Pcrtanika 5(1),12-19 (1982)

The Use of A Laser Light-Scattering Technique in Fluvial SedimentMeasurement

M. YUSOF SULAIMAN, MAAROF MOKSIN, SHAHARIN IBRAHIM and LEONG SEE KINGJabatan Fizik, Fakulti Sains dan Pengajian Alam Sekitar, Universiti Pertanian Malaysia,

Serdang, Selangor, Malaysia.

Key words: laser light-scattering; sediment concentration; photo-detecting system.

RINGKASAN

Kertas ini melapurkan pengukuran yang dibuat ke atas kepadatan enapan dengan menggunakanteknik penyerakan-cahaya laser. Dalam bentuk yang sedia ada, teknik ini mengalami beberapa masalahpraktik, tetapi sifat yang mudah dan cepat membuatkan teknik ini lebih berguna. Cara-cara untuk mengatasimasalah-masalah yang timbul turut dibincangkan.

SUMMARY

In this paper, we report the measurement of sediment concentration using a laser light-scatteringtechnique. In its present form, the technique suffers from several practical limitations but these are out­weighed by the simplicity and speed of the technique. The procedures used in overcoming the accompany­ing problems are discussed.

INTRODUCTION

The amount of sediment transported instreams and rivers can serve as a hydrologicalindicator in the study of erosion, flooding, silting,etc. Measurement of sediment transported insuspension has therefore become a crucial part ofrou tine hydrological investigations and of processstudy in hydrology. Other important studiesinclude surface run-off investigations andgeomorphology.

Measurement of suspended sediment involvestwo stages, the sampling of a portion of stream­flow, and the determination of the concentrationof sediment in the sampled volume of water.Tradi tional analytical methods of sediment­concen tra tion m easuremen t involve opera tionssuch as decanting, filtering and weighing. Althoughthese procedures are straight-forward, they aretime-consuming. Through development of newtechniques, a search is being conducted for ananalysis that is more efficient. The search hasincluded studies of response of sediment-watersuspensions to various form of radiant energy, toelectrical currents and to applied forces.

Sediment particles interact with radiantenergy to produce both scattering an d absorption.

Acoustic interactions were investigated byFlammer (1962) who showed that the attenuationof energy was affected by both particle size andconcentration. But many problems have hamperedthe application of the technique, such as drift­limited sensitivity and interference from smallamounts of air.

Brown and Ritter (1971) measured thesediment concentration using an optical method.The method, although sensitive to sedimentconcentration as low as a few milligrams per litre,is su bject to large random errors. This is becausethe intensity and spatial distribu tion of scatteredflux is a function of the indices of refraction ofthe fluid and particles that vary with time.

A gamma-ray type of sediment gauge wasdesigned by Ziegler, Papadopoulous and Sellers(1967) for battery operation at remote unattendedsites. Bu t, because of excessive drifts, high failurerates, high maintenance costs and high assemblyand installation costs, the system was abandoned.

Sediment concentration and particle sizemeasurement employing the electrical conductivityprinciple has been shown to be ex tremely accurate.In the Coulter Counter (USIACWR, 1964), theelectric field is confined by an insula ted orifice

Key to authors' names: M.Y. Sulaiman, M. Moksin, S. Ibrahim, S.K. Leong.

12

M.Y. SULAIMAN, M. MOKSIN, S. IBRAHIM AND S.K. LEONG

tha t bounds a volume of liquid of the same orderof magnitude as the volume of a single sedimentparticle. The confined field enables the instrumentto count and size individual particles as they aredrawn through the field. Because fluvial sedimentparticles have a wide range of sizes, the orifice fre.quently plugs. However, Killen (1969) eliminatedthe plugging problem by using orifices much largerthan that used in the Coulter Counter. He alsoemployed a device consisting of two orificeselectrically connected in a bridge measurement.The system includes a hydrocyclone separator thatdiverts sediment to one leg of the bridge andrelatively clear reference water to the other leg.

Beverage and Skinner (1974), constructed aspecial hydrometer that responds to smalldifferences in density. The mixture is placed insidethe hydrometer, which is then submerged in awater bath. The bath water which is free of sedi­ment, serves as density reference. Temperaturesensitivity proves to be the largest source of error.With temperature variation limited to ± 0 ..5° C, theconcentration can be measured, but only with anuncertainty of ± 50 kg.m -3.

The principle of light scattering has in the pastreceived much attention as a means of measuringthe concentration, size and shape of suspendedparticulate matter. In a system developed byBickel (1979), a laser beam from a fixed ex ternalsource that is in a definite and exactly knownpolarization state enters a scattering chambermounted on a rotating table. The scattered light isdetected and analyzed by fixed analyzing opticsand detector, as the table rotates through 360°.The laser beam is aligned along the axis of rotationand prepared into a well-defined polarization stateby several optical components mounted on thetable. The system of Diehl et al. (1979) makes useof six detectors mounted around a sample at anglesof ± 45°, ± 90° and ± 135° from the incidencelaser beam to study particulate contaminants in asample of suspended solids. The scattered signalsare analyzed by examining the pulse heights fromthese detectors simultaneously. The pulse heightsare stored in a computer. The method has applica­tion in optical categorization of samples forremote sensing purposes.

The techniques of light-scattering mentionedabove are examples of two out of the manytechniques (references to other methods can befound in Diehl et at. (1979), Bickel (1979» whichdepend principally on the properties of thescattered beam of which considerable work, boththeoretically and experimentally, has been carriedout in the past (see for instance, Spinrad et at.(1978) and the references quoted there in).

13

Although many properties of the scatterer may bestudied using this principle, the resulting systemusually requires elaborate optical arrangementsand! or several detectors, and is often cum bersome.In this paper, we report on the results of severalmeasurements of sediment concentration using alaser-light scattering device originally designed byWoolsey and Douglas (1979). Since the methoduses the principle of attenuation of electromagneticwaves by a scattering medium, the num ber ofoptical components required are small. However,as with other light-scattering techniques (Diehlet al. (1979), Bickel (1979», the concentrationis equally sensitive t the size of the particles. Wehave therefore restricted our investigations toparticles having diameter, d, in the followingranges; d .;; 50 [.lm, 50 [.lm .;; d.;; 100 /lm and100 pm.;; d.;; 150 [.lm.

In the next section we describe the principleof the method employed in this work. In thesection on the procedure and results, we establishthe correspondence between the laser scatteringdata and filter data. Finally, we show how thelaser method is applied to determine the absolu teconcentration of the above-mentioned particles.Since the concentration can only be reliablymeasured if multiple scattering is insignificant,the effect of multiple scattering on the measure­ments will be discussed in the section on pro­cedure and results.

PRINCIPLE OF lHE MElHOD

When light passes through a medium contain­ing suspended sediment, its intensity decreases as aresult of scattering and absorption. True absorp­tion represents the transfer of electromagneticenergy to the molecules of the absorbing material.

On a macroscopic scale, the interaction oflight in a medium is described by Lam bert's law.According to this law, equal fractions of lightintensity are lost when the light traverses equallengths of the medium. The behaviour can besummarized neatly by the well-known exponentialexpression,

... (1)

where I is the intensity of the light aftertraversing a distance x, 10 is its original intensityand k is the absorption constant, characteristic ofthe medium and is physically associated with thecross-section for collision. In the case of two lightbeams passing through the arrangement depicted

LIGHT-SCATTERING TECHNIQUE IN FLUVIAL SEDIMENT MEASUREMENT

where C is a constant that depends on the intensi­ties of the incident beams.

in Fig. 1, straightforward extension of equation (l)shows that

k =[

IIJIn - + C12 .•. (2)

sion is [1 - fJ. If there are n particles in the

light path, the probability of transmission is

PT = [1 - fJn. Since a.~ A and n is large,

[1 -JJ n may be written as exp [-~a J.Since

N is the number of particles per unit volume, andx is the distance traversed, N = n/Ax. Thus, theprobability of transmission PT = exp (-Nax). Butthe area a = 1fr2 where r is the radius of the particleassumed spherical. Therefore, PT = exp (-N1fr2 x)and hence k = N1fr2

•

Since the mass of the sediment per unitvolume is,

M =..i1f r3 pN3

... (4)

- ---~ ---

In practice, deviation from equation (5) isexpected for the following reasons;

... (5)

where p is the density of the particles,

k = 3 M

4 rp

showing that k is directly proportional to theconcentration M.

IT _ _ _ :- _-_ _ d2

It:=::=-==t=--======t-=-=-='--=--=-~l

,(i) the particles are not spherical and have

varying sizes,

(ii) the particles do not have the same density,although their densities do not differ verymuch from each other, *

I, 12

Fig. 1. Sample cell showing incident and trans­mitted beams (volume of cell: 1.35 x10-4 m 3 ).

If the particles are assumed to be sphericalwith radius r and their num ber per unit volume isN, then

(iii) detailed analysis shows that the abso.rptionprocess depends on the refractive indices ofthe particles,

(iv) Lambert's law is not obeyed at high con­centration due to multiple scattering.

(v) heavier particles are not suspended but settlerapidly.

This can be shown as follows. Let A be anyarbitrary cross-sectional area of the light path.Then, the probability of an interaction is Q. where

Aa is the cross-sectional area of a particle. Theprobability of no interaction and hence transmis-

k = N1fr2 ... (3) Despite these limitations, the laser scatteringprinciple caYl still be applied to sediment, contain­ing particles such as fine sand, silt or clay. Althoughthese particles have a wide range of sizes, the useof a calibration curve derived from the same typeof particles is generally sufficient and accurateenough to compensate for the deviations from theideal situation.

*The specific gravity of the particles varies between 2.61 and 2.65.

14

M.Y. SULAIMAN, M. MOKSIN, S. IBRAHIM AND S.K. LEONG

where

Equating equations (2) and (5), relates theconcentration to the light intensities, thus,

M = A ln G~J+ C ... (6)

The relationship between the concentrationand the light intensities of equation (6) is realizedin the practical scheme of Fig. 2. Here, the trans­mitted beams 11 and 12 are first converted intoelectrical signals by the photodetectors. They arethen passed through the logarithmic amplifiers,the outputs of which are connected to thedifferential amplifier. The outputs of the differen­tial amplifier V is proportional to log [ ~ J

The optical components' consist of a beam­splitter, a prism and a front-surface mirror. Thelaser light is divided by the beam-splitter intotwo mutually perpendicular beams - a directlytransmitted beam and a beam reflected at 90°.The transmitted light is allowed to fallon areflecting mirror inclined at 45°. The latter reflectsit into one of the two chambers of a sample cell(Fig. 1). The depth of water in the two chambersis d1 = 3 X lO-'m and d2 = 7 X lO-'m. Thereflected light of the beam-splitter is then totallyreflected by a prism and continues to the inclinedreflecting mirror. The mirror reflects it into theother chamber of the cell. The beams transmittedfrom the cell fall on the photodetecting unit ofFig. 2. The output of the photodetecting unit isregistered with a millivoltmeter.

attempt to develop the technique an unpolarizedHe-Ne laser was used. However, it was discoveredthat the detector output voltage showed cyclicvariations. The behaviour was caused by axialmode switching as the laser cavity expandedlongitudinally during the warm-up period afterswitch-on (Woolsey et al., 198'2).

... (7)A4rp

PHOTODETECTING UNITt

The constant factor that appears in convertingthe logarithm to the base of 10 to natural logarithmis absorbed into a proportionality constant as willbe explained later.

PROCEDURE AND RESULTS

The experimental arrangement is shown inFig. 3. A polarized He-Ne laser of wavelength632.8 nm acts as the light source. In an earlier

Muddy water samples were obtained from fivedifferent locations in the University campus.Before making each set of measurements themuddy water was throughly stirred and separatedinto suspensions of 500 ml in graduated cylinders.The suspended water was then quickly transferredto the sample cell after first mixing it thoroughly,and allowing it to settle for two minutes. Thereading of the millivoltmeter was then recorded.

VCXln(~)12

v In v, - In Va : In(~:}?-------

Differentia,

Ampli fierIn v2

In v.

L09--~tthmlc

"'-PUtl.'.

r

Fig. 2. Photodetecting unit.

t The photodetecting unit was constructed and assembled by the staff of the Physics Workshop, University of NewEngland, Arrnidale, Australia.

15

LIGHT-SCATIERING TECHNIQUE IN FLUVIAL SEDIMENT MEASUREMENT

r--,._4- -r--..!'R"'1,,9"--_+ 15 V

O· 022~F Cl - lK R 17

it

R3

Photod i 0 d e 15KVo C5

lOOK Rl + o ·01 ~F

- -R13

lOOK - -

lOOK R 7

R9 2K2

'11

II

[:

1510K

III741

R27

50 nOu t P ut

RV4

10K

lOOK R 8

R12

4 K7

Rl0

lOOK R23 R26

lOOK

C6O·Ol~F

BC109

R4

15K

R 18 lK

.]1..__....:.. Q4

lK

RV 2

R14

lOOK

100 K

Fig. 3. Experimental arrangement.

TPhotodi ode

The sample in the cell was then diluted by10% (by volume) and the voltage re-measured.The procedure was repeated until the concentra­tion of the final sample was 10% of the originalsuspension. Fig. 4 illustrates the graph of voltageagainst relative concentration for this measure­ment. Other results can be found in Ali (1980).The voltage (V-V 0) in Fig. 4 is the differencebetween the sample voltage and the reference

voltage obtained with an equal volume of distilledwater. The reference voltage was used to compen­sate for any drifts in the detecting system andthis was periodically checked usually before,during and after a course of measurements of a setof samples.

In this part of the experiment, we haveattempted to show the validity of equation (6)

16

M.Y. SULAIMAN, M. MOKSIN, S. IBRAHIM AND S.K. LEONG

RelaUve Concentration

Fig. 4. Graph of (V-Va) against relative concen­tration of sediment.

At high concentration, the curves are seen todeviate significantly from the straight line. This isdue to multiple scattering for it has been shown(Spinrad et al., (1979)) that when light interactswith particulate matter, more than half of thescattered radiant intensity is scattered at anglesless than 3° from the incident beam. If the con­centration and the size of the particles are large,this in effect causes the scattered beams to beredirected into the incident path thus effectively ... (8)Stheory = K Sexpt

The assumptions used in deriving equations(6), justifies one against equating the theoretical

slope S = .!. to the slope of the experimentalA

curve directly. In addition, the conversion fromln e to loglo involves a constant factor of 2.3 andthe absorption cross-section may not be exactlyequal to the sum of the physical sizes of theparticles. This requires one to write,

The correspondence between these laserscattering data and filtering data is established byplotting a calibration curve. Since this is dependenton the particle size, we have chosen to work withparticles having diameters in the following ranges:d~50pm, 50pm,,;; d,,;; 100pmandlOOpm,,;; d,,;; 150 pm. The categorization into these sizes isconvenient as they correspond rougWy to the sizeof clay, silt and fine sand respectively and theirseparation can be carried out by the methods ofdry and wet sieving. These techniques are describedin Ali (1980). Samples in each category wereprepared by mixing some predetermined amountof the sediment (by weight) with a known volumeof distilled water. The absolute concentration wasmade up so that the solution remained in the single­scattering region. About 1.35 X 10-4 m 3 of thesuspension was then transferred into the measuringcell and the millivoltmeter reading recordedimmediately. The suspended sediment in the cellwas then filtered thorougWy and after constantdrying, the weight determined. From the weightof the sediment and the volume of the filtrate, theabsolute concentration of the sample was calculatedand the average of the concentrations determinedbefore and after laser measurement obtained.Meanwhile, the suspension that was left over fromthe first measurement was diluted by 10% (byvolume) and the whole process of measuring anddetermining the average concentration repeated.The remaining data points were obtained inexactly the same way. Fig. 5 shows the calibrationcurve for the three ranges of particle size.

lowering the collision cross-section. Closerexamination of Fig. 4, reveals the fact that forsmaller r, the onset of multiple scattering occurs ata higher concentration than for larger r. Sinceequation (6) no longer holds in the presence ofmultiple scattering, high concentration samplesmust be diluted prior to any actual measurement.Our experience has shown that if the concentra­tion is limited to less than 0.1 kg. m- 3

, multiplescattering is insignificant for particles of size lessthan 160 pm in diameter.

• aample ...x Mmple.• umPte C

.ample 0• .ample E

,..v..voYolt,

and the problems that emerge at high concentra­tion. According to equation (6), a plot of V

I I(proportional to ln -) against M should be a

12straight line with a slope 1/A. As shown in Fig. 4,equation (6. is obeyed reasonably well at lowconcentration. The diffe~ence in the slopes of thecurves can be attribu ted mainly to the differencein the particle size since the densities of theparticles do not vary significantly. Thus theslopes serve to indicate that rA < rB < re < rn< rE where rx is the effective radius of theparticles in sample X.

17

LlGHT-SCATIERING TECHNIQUE IN FLUVIAL SEDIMENT MEASUREMENT

where M is the concentration of sediment. Fromequations (9) and (10),

This relationship enables one to use equation(6) to measure sediment concentration using thelaser light-scattering technique. Ideally, anymeasurement that is made with respect to distilledwater will help eliminate the constant factor C inequation (6) since for distilled water the particlec9ncentration is naturally zero. Assuming, thatthis is approximately the case (as justified by thesmall intercept 'V 4mV, in the graph of Fig. 5),then,

"0

<0,..

o·a-<,."m~

< 0·6I

i

V-Vo = SexptM ... (10)

15 0·4r­-<

0'2

M

... (11)

Fig. 5. Graph of (V- Vo) against absolute sedi­ment concentration for different particlesizes.

It remains now to find the value of K. Wehave determined K empirically in the followingway. Sexpt is evaluated using the cali bration curvesof Fig. 5 for the different particle sizes. The valuesof Stheory are calculated from equation (7) forr = rmax and r = rmin in each particle size range,assuming a constant density for all the particles.Finally, the corresponding K'S are obtained fromequation (8) and the average determined. Theresults of these calculations are shown in Table 1,where the average value of iZ is found to be (K)ave= 0.19. Therefore,

RELATIVE CONCENTRATION

o o·a

Sexp "'" 5 Stheory

o·a "0

... (9)

If the particle size is known, then, Stheorycan be calculated from equation (7) and M canbe determined from equation (11). In a non­dispersive medium, samples of approximatelyknown size may be obtained by using the settlingmethod. This is the technique used to sampleparticle size according to their terminal velocitycalculated from Stokes' equation. The terminalvelocity varies with the size and the density ofparticles and also with the viscosity of the medium.The latter is a function of temperature. At 15°C,the terminal velocities for the 150,1 00 and 50 pmparticles are calculated to be 1.8, 0.8 and 0.2 X10- 2 m. sec.-I respectively.

Alternatively, a calibration curve for theparticle size of interest may be used directly toconvert a measured (V- V0) value in to an M value.Table 2 compares measurements of concentrationmade in this way with values obtained by filtering:agreement is obtained within 10%.

TABLE 1

Particle diamerer Sexpt (Stheory h max (Stheory)rmin

J1m m 3 kg- 1 m 3 kg- 1 m 3 kfl

50 2.3 0.46

50 - 100 1.8 0.23 0.46

100 - 150 1.1 0.15 0.23

-Krmax Krmin K

0.20 0.20

0.13 0.26 0.19

0.14 0.21 0.18

(K)ave = 0.19

The density of the particles is taken to be 2.63 X 103 kg.m- 3 .

18

M.Y. SULAIMAN, M. MOKSIN, S.IBRAHIM AND S.K. LEONG

REFERENCES

ACKNOWLEDGMENT

ALI, M.J. (1980): Laser Scattering Measurement ofFluvialSediment, B. Sc (Hons.) Proiect Report, PhysicsDepartment, U.P.M.

We have shown how a laser light-scatteringtechnique can be used to measure sedimentconcentration. However, before the method canbe reliably applied, several considerations haveto be taken into account. This is because of itsdependence on several, physical factors, notablythe particle size. Nevertheless, because of itssimplicity, low-cost and speed, the method offersan attractive alternative in sediment concentrationmeasurement. Further work is currently beingundertaken to improve the system.

BEVERAGE, J.P. and SKINNER, J.V. (1970): Operator'sManual PS-69 Pumping Sampler. Federal Inter­Agency Sedimentation Project.

BICKEL, W.S. (1979): Optical System for Light-scatteringExperiment. App. Optics 18, 1707.

BROWN, W.M. and RITTER, J.R. (1971): SedimentTransport and Turbidity in the Eel River Basin,U.S. Geol. Survey Water-Supply Paper, 1986.

DIEHL, S.R., SMITH, D.T. and SYDOR, M (1979):Analysis of Suspended Solids by Single-ParticleScattering. App. Optics 18, 1653.

FLAMMER, G.H. (1962): Ultrasonic Measurement ofSuspended Sediment. U.S. Geol. Survey Bull.1141-A.

KILLEN, J.M. (1969): A Feasibility Study of a FieldInstrument for the Measurement of Suspended­Sediment Concentration, Univ. of Minn., St.Anthony Falls Hydraulic Laboratory Project Report.

SPINRAD, R.W., ZANEVELD, J.R.V. and PAK, H (1978):Volume Scattering Function of Suspended Particu­late Matter at Near-forward Angles: A Comparisonof Experimental and Theoretical Values. App. Optics17,1125.

USIACWR (1964): Electronic Sensing of Sediment, U.S.Inter-Agency Committee on Water ResourcesReport.

WOOLSEY, G.A. and DOUGLAS, 1 (1979): Measurementof Suspended-Sedimen Concentrations using a LaserScattering Technique, 1st National Conference onApplied Physics (Australian Institute of Physics).Capricomia Institute of Advanced Education,Rockhampton.

WOOLSEY, G.A., SULAIMAN, M.Y. and MOKSIN, M(1982): Correlation of Changes in Laser Tube Tem­perature, Cavity Length and Beam Polarization ForAn Internal-Mirror Helium-Neon Laser. (accepted forpublication in Amer. J. Phys.)

ZIEGLER, C.A., PAPADOPOULOS. J and SELLERS, B(1967): Radioisotope Gauge for Moniroting Sus­pended Sediment in Rivers and Streams. Int. 1.Applied Radiation and Isotopes, 18.

(Received 28 August 1981)

Sediment concentration(X10-' kg m-3)

TABLE 2

0.43 0.14 0.55 0.28 0.37

OAO 0.13 0~4 027 OAO

CONCLUSION

Laser scattering

Methods

Filtering

This project is supported by the Faculty ofScience Research Committee, UPM (ProjectNo. 1705/062) and we would like to thank theFaculty for providing us with the financial aid.The assistance rendered by Dr. Wan Sulaiman WanHamn, of the Soil Science Department, UPM isvery much appreciated. Finally, we would like tothank Professor Woolsey, Physics Department,University of New England, Armidale, Australiafor suggesting the problem, and the staff of theUniversi ty of New Englan d Physics Workshopfor constructing the photodetecting unit.

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