Estuaries: Monitoring and Modeling the Physical System || Monitoring Estuarine Systems

2

MONITORING ESTUARINESYSTEMS

Contents

2.1 Introduction 232.2 Bathymetric surveying 232.3 Tide gauges 252.4 Current metering 28

2.5 Thermometry 312.6 Estuarine salinity

determinations 322.7 Estuarine particulates 34

2.1 Introduction

This book considers estuaries in terms of theshape of the basin and the processes asso-ciated with the vertical rise and fall of thetide, the freshwater and tidal currents, theadvection and diffusion of salt and thermalenergy, and finally the transport of partic-ulates. Each of these subjects is dealt within a separate section of the book. The devel-opment of monitoring techniques for eachparameter is described in this chapter.

2.2 Bathymetric surveying

Bathymetric surveying demands two prac-tical requirements which, until relatively

recently, have been difficult to achieve. First,the surveyor must locate his or her positionon the sea surface and, second, he or shemust determine the depth of water at thatlocation and correct the depth onto one ofthe different data detailed below. There is awide variety of methods for estuarine posi-tion fixing which vary from simple opticalsextant fixes to electronic systems linked toorbiting satellites.

Manual position fixing

Close to the coast the observer may use sim-ple compass bearings or a vertical sextant todetermine the distance from a coastal land-mark. With less accuracy, celestial observa-tions with a sextant can be combined with

Estuaries: Monitoring and Modeling the Physical SystemJack Hardisty

Copyright © 2007 by Jack Hardisty

24 evolution and monitoring

careful timing and tables of astronomicalconstants to determine the ship’s positionat sea. These optical systems demand goodvisibility and a relatively calm sea; onlyin the hands of an experienced seamanwill reliable results be obtained from aheaving deck.

Electronic position fixing

Older electronic systems are based uponthe comparison of signals from shore-basedradio stations. The Decca Navigator systemis the primary aid of this type for shipsoperating in the seas around the BritishIsles. It consists of chains of radio stationswhich transmit low-frequency radio signals.The signals intersect in a known hyper-bolic pattern and a receiver on the shipdisplays the results on three Decometers assets of letters and numbers. Each letter andnumber identifies a line on the chart andthe intersection of lines from two of theDecometer readings allows a position to beplotted.

More recently, microwave, range andbearing, and combined systems have beenintroduced in coastal locations and effec-tively replace the line of site optical sys-tems by providing electronic distance anddirection information with respect to shore-based transmitters. Most recently, satellitenavigation systems have been introducedwhich measure the distance of one or moreorbiting or geostationary satellites. The sys-tems also gather information on the heightand position of the satellites, and can thencompute the ship’s position with remark-able accuracy. It is likely that satellitenavigation systems will supersede all ofthe other position-fixing methods in thefuture.

Manual depth determination

The depth of water at any given locationwas, until the middle of the last century,determined by the traditional lead andline soundings. In shallow water thisproved sufficiently accurate for most pur-poses and tallow wax was often insertedinto the base of the lead to recover asample of seabed sediment. However, thetechnique is laborious and can, in deepwater, be extremely time consuming andprone to inaccuracies. Linklater (1972),writing about the famous voyage of theChallenger in 1872, notes that a single deepsea depth measurement often consumeda whole day’s work and that the errorsdue to a nonvertical line or wire could beconsiderable.

Acoustic depth determination

The advent of underwater acoustics in theearly twentieth century led to the develop-ment of echo-sounders which measure thetwo-way travel time of a pulse of soundfrom the vessel to the seabed and back,and convert this to water depth. Mechan-ical echo-sounders usually employed abelt-driven or radial-arm-driven electricalstylus passing across a strip of speciallyprepared paper to provide a continuousprofile of the seabed. A higher stylusspeed, and hence faster pulse repetitionrate, was used with surveying systemsthan in conventional navigational echo-sounders, in order to provide maximumresolution of the seabed and a comparativelylarge vertical scale. Mechanical instrumentshave largely been replaced by electronicand digital devices. Typically, transmissionfrequencies lie in the band 30–210 kHz.Lower frequencies are used in deep water

monitoring estuarine systems 25

because of the attenuation of the higherfrequencies.

2.3 Tide gauges

Estuarine tides are observed as the regularrise and fall of sea-level due to thegravitational attraction of the Moon and theSun on the waters of the world’s oceans.Most estuaries experience approximatelytwo tidal cycles in each twenty-four-hourperiod and the range of the tide increasesfrom small values in the deep ocean to manymeters in certain coastal and estuarine loca-tions. For example, the Minas Basin at theupper end of the Bay of Fundy and Lac AuxFeuilles in Hudson Bay experience ranges inexcess of 16m and the Severn Estuary inEngland has only slightly smaller ranges.

The monitoring of tidal elevations in anestuary aims to measure the vertical dis-tance between the water surface and a fixeddatum. Although this may appear to be atrivial exercise in a laboratory, the situationin an estuary is complicated by, for example,the need to smooth the readings to removethe effects of short-period wind waves, therequirement to protect the system againstbio-fouling, and even the rampages of sitevandalism. The following section is basedin part upon Hardisty (1990a) and Pugh(1987 and 2004). A more detailed descrip-tion of the techniques involved in choosingand operating a tide gauge system is givenin UNESCO’s manual, IntergovernmentalOceanographic Commission (1985).

Pugh (2004) relates that, whilst IsaacNewton provided the theoretical explana-tion for tidal phenomena, it was one ofhis contemporaries, Edmond Halley, whofirst made systematic measurements of tidesin the English Channel in the late seven-teenth century. Francis Beaufort (of wind

scale fame) analyzed 30-year records of highand low water at London Docks and pub-lished the results in the British Almanac for1830. Lubbock then set out to record “theheights and times of high water over a longperiod to calculate the different variationsto be detected in the tides, the semi-diurnalrise and fall, the twice-monthly springs andneaps, the alterations caused by the vari-ation in the moon’s orbit, and to relatethem to the astronomical forces which hadcaused them” (Deacon, 1971). This ini-tiative was taken up by the Royal Soci-ety which appealed to the Admiralty whoagreed to install automatic tide gauges in thedockyards at Sheerness, Portsmouth, andPlymouth, and two sites in London in 1831.

Lubbock’s Cambridge tutor, WilliamWhewell, widened the field “to discoverthe general pattern of tides throughout theworld.” Beaufort again assisted by orga-nizing simultaneous measurements aroundBritain and Ireland in June 1834, extendingto “both sides of the Atlantic, at twenty eightor more places between Florida and NovaScotia and from the Straits of Dover to NorthCape” in 1835. James Clark Ross is recordedas obtaining tidal data whilst trapped in theice during the Arctic winter of 1848–9.

Tide poles

Tide Poles or staffs (Figure 2.1) are cheap,easy to install, and perfectly adequate fora short-term survey. The pole is graduatedand permanent installations are often foundaround harbors to assist shipping and navi-gation. It is necessary to average the readingsover a short time interval to remove theeffects of wave action. However the tediuminvolved and the errors that result if read-ings are required over a longer period of timemakes tide poles unsuitable for long-term,high frequency data.


FIGURE 2.1 Tide pole at Anchorage, Alaska(DIR 2.1).

The Palmer–Moray gauge

The vast majority of permanent gaugesinstalled during the nineteenth and twenti-eth centuries consisted of an automatic chartrecorder: the recording pen is driven by afloat which moves vertically in a well, con-nected to the sea through a relatively smallhole or narrow pipe. The limited connec-tion damps the external, short-period waveoscillations.

The basic idea was described for the RoyalSociety of London by Moray in the seven-teenth century. He proposed that a longnarrow float should bemounted vertically ina well, and that the level to which the floattop had risen be read at intervals. The first,self-recording gauge, designed by Palmer,began operating at Sheerness in the Thamesestuary. Figure 2.2 shows the componentsof such a Palmer–Moray system. The wellis attached to a vertical structure whichextends below the lowest level to be mea-sured and has one ormore small connections

to the open sea. Copper is often used toprevent fouling of the narrow connectionsby marine growth and a layer of kerosenemay be poured over the seawater in the wellwhere icing is a winter problem.

Vertical float motion is transmitted by awire or tape to a pulley and counterweightsystem. Rotation of this pulley drives a seriesof gears which reduce the motion and drivea pen across the face of a chart mounted ona circular drum. Usually the drum rotatesonce in 24 h, typically at a chart speedof 0.02mh−1; the reduced level scale maybe typically 0.03m of chart for each meterof sea-level. Alternative forms of record-ing included both punched paper tape andmagnetic tape.

Although stilling-well systems are robustand relatively simple to operate, theyhave a number of disadvantages. They areexpensive and difficult to install, requir-ing a vertical structure for mounting overdeep water. Accuracies are limited to about0.02m for levels and 2 min in time becauseof the width of the chart trace. Charts canchange their dimensions as the humiditychanges. Reading of charts over long periodsis a tedious procedure and prone to errors.Palmer–Moray gauges are being replaced bythe electrical or electronic devices detailed inthe following sections.

Pressure gauges

The hydrostatic pressure at a fixed pointbelow the water surface is related to theoverlying water depth:

P = Pa + ρgh Eq. 2.1

where P is the hydrostatic pressure, Pa isthe atmospheric pressure, ρ is the density ofseawater (about 1040 kg m−3), g is the grav-itational acceleration (9.81 m s−2), and h isthe water depth.


High water

Low water

Pully wheel

Counterweight

Dock wall

Wooden float

2–6 cm

10 15

77

6

5

4

1

20B.S.T.

O.D.N

Ht. (m)

Tide number2 4 6

Tide gauge: Humberbull fort sand

0.2–2 m

8–15 m

Tide well

Pen

Paper on drum(a)

(b)

8 1010121214148 101214

FIGURE 2.2 A Palmer–Moray stilling-well system (a) and typical tidal record (b) from the Bull Fort gauge in theHumber Estuary (after Hardisty, 1990a).

Devices which measure this pressure andthus the water depth are called pressuregauges or pressure transducers. Direct read-ing and recording systems are availablefrom a number of commercial organizations(Figure 2.3).

In many systems, the pressure transduceris kept dry in the bubbler gauge. Compressedair is fed into an upper chamber at a pressurewhich bubbles out at the submerged endof the sensing pipe. The pressure through-out the system will thus be equivalent tothe pressure at the orifice, and the system

then determines this pressure and hence thedepth of water.

Acoustic tide gauges

The time taken for a pulse of sound to travelfrom the source to a reflecting surface andback again is a measure of the distance fromthe source to the reflector. The travel time isgiven by

t = h

CoEq. 2.2


FIGURE 2.3 Deep water pressure transducergauge (DIR 2.2).

where t is the travel time (s), h is the pathlength (m), and Co is the velocity of sound(m s−1).

This principle is utilized in the acous-tic tide gauges which are being installed inmany locations as shown in Figure 2.4. Suchdevices send an acoustic pulse down a trans-mitting tube. The tube has a fixed reflectorabove the water surface which is used auto-matically to calibrate the device and thetravel time for the water surface echo is usedto measure the location of the water surfaceand hence to monitor the tidal elevations.

Online tidal information

The development of the Internet has enabledan increasing number of online tidal datasources to be accessed. There are long-term,archival datasets such as the British Oceano-graphic Data Centre, and operational sitesshowing real-time or near-real-time data,often in comparison with predictions as atthe CO-OPS sites. A selection is describedbelow.

British Oceanographic Data Centre:DIR 2.4 – The UK National Tide Gauge Net-work, run by the Tide Gauge Inspectorate,

FIGURE 2.4 Acoustic tide gauge installed atSettlement Point, Bahamas (DIR 2.3).

records tidal elevations at 44 locationsaround the UK coastTide Online: DIR 2.5 – NOAA’s NationalOcean Service (NOS) operate a large num-ber of online tide gauges around the NorthAmerican coastline and across the PacificACCLAIM: DIR 2.6 – The ACCLAIM pro-gram in the South Atlantic and southernoceans consists of measurements fromcoastal tide gauges and bottom pressure sta-tions, together with an ongoing researchprogram in satellite altimetry.

2.4 Current metering

This section considers the manual, mechan-ical, electronic, and automated techniquesthat have been developed to measure thespeed and direction of water flows in estuar-ine environments. The problem is not trivial:estuarine currents, typically, increase from


zero at slack water to maxima which canbe in excess of 2–3m s−1 at mid tide beforedecelerating, reversing, and then accelerat-ing to achieve similar values in the oppo-site direction. Estuarine current metering,unlike work in streams and rivers, musttherefore not only be capable of resolv-ing a wide range of speeds, but must alsoencompass vector as opposed to simple scalardata.

The measurement of water flow and cur-rents in estuarine environments combinesthe interest of oceanographers and hydrog-raphers with branches of the environmen-tal and engineering sciences. Backgroundinformation may be found in such textbooks as Neumann and Pierson (1966). Thetechniques separate into Lagrangian with asensor mounted at a fixed point and datadescribe time series of the flowvectors at thatpoint, and Eulerian which track the path ofa water particle.

The measurement of tidal and freshwa-ter flow in estuaries probably dates backalmost as far as the measurement of thetidal water depth itself. Early charts, forexample, included descriptions of flowsbased upon the nautical term of knots.One knot is equivalent to a flow speed of0.51m s−1. Mechanical devices embodyingsome form of rotating element which areused for water velocity measurement arecalled current meters and The Proceedings of theRoyal Society of London record descriptions ofsuch devices from Newton’s time.

Mechanical flow sensors

Rotating cup current meters work becausethe drag on the hollow hemisphere orcone is greater when its open side facesthe flow, and so there is a net torqueon the assembly that causes the cups torotate. The magnitude of the fluid velocity

FIGURE 2.5 Valeport Braystoke impellor flowmeter (DIR 2.7).

determines the speed of rotation, which isusually indicated by the number of once-per-revolution countsmade in a known timeinterval. For higher velocities, an impellor isused having its axis parallel to the fluid flow(Figure 2.5). The impellor current meter issensitive to the direction of flow particularlyif the propeller is surrounded by a shieldingcylinder.

Acoustic flow sensors

There are two, quite different, acoustictechniques for the determination of waterflow speeds and these are known as acous-tic travel time and acoustic Döppler devices.In the former, the travel time of an acous-tic pulse between a source and a receiverdepends upon three factors:

1 The distance between the source and thereceiver;

2 The speed or celerity of the sound (about1410 m s−1 in seawater); and

3 The velocity of the flow: if the flow trav-els from the source to the receiver, then thetravel time is reduced and vice versa.


(a) (b)

FIGURE 2.6 (a) Travel Time acoustic current meter and (b) Döppler shift acoustic current meter (DIR 2.8).

Acoustic travel time current meters(Figure 2.6a) measure the travel time in,for example, three dimensions to computethe current speed vector. Acoustic Döpplercurrent meters (Figure 2.6b), emit a soundpulse which is detected after it has beenreflected back from particles in the flow. Thereflected sound will evidence a Döppler fre-quency shift which depends upon the speedand direction of the flow. The instrumentsmeasure the Döppler shift and compute theflow velocity.

Electromagnetic flow sensors

It has long been appreciated that movementof a conducting medium, such as salt water,around a primary electrical coil induces anelectromotive force in a secondary coil. Elec-tromagnetic current meters and flow sensorsutilize this effect by measuring the inducedcurrent in a secondary coil encapsulated ina sensing head (Figure 2.7) to compute thecurrent velocity.

Eulerian techniques

Among special flow measurement tech-niques used in rivers, estuaries, and the

FIGURE 2.7 A range of electromagnetic currentmeters (DIR 2.9).

open sea is the salt-velocity method, in whicha quantity of a concentrated solution ofsalt is injected into the water. At eachof two downstream cross-sections, a pairof electrodes is inserted, the two pairsbeing connected in parallel. When the salt-enriched water passes between a pair ofelectrodes its higher conductivity causes theelectric current to rise briefly to a peak.The mean velocity of the stream is deter-mined from the distance between the twoelectrode assemblies and the time intervalbetween the appearances of the two currentpeaks. Rather elaborate apparatus is needed


FIGURE 2.8 The North American PORTS system (DIR 2.10).

for the automatic recording of data. Morepragmatically, floats are frequently trackedto determine mean current drift directions.

Online current data

One of themost interesting sources of onlineestuarine flow data is the Physical Oceano-graphic Real Time (PORTS) system operatedin North America (Figure 2.8).

2.5 Thermometry

The salinity and temperature of water havebeen monitored throughout recorded his-tory using manual techniques. The secondhalf of the twentieth century witnessed theintroduction of the automated and elec-tronic systems described here. This section

is based, in part, on Hardisty (1990a) andRidout (2004).

Manual temperature determinations

Most substances expand when heated, andthis property has been used for cen-turies in the manufacture of thermometers.Most commonly, “mercury in glass” ther-mometers are used. There are problems,however, associated with the use of suchthermometers for measuring the tempera-ture of estuarine waters. The thermometeris usually lowered into the water to deter-mine the temperature at the required depth.However, the reading changes during recov-ery as the instrument is hauled to the surfacethrough water which may have a differ-ent temperature and in the air. The prob-lem was resolved with the introduction of


(a)

(b)

FIGURE 2.9 Traditional oceanographic reversingthermometer (DIR 2.11).

the oceanographic reversing thermometer asshown in Figure 2.9.

Electronic thermometers

Water temperature is measured electron-ically with thermistors and thermocou-ples which sense temperature-dependentchanges in the electrical properties of con-ductors (Figure 2.10).

FIGURE 2.10 Seabird Electronics oceanographictemperature sensor for use in estuarine waters(DIR 2.12).

2.6 Estuarine salinity determinations

Early work on measuring the saltiness ofthe sea involved taking a known weightof seawater, evaporating the water, andweighing the residue (Boyle, 1693; Birch,1965). Refinements to the analysis includedsolvent extraction (Lavoisier, 1772) and pre-cipitation (Bergman, 1784) using an OceanSalinometer (Figure 2.11). Forchhammer(1865) introduced the term salinity and theconcept of measuring one parameter, chlo-ride, from which the salinity could be cal-culated. This work was supported furtherby Dittmar (1884) who analyzed samplesfrom the Challenger Expedition to estab-lish the theory of “Constant Composition ofSeawater”. These methods remained domi-nant until the 1950s when conductivity wasintroduced as a practical means to measuresalinity.

Chemical salinity determinations

Since chloride ions are one of the majorconstituents of estuarine water, and therelative proportions are constant, it isonly necessary to determine the chlorin-ity (Cl‰) of a sample. The oldest, and


100

90

80

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60

OT

AG

LIA

BD

E

302P

EA

BL2

7NY

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40

30

F

20

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22

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26

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29

30

31

Hilgerd’s oeean salinometer.

FIGURE 2.11 Hilgard’s Ocean Salinometer(c. 1880) (DIR 2.13).

still often applied, method is based uponMohr’s Cl− titration, where the chlorinecontent is determined with silver nitrateusing a potassium chromate solution asthe indicator. The salinity is then givenby the internationally agreed empiricalrelationship:

S‰ = 0.030 + 1.8050Cl‰ Eq. 2.3

Further work by Knudsen et al. (1902)resulted in a new definition which statedthat salinity was “The total amount of solidmaterial in grams contained in one kilogram

FIGURE 2.12 University of Miami salinometer onboard the Royal Caribbean cruise ship Explorer of theSea (DIR 2.14).

of sea water when all of the carbonate hasbeen converted to oxide, all the bromineand iodine replaced by chlorine and all theorganic material oxidised”.

Electronic salinity sensors

Salinity is now determined on a routine basiswith electrical instruments which measurethe conductivity of the water sample and arecalibrated to directly calculate the value ofeither the chlorinity or salinity (Figure 2.12).Various salinometers were developed duringthe 1950s and 1960s (e.g. Hamon, 1955) andnew relationships were produced for con-ductivity and salinity (Millero et al., 1977;Poisson et al., 1978). These gave rise tothewidely accepted “industry standard” sali-nometers (Autosal and Portasal) which areproduced by Guildline Instruments Ltd. andachieve a quoted accuracy of ±0.002 inpractical salinity units.


2.7 Estuarine particulates

The principles of field monitoring ofsuspended particulate matter are usuallycontained in the research papers referencedbelow, but introductory discussions may befound in such as Sternberg (1986). One ofthe better discussions of the techniques anderrors in particulate monitoring is given byVan Rijn (1993).

Particulate determinations were basedupon the range of techniques for obtaininga water sample and removing the solids. Thetechniques which measure the effect of theparticulates on acoustic or optical propaga-tion through an in situ water sample wereintroduced more recently.

Although the determination of the con-centration of SPM in estuarine waters is arelatively recent endeavor, the study of the“muddiness” or “turbidity” of water has ear-lier origins. Since the early days of oceano-graphic endeavor in the mid-nineteenthcentury, such work has utilized the Secchidisk as shown in Figure 2.13. The disk con-sists of an 8′′ (23 cm) diameter black andwhite plate which is lowered into the wateruntil no longer visible and then raised untilagain visible. The Secchi depth (of extinc-tion and usually in feet) is then recordedas an indication of the turbidity of thewater.

The Secchi depth was roughly correlatedwith the Jackson turbidity units (JTU’s)which were introduced at the end of thenineteenth century through the JacksonTube. This device is a long glass tube sus-pended over a lit candle. A sample of waterwas slowly poured into the tube until thecandle flame, as viewed from above, couldno longer be seen and the depth of waterwas taken as the turbidity in JTUs. Nowa-days, turbidity is measured in nephelometerturbidity units (NTU) with the instrumentsdescribed below.

Gravimetric methods

There are a range of techniques for the deter-mination of the concentration of particulatesin estuarine systemswhich involve samplingthewater, usually at a point and for a limitedtime interval, and then removing (throughfiltration or drying) and weighing the solidsto determine the concentration as illustratedin Figure 2.14.

Bottle sampler

Most simply, a stoppered bottle is lowered tothe required depth within the water columnand a line or electric relay is used to removethe stopper. Water and particulates flow intothe bottle which is then returned to the lab-oratory for analysis (Delft Hydraulics, 1980).There are problems associated with the nat-ural fluctuations in the actual concentrationin the field for short filling times.

Instantaneous trap sampler

The instantaneous trap sampler consists ofa horizontal cylinder equipped with endvalves which can be closed suddenly (by amessenger system) to trap a sample instan-taneously. The water is allowed to flowfreely through the open horizontal cylinderwhile the sampler is lowered to the desiredpoint. Again the disadvantages are that thesampler is incapable of detecting naturalvariations in the local concentration.

Time-integrating trap sampler

This consists of a vertically hung funnel orcylinder with a closed bottom. The devicetraps particulates in its enclosure and theopen top is closed by a door or latch forrecovery. The efficiency of such systems


Disk raised slowly to pointwhere it reappears

Disk lowered slowly until itdisappears from view

Secchi depth is midway

FIGURE 2.13 The principles of the Secchi disk for measuring in situ turbidity.

Rope to Pull CorkOut of Bottle

to float

Funnel

Plastictube

Rubber stopper

Spacing block

1 L Nalgenebottle

to anchor Nylon suspension rope

Removable500 MillilitreBottle

Surgical TubingTo secure Bottle

Weighted Base

Cork

(a) (b)

FIGURE 2.14 Techniques for sampling particulate matter in estuarine waters include (a) bottle sampler and (b)time-integrating trap sampler.

decreases from 100% in still water as thecurrent speed increases and sediment isswept out before recovery (Bloesch andBurns, 1980). A development is the streamertrap (Figure 2.14) wherein the water flowsinto the device and the particulates are

filtered out as the water exits and aredescribed by Krause (1987), and the bot-tle sampler (Figure 2.14a) where inter-nal tubes arrange for the sediment tosettle out before the water is releaseddownstream.


Pump sampling

Finally, samples of thewater can be obtainedand returned to the shore or to a survey boatusing carefully arranged pipes and pumps.Particulates in the sample are either settledout or removedwith a filter to determine theconcentration

Optical methods

Optical and acoustic methods for the deter-mination of particulate concentrations arebased on the effect of the presence of theparticles on the reflection or propagationof a beam. Optical methods separate intotransmissometers, nephelometers, and backscatter devices as illustrated in Figure 2.15.

Transmissometers

These consist of a light source, a straight lightpath, and a light detector. The amount oflight which is transmitted from the sourceto the receiver depends inversely on theconcentration of particulates (Figure 2.16):

It = k1 exp(−k2C) Eq. 2.4

where It is the intensity of the light receivedat the detector, C is the concentration ofparticulates, and k1 and k2 are calibrationcoefficients for the device and shape andcomposition of the sediment.

Ambient light

Light source Transmissometer

OBS

Nephelometers

FIGURE 2.15 The principles of optical methodsincluding back scatter and transmissometers.

Nephelometers

These consist again of a light source and adetector that aremounted at about 90◦ to theilluminated beam. The fraction of the lightthat is scattered into the detector increaseswith the concentration of particulates and isgiven by

Is = k3C exp(−k2C) Eq. 2.5

where Is is the light scattered to the detector,C is again the concentration of the particu-lates, and k2 and k3 are calibration coeffi-cients which depend again upon the shapeand optical properties of the particulates.

Back scatter devices

These consist of a light source and a detec-tor that is mounted adjacent to the source(Figure 2.17) at 180◦ to the illuminatedbeam. The fraction of the light that is scat-tered into the detector increases with theconcentration of particulates and is alsogiven by Eq. 2.5.

Acoustic methods

Single frequency acoustic backscatter mea-surements of suspended sediments in theocean were introduced in the 1980s (Younget al., 1982; Hay, 1983; Hess and Bedford,1985; Lynch 1985) using frequencies inthe range 1–4 MHz. More recently, Thorneet al. (1991) have developed a down-ward directed acoustic concentration sensorworking on backscattering principles knownas the Acoustic Profiling System. A shortpulse (10 μs) of high frequency (1–5MHz) isemitted from a transducer and some of theacoustic energy is scattered back from par-ticulates in the water column to the trans-ducer. The magnitude of the backscattered


FIGURE 2.16 Partech transmissometer for suspended solid monitoring and optical back scatter (OBS-3) device(DIR 2.15).

Acoustic Backscatter Measurements of Sediment Concentration

10

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ght a

bove

bot

tom

(cm

)

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35

–0.4 –0.2 0 0.2 0.4log Concentration (g/l)

0.6 0.8 1 1.2

20 30 40Time (seconds)

50 60 70 80

FIGURE 2.17 Acoustic backscatter device and example of suspended particulate concentration profilersobtained on the Californian continental shelf (DIR 2.16).

signal is related to the concentration, particlesize, and the time delay between trans-mission and reception. Range gating thedetected signal permits the concentrationprofile to be estimated (Vincent et al., 1991).

Remote sensing

Measuring SPM remotely relies on thefact that the reflectance of visible lightincreases with the amount of suspended


sediment in the surface waters. The satel-lite sensor used is the Advanced Very HighResolution Radiometer (AVHRR), which hasbeen in operation since the early eighties.Initially a relationship between reflectanceand the suspended sediment was foundusing field data collected on research cruises.An optical instrument was used to mea-sure reflectance at sea which mimicked themeasurements made by the AVHRR. Thesatellite images can be processed with thisrelationship to make maps of sediment con-centrations (White, 1999).

Online particulate data

There are nowadays long-term, archival dataand operational sites showing real-time ornear-real-time data for suspended sediment.Two examples are as follows:

Newfoundland: DIR 2.17 – Near-real-time data from the Canadian HumberRiver.North Sea: DIR 2.18 – West Gabbard buoyin the southern North Sea is a Smart Buoyoperated by CEFAS.

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Estuaries: Monitoring and Modeling the Physical System || Monitoring Estuarine Systems