Dynamics of the turbidity maximum zone in a micro-tidal ... · balance between the two can change along the estuary and between neap and spring tides, so that the causal mechanism

Dynamics of the turbidity maximum zone in a micro-tidalestuary: Hawkesbury River, Australia

M. G. HUGHES*, P. T. HARRIS and T. C. T. HUBBLE**Department of Geology and Geophysics (FO5), University of Sydney, Sydney NSW 2006, Australia Antarctic CRC, Australian Geological Survey Organization, University of Tasmania, GPO Box 252C,Hobart TAS 7001, Australia

ABSTRACT

Bed sediment, velocity and turbidity data are presented from a large (145 km long),

generally well-mixed, micro-tidal estuary in south-eastern Australia. The percentage

of mud in the bed sediments reaches a maximum in a relatively narrow zone centred

�30±40 km from the estuary mouth. Regular tidal resuspension of these bed

sediments produces a turbidity maximum (TM) zone in the same location. The

maximum recorded depth-averaged turbidity was 90 FTU and the maximum near-bed

turbidity was 228 FTU. These values correspond to suspended particulate matter

(SPM) concentrations of roughly 86 and 219 mg l)1, respectively.

Neither of the two existing theories that describe the development and location of

the TM zone in the extensively studied meso- and macro-tidal estuaries of northern

Europe (namely, gravitational circulation and tidal asymmetry) provide a complete

explanation for the location of the TM zone in the Hawkesbury River. Two important

factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water

discharge rate and supply of sediment to the estuary head is very low for most of the

time, and (2) suspension concentrations derived from tidal stirring of the bed

sediments are comparatively low. The ®rst factor means that sediment delivery to the

estuary is largely restricted to short-lived, large-magnitude, ¯uvial ¯ood events.

During these events the estuary becomes partially mixed and it is hypothesized that

the resulting gravitational circulation focuses mud deposition at the ¯ood-determined

salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The

second factor means that easily entrained high concentration suspensions (or ¯uid

muds), typical of meso- and macro-tidal estuaries, are absent. Maintenance of the TM

zone during low-¯ow periods is due to an erosion-lag process, together with a local

divergence in tidal velocity residuals, which prevent the TM zone from becoming

diffused along the estuary axis.

INTRODUCTION

A large number of studies over the past 25 yearshave investigated suspended sediment dynamicsin estuaries. Many of these studies report theexistence of a zone toward the head of the estuarywhere the turbidity of the water is markedlyhigher than that observed further landward orseaward. This zone is termed the turbidity max-

imum (TM) zone (see Dyer, 1986; Eisma, 1993). Inrecent times the TM zone has emerged as animportant focus for estuarine research, because itis frequently located near the fresh±salt waterinterface and consists of high concentrations ofsuspended particulate matter (SPM). These con-ditions lead to strong spatial and temporal gradi-ents in geochemical processes, which can play adetermining role in the dynamics and fate of

Sedimentology (1998) 45, 397±410

Ó 1998 International Association of Sedimentologists 397

anthropogenic inputs to estuarine systems (e.g.Morris et al., 1978; Morris et al., 1986; Turneret al., 1994).

Most research on the TM zone has focused onmeso- and macro-tidal estuaries, where tidalcurrents are strong (>1 m s)1) and SPM concen-trations are large (Table 1). In many cases, thesuspensions are well in excess of 10 g l)1, classi-®ed as `¯uid mud' (see Einstein & Krone, 1962).

Two mechanisms have been proposed for thedevelopment of a TM zone in meso- and macro-tidal estuaries. The ®rst involves the residualcurrents associated with gravitational circulationin partially mixed estuaries. In these estuaries thesalinity distribution drives a residual bottom ¯owdirected landward along the estuary axis and aresidual surface ¯ow directed seaward. SPMsupplied to the estuary head travels seaward inthe surface waters until it begins to ¯occulate andsettle deeper into the water column. Then it iscaught in the landward ¯owing bottom currentand transported back towards the estuary head. Anull point exists where the velocity of thelandward ¯owing estuarine bottom water equalsthe seaward ¯owing river water. This null pointtypically occurs near the landward limit of thesalt intrusion, and it is here that ®ne sedimentaccumulates and undergoes tidal re-suspensionto produce a TM zone (Dyer, 1994).

The second mechanism proposed for the de-velopment of a TM zone in meso- and macro-tidalestuaries involves the distortion of the tide waveassociated with non-linear interactions betweenthe tide and channel morphology. Distortioncauses ¯ood currents to be stronger and of shorterduration than ebb currents. This asymmetryincreases towards the head of the estuary and a

net transport of sediment in a landward directionresults. A null point exists where the seawarddirected river ¯ow has a transport competencyequal to that of the ¯ooding tide. It is at this nullpoint, located somewhere landward of the pointwhere the tide becomes signi®cantly distorted,that ®ne sediment accumulates to produce a TMzone (Dyer, 1986).

In general, gravitational circulation is mostimportant in meso-tidal estuaries, whereas tidalasymmetry is of primary importance in macro-tidal estuaries. It is noteworthy, however, that inmany of the meso- and macro-tidal estuariesstudied to date, both residual circulation andtidal asymmetry are coincident phenomenon. Thebalance between the two can change along theestuary and between neap and spring tides, sothat the causal mechanism for the focused accu-mulation of ®ne sediment in a TM zone is notalways straightforward (Dyer, 1986). In bothcases, however, tidal resuspension of ®ne sedi-ment in the zone of accumulation leads tomaximum turbidities. In this regard, the presenceof readily entrainable slackwater ¯uid muds,which are characteristic of these meso- andmacro-tidal estuaries (Kirby, 1988), clearly con-tributes to the existence of a TM zone.

Investigations of TM zone suspended sedimentdynamics in micro-tidal estuaries are signi®cant-ly fewer in number than for meso- and macro-tidal estuaries. Of the few published studies, allreport SPM concentrations in the TM zone of lessthan 1000 mg l)1 (Table 1). In particular, theorigin, persistence and dynamics of the TM zonein micro-tidal estuaries is not well established.Gravitational circulation and tidal asymmetry dooccur in these estuaries, but it is not known if the

Table 1. Some previously reported values for SPM concentration in the TM zone.

EstuarySpring tiderange

SPM concentrationTM zone* Reference

Meso- Macro-tidalWeser River 3á8 m 1á5 g l)1 Grabemann & Krause (1989)Fly River 4á0 m 5±30 g l)1 Wolanski et al. (1995)Tamar River 4á5 m 10 g l)1, 26 g l)1 Uncles & Stephens (1993)Gironde River 5á0 m 10 g l)1 Allen & Castaing (1973)Severn River 12á3 m 20 g l)1 Kirby (1988)

Micro-tidalJames River 0á7 m 100±270 mg l)1 Nichols (1993)Varde A Estuary 1á6 m 100±1000 mg l)1 Bartholdy (1984)Cooper River 2á0 m 40±100 mg l)1 Althausen & Kjerfve (1992)

* These ®gures are near bed values, except in the case of the Tamar River, where the ®rst is the depth-averaged valueand the second is the near bed value.

398 M. G. Hughes et al.

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 397±410

resulting sediment transport is suf®cient to leadto continuous deposition of ®ne sediments withina narrow zone in the estuary. Moreover, the lackof regularly occurring, high-concentration, slack-water suspensions (or ¯uid muds) in theseestuaries also means that the mechanism androle that tidal resuspension plays will be some-what different to that observed in meso- andmacro-tidal estuaries. The purpose of this paperis to increase our knowledge of sediment dynam-ics in the TM zone of micro-tidal estuaries, bypresenting the results from a study of the micro-tidal Hawkesbury River estuary.

THE HAWKESBURY RIVER

The Hawkesbury River drains into the TasmanSea on the eastern coast of New South Wales,Australia (Fig. 1). The total catchment area isabout 23 000 km2. Fresh water discharge into theestuary is modest. Analysis of 32 years of recordsfrom Penrith weir (provided by NSW Departmentof Land and Water Conservation), located 20 kmupstream of the tidal limit, indicates that themean fresh water discharge into the estuary is46á48 m3 s)1 whereas the median is only2á85 m3 s)1 (Table 2). The fact that the medianis so much smaller than the mean indicates that¯uvial ¯ow into the estuary is highly skewedtowards small discharges. The general pattern offresh water discharge is one of extended low-¯owconditions punctuated by aseasonal, short-lived,large-magnitude ¯oods. Floods often have dis-charge rates up to 3 orders of magnitude largerthan the mean.

Tides at the coast are mixed semi-diurnal witha mean spring range of 1á32 m and a mean neaprange of 0á78 m. The maximum tidal range is1á92 m. The tide-affected part of the HawkesburyRiver is �145 km long, with the tidal limitlocated at the Grose River con¯uence (Fig. 1).This section can be divided into two reaches: (a) a¯uvio-tidal reach, where the water is fresh andthe ¯ow is mostly tidal, except during large ¯oodsand (b) an estuarine reach, where the water isbrackish to saline and the ¯ow is always tidal,although it is modi®ed considerably during large¯oods. The estuarine reach occupies from c. 0±75 km from the estuary mouth and the ¯uvio-tidalreach occupies 75±145 km from the estuarymouth.

The estuarine reach occupies a drowned rivergorge carved into Triassic Hawkesbury sandstone(Roy et al., 1980). The channel is ¯anked locallyby steep bedrock valley walls, talus slopes or lat-erally restricted back-plain swamps with fringing

Fig. 1. Map of the HawkesburyRiver showing locations of perma-nent tide gauges and instrumentdeployment sites. Tide Gauges 1±6refer, respectively, to the followingsite names: Spencer, Gunderman,Webbs Creek, Sackville, Ebenezerand Windsor.

Table 2. Summary statistics of fresh water dischargemeasured on the Nepean River at Penrith weir duringthe period 1960±1992 (raw data provided by NSW De-partment of Land and Water Conservation).

PercentileDischarge(m3 s)1)

10 0á6750 2á8590 79á53

Mean 46á48Maximum 11 267á19

Dynamics of turbidity maximum zone in a microtidal estuary 399


mangrove stands. Channel morphology includestidal levees, inter-tidal and shallow sub-tidal barsand point bars attached to channel margins andlimited inter-tidal and shallow sub-tidal mud¯ats located mid-channel (Hubble & Harris, 1994).The ¯uvio-tidal reach consists of a channel that ismoderately incised into bedrock or Tertiary sed-iment and is abutted by levee banks and ¯oodplains of variable width (Hubble & Harris, 1994).Channel morphology mostly appears to be of a¯uvial nature, shaped by the infrequent ¯oodevents rather than by the tide (Hubble & Harris,1994; Hughes & Callaghan, 1995).

The channel width decreases at an exponentialrate, from roughly 3500 m at the entrance toBroken Bay to 150 m at the tidal limit (Fig. 1). At35 km inland the channel width has decreased to� 10% of its width at the entrance and at 75 kminland it has decreased to �5%. Beyond 75 kmthe channel width remains nearly constant at100±200 m. The thalweg depth displays a steady,gradual decrease from 15 to 20 m in Broken Bayto roughly 10 m at 100 km inland. Beyond100 km it decreases more quickly to < 2 m depthat the tidal limit (Hughes & Callaghan, 1995).

FIELD AND ANALYTICAL METHODS

Field data reported here was collected betweenMay 1992 and June 1994. Throughout this periodthe fresh water discharge was indicative ofextended, low-¯ow conditions.

Bed sediment

Bed sediments were sampled from channel cross-sections spaced at intervals of �5 km along theestuary. A total of ®ve samples were collectedfrom each cross-section: three samples from thesub-tidal channel and one from the inter-tidalzone on each bank. The samples were wet sieved,dried and weighed to determine the percentage ofmud (<63 lm), sand (63 lm to 2 mm) and gravel(>2 mm). The equivalent size distribution of thesand and mud fractions were determined using asettling column and laser diffraction particle sizer(Hubble & Harris, 1994).

Water level

Water level data from seven permanent tidegauges on the Hawkesbury River were providedby the NSW Public Works Department. Thegauges are located at 28, 49, 61, 97, 112 and

126 km from the estuary mouth (Fig. 1). A tidegauge in the entrance to Sydney Harbour, 30 kmto the south of the Hawkesbury River, was used torepresent the tide at the coast. The records arecontinuous time series with a 15 minute samplinginterval. Twenty-nine days of record were select-ed from each location for harmonic analysis usingthe method of Foreman (1977). The amplitudeand phase of 33 tidal constituents were resolved.

Salinity, turbidity and current velocity

Longitudinal pro®les of salinity and turbiditywere measured on two occasions (26 March 1993and 7 April 1993) by vertical sampling of thewater column at 3 km intervals along the estuaryas near as possible to the time of local high waterslack. The water samples were collected with aNiskin Bottle from the water surface, mid depthand 1 m above the bed. Salinity, temperature andturbidity of the samples were determined imme-diately on deck using a WTW LF196 temperature/conductivity meter and a Hach 2100P turbiditymeter.

Long-term records of the temperature, salinity,current speed and turbidity were obtained usingAanderaa RCM7 current meters ®tted with Sea-tech transmissometers. The instruments weredeployed in taught-line moorings with the sen-sors situated 1 m above the bed. Data was loggedat 10 min intervals and the instruments wereserviced fortnightly. This was necessary to avoidproblematic soiling of the transmissometer win-dows.

A total of 19 sites were occupied to providerepresentative coverage of the longitudinal andcross-channel dimensions of the estuary. Someareas of the estuary are, however, under-repre-sented due to the inability to moor instruments inestablished trawling grounds. All sites wereoccupied for at least one spring±neap tide cycle,with most sites occupied for up to eight cycles.Because it is the longitudinal estuary dimensionthat is of interest here, the data from only eight ofthe deployment sites are presented. At theseparticular sites the instruments were moored inthe channel thalweg. Site locations are shown inFig. 1 and time periods chosen for analysis arelisted in Table 3. Although the time periodsanalysed do not correspond between all sites,the pattern of velocity and turbidity at all sites ishighly repetitive in time from one spring±neaptide cycle to the next (Hughes & Callaghan, 1992,1993, 1994). Hence, it is reasonable to comparedata from different time periods provided the



comparison is based on a record length incorpo-rating at least one spring±neap tide cycle.

To permit comparison with previous studies,the transmissometers were calibrated for turbidityusing 10 formazin calibration standards rangingbetween 2 and 200 formazin turbidity units(FTU). The instrument response was linear overthe calibration range and was found to be consis-tent between calibrations. Least squares regres-sion equations were ®tted to the calibration datato determine transform equations relating turbid-ity to light transmission. These transform equa-tions all had r2 values of 0á99 or greater.

It was not feasible to undertake an in situcalibration of the transmissometers for SPMconcentration, because of the long deploymentperiods. For this reason only turbidity is reportedhere. It is worth noting, however, that the generalrepeatability of the turbidity records from onespring±neap cycle to the next at all sites stronglysuggests that the relationship between turbidityand SPM concentration is reasonably consistentthrough the estuary and through time (Hughes &Callaghan, 1992, 1993, 1994). To enable anapproximate comparison to be made betweenthe data reported here and other studies theSPM concentration (mg l)1) is equal to roughly0á96 times turbidity (FTU). This is based onlaboratory calibrations of the transmissometerswith bed sediment collected adjacent to theinstrument deployment sites (Hughes & Callag-han, 1995).

RESULTS

Distribution of bed sediments

The beaches lining much of the foreshore ofBroken Bay are composed of clean marine sands,

as is the large subaqueous ¯ood tide delta thatoccupies the ¯oor of Broken Bay and extendslandward into the estuary (Roy et al., 1980).Between Broken Bay and 14 km from the estuarymouth the intertidal zone consists almost entirelyof bedrock valley walls, thus sediment was onlysampled from cross-sections landward of thispoint. Sub-tidal mud is typically dark olive greyand organic rich (Roy, 1983) with a mean grainsize ranging from 7á2±13á9 lm (medium silt) atthe current meter sites based on laser particle sizeanalyses. The sand fraction was composed ofangular to moderately rounded, quartzose, medi-um-well-sorted, ®ne to coarse sand, with a meangrain size ranging from 0á2±0á6 mm based onsettling column analyses.

Most of the inter-tidal mud deposited in theestuary occurs in a zone located 14±60 km fromthe estuary mouth (Fig. 2a). The proportion ofmud in samples collected from this zone isgenerally greater than 50%, and reaches a maxi-mum of greater than 95% in a narrower zonelocated 36±50 km from the estuary mouth. In the¯uvio-tidal reach, 75±145 km from the estuarymouth, the mud content of the inter-tidal zone isgenerally less than 30%, although some isolatedcases reach 40±95% (Fig. 2a).

Sub-tidal mud content exceeds 40% in theseaward section of the estuary (Fig. 2b) and

Fig. 2. Cross-sectionally averaged composition of (a)inter-tidal and (b) sub-tidal sediments as a function ofdistance from the estuary mouth.

Table 3. List of instrument deployment sites and thedeployment period reported here.

SiteNumber

Distance fromestuary mouth(km)

Deploymentperiod

HR3 58á8 03/06/92±18/06/92HR5 55á0 03/06/92±18/06/92HR6 51á5 07/05/92±21/05/92HR8 49á5 02/12/92±17/12/92HR9 42á0 02/12/92±17/12/92HR10 39á0 02/12/92±17/12/92HR11 34á5 02/12/92±17/12/92HR19 21á5 24/05/94±09/06/94



reaches a maximum of 80% at 32 km from theestuary mouth. In the ¯uvio-tidal reach the mudcontent of the sub-tidal zone is generally less than10%, although some isolated cases reach 40%.

The presence of gravel size material in thesamples was rare (Fig. 2a and 2b). Where itoccurred, it was composed entirely of shelldetritus in the estuarine reach of the river andentirely of lithic material further landward.

Tidal water levels and velocities

The water level records indicate that a change intide behaviour occurs at c. 50 km from the estuarymouth. Ampli®cation of the tide range in thedirection of tide propagation occurs seaward ofthis point and damping occurs landward(Fig. 3a). The maximum spring tide range increas-es from 1á92 m at the estuary mouth to a maxi-mum of 2á10 m at Gunderman (49 km from theestuary mouth), and then decreases to zero at thetidal limit (145 km from the estuary mouth).Hughes (1992) showed that this behaviour isconsistent with that expected for a damped,linear progressive wave propagating through achannel of exponentially decreasing cross-sec-tional area. He also found, however, that friction-

al effects landward of 60 km from the estuarymouth caused non-linear tide behaviour to de-velop. The resulting tidal asymmetry is manifestby a weaker, longer-duration ebb and a stronger,shorter-duration ¯ood (Fig. 3a).

The tidal ampli®cation and damping observedat the seaward and landward ends of the estuarylargely results from the shoaling and frictionaldamping of the diurnal and semi-diurnal tideconstituents, particularly K1 and M2 (Fig. 3b and3c). The importance of friction and other non-linear effects on the tide as it propagates beyond50 km from the estuary mouth is demonstrated bythe strong growth in the quarter-diurnal, shallowwater tide constituents (Fig. 3d). The values oftwo parameters commonly used to characterizenon-linear tide behaviour (see Friedrichs & Au-brey, 1988) are listed in Table 4. The relativephases of M2 and M4, 2GM2±GM4, measured atSpencer and Gunderman are between 180° and360°. This indicates that the seaward reach of theestuary has tidal asymmetry favouring shorter ebbdurations, however, the small values for theamplitude ratio of M2 and M4, aM4/aM2, indicatesthat the degree of this tidal asymmetry is small.Landward of 50 km from the estuary mouth thedegree of tidal distortion grows steadily due to

Fig. 3. (a) Spring tide range, and¯ood and ebb duration as a functionof distance from the estuary mouth.Amplitude of the important (b) di-urnal (c) semi-diurnal and (d) quar-ter-diurnal tidal constituents as afunction of distance from the estu-ary mouth.



the combined effect of a growing aM4 and adampening aM2 (Table 4; Fig. 3c and 3d). More-over, as the values of 2GM2±GM4 lie between 0°and 180° the sense of this distortion switches tofavour shorter ¯ood duration.

Summary statistics of the peak tidal velocitiesare consistent with the tide behaviour inferredfrom the water level data (Table 5). In the ®rst50 km from the estuary mouth ebb velocities aremarginally stronger and of shorter duration than¯ood velocities, whereas further landward ¯ood

velocities are signi®cantly stronger and of shorterduration than ebb velocities.

Salinity

The longitudinal salinity pro®les surveyed onceduring neaps and once during springs are shownin Fig. 4. The total fresh water discharge rate intothe estuary from all major tributaries amounted to6á4 m3 s)1 and 3á4 m3 s)1 for the neap and springsurvey, respectively; thus the data is indicative of

Fig. 4. Longitudinal salinity pro-®le during (a) neaps and (b)springs. Results from the surface,mid depth and 1 m above the bedare indicated by a dashed, thinand thick line, respectively.

Table 4. Indicators of non-linear tide behaviour.

StationName

Distance fromestuary mouth(km) aM2/h aM4/aM2 2GM2±GM4(deg)

Spencer 28 0á033 0á0072 304Gunderman 49 0á039 0á0045 341Webbs Creek 61 0á040 0á0137 48Sackville 97 0á039 0á0494 65Ebenezer 112 0á037 0á0916 66Windsor 126 0á053 0á1156 68

Table 5. Summary statistics for peak tidal velocities measured over a spring±neap tide cycle.

Distance from

estuary mouthPeak ¯ood velocity (m s)1) Peak ebb velocity (m s)1)

Site Number (km) Mean Max Min Mean Max Min.

Sackville* 97á0 0á67 0á79 0á55 0á30 0á42 0á13Webbs Cr* 64á0 0á55 0á70 0á39 0á28 0á42 0á12HR3 58á8 0á48 0á60 0á40 0á34 0á56 0á16HR5 55á0 0á39 0á55 0á35 0á43 0á68 0á25HR6 51á5 0á39 0á52 0á34 0á22 0á42 0á11HR8 49á5 0á36 0á43 0á29 0á42 0á50 0á34HR9 42á0 0á25 0á31 0á19 0á48 0á68 0á27HR10 39á0 0á35 0á41 0á23 0á44 0á62 0á25HR11 34á5 0á40 0á51 0á26 0á41 0á58 0á19HR19 21á5 0á51 0á75 0á33 0á37 0á53 0á23

*Unpublished data provided by NSW Public Works Department's Manly Hydraulics Laboratory.



low fresh water ¯ow conditions. On both occa-sions the water column was well mixed withdifferences in salinity between surface and bot-tom waters typically less than 1 p.p.t. The limit ofsalt intrusion into the estuary was 73 and 75 kmfrom the estuary mouth for neaps and springs,respectively.

Intra-tidal variation in salinity at a ®xed pointin an estuary arises largely from advection of thelongitudinal salinity pro®le by the ¯ood and ebbof the tide. Maximum values of the semi-diurnalsalinity range recorded during the instrumentdeployments is shown in Fig. 5. The maximumrange observed was 10 p.p.t. at Site HR10 (39 kmfrom the estuary mouth).

Longitudinal turbidity pro®le

The depth-averaged longitudinal turbidity pro®lewas surveyed once during neaps and once duringsprings (Fig. 6). During springs a clear maximumin the depth-averaged turbidity, reaching 90 FTU,was measured at a distance of 33 km from theestuary mouth. The near bed turbidity at the samelocation was 228 FTU. During neaps the turbiditypro®le was ¯at and featureless (Fig. 6).

The turbidity time series at each long-terminstrument deployment site displayed a highdegree of autocorrelation, relating to tidal period-icity, thus the following procedure was employedto obtain a satisfactory measure of the mean andstandard error of turbidity: a 24 h 50 min runningmean was calculated for the time series from eachdeployment site, which was then sub-sampled atnoon of each day to provide a daily averagedturbidity. The means and standard errors of thedaily averaged turbidities obtained from theinstrument deployment sites are consistent withthe results of the longitudinal surveys (Fig. 6).

The daily averaged turbidity at Sites HR11 andHR10 (34á5 and 39 km from the estuary mouth) isstatistically signi®cantly higher than sites locatedeither seaward or landward (Fig. 6). This indi-cates that the TM zone identi®ed in the one-offlongitudinal surveys is a persistent feature in thispart of the estuary.

Turbidity±velocity patterns

Three-day records of turbidity and velocity dur-ing springs for each of the instrument deploymentsites are shown in Fig. 7. During the ¯ooding tidebeginning late 4 June and continuing into 5 June,the turbidity at Site HR3 increased from abackground value of 5 FTU to reach a maximumof 15 FTU (Fig. 7a). The peak turbidity occurredwhen the velocity reached its maximum and anelevated turbidity was sustained through theremainder of the ¯ood. Shortly before high waterslack the turbidity decreased, but only to 10 FTU.The largest ebb of the day followed the largest¯ood and produced a narrow peak in turbiditythat occurred at the time of maximum ebbvelocity. Unlike the ¯ood, the turbidity levelduring the ebb was not sustained and fell rapidlyto the background value at low water slack(Fig. 7a). The second ¯ood of the day wasmarginally smaller than the ®rst. The peakturbidity again reached 15 FTU but, in contrastto the ®rst ¯ood, this value was not sustained and

Fig. 6. Depth-averaged longitudinal turbidity pro®lemeasured during springs (solid line) and neaps (dashedline). Squares with error bars show means and standarderrors of the daily averaged turbidities measured at theinstrument deployment sites. The turbidity time seriesat each site displayed a high degree of autocorrelation,relating to tidal periodicity; thus the following proce-dure was employed to obtain a satisfactory measure ofthe mean and standard error: a 24 h 50 min runningmean was calculated for the time series from each de-ployment site, which was then sub-sampled at noon ofeach day to provide a daily averaged turbidity.

Fig. 5. Semi-diurnal salinity range during springs as afunction of distance from the estuary mouth.



fell rapidly to the background value at high waterslack. The following ebb was the smallest of theday and was not capable of entraining bedsediment to an elevation of 1 m above the bed.This pattern, of strong ¯ood currents associatedwith high and sustained turbidity levels but weakebb currents associated with low turbidity levels,is repeated on successive tides (Fig. 7a).

The turbidity±velocity pattern at Site HR5 isalmost the same as that described above for SiteHR3, but in this case there was elevated turbidityduring the second ebb of the day (Fig. 7b). Notethat the ebb velocities at Site HR5 are greater thanthose at Site HR3 (Fig. 7b).

Turbidity±velocity patterns at Site HR6(Fig. 7c) differ from Site HR3 in two ways: the

®rst is that the turbidity level at this site isrelatively high (exceeding 50 FTU), probablycaused by the larger amount of ®ne grainedsediment available for suspension in this sectionof the estuary. The second difference is that thepeak turbidity relating to the largest ebb of the daywas roughly equal to or greater than the turbidityassociated with the largest ¯ood of the day.Notice, however, that high turbidities were notsustained during the ebb to the same degree thatthey were during the ¯ood.

The turbidity±velocity pattern at Site HR8 isvery similar to Site HR5 (Fig. 7d), except that theturbidity level at this site was generally higherand there was a clear peak in turbidity with every¯ood and ebb tide. Note that for comparison

Fig. 7. Simultaneous time series ofvelocity (thick line) and turbidity(thin line) measured during springsat Sites (a) HR3 (b) HR5 (c) HR6 (d)HR8 (e) HR9 (f) HR10 (g) HR11 and(h) HR19. Positive velocities are inthe ¯ood direction and negative ve-locities are in the ebb direction.



purposes the largest ¯ood velocities occur on themorning of 12 December at Sites HR8 to HR11(Fig. 7d,e,f and g).

At Site HR9 the turbidity during the largest¯ood of the day increased from the backgroundvalue gradually with increasing velocity, reachinga peak at the time of maximum velocity (Fig. 7e).The turbidity level was sustained during theremainder of the ¯ood, with only a small decreasein turbidity at high water slack. In contrast,during the ebb the turbidity quickly reached amaximum together with the velocity, then for theremainder of the ebb the turbidity decreasedsteadily to the background value at low waterslack (Fig. 7e). This pattern was repeated nearlyexactly every semi-diurnal tide cycle (thus therewas little diurnal variation at this site).

The pattern in turbidity±velocity at Sites HR10and HR11 is very similar to Site HR6 (Fig. 7f and7 g) except that turbidity levels during the largestebb were not equivalent to those on the largest¯ood. At Site HR19 (Fig. 7h) turbidity increasedrapidly on the ¯ood, reaching a peak before themaximum velocity was attained, then it de-creased steadily to the background value at highwater slack. In contrast, during the ebb theturbidity increased slowly from the backgroundvalue to reach its peak after the velocity reachedits maximum, then the turbidity dropped to thebackground value again at low water slack. Thispattern is almost the reverse of that described forSite HR9 (Fig. 7e; see above).

DISCUSSION

A TM zone exists in the micro-tidal HawkesburyRiver estuary and it is persistently located be-tween 30 and 40 km from the estuary mouth. TheTM coincides with an inter-tidal and sub-tidalmud deposition zone, located between 20 and50 km from the estuary mouth, which has expe-rienced signi®cant shoaling (i.e. net mud accu-mulation) in the past 60 years (Gardiner, 1993).Two points of discussion arise from these obser-vations. The ®rst concerns the processes respon-sible for focusing mud accumulation and creatinga TM zone (i.e. what is the origin of the TMzone?). Secondly, what are the tidal resuspensionprocesses responsible for maintenance of the TMzone over time?

Origin of the TM zone

The existing models for meso- and macro-tidalestuaries predict that the focus for mud accumu-

lation and the location of the TM zone should besome tens of kilometres further landward ofwhere it is actually found in the HawkesburyRiver estuary. The gravitational circulation modelwould place the TM zone at a null point locatednear the limit of salt intrusion and for most of thetime this occurs between 60 and 80 km from theestuary mouth. The tidal distortion model wouldplace the TM zone at a null point locatedsomewhere landward of the point where non-linear tide behaviour and velocity asymmetry ®rstbecome apparent; this would be somewherelandward of 50 km from the estuary mouth.

It is interesting to note that the location of theTM zone coincides with the largest semi-diurnalsalinity range, which is consistent with observa-tions reported by Althausen & Kjerfve (1992) inthe micro-tidal Cooper River estuary. Theseauthors proposed that ¯occulation/de¯occulationprocesses, in association with the large salinityrange, might be important in producing a TMzone. This explanation cannot be applied to theHawkesbury River, however, since salt-induced¯occulation is complete at 8 ppt or less (Eisma,1993) and the salinity in the TM zone is 15±25p.p.t. What process, then, has given rise to theHawkesbury TM zone?

While it seems clear that the location of the TMzone is linked closely to the focus of net mudaccumulation through tidal resuspension pro-cesses, the mechanism responsible for focusingmud accumulation in the ®rst place is stilluncertain. Probably no process acts to focus netmud accumulation during periods of extendedlow ¯uvial ¯ow conditions. During these periodsthe estuary is well mixed so that vertical gravita-tional circulation will be either weak or com-pletely absent. Moreover, the supply of sedimentto the head of the estuary during these periods isalso very low; values of SPM concentration in the¯uvio-tidal reach are typically less than 5 mg l)1

(Hooper & Humphreys, 1993). It is thereforeproposed that the mechanism for focusing mudaccumulation in the TM zone operates duringinfrequent, moderate to large ¯uvial dischargeevents.

During such events the estuary becomes par-tially strati®ed, gravitational circulation is welldeveloped and the saline intrusion limit ispushed tens of kilometres seaward from its usuallocation (Fig. 1; Wolanski & Collis, 1976; Kjerfveet al., 1992; Hughes & Callaghan, 1995). More-over, substantial amounts of ®ne suspendedsediment are delivered to the head of the estuaryduring these events. Williams et al. (1993) report



a suspended sediment load of 210 000 t deliveredto the estuary during a single ¯ood in August1991. No ®gures are available for the annual loadof suspended sediment delivered to the estuary,but the typical situation of extended low ¯owconditions together with very low SPM concen-trations in the river suggests that signi®cantsediment supply to the estuary is restricted tolow-frequency, moderate to large-magnitude, dis-charge events.

We hypothesize that it is the gravitationalcirculation established in the estuary duringmoderate to large discharge events that serves tofocus mud accumulation at the TM zone. Thefresh water discharge rate required to set up agravitational circulation suitable for focusingmud accumulation at c. 35±40 km from theestuary mouth is unknown, but Wolanski & Collis(1976) present data that shows the near bed saltintrusion limit was pushed seaward to a position38 km from the estuary mouth when the fresh wa-ter discharge into the estuary reached 73 m3 s)1.Although this discharge rate is not substantiallygreater than the mean, for nearly 90% of the timeriver discharge is signi®cantly less than this value(Table 2). In this context therefore it represents asigni®cant perturbation from the norm.

Given that the hypothesized mechanism forfocusing mud accumulation in the HawkesburyRiver estuary is inoperative for most of the timeand the fresh water discharge into the estuary isgenerally episodic (aseasonal), there is not ex-pected to be any seasonal variability in the TMzone like that described for the Tamar Riverestuary (Uncles et al., 1994). Indeed, it appearsthat in the Hawkesbury River the position of theTM zone is remarkably stable, migrating no morethan a semi-diurnal tidal excursion length fromthe zone of maximum mud content in the bedsediments. The question thus arises ± whatprevents the mud being dispersed from the¯ood-determined focus of accumulation by tidalprocesses operating during the extensive inter-vening (low-¯ow) periods?

Maintenance of the TM zone

Although patterns in the turbidity time series arehighly repeatable through several tide cycles,there is no direct and consistent relationshipbetween the magnitudes of turbidity and watervelocity (Fig. 7). The reason for this must be duein part to the fact that the measured turbidity at asite represents both entrainment of local bedsediment and advection of sediment already in

suspension. Entrainment is complicated by anumber of factors peculiar to cohesive sediments.Mehta et al. (1989) describe three modes ofcohesive sediment entrainment: surface erosion,mass erosion and resuspension of a stationarysuspension (or ¯uid mud). Given the fact thatSPM concentrations in the Hawkesbury Rivernever reach values suf®cient to develop ¯uidmuds, the latter mechanism cannot be important.In the cases of surface erosion and mass erosionthe threshold of entrainment for the sediment isstrongly dependent on the character of the bed asa whole, in particular its water content andcompaction (Mehta et al., 1986). Mud depositsde-water and compact with time until the excessshear stress exerted by the current is suf®cient tore-initiate erosion.

The more àged' (de-watered and compacted)the bed is the more sustained the current shearneeds to be before erosion and resuspension ofthe sediment is initiated. The unsteady nature oftidal currents and the variable resistance of àged'beds results in phenomena known as thresholdand erosion lags (Dyer, 1994), which serve toproduce a phase shift between turbidity andvelocity that will vary between sites in an estuaryand with diurnal and spring±neap variations intidal energy. The fact that virtually no sediment issuspended in the estuary during neaps (Fig. 6)suggests that the bed of the estuary becomesàged' to some degree, at least fortnightly. Fur-thermore, àgeing' of the bed probably takes placealso on a semi-diurnal time scale, since low tidalcurrent speeds mostly correspond with low tur-bidity levels (Fig. 7).

This àgeing' of the bed effects tidal resuspen-sion of sediment in the TM zone during springs.Here, sediment is mainly eroded on the ¯ood tide,when peak velocities are sustained for a period oftime, whereas minimal resuspension occurs dur-ing the relatively short-duration peak ebb ¯ows(Fig. 7f and g). This is despite the fact that peakebb velocities are larger than peak ¯ood velocitiesat these sites (Table 4). This may be considered asa type of erosion lag process (terminology of Dyer,1994). An important consequence of this is thatdownstream and within the TM zone the netsuspended sediment transport vector is directedlandward (see below).

The times of maximum turbidity and maximumvelocity on the ¯ooding tide coincide at the TMzone sites (Fig. 7f and g), suggesting that thesource of the turbidity is local resuspension.Further landward, however, there is a phase lagbetween maximum turbidity and velocity that



increases with distance along the channel, sug-gesting that suspended sediment is advectedlandward from the TM zone (Fig. 7d and e). Notethat these sites are within one tidal excursionlength of the TM zone (see below). This explana-tion implies that most of the SPM (turbidity)observed at sites outside the TM zone representsadvection from the TM zone rather than tidalresuspension of local bed sediment. A shortdistance upstream of the TM zone (c. 42 km fromthe estuary mouth) the peak in turbidity coincideswith the last portion of the ¯ood tide duringsprings (Fig. 7e). Turbidity remains relativelyhigh during high water slack and a secondturbidity peak coincides with the ®rst portion ofthe ebb (Fig. 7e). This site is also where peak ebb

velocities are the greatest and peak ¯ood veloci-ties are the weakest (Table 5). The end result ofthis turbidity±velocity pattern is that the net SPMtransport vector at this site is directed seaward,creating a barrier to effective transport andlandward dispersion of the TM zone.

The combined effect of the erosion lag processand local divergence in tidal current velocityresiduals is summarized in Fig. 8. The followingare shown in order from top to bottom: (1) netdirection of suspended sediment transport (2)tidal excursion length (3) daily averaged turbidity(4) percentage of mud in bottom sediments (5)maximum ¯ood directed tidal current velocityand (6) maximum ebb directed tidal currentvelocity. (1) was calculated from the product ofthe velocity and turbidity records integrated overthe duration of a spring±neap tide cycle and (2)was determined from the semi-diurnal salinityrange and local gradient of the longitudinalsalinity pro®le. (3) to (6) represent data alreadyshown in previous ®gures and tables. The localityof the TM zone is maintained by net landwardtransport of suspended sediment in the lowerestuary (an erosion lag mechanism) coupled withnet seaward transport in the middle estuarywhere there is a local divergence in tidal velocityresiduals. Together, these processes serve to trapthe TM zone to well within one tidal excursionlength of the ¯uvial ¯ood determined point ofmaximum mud accumulation in bottom sedi-ments.

SUMMARY

The origin of the TM zone in the micro-tidalHawkesbury River estuary cannot be explainedpurely by either of the two available theories:gravitational circulation and tidal asymmetry.The predicted location of the TM zone, based onthese two theories, is some tens of kilometreslandward of the actual location for probably 90%of the time. It is hypothesized here that thelocation of the TM zone is controlled largely byepisodic ¯uvial ¯ood events, which shift the nearbed salt intrusion limit to about 35 km seaward ofits normal (low ¯uvial ¯ow) position and causegravitational circulation within the estuary. Mudis deposited at this location during the ¯uvial¯ood and at other times it is resuspended by tidalprocesses. The data presented here shows thatduring the long intervening periods between¯uvial ¯ood events the location of the TM zoneis trapped at its ¯ood-determined position. This

Fig. 8. Summary diagram showing daily averaged tur-bidity, cross-sectionally averaged percentage mud inbottom sediments, maximum ¯ood tidal current speedand maximum ebb tidal current speed, relative to dis-tance from the estuary mouth. The approximate loca-tion of the TM zone, tidal excursion length and netdirection of suspended sediment transport at each in-strument deployment site are indicated. The tidal ex-cursion length was estimated from the semi-diurnalsalinity range and local gradient of the longitudinalsalinity pro®le. The net sediment transport directionswere derived from the product of the turbidity andvelocity time series integrated over the duration of aspring±neap tide cycle.



appears to be caused by the coexistence of anerosion lag process, which favours landwardtransport of SPM, and a local divergence in tidalvelocity residuals, which favours seaward trans-port of SPM. This trapping mechanism preventsmud from being transported to the modal nullpoint location (which is related to low rather thanaverage ¯uvial ¯ow conditions in this case),where we might normally expect the TM zoneto be located. It appears that the TM zone in thismicro-tidal estuary is not nearly as mobile as itsmeso- and macro-tidal counterparts.

There remain several important issues thatrequire further investigation. The mechanics ofthe hypothesized gravitational circulation duringmoderate±large discharge events needs to beestablished by ®eld observation and the modalposition of the ¯ood-determined salt intrusionlimit (null point) needs to be veri®ed. Further-more, the relative amounts of SPM derived fromlocal resuspension and advection need to bedetermined for sites located in the TM zone (e.g.Sites HR10 and HR11) and for sites locatedimmediately landward (e.g. Site HR9), in order toestablish the importance of erosion lag (àging' ofthe bed) as a mechanism for trapping the TM zone.

ACKNOWLEDGMENTS

Financial support and permission to publish datawas granted by Water Resources Planning Branch,Sydney Water Board through their Hawkesbury-Nepean River Sediment Dynamics Study. DavidBrown (Manly Hydraulics Laboratory) providedthe tidal velocity data from Sackville and WebbsCreek and Kerryn Stephens and Simon Williamsprovided the river discharge data from Penrithweir. Special thanks to John Watkins and Ro-chelle Callaghan who assisted during all of thelong hours spent in the ®eld ± their companion-ship and good humour contributed greatly to thesuccessful completion of the ®eld programme.

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Manuscript received 22 July 1996; revision accepted 29July 1997.



Documents

Dynamics of the turbidity maximum zone in a micro-tidal ... · balance between the two can change along the estuary and between neap and spring tides, so that the causal mechanism