A guide for carousel experiments on cohesive sediment

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TechnischeUniversiteit Delft

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A guide for carousel

experiments on cohesive

sediment

R. van der Ham Report no. 2-96

Faculteit der Civiele Techniek

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A guide for carousel experiments

on

cohesive sediment

March 1996

R.van der Ham Hydromechanics Section

Hydraulic and Geotechnic Engineering Division Department of Civil Engineering

Delft University of Technology Delft,The Netherlands

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Acknowledgements

This work was supported by the Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organisation for Scientific

Research.

I would like to thank Cees Kranenburg and Han Winterwerp who provided valuable comments and constructive criticism.

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1 INTRODUCTION ••••.•••••••••••••••.•••••.•.••.••••••••••••••••••••••••••••••••••.••••••••••••.•..••.•.••••1 ••••••.••.•..•..•..•

2 CHARACTERISATION OF COBESIVE SEDIMENT 2

2.1PHYslCO<HEMICAL PROPERTJES 2

2.2ERODIBILITY OF MUD 3

2.2.1Hydromechanical approach 4

2.2.2 Soil mechanical approach 4

2.2.3Liquefaction 6

3 mE CAROUSEL IN mE LABORATORY OF HYDROMECHANICS ••.•••••.•.•..•....••.7••

3.1DESCRIPTIONOF THE CAROUSELANDANOPERATING STRATEGY FOR TOP-LIDANDFLUME

SPEEDS 7

3.2FLOW CONDmONS IN THE CAROUSEL 8

3.3THE INSTRUMENTSUSED 9 3.4POSmONING AND SAMPLINGFREQUENCYOFTHE TURBIDITY METERSANDTHE

ELECTROMAGNETJCFLOW METERS 10

4 THE BASIC MEASURING PROGRAMME ••••••.•••••••••••••••••.•••..•.•.•••...•.•••••••••••.•.•.•.12•.•.•••.

4.1SAMPLING 12

4.2TREATMENT OF THECOHESIVESEDIMENT 12

4.3PHYSICO-CHEMICAL AND RHEOLOGICALPROPERTJES 13

4.4EROSIONAND DEPOSmON BEHAVJOURIN THECAROUSEL 14

4.5SETTLING/CONSOLIDATJON TESTS 14

5 DATA PROCESSING PROCEDURE ...•...••••••••••.•••••••••..•••••••••.••••.••••••.•••...•..•.••15••.••.•.••

STAGE 1 RAwDATA 15

STAGE2VALIDATIONANDCALIBRATION 16

STAGE3PRiMARYDATA 16

STAGE4COMP ARIS0N ANDINTERPRETATION 16

LITERATURE 17

APPENDIX A CHARACTERISTICS OF THE INSTRUMENTS USED ...•...•••••..•..•..•...•...1

APPENDIX B METHODOLOGY OF SEDIMENT ANALYSES ••••••••••••••••.•...•...•.•.••..•nr...

APPENDIX C MATLAB ROUTINES FOR CALmRATION AND DETERMINATION

OF PROCESS VARIABLES V

APPENDIX D REGISTRATION FORM FOR NATURAL COHESIVE SEDIMENT ON BE HALF OF EXPERIMENTAL RESEARCH (LABORATORY OF

HYDROMECHANICS) VI

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Figure 1.1 Carousel of Laboratory of Hydrornechanics, Delft University of Technology

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1 Introduction

Carousels have been developedin an attempt to approach the in situ hydrodynamic conditions and to obtain arealistic simulation of the erosion and deposition processes of cohesive sediments. The main advantage of a carousel over straight flumes is that efIects of inflow and outflow are absent and no circulation pumps are needed which would break up sediment flocs. Figure 1.1 represents the flume, of the carousel type,

in the Laboratory of Hydromechanics Delft University of Technology. The purpose of this guide is to contribute to carousel experiments on natura! cohesive sediment in the Laboratory of Hydromechanics. The use of standardised international accepted methodologies is advocated herein, thereby followingthe EC-MAST-Iprogramme (1993), in order to make comparisons possible between present results and the results obtained at other institutes.

Chapter 2 of this guide presents a short description of the characteristic properties of mud and their role in the erodibility of the mud.Some differences between the soil mechanical approach and the hydromechanical approach are explained. Chapter 3 briefly describes the carousel and the most important properties of flow structure in it.Anoperating strategy is provided, which minimizes undesirable secondary flow cells (Booij,1994) and the minimum measuring frequency needed for calculating the Reynolds shear stress and the turbulence intensities. A Measuring Programme is presented in Chapter 4 which followsthe main lines of the Delft Hydraulics measuring programme, which reflects many years of experience with carousel

experiments on natural mud (Winterwerp, 1992). Chapter 5 presents a proposal for a data processing procedure,and a set up of a simple database. The database contains the data sets obtained during the carousel experiments and additional (rheological) experiments.

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1. Physico-chemical properties

of the over flowing fluid

Chlorinity Oxygen content pH

Sodiwn Adsorption Ratio (SAR) Temperature

Redox Potential

3.Water-bed exchange processes

Critical shear stress for deposition

Critical shear stress for erosion as a function of the sediment concentration

Erosion rate as a function of sediment concentration

Settling velocity distribution at various sediment concentrations (Sedimentation balance)

Consolidation coefficient

Permeability as a function of sediment concentration (Capillary Suction Time, (CST) NEN 6690)

IInprnproLimits

2.Physico-chemical properties ofthe mud

Chlorinity Redox Potential Grain size distribution of Oxygen content Sand content detlocculated sediment pH Organic content Cation Exchange Capacity Temperature (EPS content) (CEC)

Specific surface area_(SSA)

Bulk density

Viscosity at various conc.

Bingham strength, Yield strength at various sediment concentrations

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2 Characterisation

of cohesive sediment

Cohesive sediment is composed of sand, silt, clay, organic compounds and organisms.

The composition may vary in time and space due to chemical and biological processes and physical sorting processes. A certain type of sediment can be defined and

characterized, to a certain extent, by its physico-chemical properties and its (time

dependent) rheological behaviour, see Table 2.1 (EC MAST-I, 1993).The objective of research on cohesive sediment transport is to relate the physico-chemical properties

and the rheological properties of the sediment toits behaviour during processes of

interest, which herein are the processes involvedin the erosion of dense beds and

soft deposits, and entrainment of fluid mud under shear flow conditions.

2.1 Physico-chemical properties

The most basic physico-chemical properties are the dry density of the solids,the organic content, the partiele size distribution, the specific surface area, and some

chemical and mineralogical properties. These properties are determined with standardised methods in order to make comparisons possible between the results obtained by other researchers (see EC MAST-I, 1993).The methodology ofthe

analyses of the physico-chemical properties carried out in the Laboratory of Hydromechanics isbriefly discussed in Appendix B.

The Delft Hydraulics carousel research on the relation between the physico-chemical properties of different types of mud from the Netherlands and their erosion

behaviour forms a goodstarting-point, Therefore a limited summary of their findings is presented herein. For a comprehensive discussion the reader is referred to

Winterwerp (1992) and Mulder (1991). Delft Hydraulics found that the mineralogical compositions of sediment samples from the Western Scheldt, EmslDollard, Loswal Noord,Hollands Diep,Lake Ketel, Meuse and the Biesbosch do not differ much. The absence ofmontmorillonite, a clay with a high specific surface area, is noteworthy. The organic content of the cohesive sediment is fairly high, 5-12%,the oxygen content is low, and because of that the redox potentialof the bulk of the sediment is negative. The sand fraction (>63 urn) ofthe different types of mud varies widely (7-69%) and the lutum fraction

«

2 urn) varies about by a factor 2 (14-30%).

The sediments described above were eroded,by means of a stepwise increasing current. Winterwerp (1992)reported that the critical shear strength for erosion "te,

which is about 0.1-1.0 Pa, appears to increase with increasing density of the

sediment bed. The density increase with depth was found to correlate with the sand fraction, to a certain extent. Whenever the sand fraction was larger than

approximately 50% (by mass), segregation and sorting effects occurred which hampered the interpretation of the experimental results. An increase of the consolidation period from 1 to 7 days, leads to an increase in bed density and in "te

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resistance against erosion. Furthermore, some correlation has been found between the specific surface area (SSA) and the lutum fraction. In conclusion Winterwerp (1993) found that the accuracy of the results, apparently inherent to the carousel

experiment procedure applied, istoo low to distinguish between the sediments

and/or to correlate the erosion results to one of the physico-chemical bulk parameters other than the density and the sand content.

These findings show that carousel research is less suitable to discriminate between the erosion behaviour of the different types of'natural' muds from the Netherlands. The time histories of the suspended sediment concentrations during the carousel experiments were described fairly well for all cohesive sediment samples with two simpIe 'best fit' empirical relationships for the erosion rate (E)and 'tewithin a factor

2 to 3 (Winterwerp, 1993):

The erosion of mud/sand mixtures has been studied in the and in the field by Torfs (1995) and Williamson (1994), respectively. The laboratory experiments on a

mixture of sand (D50=230urn) and fines showed an increasing erosion resistance

with an increasing percentage of fines. Fines are defined here as sediment particles with a diameter smaller than 63 um.The fines fill up the pores between the sand grains and form electro-chemical bonds. At a critical percentage of fines in the

mixture (10-20%)the sand grains loose contact,and the sediment should be treated

as cohesive. Torfs (1995) concludes that layered bed structures are always to be expected in cases of absence ofbiological activity, initial mud concentrations below the gel point, or high settling rates. In the field layered bed structures are often

encountered (Williamson,1994). In research on natural cohesive sediments

segregation effects should be reckoned with since natural sediments are almost always a mixture of fines and sand.

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2.2 Erodibility of mud

The mud properties of importance for erosion and entrainment of mud depend

amongst others on the types of failure mechanisms that are responsible for the break up of the intern al bed structure and the erosion of the sediment bed. The failure mechanisms are plastic deformation, swell, shearing, crack forming, fluidization and liquefaction.

For the moment, there is a difTerencein approach of the erosion of sediment beds between researchers with different backgrounds, that is, soil mechanics and

hydromechanics. The most clear distinction is that in soil mechanics the focus is on applicability of typical clay properties, like liquid limit and plastic limit,for the description of the behaviour of mud, with promising results. In hydromechanics the mud is looked upon as, generally speaking, a rheological 'fluid'with a time and shear rate dependent internal structure. It will be made acceptable herein that the

approaches are supplementary (see also Winterwerp, in EC-MAST-IIG8, 1995).

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2.2.1Hydromechanica! approach

The hydromechanical approachis weUrepresented by the latest Kranenburg and Winterwerp model (1996)or the Mehta & Srinivas model (1993),(see also Reuber,

1994).These models consider the entrainment of a fluid mud layer into a overflowing turbulent water layer, the so-called'mixed layer',under shear flow conditions. The

starting point of these models is the balance equation of the turbulent kinetic energy (TKE).TKE is produced by shear near the walls and top-lid of the carousel and possibly at the fluid-mud water interface. The interaction between the TKE, the

entraiment process and the rheology of the fluid mud is incorporated in the Kranenburg model by the followingprocesses. Ifthe viscosity of the fluid mudis high, the fluid mud is dragged along with the mixed layer and the sounding of turbulent eddies into the mud is hampered and the entrainment rate is smalI. If the difference in velocity between fluid mud and overflowingwater is enlarged, the

turbulence becomes dominant which results in an even further increase in velocity difference and thereby increased entrainment rates. This process is self reinforcing. The yield strength is assumed to reduce the shear production of TKE,because part of the deformation work is needed for the break up of the sediment structure and therefore it reduces the entrainment rate. A part of the TKE is needed for keeping the sediment in suspension. High settling veloeities will reduce the TKE and thereby reduce the entrainment rate. In conclusion Kranenburg (1996) states that for the entrainment of soft mud beds a number of properties are of importance of which are

mentioned herein:

• Bulk density of the fluid mud (p) • The yield stress of the sediment bed (ty)

• The viscosity of the fluid mud for the applied shear rates (u)

• The (hindered) settling velocity as a function of the suspended sediment concentration (w.)

These properties depend amongst others on the stress history of the sediment bed and the typical mud properties and they should be determined from appropriate tests.

2.2.2SoHmechanica} approach

The soil mechanical approach is represented by the Van Kesteren erosion model (Bisschop,1993).This model takes both drained and undrained failure mechanisms into account. Surface erosion is associated with swell,a drained process,and bulk erosion is associated with locally exceeding of the undrained strength in the sediment bed. In both cases the turbulent shear stress is much smaller than the peak strength as weUas the remoulded strength of the top layer of the sediment bed. For exa-nple a homogenous cohesive sediment layer of mud with an undrained peak strength of 200 Pa and a remoulded strength of 100 Pa is eroded by a current at a

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~ 2~~""," 10 100 1000 Shear strength (Pa) Source:Van Kesteren'94

Figure 2.1 The types oferosion that can occur for an imaginary sediment as a function ofpeak strength and average turbulent shear.

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turbulent shear stress of2 Pa. Bulk erosion occurs at 4 Pa (Van Kesteren, see also Figure 2.1).

Important properties in the surface erosion model are: the diffusion rate of the 'water over pressure', the critical softening depth necessary for scouring off a sediment layer, and the total amount of water necessary to reach a certain water content that will allow break up ofthe bed structure (Van Kesteren 1991, Bisschop 1993). The investigation into this type of erosion requires the determination of the following properties:

• Capillary Suction Test value (relative value) (CST) • Consolidation coefficient (cv)

• Water content (w)

• Peak shear strength (ap),determined with a vane test • Liquid Limit (method Casagrande) (LL)

• Plastic Limit (PL)

For a description of the last two properties the reader is referred to Verruijt (1983) and Bisschop (1993). These properties allow the testing ofthe Van Kesteren surface erosion formula (Bisschop,1993):

Cv V =(r-r )----....:...._----, er 10Dso(1+ 2.65wt (up - 10)

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Veis similar to an entrainment velocity but it is of course a few orders of magnitude smaller, "tcr is the critical shear stress for erosion,"tis the bottom shear stress. D50 is

the median partiele diameter and 'a' is a constant. The factor 1ODso(1+2.65wt is a

measure for the depth over which the sediment has to be weakened before erosion can occur. The factor (up -IO)is a measure for the increase in water content (swelling) needed before the top layer of the bed can be eroded.

Bulk erosion is associated with local failure of the sediment bed, for example crack forming. Parts of the sediment are torn apart from the bed and are brought into suspension. If the failure takes place easily,the bulk erosion rate could be controlled by the TKE. In that case,an entrainment model based on the TKE budget equation could be appropriate to describe bulk erosion and the hydromechanical approach is supplementary even for erosion of sediments with high peak strengths. Local failure can result from high localloading due to turbulent pressure fluctuations for example as described in Bisschop (1993),or from localisation of stresses due to

inhomogeneous material behaviour. Until now no model is known to the author that describes such mechanisms quantitatively.

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Figure 2.2 Above:a compact bed (OCR> 1) will increase in volume if external

loading is applied.As a result of the increase in bed volume, the water content of

the bed increases, and the bed weakens (Van Kesteren, 1994). Below:a soft bed

(OCR = 1) will keep the same volume if external loading is applied. The bonds

between the sediment floes break down and the bed weakens. As a result the

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2.2.3 Liquefaction

If the external dynamicalloading induced by waves,for example,exceeds the yield strength of the bed deformation will take place.Liquefaction will occur in a soft bed with an over consolidation ratio of about one (OCR

=

1) if the sediment structure

breaks-up and a build-up of excess pore water pressure is expected. The excess pore

pressure is maintained as long as loading is applied and the restructuring of the bed is prohibited. The bed is now easily transported or entrained by a current (DeWit,

1994).When the externalloading stops it depends on the ratio of the time seales of the restructuring of the bed and the diffusion of the water over pressure whether the

mud becomes more consolidated than before liquefaction.

The liquefaction process of the sediment bed can be described by the poro-elastic

'Spierenburg model' (see De Wit, 1994) or by a simplified form of the former model (Van Kessel, 1996).Ifthe bedstructure istoo dense (OCR» 1),the volume ofthe

sediment bed will increase under deformation and pore under-pressure will occur (see Figure 2.2). In that case it depends on the water pressure diffusion whether the under pressures allow for further deformation.

In the carousel the effects of dynamicalloading are assumed to be small because of the absence of surface waves,but is still subject of investigation. The fluctuating instantaneous turbulent shear stress and fluctuating pressure could contribute to liquefaction of the sediment.

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motor toplid

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motor annular flurne

R=1.85 m

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3 The carousel in the Laboratory of

Hydromechanics

The experimental set-up in the Laboratory of Hydromechanics is described herein: the lay-out of the carousel and an operating strategy, the flow characteristics in the

carousel, the instruments that can be used, the positioning ofthe high-frequency

(and therefore highly sensitive) instruments, and the sample frequency that should be applied.

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3.1 Description of the carousel and an operating strategy lor top-lid and [lume speeds

The fluid within the flume is driven by a ring shaped rotating top lid,while the annular flume is rotating in the opposite direction in order to obtain uniform distributions of tangential velocity and bottom shear stress. A schematic representation of the carousel is shown in Figure 3.1. The carousel has a mean radius of 1.85 m,the flume has a width of 0.3 mand a height of 0.4 m.

DifTerences in tangential flow velocities in the carrousel cause differences in centrifugal forces which result in secondary flow cells in radial direction. The optimum flow conditions in the carousel are defined herein as 'minimal radial flow veloeities near the bottom ofthe flume',

For the optimised value of the ratio of the angular veloeities Booij (1994) found the empirica! relationship:

(1), +1

=

-1.17~ (3.1)

(1)I b

OOt and er(l/s) are the angular veloeities of the top lid,h (m) is the height of the fluid

in the flume and b (m) the width of the flume. Although the secondary flow

circulations are relatively small, the vertical veloeities near the side walls in many cases are larger than the fall velocity of the sediment. Therefore deposition

experiments should be interpreted with care.

The friction velocity of the flume,u. ,can be estimated from the following relationship (Booij,1994): -5

~ 2.0,'8

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Uav - UI

=

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==

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=

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±

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(3.2) u. IC ao

ur is the flume velocity and isequal to oocR, Uav is the averaged velocity, Ótis the

turbulent wall boundary layer thickness, ao(m) is an integration constant. À (-) is the friction coefficient and defined as:

Booij argues that for smooth flow the ratio of the inner boundary layer thickness (116 Ót) and the integration constant aois in the order of 5000, and the influence of the friction velocity on the logarithm of this ratio is relatively small. From the

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momentum balance and the assumption of a constant friction coefficientthe average velocity can be found:

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b

When LDAmeasurements are compared with these findings, it is found that too high an averaged velocity is predicted with an average error of 10%.Better

agreement is found if the ratio of the bottom to top lid friction coefficientsis set to approximately 1.3 (C. Kranenburg, pers. comm.). The LDA measurements gave as estimates for the friction coefficients:

(3.3) Àfl"- ::z 1.8E-3 Àtoplid =1.4E-3 ±10%

(U

av -tlJfR) Re

=

2 x 105 - 4 x 105 ±lO% v (3.4)

These values are in approximate agreement with the ratio of 1.3 and with the theoretical value of 1.6 E-3.Furthermore, Booijmentions th at the often used

relationship for the friction velocity found by Mehta and Partheniades (1973) results in too low values compared to the LDAmeasurements. This must be kept in mind when our findings are compared to the experimental results of other investigators.

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3.2 Flow conditions in the carousel

The flow conditions in the carousel can be compared best with wind driven shear flow conditions,although no surface waves are present. The water surface is fixed by a rigid toplid,and sidewalls are present. The turbulence in the carousel is more intense for a given bottom shear stress, compared to the field, due to the generation of extra turbulence near the sidewalls and the toplid.

Secondary circulations are present in the cross sections of the carousel and hamper the interpretation of the deposition experiments. In the optimal situation these circulations have veloeities of the order of 3% of the longitudinal velocity (Booij,

1994). In view of the magnitude of the secondary circulations it can be assumed that they are not of importance for the turbulence structure near the bottom, which is responsible for the flocbreak up and resuspension of the sediment. On the other hand, the vertical veloeities in many cases are larger than the fall velocity of the sediment. Therefore, it is difficult to teIl in advance whether the deposition process in the carousel is very different from the 'natural' process. If, for example,the deposited sediment is accumulated near the sidewalls of the carousel while the deposited material in the field is distributed equally, the interpretation of the experiments should be done with care. The experience with deposition experiments at Delft Hydraulics shows that their carousel (R=1.2 m) is less suitable, and the carousel at Delft University doesn't differ that much from the Delft Hydraulics one (pers. comm.Winterwerp).

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Turbulence structure

The hydrodynamic conditions near the bottom of the carousel should be chosen similar to the conditions in the field. The roughness of sediment beds in the field shows a large spatial variability. A mean roughness height of 0.03 m is common for mud flats, but this value is mainly due to tidal gullies and steep ridges. Large parts of the surface have typical roughness heights of 1 mm, so that the bottom shear stress on these parts can be much lower.Turbulence generated 'upstream' may have an influence on the local turbulent shear acting on the sediment bed. The absence of

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gullies and ridges in the carousel must be taken into account when the results obtained inthe laboratory for a certain bottom shear stress are applied to the field.

The bed shear stress shows a large temporal variability. The main production of turbulent kinetic energy and turbulent shear isproduced during so-called 'bursts',

with peaks in shear stress up to 10 times the mean (Nieuwstadt, 1992). The burst frequency fi,scales with the parameters of the outer flow: the water depth hand mean velocity Urn. fi,may be estimated by the following relationship (Booij,1992) :

fb =O.2~'" (3.5)

The mean veloeities in the model are comparable to the mean veloeities in the field but the water depth is drastically reduced. Therefore fi"modelwill always be larger

than fi"field. It is not known if this difference in burst frequency influences the

weakening or strengthening of the sediment bed.

It is assumed that the magnitude of the peaks in the bed shear stress in the model are not different from th at in the field. The turbulent fluctuations (Ui I)always seem more or less normally distributed (Booij,pers. comm.): the kurtosis of UiI is

approximately 3:

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It is assumed that the fluctuations of the bottom shear stress are also more or less normally distributed and independent of the Reynolds-number of the flow.As the higher peaks may be responsible for a significant part of the erosion process, this assumption may be critical and requires further research .

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3.3 The instruments used

The instruments used and the properties recorded are summarized herein, more details on reproducibility and accuracy of the instruments are given in appendix A. The following instruments are used:

• The density profile of the consolidated bed is measured by means of an Electrical Conductivity Probe (ECP)

• The yield strength profile of the consolidated bed is measured by means of a highly sensitive sounding method (Van Kessel, 1996)

• The turbidity is measured at two levels by means of high frequency turbidity meters (FOSLIMs)

• The suspended sediment concentration is measured at two levels by taking water samples frequently

• The tangential and vertical velocity components are measured at two levels by means of high frequency electromagnetic flow meters (EMF)

• The rotational veloeities of the top lid and of the flume are recorded

• The height of the mud· water interface is recorded on video by a CCD-camera. The yield strength profile of soft deposits can be determined by means of a newly developed method based on a sounding technique (Van Kessel, 1996). This technique has been applied successfully in a graduated cylinder. The technique is based on the

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Experimental set-up:FOSLIM and EMF probes

F LIM

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resistance of a sediment bed against a concentrated load: a vertical rod with a certain mass penetrates very slowlyinto the soft bed by means of its own weight,

while an electronic balance records which part of its weight is 'carried' by the bed. The stronger the sediment is, the larger the part of the weight it carries.

All electrical signals, except those from the CCD-camera, are recorded by a personal computer equipped with a DAP data acquisition board and DaisyLab software.

3.4 Positioning and sampling frequency of the turbidity meters and the electromagnetic flow meters

The positions of the turbidity meters (FOSLIMs) and the electromagnetic flow meters (EMF) in the carousel are of great importance, as mutual interference could disturb the measurements. Combinations of one EMF and one FOSLIM are placed at two levels above the sediment bed (see Figure 3.2).

The FOSLIMs are located opposite to the EMF sensors, inorder to make the

measuring volume as small as possible (~5 cmê).Tests were performed to determine whether the electromagnetic field of the EMF is disturbed by the FOSLIM or not. They show no disturbance of EMF signal, even at small distances of approximately 2 cm between the fibre heads and the EMF ellipsoid. The flow field and concentration distribution are probably somewhat disturbed by both the EMF ellipsoid and the optical fibres, but these disturbances are assumed to be small in comparison to the fluctuations that are responsible for the vertical exchange processes.

The sampling frequency is set to 20 Hz,twice the maximum filter frequency of 10 Hz applied for the EMF and FOSLIM sensors.The Taylor micro sealeXis a measure for the smallest scale contributing to the exchange processes of momentum and

sediment, and therefore the smallest scale of interest. It can be estimated from the followingformula for isotropie turbulence (Booij, 1992):

X=~2: V=5~ (3.7)

Where k is the turbulent kinetic energy and Ethe dissipation rate, v the kinematic

viscosity of the fluid, u the mean velocity, and z the distance from the bottom. Table 3.1 gives Xfor a few combinations of velocity and distance from the bottom:

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velocity u (mIs) 0.2 0.2 0.2 0.6 0.6 0.6 distance z (m) 0.05 0.1 0.2 0.05 0.1 0.2 Taylor micro scale (m) 2.9E-3 4.0E-3 5.7E-3 1.6E-3 2.3E-3 3.3E-3 Desired Frequency (Hz) 70 50 35 375 260 180 *v=1.3E-6m2/s

A minimum value for the sampling frequency is 14of the values given in the last column of Table 3.1(R. Booij,pers. communication) and this leads torelatively high sampling frequencies. This implies that, although mainly large turbulent structures are responsible for the vertical exchange processes, speetral analyses of the

measured signals must prove if the high frequency contribution to the vertical exchanges may be neglected.

Another measure for the measuring frequency is the Brunt-Väisälä frequency:

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I

N2

=

_K_

op

p

a

Nis the eigen-frequency of a fluid parcel in a stratified stagnant water situation. When the sediment is mixed over the water column low frequency internal waves are to he expected.Only at a pronounced density interface with a density gradient of 100 kg/rn"per 0.01 m the eigen-frequency N hecomes as large as 10 Hz. The effective measuring frequency is sufficiently high with respect to this value.

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4 The basic measuring

programme

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The basic measuring programme for carousel experiments at the Laboratory of Hydromehanics followsthe rnain lines of Delft Hydraulics measuring programme (see Winterwerp, .lJll!.gl. Sorne changes have been made in the programme because of new insights and differences in approach.The present programme focusesmainly on quantitative descriptions of processesinvolved in the erosion of natural sediment

beds while the Delft Hydraulics programme has been designed to characterise the

transport behaviour of different types of cohesive sediments from the Netherlands.

Furthermore, the present programme is extended with a number of mechanical tests, such as capillary suction test and rheological test (see also Chapter 2).

I

4.1 Sampling

1 The selection of the sample area

Beforehand some samples must be taken and analysed on contaminants, organic content and grain size distribution. Large sand contents (>50%) might hamper erosion experiments due to segregation efTects(Winterwerp, 1992). Large organic contents might influence the reproducibility of the behaviour of the mud due to degeneration of the organic material. Tests which could be performed in order to monitor the mud are small scale consolidation tests and redox measurements.

With reference to the Dutch Arbo-law, the form 'Registration form for natural cohesive sediment on behalf of experimental research' must be filled in and distributed (see Appendix D).

The selection criteria for taking samples must be selected in advance, as the sediment composition at alocation can change within a few weeks,the time between determining the location and the actual sampling should be as short as possible.

2 Sampling

100 kg dry material iscollected which is about 300 kg wet material or 250 1. Samples are collected with a 'Van Veen'grab from the upper 0.15 m ofthe sediment layer. In this way 8 containers of 40 I are filled.The water is pumped from mid depth and stored in 35 containers of 50 I each or is prepared using tap water.

3 Description of the area

During the sampling the conductivity, temperature, pH, the redox-potential and oxygen content of the sediment are measured. Some photographs of the sediment bed are made and the location of the samples is recorded.

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'

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4.2 Treatment of the cohesive sediment

The cohesive sediment is treated according to the pamphlet Working with Natural Cohesive sediment' of the Laboratory of Hydromechanics (1992). The sediment is sieved with a lmm lattice, if necessary, and it is kept at a low temperature and in the dark. The sediment mixing period needed to obtain a suspension which is in chemical equilibrium,is estimated at one or two weeks.

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I

The speed of the mixer should be set to a low value, as high shear rates tend to break up the floc structure. In case of placed bed experiments the mud is put directly into the carousel.

I

4.3 Phvsico-chemical and rheological properties

A standard procedure for characterising a cohesive sediment was developed within the framework of the EC MAST-l programme, the physico-chemical properties that should be determined are given in Table 2.1. The standard procedures and the performing institutes are:

• Deflocculate the cohesive sediment and determine the grain size distribution with a Sedigraph (by Delft Geotechnics)

• Determine the specific surface area (SSA) (by GD)

• Determine the organic content, the sand content, the oxygen content (by GD) • Determine the pH, the redox potential, temperature and salinity (TUD) • Measure SAR and CEC (by GD)

• Make some photographs of the sediment with a microscope (TUD)

• Determine the amount of EPS and the remainder of the organic content (Micro biology, Amsterdam University)

The rheological properties for carousel experiments on soft soils and fluid mud (OCR

=

1) (TUD)

• The yield stress of the sediment bed as a function of depth (ty)

• The viscosity of the fluid mud for the applied shear rates (u)

•

The (hindered) settling velocity as a function of the suspended sediment concentration (w.)

I

The soil mechanical properties for carousel experiments on dense sediment beds (OCR» 1) (GD)

• Liquid Limit (method Casagrande) (LL) • Plastic Limit (PL)

• Capillary Suction Test value (relative value) (CST) • Consolidation coefficient (cv)

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• Water content (w)

The determination of the properties should be performed on sediment samples representative of the bulk. When the sediment is not mixed before it is brought into the carousel, as is the case for placed bed experiments, measures should be taken in order to find out if the sample represents the bulk. In that case sample techniques should be developed.

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4.4 Erosion and deposition. behaviour in the carousel

The design of the experiments depends amongst others on the objectives ofthe research and the possibilities of the instrumentation. Roughly, the following types of carousel experiments can be distinguished:

1. erosion or entrainment experiments on fluid mud 1 to 5 hours consolidation time, ty <1 Pa

2. erosion experiments on soft beds

1 to 7 days consolidation time which allows for the build up of a (firm)internal structure: ty

=

1-100 Pa

3. erosion experiments on placed beds

"ty> 100 Pa

4. deposition experiments

It is assumed th at the conditions in the carousel are not ideal for deposition experiments (Booij 1994,Winterwerp personal communication).

The deposition experiments in the carousel, however, play an important

monitoring role. After each erosion experiment a deposition experiment can be performed to find out if the deposition behaviour of the sediment, which is closely related to the flocculation capabilities of the sediment, has changed during the course of time.

Figure 2.1 presents a fair insight in the types of erosion that can occurduring the erosion experiments on a typical sediment bed for different yield strengths.

I

4.5 Settling/Consolidation. tests

In this test the settling velocity is determined under hindered settling conditions. The test is started with a homogeneous suspension of 5 gil in a graduated

column. The height of the interface between the clear water and the mud is recorded visually, from which the settling velocity can be calculated. After the test the bed density profile will be measured with the ECP meter and the strength profile with the sounding method.

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5 Data processing procedure

The carousel experiments together with the additional experiments, like rheological experiments and sounding experiments, result ina large number of data files,plots, and observations. In order to keep the large number of data accessible, the raw data isplaced in a database. After that the data is validated, calibrated, and reduced in magnitude, in a way that the experiments can be analysed and compared with models and results from research which has been carried out previously at the Hydromechanics Laboratory and at other research institutes.

The following stages can be distinguished:

Sta~e1 7 Sta~e 2 7 Sta~e 3 Raw data Validation and Primary data

Calibration Sta~e4 Comparisonwith numerical models

I

and previous measurements

I

Stage 1Raw data

The signals from measuring devices in the carousel are sampled and stored during the experiments with a frequency of twice the hardware filter frequency. Apart from that, the signals are filtered on-line with a DaisyLab software specification. The filtered signals can be sampled and stored again at much lower frequencies. This data set can be used for quick analyses in between the experiments on basis of which decisions can be made about the adjustment of coming experiments or the repetition of experiments that have been carried out previously.

Furthermore, before and after each carousel experiment additional small seale experiments, like consolidation experiments and rheological experiments, are carried out in order to support the carousel experiments. These additional experiments result in data sets.

All data-sets are placed in the following type of data-base:

I

Database

CAROUSEL EXPERIMENTS 'NAME'

EXP. SUB-EXPo DATE and TIME DESCRIPTION LOCATION Start Stop

El EI_CARR.ase rnmIdd hh:mm hh:mm 40Mb, EMFIX, CD SV74_# EMFIY, etc. EI_ECP _A.ase EI_ECP _B.ase El_Sound.ase etc. E2 etc.

I

The easiest way is to make this database is to use ACCESS 2.# which is available at the server dutcvs5.

All data stored on the hard disk of the 'carousel-computer' and the data from the additional measurements are backcd-up on a daily basis on tape. After the experiments these tapes can be put on CD with a SV74 code.

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Stage 2 Validation and Calibration

The accuracy of the data concerning for example; the velocity, the suspended

sediment concentration and the bed density profile, depends amongst other things on the characteristics of the instruments, and their calibrations. The calibrations of most instruments are known, except for the recently developed FOSLIM (by Delft Hydraulics). The FOSLIM has to be calibrated 'insitu', each experiment, because it is assumed that the FOSLIM is highly sensitive for changes in floc size and floc density (see also Appendix A).

Appendix C provides in an example of a MATLAB routine that has been used for the processing of the raw data from field experiments which were carried out in the EmslDollard estuary. The same routine can be used for the processing of data form turbulence measurements in the carousel.

Stage 3Primary data

For each type of experiment the results can be presented in plots, for example, as follows:

Deposition experiment

• Time histories of concentrations and bed shear stress • Dry density profile of the bed

• Yield strength profile of the bed • Bed height during the experiment Erosion experiments

• Time histories of concentration and (estimated) bed shear stress • Mean concentration 0 (estimated) bed shear stress curves

• Time histories of Rib and Rif (for definitions; see Appendix E) • Ruw0 Rifand Rcw0 Rif curves (for definitions; see Appendix E)

• Time histories of the turbulence intensities and shear stress !L:, W'2, u'w' at

approximately 5 and 15 cm above the bed

I

Stage 4 Comparison and interpretation

The experimental results can be compared to results obtained from previous carousel experiments carried out with mud from the EmslDollard by Delft Hydraulics, or with carousel experiments carried out with mud from the Caland Channel in the

Laboratory of Hydromechanics, or with field experiments carried out in the EmslDollard. The results of entrainment of soft deposits can also be compared to numerical computations carried out with an entrainment model (Kranenburg, 1996) or a lDV kog model including gravity efTects(Uittenbogaard, 1996).

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Literature

I

Lau, Y.L., Krishnappan, B.G.,'Does Reentrainment Occur during Cohesive Sediment Settling?', J.Hydraul.Eng. ASCE 120(2),236-244, 1994

Laurence, D.,Maupu, V., Galland J.C., Teisson, C.,'A sediment laden open channel flow simulation with recent Reynolds stress-flux transport models', Communications

HE-41192.33,EDF, Laboratoire Nationale d'Hydraulique Department, 1992

K~sel, T.van, Kranenburg, C.,Wave-induced liquefaction and transport of mud on inclined beds', Proc. PECS '96,Den Haag, The Netherlands, (in prep.)

Kranenburg, C.,'The Fractal structure of Cohesive Sediment Aggregates', Estuarine,

Coastal and ShelfScience, Vo1.39,p.451-460, 1994

Kranenburg, C.,Winterwerp, J.C.,'Erosion offluid mud layers, Part 1,entrainment mode!',J. Hydraul.Eng. (accepted)

Krishnappan, B.G.,Engel, P.,'Critical shear stresses for erosion and deposition of

fine suspended sediments in the Fraser river', INTERCOH, Ontario, 1994

Leussen, van,W., Winterwerp, J.C., 'Laboratory experiments on sedimentation of fine-grained sediments: A State-of-tha-Art review in the light of experiments with the Delft Tidal Flume' ,Coastal and Estuarine studies, Vo1.38,Residual Currents and Long-Term Transport, Ed. R.T. Cheng,p.241-259, Springer-Verlag 1990 Leussen, van, W.,' Estuarine macro flocs and their role in fine-grained sediment transport, Ph.D. thesis Utrecht University, Department of Earth Science, Utrecht,

The Netherlands. 1994

Leussen, van, W., 'Aggregation ofparticles, settling veloeities of mud flocs - a

review', Physical processes in Estuaries, Eds. Dronkers, J., Leussen, van, W., p.427-446, Springer-Verlag 1988

Mehta, A.J., Partheniades, E., 'Depositional behaviour of cohesive sediments', Techn.

Rep. no 16,Coastal and Oceanographic Engineering Department, Universityof

Florida, USA,1973

Metha, A.J., 'Laboratory studies on cohesive sediment deposition and erosion', Physical Processes in Estuaries, Eds. Dronkers. J., Leussen. van, W., p.427-446,

Springer-Verlag 1988

Parchure, T.M.,Mehta, A.J., 'Erosion of soft cohesive sediment deposits' , J. of Hydr. Eng. Vol. 111, No. 10,1985

Partheniades, E., Cross, R.H.,Ayora, A.,'Further results on the deposition of cohesive sediments', Proc., l I'" Conf. on Coastal Engrg., London, UK,2 ,723-742 Torfs, H. Erosion ofmud/sand mixtures, Ph.D. thesis Leuven University, Belgium,

1995

Verruijt, A., 'Grondmechanica', Delft University Press, Delft, 1983

Williamson, H.J.,'Recent Field Measurements of erosion shear stress using ISIS', Proc. INTERCOH '94,Wallingford, UK (in prep.)

Winterwerp, J.C., 'A simple flocculation model for cohesive sediment', abstract for

the 25th International Conference on Coastal Engineering Florida September 1996,

Delft Hydraulics, 1996

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Winterwerp, J.C., Kranenburg, C.,'Erosion offluid mud layers. Part 2,Experiments

and model validation', J. Hydraul.Eng. (accepted)

Wit, P.J. de,'Liquefaction ofCohesive Sediments caused by Waves', Ph.D. thesis

Delft University of Technology,The Netherlands, 1994

RSWIDELFT HYDRAULICS REPORTS

I

Bisschop, F., 'Report and evaluation of erosion experiments on clay and mud', Delft

Hydraulics, BAGT 531, 1993 (in Dutch)

Cornelisse, J. M.,Kuijper, C.,Winterwerp, J.C. 'Erosion and deposition

characteristics ofnatural muds, Sediments from the Eems-Dollard (Eems Harbour)',

Rijkswaterstaat) Waterloopkundig laboratorium, Report 26, 1990

Cornelisse, J. M., C Kuijper, C.,Winterwerp, J.C.:In situ erosie meters' (in Dutch),

Rijkswaterstaat) Waterloopkundig laboratorium, Slibonderzoek, Report 45, 1993 Kesteren, W.G.M. van, 'Preliminary study mud dredging, phase 1',BAGT 468, Delft Hydraulics, 1991 (in Dutch)

Kuijper, C.,Cornelisse, J. M.,Winterwerp, J.C. 'Erosion and deposition characteristics ofnatural muds, Sediments from the Eems-Dollard (Delfzijl)',

Rijkswaterstaat/ Waterloopkundig laboratorium, Report 35, 1990

Mulder, H.P.J., 'Properties ofmud from two sites in the Ems/Dollard estuary on basis of Laboratory measurements' (in Dutch), Rijkswaterstaat, GWAO-91.12006,

1991

Uittenbogaard, R.E.,et.al., '3D Cohesive Sediment Transport, A preparatory study about implementation in DELFT3D' ,Delft Hydraulics, VRI251.96/Z1022, 1996 Winterwerp, J.C., J.M. Cornelisse, C. Kuijper, 'Erosion ofnatural sediments from The Netherlands, Analysis of laboratory experiments', Rijkswaterstaat/

Waterloopkundig laboratorium, Report 38, 1992

Winterwerp, J.C., J.M. Cornelisse, C. Kuijper, 'Erosion ofnatural sediments from The Netherlands, Model simulations and sensitivity analysis', Rijkswaterstaat/ Waterloopkundig laboratorium, Report 43, 1993

I

MAST-REPORTS

I

Variouproperties to charas authors, 'On thcterisee methodology and accuracy ofmeasuring physico-chCohesive Sediments', EC MAST-I, Coastal emical morphodynamics, 1993

Various authors, 'OVERALL WORKSHOP abstracts-in-depth', EC MAST-2, G8 Coastal Morphodynamics, 1993

Various authors, 'FINAL OVERALL MEETING project summary abstracts-in-depth

institutes' reference list, EC MAST-2,G8 Coastal Morphodynamics, 1995 Vriend, H.J. de, 'FINAL SCIENTIFIC REPORT', EC MAST-2,G6 Coastal Morphodynamics, 1992

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LABORATORY OF HYDROMECHANICS REPORTS

Booij, R. 'Turbulence in hydraulics' (in Dutch), Lecture notes b82, Department of Civil Engineering, Delft University of Technology, 1992

Booij, R. 'Measurements of the flow field in a rotating annular flume', Communications on Hydraulic and Geotechnical Engineering, Report 94-2,

Department of Civil Engineering, Delft University of Technology, 1994

Fontijn, H.L., 'A guideline for working with natural mud in the Laboratory of Hydromechanics' (in Dutch), Department of Civil Engineering, Delft University of Technology, 1994

Kessel, T. van, 'Small-scale sounding tests on soft saturated cohive soils',Report 7-96, Delft University of Technology, Department of Civil Engineering, 1996

Kranenburg, C.,'Anentrainment model for fluid mud', Report 93-10, Delft University of Technology, Department of Civil Engineering, 1993

Merckelbach, L.M., 'Consolidation theory and rheology of mud, A literature survey',

Report 9-96, Delft University of Technology, Department of Civil Engineering, 1996 Reuber, J.,'Physical and numerical modelling of the erosion of fluid mud layer due to entrainment', Report no. 8-94,Delft University of Technology, Department of Civil Engineering, 1994

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Appendix A Characteristics

of the instruments

used

I

High Frequency Turbidity meter (FOSLIM)

The high-frequency turbidity meter was recently developed at Delft

Hydraulics and is based on the principle of adsorption and reflection of light by the particles in suspension. The FOSLIM has its measuring volume between two optical fibres. lts range and measuring volume vary with the distance between the fibre heads, but they are normally about 1 . 10 kg/rnê and 1 cmêrespectively. The standard deviation is about 50 mgll] (has to be verified by experiments). After use under field conditions no significant fouling was found and daylight disturbance is prevented by the very narrow daylight filter which is implemented in the electronics. The main advantage of this newly developed instrument is that it can measure the turbidity and its fluctuations 'undisturbed' and instantaneously with a relatively high frequency of 10 Hz.

The accuracy of turbidity meters depends largelyon the calibration

technique used, because the translation from turbidity to concentration is complex.Two suspensions with the same sediment concentration, but with different sized and shaped sediment particles or floes can show a different turbidity. One can expect that a standard deviation for the measurements of approximately 5% can be achieved when the FOSLIMs are calibrated

continually during the experiments and the measuring range is adjusted to the expected concentrations.

High frequency Electromagnetic Flow meter (EMF)

The EMF is suitable for measurements in cohesive sediment suspensions: its signal has been found invariant to the suspended sediment concentration (de Wit, 1992). The standard deviation of the EMF is 1%over the selected fuIl scale and 0.5 cm/s for absolute veloeities below 20 cm/s, the zero stability is better than 1 cm/s a day. The sensor has an ellipsoidal shape (lIx33 mmê) with the cylindrical measuring volume just in front of it. The measuring depth is approximately 0.5 cm. The EMF meters must be kept 15 cm apart in order to prevent interference and must be grounded to avoid zero drift during the experiment or translation. To prevent ground loops an isolation transformer is recommended. For more detailed specifications the reader is referred to the technicaImanual (Delft Hydraulics).

Density meter (ECP)

The conductivity probe (Electrical Conductivity Probe) is used for

determining the density profile of the sediment bed. lts operation range is from 250 to 1300 kg/m> and it has an accuracy of 10% (De Wit, 1992). It can be used only in saline water and a vertical variation in organic content in the top layer of the sediment bed reduce its accuracy substantially

(Cornelisse, 1993).

The principle of the ECP is based on the decrease in conductivity of water with a constant composition when sediment is added. The measuring volume is 3 mm-. The ECP has four separate electrodes and is equipped with an alternating current in order to eliminate polarisation effects.

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When the composition of the water, temperature or the sediment composition changes, due to varying organic contents for example, the

accuracy of the ECP can decrease markedly. Cornelisse-found a difference up to 200 kg/m>between the results from an acoustic density probe and the ECP when used in the Ketelmeer (Cornelisse, 1993). In the laboratory the sediment is mixed thoroughly before it is added to the carousel and the temperature is nearly constant, therefore it can be assumed th at the ECP provides reliable measurements, though when segregation is observed the

measurements must be interpreted with care.

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Appendix B Methodology of sediment analyses

(Laboratory of Hydromechanics)

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Piltration. of water samples

A common method of measuring the sediment concentration in a water

sample is filtration. DeWit and Mootdeveloped a method for a very accurate

determination of the concentration (De Wit, 1992). They also mention that:

'itis advised to consider beforehand how accurate the determination has to

be, if a rough estimation is good enough, several actions inthe procedure

may be omitted'.

The method:

1. Place the filter on the support plate of the filter funnel and rinse it with

distilled water in order to remove all loose or soluble material.

2. Place the filter with asmooth-tip forceps in a clean Petri dish and dry the

filter in an oven for half an hour at 100 OC.From now on the filter should

only be toughed carefully by a smooth-tip forceps.

3. Remove the filter from the oven and place it in a desiccant for two hours

in order to cool down and to prevent the adsorption 0f water. If the colour

of the gel is pink, the gel is fully saturated an it should be dehydrated

first. The gel is bleu in a dehydrated state.

4. Take the filter out ofthe Petri dish and weight the filter as accurate as

possible and record its weight: Wfil.

5. Place the filter on the support plate of the filter funnel and replace the

upper chamber. Clamp the filter funnel together. Apply the vacuum very

shortly to avoid tearing the filter. Pour the weU-mixed sample with a volume Vs into the upper chamber. Rinse several time s with distilled

water to ensure that all sediment is at the filter and no salt has remained

in the filter.

6. Remove very carefully the filter from the support plate and put it in a

Petri dish. Place the Petri dish with the filter in an oven for half an hour

at 100 OC.

7. Remove the filter from the oven and place it in the desiccant for two hours.

8. Weight the filter carefully an record its weight: Wtot.

9. Calculate the concentration:.

C-_W,01 - Wfil (C.1)

Vs

The standard deviation of this method for the determination of the

suspended sediment concentration for 0.1 g China Clay suspended in 100 ml

distilled water was 1%. Defining the accuracy as three times this value the

accuracy becomes 3%.

For economy, the contribution of steps 1,2 and 3 to the accuracy can be

ignored. A method which can be used here is weighing the filters before and

after the rinsing and dehydration. If the difTerencein weight is smaU and

there is no great loss in accuracy, itis allowed to omit steps 1,2, and 3. The

sampling procedure itself, that is: comparison of the FOSLIM readings in

one location with the samples taken from alocation nearby, probably leads

to significant larger standard deviations.

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The membrane filters used are manufactured by Schleicher& Schuell (ME23) with a pore size of 0.15 mmo For more information on the filters is

referred tothe manufacturer. The filter funnel (Nalgene, Cat. No.315-0047)

is designed for filtration of liquids under fulI vacuum using 47 or 50 mm membrane filters.

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Conductivity measurements (density profile)

The ECP has tobe calibrated before and after each experiment with the

same pore water, sediment and at the same temperature. The procedure for

the calibration is:

• Clean the probe with a brush and tap water

• PIace the probe in the pore water and record the reading (Uelean)

• Place the probe in pore water with the maximum concentration (Cmax)

that is expected in the experiment and record the reading (Umax)(The volumetrie fraction should be less than 50%)

The concentration C can be derived with the following formula:

I

C

=

Cmax--""-=--

u.:

U -Uclear

Umax - Uc/ear U

(C.2)

The accuracy of this measuring technique is very sensitive to the quality of the calibration, if the calibration is carried out at a temperature different or with different pore water, the standard deviation can easily exceed 100 kg/m" (Cornelisse, 1993). Thorough mixing of the sediment sample is therefore very important and segregation effects should be observed very closely.

I

For information on pH-measurements, temperature measurements, and

weighing the reader is referred to De Wit (1992)

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Appendix C MATLABroutines for calibration

and determination of process variables

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%INPUT FROM THE USER

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Experiments and data-processing procedures

FREQ= ; REDUCT= ; FILT =;

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SPR_FOS=; SPK_FOS=SPR_FOSIFREQ; SPR_EMF=; SPK_EMF=SPR_EMFIFREQ; SPK_MEAN=;

% FREQ is the sampling frequency;

%REDUer averaging period before storage (sec.)

% FILT is the moving avofilter period used in the % procedure turbul.m in order to discriminate % fluctuations from the signals (sec.)

% Spike criterion (gIl/s)= max. increase rate of C. % Spike criterion (mls"2)= max.increase rate of C.

I

% The spike is replaced by an average value of % the surrounding points over the period SPK_MEAN % (sec.)

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% OFFSETs of EMF's and FOSLIMs obtained by means of calibrations:

% (Columns 1-10)

% The calibration is the following:

% VARIABLE[mls) or [kglm3] = (SIGNAL(V) - OFFSET (V)/RESPONSE(Vs/m)/(Vm3/kg)

I

OFF_TIME=;

OFF EMFIX_- = ._'_-OFF EMFIY= ._,

OFF EMF2X=_- ._,OFF EMF2Y= ._- _,

OFF EMF3X=_- ._,OFF EMF3Y=_- ._,

OFF_FOS1= ; OFF_FOS2= ; OFF_FOS3= ;

I

% The RESPONSE ofEMFs and FOSLIMs obtained by means of calibrations:

% (Columns 1-10)

I

RE SP_TIME=;

RESP EMFIX=_- ·_'-RESP EMFIY=·_,

RESP EMF2X=_- ·_'-RESP EMF2Y=·_,

RESP EMF3X=_- ._'-RESP EMF3Y=·_,

RESP_FOSI= ; RESP_FOS2=; RESP_FOS3=;

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% File locations and file names

I

Dir_r=sprintf('C:\\dir\\'); Dir_w=sprintf('C:\\dir\\'); Experiment=sprintf('N '); Year=sprintf('yy'); Julianday=sprintf('jd'); Num=; Column= ; ii=; FID=3;

% Directory raw data 'C\\data' for example %Directory results (note the double slash) % File identification:p=pole,b=bridge

% File identification year:95 or 96 for example %File identification Julian day:00 1 = 1 January % File-length

% Numb of columns is a1ways 10, if otherwise the % alterations of procedures turbul/calibr are necessary % ii is here the number of the first file for example 0

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%ii=O

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Check if the averaging period REDUCT is oke.

-I

Dummy 1=NumlREDUCTIFREQ;Dummy2=round(Dummyl);

Check=Dummy2-Dummy I; ifCheck-=O

fprintf('file lenght in sec.is not a whole number of times REDUC1\n');break, end

end

I

end

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %MAIN PROGRAM %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% This programme reads the data from input files,CALmRates thedata.removes SPIKES % and calculates the averaged variables and a number of TURBULent quantities,

% the output is stored in ascii-files

% The raw data input file is expected to contain JOcolumns in which are stored:

% EMFIX EMFIY EMF2X EMF2Y EMF3X EMF3Y FOSLIMI FOSLIM2 FOSLIM3

I

% The outputfiles contain:

%AVname.asc Averaged values ofvelocity and concentr.

% SPname.asc Number of Spikes encountered % UWname.asc Reynolds stresses

% CWname.asc Turbulenttransport of sediment vertical direction % CUname.asc horizontal direction

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% X2name.asc Turbulence intensities

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Main program

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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Call User input file u_input

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% Extravariables used for averaging procedures

AV=REDUCT*FREQ; % AV is the number of points used for a average value (for Ndum=NumlAV; % example =1200 ifREDUCT is60 secand the FREQ = 20Hz)

I

while FID-=-1;

ii

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %read the data

% it says:as long as there are files to read, continue u_read

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% calibrate the data

% Fill up the calibration vectors:

I

C_OFF(I)=OFF _TIME;

C_OFF(2)=OFF_EMFIX;C_OFF(3)=OFF _EMFI Y;C_OFF(4)=OFF_EMF2X;

C_OFF(5)=OFF_EMF2Y;C_OFF(6)=OFF_EMF3X;C_OFF(7)=OFF_EMF3Y;

C_OFF(8)=OFF_FOS 1;C_OFF(9)=OFF_FOS2;C_OFF( IO)=OFF_FOS3;

I

C_RESP( I)=RESP_TIME;

C_RESP(2)=RESP_EMFI X;C_RESP(3)=RESP_EMFI Y;C_RESP(4)=RESP_EMF2X;

C_RESP(5)=RESP_EMF2Y;C_RESP(6)=RESP_EMF3X;C_RESP(7)=RESP_EMF3Y;

C_RESP(8)=RESP_FOSl;C_RESP(9)=RESP _FOSI;C_RESP(IO)=RESP _FOS3;

% Detennine the calibrated data file called DATA for i=I:Num

(38)

I

DAT A(i,:)=(DAT A(i,:)-C_OFF).IC_RESP; end

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%remove the spikes according the spike criteria u_spikes

%save file

fname=sprintfï'sp'); u_save

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% calculate the average values of the signals over REDUCT sec.

I

for j=l:Column

for i=AV:AV:Num

result(i/AV j)=mean(DATA(i-AV+ l:ij»; end

end

I

%save file

fname=sprintfî'av'); id= 1; u_save result=zeros(Ndum,Column);

I

%%%%o/~~%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Make a Gauss filter (processes vector:f).

% Gauss curve for the convolution product

% f(x) = 1/ sqrt(2 pi sigma"2) * exp ( -(x-mu)"2 / 2 sigma "2)

I

sigma=FREQ*FIL T/2; N=7*sigma+ 1; mu=N/2; for i=l:N

f( 1,i)= 1/«2 *pi)"O.5*sigma)*exp(-((i-mu)"2)/(2 *sigma"2»;

end

.1

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Calculate the turbulent fluctuations by subtracting tbe running % mean (x_m) from the original signal.x_m is calculated from the % convolution product witb the vector f.

I

overlap=(N-l)/2;

for j=2:Column

x_m=conv(DATA(:j),f);

x_m=x_m(overlap+I:Num+overlap);

fluct(:j)=DATA(:j)-x_m';

clearx_m end

%overlap that will result from the conv. product % i= 1:fluctuations of time sign.are not relevant % running mean x_m

%remove overlaps

% calculate the fluctuations

% we don't needx_m for future calc.

I

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Calculate REYNOLDSTRESSES

A guide for carousel experiments on cohesive sediment

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A guide for carousel

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A guide for carousel experiments

on

cohesive sediment

Acknowledgements

CONTENTS

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