Water for a Healthy Country

N AT I O N A L R E S E A R C H

FLAGSHIPS

An overview of the hydrodynamicsof the Coorong and Murray Mouth

Water levels and salinity – key ecologicaldrivers

Ian T Webster

AN OVERVIEW OF THE HYDRODYNAMICS OFTHE COORONG AND MURRAY MOUTH

Water levels and salinity – key ecological drivers

Ian T Webster

ISBN: 0 643 09259 5

Water for a Healthy Country is one of six National Research Flagships established by CSIROin 2003 as part of the National Research Flagship Initiative. Flagships are partnerships ofleading Australian scientists, research institutions, commercial companies and selectedinternational partners. Their scale, long time frames and clear focus on delivery and adoptionof research outputs are designed to maximise their impact in key areas of economic andcommunity need. Flagships address six major national challenges; health, energy, light metals,oceans, food and water.

The Water for a Healthy Country Flagship is a research partnership between CSIRO, state andAustralian governments, private and public industry and other research providers. TheFlagship aims to achieve a tenfold increase in the economic, social and environmental benefitsfrom water by 2025.

© CSIRO 2005 All rights reserved.

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, nopart may be reproduced by any process without prior written permission from theCommonwealth.

Citation: Webster, I.T., 2005. An Overview of the Hydrodynamics of the Coorong andMurray Mouth. CSIRO: Water for a Healthy Country National Research Flagship

DISCLAIMER

You accept all risks and responsibility for losses, damages, costs and other consequencesresulting directly or indirectly from using this publication and any information or materialavailable from it.

To the maximum permitted by law, CSIRO excludes all liability to any person arising directlyor indirectly from using this publication and any information or material available from it.

For more information about Water for a Healthy Country Flagship visit <www.csiro.au/healthycountry/> or the National Research Flagship Initiative <www.csiro.au>.

Foreword

The environmental assets of the Coorong, Lower Lakes and Murray Mouth (CLLAMM)region are currently under threat as a result of the ongoing changes in the hydrological regimeof the Murray–Darling River. While a number of initiatives are underway to halt or reversethis environmental decline, such as the Murray-Darling Basin Commission’s Living Murrayinitiative, rehabilitation efforts are hampered by the lack of knowledge about the links betweenflows and ecological responses in this system.

As a component of the Water for a Healthy Country National Research Flagship, CSIRO hasdeveloped a collaborative research program with the aim of producing a decision-supportframework for environmental flow management for the CLLAMM region. This involvesunderstanding the links between the key ecosystem drivers for the region (such as water leveland salinity) and key ecological processes (generation of bird habitat, fish recruitment, etc.). Asecond step will involve the development of tools to predict how ecological communities willrespond to manipulations of the ‘management levers’ for environmental flows in the region.These include flow releases from upstream reservoirs, the Lower Lakes barrages, and the UpperSouth East Drainage scheme, and dredging of the Murray Mouth. The framework will attemptto evaluate the social, economic and environmental trade-offs for different scenarios ofmanipulation of management levers, as well as different climate scenarios in the Murray–Darling Basin.

The research program brings together several institutions as well as researchers from a range ofbackgrounds. CSIRO provides a core of expertise in the fields of hydrodynamics,biogeochemistry and socioeconomics. Knowledge about the links between ecological driversand ecosystem responses will be developed with the CLLAMMecology research cluster, apartnership between the University of Adelaide, Flinders University and SARDI AquaticSciences supported by the Flagship Collaboration Fund.

This report is part of a series summarising the output from the Water for a Healthy CountryCoorong, Lower Lakes and Murray Mouth program. Previous reports and additionalinformation about the program can be found at www.csiro.au/csiro/channel/ich4.html.

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

v

Contents

Foreword ...................................................................................................................................... iii

Glossary and abbreviations ......................................................................................................... vi

Acknowledgments ....................................................................................................................... vii

Executive summary .................................................................................................................. ix

1. Introduction ............................................................................................................................ 1

2. The Murray Mouth .................................................................................................................. 3

2.1 Mouth geomorphology ..................................................................................................... 3

2.2 Sediment dynamics .......................................................................................................... 4

2.3 Mouth Opening Index ....................................................................................................... 5

3. The Coorong ........................................................................................................................... 8

3.1 Hydrodynamics ................................................................................................................ 8

3.2 Water levels ..................................................................................................................... 8

3.3 Tides ................................................................................................................................. 9

3.4 Wind-driven level changes ............................................................................................. 10

3.5 Lower frequency water level changes ............................................................................ 12

3.6 Salinity ............................................................................................................................ 15

3.7 Stratification .................................................................................................................... 18

4. Summary and conclusions ................................................................................................. 21

References................................................................................................................................ 22

vi

Glossary and abbreviations

AHD Australian height datum

CFMI Computational Fluid Mechanics International

CLLAMM Coorong, Lower Lakes, and Murray Mouth

continental shelf waves waves of period days or longer propagating around the coast

E&WS Engineering & Water Supply, SA State Department

geomorphology the study of the origin, evolution, and configuration of the naturalfeatures of the earth’s surface

hydrodynamics the study of the dynamics of water flow

hypersaline salinity above that of sea water

isothermal of uniform temperature

MDBC Murray-Darling Basin Commission

MOI Mouth Opening Index

Murkey Model water transport model for the River Murray in South Australia

stratification layering of properties in water column

salinity salt content of water measured in gL-1 (salinity of full sea water~35 gL-1)

shear stress the force of water flowing over the sea bed

tidal prism the volume of water exchanged between high and low tide

turbid thick or opaque with suspended matter; not clear; cloudy, muddy

USED Upper South East Drainage

WBM Oceanics engineering and environmental consulting company

wind stress the force of the wind on the water surface

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

vii

Acknowledgments

The author would like to acknowledge the provision of water level data for Victor Harbor byFlinders Ports Pty Ltd. (Greg Pearce) and meteorological data by the Bureau of Meteorology(Lynda Garlick). Several useful reports and the time series of Mouth Opening Index weremade available by the Murray-Darling Basin Commission (Julianne Martin). The salinity andwater level data were obtained from the Surface Water Archive (SA Department for Water,Land & Biodiversity Conservation). The author is also grateful to Mike Geddes and toSebastien Lamontagne for providing comments on the draft report.

viii

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

ix

The Coorong, Lower Lakes, and the MurrayMouth form a terminal lake system at themouth of the River Murray in SouthAustralia. A line of barrages inside the mouthseparate the Lower Lakes (Alexandrina andAlbert) from the saline waters of theCoorong which exchange with the seathrough the Murray Mouth. The Coorongand the Lower Lakes are of national andinternational conservation status, especiallyfor birds, and are listed as Ramsar Wetlands.

Reduced river flows in the River Murray inrecent years and the associated increasedlikelihood of mouth closure are regarded as athreat to the ecological function of theCoorong through:

� the propensity for higher salinities in thesystem;

� alterations to the water level regime; and

� blocking of fish migration pathways.

Water level is a key environmental attributethat determines the availability of physicalhabitat for birds and for other aquatic life.Similarly, all aquatic organisms havetolerance limits to salinity levels and theirdegree of variation, so the salinity regimewithin the Coorong is a major determinantof where and how well such organisms canlive and prosper.

The physical environment of the Coorongcan be altered through human manipulationincluding dredging the mouth and byaltering the timing and magnitude of barragedischarges and releases from the UpperSouth East Drainage scheme (USED) intothe South Lagoon. This report considers thefactors that determine water levels andsalinity within the Coorong.

SUMMARY OF MAIN FINDINGS

1. Water levels in the Coorong undergo aseasonal cycle of up to ~0.7 m in range,with higher levels tending to occur in latewinter – early spring, and lower levels inlate summer – early autumn. Thisseasonal variation is due to a combinationof variation in sea level outside the mouthand back-up due to discharge throughthe barrages.

2. Shorter-term water level variations of~±0.05 m typically are due to tilting ofthe water surface by the wind. Tidal levelvariation is important near the mouth.

3. Salinities increase along the Coorongfrom the mouth through to the SouthLagoon where salinity reaches severaltimes that of sea water. The sloshing ofwater in and out of the South Lagoonassociated with seasonal water levelvariation is a key determinant of thesalinity.

4. Salinity stratification has been observedin the North Lagoon and has thepotential for negative ecologicalconsequences, but its causes are poorlyunderstood.

5. Variations in water level and salinity andthe exchange of other material within theCoorong are inextricably linked.Manipulation of water levels throughdischarge variation or mouth dredgingwill have consequences for the salinityregime and for the concentrations ofsubstances such as nutrients.

Executive summary

x

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

1

The Coorong, Lake Alexandrina, LakeAlbert and Murray Mouth form a terminallakes system at the mouth of the RiverMurray in South Australia (Figure 1). A lineof barrages inside the mouth separate thelower lakes (Alexandrina and Albert) fromthe saline waters of the Coorong whichexchange with the sea through the MurrayMouth. The barrages prevent seawater fromentering the Lakes and lower Murray, andmaintain higher water levels in the naturallyshallow lakes. During times of high dischargein the Murray, significant volumes of freshwater pass the barrages and these flows havebeen deemed essential for maintaining theMurray Mouth in an open condition. Overthe past decades, flows in the River Murrayhave been reduced – mainly due to irrigationabstraction – to a fraction of previousvolumes and it would appear that thelikelihood of mouth closure has increasedover this time as a consequence.

The Coorong is a lagoon system severalkilometres wide that follows the coast formore than 100 km from the mouth. It isdivided into two lagoons, the North andSouth lagoons (Figure 1). The Coorong andthe Lower Lakes are of national andinternational conservation status, especiallyfor birds – the Coorong being ranked withinthe top six waterbird sites in Australia. Thesewater bodies are Ramsar listed and also asone of six Significant Ecological Assetsidentified in the Living Murray Initiative<www.thelivingmurray.mdbc.gov.au/>.Reduced river flows and the associatedincreased likelihood of mouth closure areregarded as a threat to the ecologicalfunction of the Coorong through:

� the propensity for higher salinities in thesystem;

� alterations to the water level regime; and

� blockage of fish migration pathways.

Water level is a key environmental attributethat determines the availability of physicalhabitat for birds and other aquatic life.Similarly, all aquatic organisms havetolerance limits to salinity levels and theirdegree of variation, so that the salinityregime within the Coorong is a majordeterminant of where and how well suchorganisms can live and prosper.

The physical environment of the Coorongcan be altered through human manipulationin a number of ways. Drivers include:

� dredging the mouth when it is threatenedby closure; and

� altering the timing and magnitude offlows past the barrages and releases fromthe Upper South East Drainage scheme(USED) into the South Lagoon at SaltCreek.

As a step in understanding the consequencesof management manipulation on thephysical environment of the Coorong andultimately on its ecology, this reportconsiders the dynamics of current water leveland salinity variations within the Coorong.Water level variation in the Coorong is a keydeterminant of habitat suitability and is alsoshown to be an important driver of waterexchange and salinity. Barrage discharges alsohave an impact on causing water levelvariations besides their role in providingfresh water to the system and maintaining anopen mouth.

The report has been prepared as part of thefirst stage of the Coorong, Lower Lakes, andMurray Mouth (CLLAMM) Project in theRiver Murray Theme of Water for HealthyCountry. The aims, rationale, and context ofthe CLLAMM project are outlined byLamontagne et al. (2004). Future work inthe project will develop the links between thebiophysical environment and ecological

1. Introduction

2

outcomes as well as assess the socioeconomicbenefits of alternative management strategiesfor the CLLAMM system.

Exchanges between the Murray Mouth andthe Coorong are caused by sea levelvariations, tidal variation, winds, andfreshwater discharge through the barrages. Inturn, these factors, impact on salinity levels

throughout the Coorong and on how othercontaminants including nutrients areexchanged along its length. The followingdiscussion presents features of thehydrodynamics of the Coorong itselfincluding water levels, salinity andstratification all of which are importantphysical characteristics.

Figure 1. The Coorong, Lower Lakes and Murray Mouth.

GOOLWA

SALT CREEK

WELLINGTON

0 10 20 Kilometres

Lake Alexandrina

Lake Albert

North Lagoon

South Lagoon

River Murray

Mundoo Barrage

Ewe Island Barrage

Tauwitchere Barrage

GoolwaBarrage

Murray Mouth

Southern Ocean

Boundary Creek Barrage

N

Woods Well

Long Pt

Dodd Pt

Mark Pt

Pelican Pt

The Needles

The Narrows

Younghusband Peninsula

Salt Creek Bay

Bul Bul Basin

Tea Tree Crossing

Policemans Pt

Villa de Yumpa

Parnka Pt

Robs Pt

Meningie

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

3

We address the dynamics of the opening ofthe mouth first. As the connecting channelbetween the sea and the Coorong system, thedegree of openness of the mouth is of criticalimportance:

� in the way that the system exchangeswater with the sea; and

� in setting water levels within the system.

2.1 MOUTH GEOMORPHOLOGY

The Murray Mouth once connected a largeestuarine system covering almost 750 km2,including the present Lake Alexandrina, LakeAlbert and Coorong, to the ocean (Bourman& Barnett 1995). It has always beenrelatively narrow, but has been and continuesto be extremely dynamic. The width of themouth has varied from being severalhundred metres during flood flows (Walker2002), to closed off completely in 1981 andalmost closed in 2003. In the 1830s, itsposition was 1.6 km from its presentposition; it has been observed to move 14 min 12 hours (Bourman 2000). Given micro-tidal conditions and domination of waveenergy along the coast, the system could beexpected to have a flood-tide delta (a deltalandward of the mouth) (Harvey 1996).Since the 1830s, the mouth has depositedand eroded significant volumes of sedimentfrom this delta. The tendency of theentrances of such coastal lagoon systems toclose has been observed in many seasonallyopen lagoons around Australia, but it is clearthat the construction of the barrages and theregulation of the River Murray haveexacerbated this tendency over the lastcentury.

Whereas prior to barrage construction thetidal prism was estimated to be ~20 000 MLduring spring tides (Walker 1990), the tidalprism has been reduced by an estimated 87%to 96% (Harvey 1996). Further, with theregulation of the River Murray and greatlyincreased water abstraction over the lastcentury, the freshwater flows through theMouth have also been greatly curtailed.Close (1990) reported that outflows throughthe Murray Mouth had reduced to a third ofnatural flows; since then they have reducedfurther. Reduced tidal and freshwater flowsthrough the Murray Mouth appear to haveresulted in major morphological changes innearby channels. These changes include:

� stabilisation of the flood-tidal delta (BirdIsland); and

� sedimentation and increased constrictionof the channels through the mouthestuary region including GoolwaChannel, Tauwitcherie Channel, and theMurray Mouth itself.

A number of people have described thealterations of the geomorphology of theMurray Mouth, particularly how it haschanged over the years since barragecompletion in 1940 (Bourman & Barnett1995, Harvey 1996, Bourman 2000).

We expect that the key factors determiningmorphological condition of the MurrayMouth and its adjacent channels are the:

� freshwater flow past the barrages; and

� the coastal conditions, including the waveclimate, littoral transport, and sea levelincluding the tides.

2. The Murray Mouth

4

Aeolian processes may also be of significancewith large quantities of sand moving alongthe modern shoreline and being blown fromthe dunes and into the channels (Bourman1986). How important this mechanism is forrestricting channel flows has not beenaddressed.

2.2 SEDIMENT DYNAMICS

The dominant sediments in the MurrayMouth area are sands comprised of mineralmaterial and shell fragments. They aretransported by:

� saltation; or

� suspension into the water column andadvection by the water flow.

Saltation occurs when the current is notstrong enough to lift the sediment grainsinto the water column, but is strong enoughto cause grains to ‘bounce’ along the bottomcausing bed-load transport. Othercontributors to bed-load transport are rollingand sliding of sand grains over the bedsurface. Both bed-load transport andsuspension require a minimum bottom shearstress to be exerted on the bed which meansthat the flow over the bottom must bevigorous. Such critical shear stresses for sandtransport depend on the sand propertiesincluding grain size distribution and specificgravity. The shear stress of the flow over thebottom increases as approximately the squareof the flow speed. When a mean current andwaves occur together, the bottom stressincreases as more than the sum of the stressesdue to the waves and currents consideredseparately (Grant & Madsen 1979). Thusthe presence of waves in a coastalenvironment can enhance considerably theability of currents to mobilise sediments.

Bed-load transport and sand suspension ratesare typically considered to be proportional tothe amount by which the critical shear stressis exceeded raised to the power of 1.5 ormore (e.g. van Rijn 1993). This implies that

sediment transport increases by the ‘excess’flow speed raised to at least the third power.Consequently, sand transport is very muchmore effective during times of energeticcurrents or waves than during times whenthe flows are close to the critical thresholds.Being more dense than water, suspendedsediment settles to the bottom so that in ahydrodynamically active environment,sediment suspension and deposition occursimultaneously. If local suspension ratesexceed the rate of settlement, then there is atendency towards erosion of the bed andsimilarly if settlement rate exceeds suspensionrate then the tendency is towards deposition.Bed-load transport can also resupply orremove sand from an area of the bed, sowhether net erosion or net deposition occursdepends on whether the total rate of supplyof sediment to the bed area (includingsuspended and bed-load transport) is lessthan or exceeds the rate of sediment removal.In coastal environments, erosion tends tooccur in areas of high hydrodynamic energy(waves and currents) and deposition tends tooccur in areas of lower hydrodynamic energy.

A modelling study has been undertaken byWBM Oceanics to examine the interplaybetween waves and flow in forming theMouth and its connecting channels Goolwaand Tauwitcherie (WBM Oceanics 2003).This study models the hydrodynamicsaround the Murray Mouth and the resultingsand transport. Equations in the sandtransport model incorporate the abovegeneral principles of sediment transport. Thestudy includes comparisons betweenmeasurements and model simulations, andgives results that are consistent with thefollowing conceptual picture of the sedimentdynamics in the vicinity of the MurrayMouth.

During times of low or no water flowthrough the barrages, flows through theMurray Mouth are dominated by tidal flows.The tidal water level and tidal current

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

5

pattern is highly asymmetric through themouth and estuary region with the flood tidehaving higher current speed and shorterduration than the ebb tide. The stronglyincreasing function of flow speed of sandtransport means that , the flooding tidetransports more sand than the ebbing tideeven though its duration is less. Theconsequence is that under zero or no flowsthrough the barrages there is a tendency forsand to be transported through the mouthand to be deposited in the inside sand deltawhere current speeds are reduced. Themodelling and measurements presented byWBM Oceanics (2003) show that thistransport is likely to be particularly intensiveunder conditions of high waves and springtides. In a two-week period between 14 and28 May 2002, which included a storm withsignificant strongly increasing function offlow speed wave heights (~4 m) and springtides, approximately 46 000 m3 of sand wasdeposited in the sand shoals and channelsinside the mouth. Some of this sand appearsto have come from scouring and widening ofthe mouth itself. In effect, the oscillatorycurrents associated with the high waves serveto help mobilise and suspend the sedimentsso they can be more effectively transportedby the tidal currents.

It is certain that the nature of sand transportalong the open coast on either side of theMurray Mouth plays an important role ingeomorphology:

� coastal winds drive long-shore currents(littoral drift);

� swell waves that arrive on a beach at anoblique angle can contribute to littoraldrift;

� high waves during storms suspendsediments and these are carried by thelittoral drift resulting in long-shore sandtransport.

A study by Chappell (1991) estimated thatbetween 1940 and 1990, 3849 significantstorms transported an average of 260 000 m3

of sand along the coast near the MurrayMouth. Such sand transport has thepotential to supply sand for transportthrough the mouth as well as to causechanges to the position and geomorphologyof the mouth itself. Chappell (1991)estimates that the volume and direction ofsand transport along the coast is quitevariable between years; Harvey (1996) showsthat the direction of migration of theposition of the mouth is also highly variablebetween years. However, Harvey (1996) alsonotes that the correlation between littoralsand drift and Mouth migration isinconclusive.

2.3 MOUTH OPENING INDEX

Significant freshwater flow through thebarrages alters the sediment transportdynamics in the Murray Mouth. Inparticular, this flow adds to the ebbing tidalflow and subtracts from the flooding flow.Provided the freshwater flow is large enough,the outward transport of sand on the ebbtide through the mouth will be larger thanthe inward transport on the flood, and themouth and inner channels will tend to clear.Walker and Jessup (1992) and Walker(2002) have examined the degree of mouthopening as it is affected by flows through thebarrage. The index they use for degree ofopenness is the ratio of the square of thesemi-diurnal tidal amplitudes measured atGoolwa Barrage and on the open coast atVictor Harbor (see section Tides below). Anempirical relationship between the opennessindex in month t (R

t) and the discharge in

gigalitres (GL) in month t was determinedas:

Rt = 0.8 R

t-1 + 0.0002F

t-2(1)

6

In other words, the index depends on theindex in the previous month and the flowthrough the barrages two months previously.It is not clear why the index should dependon flows two months previously and notthose from the previous month, but it isempirical and does not include other factorssuch as waves or storm activity. The R2 valueof 0.46 obtained for the comparison betweenthe measured and modelled index reflects themodel’s goodness of fit; the model capturesthe major features of the seasonal andinterannual variation in the measured index.This index has since been incorporated intothe MDBC Murkey Model as the MouthOpening Index (MOI); that is, MOI = R

t

(Close 2002).

An MOI near zero represents a mouth that isnearly shut; an MOI of unity signifies amouth that is sufficiently open that itrepresents little restriction to tidal exchange.The MOI typically starts to rise withincreased barrage flows after mid-year andreaches a maximum when flows reducesubstantially around the beginning of thenext year. During the low-flow periodbetween the beginning of the year and mid-year, the MOI gradually declines. Thisdecline is probably due to the gradualsedimentation of the mouth and insidechannels when the tides pump sediment intothe system (see above).

Despite periods of historically low flow, theMurray Mouth has only closed once in1981; it also almost closed in 2002. In 2002,a channel was maintained to the sea bydredging. Figure 2 shows both these times ashaving a small MOI. However, a low MOIdoes not necessarily result in mouth closure:

� in 1983 the mouth did not close despiteshowing a similar MOI to 1981.

� in 1968, it had an even smaller MOIand did not close.

Close (2002) suggests that the MOI is bestconsidered as an indicator of risk of mouthclosure and presents discharge scenarios thatare designed to reduce its risk.

Walker (2002) also considers the long-termconveyance of tidal flows through theGoolwa and Tauwitcherie Channels. Theabove analysis represents the conveyancethrough the Goolwa Channel. A similaranalysis for Tauwitcherie Barrage showsmaximum yearly measured indices of mostlyless than 0.3 which is considerably smallerthan those measured at Goolwa Barrage. Thelatter are typically similar to the modelledindices shown in Figure 2. Walker suggeststhat the difference in the tidal transmissionindices is due to Goolwa Channel being usedto carry most of the discharge from thebarrages and resulting in its being an

Figure 2. Mouth Opening Index (MOI) calculated using equation 1 and discharges through thebarrages estimated using the Murkey model (beginning of 1980 to the end of 2003).

Murray Mouth

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

MOI

0.0

0.2

0.4

0.6

0.8

1.0

Barra

ge d

ischarg

e (G

L/m

o)

0

500

1000

1500

2000

2500

3000

MOI

Discharge

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

7

efficient channel to the ocean. This iscertainly partly true, but TauwitcherieChannel also connects to a body of waterhaving a much larger surface area (theCoorong), so that a larger volume of waterwould need to flow through it to cause aspecified water level change. Walker alsosuggests that his analysis shows that tidaltransmission through Tauwitcherie Channelshows an overall downward trend throughthe 1990s and attributes this to a consistentsediment build-up. Such a trend is alsomeasured through Goolwa Channel, but isnot so marked.

8

3.1 HYDRODYNAMICS

Largely in response of the need to investigatethe long-term effects of drainage from theUSED scheme into the southern end of theCoorong, Computational Fluid MechanicsInternational (CFMI) undertook modellingof the hydrodynamics and salinity levels inthe Coorong lagoons at the beginning ofJuly, 1992. The description and validation ofthis modelling are reported in CFMI (1992).Two further reports (Reports CFMI 1998,CFMI 2000) present further scenariosimulations using the same model.

Much of the conceptual descriptionpresented here derives directly from the workreported in the 1992 CFMI report. Themodels applied to the Coorong wereESTRAFPH for the hydrodynamics andSALTIE which simulates salt transport andsalinity distributions. ESTRAFPH is a one-dimensional model that describes the time-dependent water levels and flows along theCoorong as they respond to measured waterlevels at Pelican Point (near the northern endof the North Lagoon, see Figure 1) and towind. SALTIE, also a one-dimensionalmodel, integrates the salinity transportequation using exchange coefficients derivedfrom ESTRAFPH.

The Coorong naturally splits into North andSouth Lagoons at Parnka Point (see Figure1). There are several channel sections oneither side of Parnka Point (the Narrows)that are very narrow (~100 m) and shallow,and which represent the main restriction forwater exchange between the two lagoons.The length of the North Lagoon to PelicanPoint is 48 km versus a length of 40 km forthe South Lagoon if the effective end to thelagoon is chosen to be 6 km south-east of

Sand Spit Point near Tea Tree Crossing asdoes CFMI (1992). At zero water elevation(AHD), the average widths of the North andSouth Lagoons are 1.5 km and 2.5 km,respectively, whereas the average depths are1.2 m and 1.4 m, respectively (CFMI 1992).

3.2 WATER LEVELS

According to Noye (1975), changes in thewater levels in the Coorong can be,

… classified into three main types; wind-induced short period changes of a foot or so(sic), with a time scale of days, in whichopposite ends of each lagoon move out ofphase with each other; short period increasesin levels in North Lagoon which occurwhen the barrages at the north end of theCoorong are opened for several days at atime; and seasonal variations of up to fourfeet (sic).

Noye 1975

Water level changes in Encounter Bayincluding tides are also acknowledged aspenetrating through the Murray Mouth toinfluence levels within the Coorong.

We first examine the relationship betweenwater levels in Encounter Bay and in theMurray Mouth. Water level variations occuracross a spectrum of time scales ranging fromthose associated with wind waves (~seconds), swell (~tens of seconds), tides (~1 day) and low frequencies (>1 day). Low-frequency water level changes in EncounterBay are characteristically of two types (Provis& Radok 1979). The first type is due to thepassage of weather systems and associatedcontinental shelf waves. These have periodsbetween 1 and 20 days. The second type offluctuation with periods between 20 and 365

3. The Coorong

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

9

days have an uncertain origin, but appear tobe due to sources in the Southern Ocean orin the interaction between the continent andthis ocean.

In general, the impact of water levelvariations imposed on the Coorong dependson the frequency of variation of these waterlevels and the degree of opening of themouth. For a given amplitude of water levelfluctuation within an enclosed basin such asthe Coorong, a high-frequency fluctuationmust exchange the same volume of watermore quickly than a low-frequencyfluctuation. Since the friction associated witha current is approximately proportional tothe flow speed squared, a greater frictionalforce must be overcome in a connectingchannel (the mouth) for a high-frequencylevel fluctuation to occur than for a low-frequency one. Consequently, low-frequencywater level variations in Encounter Bay suchas those associated with the passage ofweather systems penetrate more effectivelythrough the mouth and along the Coorongthan more rapid level fluctuations such asthose due to the tides. Similarly the widthand depth of the mouth affect its ability totransfer water level fluctuations into theCoorong.

3.3 TIDES

The tides along the coast of Encounter Bayare described as being micro-tidal in rangeand semi-diurnal with a moderate diurnalinequality (Short & Hesp 1975). Tidalranges vary between ~1 m during springtides to ~0.2 m during neap tides. When theMurray Mouth is open the tides penetrateinto the Murray estuary region causing waterlevel variations.

Walker and Jessup (1992) have undertakenan analysis on the opening of the MurrayMouth and how it relates to flow throughthe barrages (see also section Mouth OpeningIndex). This analysis uses relative tidal energyof the semi-diurnal tidal component (12-hour period) at Goolwa as a surrogate for

mouth opening. Relative tidal energy, R, isthe ratio of the square of the tidal rangebetween measurements obtained within theCoorong–Murray Mouth system and thoseon the coast at Victor Harbor 25 km fromthe mouth. Results presented by Walker(2002) extending this analysis, show that Rfor Goolwa varies between 0 (mouth closed)and 1.0 (mouth ‘fully’ open). Visualinspection would suggest a median of ~0.4.However, for the limited measurementsobtained at Tauwitcherie Barrage, themaximum value for R is ~0.5 and theindicated median would be less than 0.2.Walker (2002) has further suggested that areduction in R after 1990 is indicative ofsediment build-up in both the MurrayMouth and Tauwitcherie Channel. Onewould expect that R for the diurnal (24-hour) period would be larger than that forthe semi-diurnal component. Nevertheless,these results suggest that the tidal ranges atthe north-western end of the Coorong aremuch less than those on the open coastwhere daily tidal ranges vary between ~0.2and 1.1 m depending on whether the springor neap tidal phase is prevailing.

For a value of R = 0.2 and a tidal range of1 m (amplitude of 0.5 m) at Victor Harbor,the amplitude of the tide at the north-western end of the North Lagoon would be~0.2 m. Applying a simple model of tidalpropagation using the calibrated frictionparameters determined by CFMI (1992), theamplitude of this tide would be reduced by afactor of five at the south-eastern end of theNorth Lagoon if it is assumed to have asemi-diurnal period. By contrast, a waterlevel oscillation of 0.2 m amplitude andhaving a period of 48 hours would bereduced in amplitude by a factor of 0.6illustrating the increase in efficiency of waterlevel transmission along a water body as itsperiod of fluctuation becomes greater. CFMI(1992) has noted that the tides do extendwell down into the North Lagoon furthersouth than Robs Point.

10

3.4 WIND-DRIVEN LEVELCHANGES

Wind blowing on the water surface exerts aforce which pushes water in the downwinddirection. This causes the water to pile upagainst the downwind shore and results in anupwards tilt of the water surface from theupwind to the downwind end (Figure 3).Once the wind has been blowing for longenough, an equilibrium is established inwhich the force of the wind on the watersurface is counterbalanced by the pressuregradient associated with the water tilting inthe opposite direction. If the wind speeddrops to zero, then the system relaxes and thecurrents would flow along the Coorong inthe opposite direction.

In the short term (less than a few days),water levels in the North and South Lagoonsrespond individually, but eventually theentire length of the Coorong tilts in onedirection due to flow between the lagoons.At equilibrium, the water level near theMouth would be that of the sea.

For a simple lagoon closed off at the ends,and of uniform width and depth, a simplehydrodynamic analysis would suggest thatthe steady-state relationship between windset-up (η) and wind speed along the lagoon(W) would be:

η ∼ W 2 (2)

where ρa ~ 1.2 kgm-3 is air density, ρ

w ~ 1000

kgm-3 is water density, L is lagoon length,C

D ~ 0.0015. C

D is the drag coefficient for

wind over water, g ~ 9.8 ms-2 is gravitationalacceleration, and H is water depth. Thus, theinverse proportionality with water depthmakes the shallow Coorong susceptible tolarge wind set-ups.

Due to the constriction in the channels nearParnka Point, the initial response of theNorth and South Lagoons occurs as if thesetwo basins had closed ends. In their analysisof the response of the North Lagoon towind, Noye and Walsh (1976) treated thislagoon as a closed system and obtained goodagreement between their model andmeasured water levels. Figure 3 (short-termwater level) shows how such a two-lagoonsystem might respond to a wind blowingalong its length. Using the above formula, a5 ms-1 wind blowing along the Coorongwould cause a set-up in such a two-lagoonsystem of 0.09 m in the North Lagoon and0.07 m in the South Lagoon. The time thatthe wind needs to blow for the set-up toestablish itself is of the order of 4 hours. Ofcourse, the wind over the Coorong changesdirection and speed with time, causing thewater levels along the lagoons to vary inresponse. In CFMI (1992), measurementsand model simulations show that the waterlevel response of the lagoons is approximatelyin accordance with the response predicted byequation 2.

Figure 3. Water level response to wind and to wind stress along the channel in the Coorong.

ρaLC

D

2ρw

gH

Wind

Flow from mouth

Flow through Narrows

Sea level

Equilibrium water level

Tauwitcherie Channel Northern Lagoon Parnka Point Southern Lagoon

Short term water level

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

11

Figure 4 shows the high-frequency waterlevel record for Sand Spit Point (near SaltCreek in the South Lagoon). These data wereobtained by subtracting the low-frequencyfluctuations having periods greater than 10days (including the mean) from the timeseries of daily averaged water levels. Alsoshown are the water levels at Sand Spit Point‘predicted’ using equation 2 with measuredwind speeds at Meningie. It is apparent thatthe variations in the wind stress account formost of the water level fluctuations at thislocation in the South Lagoon. Further, themagnitude of these wind-driven fluctuationsbetween 1 and 10 day periods are mostly oforder ±0.05 m. The storm event in mid-September shows water levels at Sand SpitPoint to rise above 0.15 m for approximatelyone day. Storm surges can increase waterlevels outside the mouth by up to 1.5 m(Bourman & Harvey 1983) and it is likelythat such a surge would penetrate into andalso cause substantial short-term increases inwater levels in the Coorong.

Treating the lagoons as separate water bodiesfor wind response is likely to be reasonableprovided the duration of the wind event isnot longer than a few days. The difference inwater level between the two sides of theconnecting channel at Parka Point will causewater to flow from the lagoon on the upwindside to the lagoon on the downwind side at arate that depends inversely on whether thewater level at the Narrows is relatively deep(winter–spring) or shallow (summer–autumn). Eventually if the wind blows longenough, the water level on both sides ofParnka Point will equilibrate. Likewise, flowthrough the Murray Mouth will eventuallycause the water level at the northern end ofthe Coorong to equilibrate to that ofEncounter Bay. In effect, the tilt in the watersurface along the Coorong would respond asif the length of the lagoon were the totallength of the Coorong rather than length ofeach of the North and South Lagoons takenseparately. In this circumstance, one mightexpect that the water level response at thesouthern end of the Coorong would beaugmented (see equilibrium water level,Figure 3). The model results presented byCFMI (1992) did not address how long thewind would need to blow steadily for thiscircumstance to occur.

Figure 4. Comparison between measured and modelled ‘high’-frequency water level variationsat Sand Spit Point (near Salt Creek, South Lagoon).

1999

Sep Oct Nov

Wa

ter

leve

l (m

)

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Measured level

Predicted level (Eq. 2)

12

3.5 LOWER FREQUENCY WATERLEVEL CHANGES

Monthly averaging (see Figures 5 & 6)removes most of the fluctuations associatedwith the passage of weather systems as well asthe tidal fluctuations. Pronounced seasonalfluctuations occur in water levels in theCoorong (see Figures 5 & 6). At Goolwa, theseasonal variation in water level was often~0.5 m and at Robs Point it was often~0.7 m. High water levels tend to occur inlate winter–early spring at Goolwa (and atRobs Point), whereas lowest water levels tendto occur in late summer–early autumn.

It is apparent that the seasonal variation inwater levels is associated with variation in thewater level on the coast as well as variation indischarge through the barrages (Figure 6).The lowest water levels in the Coorong occurwhen the flow through the barrages is aminimum and when the sea level at theMurray Mouth is depressed. Highest waterlevels occur several months before the time ofmaximum discharge which occurs on averagein spring. This phase shift is certainly due inpart to the maximum of the coastal waterlevel occurring in mid-year. Note that theperiod over which the monthly VictorHarbor water levels were calculated was1998–2003 versus 1985–2002 for themonthly Goolwa levels and the barragedischarges. On the assumption that theaverage coastal water level variation wouldhave been similar to that shown in Figure 6,it would seem that on average the seasonalcoastal water level variation has a similarimportance to those induced by the barrageflows. Certainly, through the low dischargeperiod between February and June, it is thechange in the coastal sea level that appears tocause Coorong water levels to increase,whereas the levels later in the year may bemore due to elevated barrage discharge atthis time.

On an open coast such as that near theMouth of the Coorong, waves running upthe beach would be expected to cause anincrease in average water level over thatrecorded at Victor Harbor of perhaps tens ofcentimetres (Nielsen 1988). The swell isdominated by waves from the south west andpeaks during April to September (Short &Hesp 1980) so wave set-up makes acontribution to the rise in water levels in theCoorong during this time. How such waveset-up would interact with a channel throughthe beach such as at the mouth is notknown.

The flows through the barrages cause a waterlevel change in the Coorong by increasingthe outflow through the mouth. Theincreased flow requires an increasedhydraulic head (water level) in the landwardside of the mouth in order to overcome theincreased friction. Thus, for a given channelcross-section one might expect that higherdischarges would be associated with higherwater levels in the Coorong and this seems tobe what happens. For example, the relativelylarge discharges of 1989 and 1990 areassociated with water levels at Goolwa of~0.5 m, whereas during the low dischargeyears of 1994 and 1997 water levels were ~0.2 m.

Elevated flows appeared to cause even higherwater levels at Robs Point. We suggest thatthis phenomenon must be due to themanner of discharge through the barrages.Discharge through Tauwitcherie Barragewould tend to elevate water levels at RobsPoint above those caused by dischargethrough Goolwa Barrage due to the way inwhich the channels from the two barragesconnect to the mouth.

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

13

Figure 5. Measured monthly averaged water levels at Goolwa Barrage, Robs Point and VictorHarbor. Also shown are calculated discharges through the barrages.

Figure 6. Averaged measured monthly water levels at Goolwa (1985–2002) and Victor Harbor(1998–2003. Also shown are averaged monthly discharges through the barrages (1985–2002).

1985 1990 1995 2000 2005

Wa

ter

leve

l (m

)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Ba

rra

ge

dis

ch

arg

e (

GL

/mo

)

0

2000

4000

6000

8000

10000

Goolwa

Robs Point

Victor Harbor

Discharge

Jan Mar May Jul Sep Nov Jan

Wa

ter

leve

l (m

)

-0.1

0.0

0.1

0.2

0.3

0.4

Ba

rra

ge

dis

ch

arg

e (

GL

/mo

)

0

200

400

600

800

1000

Goolwa (1985-2002)

Victor Harbor (1998-2003)

Discharge (1985-2002)

14

Both Robs Point in the North Lagoon andSand Spit Point in the South Lagoon arenear the southern ends of their respectivelagoons. It is apparent that for most of theyear, water levels at both sites track oneanother fairly closely (Figure 7). Variations inwater levels at both sites occur due tovariations in water levels imposed at the coastas well as by variations in barrage discharge.Differences in water levels between the twolocations would be due in part to tilt of thewater surface caused by a component of thewind blowing along the length of theCoorong (see Figure 7 showing the waterlevel difference between the two locationsthat would occur due to wind tilt calculatedusing equation 2). For ten -day averagedwind stresses, the elevation difference due tocalculated wind tilt shows a distinct seasonalvariation. The dominant southerly or south-easterly wind directions of summer areestimated to cause water levels towards thesouthern end of the South Lagoon to bedepressed by ~0.05 m compared to thewinter condition.

There is a consistent and significantdifference between water levels at Robs Pointand at Sand Spit Point that occurs throughJanuary to April in the record shown as wellas in longer time series of levels at the twolocations. The pattern of water leveldepression of ~0.2 m at Sand Spit Pointduring this time is not consistent with itsbeing simply due to wind tilt. This is thetime of the year when water levels in the

Coorong are at their lowest and the channelconnecting the North and South Lagoons ismost restricted.

... this channel becomes very shallow and atone place during most summers the waterlevel falls sufficiently for a sand bar(Parnka Crossing) to become completelyexposed, thereby completely separating thewaters of North and South Lagoon.

Noye 1975

A measured cross-section near Parnka Pointpresented by CFMI (1992) shows amaximum water depth of ~0.3 m AHD, sowhen the water level in the system reduces tothis height, the flow through the channel atthis time would be so restricted (if it occursat all), that it would not be able to replaceevaporative loss in the South Lagoon. Thedivergence of water levels between the Northand South Lagoons would suggest theywould effectively be separated when waterlevel reduces to ~0 m AHD. A secondpossibility is that wind events in the SouthLagoon would raise the level at its north-westend sufficiently to cause water to spillthrough the channel into the North Lagoon.Due to the prevalence of southerlies andsouth-easterlies in summer such spilling islikely to result in more water beingtransported from the South to the NorthLagoon rather than the other way round.Either way water levels in the South Lagoondrop significantly below those in the NorthLagoon at this time.

Figure 7. Measured water levels at Robs Point (North Lagoon) and at Sand Spit Point (SouthLagoon) presented as ten-day running averages. Also shown are the water levels at Sand SpitPoint due to wind tilt (equation 2).

1998 1999 2000 2001 2002 2003

Wa

ter

leve

l (m

)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0 Robs Point

Sand Spit Point

Sand Spit Point wind tilt

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

15

3.6 SALINITY

Salinities have been measured along theCoorong over a number of years. Salinity isimportant because it is:

� a significant component of the physico-chemical environment – most aquaticflora and fauna thrive over limitedsalinity ranges; and

� an important indicator of the exchangeprocesses that occur in a water body.

The salinity distribution along the Coorongis determined by:

� the balance of freshwater input into thesystem;

� the flow through the barrages;

� precipitation;

� loss of water through evaporation; and

� the exchange (mixing) of water along thelagoons and between the lagoons.

Groundwater input also has a potential (butunknown) effect on salinity. The freshwaterinput through the barrages tends to lowersalinities at the north-west end of theCoorong, whereas the excess of evaporationover precipitation tends to increase salinityalong its length. The overall salinity patternin the Coorong is for salinity to increasefrom its north-west towards its south-eastend. Depending on the flow through thebarrages, salinities can be near zero at thenorth-west end of the Coorong. Typically,salinities can be several times that of seawaterat its other end due to the excess ofevaporation over precipitation. All the factorsaffecting salinity combine to not only set theaverage yearly salinities along the Coorong,but also to cause a significant seasonal cycle.Geddes and Butler (1984), and Geddes(1987) described measurements of salinityalong the North and South Lagoons andrelate the distribution and variations insalinity to flows through the barrages.

Release of freshwater typically during June toOctober and peaking in August causes thewater near Pelican Point to become relativelyfresh with salinities of less than 5 gL-1

measured on occasion. One might expectthat the actual salinities in this zone to bedependent not only on the volume andduration of the flow through the barrages,but also on the degree of opening of theMurray Mouth. A relatively open mouthwould allow more vigorous mixing ofseawater through the mouth and would tendto increase salinities.

Along the length of the Coorong, salinitiesare impacted by the relative rates ofprecipitation and evaporation. Evaporationrates from the Coorong have been estimatedfrom measured pan evaporation ratesadjusted by a factor of 0.8 (CFMI 1992).Measurements presented for the region overthree years by CFMI show that panevaporation rates tend to be a minimumduring winter (May to August) when ratesare ~2 mmd-1 and a maximum in summer(November to February) with rates of~7 mmd-1. Rainfall shows the inverse patternwith rates of ~3 mmd-1 in winter and lessthan ~1 mmd-1 in summer. Thus, exceptduring winter months, the Coorong loseswater across its surface due to the dominanceof evaporation over precipitation. Thisimbalance causes the salinity to increase.

According to CFMI (1992), mixing alongthe North and South Lagoons and betweenlagoons is mainly caused by the winds whichcause water to ‘slosh’ back and forth. In theprocess, some water is left behind and mixeswith the ‘local’ water. The net effect of such aprocess is to tend to cause the salinity tobecome more uniform along the Coorong.However, the exchange of water past ParnkaPoint is restricted, allowing evaporation toconcentrate salinities in the South Lagoon towell over that of seawater.

16

We now consider some details of the salinitydynamics as illustrated by measurementsobtained at three locations along theCoorong over a four-year period anddownloaded from the Surface Water Archive(2004). There is a very evident generalincrease of salinity along the Coorong fromnorth to south (see Figure 8), but the salinitybehaviour at Mark Point, Sand Spit Pointand Parnka Point is quite distinct. Thesalinity at Mark Point ranges from near zeroat times when the elevation through thebarrages is relatively high in the spring–summer of 1998/1999 and 2000/2001.Salinities increase to levels at or exceedingthose of seawater (~35 gL-1) when thedischarge was near zero during the summer–autumn of 1998, 2000, and for most of2002 when the discharge was zero.

During this time, salinity at Salt Spit Pointvaried between 80 gL-1 and 140 gL-1 with anaverage of 105 gL-1; that is, salinities variedbetween greater than twice seawater salinityto four times seawater salinity. Salinity showsa pronounced seasonal cycle of ~40 gL-1 withhighest salinities occurring in March–Apriland lowest occurring in September–November.

Why does the salinity vary the way it doesseasonally and what sets the overall salinitylevel?

Near the middle of the low-salinity time inOctober 2001, the salinity in the SouthLagoon was ~80 gL-1 and it increased to~125 gL-1 in April 2002. Assuming anestimated average evaporation rate of5 mmd-1and an average precipitation rate of1 mmd-1 over this six-month period(measured at Salt Creek) would result in thenet loss of ~700 mm of water. DuringOctober to December, 2001, there wasinflow through Salt Creek of water havingsalinity 10–12 gL-1. Averaged over the area ofthe South Lagoon, this inflow wouldcontribute ~100 mm of water which is muchfresher than the water already present so wecould consider that the net loss of fresh water(including the inflow) was 600 mm duringthe October–April period. Assuming nomixing between lagoons, then a loss of 600mm from a water column of initial depth1.9 m would cause the salinity in the SouthLagoon to concentrate to 115 gL-1 – similarto the salinity measured in April 2002. Thus,evaporation is sufficient to explain theincrease in salinity in the South Lagoonduring the summer season.

1998 1999 2000 2001 2002 2003

Salin

ity

0

40

80

120

160

200

Dis

ch

arg

e (

GL

/mo

)

0

200

400

600

800

1000

Mark Point

Parnka Point

Sand Spit Point

Discharge

Figure 8. Measured salinity at Mark Point (North Lagoon) and at Sand Spit Point (SouthLagoon) presented as ten-day running averages. Also shown is the discharge through thebarrages.

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

17

We now consider the issue of how thesalinity decreases again during the wintermonths. Consider the time period April2002 to October 2002. For this period,evaporation (~400 mm) still exceedsprecipitation (~300 mm), but the net loss tothe system is reduced to ~100 mm. Duringthis time, the water level in the SouthLagoon increased from ~-0.2 m (AHD) to~0.4 m, a change of 0.6 m. Assuming thatthe water required to achieve this water levelchange comes from the North Lagoon with asalinity of ~40 gL-1 and assuming that thiswater mixes with the water of salinity~125 gL-1 that was in the South Lagoon inApril 2002, then the salinity of the mixturewould be ~95 gL-1. This salinity is similar tothe salinity measured at Sand Spit Point inSeptember–October 2002. During this timeperiod, the flows recorded for Salt Creekwere small.

Measurements made by E&WS (CFMI1992) of salinity at Salt Creek near Sand SpitPoint showed a similar seasonal cycle ofsalinities to those shown in Figure 8, butwith lower salinities overall. During 1985–1988, the average salinity was ~85 gL-1

dropping to ~60 gL-1 during 1990–1992.Overall, barrage discharges in 1985–1988were not dissimilar to those for the period1998–2003, but discharges were unusuallylarge in the early 1990s (Figure 2).

The salinities measured at Parnka Pointduring the four-year period largely fall inbetween those measured at Mark Point andSand Spit Point. In effect, the salinity atParnka Point largely reflects mixing andexchange between the relatively high salinitywaters of the South Lagoon and the lowersalinity waters of the North Lagoon and alsoillustrates a pronounced seasonal cycle. For aperiod in summer, salinities at Parnka Pointreach and follow those at Salt Spit Point.Later in the summer, the North and SouthLagoons become separated due to low waterlevels in the channel connecting them andthe salinities in the two lagoons diverge.

This simple analysis demonstrates features ofhow the seasonal cycle of salinity variationwithin the South Lagoon are related to:

� net evaporation rates;

� salinities in the North Lagoon; and

� water level changes in the system.

It is an approximate analysis only. Clearly ifwater was being evaporated from the SouthLagoon and was simply replaced by an inflowof saline water from the North Lagoonduring the winter months, there would be anet accumulation of salt in the South Lagoonevery year and its salinity would continue torise year after year. There was a net input ofsalt into the South Lagoon between 2001and 2002, but the average salinity of thelagoon was less in 2001 than it was in 1999.The mechanisms that allow the lagoon tolose salt are:

� two-way mixing between the North andSouth Lagoons – included in thehydrodynamic modelling of the Coorong(CFMI 1992); and

� the outflows from the South Lagoon thatwould occur if the seasonal range in waterlevel variation were greater than the netrate of water loss from the lagoonincluding evaporation, precipitation andSalt Creek inflows.

As with the seasonal variation in salinities,the average salinities in the South Lagoonwill be determined by:

� net evaporation rates;

� salinities in the North Lagoon;

� water level changes; and

� long-channel mixing.

Estuarine salinities, that is salinities of lessthan or equal to those of seawater, have notbeen recorded in the South Lagoon since thefloods of 1975 (Geddes & Butler 1984,Geddes 2000) when salinities along theNorth Lagoon were lower than normal.

18

3.7 STRATIFICATION

Stratification in water bodies occurs wheneither the water temperature or the salinityvary with depth through the water column.The presence of stratification in the watercolumn implies that the water column iseither not completely mixed or perhaps thatmixing is not occurring vigorously enough tonegate a stratifying tendency such asdifferential absorption of solar radiation.Further, the density of water is dependent onboth its temperature and its salinity so thatstratification in temperature and/or salinityindicates that the density of water changeswith depth from the surface. Under normalconditions, the water column is either notstratified (indicating sufficiently vigorousvertical mixing) or it is stably stratified; thatis, the density increases with depth. A stablystratified water column is more difficult tomix than a uniform water column becausewith stable stratification, energy is requiredto ‘lift’ parcels of relatively dense waterupwards or equivalently to ‘push’ water ofrelatively low density downwards. Thus,stratification indicates incomplete mixing ofthe water column as well as being acondition that inhibits mixing.

Decomposition processes in the watercolumn or on the bottom can deplete oxygenin the water column. If the water column isactively mixing, then this water can bereplenished with oxygen through transferacross the water surface.

In stratified conditions, the absence ofvigorous vertical mixing may allow the waterto become seriously depleted in oxygen(hypoxic) or even anoxic. Hypoxic or anoxicconditions can be deleterious to bottomdwelling organisms. Also, anoxic conditionscan lead to the release of phosphorus andammonia from bottom sediments andthereby exacerbate eutrophication. In thehypersaline waters of the South Lagoon, thesaturation concentration of oxygen would be

low thereby increasing the propensity ofserious oxygen depletion during stratifiedconditions.

A second effect of stratification is to reducethe amount of friction between water layers.This reduction allows water layers to slideover one another more easily so that theresponse of the currents to wind blowingover the water surface is modified. This willin turn affect the way in which salinity andnutrients are transported along the Coorong.

One study describes the stratificationbehaviour of the Coorong (Holloway 1980a,1980b). Holloway showed that the Coorongstratifies in both temperature and in salinity.Measurements over a year demonstratedthree types of stratification:

� maintenance of isothermal conditions;

� stratification formation and decay; and

� salinity stratification.

Maintenance of isothermal conditions

The maintenance of isothermal conditions inthe water column is the most common typeof stratification. In this case, either the windis sufficiently strong or the energy inputacross the water surface is sufficiently lowthat the wind is capable of maintaining thewater column in a mixed condition.

Stratification formation and decay

Stratification formation and decay typicallyoccurs on a diurnal basis. Early in themorning, the water column may beisothermal, but as the sun rises thetemperature of the near surface waters heatsup and stratification is established. Seabreezes later in the day (or perhaps thepassage of a weather system) are sufficientlystrong that the water column becomes fullymixed. Holloway demonstrates that thedestruction of stratification under thesecircumstances is largely predictable and

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

19

typically occurs when an energy flux ratioexceeds a critical value. This ratio depends onthe wind speed cubed divided by thebuoyancy flux times the water depth. Thebuoyancy flux is calculable from the netinput of heat energy across the water surface(solar radiation, thermal emission, heatconduction, evaporative heat loss) and theturbidity of the water column. The value ofthe ratio decreases with increasing waterdepth and increases with turbidity so thatthe susceptibility of the water column toremain stratified under a specified windspeed is greater in the deeper parts of theCoorong and increases when the water ismore turbid.

Application of this analysis demonstrates thatunder conditions of maximum summertimeheating in a system that has a water depth of3 m and a light extinction coefficient of 1 m-

1, (moderately turbid) stratification would beestablished in the water column for windspeeds less than ~4 ms-1 or ~14 kmh-1.

During the night, the water surface cools dueto evaporation, long-wave (thermal) emissionto the sky, and conduction of heat betweenwater and atmosphere. More often than not,one would expect that the heat loss duringthe night would be similar to the heat gainduring the day so that the net change inthermal energy of the water column over 24hours would be close to zero. In thesecircumstances, any stratification that hadformed in the water column after the day’sheating would be eroded at night and thewater column would have mixed from top tobottom by dawn. If mixing does occurthrough the water column on a daily basis,then the likelihood of significant oxygendepletion in bottom waters is diminished.

It is apparent from studies in slow flowingsections of the River Murray near Lock 1(Bormans & Webster 1998) that thermalstratification can persist for several days in a

row. This situation is associated with thepassage of weather systems over the region.For example, if cold, cloudy conditions arefollowed by a change to finer, warmerweather, the net amount of heat absorbed bythe water column can continue to increase soallowing for the maintenance of thermalstratification. If winds are sufficientlyvigorous, thermal stratification might bedestroyed despite favourable heatingconditions. Condie and Webster (2001)provide an analysis that demonstrates howstratification might or might not bemaintained under prescribed daily heatinputs and wind conditions.

Salinity stratification

The third class of stratification in theCoorong identified by Holloway is salinitystratification. Holloway (1980a) identifiedseveral occasions during a year-long studyundertaken at Long Point when salinities inthe surface layer were markedly differentfrom those in the deeper part of the watercolumn. Long Point is located approximatelymid-way between the Mouth and ParnkaPoint. Development of stratification isattributed to either:

� the advection of a relatively fresh layer ontop of an existing brine layer; or

� from the advection of a brine layerbeneath an existing fresher layer.

Overall, it might be expected that the brinelayer originates towards the south-east end ofthe North Lagoon and perhaps even fromthe South Lagoon, whereas the fresher layercould result from either fresh water flowsover the barrages or from seawateroriginating from the mouth.

Holloway describes two occasions on whichsalinity stratification was evident. The first ofthese occurred in January 1977 and lastedfor at least a day (the near-surface salinitysensor failed after this time). The difference

20

in salinity between the upper and lower partsof the water column was measured to be ~12gL-1. The second occasion occurred in July1976 and lasted for some nine days with asalinity difference of up to 20 gL-1 throughthe water column. On this occasion, thebottom layer was measured to flow in anapproximately northwards direction out ofthe Coorong, but the surface layer oscillatedback and forth apparently following thedirection of the wind.

This behaviour illustrates how thestratification can cause the motions in theupper and lower parts of the water columnto decouple from one another. On his moreintermittent surveys, Geddes (1987) notedthat the water column was usually isohaline,but that there were occasions whensignificant stratification was measured. Morerecent measurements at a series of stationsalong the North Lagoon by Geddes (2005)showed the lagoon to be:

� mostly vertically mixed in salinity duringtransects in September 2003 and July2004; but

� to contain significant verticalstratification along most of the lagoonlength in April 2004.

The few measurements of salinity structurein the South Lagoon (Geddes & Butler1984, Geddes & Hall 1990) do not showevidence of stratification. However, weconsider that it is probable that stratificationdoes occur from time to time. Anopportunity for development ofstratification would occur when relativelyfresh water flows into the South Lagoon pastParnka Point in mid year. Other fresh waterinputs from Salt Creek or throughprecipitation could result in stratification.Stratification could also occur due to the

inflow of relatively high salinity water fromthe shallow areas along the sides of thelagoon and from the shallow Bul-Bul Basinto the south of Salt Creek. In shallow depths,evaporation would tend to increase saltconcentrations more than in the deepersections of the lagoon.

Depending on the rate of bacterialdegradation of organic matter in bottomwater and on the sediment surface,stratification that persists for nine days (oreven longer) may be of sufficient duration tocause serious depletion of oxygen in bottomwaters . At the stations with significantstratification in April 2004, Geddes (2005)measured bottom oxygen concentrations tohave been up to 5 mgL-1 lower than thosemeasured at the surface.

Holloway (1980a) has also noted that salinitystratification was associated with ‘heatpooling’. Heat pooling occurs when thewater column is sufficiently clear thatbottom water heats up due to the absorptionof solar radiation during the day. Unlike thecase of thermal stratification for which thewhole of the water column tends to cool andmix during the night, the salinitystratification may be sufficiently strong toprevent vertical mixing. Although surfacewaters may cool during the night, bottomwaters do not. Holloway’s measurementssuggest that bottom water temperatures wereseveral degrees warmer than they would havebeen without the stabilising effects of thesalinity stratification. Since bacterialdegradation processes increase withtemperature, the heat pooling effect wouldtend to exacerbate the tendency of theCoorong to experience anoxic bottom watersunder conditions of salinity stratification.

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

21

4. Summary and conclusions

This report has considered features of thehydrodynamics of the Coorong and MurrayMouth particularly as these affect importantelements of habitat: water level variationsand salinity in the system. It is evident thatlonger period variations (> 5 days) in waterlevel in the North and South Lagoons areaffected by sea level variation as well as bydischarges of fresh water through thebarrages. As long as the mouth is open, theseasonal variation in sea level appears topenetrate the length of the Coorong and isnot likely to be strongly influenced by thedegree of mouth opening.

Conversely, the water level response of theCoorong to barrage discharges is likely to beaffected by:

� the barrage that is discharging; and

� the degree of channel constriction in themouth region.

Significant shorter-period water levelvariations are caused by the wind which tiltsthe water along the lagoon basins one way orthe other and by the tides which penetrateinto the northern end of the North Lagoon.

Salinity along the Coorong is affected by:

� barrage discharges which supply freshwater to the northern end of the system;

� evaporation;

� precipitation; and

� drainage from the USED.

The distribution of salinity along theCoorong is determined by flows and mixingprocesses in the system. Wind and tidal flowsnear the mouth are important agents forlong-system exchange. Longer-period waterlevel variations associated with sea levelchanges and barrage discharges are also animportant agent. The rise and fall of waterlevels act to pump water from one part of theCoorong to another. The balance betweenevaporation and water exchange caused bywater level change results in a seasonal cycleof salinity variation in the South Lagoon anddetermines overall salinity levels. Thus,manipulation of water levels in the systemthrough dredging of the mouth or varyingbarrage discharges has implications for thesalinity regime and for the exchange of othermaterials such as nutrients.

The occurrence of significant salinitystratification has been observed in theCoorong, but its dynamics are not welldescribed. Through its potential to allowbottom water to become depleted in oxygen,persistent stratification could have significantdeleterious impacts. Salinity stratificationcertainly arises as a consequence of waters ofdiffering salinity sliding over one anotherand is diminished by vertical mixingincluding by the wind. It is likely thatmanipulation of water levels and salinity inthe Coorong will also affect the propensity ofthe system to stratify, but how this wouldoccur is not known at present.

22

Bormans M & Webster IT 1998, ‘Dynamicsof temperature stratification in lowlandrivers’, Journal of Hydraulic Engineering,vol. 124, no.10, pp.1059–1063.

Bourman RP & Harvey N 1983, ‘TheMurray Mouth flood tidal delta’,Australian Geographer, vol. 15,pp. 403–406.

Bourman RP 1986, ‘Aeolian sand transportalong beaches’, Australian Geographer,vol.17, pp. 30–34.

Bourman RP & Barnett EJ 1995, ‘Impactsof river regulation on the terminal lakesand mouth of the Murray River, SouthAustralia’, Australian Geographical Studies,vol. 33, no. 1, pp.101–115.

Bourman RP 2000, ‘Geomorphology of theLower Lakes and Coorong’, in A Jensen,M Good, P Harvey, P Tucker & M Long(eds), River Murray Barrages -Environmental Flows, , Murray-DarlingBasin Commission, Canberra, Australia.

CFMI 1992, Mathematical Modelling of theHydrodynamics and Salinity in theCoorong Lagoons, report CNG-1-12-12/92 prepared for the Engineering andWater Supply Department, SouthAustralia.

CFMI 1998, Long-Term Salinity Trends inthe Coorong Lagoons, report NRC-2-06-98 prepared for the Natural ResourcesCouncil, South Australia.

CFMI 2000, Long-Term Salinity Trends inthe Coorong Lagoons (Part 2), report PIR-1-02/2000 prepared for PrimaryIndustries & Resources, South Australia.

Chappell J 1991, Murray Mouth LittoralDrift Study, report prepared for theEngineering and Water SupplyDepartment, South Australia.

Close A 1990, ‘The impact of man on thenatural flow regime’, in N Mackay & DEastburn (eds), The Murray,Murray-Darling Basin Commission, Canberra,Australia.

Close A 2002, Options for Reducing the Riskof Closure of the River Murray Mouth,Murray-Darling Basin CommissionTechnical Report 2002/2.

Condie SA & Webster IT 2001, ‘Estimatingstratification in shallow water bodiesfrom mean meteorological conditions’,Journal of Hydraulic Engineering, vol.127, no. 4, pp. 286–292.

Geddes MC & Butler AJ 1984,‘Physicochemical and biological studies ofthe Coorong lagoons, South Australiaand the effect of salinity on thedistribution of the macrobenthos’,Transactions of the Royal Society of SouthAustralia, vol. 108, no. 1, pp. 51–62.

Geddes MC 1987, ‘Changes in salinity andin the distribution of macrophytes,macrobenthos, and fish in the Cooronglagoons, South Australia, following aperiod of River Murray flow’, Transactionsof the Royal Society of South Australia, vol.111, no. 4, pp. 173–181.

Geddes M & Hall D 1990, ‘The MurrayMouth and Coorong’, in N Mackay & DEastburn (eds), The Murray, Murray-Darling Basin Commission, Canberra,Australia.

Geddes M 2000, ‘Fish and invertebrates’, inA Jensen, M Good, P Harvey, P Tucker& M Long (eds), River Murray Barrages -Environmental Flows, Murray-DarlingBasin Commission, Canberra, Australia.

Geddes MC 2005, Ecological Outcomes forthe Murray Mouth and Coorong from theManaged Barrage Release of September-

References

WA

TE

R F

OR

A H

EA

LTH

Y C

OU

NT

RY

– C

oo

ron

g a

nd

Mu

rray

Mo

uth

hy

dro

dy

na

mic

s

23

October 2003, report prepared for theDepartment of Water, Land, andBiodiversity Conservation; SouthAustralian Research and DevelopmentInstitute (Aquatic Sciences), Adelaide.SARDI Aquatic Sciences Publication No.RD03/0199-2.

Grant WD & Madsen OS 1979, ‘Combinedwave and current interaction with arough bottom’, Journal of GeophysicalResearch, vol. 84, no. C4,pp. 1797–1808.

Harvey N 1996, ‘The significance of coastalprocesses for management of the RiverMurray Estuary’, Australian GeographicalStudies, vol. 34, no. 1, pp. 45–57.

Holloway PE 1980a, Vertical TemperatureStructure in Shallow Water, PhD thesis,Flinders Institute for Atmospheric andMarine Sciences, The Flinders Universityof South Australia.

Holloway PE 1980b, ‘A criterion for thermalstratification in a wind-mixed system’,Journal of Physical Oceanography, vol. 10,no. 6, pp. 861–869.

Lamontagne S, McEwan K, Webster I, FordP, Leaney F & Walker G 2004, Coorong,Lower Lakes and Murray Mouth.Knowledge Gaps and Knowledge Needs forDelivering Better Ecological Outcomes,Water for a Healthy Country NationalResearch Flagship CSIRO: Canberra.

Nielsen P 1988, ‘Wave setup: A field study’,Journal of Geophysical Research, vol. 93,no. C12, pp. 15 643–15 652.

Noye BJ 1975, The Coorong, Department ofAdult Education, University of Adelaide,Pub. No. 39.

Noye BJ & Walsh PJ 1976, ‘Wind-inducedwater level oscillations in a shallowlagoon’, Australian Journal of Marine andFreshwater Research, vol. 27,pp. 417–430.

Provis DG & Radok R 1979, ‘Sea-leveloscillations along the Australian coast’,Australian Journal of Marine andFreshwater Research, vol. 30,pp. 295–361.

Short AD & Hesp P 1980, CoastalGeomorphology and Hydrodynamics of theSouth East Coast Protection District ofSouth Australia, report prepared for SouthAustralian Coast Protection Board.

Surface Water Archive 2004,<www.dwlbc.sa.gov.au/water/technical/surface_water_archive/a1pgs/>, SADepartment for Water, Land &Biodiversity Conservation

van Rijn LC 1993, Principles of SedimentTransport in Rivers, Estuaries and CoastalSeas, Aqua Publications, Amsterdam.

Walker DJ 1990, ‘The role of river flows inthe behaviour of the Murray Mouth’,South Australian Geographical Journal,vol. 90, pp. 50–65.

Walker DJ & Jessup A 1992, ‘Analysis of thedynamic aspects of the River MurrayMouth, South Australia’, Journal ofCoastal Research, vol. 8, no. 1, pp. 71–76.

Walker DJ 2002, The Behaviour and Futureof the Murray Mouth, Centre for AppliedModelling in Water Engineering, TheUniversity of Adelaide.

WBM Oceanics 2003, Murray River Mouth– Morphological Model Development Stage2 – Model Set Up, Calibration andVerification, report prepared for Murray-Darling Basin Commission & SADepartment for Water, Land &Biodiversity Conservation, Brisbane,Australia.

WA

TE

R F

OR

A H

EA

LT

HY

CO

UN

TR

Y –

Co

oro

ng

an

d M

urra

y M

ou

th h

yd

rod

yn

am

ics