusions............................................ 27

8. Conclusions and recommendations .......................................................... 278.1 Field measurements..................................................................... 278.2 Measurements at the River Tees.................................................. 28

8.2.1 Pre-dredging phase ......................................................... 288.2.2 Dredging phase............................................................... 288.2.3 Post-dredging phase ....................................................... 288.2.4 Minipod measurements .................................................. 29

8.3 Measurements at Harwich ........................................................... 298.3.1 Pre-dredging phase ........................................................ 298.3.2 Dredging phase............................................................... 308.3.3 Post-dredging.................................................................. 308.3.4 Minipod measurements .................................................. 31

8.4 Findings and their Application .................................................... 318.5 Recommendations ....................................................................... 31

9. Acknowledgements .................................................................................. 32

10. References ................................................................................................ 33

TablesTable 1 Timetable of field measurementsTable 3 Surface sediment properties – Dredging phaseTable 4 Surface sediment properties : Post-dredging phaseTable 5 Surface sediment properties – Pre-dredging phaseTable 6 Surface sediment properties – Dredging phaseTable 7 Surface sediment properties – Post-dredging phase

FiguresFigure 1 Location PlanFigure 2 Silt content versus critical shear stress - TeesFigure 3 Bulk density versus critical shear stress - TeesFigure 4 Silt content versus bulk density - TeesFigure 5 Tees disposal site

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Contents continued

Figure 6 Example of turbidity and wave data - TeesFigure 7 Dredged material placements at the Tees disposal siteFigure 8 Particle vector diagram for a placement at the Tees disposal siteFigure 9 Example ABS burst data from the Tees disposal siteFigure 10 Harwich Harbour and disposal siteFigure 11 Silt content versus bulk density - HarwichFigure 12 Bulk density versus critical shear stress - HarwichFigure 13 Silt content versus bulk density - HarwichFigure 14 Example of turbidity and wave data - HarwichFigure 15 Turbidity levels before, during and after dredging - HarwichFigure 16 Particle vector diagram for a placement at the Harwich disposal siteFigure 17 Example ABS burst data from the Harwich disposal siteFigure 18 Probability of shear stress exceedance at the disposal sitesFigure 19 Predicted suspended sediment concentration and observed turbidity at

Threshold, simulation of dispersion of fine material initially releasedin water column

Figure 20 Predicted suspended sediment concentration and observed turbidity atThreshold, simulation of dispersion of fine material resuspended fromthe bed

Figure 21 Comparison of observed turbidity and observed wave conditionsduring placement by W.D.Fairway

Figure 22 Comparison of observed turbidity and observed near bed tidal currentspeed during placement by W.D.Fairway

PlatesPlate 1 Minipod aboard RV Corystes prior to deploymentPlate 2 Minipod instrumentationPlate 3 Syringe water samplersPlate 4 The NIOZ corer being deployed from RV CirolanaPlate 5 ISIS head unit positioned on a typical core samplePlate 6 SedErode being deployed in both sleeve and tray modesPlate 7 Sediment surfaces as collected and after surface smoothingPlate 8 An example of bed layering at the Tees disposal sitePlate 9 Interface between material types - Landguard FortPlate 10 Example of exposed surface following WD Fairway placement

AppendicesAppendix 1 ISIS – Instrument for Shear stress In-SituAppendix 2 SedErode – Sediment Erosion Device

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1. INTRODUCTION

1.1 BackgroundLittle is known about the physical properties of dredged material once placed at an offshore disposal site.Few specific measurements have been made, particularly of maintenance dredged muddy material and themain source of information tends to arise from anecdotal information from Port Authorities and DredgingContractors during the course of their management of disposal activities.

Dredged material is likely to consist of a mixture of mud, sand, gravel and stone in varying proportions.The dredging process itself will change many of the in-situ bed sediment properties of the material. Thismay be by mixing (so any 3-D structure will be lost), the addition of water, loss of fine material, or achange in the chemical environment (e.g. anoxic to an oxic state) or break-up of biological algal growth,burrows, and/or secretions. Other changes may include pH, salinity, pore water pressure, temperature,density, texture and colour.

HR Wallingford was commissioned by MAFF in 1995 to carry out a study aimed at investigating thephysical properties of dredged material and the importance of these properties with regards to disposal atoffshore locations. This report summarises the work done as part of this project.

1.2 ObjectivesPrediction of the behaviour and ultimate fate of dredged material placed on the sea or estuary bed isrequired if informed decisions are to be made concerning legislation and licensing of disposal methods andsites. The purpose of the research undertaken for this study is to improve knowledge of the properties ofreal dredged material. This information will allow predictive methods to be made more reliable in theirrepresentation of erosion and transport. This, in turn, will enable those responsible for legislation andlicensing to make informed decisions concerning the environmental impact of disposal in a wide range ofsituations. Most, if not all, existing models assume homogeneous material, either cohesive (mud) orcohesionless (sand and gravel). In reality much of the material dredged around the UK is a mixture ofmaterial types ranging from a slurry arising from maintenance dredging in silts to mixtures of rock andclay generated with bucket dredgers. The physical properties of these mixtures, particularly the erodibilityafter placement, are little known. Determining the erosion and consolidation characteristics of thematerial, once place on the bed, is an important step in predicting the environmental impact of theplacement of the material.

From the MAFF dredging licence database for the UK (1985 to 1994) approximately 85% of the totaltonnage of material placed at disposal sites is generated from maintenance dredging. The remaining 15%is the result of capital dredging work. For this reason the research undertaken during the course of thisstudy focuses on the behaviour of maintenance material.

1.3 ApproachIn order to obtain the best measurement of the critical erosion threshold of a surface layer the measurementshould be made in-situ. At the time of writing the proposal for this study there were no devices availablefor making sub-tidal measurements of the critical erosion threshold. It was therefore originally proposedthat large bulk samples of dredged material be transported from the dredging site to HR for investigation ina flume.

A concern regarding the flume type experiment was the likelihood that the physical properties of thematerial may have significantly changed as a result of being handled and transported over a period of time.For this reason alternative approaches were investigated. During the course of early minipod deploymentsa large diameter NIOZ corer was deployed and was found to provide a suitable in-situ sample of materialon which to use the ISIS instrument for measuring erodibility of material. This combination of instruments

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proved to be highly flexible and the large scale laboratory experiments were replaced with on board shipmeasurements which were able to represent more closely the in-situ physical properties of the material.

It was thus agreed that the investigations should be based around obtaining large undisturbed seabed coresfrom an area over and surrounding the disposal site of interest. The critical erosion threshold of theexposed surface would be measured by use of the HR in-situ erosion bell. The physical properties of thematerial were determined prior to dredging, within the hopper of the trailer dredgers used for the dredgingand post placement at the disposal site. For material collected from the pre-dredging and post-dredgingphases of the dredging cycle the HR erosion bell ISIS (Appendix 1) and its development SedErode(Appendix 2) would be used. For material collected during the dredging phase, from the hopper of thedredger, SedErode was to be used.

In parallel with the measurement of the physical properties of dredged material a series of fieldmeasurements were carried out at the disposal sites. These were undertaken by CEFAS, Lowestoft toexamine the nature of the hydrodynamic and sediment environments that occur at the disposal sites atwhich the physical properties of the placed material were being measured.

1.4 Technical Report Source ListThe studies summarised in this report have been described in more detail in previous Technical Reports.The full list of these reports is as follows:

TR 14 Properties of Dredged Material. Erosion shear stress measurements on seabed cores taken fromSellafield mud patch 26 May – 31 May 1996.

TR 17 Development of SedErode, Instrument for in-situ mud erosion measurements

TR 21 Properties of Dredged Material. Review of available measurement techniques for determiningphysical properties.

TR 44 Shear stress measurements on seabed cores taken from Lowestoft Harbour.

TR 46 Properties of Dredged Material, Measurement of sediment properties of dredged material fromHarwich Harbour.

TR 47 Properties of Dredged Material, Minipod deployments at the Roughs Tower disposal site.

TR 53 Properties of Dredged Material, Harwich minipod deployments – Winter 1997.

TR 54 Properties of Dredged Material, Measurement of sediment properties of dredged material from theTees estuary.

TR 61 Properties of Dredged Material, Minipod deployments at the Tees disposal site.

TR 63 Measurement of sediment properties of reclamation material from Parkstone Yacht Club.

TR 72 Beneficial Use of Dredged Material, North Shotley.

1.5 Report structureThe remainder of this report is in six chapters. The importance of the study in terms of knowledgeenhancement is described in Chapter 2. The state of knowledge of sediment properties is discussed inChapter 3. In Chapter 4 the instrumentation and methodology used in the study is described. The results

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of the field measurements are presented in Chapter s 5 and 6. Chapter 7 describes some applications of thefield measurement results. The conclusions arising from the study are presented in Chapter 8.

2. KNOWLEDGE ENHANCEMENT

2.1 Literature reviewAs part of this study a review of available measurement techniques for determining the physical propertiesof sediments (including dredged material) was carried out (Reference 3). The objectives of the literaturereview were to review the techniques and methods that are presently used to evaluate sediment propertiesand to evaluate laboratory and field measurements that relate to dominantly cohesive or mixed cohesivesediments. The review also aimed to highlight any changes that may result during dredging of cohesivebased sediments and to relate this to any relevant literature.

In the review the laboratory and field measurement of the physical properties of dredged material aredivided into two categories, inherent properties and behavioural properties. In the inherent propertiescategory the measurement of density, density profile, vertical structure, grain size distribution, chemicalcompound and environment and rheological parameters are discussed. In the behavioural propertiescategory the measurement of erosion, settling and consolidation are discussed.

Finally the review summarises a number of other studies undertaken relating to dredged material.

• The use of radioactive sediment tracers to study the release, transfer properties and distribution overthe seabed of dredged materials at sites in France and Belgium.

• Stiff or highly plastic clays often form clay balls, which slow down the transport process. Tests werecarried out to simulate of the degradation of clay balls in dredge pipes by exposing clay lumps toagitation in a rotating drum.

• The geotechnical properties needed for a complete and adequate evaluation when considering dredgingrequirements are widespread. A computer program providing guidance of sampling and testingtechniques that can be used at a single exploration site during a subsurface investigation for dredgingprojects has been written.

• A study that defines the standard dredging related descriptors that may be used to give an indication ofthe dredging requirements of in-situ sediments.

2.2 Issues associated with dispersion from offshore disposal sitesOffshore disposal sites are generally monitored by regular bathymetric surveys. The results of such asurvey may indicate that dredged material disposed of at the site is remaining on the seabed and that thesite is non-dispersive for that particular material. Alternatively the survey may show that the bathymetryhas not significantly changed since the previous survey despite the disposal of a large volume of dredgedmaterial. In this case the site is deemed dispersive and the material has dispersed from the disposal siteeither during the disposal process or from the seabed some time after disposal.

Dredged material that disperses from a disposal site is likely to be re-deposited elsewhere. It may takesome time to find the new deposition site and during this period may influence local suspended solidsconcentrations significantly, possibly leading to the smothering of nearby shellfish beds. The rate ofdispersion of material from a site is therefore of particular interest as is the potential for material todisperse from a site under extreme conditions.

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2.3 Consolidation of dredged materialFrom bulk density measurements carried out on the surface layer of cores retrieved from the seabed anindication of the level of consolidation of dredged material placed at the site can be obtained. Given thedynamic nature in which the dredged material is normally placed at a site the issue of consolidation is onlylikely to be important at low energy sites.

2.4 Beneficial use of dredged materialBeneficial uses of dredged material may include use for coastal defence, nature conservation initiatives orhabitat creation. Before any decisions are made regarding beneficial use applications the suitability of thesupply material must be determined. If, for example, the dredged material is to be used for a habitatcreation scheme information relating to the physical and chemical properties of the material (density,resistance to erosion, contaminants etc.) must first be sought. The potential for dispersion of material fromthe site also needs to be considered.

3. PREVIOUS INFORMATION ON PROPERTIES OF DREDGED MATERIAL

Historically, sediments have been treated as either muds or sands because the characteristics and resultantbehaviour are very different. As a result there are well-established sediment transport formulas for non-cohesive sediments, and some for muddy sediments, but few parameterisations exist for true sedimentmixtures. Most literature references to mixed sediment behaviour is incidental and describes the effect ofadding sand to mud or vice versa, but still treating the sediment as if it were a mud or a sand.

3.1 Processes of erosion and deposition of sedimentThe amount of sediment carried in suspension in the water column is principally dependent on two factors:

• The ability of the currents (and/or waves) to resuspend material from the bed• The ability of the currents (and/or waves) to keep material in suspension

For non-cohesive sediments the first of these factors can be examined by considering the sum of forces onan individual grain. Such a procedure leads to the conclusion that when the drag force exerted on the fluidby the grain exceeds a certain threshold, the grain will move. The point at which the grain moves can becalculated analytically for different grain sizes, assuming certain packing and grain geometry, to give athreshold function dependent on grain size. This function is classically stated in terms of a threshold ofbed shear stress. Shields (Reference 4) conducted a large number of laboratory experiments over a varietyof sediment types, and produced the well known graphical results linking non-dimensional flow conditionsto non-dimensional erosion threshold.

For cohesive sediments classical theory adopts the same approach to the threshold of erosion of material.However, for cohesive material the balance of forces is not between the weight of particle and the dragforce, but between the attractive physico-chemical forces imposed on a particle by its neighbours and thedrag force. The cohesive attraction experienced by an individual particle is not well described by scienceand therefore the threshold at which erosion can occur tends to be obtained through laboratory, or morerecently in situ, experiments on mud samples.

Once the critical threshold has been exceeded, classical theory (e.g. Reference 5) gives the rate of erosionof sediment (by mass) of both non-cohesive and cohesive material to be proportional to excess bed shearstress above the threshold,(τ-τe), with the constant of proportionality again dependent on the substance inquestion, and therefore empirically determined. The coarser sand particles do not tend to move very farupwards through the water column, because of their higher settling velocities, and sediment transport forthese particles cannot be thought of as strict suspension but as a series of leaps and jumps. For this reasonthe transport of such sediment along the bed has often been described by holistic empirically-based

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equations which lump together the processes of erosion/deposition and advection of such particles – e.g.Van Rijn (References 6 and 7).

In an analogous manner, the settling of material from suspension onto the bed is also described by classicaltheory using a critical threshold of bed shear stress for deposition. Once bed shear stress falls below thisthreshold, the rate of deposition (by mass) to the bed is proportional to the product of the settling velocityand the concentration field. Krone (Reference 8) gives the constant of proportionality as (1-τ/τd).Essentially this is a linear interpolation between the state of no flow (τ=0) when material settles at the ratedetermined by the concentration and the settling velocity and the threshold state when no settling can occur(τ=τd).

The principal of a threshold for deposition is open to question in the physical sense. Its use prescribes thedeposition of material at levels of bed shear stress greater than the erosion threshold, which observationshave shown not to be the case. This discrepancy results from the use of bed shear stress to describeresuspension/deposition, rather than turbulence. It can be demonstrated that bed shear stress isproportional to the intensity of turbulence at the bed under clear water conditions. However, use of bedshear stress as the fundamental parameter describing deposition does not allow for the damping effect ofincreasing concentration on the turbulence field itself. A more physics-based approach is to consider theturbulence-induced movement of sediment particles near the bed, the so-called gradient diffusionapproach.

For sand suspensions, the settling flux of grains near the bed can be equated to the counterbalancingupward turbulent diffusion. If some simplifying assumptions are made about the variation of eddydiffusivity with distance from the bed, then a “saturation” through-depth concentration profile can bederived. Different assumptions produce different types of profile, such as the well known Rouse profile orthat proposed by Van Rijn (Reference 7). The shape of such profiles is further (empirically) dependent onthe erosion threshold of bed shear stress. This approach gives different behaviour to the classical approachsince if a saturated concentration experiences any reduction in current speed, even at very high speeds,there will be reduced concentrations in the water column and therefore deposition. Using the sameapproach to derive a similar concentration profile for mud is prevented by the uncertainty concerning theprocesses that occur in muds very near to the bed. However recent work by Galland et al (Reference 9)and Winterwerp (Reference 10) has shown that the depth-averaged “saturated” concentration can bederived which increases with bed shear stress and decreases with depth and settling velocity.

The approach of equating upward turbulent diffusion with the settling flux is itself a simplification of thereal system as it represents a time and space averaged view of turbulent diffusion “events” and assumesthat the scale of these events is small compared to the scales of the relevant situation. In reality the systemconsists of a series of discrete random turbulent “bursts”, which carry sediment away from the bed,followed by a compensating “inrush” of fluid towards the bed. Hogg et al (Reference 11) have shown thatby considering the processes involved in individual “bursts”, the gradient-diffusion approach can be re-derived.

3.2 Dispersion of dredged materialStudies have been undertaken to track the dispersion of material in suspension away from a dredgingoperation. Methods of plume tracking have included adding tracers to the load prior to disposal, usingwater-sampling techniques and using a vessel mounted Acoustic Doppler Current Profiler (ADCP). AnADCP, when used in backscatter mode, can give a visual indication of plume size and concentration.These approaches have been reviewed in a recent CIRIA study (Ref RP600).

Computer models can be used to predict the dispersion of dredged material from a point source. Theability of such models to make an accurate prediction is limited by the reliability of the dispersionalgorithms within the model. Such algorithms are based on information acquired from measurementsmade in the field. Further field measurements of plume dispersion will provide information that will allow

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computer models to be made more reliable in their predictions of erosion and transport. The Consortiumof Dutch Dredging Contractors (VBKO) are funding research and field measurements in this area(Reference 12).

3.3 Laboratory tests on mudThe vast majority of laboratory tests on mud have been aimed at measuring the three principal physicalproperties of settling, consolidation and erosion.

In general settling and consolidation tests have been carried out using settling columns whereby the mud ismixed into a suspension before being introduced to the top of the column filled with water. From theextraction of small water samples at various heights in the column over a period of time informationrelating to the settling characteristics can be obtained. From the regular measurement of the height of thesettled mud/water interface information relating to the consolidation characteristics can be obtained.Information relating to the density structure of the settled mud bed can be obtained from sampling orprofiling with a non-intrusive device such as a nuclear transmission probe.

Laboratory measurements of critical erosion threshold have been made in straight, annular and waveflumes. There are many methods for forming a mud bed in the working section of a flume. Essentiallythese consist of either physically shovelling and levelling the mud bed or settling it out of suspension byfirst filling the flume with a mud slurry. Both of these methods have drawbacks in terms of howrepresentative they are of a naturally deposited and consolidated mud bed. Mud beds settled fromsuspension have low bulk densities unless left to consolidate for long periods. Mud beds of this type aregenerally untypical of a natural exposed mud surface, but may more closely represent dredged material onthe bed that has been recently disposed of from a dredger. Higher mud densities may only be achieved bythe shovelling and levelling technique, the process of which invariably changes the structure of thesediment. Where it is not practicable to obtain large bulk samples of material it may be possible to use abox corer to obtain smaller samples that may be investigated in a modified flume. However, erosion of themud surface in a box core may often be controlled by the box core itself unless a sub area of the coresurface can be isolated.

3.4 Laboratory tests on sandLaboratory tests carried out to measure the physical properties of sand are similar to those carried out onmud, as briefly described above. As sand is non-cohesive the settling, consolidation and erosion propertiesare quite different to those of mud. In an erosion flume, flowing water over a bed of sand will tend togenerate bed load transport in the form of sand waves that move downstream rather than the sand particlesbeing lifted in suspension, as would be the case for mud.

3.5 Laboratory tests on mud/sand mixturesNatural sediments rarely consist uniformly of either mud or sand and the majority of estuarine sedimentsare comprised of a combination of grain sizes comprising sands, silts and clays, and even gravels. Themineralogical make-up is also varied as sand is primarily quartz, but clay particles are comprised of metalsilicates in different chemical forms. Other sediment constituents may include organic particles andpolymers, oil, and shell fragments.

The fabric of the sediment becomes important when considering sediment mixtures because thesesediments are commonly deposited in layers, with a degree of vertical sorting. This means that naturalundisturbed sediment may be comprised of layers of mud and sand and thus its sediment transportproperties will reflect it’s structural composition. The sediment may then undergo mechanical andbiological reworking which will break up the vertical structure, as well as compaction, consolidation,erosion and further deposition. Mixed sediments are thus classified as homogeneous or layered sedimentand are in practice a complex, three-dimensional combination of textural and sedimentologicalconstituents.

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Few laboratory tests have been carried out to measure the physical properties of mud/sand mixtures. Testprocedures for measuring properties relating to deposition, consolidation and erosion would be similar tothose described for mud above.

3.6 Erodability tests on mudflatsDevices used for carrying out erodability tests on inter-tidal mudflats are generally portable devices thatcan make a measurement relating to erodability within a few minutes. Some instruments measure thecritical shear stress directly by pumping an eroding medium over the exposed surface at progressivelyhigher discharges. From the measurement of the turbidity of the eroding medium the point at whicherosion begins can be identified by a turbidity jump. From the associated discharge the critical shear stresscan be determined.

Other devices measure the erodability indirectly. For example the bed strength may be determined by themeasurement of the acoustic shear wave velocity. Though this is not a direct measurement of the criticalerosion threshold the measured data can be calibrated in order that it can be inferred.

3.7 In-situ erosion flumesAn in-situ erosion flume tends to be a larger, less portable device that is placed on an inter-tidal mud bed atlow water and left in place until the following low water when it may be recovered. Generally thesedevices have the disadvantage that only one measurement of critical shear stress can be made during eachtidal cycle. They do however have the advantage that they cover larger areas of the bed than truly portabledevices.

3.8 Anecdotal evidenceMost licensed disposal sites around the coast of the UK have been used on a regular basis for decades. It iscommon for these sites to be surveyed regularly to ensure that any placements of material are not posing athreat to navigation. The majority of these sites show only very small changes in bathymetry despite theplacement of large quantities of maintenance dredged material. This indicates that the sites are dispersive.An example of such a site is the Roughs Tower at Harwich. Disposal of material arising from capitaldredging may lead to significant changes and even the eventual filling of a site.

4. FIELD MEASUREMENTS

Prediction of the behaviour and ultimate fate of dredged material placed on the sea or estuary bed isrequired if informed decisions are to be made concerning legislation and licensing of disposal methods andsites.

The purpose of the field measurements undertaken for this study is to improve knowledge of the propertiesof real dredged material. This information will allow predictive methods to be made more reliable in theirpredictions of erosion and transport. This, in turn, will enable those responsible for legislation andlicensing to make informed decisions concerning the environmental impact of disposal in a wide range ofsituations.

Two primary methods were used in the collection of the field measurements during the course of thisstudy. Firstly, data was collected relating to the physical properties of dredged material during the threephases of the dredging cycle i.e. before, during and after dredging. Secondly the hydrodynamic andsuspended sediment regimes that existed at the disposal sites were measured.

In the case of the measurement of the physical properties of dredged material the field data collected wasbased on obtaining large undisturbed seabed cores from an area over and surrounding the disposal site ofinterest. The critical erosion threshold of the exposed surface was measured by the application of the HRin-situ erosion bell, ISIS and its development SedErode

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Field measurements to examine the nature of the hydraulic environment that occurs at the disposal siteswere undertaken by CEFAS, Lowestoft using a specially developed seabed lander called a ‘minipod’.

The timetable of fieldwork carried out during the course of this study is provided in Table 1. The fieldmeasurements are described in detail in References 13, 14, 15, 16, 17 and 18, and summarised in Sections4.1 and 4.2 below. The results of the field measurements are provided in Chapter 5.

4.1 Minipod deploymentsThis section describes the MAFF CEFAS minipod and its associated instrumentation.

The minipod is a bottom lander developed by CEFAS (Centre for Environment, Fisheries and AquacultureScience, an agency of MAFF (Ministry of Agriculture, Fisheries and Foods). The minipod was originallydesigned to enable an estimate of the sediment concentration near to the seabed to be made. The purpose ofthe minipod deployments made during this study was primarily to characterise the hydrodynamicconditions that exist at the site as an aid to predicting the likely level of dispersion of the material beingplaced at the disposal site. The duration of the minipod deployments was typically five to six weeks.Plate 1 shows a minipod aboard the MAFF research vessel RV Corystes prior to deployment.

4.1.1 InstrumentationEach minipod was fitted with a set of sensors to provide information relating to the hydrodynamicconditions that exist at the deployment site. Generally all deployments were fitted with the same set ofinstruments. Different logging regimes may be triggered by changes in significant wave height. Each ofthe minipod instruments are briefly described below.

Current meterA single Marsh McBirney electromagnetic current meter was used to measure current speeds in the X andY horizontal planes. From this information, and knowing the orientation of the minipod, current velocitiesand directions can be derived. The current meter was typically fixed at a height of about 0.4 m above thebed. The current meter is shown fitted to a minipod in Plate 2.

Suspended sediment sensorPoint measurements of the suspended sediment concentration were obtained by the use of a MiniatureOptical Backscatter Sensor (MOBS). For each of the minipod deployments two MOBS were fitted to theminipod to measure the suspended sediment concentration at two heights above the bed. The MOBS weretypically fixed at heights of 0.54 m and 0.72 m above the bed. The MOBS are shown fitted to a minipod inPlate 2.

Acoustic backscatter sensorA two-frequency (1 MHz and 6 MHz) Acoustic Backscatter Sensor (ABS) was used to measure thevertical profile of suspended sediment concentration between the sensor and the bed. The data provided bythe ABS, which is in terms of nominal backpressure, may also be used to make an estimate of the sedimentsize distribution. The ABS was typically fixed at a height of 0.8 m above the bed. The ABS is shownfitted to a minipod in Plate 2.

Pressure sensorTidal elevation and wave statistics (significant wave height, wave period and bed orbital velocity) werederived from a Digiquartz pressure sensor installed at a height of 1.76 m above the bed.

Syringe water samplerTo provide water samples with which to aid the calibration of the optical sensors a number of syringewater samplers were fitted to the minipod. The samplers, which have a capacity of 1.8 litres, operate by

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moving a piston within a cylinder to draw water from an intake nozzle fixed to one of the minipod legs atthe same height above the bed as the MOBS are fitted. The control system allows each syringe to beprogrammed independently, firing when a certain criteria is met. Plate 3 shows syringe water samplesmounted on a minipod.

Sediment trapTo provide a sample with which to calibrate the acoustic backscatter sensors sediment traps have beenused. The sediment traps (mini booner tubes) are fixed to one of the minipod legs close to the bed. Thetubes are designed to trap any suspended sediment passing through the tube. Once the minipod has beenrecovered from the seabed the sediment may be removed.

The more recent minipod deployments have utilised timed booner tubes. These have only lately beendeveloped and are able to collect many discrete samples during the period of the deployment. The tubeincorporates a number of plastic discs that may be released to descend to the surface of the existingsediment layer in the tube. Any subsequent trapped material is therefore isolated from samples from aboveand below. The instrument may be programmed to release a disc either at a certain time, or following anevent of a certain magnitude. Levels of wave height, current speed and suspended solids concentrationmay be used to trigger the release of a disc. A timed booner tube is shown in Plate 2.

Auxiliary sensorsAll minipod deployments incorporated a standard set of auxiliary sensors for measuring the alignment ofthe minipod on the seabed i.e. pitch, roll, and compass orientation. Water temperature was also recorded.The pitch and roll sensors can be used in conjunction with the compass to determine if the minipod hasmoved on the seabed during a deployment. The true current direction is determined from the current meteru and v components of velocity corrected based on the measured compass reading i.e. corrected for theminipod orientation.

4.1.2 Data logging configurationThe data recorded by a minipod is stored on an internal hard disk that is downloaded following recoveryfrom the seabed. The duration of the deployment, the number and type of instruments fitted and the size ofthe data storage disk are all factors that determine the frequency at which data is stored. Each of theinstruments can be configured to switch on and off at pre-determined intervals. In most cases the burstinterval was 10 minutes every 30 minutes (i.e. switched off for 20 minutes). Whilst an instrument wasswitched on, the frequency at which measured data was recorded varied between 1Hz and 5Hz.

4.2 In-situ erosion threshold measurementsThis section describes the methods of sample collection and the measurement of the critical erosionthreshold.

4.2.1 NIOZ corer sampling methodologyFor the measurement of the critical shear stress of material relating to the pre-dredging and post-dredgingphases of the dredging cycle a NIOZ corer was used to collect undisturbed bed samples.

The NIOZ corer, which is owned by CEFAS, is unique in terms of being capable of collecting largeundisturbed seabed samples. The core sleeves are approximately 325mm in diameter and 555mm deep.The size of the corer itself is 2.5m long, 1.5m wide by 2.6m high and has a weight in excess of 460kg. Inorder to deploy the corer over the side of a vessel a clearance of at least 8.5m is required between the deckand the main lifting boom.

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During the course of this study over 100 seabed samples were collected with the NIOZ corer deployedfrom the MAFF research vessels RV Cirolana and RV Cirolana. The NIOZ corer is shown being deployedfrom RV Cirolana in Plate 4.

The procedure adopted for the measurement of the critical shear stress of the exposed surface of the coresample is to first siphon off any overlying water prior to planting one of the two HR erosion bell devicesISIS or its development SedErode. These devices are briefly described in the sections below and in detailin Appendices 1 and 2 and References 1 and 2.

The NIOZ corer was first used to collect undisturbed bed samples during a trial carried out at the Sellafieldmud patch in May 1996 (Reference 3). The trials showed that the sampling and shear stress measurementmethodology briefly described above was practical. Previous trials carried out by MAFF have shown thatthis procedure of retrieving bed samples caused minimal disturbance to the exposed surface. At presentthe use of the NIOZ corer is believed to be the only means whereby large surface area undisturbed samplescan be obtained from sub-tidal sites.

The cores were generally retrieved from the seabed in batches and then stockpiled. This meant that insome cases the instrument was not planted on the test surface for up to six hours after the core had beenretrieved from the seabed. This was not considered to have any significant effect on the determination ofthe critical shear stress, as the ambient water that covered the test surface upon retrieval remained in placeup until a few minutes before the test commenced.

The use of ISIS within NIOZ core tubes was achieved by the development of a specially designed supportframe. This frame allowed the ISIS head unit to be supported and positioned within the NIOZ core sleeve.The design of the frame also allowed for different depths of sediment within the cores to be accessible tothe ISIS head unit. Plate 5 shows the ISIS head unit positioned on a typical core sample, and the samesample after testing with the core sleeve removed.

The use of SedErode on seabed material collected with the NIOZ corer was achieved by one of twomethods depending on whether or not the retrieved core sample was full or not. As the diameter of theSedErode flange was greater than the diameter of the core sleeves, SedErode could only be applied directlyon full cores. In these cases the sediment surface was skimmed level with the rim of the sleeve. Thisprovided a flat surface onto which SedErode was planted, the rim of the sleeve acting as a support for theinstrument. If, on the other hand, the core sample was not full then a representative sample was transferredfrom the sleeve into a specifically designed test tray and carefully smoothed. The SedErode instrumentwas then planted on the resulting surface. Plate 6 shows SedErode being deployed in both sleeve and traymodes.

Where a surface was to be tested with both ISIS and SedErode the SedErode test was generally carried outfirst on full core samples. If the core was not full then the ISIS test was carried out first.

4.2.2 Hopper sampling methodologyFor the measurement of the critical shear stress of material relating to the dredging phase of the dredgingcycle samples were collected from the dredger hopper.

Samples were collected from the hopper of the dredger by suspending galvanised steel buckets in thehopper prior to the commencement of dredging. When dredging had finished the buckets were retrievedfrom the hopper and carried to the area of the ship where SedErode was set up. In most cases two bucketsamples were collected from each dredging load. Prior to each SedErode measurement the bucket samplewas transferred into a specially designed test tray and carefully smoothed.

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4.2.3 Critical shear stress measurement procedureDuring the course of this study both ISIS and its development SedErode were used for the measurement ofcritical shear stress. The measurement procedure was very similar for both instruments.

Local ambient seawater was used as the eroding medium for each of the tests. A bulk sample of this waterwas collected and where possible, left overnight to allow any sediment to settle out of suspension and todeaerate. Clearer water was then decanted from the bulk sample prior to use in the instruments as andwhen required. The basic test procedure for the critical erosion measurement runs was as follows:

1. The test surface was prepared for measurement by first removing any large surface debris andoverlying water. Some samples required varying degrees of surface smoothing which was done verycarefully by hand to minimise the disturbance, whilst ensuring an even representative sediment surfacefor testing. Overfull core samples were skimmed level with the top of the core sleeve. Plate 7 showsan example of the sediment surfaces as collected, and after surface smoothing.

2. The instrument was then positioned onto the test mud surface, and the system was filled with ambientseawater. In the case of ISIS the head unit was manually positioned close to the bed, with the gapbetween the two ranging between about 7 and 10 mm. The distance between the ISIS bell head andthe mud surface was measured accurately (to within ±0.25 mm) using 2 ultrasonic transducers locatedwithin the head unit and positioned over 2 different areas of the test surface. The gap was taken as theaverage of these 2 measurements. In the case of SedErode the gap is fixed at 5.8 mm, the distancebetween the underside of the support flange and the head unit. The nephelometer turbidity sensor wasthen zeroed, and logging commenced.

3. The flow of water through the system was then increased in controlled steps to apply increasing shearstress steps to the mud surface. The lowest discharge setting (i.e. the lowest applied shear stress) wasapplied for 3 to 4 minutes to allow the water within the recirculating system to become fully mixed,and a baseline turbidity to be established prior to applying increased shear stresses and monitoring theerosion response. Higher applied shear stresses then resulted in increasingly larger amounts of bedmaterial being removed from the test surface and therefore an increased turbidity within the system.This confirmed that surface bulk erosion had occurred.

Each ISIS measurement run took between 40 minutes to 1 hour to complete. In comparison a SedErodemeasurement was generally completed within 20 minutes.

4.2.4 Other measurements and observationsPrior to an erosion measurement being made a photographic record was taken of the test surface and asmall scrape of the top 2mm of the sediment was taken (typical sample mass 25-50 g). The sedimentsample was taken from the outer edge so as not to interfere with the test surface. The sediment sampleswere double-bagged in polythene and returned to HR Wallingford for subsequent laboratory analysis forbulk density, mud and sand content and determination of particle size distribution. Finally, whenapplicable, a series of five shear vane measurements were taken from around the perimeter of the coresurface using a Pilcon hand shear vane. The shear vane, which was 33mm in diameter and 50mm long,was pushed into the test surface to a depth of 70mm. Generally, shear vane measurements were notpossible as the material was either too coarse (in the case of the disposal site cores), or of insufficientdensity to register a reading (in the case of the harbour and hopper samples).

4.3 Summary of field measurementsField measurements undertaken during the course of this study were primarily carried out at the River Teesand at Harwich, both on the East Coast of England. The main reason for this choice was because themaintenance dredging operations at the two sites were quite different. At the River Tees, the Tees and

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Hartlepool Port Authority (THPA) operate two trailing suction hopper dredgers and a smaller grab dredger.Dredging is carried out on a more or less daily basis, with regular placements of small amounts of materialat the disposal. In contrast at Harwich, dredging does not take place continually but as campaigns 5 to 6times a year. In this case larger amounts of material tend to be placed at the disposal site in relatively shortperiods of time. Harwich Haven Authority (HHA), who do not have their own dredging fleet, utiliseDredging Contractors to undertake the work. During the course of this study all dredging undertaken atHarwich was carried out by Westminster Dredging.

Other reasons for choosing these two sites were: -

• Both sites are on the East Coast of England and within two days sailing of Lowestoft, the base port forthe MAFF research ships.

• The water depths at the two sites are quite different. At the Tees disposal site the depth of water isabout 33m. At Harwich the water depth at the Roughs Tower disposal site is about 13m. Thisdifference in water depth, and hence current speeds, was considered likely to give quite differentresults in terms of dispersion at the two sites.

Sediment propertiesIn-situ measurements of the critical erosion threshold of material collected from various locations aroundHarwich Harbour and the River Tees have been made. The material was collected from various locationsto represent the three phases of the dredging cycle i.e. pre-dredging, dredging and post-dredging.

Seabed cores were taken from the MAFF research vessels RV Cirolana and RV Corystes during six shortcruises that took place between April 1996 and December 1997. In addition to seabed cores samples ofdredged material were taken from a dredger hopper at each of the sites. During the course of the samplingcampaigns at Harwich and the Tees a total of 123 sediment samples were obtained. From these 123samples a total of 94 critical shear stress measurements were obtained. Some of the measurements werenot made on the exposed surface but on sub-samples taken from different levels within the core.

Hydraulic environmentDuring the course of the study minipod deployments were made at the Tees Inner disposal site (2deployments) and at the Harwich Roughs Tower disposal site (7 deployments). The duration of theminipod deployments was typically five to six weeks.

In the following chapter the results of the field measurements made at Harwich and the Tees aresummarised. For clarification the results are divided into for sub-sections.

• Tees physical properties (Section 5.1)• Tees hydraulic environment (Section 5.2)• Harwich physical properties (Section 6.1)• Harwich hydraulic environment (Section 6.2)

5. MEASUREMENTS AT THE RIVER TEES

A comprehensive set of results from each of the field measurement exercises is provided in detail in theassociated technical reports (References 16 and 18). In this report selected examples of the measured dataare presented. In the following sub-sections the measurements are described, summarised andrelationships discussed where appropriate.

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5.1 Tees physical propertiesSeabed cores were taken from the River Tees and the inner disposal site from the MAFF research vesselRV Cirolana during cruises in December 1996, January 1997 and November 1997. During the threesampling campaigns a total of 35 core samples were obtained. Of these 35 samples 23 were considered tohave surfaces suitable for testing with ISIS, i.e. of sufficient depth and reasonably planar. DuringNovember 1997 a further 11 surface shear stress measurements were made with SedErode, a developmentof ISIS. In most of these cases measurements were also made with ISIS in order to provide a comparisonof results obtained from the two instruments. During September 1997 a total of 23 erosion thresholdmeasurements were made on material taken from the hopper of the Tees and Hartlepool Port Authority(THPA) trailer dredger Heortnesse using the SedErode instrument.

SedErode, which is a more recently developed miniaturised version of ISIS, was not available for useduring the December 1996 and January 1997 cruises. The two instruments were used side-by-side duringthe November 1997 cruise only. Each instrument has its own particular application. ISIS was specificallymodified in March 1996 to allow surface shear stress measurements to be made directly onto the bedmaterial collected with the NIOZ corer operated by CEFAS. SedErode, which is principally designed tooperate on exposed inter-tidal mud banks, was modified in August 1997 to allow materials collected in aspecially designed erosion tray to be tested. This was particularly useful aboard the dredger where thematerial collected from the hopper was fluid and so could be readily poured from the collecting vessel intothe erosion tray.

5.1.1 Summary of dataA brief summary of the results are provided in each section below with appropriate references to tables andfigures where more detailed results may be found.

Pre-dredging phaseFor measurements on material representing the pre-dredging phase of the dredging cycle core sampleswere collected from within the navigation channel of the lower River Tees in an area adjacent to the ICIPetrochemicals No 2 jetty (typically 54° 35.787′ N, 01° 10.364′ W). This is an area of the river thatrequires regular dredging to remove accumulations of silt. During the coring exercise it was noted that thepenetration depth of the corer was quite variable suggesting that some areas of the channel are moresusceptible to accretion than others. It is also understood that this reach of the river had recently beendredged by THPA. The material collected from the bed of the river was observed to be black/brown incolour and very gelatinous in consistency.

The results of the erosion tests and subsequent laboratory analyses are given in detail in Appendix 4 ofReference 16 and summarised below and in Table 2.

Critical shear stress measurements made on this material gave results varying between 0.03 N/m2 and 0.37N/m2. The material collected generally composed 95% silt (particles less than 63 microns). The bulkdensity of the samples tested varied between 1380 kg/m3 and 1520 kg/m3. The median grain size of thesamples was about 7 µm and the mud/sand mixture varied between about 87% and 97% mud.

Dredging phaseThe hopper measurements (i.e. the dredging phase of the dredging cycle) were made aboard the THPAtrailing suction dredger “Heortnesse”, whilst dredging off of South Bank No 6 jetty, a region of the Riverwhere the bed is predominantly silt. This area of the river, which also requires regular dredging to removeaccumulations of silt, is some 700m further upstream than the pre-dredging coring locations discussedabove. During the period of the measurements seven hopper loads were sampled. The hopper sampleswere observed to be fluid and gelatinous in composition. This gelatinous consistency was also observedduring the pre-dredging measurements, although these appeared to be more consolidated and of a higherbulk density.

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The critical erosion shear stress (τcr) of the hopper material, measured using SedErode, ranged between0.29 N/m2 and 0.69 N/m2. The bulk density of the samples tested with SedErode varied between 1400kg/m3 and 1660 kg/m3. The average bulk density of the samples tested was about 1480 kg/m3. Themedian grain size ranged between about 5 µm and 20 µm and the mud/sand mixture varied between about65% and 90% mud.

It is interesting to note that the measured ranges for both the density and the τcr are slightly higher thanmeasured on the pre-dredging samples described above. This is despite the observation that the pre-dredging samples appeared to be more consolidated. However, the material was dredged from a differentsite.

Post-dredging phaseIn order that the best targeting of dredged material placements at the Inner Tees disposal site could bemade information was provided by THPA regarding the location of recent placements of dredged material.Despite having this information to hand very little cohesive sediment was found at the site. The reason forthis may be due to a large proportion of the fine material being dispersed from the site before reaching theseabed and the very weak strength of the material once it accumulated on the bed. On a few occasions avery thin film of cohesive sediment was observed on the surface of the core sample. Due to the very lowdensity of this material it is possible that the intrusive action of the coring procedure was disturbing thisthin sediment layer causing it to be washed into suspension.

The surface of the cores retrieved from the disposal site generally comprised fine sand, silt and smallparticles of coal. Sectioning the cores clearly showed layers of fine sand separated by thinner layers ofsmall granules of coal. This feature of bed structuring was observed to varying degrees in all of thedisposal site cores. An example of the bed layering is shown in Plate 8. It is assumed that the coal is nownaturally present offshore in Tees Bay rather than the dredgers transporting it to the site from the estuary.


From the measurements made on surface material found at the disposal site (i.e. the post-dredging phase ofthe dredging cycle) the critical shear stress for the initiation of erosion was found to vary between about0.04 N/m2 and 0.24 N/m2. Generally the material found was a mixture of fine sand, silt and smallfragments of coal. Although this was not a particularly cohesive surface, erosion threshold measurementswere successfully carried out nevertheless. The bulk density of the samples tested varied between 1400kg/m3 and 2150 kg/m3. The median grain size ranged between about 110 µm and 395 µm and themud/sand mixture varied between about 4% and 35% mud.

The measurements of critical shear stress on the post-dredging samples were generally lower than thosemeasured on the samples representing the pre-dredging and the dredging phases of the dredging cycle.This was due to the low level of cohesion of the samples. The average mud content of these samples was14% compared with 95% mud and 78% mud for the river and hopper samples respectively.

5.1.2 Analysis of dataFigure 2 shows the determined silt content plotted against the measured critical shear stress for the Teessamples. Generally the higher the silt content of the sample the greater was the resistance to erosion. Thematerial that was least resistant to erosion were the seabed samples collected from the disposal site. Thehighest resistance to erosion was encountered with the samples taken from the dredger hopper. Thematerial with the highest silt content was that collected from the bed of the River. In this case the

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resistance to erosion was not as great as that measured on material from the dredger hopper. The reasonfor this being the case is not clear. As mentioned in the section above the hopper material was observed tobe particularly gelatinous though still fluid in composition. It is believed that it was this gelatinousproperty that enhanced the natural resistance of the material to erosion.

Figure 3 shows the sample bulk density plotted against critical shear stress for each of the Tees samples.As the physical characteristics of the samples from each of the three material groups are different nooverall relationship may be identified. Within each of the individual groups there is a large variability incritical shear stress with respect to bulk density. This variability is greater for those samples collectedfrom the dredger hopper.

During the pre-dredging measurements three erosion tests were carried out with both ISIS and SedErodeon the same bulk sample. The comparative results, which are shown in Figure 3, show that ISIS wasconsistently returning a higher critical shear stress than SedErode. Further inspection of the test detailsshowed that for the two samples with a bulk density of about 1400 kg/m3, the ISIS test was in fact carriedout over 24 hours after the SedErode test. In the case of the sample with a bulk density of about 1500kg/m3 the SedErode test was carried out 17 hours after the ISIS test, but was a remould of the surfacematerial in the erosion tray. This limited comparison data suggests two things. Firstly, that leaving thesample overnight results in the exposed surface having a greater resistance to erosion, and secondly thatthe process of transferring a sample into the erosion tray and subsequent remoulding has the effect ofreducing the resistance to erosion.

Figure 4 shows the relationship between bulk density plotted against percentage silt as determined fromlaboratory analysis. This demonstrates the effect of the high sand content (65% to 95%) increasing thedensity at the disposal site (1400 to 2200 kg/m3). The composition of the material collected from thehopper of the dredger varied between about 60% and 90% silt. The dredging operation itself had verylittle effect on the bulk density of the material, this being about 1450 kg/m3 for both the harbour andhopper samples. In the case of the pre-dredged material collected from within the river the silt content wasconsistently about 95%.

5.2 Tees hydraulic environmentDuring the course of this study two minipods were deployed from the MAFF research vessel RV Cirolanaat the Tees Inner disposal site. The deployments were left on the seabed for a period of six weeks duringthe winters of 1995/1996 and 1996/1997. The minipods were located to the north west of the disposal sitesome 2.25km from the general placement area as shown in Figure 5.

5.2.1 Summary of dataAt each of the sites there was a repeating pattern in terms of the level of turbidity during the tidal cycledemonstrating the natural variability of the suspended load at the site. The data also shows that duringspring tide periods the suspended solids concentrations were higher than those during neap tide periods.The data shows very clearly that the major influencing factor on the level of near-bed suspended solidsconcentration are the associated wave conditions. Figure 6 shows the recorded Miniature OpticalBackscatter Sensor (MOBS) and wave data for the winter 1995/1996 deployment. The MOBS data showsthat the sensor that is lower in the water column (closer to the bed) consistently records a higher level ofturbidity. The figure shows that the largest waves recorded during the period of the deployment had asignificant wave height of about 0.5m.

During each of the periods of minipod deployment dredged maintenance material from the River Tees wasplaced within the general placement area of the disposal site. On only one occasion was there a possibilitythat the effect of material placement may have been detected at the minipod site some 2.25km away. Thiswas during the winter 1996/97 deployment when the minipod detected an increase in the level ofsuspended solids over a period of about 4 hours with no substantial increase in the associated significant

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wave height. This period of increased concentration coincided with the passage of a plume of placedmaterial based on current speeds and directions measured by the minipod.

During the period of the winter 1996/97 deployment it was found that the majority of the plumes trackedfrom the disposal site were passing well south of the minipod location. Therefore, prior to the winter1997/98 deployment a special request was made to the Tees and Hartlepool Port Authority to place alimited number of loads in the northern corner of the disposal site, some 1 km to the north of the generalplacement area. During the 10 days after deployment six loads of dredged material were placed at therequested location. Analysis of the recorded suspended solids data showed no evidence of passing plumesof dredged material being detected.

A likely reason for the passing plumes not being detected by the minipod instrumentation is the relativelyshort burst length in relation to the burst interval. For the Tees minipod deployments the burst length was8½ minutes with an interval between bursts of 1 hour i.e. recording for 14% of the time. It thereforefollows that it is quite possible that in some cases the passage of the plume of dredged material was simplymissed by the instrumentation. Advances in computer technology meant that minipod deployments atHarwich in November 1997 were able to utilise larger storage disks that allowed the burst length to beincreased to 10 minutes with a reduced interval of 30 minutes (i.e. recording for 33% of the time).

5.2.2 Analysis of dataThe data recorded by the Miniature Optical Backscatter Sensors (MOBS) from the River Tees deploymentsis presented in terms of an output voltage as no calibration between voltage and suspended solidsconcentration was available. Nevertheless the time series plots of suspended solids concentrationpresented in Reference 18 demonstrate that the magnitude of the suspended load varies considerablydepending on the stage of tidal cycle, the stage of the spring/neap cycle and the associated waveconditions. This is the natural variability that exists at the site.

Figure 6 shows the turbidity and wave data recorded by the winter 1995/1996 deployment. The figureclearly demonstrates the relationship between turbidity and wave activity.

Of particular interest is the magnitude of the turbidity generated by a particular height wave occurring atthe site where the water depth is about 36 m. Prior to the occurrence of the largest waves on 19/02/96 therecorded turbidity had not been over scale. The effect of the orbital velocity associated with the largestwaves was to create a shear stress at the bed that was high enough to erode and resuspend bed material thatwould not normally be disturbed by smaller waves. As the wave height reduced the suspended materialwas allowed to settle back onto the bed as a fresh, low density, deposit. As the density of the new depositis low it is a source of material that may now be resuspended by much smaller waves than those thatoriginally eroded it from the seabed. Figure 6 shows that the height of the next group of notable waves isonly about 0.1m. These waves, though fairly small, suspend sufficient material from the seabed to give offscale readings on both of the MOBS sensors. Prior to the arrival of the large waves of 19/02/96 therewould not have been sufficient material on the seabed in the form of a new deposit for these smaller wavesto give an over scale reading. This illustrates an important mechanism of untypical large waves to generatea sediment source that remains available for resuspension by smaller waves for 3 to 4 weeks before eitherbeing dispersed from the site or having undergone sufficient consolidation to resist erosion.

During each of the periods of minipod deployment dredged maintenance material from the River Tees wasplaced within the general placement area of the disposal site. The Tees and Hartlepool Port Authority(THPA) provided HR with details of the dredging and disposal activity at the site during the period of eachof the deployments. Figure 7 gives details of each of the dredged material placements made during theperiod of the winter 1995/1996 deployment in terms of hopper volume. The figure shows that the majorityof the 190,000 m3 of maintenance material was placed at the disposal site by the THPA trailing suctiondredger Heortnesse and that the typical dredger load is between 200 m3 and 1,700 m3. It is interesting tonote that during the period of the large wave activity discussed above no material was placed at the

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disposal site. The reason for this is simply that the sea conditions were too severe for the dredging fleet tooperate.

Using the current speed and direction data measured by the minipod a progressive vector diagram (PVD)was generated for each load of dredged material placed at the disposal site. Each PVD gave a particletrack covering a period of 5 to 7 hours after its release at the disposal site.

From the analysis of the PVD tracks and the associated turbidity records from the two Tees minipoddeployments there was only one occasion that was identified where the effect of a placement may havebeen detected. During the winter 1995/96 deployment the minipod instrumentation detected an increase inthe level of suspended solids over a period of about 4 hours with no substantial increase in the associatedsignificant wave height. This period of increased concentration coincided with the passage of a plume ofplaced material based on current speeds and directions measured by the minipod. The derived PVD forthis placement of material and the associated turbidity and wave data is shown in Figure 8. In this case thedredger log describes the load to be sand. In this case the released plume of dredged material will havebeen subject to dispersion by waves and currents for between 5 and 6 hours prior to passing the minipodsite. By making some basic assumptions and applying simple theory an estimate of the distance overwhich the dredged material will have dispersed may be made. Assuming a constant current speed of0.1 m/s, a water depth of 33 m and a settling velocity of 5 mm/s, fine sand (d50=0.1 mm) would take about2 hours to reach the bed and would be dispersed over a longitudinal distance of about 650 m. This isconsidered to be an overestimate as samples taken from the Tees disposal site suggest that the medianparticle size of the bed material is significantly larger that that assumed above. Based on this simpleassessment it is unlikely that any sand placed in the central area of the disposal site would still be insuspension by the time that the plume passed the minipod site.

The direction of the main flood and ebb tide is shown in Figure 5. This shows that it is unlikely that anymaterial released within the general placement area will be carried past the minipod sites by the main runof the ebb tide. With this in mind, during the winter 1996/97 minipod deployment, a special request wasmade to the THPA to place a limited number of loads in the northern corner of the disposal site, some 1 kmto the north of the general placement area. During the 10 days after deployment six loads of dredgedmaterial (both sand and silt) were placed at the requested location. Analysis of the recorded suspendedsolids data showed no evidence of passing plumes of dredged material being detected.

Figure 9 shows an example of the ABS burst data recorded during the winter 1996/97 deployment. In thiscase the burst mean significant wave height is about 0.55m in a water depth of about 38m. The effect ofthe wave-generated near-bed turbulence on the suspended solids concentration is quite marked even in thisdepth of water. The ABS data shows that for larger waves the associated near-bed orbital velocity liftsmore material into the water column than for smaller waves. The effect of each individual wave can beseen as a separate peak in the ABS record.

6. MEASUREMENTS AT HARWICH

A comprehensive set of results from each of the field measurement exercises is provided in detail in theassociated technical reports (References 14, 15 and 17). In this report selected examples of the measureddata are presented. In the following sub-sections the measurements are described, summarised andrelationships discussed where appropriate.

6.1 Harwich physical propertiesSeabed cores were taken from Harwich Harbour and the Roughs Tower disposal site from aboard theMAFF research vessels RV Cirolana and RV Corystes during cruises in April 1996, December 1996,January 1997, November 1997 and December 1997. In addition to seabed cores samples of dredgedmaterial were taken from the hopper of the Westminster Dredging trailing suction dredger Sospan whilstworking in the Harbour in December 1997. During the six sampling campaigns a total of 67 sediment

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samples were obtained. From these 67 samples a total of 43 critical shear stress measurements wereobtained using the HR erosion bell ISIS and its development SedErode. In some cases a shearmeasurement was not possible as the corer failed to yield sufficient material. In other cases the exposedsurface of the core was not considered to be sufficiently planar.

SedErode, which is a more recently developed miniaturised version of ISIS, was not available for use untilthe December 1997 cruise. Each instrument has its own particular application. ISIS was specificallymodified in March 1996 to allow surface shear stress measurements to be made directly onto the bedmaterial collected with the NIOZ corer operated by CEFAS. SedErode, which is principally designed tooperate on exposed inter-tidal mud banks, was modified in August 1997 to allow materials collected in aspecially designed erosion tray to be tested. This was particularly useful aboard the dredger where thematerial collected from the hopper was fluid and so could be readily poured from the collecting vessel intothe erosion tray. During the December 1997 measurements in Harwich Harbour both instruments wereused, though not on the same samples. ISIS was deployed on the exposed surface of the cores andSedErode was used on a sub-sample of material taken from a lower layer.

6.1.1 Summary of dataA brief summary of the results are provided in each section below with appropriate references to tables andfigures where more detailed results may be found.

Pre-dredging phaseDuring each of the two visits to Harwich Harbour three cores were taken from each of three sites. Theharbour locations were adjacent to Trinity Berth 7 at Felixstowe, 100m off shore of Trinity Berth 7 and tothe side of the navigation channel adjacent to Landguard Fort. The 3 Harbour coring sites are shown inFigure 10.

Critical shear stress measurements were carried out on the exposed surface of all 18 of the harboursamples. Two of the measurements failed due to poor sealing between the ISIS head and the test surface.Due to the relatively low density of the material several of the cores were overfull upon retrieval. Thesecores required the surface to be skimmed level with the top of the sleeve prior to testing with ISIS.

Several of the retrieved cores showed a distinct interface between two material types of quite differentdensities. On these occasions, where possible, critical shear stress and subsequent laboratory measurementswere made on the exposed surface with ISIS and on the underlying denser layer with SedErode. Anexample of this feature is shown in Plate 9. This core (Core F7) was retrieved from the Landguard Fortsite in January 1997. The density of the surface layer varied between 1225 kg/m3 and 1504 kg/m3 incontrast with that of the lower layer, which varied between 1350 kg/m3 and 1610 kg/m3.


The grain size analysis of the harbour samples yielded mud fractions (particles less than 63 microns)ranging between about 91% and 99% by weight. The bulk densities reflected the variability in the samplecomposition, with values between about 1230 kg/m3 and 1610 kg/m3. The median grain size rangedbetween about 5 µm and 11 µm.

The critical erosion shear stress (τcr) of the exposed surface measured using ISIS ranged between about0.005 N/m2 and 0.075 N/m2. The SedErode measurements carried out on material from the lower layeryielded a τcr ranging between 0.16 N/m2 and 0.32 N/m2. It should be noted that the SedErodemeasurements were generally carried out on higher density, sub-surface layers. The measured stresses are

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typical for soft to medium consistency marine cohesive sediment, and the range reflects the differences indensity, sand content and biological activity within the sediment.

The results of the laboratory analysis showed that regardless of the sample density the composition interms of particle size was very similar from one sample to another. The results generally showed atendency for greater silt content by weight for higher density samples. Where the carbon content wasmeasured by loss on ignition the results showed that the carbon content was generally higher by 1% or 2%in those samples taken from beneath the exposed surface.

Dredging phaseThe erodability measurements relating to the dredging phase of the dredging cycle at Harwich were madeon samples collected from the hopper of the trailing suction hopper dredger Sospan during December1997. During the period of the measurements the vessel, operated by Westminster Dredging, was carryingout foreshore recharge works at Shotley Point in Harwich Harbour. This placement is being monitoredunder another MAFF funded research study at HR (Reference 19). The Sospan was dredging in theFelixstowe berths. This is an area that is subject to maintenance dredging 5 to 6 times a year to removeaccumulations of silt.

The hopper samples were observed to be very similar in composition to the pre-dredged samples retrievedby coring. The main difference between the two material types was the fluidity. The hopper materialresembled thick soup that could readily be poured whereas the core samples had an appearance moretypical of an in-situ cohesive material.


The grain size analysis of the hopper samples yielded mud fractions ranging between about 96% and 99%by weight. The bulk densities reflected the variability in the sample composition, with values between1260 kg/m3 and 1450 kg/m3. The median grain size for all samples tested in the laboratory was about 5µm.

The critical erosion shear stress (τcr) measured using SedErode ranged between 0.06 N/m2 and 0.29 N/m2.This range of shear stresses is similar to that measured on the pre-dredged material (0.005 N/m2 to 0. 0.32N/m2).

Post-dredgingOver the course of the four visits to the Roughs Tower disposal site 39 seabed cores were collected. Thelocation of the disposal site is shown in Figure 10. The coring locations were generally chosen based oninformation from Harwich Haven Authority and from sidescan sonar surveys.

Of the 39 cores retrieved from this site only 7 critical shear stress measurements were made. The primaryreason for the small number of erodability measurements was that most of the exposed surfaces retrievedwere unsuitable for the measurement devices to be planted on. The surface of the cores retrieved from thedisposal site generally comprised clean sand and gravel with some shell fragments. On occasions denseclay was interspersed with the sand and gravel.

Sectioning the cores often revealed dense cohesive material beneath the exposed surface. The reason forabsence of significant amounts of cohesive sediment is probably due to a large proportion of the finematerial being dispersed from the site before reaching the seabed. For material that does reach the seabeddispersion is also rapid. The relatively shallow water depths and high current speeds that exist at the sitesupport the theory that the site is highly dispersive.

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The grain size analysis of the surface samples retrieved from the disposal site varied immensely. Mudfractions (particles less than 63 microns) ranged between about 0.5% and 85% by weight. The bulkdensity of the samples ranged between about 1600 kg/m3 and 2500 kg/m3. The median grain size rangedbetween about 5 µm and 21500 µm, again reflecting the variability in the surface material.

The limited number of erosion measurements made using ISIS on materials from the disposal site gavecritical erosion shear stresses (τcr) ranging between about 0.09 N/m2 and 1.26 N/m2.

6.1.2 Analysis of dataFigure 11 shows the measured silt content plotted against critical shear stress (τc) for each of the Harwichsamples. The figure shows that the material from the three phases of the dredging cycle falls into twodistinct groups. The material collected from the Harbour and from the dredger hopper shows similarproperties, particularly in terms of silt content, which varies between about 90% and 100%. The figurealso shows the large variability in the physical properties of the material found at the disposal site.Generally this material comprised greater than 80% sand interspersed with fine silt and shell fragments.

The material that was least resistant to erosion was the exposed surface layer of the Harbour samples, theaverage τc being about 0.05 N/m2 measured with ISIS. The material collected from the hopper of thedredger had an average τc about 0.17 N/m2 measured with SedErode. The material group with the highestresistance to erosion was the sub-surface layer of the Harbour samples, the average τc being about 0.30N/m2 measured with SedErode.

Figure 12 shows the bulk density plotted against critical shear stress (τc) for each of the Harwich samples.The figure shows that although there is no overall relationship, within the four distinct groups of datapoints there is generally a trend for τc to increase with bulk density. The obvious exception to this is thematerial collected from the disposal site where, for the limited data provided, no relationship appears toexist. The τc for the disposal site samples, measured with ISIS, varied between about 0.1 N/m2 and 0.4N/m2 with only a small variation in the bulk density.

Figure 13 shows the bulk density plotted against silt content for each of the Harwich samples. The figureagain demonstrates the strong similarity between the Harbour and dredger hopper samples in terms of bothdensity and composition. Compared to materials tested from other locations, such as the Tees (Figure 4)there is very little scatter. In the case of the Harwich samples the dredging operation appears to have hadthe effect of reducing the bulk density of the material. For the hopper samples the bulk density wastypically 1350 kg/m3 compared to 1500 kg/m3 for the harbour samples.

6.2 Harwich hydraulic environmentDuring the course of this study seven minipods have been deployed around the periphery of the RoughsTower disposal site from the MAFF research vessels RV Cirolana and RV Corystes. The deploymentswere left on the seabed for a period of between six and ten week during the winters of 1995, 1996 and1997. The minipods were located around the periphery of the disposal site, between about 1km and 5kmfrom the general placement area as shown in Figure 10.

There were a number of problems encountered at Harwich during the minipod deployments. The first twominipods were deployed for a period of about 9 weeks during early 1996. Soon after the data had beendownloaded from the minipods it was clear that both minipods had fallen over within an hour of each otherduring a large storm in the early hours of 6 February. The significant wave height at the time wasmeasured by the minipods to be about 1.8 m. This was particularly unfortunate, as the Harwich

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maintenance dredging campaign had only started the day before. For future deployments at Harwich theminipod footplates were enlarged and additional lead weight fitted to provide a more stable platform.

A second problem was encountered during the winter 1995 and 1996 deployments with the MOBSturbidity sensors reading over-scale for a short period during most tides. Readings were continually over-scale, particularly on the lower sensor, during periods of large wave activity. For the three winter 1997deployments improved MOBS hardware was installed that allowed the sensor gain to switch automaticallybetween two different ranges depending on the magnitude of the suspended solids concentration at anygiven time.

6.2.1 Summary of dataAt each of the Harwich minipod sites there was a repeating pattern in terms of the level of turbidity duringthe tidal cycle demonstrating the natural variability of the suspended load at the site. The data also showsthat during spring tide periods the suspended solids concentrations were higher than those during neap tideperiods. The data shows very clearly that the major influencing factor on the level of near-bed suspendedsolids concentration is the associated wave conditions. Figure 14 shows the recorded Miniature OpticalBackscatter Sensor (MOBS) and wave data for a winter 1997 deployment. The MOBS data shows that thesensor that is lower in the water column (closer to the bed) consistently records a higher level of turbidity.The figure shows that the largest waves recorded during the period of the deployment had a significantwave height of about 2.5m.

During each of the periods of minipod deployment maintenance dredging material from Harwich Harbourwas placed at the Roughs Tower disposal site. The only occasion when the effect of material placementwas detected was during the winter 1997 deployments. In this case the Threshold minipod, which wasdeployed some 5km downstream of the disposal site (Figure 10), measured concentrations during theperiod of material placement that were consistently higher than the normal background level.

This dredging campaign was untypical for Harwich in terms of the amount of material placed at the siteduring each disposal event. The hopper volume of the type of ship normally used in a harbourmaintenance dredging exercise is between about 3,500 m3 and 6,500 m3. In comparison the hopper volumeof the vessel WD Fairway used during December 1997 is about 23,500 m3. The total amount of materialplaced was about 128,000 TDS (Tonnes Dry Solids). At this time the WD Fairway was the largest andfastest dredger of its type in the world.

The effect of the disposal activity was clearly seen in the turbidity data record as shown in Figure 15. Priorto and soon after disposal activities the level of turbidity measured was relatively low with little variationthrough the tidal cycle.

The increase in turbidity was only detected at the end of the ebb tide and during the early flood tide. Noincreases were detected at high water slack which would be expected if the plume passed back again on thesubsequent flood tide, suggesting that the material within the plume disperses to background levels withina single tide.

The most likely reason for the majority of the passing plumes not being detected by the minipodinstrumentation is the relatively short burst length in relation to the burst interval, particularly for theearlier deployments. For the winter 1995 minipod deployments the burst length was 10 minutes with aninterval between bursts of 1 hour i.e. recording for 17% of the time. It is therefore likely that in manycases the passage of the plume of dredged material was not seen by the recording instrumentation. For thewinter 1996 deployments the burst length was 8½ minutes with a burst interval of 1 hour. Advances incomputer technology meant that the winter 1997 deployments were able to utilise larger storage disks thatallowed a burst length of 10 minutes with a reduced interval of 30 minutes (i.e. recording for 33% of thetime).

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6.2.2 Analysis of dataThe data recorded by the Miniature Optical Backscatter Sensors (MOBS) from the Harwich deployments ispresented in terms of an output voltage as no calibration between voltage and suspended solidsconcentration was available. Nevertheless the time series plots of suspended solids concentrationpresented in Reference 15 demonstrate that the magnitude of the suspended load varies considerablydepending on the stage of tidal cycle, the stage of the spring/neap cycle and the associated waveconditions. This is the natural variability that exists at the site. As described in Section 6.2 above much ofthe MOBS data recorded during the early deployments at Harwich was over-scale for a short period duringmost tides. Readings were continually over-scale, particularly on the lower sensor, during periods of largewave activity. For the three winter 1997 deployments improved MOBS hardware was installed thatallowed the sensor gain to switch automatically between two different ranges depending on the magnitudeof the suspended solids concentration at any given time.

Figure 14 shows the turbidity and wave data recorded during a winter 1997 deployment at the RoughsTower disposal site where the water depth is about 12 m. The figure clearly demonstrates the relationshipbetween turbidity and wave activity. The figure also shows that the near-bed suspended solidsconcentration is unaffected by waves with a significant wave height of less than about 1m. This gives anindication of the composition of the exposed surface of the seabed.

During each of the periods of minipod deployment dredged maintenance material from Harwich Harbourwas placed within the general placement area of the disposal site. Harwich Haven Authority (HHA)provided HR with details of the dredging and disposal activity at the site during the period of each of thedeployments. During the maintenance dredging campaigns that took place during the winter of 1995, 1996and 1997 approximately 425,000 TDS, 95,500 TDS and 128,000 TDS respectively was placed at thedisposal site.

Using the current speed and direction data measured by each of the minipods a progressive vector diagram(PVD) was generated for each load of dredged material placed at the disposal site. Each PVD gave aparticle track covering a period of 5 to 7 hours after its release at the disposal site. The PVD analysisshowed that material placed at the disposal site would be unlikely to pass the minipod sites, as theminipods were not in line with the general direction of the main flood and ebb tide. An exception to thiswas the winter 1997 minipod deployment at Threshold, which did appear to detect the dispersion ofmaintenance material from the disposal site.

During four days in December the WD Fairway dredger placed 23 loads of maintenance silt at the RoughsTower disposal ground. The vessel is a trailing suction hopper dredger with a hopper capacity of 23,425 m3

and is currently one of the largest and fastest of its type in the world. The total amount of material placedwas about 128,000 TDS (Tonnes Dry Solids) giving an average load of about 5,500 TDS. For previousmaintenance dredging campaigns smaller dredgers have been used with hopper volumes of about 8,000 m3.The material was placed at the location shown in Figure 10.

Figure 15 shows that prior to the disposal commencing the turbidity is generally low with little variationthrough the tidal cycle. The exception to this is the peak observed at low water at midday on 8 December.This peak, which was also detected by the other minipods, is considered to be due to the large waves thatoccurred earlier in the day on the previous rising tide. Any material resuspended from the bed during thistime will have settled back to the bed during the slack water period around low water.

Within hours of the first material placement the turbidity can be seen to increase to a peak level during thelate ebb tide. Detailed analysis of the background turbidity data showed that for this tide type the level ofturbidity at the Threshold site normally peaks during the early flood tide. This suggests that theconcentration increase is not due to the natural variability that exists at the site. The near-bed turbiditycontinued to be high until about 12 hours after the last of the 23 placements at which time it returned to alevel similar to that observed before the dredging had started. Figure 14 shows that the significant wave

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heights measured during the period of material placement were no greater than those measured during thepreceding 14 days. This gives confidence to the theory that the observed increases in turbidity are due tothe material placements rather than any other hydrodynamic forces existing at the time.

During the period of disposal both the magnitude and variability of the turbidity was significantlyincreased to a level not dissimilar to that generated by the large storm that occurred the following week.During this storm significant wave heights of 3.2 m were measured in a water depth of 13-15 m. The near-bed orbital velocity was about 1 m/s.

Figure 16 shows a progressive vector diagram (PVD) based on the Threshold minipod current meter datafor a material placement released at the disposal site at 2 hours before high water. The track shows that theplume would be carried close to the minipod site. Since the minipod is approximately 5 km from thedisposal site it must be assumed that small local variations in current speed and direction would existbetween the two sites. The PVD also demonstrates the south-easterly residual current that exists at theminipod site. The tidal excursion from the minipod site can be seen to be about 10 km. Based on thisinformation any material being placed at the disposal site would be carried towards, and past, the minipodsite during the late ebb tide. The data recorded by the minipod indicates that the plume is not carried backagain on the following flood tide. There are two possible reasons for this. Firstly, the material within theplume may have dispersed to background levels within a single tide. Secondly, the plume may have beencarried away from the minipod site by the residual current.

Figure 17 shows an example of the ABS burst data recorded by the Threshold minipod during the winter of1997. The ABS data shows what is assumed to be a sand wave moving past the minipod during the 10minute burst. During this period of large waves on and around 16 December sand waves were oftenobserved at the Threshold site in particular.

6.3 Coring at the WD Fairway placement siteSampling has never been undertaken immediately after placement for logistical reasons (ie a suitablevessel was not available on site at the same time that disposal operations were occurring). The shortestinterval between a significantly large placement and sampling at the placement site was about 9 days.Fortunately this occasion was after the placements by the trailing suction hopper dredger WD Fairway.NIOZ core samples taken at the WD Fairway placement site showed no evidence that the recently dredgedmaterial was still at the site. In general the samples collected showed the exposed surface material to beclean gravel with some coarse sand and shell fragments. This type of material is typical of the naturalseabed at the site as opposed to the remnants of an old maintenance material placement. An example ofthe surface material is shown in Plate 10.

Laboratory analysis of the surface samples showed the composition of the material to be about 98% sandand 2% silt. The median particle size of the samples was about 8.5 mm.

7. FINDINGS AND THEIR APPLICATION

The aim of the research undertaken for this study is to improve knowledge of the properties of dredgedmaterial with particular regards to the manner in which it may disperse form offshore disposal sites. Thisinformation will allow predictive methods to be made more reliable in their representation of erosion andtransport. Most, if not all, existing models assume homogeneous material, either cohesive (mud) orcohesionless (sand and gravel). In reality much of the material dredged around the UK is a mixture ofmaterial types ranging from a slurry arising from maintenance dredging in silts to mixtures of rock andclay generated with bucket dredgers. The physical properties of these mixtures are little known.Determining the erosion and consolidation characteristics of the material, once place on the bed, is animportant step in predicting the environmental impact of the placement of the material.

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7.1 Sediment variabilityThe measurements relating to the physical properties of dredged material have shown that there isconsiderable variability in the critical shear stress for erosion of an exposed surface. This is not only thecase for one site to another but also for successive samples collected from a given site. For this reason it isdifficult to derive any definitive relationships between, for example, bulk density and critical shear stressfor erosion.

In terms of the overall findings of this study it is evident that making assumptions about the erodability ofdredged material placed at a disposal site is difficult without making measurements in the field similar tothose presented in this report. The hydrodynamic conditions that exist at the Roughs Tower disposal site atHarwich result in the site being particularly dispersive, i.e. the vast majority of the dredged material placedat the site is transported away before the material even reaches the bed. Furthermore, the majority ofmaterial that does reach the bed is resuspended and transported away from the site within a few days.Based on the measurements of critical erosion stresses on the in-situ material and the material within thehopper these observations are not surprising. The question being whether there would be any significantdifferences in terms of dispersion and potential impact if the material were considerably weaker at thedisposal site than in its previous states. At the Roughs Tower site the hydrodynamic regime is such thatthis is probably not that significant at a different site, where a more benign regime existed then this mightbe a sensitive issue.

7.2 Probability of erosionFrom the current speed data recorded by the minipods at each of the disposal sites and the predicted annualwave climate the probability of a given combined wave and current induced bed shear stress beingexceeded was determined. Figure 18 shows the probability of exceedance graphically for each of the twosites.

In the case of the Tees disposal site the maximum measured critical erosion shear stress (τc) was 0.24N/m2. It can be seen from the graph that this shear stress is exceeded about 8% of the time. In the case ofthe Harwich disposal site the maximum measured shear stress (1.26 N/m2) is exceeded about 35% of thetime. It should be noted that for the Harwich measurements the maximum of 1.26 N/m2 was untypical andnot representative of the maintenance material placed at the site. Generally the measured τc was less than0.5 N/m2 which can be seen to be exceeded about 75% of the time. It is therefore not surprising that littleevidence of recent material placements at the disposal sites was found.

Although definitive quantitative relationships may not be readily derived from the measurements made atHarwich and the Tees, from the critical shear stress measurements made, an indication of the currentspeeds likely to be required to resuspend the dredged material may be determined. For example, the shearstress required to erode the exposed surface at the Harwich disposal site varies between about 0.09 N/m2

and 1.26 N/m2, and between 0.04 N/m2 and 0.24 N/m2 at the Tees site. From these shear stresses anassociated near-bed current speed can be calculated.

Minipod deployments at each of the sites included instrumentation for measuring the current speed. Thecurrent meters were generally installed at a height of 0.4 m above the bed. The algorithm for calculatingthe current speed at 0.4 m above the bed for a given measured shear stress is shown below.

2*Uc ρτ =

Where cτ = Measured shear stress

ρ = Water density = 1025 kg/m3

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⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

*

lnz

z

kUU

Therefore 2

2

0

ln

U

z

z

kc

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛= ρτ

Where k = Bed roughness = 0.4z = Height above bed = 0.4m

0z = 0.0003

Therefore17.3cU

τ=

It therefore follows that for a measured τc of 0.09 N/m2 at the Harwich disposal site the current speed at aheight of 0.4 m above the bed of 0.17 m/s would be required to erode the exposed surface. Thiscalculation assumes a water density of 1025 kg/m3. For a measured τc of 1.26 N/m2 the required currentspeed at a height of 0.4 m above the bed is 0.63 m/s. The Harwich minipod data presented in Reference15, shows that for the lower critical shear stress the equivalent current speed is exceeded on all tides. Thecurrent speed required to generate the higher critical shear stress was exceeded for up to one hour duringthe early flood of most large spring tides.

In the case of the shear stresses measured at the Tees the associated current speeds at 0.4 m above the bedwould be 0.11 m/s and 0.27 m/s respectively. The likely occurrence of these current speeds are similar tothat observed at Harwich in that the lower current would be experienced on nearly all tides but the highercurrent would be exceeded only on large spring tides (Reference 18).

The primary reason for the variability in the measured τc, particularly at the Harwich site, is thecomposition of the bed material both in terms of silt/sand content and bulk density. On most occasionsthree seabed cores were obtained from each targeted location. Both the erodability and the laboratorymeasurements often showed significant variations from one sample to another. This shows that thecomposition of the exposed surface at the disposal site can vary considerably within a small sampling area.

This is more the case at the disposal site than within either the river or the harbour (pre-dredged material)where the spatial variability in composition and density was far less. Consequently there was littlevariability in the measured critical shear stress of these samples. This was also the case for the materialtaken from the dredger hopper, which was naturally well mixed.

7.3 Application in numerical modelling

7.3.1 IntroductionDuring the dredging December 1997 campaign described in Chapter 6 the W.D. Fairway placed a total of128,000 TDS (Tonnes dry solids) at the Roughs Tower disposal site over a period of four days. Aproportion of the placed material was released into the water column, forming a plume of sediment whichthen dispersed under the influence of the hydrodynamic conditions. A Minipod deployed some 5km to theNE at Threshold (Figure 10) was recording during the placement, and took measurements of wave height,

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current speed and direction, water level and suspended sediment concentration, although the latter is atpresent uncalibrated. HR had previously established flow and plume models for the Roughs Tower site andthese tools have been used to predict the observed turbidity variations at the minipod site.

7.3.2 Description of the placementThe method of placement of material by the Fairway consisted of the opening of the bottom doors of thedredger, the material descending to the bed under the force of gravity. The method usually results in themajority of sediment reaching the sea bed, unless the water depth is particularly deep, but there will alwaysbe some loss of material into the water column, say of the order of 5%. In effect there are two sources ofmaterial – the plume of sediment initially lost into the water column, and the sediment on the bed whichwill be resuspended by waves and currents over a longer time frame, depending on the hydrodynamicconditions on the sea bed. The rates of release of either of these two sources are generally not predictablewith any certainty as the current state of knowledge of the processes occurring during bottom disposal isnot sufficient.

The Fairway placement consisted of 23 placements of 23,500m3 over 4 days. Although the cycle timevaried over the course of the operation, on average placement occurred every 3.5 hours.

7.3.3 MethodologyThe hydrodynamic information for the sediment transport model came from a 3D flow model of the OuterThames. Two simulations of dispersion from the Fairway placements were carried out. Both simulationsmodelled the dispersion of the 23 individual placements over a period of 10 mean spring tides.

The first simulation modelled the dispersion of sediment initially lost into the water column and assumedthat 5% of the placed material was released into the water column on each placement with this materialbeing released over the course of 1 minute. The second simulation modelled the release of the remainingsediment sediment resuspended from the bed under the combined action of waves and currents. It wasassumed that the sediment resuspension from the material at the sea bed was proportional to the (combinedwave and current) bed shear stress minus the critical shear stress for erosion, with the average rate ofresuspension such that a single placement load would be resuspended over the course of a single tide.

7.3.4 ResultsFigure 19 shows the suspended sediment concentrations predicted at Threshold for the material releasedinto the water column together with the MOBS uncalibrated voltage output from the Minipod for theperiod of the simulation. The Figure shows two The model reproduces the observed peaks in suspendedsediment seen at Low Water. However, it can be seen that the results of the model show a steady increasein concentrations over the first part of the simulation and a decline over the latter half while the observedconcentrations appear to vary erratically over the period simulated. The simulated concentrations atThreshold appear to drop away less rapidly than the observations at the end of the placement operations(approximately tide 7).

Figure 20 shows the suspended sediment concentrations predicted at Threshold for the materialresuspended from the bed together with the Mobs uncalibrated voltage output from the Minipod for theperiod of the simulation. The figure shows that that again the model reproduces the observed peaks insuspended sediment concentration seen at Low Water. The response of the simulated concentrations stilldoes not reproduce any of the erratic nature of the observations well, but the initial increase inconcentrations at the start of the simulation and the die off of concentrations after placement stops(approximately tide 7.0) are reproduced well.

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7.3.5 Discussion and conclusionsThe model results reproduced the general behaviour of the observations – the peaks at Low Water, the riseand fall of the peaks with the start/end of the placement operations – but were not able to reproduce thedetail of the observations.

One possibility is that the observed variation in suspended sediment concentrations at Threshold is causedby wave action, either causing rapid resuspension in a way not reproduced by the model or generallyincreasing background concentrations. Figure 21 shows the variation in wave height and suspendedsediment activity over the placement period at Threshold. It can be seen that there is little correlationbetween wave action and observed suspended sediment concentrations.

A further consideration is the variation in current patterns in the vicinity of Roughs Tower and Thresholdover the course of the placement operations. The variation in speed measured at the Threshold Minipodnear the bed over the placement period is shown in Figure 22. The low peaks in suspended sedimentconcentrations at approximately 6:00 am on the 11 and 12 of December (at tide 3.5 and 5.5 in the model)are matched by correspondingly low peak speeds on the ebb tide just before the Low Water. Similarly, thehigher peaks are generally matched by higher peak ebb speeds. The exception to this is the peak inconcentrations at 2:00 am on the 10 December (tide 1.5 in the model) which corresponds to a previousrelatively small ebb tide peak speed. This temporal variation in current speed between tides wouldinfluence both the advection of suspended sediment and the resuspension of deposited material and may inpart account for the variation in observed turbidity. In the model a repeating tide was used.

Although the general behaviour of sediment can be reproduced by the methodology described above, thevariation of observed suspended sediment concentrations at Threshold appears to be linked with processesnot represented within the model simulation such as the variation in current speeds between tides. Futureresearch into this type of study would benefit from the incorporating the observed tidal fluctuations overthe placement period rather than the use of repeating tides of a single type.

8. CONCLUSIONS AND RECOMMENDATIONS

8.1 Field measurements1. Field measurements undertaken during the course of this study were primarily carried out at the River

Tees and at Harwich, both on the East Coast of England. These two sites represented quite differentmaintenance dredging operations and hydrodynamic conditions.

At the River Tees, the Tees and Hartlepool Port Authority (THPA) operate two trailing suction hopperdredgers and a smaller grab dredger. Dredging is carried out on a more or less daily basis, with regularplacements of small amounts of material at the disposal. The depth of water at the disposal site isabout 33m.

At Harwich, dredging does not take place continually but as campaigns 5 to 6 times a year. In thiscase larger amounts of material tend to be placed at the disposal site in relatively short periods of time.Harwich Haven Authority (HHA), who do not have their own dredging fleet, utilise DredgingContractors to undertake the work. At Harwich the water depth at the Roughs Tower disposal site isabout 13m.

2. Seabed cores were taken from the MAFF research vessels RV Cirolana and RV Corystes during sixshort cruises that took place between April 1996 and December 1997. In addition to seabed coressamples of dredged material were taken from a dredger hopper at each of the sites. During the courseof the sampling campaigns at Harwich and the Tees a total of 123 sediment samples were obtained.From these 123 samples a total of 94 critical shear stress measurements were obtained using the ISIS

�� 28 SR 517 13/09/02

and SEDERODE in-situ erosion instruments. Some of the measurements were not made on theexposed surface but on sub-samples taken from different levels within the core.

3. During the course of the study minipod deployments were made at the Tees Inner disposal site (2deployments) and at the Harwich Roughs Tower disposal site (7 deployments). The duration of theminipod deployments was typically five to six weeks.

4. Also obtained as part of the field studies were photographic records, analysis of samples of the topsediment layer for bulk density, mud /sand content and particle size distribution and shear vanemeasurements.

8.2 Measurements at the River Tees

8.2.1 Pre-dredging phase5. Core samples were collected from within the navigation channel of the lower River Tees in an area that

requires regular dredging to remove accumulations of silt. The material collected from the bed of theriver was observed to be black/brown in colour and very gelatinous in consistency.

6. Critical shear stress measurements made on this material gave results varying between 0.03 N/m2 and0.37 N/m2. The material collected generally composed 95% silt (particles less than 63 microns). Thebulk density of the samples tested varied between 1380 kg/m3 and 1520 kg/m3. The median grain sizeof the samples was about 7 µm and the mud/sand mixture varied between about 87% and 97% mud.

8.2.2 Dredging phase7. The hopper measurements (i.e. the dredging phase of the dredging cycle) were made aboard the a

trailing suction dredger whilst dredging a region of the River where the bed is predominantly silt.During the period of the measurements seven hopper loads were sampled. The hopper samples wereobserved to be fluid and gelatinous in composition.

8. The critical erosion shear stress (τcr) of the hopper material, measured using SedErode, ranged between0.29 N/m2 and 0.69 N/m2. The bulk density of the samples tested with SedErode varied between 1400kg/m3 and 1660 kg/m3. The average bulk density of the samples tested was about 1480 kg/m3. Themedian grain size ranged between about 5 µm and 20 µm and the mud/sand mixture varied betweenabout 65% and 90% mud.

9. The measured ranges for both the density and the τcr were slightly higher than measured on the pre-dredging samples. This is despite the observation that the pre-dredging samples appeared to be moreconsolidated. However, the material was dredged from a different site.

8.2.3 Post-dredging phase10. Very little cohesive sediment was found at the site. The surface of the cores retrieved from the

disposal site generally comprised fine sand, silt and small particles of coal.

11. From the measurements made on surface material found at the disposal site (i.e. the post-dredgingphase of the dredging cycle) the critical shear stress for the initiation of erosion was found to varybetween about 0.04 N/m2 and 0.24 N/m2. The bulk density of the samples tested varied between 1400kg/m3 and 2150 kg/m3. The median grain size ranged between about 110 µm and 395 µm and themud/sand mixture varied between about 4% and 35% mud.

12. The measurements of critical shear stress on the post-dredging samples were generally lower thanthose measured on the samples representing the pre-dredging and the dredging phases of the dredgingcycle. This was due to the low level of cohesion of the samples. The average mud content of these

�� 29 SR 517 13/09/02

samples was 14% compared with 95% mud and 78% mud for the river and hopper samplesrespectively.

13. Generally the higher the silt content of the sample the greater was the resistance to erosion. Thematerial that was least resistant to erosion were the seabed samples collected from the disposal site.The highest resistance to erosion was encountered with the samples taken from the dredger hopper.The reason for this being the case is not clear although the gelatinous nature of the hopper materialmay have enhanced the natural resistance of this material to erosion.

14. Within each of the individual groups there is a large variability in critical shear stress with respect tobulk density. This variability is greater for those samples collected from the dredger hopper. As thephysical characteristics of the samples from each of the three material groups are different no overallrelationship may be identified.

15. Leaving the sample overnight was found to result in the exposed surface having a greater resistance toerosion. Furthermore the process of transferring a sample into the erosion tray and subsequentremoulding has the effect of reducing the resistance to erosion.

16. The dredging operation itself had very little effect on the bulk density of the material.

8.2.4 Minipod measurements17. For each of the minipod deployments sites there was a repeating pattern in terms of the level of

turbidity during the tidal cycle demonstrating the natural variability of the suspended load at the site.The data also shows that during spring tide periods the suspended solids concentrations were higherthan those during neap tide periods.

18. The data shows very clearly that the major influencing factor on the level of near-bed suspended solidsconcentration are the associated wave conditions. The largest waves recorded during the period of thedeployment had a significant wave height of about 0.5m.

19. During each of the periods of minipod deployment dredged maintenance material from the River Teeswas placed within the general placement area of the disposal site. On only one occasion was there apossibility that the effect of material placement may have been detected at the minipod site some2.25km away. This may be due to the relatively short burst length in relation to the burst interval.

20. The field measurements illustrated a mechanism whereby storm waves generate a sediment source thatremains available for resuspension by smaller waves for 3 to 4 weeks before either being dispersedfrom the site or having undergone sufficient consolidation to resist erosion.

8.3 Measurements at Harwich

8.3.1 Pre-dredging phase21. Cores were taken from locations adjacent to Trinity Berth 7 at Felixstowe, 100m off shore of Trinity

Berth 7 and to the side of the navigation channel adjacent to Landguard Fort.

22. The grain size analysis of the harbour samples yielded mud fractions (particles less than 63 microns)ranging between about 91% and 99% by weight. The bulk densities reflected the variability in thesample composition, with values between about 1230 kg/m3 and 1610 kg/m3. The median grain sizeranged between about 5 µm and 11 µm.

23. The critical erosion shear stress (τcr) of the exposed surface measured using ISIS ranged between about0.005 N/m2 and 0.075 N/m2. The SedErode measurements carried out on material from the lower layer

�� 30 SR 517 13/09/02

yielded a τcr ranging between 0.16 N/m2 and 0.32 N/m2. It should be noted that the SedErodemeasurements were generally carried out on higher density, sub-surface layers. The measured stressesare typical for soft to medium consistency marine cohesive sediment, and the range reflects thedifferences in density, sand content and biological activity within the sediment. The ISISmeasurements indicated a very weak, readily eroded overlying surface layer.

24. The results of the laboratory analysis showed that regardless of the sample density the composition interms of particle size was very similar from one sample to another. The results generally showed atendency for greater silt content by weight for higher density samples.

8.3.2 Dredging phase25. The erodability measurements relating to the dredging phase of the dredging cycle at Harwich were

made on samples collected from the hopper of the trailing suction hopper dredger Sospan whiledredging in Felixstowe berths.

26. The hopper samples were observed to be very similar in composition to the pre-dredged samplesretrieved by coring. However, while the hopper material resembled thick soup that could readily bepoured, the core samples had an appearance more typical of an in-situ cohesive material.

27. The grain size analysis of the hopper samples yielded mud fractions ranging between about 96% and99% by weight. The bulk densities reflected the variability in the sample composition, with valuesbetween 1260 kg/m3 and 1450 kg/m3. The median grain size for all samples tested in the laboratorywas about 5 µm.

28. The critical erosion shear stress (τcr) measured using SedErode ranged between 0.06 N/m2 and 0.29N/m2. This range of shear stresses is similar to that measured on the pre-dredged material (0.005 N/m2

to 0. 0.32 N/m2).

8.3.3 Post-dredging29. Cores were collected from locations at Roughs Tower targetted using information from Harwich

Haven Authority and from sidescan sonar surveys.

30. The surface of the cores retrieved from the disposal site generally comprised clean sand and gravelwith some shell fragments. On occasions dense clay was interspersed with the sand and gravel. Thegrain size analysis of the surface samples retrieved from the disposal site varied immensely. Mudfractions (particles less than 63 microns) ranged between about 0.5% and 85% by weight. The bulkdensity of the samples ranged between about 1600 kg/m3 and 2500 kg/m3. The median grain sizeranged between about 5 µm and 21500 µm, again reflecting the variability in the surface material.

31. The limited number of erosion measurements made using ISIS on materials from the disposal site gavecritical erosion shear stresses (τcr) ranging between about 0.09 N/m2 and 1.26 N/m2.

32. The results generally displayed a trend for τc to increase with bulk density. The exception to this wasthe material collected from the disposal site where, for the limited data provided, no relationshipappeared to exist. The τc for the disposal site samples, measured with ISIS, varied between about 0.1N/m2 and 0.4 N/m2 with only a small variation in the bulk density.

33. There was a strong similarity between the Harbour and dredger hopper samples in terms of bothdensity and composition. Compared to materials tested from other locations, such as the Tees therewas very little scatter.

�� 31 SR 517 13/09/02

34. In the case of the Harwich samples the dredging operation appeared to have had the effect of reducingthe bulk density of the material. For the hopper samples the bulk density was typically 1350 kg/m3

compared to 1500 kg/m3 for the harbour samples.

8.3.4 Minipod measurements35. At each of the Harwich minipod sites there was a repeating pattern in terms of the level of turbidity

during the tidal cycle demonstrating the natural variability of the suspended load at the site. The dataalso showed that during spring tide periods the suspended solids concentrations were higher than thoseduring neap tide periods. The major influencing factor on the background level of near-bed suspendedsolids concentration was the associated wave conditions.

36. During each of the periods of minipod deployment maintenance dredging material from HarwichHarbour was placed at the Roughs Tower disposal site. The only occasion when the effect of materialplacement was detected was during the winter 1997 deployments when a higher rate of input occurred.In this case the Threshold minipod, which was deployed some 5km downstream of the disposal sitemeasured concentrations during the period of material placement that were consistently higher than thenormal background level.

37. NIOZ core samples taken at the WD Fairway placement site showed no evidence that the recentlydredged material was still at the site. In general the samples collected showed the exposed surfacematerial to be clean gravel with some coarse sand and shell fragments. This type of material is typicalof the natural seabed at the site as opposed to the remnants of an old maintenance material placement.

8.4 Findings and their Application38. The measurements relating to the physical properties of dredged material have shown that there is

considerable variability in the critical shear stress for erosion of an exposed surface. This is not onlythe case for one site to another but also for successive samples collected from a given site. For thisreason it is difficult to derive any definitive relationships between, for example, bulk density andcritical shear stress for erosion.

39. It is evident that making assumptions about the erodability of dredged material placed at a disposal siteis difficult without making measurements in the field similar to those presented in this report.However, it has been observed that the form of silty maintenance material after placement at openwater disposal sites is substantially weakened compared to its in-situ or in-hopper state. This impliesthat rapid dispersion of this material may be expected at open water sites.

40. By characterising placed sediment using representative parameters, numerical dispersion modellingcan be used effectively to provide the targetted positioning of minipods. Furthermore, the use ofnumerical dispersion tools in conjunction with field data can produce important information regardingthe nature of the short and long term dispersion of material from placement.

8.5 Recommendations1. Minipod monitoring is proposed for use at other sites where a requirement to monitor exists or as a

basis for research. The technique can be expensive but provides a long term record of information atpresent unmatched by other monitoring approaches. The deployment of minipods should be supportedby targetted observations and numerical modelling.

2. The work undertaken in this study has shown that for dispersion studies regarding the placement ofsoft muddy maintenance material at open water sites, it should be assumed that the materialimmediately after placement has a very low critical shear stress for erosion.

3. It is recommended that similar studies be undertaken to obtain the nature of properties immediatelyafter placement at non-dispersive sites where significant amounts of material can be found on the bed.

�� 32 SR 517 13/09/02

4. It is further recommended that research into the properties of placed material at dispersive sites beconsidered. In particular, investigation of the nature of the near bed layer found immediately afterplacement would be most useful in aiding the prediction of dispersion of plumes resulting frommaintenance disposal.

9. ACKNOWLEDGEMENTS

HR would like to thank the staff of MAFF EG CEFAS, the crews of the MAFF research vessels RVCirolana and RV Corystes for their invaluable assistance, co-operation and hospitality during the course ofthis study. Similar thanks are also due to the crews of the dredgers Heortnesse (Tees and Hartlepool PortAuthority) and Sospan (Westminster Dredging).

�� 33 SR 517 13/09/02

10. REFERENCES

1. Williamson H J and Ockenden M C (1996). ISIS: An instrument for Measuring Erosion Shear StressIn-Situ. Estuarine, Coastal and Shelf Science 42, pp1-18.

2. Development of SedErode, Instrument for in-situ mud erosion measurements. HR Wallingford. ReportTR 17.

3. Properties of Dredged Material. Review of available measurement techniques for determiningphysical properties. HR Wallingford. Report TR 21.

4. Shields A (1936). Application of similarity principles and turbulence research to bed-load movement.Translated from: "Anwendung der Aehnlichkeitsmechanik und der Turbulenzforschung auf dieGeschiebebewegung", Mitteilungen der Preussischen Versuchsanstalt fur Wasserbau und Schiffbau,Berlin, 1936, by W P Ott and J C van Uchelen, Soil Conservation Service, Cooperative Laboratory,California Institute of Technology, Pasadena, California.

5. Partheniades E., Erosion and Deposition of cohesive soils, Journal of the Hydraulics Division, ASCE,volume 91, No.HY1, pp105-139, 1965.

6. Van Rijn L.C.,Sediment Transport: Part I: bed load transport, Proceedings of the ASCE Journal ofHydraulics Division, volume 110, HY10, 1431-1456, 1984.

7. Van Rijn L.C., Sediment Transport: Part II: suspended load transport, Proceedings of the ASCEJournal of Hydraulics Division, volume 110, HY11, 1613-1641, 1984.

8. Krone R.B., Flume studies of the transport of sediment in estuarial shoaling processes, Final Report,Hydraulic Engineering and Sanitary Engineering Research Laboratory, University of California,Berkely, USA, 1962.

9. Galland J.C., Laurence D. and Teisson C., Simulating turbulent vertical exchange of mud with aReynolds stress model, In: Proceedings of the 4th Nearshore and Estuarine Conhesive sedimentTransport Conference, INTERCOH’94, Wallingford, UK, ed. T.N. Burt, W.R. Parker and J.Watts,John Wiley and Sons, pp417-428.

10. Winterwerp J.C., Uittenbogaard R.E. and de Kok J.M., Rapid Siltation from saturated mudsuspensions, Proceedings of the 5th Nearshore and Estuarine Cohesive sediment Transport Conference,INTERCOH’98, in press.

11. Hogg, A.J., Huppert E.H., and Soulsby R.L., The dynamics of particle-laden fluid elements, IN:Sediment transport Mechnisms, In: Coastal Environments and Rivers, Euromech 310, ed. Belogey M.,Rajaona R.D. and Sleath J.F.A., World Scientific, Singapore, pp64-78, 1994.

12. Model Development for the Assessment of Turbidity caused by Dredging, Volume 1, TechnicalReport, HR Wallingford Report EX 3998, 1999.

13. Properties of Dredged Material. Erosion shear stress measurements on seabed cores taken fromSellafield mud patch 26 May – 31 May 1996. HR Wallingford. Report TR 14.

14. Properties of Dredged Material, Measurement of sediment properties of dredged material fromHarwich Harbour. HR Wallingford. Report TR 46.

�� 34 SR 517 13/09/02

15. Properties of Dredged Material, Minipod deployments at the Roughs Tower disposal site. HRWallingford. Report TR 47.

16. Properties of Dredged Material, Measurement of sediment properties of dredged material from theTees estuary. HR Wallingford. Report TR 54.

17. Properties of Dredged Material, Harwich minipod deployments – Winter 1997. HR Wallingford.Report TR 53.

18. Properties of Dredged Material, Minipod deployments at the Tees disposal site. HR Wallingford.Report TR 61.

19. Beneficial Use of Dredged Material, North Shotley. HR Wallingford. Report TR 72.

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Tables

�� SR 517 13/09/02

�� SR 517 13/09/02

Tab

le 1

Tim

etab

le o

f fie

ld m

easu

rem

ents

1996

1997

JF

MA

MJ

JA

SO

ND

JF

MA

MJ

JA

SO

ND

Min

ipod

122

- T

ees

Dis

posa

l Site

Min

ipod

139

- T

ees

Dis

posa

l Site

Min

ipod

120

- H

arw

ich

Rou

ghs

Tow

er S

outh

Min

ipod

121

- H

arw

ich

Rou

ghs

Tow

er N

orth

Min

ipod

137

- H

arw

ich

Rou

ghs

Tow

er N

orth

Min

ipod

138

- H

arw

ich

Rou

ghs

Tow

er S

outh

Min

ipod

157

- H

arw

ich

Rou

ghs

Tow

er W

est

Min

ipod

158

- H

arw

ich

Sle

dway

Min

ipod

159

- H

arw

ich

Thr

esho

ld

Sel

lafie

ld IS

IS T

rials

Par

ksto

ne S

edE

rode

Low

esto

ft IS

IS

Har

wic

h P

re-D

redg

ing

Pha

se IS

IS /

Sed

Ero

de

Har

wic

h D

redg

ing

Pha

se S

edE

rode

Har

wic

h P

ost-

Dre

dgin

g P

hase

ISIS

/ S

edE

rode

Tee

s P

re-D

redg

ing

Pha

se IS

IS /

Sed

Ero

de

Tee

s D

redg

ing

Pha

se S

edE

rode

Tee

s P

ost-

Dre

dgin

g P

hase

ISIS

/ S

edE

rode

�� SR 517 13/09/02

Table 2 Surface sediment properties – Pre-dredging phase

Date SiteISIS /Sed

Bulk Density(Kg.m-3) τa Nm-2 τb Nm-2 τcr Nm-2 %carbon d50 (µm) %sand %mud

24/01/97 F1 I 1339 0.034 0.046 0.040 - 7.1 4.9 95.1

24/01/97 F2 I 1227 - - - - 7.6 6.3 93.7

24/01/97 F3 I 1397 0.033 0.043 0.048 - 7.2 5.4 94.6

24/01/97 F4 I 1472 0.045 0.049 0.047 - 10.0 8.3 91.7

24/01/97 F5 I 1482 0.039 0.046 0.042 - 9.4 7.8 92.2

24/01/97 F6 I 1476 0.040 0.052 0.046 - 10.2 8.1 91.9

24/01/97 F6B** I No sample 0.049 0.051 0.050 - - - -

24/01/97 F7 I 1439 0.041 0.047 0.044 - 8.6 7.7 92.3

24/01/97 F7B** I 1535 0.034 0.047 0.041 - 6.8 4.0 96.0

24/01/97 F8 I 1469 0.045 0.065 0.055 - 7.6 6.5 93.5

24/01/97 F9 I 1504 0.057 0.093 0.075 - 11.1 8.3 91.7

19/12/97 19H1 I 1500 0.051 0.057 0.054 9.83 6 4.4 95.6

19/12/97 19H1B** S 1610 0.15 0.24 0.20 9.85 6 5.5 94.5

19/12/97 19H2 I 1420 0.007 0.008 0.008 7.06 7 6.6 93.4

19/12/97 19H2B** S 1520 0.24 0.30 0.27 11.68 6 4.6 95.4

19/12/97 19H3 I 1450 - - - 8.63 5 2.6 97.4

19/12/97 19H3B** S 1550 0.31 0.34 0.32 10.53 6 4.1 95.9

19/12/97 19T1 S 1450 0.14 0.23 0.18 11.74 5 1.6 98.4

19/12/97 19T2 I 1270 0.072 0.079 0.076 10.11 5 1.1 98.9

19/12/97 19T2B** S 1350 0.26 0.29 0.28 13.90 5 0.9 99.1

19/12/97 19T3 S 1340 0.23 0.28 0.26 12.03 5 1.1 98.9

19/12/97 19T3B** S 1440 0.14 0.18 0.16 12.01 5 0.9 99.1

19/12/97 19L1 I 1460 0.000 0.007 0.004 11.97 5 2.8 97.2

19/12/97 19L1B** S 1500 0.26 0.36 0.31 12.31 6 4.9 95.1

19/12/97 19L2 I 1440 0.070 0.080 0.075 12.24 6 4.8 95.2

19/12/97 19L2B** S 1460 0.24 0.29 0.27 11.98 5 2.7 97.3

19/12/97 19L3 I 1420 0.055 0.062 0.059 9.46 6 5.5 94.5

19/12/97 19L3B** S 1410 0.29 0.35 0.32 11.17 6 4.7 95.3

Key: * denotes poor measurement resolution (high shear stress steps)- no measurement madeB** denotes core base sample

�� SR 517 13/09/02

Table 3 Surface sediment properties – Dredging phase

Date SiteISIS /Sed


15/12/97 151A S 1420 0.00 0.14 0.07 6.56 6 2.2 97.8

15/12/97 151B S 1350 0.14 0.17 0.15 8.59 5 1.9 98.1

15/12/97 151C S 1390 0.16 0.23 0.20 6.80 6 4.1 95.9

15/12/97 151D S 1260 0.13 0.22 0.17 9.76 5 3.4 96.6

16/12/97 161A S 1450 0.17 0.25 0.21 9.03 5 1.7 98.3

16/12/97 162A S 1400 0.00 0.13 0.07 9.76 5 2.0 98.0

16/12/97 162B S 1290 0.27 0.32 0.29 11.82 5 1.9 98.1

16/12/97 163A S 1450 0.16 0.23 0.19 10.93 5 0.8 99.2

16/12/97 163B S 1370 0.00 0.12 0.06 10.44 5 1.1 98.9

16/12/97 164A S 1360 0.15 0.22 0.19 11.23 5 0.9 99.1


�� SR 517 13/09/02

Table 4 Surface sediment properties : Post-dredging phase

Date SiteISIS /Sed


03/04/97 MP22 I - 0.90 1.62 1.26* - - - -

02/12/96 H1 I 1960 - - - - 190 97.1 2.9

02/12/96 H2 I 2220 - - - - 496 84.8 15.2

02/12/96 H3 I 1960 - - - - 329 69.8 30.2

02/12/96 H4 I 2130 - - - - 783 90.9 9.1

02/12/96 H5 I 2380 - - - - 5055 39.1 0.9

02/12/96 H6 I 1650 0.07 0.163 0.12 - 60 47.7 52.3

02/12/96 H7 I 2050 - - - - 2834 99.5 0.5

02/12/96 H8 - - - - - - - - -

02/12/96 H9 - - - - - - - - -

04/12/96 RT1(R) I 1690 - - - - 45 47.0 53.0

04/12/96 RT1(S) I 2180 - - - - 6225 86.1 13.9

04/12/96 RT2(R) I 1650 0.74 1.25 1.00 - 23 34.6 65.4

04/12/96 RT2(S) I 1960 - - - - 237 95.4 4.6

04/12/96 RT2 6cm I 1760 - - - - 19 30.9 69.1

25/01/97 C1 I 1914 - - - - 4 15.7 84.3

25/01/97 C2 I 1941 - - - - 4 14.6 85.4

25/01/97 C3 I 2069 - - - - 6 29.9 70.1

25/01/97 S1 I 1959 - - - - 333 59.5 40.5

25/01/97 S1B** I 1697 0.05 0.13 0.09 - 30 36.5 63.5

25/01/97 S2 I 2125 - - - - 8756 90.9 9.1

25/01/97 S2B** I 1808 - - - - 40 39.7 60.3

25/01/97 S3 I 2092 - - - - 1626 99.1 0.9

25/01/97 S3B** I 1990 - - - - 9250 64.5 35.5

12/11/97 RT61A I 2480 - - - 0.61 1115 98.9 1.1

12/11/97 RT61B I 2100 - - - 1.10 816 98.9 1.1

12/11/97 RT61C I 2420 - - - 0.29 881 99.7 0.3

12/11/97 RT62A I 1630 0.39 0.43 0.41 8.79 10 22.9 77.1

12/11/97 RT62B I 1680 0.27 0.30 0.29 8.66 21 37.5 62.5

12/11/97 RT62C I 1610 0.25 0.28 0.26 11.28 15 28.2 71.8


�� SR 517 13/09/02

Table 4 Surface sediment properties : Post-dredging phase Continued

Date SiteISIS /Sed

Bulk Density(Kg.m-3) τa Nm-2 τb Nm-2 τcr Nm-2 %carbon

d50(µm)

%sand %mud

21/12/97 TRA - - - - - - 21452 99.3 0.7

21/12/97 TRB - - - - - - 9315 98.2 1.8

21/12/97 TRB*** - - - - - - 735 61.1 38.9

21/12/97 TRC - - - - - - 6683 87.8 12.2

21/12/97 PATCH2A - - - - - - 9393 98.1 1.9

21/12/97 PATCH2B - - - - - - 6948 99.5 0.5

21/12/97 FDPOSA - - - - - - 134 53.0 47.0

21/12/97 FDPOSA*** - - - - - - 5 2.8 97.2

21/12/97 FDPOSB - - - - - - - - -

21/12/97 FDPOSC - - - - - - 6598 96.5 3.5

21/12/97 FDPOSD - - - - - - 7795 96.9 3.1

21/12/97 FDPOS2A - - - - - - 9457 99.4 0.6

21/12/97 FDPOS2B - - - - - - 4609 97.8 2.2

21/12/97 FDPOS2B*** - - - - - - 16 46.4 53.6

21/12/97 FDPOS2C - - - - - - 8802 98.1 1.9

21/12/97 CLAY2A - - - - - - 7909 98.3 1.7

21/12/97 CLAY2A*** - - - - - - 5 16.5 83.5

21/12/97 CLAY2B - - - - - - 1261 80.1 19.9

21/12/97 CLAY2B*** - - - - - - 5 19.8 80.2

21/12/97 CLAY2C - - - - - - 1997 98.1 1.9

21/12/97 CLAY2C*** - - - - - - 8 27.6 72.4

Key: * denotes poor measurement resolution (high shear stress steps)- no measurement madeB** denotes core base sample*** denotes clay sub-sample

�� SR 517 13/09/02

Table 5 Surface sediment properties – Pre-dredging phase

Date SiteISIS /Sed

Bulk Density(Kg.m-3) τa Nm-2 τb Nm-2 τcr Nm-2 %

carbond50

(µm)% sand %mud

09/11/97 TB1 S 1380 0.13 0.17 0.15 17.8 7 5.9 94.1

09/11/97 TB2 - 1450 - - - 16.6 8 12.5 87.5

09/11/97 TB3 I 1460 0.26 0.26 0.26 18.0 7 5.9 94.1

09/11/97 TB3 S 1440 0.33 0.36 0.34 14.4 7 2.7 97.3

09/11/97 TB4 S 1400 0.16 0.21 0.19 17.9 7 5.1 94.9

09/11/97 TB5 I 1490 0.23 0.27 0.25 15.0 7 7.9 92.1

09/11/97 TB5 S 1490 0.14 0.21 0.18 17.1 7 7.2 92.8

09/11/97 TB6 I 1280 0.03 0.03 0.03 15.8 7 4.1 95.9

09/11/97 TB6 S 1450 0.14 0.23 0.18 15.5 6 3.7 96.3

09/11/97 TB7 I 1520 0.17 0.18 0.18 15.4 7 4.9 95.1

09/11/97 TB7 S 1520 - - - 15.4 7 4.9 95.1

09/11/97 TB8 I 1410 0.28 0.34 0.31 14.8 7 4.5 95.5

09/11/97 TB8 S 1410 0.13 0.21 0.17 14.8 7 4.5 95.5

09/11/97 TB9 I 1390 0.35 0.39 0.37 18.9 7 3.3 96.7

09/11/97 TB9 S 1390 0.19 0.23 0.21 18.9 7 3.3 96.7

Key: - no measurement made

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Table 6 Surface sediment properties – Dredging phase

Date SiteISIS /Sed

BulkDensity(Kg.m-3)

τa Nm-2 τb Nm-2 τcr Nm-2 %carbon

d50(µm)

%sand %mud

09/09/97 9Drag1* - 1970 - - - 9.37 32 45.2 54.8

09/09/97 9M1 S 1520 0.34 0.37 0.35 16.14 10 20.9 79.1

09/09/97 92A S 1440 0.47 0.53 0.50 16.12 9 23.0 77.0

09/09/97 92B S 1420 0.23 0.42 0.32 17.43 7 13.3 86.7

09/09/97 92C S 1410 0.51 0.57 0.54 17.37 7 11.5 88.5

09/09/97 92E S 1460 0.67 0.71 0.69 18.56 8 18.6 81.4

09/09/97 92F S 1560 0.25 0.34 0.30 14.03 13 17.0 83.0

09/09/97 93A S 1500 0.25 0.33 0.29 15.21 17 35.6 64.4

09/09/97 93E S 1410 0.27 0.31 0.29 16.93 8 13.8 86.2

09/09/97 93F S 1450 0.28 0.39 0.34 18.22 9 18.1 81.9

10/09/97 101Drag* S 1480 0.28 0.36 0.32 18.16 7 9.66 90.34

10/09/97 101A S 1560 0.29 0.36 0.33 17.00 15 33.7 66.3

10/09/97 101B S 1510 0.30 15.91 14 30.7 69.3

10/09/97 101E S 1500 0.31 0.37 0.34 16.80 9 20.3 79.7

10/09/97 101F S 1660 0.31 0.38 0.35 15.23 10 23.0 77.0

10/09/97 102AB S 1520 0.43 0.45 0.44 16.84 11 29.2 70.8

10/09/97 102D S 1420 0.45 0.48 0.47 16.99 8 15.6 84.4

10/09/97 102E S 1490 0.30 0.42 0.36 17.05 11 24.8 75.2

10/09/97 102F S 1400 0.29 0.35 0.32 17.69 8 16.2 83.8

10/09/97 103AC S 1490 0.40 0.47 0.44 16.52 10 25.0 75.0

10/09/97 103DE S 1400 0.61 0.66 0.64 16.69 8 16.2 83.8

10/09/97 104A S 1470 0.55 0.59 0.57 18.38 8 24.0 76.0

10/09/97 104B S 1380 0.31 18.59 8 18.9 81.1

10/09/97 104C S 1500 0.30 16.99 11 29.3 70.7

10/09/97 104Drag* - 1440 - - - 18.46 6 3.4 96.6

Key: - no measurement made* indicates bulk sample retrieved from draghead – otherwise hopper sample

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Table 7 Surface sediment properties – Post-dredging phase

Date SiteISIS /Sed

BulkDensity(Kg.m-3)

τa Nm-

2τb Nm-

2τcr

Nm-2 %carbonD50(µm)

%sand %mud

05/12/96 T1 I 1600 0.21 0.26 0.24 - 135 81.6 18.4

05/12/96 T2 I 1810 0.19 0.29 0.24 - 130 89.6 10.4

05/12/96 T3 I 1380 0.06 0.14 0.10 - 395 82.9 17.1

06/12/96 T4 I 1810 0.07 0.09 0.08 - 160 85.6 14.4

06/12/96 T5 I 1730 0.12 0.16 0.14 - 100 66.1 33.9

06/12/96 T6 - - - - - -

06/12/96 TA I 2150 0.17 0.29 0.23 - 220 90.2 9.8

06/12/96 TB I 1940 0.03 0.07 0.05 - 190 89.0 11.0

06/12/96 TC I 1900 - - - - 150 96.9 3.1

20/01/97 T1B I 1776 0.07 0.21 0.14 - 112 82.6 17.4

20/01/97 T2B I 1558 0.10 0.22 0.16 - 122 87.5 12.5

20/01/97 T3B I 1471 0.15 0.21 0.18 - 168 83.9 16.1

21/01/97 T4B I 1857 0.06 0.07 0.06 - 133 93.5 6.5

21/01/97 T5B I 1741 0.06 0.09 0.08 - 146 95.9 4.1

21/01/97 T6B - 1511 - - - - 145 95.6 4.4

21/01/97 T7B - 1754 - - - - 151 91.0 9.0

22/01/97 T8B I 1746 0.05 0.11 0.08 - 136 85.9 14.1

22/01/97 T9B I 1750 0.06 0.07 0.06 - 128 89.5 10.5

22/01/97 T10B I 1405 0.14 0.23 0.18 - 148 66.0 34.0

22/01/97 T11B I 1772 0.06 0.11 0.09 - 128 88.8 11.2

22/01/97 T11Bmud I 1707 - - - - 75 57.8 42.2

08/11/97 Target1/1 I 1640 0.05 0.07 0.06 - 156 89.0 11.0

08/11/97 Target1/2 S 1830 0.00 0.14 0.07 - 153 89.4 10.6

08/11/97 Target2/1 S 1880 0.14 0.21 0.17 - 160 95.2 4.8

08/11/97 Target2/2 - 1840 - - - - 144 94.4 5.6

08/11/97 Target3/1 S 1790 0.13 0.20 0.17 - 148 90.0 10.0

08/11/97 Target3/2 - 1740 - - - - 197 83.0 17.0

Key: - no measurement made

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Figures

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Figure 1 Location Plan

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Figure 2 Silt content versus critical shear stress - Tees

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Figure 3 Bulk density versus critical shear stress - Tees

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Figure 4 Silt content versus bulk density - Tees

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Figure 5 Tees disposal site

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Figure 6 Example of turbidity and wave data - Tees

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Figure 7 Dredged material placements at the Tees disposal site

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Figure 8 Particle vector diagram for a placement at the Tees disposal site

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Figure 9 Example ABS burst data from the Tees disposal site

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Figure 10 Harwich Harbour and disposal site

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Figure 11 Silt content versus bulk density - Harwich

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Figure 12 Bulk density versus critical shear stress - Harwich

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Figure 13 Silt content versus bulk density - Harwich

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Figure 14 Example of turbidity and wave data - Harwich

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Figure 15 Turbidity levels before, during and after dredging - Harwich

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Figure 16 Particle vector diagram for a placement at the Harwich disposal site

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Figure 17 Example ABS burst data from the Harwich disposal site

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Figure 18 Probability of shear stress exceedance at the disposal sites

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Figure 19 Predicted suspended sediment concentration and observed turbidity at Threshold, simulation ofdispersion of fine material initially released in water column

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Figure 20 Predicted suspended sediment concentration and observed turbidity at Threshold, simulation ofdispersion of fine material resuspended from the bed

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Figure 21 Comparison of observed turbidity and observed wave conditions during placement byW.D.Fairway

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Figure 22 Comparison of observed turbidity and observed near bed tidal current speed during placementby W.D.Fairway

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Plates

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Plate 1 Minipod aboard RV Corystes prior to deployment

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Plate 2 Minipod instrumentation

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Plate 3 Syringe water samplers

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Plate 4 The NIOZ corer being deployed from RV Cirolana

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Plate 5 ISIS heas unit positioned on a typical core sample

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Plate 6 SedErode being deployed in both sleve and tray modes

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Plate 7 Sediment surfaces as collected and after surface smoothing

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Plate 8 An example of bed layering at the Tees disposal site

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Plate 9 Interface between material types – Landguard Fort

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Plate 10 Example of exposed surface following WD Fairway placement

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Appendices

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Appendix 1

ISIS – Instrument for Shear stress In-Situ

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Appendix 2

SedErode – Sediment Erosion Device

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SedErode - Sediment Erosion Device

1. SedErode Instrument

SedErode comprises a head unit and a control box. The instrument is used to apply a knownapplied shear stress to a planar surface of cohesive sediment and detect the onset of surfaceerosion.

The head section comprises a recirculating system which generates radial flow with a knownshear stress, across a 0.09m diameter area of sediment surface, via the specially designed bellhead. The recirculating system is mounted on a 0.4m diameter baseplate, with a short thin-walledtube which is pushed into the sediment to position the bell head above the mud surface. Anephelometer is used to continuously monitor the recirculating water turbidity.

The control box contains a rechargeable battery supply, discharge (ie applied shear stress) control,and logging ports for applied shear stress and turbidity monitoring. Figure A1.1 shows theSedErode instrument being deployed on a typical mudflat.

The applied shear stresses generated by SedErode under different operating conditions have beenmeasured using hot film shear stress probes (Graham et al, 1992) at the University of Plymouth.This data has been used to generate a calibration equation relating the discharge though thesystem and the gap to the applied shear stress.

2. Operation

The head unit is carefully positioned over a typical planar area of sediment under investigation.The unit is filled with clear local water and the recirculating system is bled of any air. Thenephelometer is zeroed, and shear stress and turbidity logging is commenced. The water isrecirculated over the test site first at a very low shear stress to allow even mixing of therecirculating water and establish a turbidity baseline prior to erosion testing. A series ofcontrolled increasing shear stress steps are then applied, typically 1-2 minutes duration whilstmonitoring of the turbidity. The onset of erosion occurs when there is a sharp increase in theturbidity, which corresponds to the removal of surface material into the recirculating water. Themeasurement time typically takes about 15 minutes.

3. Results and Interpretation

The output from the SedErode measurements is a time series of applied shear stress and turbidityin the system. Figure A1.2 shows a typical example of the results obtained using SedErode. Theturbidity can be directly calibrated against the concentration of the mud under test, and this recordgives the sediment response to the applied shear stress steps. Analysis of the time series data thenyields the onset of surface erosion, and the erosion rate as material is removed from the surface.The interpretation as to the definition of erosion depends on the application of the results, but formost engineering purposes it is practical to consider that erosion occurs when there is "bulksurface rupture", which continues when a higher shear stress is applied. Another definition ofcohesive sediment erosion is "benign erosion", when loose surface deposits are removed and thistype of erosion results in small discontinuous turbidity increases. The use and interpretation ofthe SedErode instrument and results can be chosen by the user and varied to investigate specificerosion characteristics of cohesive sediments.

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4. Applications

SedErode can be used on any cohesive-based sediment to measure the surface erosion shearstress. Measurements have enabled the relationship between erosion shear stress and simplermeasures of sediment properties, such as density and sand content, to be investigated. The criticalsurface erosion shear stress is a fundamental parameter for input into predictive models of coastaland estuarine cohesive sediments. Example of sites are:

- intertidal mudflats - river banks - coastal areas - sewer and drainage systems - saltmarshes - reservoirs

To date SedErode (and its predecessor ISIS) have been used to measure in excess of 130measurements of surface critical erosion shear stress at sites covering the Dollard Estuary(Netherlands), 3 sites in Severn Estuary (UK), Humber Estuary (UK), Tollesbury Creek, Essex(UK), and Mersea Island, Essex (UK).

6. References

Graham D I, James P W, Jones T E R, Davies J M, Delo E A. (1992). Measurement andprediction of surface shear stress in an annular flume. ASCE Journal of Hydraulic Engineering,Vol 118, No 9, 1992.

ETSU (1992). In-Situ Erosion of Cohesive Sediment. Energy Technology Support Unit,Harwell, UK, Report No. ETSU-TID-4112.

Williamson H J and Ockenden M C (1996). ISIS: An Instrument for Measuring Erosion Shearstress In-Situ. Estuarine, Coastal and Shelf Science Vol 42, pp1-18.

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properties of dredged material - gov.uk

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