Sediment transport through a tidal creek

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Estuarine, Coastal and Shelf Science 109 (2012) 116e132

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Estuarine, Coastal and Shelf Science

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Sediment transport through a tidal creek

Malcolm O. Green*, Nicole J. HancockNIWA (National Institute of Water and Atmospheric Research), P.O. Box 11-115, Silverdale Road, Hamilton, New Zealand

a r t i c l e i n f o

Article history:Received 1 February 2012Accepted 23 May 2012Available online 4 June 2012

Keywords:sediment transportestuarytidal creekwavesNew Zealand

* Corresponding author.E-mail address: [email protected] (M.O. Green)

0272-7714/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.ecss.2012.05.030

a b s t r a c t

A 3-month field experiment was conducted at Henderson Creek, New Zealand. The data show how tidalcreeks that are an extension of the freshwater drainage network (as opposed to tidal creeks that are partof an estuarine distributary network with no direct connection to the land) variously import, export anddeposit fine sediment sourced from both landward and seaward of the creek, depending on the wind andfreshwater runoff, and modulated by the tide.

During freshwater spates, saltwater was largely displaced from the tidal creek at low tide, and sedi-ment sourced from the land was deposited inside the tidal creek and exported to the wider estuarybeyond the base of the creek. In one spate, during which 80 mm of rain fell in less than one day, 580 t ofsediment was sourced from landward of the tidal creek, and a maximum of 33% of this was exported tothe wider estuary.

Between rainstorms when it was calm, sediment was returned from the wider estuary by tidalcurrents (but not necessarily the same sediment that was exported during spates), and sediment wasalso eroded from the middle reaches of the tidal creek and transported to the upper reaches, where itwas deposited. The up-estuary deposition is explainable in Lagrangian terms as a type of settling lag,which results in an asymmetrical response of suspended-sediment concentration to current speed in thetidal creek.

The return of sediment to the tidal creek between spates was greatly enhanced by wind waves thatresuspended sediments from the intertidal flats of the wider estuary, with that sediment being trans-ported by tidal currents into the tidal creek where it was deposited, largely in the middle reaches. Thereis a broad consensus that waves drive a net loss of sediment from intertidal flats to offshore, whichreverses a net accumulation of sediment on intertidal flats during calm weather. In contrast, waves onthe intertidal flats outside the mouth of Henderson Creek initiate net landward transport of sediment offthe intertidal flats and into Henderson Creek, although seaward fluxes off the flats into the interior of thewider estuary may also occur at the same time. The sediment that is transported into the middle reachesof the tidal creek during windy periods is then available for relatively slower removal to the upperreaches of the tidal creek by tidal currents on a slow, but regular, basis.

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1. Introduction

In New Zealand, almost all of the largest rivers connect to theocean through very small estuaries and hence the bulk of theirrespective loadspass directly into theocean (Hicks et al., 2011). Theseinclude theWaiapu,Waipaoa, Hikuwai andWhanganui rivers on theNorth Island, and the Arawhata, Hokitika and Haast rivers on theSouth Island. In contrast, larger estuaries, including the largestestuaries in New Zealand (Kaipara Harbour, Manukau Harbour,WaitemataHarbour and Pauatahanui Inlete all on the North Island),

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are fed by smaller rivers and streams that typically discharge into thewider estuary through tidal creeks. These tidal creeks convey thebulk of the freshwater runoff and associated terrestrial sedimentseroded fromsurrounding catchments. Terrestrial sedimentsewhichinclude legacies of earlier catchment deforestation and sedimentfrom present-day sources associated with agriculture, plantationforestry andurbanisatione are recognised bymanagement agenciestoday as being the main threat to New Zealand’s estuarine ecosys-tems. Catchment deforestation, which began 800 years ago with theinitial colonisation by Polynesians (McGlone, 1983) and whichaccelerated 150 years ago with European colonisation, has signifi-cantly increased sediment runoff. At the peak of deforestation in themid-1800s to the early 1900s, catchment sediment runoff increasedbyat least an order ofmagnitude over natural levels, especially in the

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M.O. Green, N.J. Hancock / Estuarine, Coastal and Shelf Science 109 (2012) 116e132 117

highly erodible soft-rock hill country of the North Island (Hicks et al.,2011). Spikes in sediment runoff of up to 100-fold immediatelyfollowing clear-felling of native forest have been observed(O’Loughlin and Pearce, 1976). Sale (1978) noted that New Zealand’stypically steep relief, large annual rainfall andweathered regolith areexacerbating factors. In response, estuarine sedimentation rateshaveincreased by up to a factor of 100 compared to pre-deforestationrates. For example, Sheffield et al. (1995) found at Whangamataestuary, on the east coast of the North Island, a rise from pre-Polynesian sedimentation rates of 0.1 mm/year to 0.3 mm/yearunder Polynesian agriculture and to 11 mm/year after Europeanclearance of native forest. Other examples include Hume and Dahm(1991), Swales et al. (2002) and Hume and McGlone (1986). Theecological effects of terrestrial sediments include smothering ofinfauna by sediment “slugs”deposited in the aftermathof rainstorms(Norkko et al., 2002; Thrush et al., 2003, 2004); chronic effects onphysiological condition and behaviour of macrofauna (Lohrer et al.,2004); clogging of the feeding structures of suspension-feedinganimals (Ellis et al., 2002); reduced light levels; and reducedecological heterogeneity through direct effects of sediment onhabitat causing a lowering of species diversity and abundance,functional species shifts, and greater vulnerability to invasive species(Ellis et al., 2004). Sediment-transport models that simulate thetransport, dispersal and deposition of fine-grained terrestrial sedi-ment in estuaries are required for ecological risk assessment,prediction of long-term morphological change (including shoalingand mangrove spread), estimation of the efficacy of sediment-control measures that may be used in the catchment, and setting ofcatchment sediment-runoff limits. Spatially and temporally resolvedfield data are needed to calibrate and validate models. In the NewZealand case, models need to pass terrestrial sediment runoff fromthe catchment through tidal creeks at the base of freshwater streamnetworks and into the adjacent wider estuary.

There does not appear to be a precise meaning to the term“tidal creek” in the literature. In its broadest sense, and followinga consensus published on Wikipedia, a tidal creek is characterisedby its reducing to a muddy channel with little or no flow at lowtide, but often with significant depth of water at high tide. Shao(1988; reported in Shi and Chen, 1996) identified four types oftidal creek network, distinguishing between tidal creeks formedby tidal scouring and convergence, and tidal creeks that conveyfreshwater runoff from the land. From a functional point of view,there are fundamental differences between tidal creeks that arean extension of the freshwater drainage network, and tidal creeksthat are part of an estuarine distributary network with no directconnection to the land (Ralston and Stacey, 2007). The sedimentdynamics of tidal creeks is often understood in the context of thelatter, especially in terms of the connections to saltmarshes andmangroves, and the way they carry water, sediments, nutrientsand organic matter to and from those habitats (e.g., regardingsaltmarshes: Ward, 1981; Wolaver et al., 1988; French andStoddart, 1992; Leonard et al., 1995; Voulgaris and Meyers,2004; and regarding mangroves: Wolanski et al., 1980; Mazdaet al., 1995; Kitheka et al., 2002).

Compared to tidal creeks that are part of an estuarine distrib-utary network, the sediment dynamics of tidal creeks that are anextension of the freshwater drainage network are less wellunderstood. Mitchell et al. (2008) and Mitchell et al. (2004) ana-lysed data from a channel at the tidal limits of a semi-enclosed tidallagoon in southern England (Pagham Harbour, West Sussex). Theyidentified net landward sediment transport, with spreading ofsediment from the channel to adjacent intertidal flats, driven bylocal tidal asymmetry and modulated, in a number of ways, byvariable freshwater discharge through a tidal flap gate at the headof the channel. Ralston and Stacey (2007) described sediment

transport in a mudflat channel that connects directly to a smallwatershed at the landward end and that opens into Central SanFrancisco Bay at the seaward end. They found that sedimenttransport in the channel at diurnal timescales was dominated bytidal advection and dispersion of a tidal salinity front, but was alsomodulated by wind, through the action of wind waves, and byrainfall. Due to the particular geometry of the site, freshwaterrunoff during rainstorms had the greater effect on sedimenttransport, by supplying both buoyancy, which altered the channelbaroclinic dynamics, and fine sediment to the system. The resultwas greatly increased export of sediment from the channel duringstorms compared to between storms, when direction of net sedi-ment transport depended on the strength of the tidal forcing. Leeet al. (2010) noted that high river discharge combined with tidalcurrents rapidly export sand and mud through the braided chan-nels of the Skagit River (Washington, USA) intertidal flats and out tothe flat edge, but that during winter storms, when sediment runoffis high, mud layers can be left intact on the flats. Recognising thatintertidal flats in which a river supplies significant buoyancy,kinetic energy and sediment inputs have not been well studied (asopposed to flats where sediment sources are either from offshore,from an adjacent shore-parallel tidal channel, or absent, and mainexternal forcings are tides and waves), Chen et al. (2010) dida model study of sediment transport across an intertidal flat whereriver forcing is important. The model geometry was an idealisationof the intertidal region at the mouth of the Skagit River. They foundthat the bulk of the sediment delivered with freshwater is trappedin the intertidal zone (on the flats and in the channel that dissectsthe flats), and that trapping increases with sediment settlingvelocity. Landward of the waterline, the channel flow was found tobe, in effect, a seaward extension of the river, with bed shear stressdirected seaward. Seaward of the waterline, channel flow was tidaland ebb-dominant, due to the way the intertidal flats drain. Sedi-ment trapping resulted from flow deceleration across thefluvialetidal transition, and was enhanced by a reduction in bedshear stress caused by water-column stratification.

As part of a model investigation of long-term (decadal) trendsin sedimentation and associated ecological effects, Green (2008a)predicted that the fraction of the terrestrial sediment load thatdeposits within tidal creeks that discharge to the Central Waite-mata Harbour (to the west of the Auckland central businessdistrict, New Zealand; see below) decreases with an increase infreshwater runoff. Under the highest freshwater discharges,virtually the entire terrestrial sediment load was predicted to flushthrough the tidal creeks and into the wider estuarine receivingwaters. In effect, the tidal creek in these conditions becomes anextension of the river, which Chen et al. (2010) also recognised onthe Skagit River intertidal flats. Under light rainfall, practically theentire terrestrial sediment load was predicted to be sequestered inthe tidal creeks. In this paper we show measurements from a tidalcreek designed to test a hypothesis that was broadly motivated byGreen, (2008a) model predictions and by observations such asthose by Ralston and Stacey (2007), viz., that freshwater dischargeis a principal control on the flushing of terrestrial sedimentsthrough tidal creeks that are an extension of a freshwater streamnetwork. The data demonstrate how sediments sourced from bothlandward and seaward move through and deposit within the tidalcreek. The results demonstrate sediment-transport processes ina tidal creek that is intermittently perturbed by freshwater runoff,and add to our understanding of how tidal creeks that are anextension of the freshwater drainage network convey andsequester sediments. Of particular interest are estimates of thefraction of the terrestrial sediment runoff that is exported from thetidal creek during individual events, which does not seem to havebeen measured before.

M.O. Green, N.J. Hancock / Estuarine, Coastal and Shelf Science 109 (2012) 116e132118

2. Locations

The field experiment was conducted at Henderson Creek, whichis a tidal creek located 10 km to the west of Auckland City centralbusiness district on the east coast of the North Island of New Zea-land. (Fig. 1 shows all locations.) Henderson Creek drains to thenorth into the Central Waitemata Harbour.

The Central Waitemata Harbour is a drowned-valley estuarywith a surface area of 80 km2. It opens into the Hauraki Gulf, whichis sheltered from ocean waves, and comprises intertidal flatsaround the fringes, expanses of subtidal flats (to 5 m deep), deeperchannels (10e15 m deep) and tidal creeks (Whau River, Henderson

Fig. 1. Location maps. (a) North Island, New Zealand. (Base map: Tumonz, Vision Softwacopyright 2011 Google. Imagery copyright 2011 Cnes/Spot Image, DigitalGlobe, GeoEye, No

Creek) that discharge along the southwestern shore. The tide issemidiurnal with range 1.8e3.0 m (neapespring), and the watercolumn in the main body of the Harbour is typically well-mixed.The Harbour receives runoff from a 205 km2 land catchment,about half of which discharges through Henderson Creek. Bedsediments in the Harbour typically contain less than 10% mud, andthe median grainsize is 130e170 mm; bed sediments in the tidalcreeks are finer (Swales et al., 2008).

From the mouth of Henderson Creek south to the confluencewith Swanson Creek is w4 km. This reach of the tidal creek isroughly funnel shaped and is surrounded by high, bush-clad hills tothe west and north and by the lower land of Te Atatu Peninsula to

re.) (b) Central Waitemata Harbour. (c) Henderson Creek. (Base maps: Google Maps,rth Shore City Council, Terralink International Ltd.)


the east. A low-lying, mangrove-covered sand spit at the tip of TeAtatu Peninsula borders the outer northeastern margin of the tidalcreek, beyond which lies the Central Waitemata Harbour. The innerreaches of the tidal creek are narrowwith sinuous channels, muddybanks and high intertidal flats covered with dense, maturemangroves. South of the confluence, Henderson Creek extends forw4 km until it becomes primarily fresh, and Swanson Creek like-wise extends for w2 km.

Measurements of water depth, currents, suspended-sedimentconcentration and conductivity/temperature were made at twolocations: station Mouth at the mouth of Henderson Creek, andstation Confluence 100 m seaward (north) of the confluence withSwanson Creek. Measurements of water depth, waves andsuspended-sediment concentration were made at station Sandflaton an intertidal sandflat in the Central Waitemata Harbour 400 mto the east of the tip of Te Atatu Peninsula sand spit.

3. Data acquisition

At station Mouth, a DOBIE-OCT (measuring pressure witha Druck ceramic pressure sensor, infrared optical backscatter witha Seapoint optical backscatter sensor [OBS], and conductivity andtemperature with a Greenspan EC250 CT sensor) and a TeledyneRDI ADCP (measuring pressure and three orthogonal componentsof the current in a vertical profile) were bottom-mounted near thedeepest part of a subtidal channel that is 340 m wide at high tide(Fig. 2a). The ADCP was levelled on the seabed using a spirit level.To the west, the channel butted up to a high bush-clad bank. To theeast, the channel was bordered by narrow intertidal flats along TeAtatu Peninsula sand spit. This reach of Henderson Creek is mostlyprotected from wind by surrounding land apart from an exposureto the north around to the northeast over a fetch of 2e4 km.

At station Confluence a DOBIE-OCT and a Teledyne RDI ADCPwere bottom-mounted near the deepest part of a subtidal channelthat is 140 m wide at high tide (Fig. 2b). Again, the ADCP was lev-elled using a spirit level. The site was bordered by steep muddybanks and high intertidal flats covered with mangroves, making itcompletely protected from the wind.

At station Sandflat a DOBIE-O (measuring pressure and infraredoptical backscatter) was placed on a broad, sandy intertidal flat.This sector of the Central Waitemata Harbour is exposed to windfetches of 2e7 km from the northeast around to the southeast.

Data were collected in bursts over a period of 3 months, from 8April 2010 to 24 June 2010. There are various gaps in the record dueto instrument servicing, some instrument failures, and the need onone occasion to remove instruments to allow the passage of a superyacht launched from a factory upstream of the confluence.

Pressure was sampled by each DOBIE at 10 Hz with 1024 pointsper burst (burst duration 102.4 s) and a burst interval of 15min. The

0 50 100 150 200 250

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pressure sensor was mounted 20 cm above the bed at all stations(Sandflat, Mouth, Confluence). Water depth was computed fromthe pressure data using the hydrostatic equation with a constantnominal water density (1025 kg/m3) and nominal atmosphericpressure (14.7 psi). Wave statistics were calculated for the Sandflatdata only, by applying linear wave theory to the burst pressure data.

Infrared optical backscatter was sampled on the same scheduleas pressure. The OBS was mounted 80 cm above the bed at Mouthand Confluence; at Sandflat it was mounted 30 cm above the bed.The voltage output by the OBS is related to suspended-sedimentconcentration by a linear relationship for the range of concentra-tions encountered. Each sensor’s gain and offset were determinedby calibration in a turbidity tank using sediment acquired from theintertidal flat near a boatramp on Te Atatu Peninsula, approxi-mately midway between the mouth of Henderson Creek and theconfluence with Swanson Creek. The sediment used in the cali-bration tests had mean grainsize of 10e30 mm, which closelymatches the mean grainsize of sediments in water samplescollected by Swales et al. (2008) 1 m above the bed at the mouth ofHenderson Creek during a period with low freshwater runoff.Hence, “suspended-sediment concentration” herein can be inter-preted as concentration of 10e30 mm particles.

Conductivity and temperature were sampled on the sameschedule as pressure. Salinitywas calculated fromconductivity usingthe equation of state by Fofonoff and Millard (1983) and is reportedusing the Practical Salinity Scale. Salinity therefore has no units.

At both Mouth and Confluence, the current was sampled by theADCP in bursts at an interval of 15 min. The blanking distance was0.44 m at Mouth and 0.25 m at Confluence. At both stations thewater column above the blanking distance was divided into 0.1 msampling bins. Water depth was calculated from pressure using theTeledyne RDI software, taking instrument height off the bed intoaccount. The height of the centre of each bin was calculated, takingblanking distance into account. Bins beyond the water depthrecorded by the ADCP pressure sensor were discarded.

Burst- and depth-averaged horizontal current speed U anddirection were calculated from the measured horizontal velocitycomponents by firstly averaging over the burst and then secondlyaveraging between the blanking distance and the water surface.The current was virtually bidirectional, flowing either along-channel seaward (current speed denoted as positive) or along-channel landward (current speed denoted as negative).

Water-level data were obtained at station Oratia, which is anAuckland Council monitoring site in the freshwater section ofHenderson Creek, 6.5 km upstream (south) of the confluence withSwanson Creek.

Rainfall data were obtained from station Blewitt Gully, which isan Auckland Council rainfall station located in the hills of theSwanson and Henderson Creek catchments.

300 350 0 50 100 150from true right bank

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4. Results

Fig. 3 shows the basic dataset, consisting of burst-averagedwater depth h, (signed) current speed U, and suspended-sediment concentration C at the Mouth and Confluence sites;and significant wave height Hsig, wave-orbital speed at the bedUsig, b (corresponding to mean spectral wave period and significant

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Fig. 3. Basic 3-month dataset. (a) Water depth h, Confluence and Mouth sites. (b) Water leconfluence with Swanson Creek. (c) Cumulative rainfall over the 3-month deployment perMouth sites. (e) Suspended-sediment concentration C, Confluence and Mouth sites. (f) Saliniorbital speed at the bed Usig, b, Sandflat site. (i) Suspended-sediment concentration C, Sand

wave height), and suspended-sediment concentration C at theSandflat site.

The tide was semidiurnal with a notable springeneap cycle.The mean depth was 3.0 m and the range was 1.4e4.6 m atConfluence; mean depth was 2.1 m and range 0.5e3.7 m at Mouth.Water level at Confluence typically lagged water level at Mouth by15e30 min.

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Fig. 4. Data from the event (denoted by “Event” in Fig. 3), which includes two rainstorms and an intervening period with no rain. Altogether, the event comprises 7 tidal cycles(highelowehigh). (a) Water depth h. (b) Cumulative rainfall over the 3-month deployment period. (c) Water level measured at the Oratia freshwater monitoring site 6.5 kmupstream (south) of the confluence with Swanson Creek. (d) Salinity. (e) Current speed U (negative denotes along-channel landward). (f) Instantaneous mass flux of water. (g)Suspended-sediment concentration C. (h) Instantaneous sediment transport q.


There were 12 rainfall events in the 3-month deploymentperiod, during which a total of w500 mm of rainfall fell. Rainfallevents ranged from <1 day to 7 days’ duration. Increases in waterlevel at Oratia (the freshwater section of Henderson Creek) coin-cided with rainfall, with faster and larger rises in water level cor-responding to more intense, heavier rainfall. Oratia water levelincreased within an hour of the commencement of rain and begandropping within 2 h of the cessation of rain. It took 36e60 h forOratia water level to return to the baseline level after rain. Partic-ularly during the latter half of the 3-month deployment period,Confluence low-tide water levels were raised by 0.1e0.35 m for upto 3 tides compared to when it was not raining. Compared toConfluence, water levels at Mouth were not as noticeably affectedby rainfall.

Current speeds at Confluence and Mouth variedbetween �80 cm/s (flood) and 100 cm/s (ebb), which includes

a freshwater component. There is no obvious difference in currentspeeds at Confluence compared to at Mouth at the scale of Fig. 3.When it was not raining, slackwater occurred at approximatelyhigh and low tides, and peak flows occurred at approximately mid-rising and mid-falling tides, indicating a standing tide wave.

Suspended-sediment concentration at Confluence and Mouthvaried over the tidal cycle. C was higher during spring tidescompared to during neap tides, and C was highest during and afterrainfall, reaching 800e1000 mg/L. During dry weather, C wastypically less than 50mg/L and higher at Confluence compared to atMouth.

At the Sandflat site, in the main body of the Central WaitemataHarbour, waves were present when the wind blew from thenortheast around to the southeast, over fetches of 2e7 km. Thehighest measured Hsig was w60 cm; mean spectral period was0.5e2.5 s; and the highest measured Usig, b was w35 cm/s.


Suspended-sediment concentration C at the Sandflat site wasvirtually zero when there was no or little wind (and Hsig wasconsequently zero or very small), and increased with increasingUsig, b, indicating resuspension of bed sediment locally by thewaves. The highest measured C at Sandflat was w375 mg/L.

Salinity varied over the tidal cycle, and there was a longitudinalgradient in salinity, with higher salinities towards the mouth ofHenderson Creek. Salinity reduced when freshwater runoffincreased (as indicated in Fig. 3 by water level at Oratia). Salinitywas generally lower in the second half of the deploymentcompared to in the first half, when the Oratia baseflow water levelshifted higher.

4.1. Suspended-sediment transport

The instantaneous suspended-sediment transport at time t wasestimated from the Mouth and Confluence data as:

qðtÞ ¼ UðtÞCðtÞAðtÞ (1)

where U is burst- and depth-averaged current speed as describedpreviously, C is burst-averaged suspended-sediment concentrationat the measurement level converted from mg/L to mg/cm3 byassuming a sediment particle density of 2.65 g/cm3, and A is thecreek cross-sectional area, which varied over the tidal cycle andwhich was estimated from Fig. 2. q(t) is the transport integratedacross the width of the tidal creek, and has units of mass (of sedi-ment) per time. Negative q denotes landward transport.

More sophisticated methods for estimating q take account ofvertical distributions of current speed and suspended-sedimentconcentration (e.g., Ralston and Stacey, 2007). q estimated by Eq.(1) may contain error if there is vertical structure in the watercolumn and/or lateral structure across the width of the tidal creek(e.g., Kjerfve et al., 1981). Errors in estimates of q are discussedbelow.

Fig. 5. Data from the event (denoted by “Event” in Fig. 3), which includes two rainstorms anfrom Fig. 4). (b) Cumulative sediment transport over the event. (c) Cumulative sediment trindividual flood period. (e) Cumulative sediment transport over each individual ebb period

4.2. Event

Fig. 4 shows data during an event that included two rainstorms.Fig. 3 shows where the event occurred during the 3-monthdeployment; the event is denoted by “Event” in Fig. 3, the firstrainstorm is denoted by “Rainstorm (1)”, and the second rainstormis denoted by “Rainstorm (2)”.

During the event, there was 80 mm of rainfall during the firsttidal cycle (where a tidal cycle is defined as high waterelowwaterehigh water), followed by 5 tidal cycles with no rainfall,and terminated by one tidal cycle during which an additional30 mm of rain fell (Fig. 4a, b). Unfortunately, the optical backscattersensor at station Confluence then failed. This was not remedieduntil near the end of the 3-month deployment. The tide rangeduring the event was average, meaning it was between the springand neap tide ranges. The first day of the event experienced themost rainfall of all days in the 3-month deployment.

The water level at Oratia (the freshwater section of HendersonCreek) responded to the rainfall by rising rapidly (as it was raining),then beginning to drop within hours of the rain ceasing (Fig. 4c). Atlow tide when the Oratia water level was high, the salinity at bothConfluence and Mouth was zero, indicating that the low-tidechannel at Henderson Creek was conveying freshwater only, atleast as far as station Mouth; at high tide when the Oratia waterlevel was high, the longitudinal salinity gradient between Conflu-ence and Mouth re-established (Fig. 4d). Over the 5 tidal cycleswith no rain, the low-tide longitudinal salinity gradient graduallyre-established, and the high-tide longitudinal salinity gradientgradually reduced through an increase in high-tide salinity atConfluence.

U at both stations (Confluence and Mouth) responded to rainfallby becoming larger on the ebb tide and smaller on the floodcompared to the 5 tidal cycles with no rain (Fig. 4e). In addition, theduration of ebb-directed flow was lengthened when it was rainingcompared to when it was not raining, to the extent that, during the

EQ

d an intervening period with no rain. (a) Instantaneous sediment transport q (repeatedansport over each individual tidal cycle. (d) Cumulative sediment transport over each.

Table 1Instantaneous sediment transport q integrated over the ebb tide (QEBB), the subse-quent flood tide (QFLOOD), and the whole tidal cycle (QTIDE) during the eventcomprising two rainstorms (one at the start and one at the end of the event) and 5intervening tidal cycles with no rain (see Fig. 3). DQTIDE ¼ QTIDE at station Confluenceminus QTIDE at station Mouth, where negative denotes sediment erosion from thebed in the reach between Mouth and Confluence. All units are tonnes.

Event

Tidal cycle Mouth Confluence DQTIDE

QEBB QFLOOD QTIDE QEBB QFLOOD QTIDE

1 (rain) 275 �87 187 613 �33 580 3922 353 �148 205 565 �128 437 2323 113 �87 26 142 �106 36 104 44 �83 �39 39 �77 �38 15 75 �38 37 52 �50 2 �356 23 �63 �40 23 �75 �52 �127 (rain) 615 �128 487 580 �19 561 74

Table 2Instantaneous sediment transport q integrated over the ebb tide (QEBB), the subse-quent flood tide (QFLOOD), and the whole tidal cycle (QTIDE) during periods when itwas calm and dry. DQTIDE ¼ QTIDE at station Confluenceminus QTIDE at station Mouth,where negative denotes sediment erosion from the bed in the reach betweenMouthand Confluence. All units are tonnes. Periods for which transport is given are shownin Fig. 3.

Calm and dry

Tide Mouth Confluence DQTIDE


Neap (1) 8.3 �8.1 0.2 5.1 �6.8 �1.7 �1.9Neap (2) 6.5 �8.9 �2.4 5.3 �7.9 �2.6 �0.2Average 11.0 �11.2 �0.2 8.7 �8.6 0.1 0.3Spring (1) 18.5 �26.9 �8.4 15.1 �30.4 �15.2 �6.8Spring (2) 27.5 �29.6 �2.1 20.8 �32.5 �11.7 �9.6


first rainstorm in particular, U barely reversed (i.e., turned land-ward). These conditions in combination resulted in a net seawardflux of water from the tidal creek during the rainstorms, relaxingback towards zero during the 5 tidal cycles following the firstrainstorm. This can be judged from Fig. 4f, which shows theinstantaneous mass flux of water (m3/s), estimated simply as U(t)A(t), and realising that the net flux of water over each phase of thetidal cycle (flood, ebb) is the area underneath the correspondingcurve. The obvious interpretation of the large net seaward flux ofwater combined with the low salinities during rain is that itcorresponds to freshwater runoff from the catchment.

Suspended-sediment concentration at both stations (Conflu-ence and Mouth) was largest during and immediately after rain(Fig. 4g). Since U was largest and primarily directed seawards atthese times, the instantaneous sediment transport q was alsolargest and primarily directed seawards (Fig. 4h).

Fig. 5 shows various estimates of cumulative and net sedimenttransport during the event. Panel a in Fig. 5, which shows instan-taneous transport q, is the same as panel h in Fig. 4 to provide a linkbetween the two figures.

Fig. 5b shows QCUM at each time t over the entire event, whereQCUM is the cumulative sediment transport over the period t1 to t2:

QCUM ¼Zt2

t1

qðtÞdt (2)

QCUM has units of mass (tonnes), and a negative sign denoteslandward-directed transport. For QCUM shown in Fig. 5b, t1 in Eq. (2)is the start of the event. QCUM at the end of the event is the netsediment transport over the event, which is given the symbolQEVENT. At station Confluence QEVENT was 1533 t directed seawardsand at station Mouth QEVENT was 863 t, also directed seawards(Fig. 5b). These numbers indicate that, integrated over the entireevent (which included two rainstorms and an intervening periodbetween those rainstorms), 863 t of sediment was exported fromHenderson Creek to the Central Waitemata Harbour and 670 t ofsediment was deposited in the reach between Confluence andMouth.

Fig. 5c, d and e are intended to show how QEVENT arose. Fig. 5cshows QCUM at each time t over each individual tidal cycle in theevent. Here, t1 for each tidal cycle is the time of high water, andQCUM at the end of each tidal cycle (i.e., at the next highwater) is thenet sediment transport over the tidal cycle, which is given thesymbol QTIDE. Likewise, Fig. 5d shows QCUM at each time t over eachindividual flood period in the event (and the last estimate of QCUM isgiven the symbol QFLOOD), and Fig. 5e shows QCUM at each time tover each individual ebb period in the event (and the last estimateof QCUM is given the symbol QEBB).

During the first rainstorm, QEBB was much greater than QFLOOD,resulting in QTIDE being directed seawards at both stations. Thispattern was repeated during the tidal cycle following the firstrainstorm. Then, over the next 4 tidal cycles with no rain, QEBBreduced and QFLOOD also reduced, but at a lesser rate, resulting inQTIDE turning negative (sediment transport directed landward). Thesecond rainstorm repeated the pattern in the first rainstorm, withQEBB greatly exceeding QFLOOD, resulting in QTIDE being directedseawards at both stations. Table 1 tabulates QEBB, QFLOOD and QTIDE,and also shows DQTIDE, defined as QTIDE at station ConfluenceminusQTIDE at station Mouth. DQTIDE can be interpreted as the mass ofsediment deposited (positive) or eroded (negative) betweenstations Mouth and Confluence over the entire tidal cycle.

Table 1 shows that during and in the immediate aftermath of thefirst rainstorm (i.e., the first two tidal cycles in the event), 1017 t ofsediment was transported seawards through Confluence, 624 t of

sediment was deposited in the reach between Confluence andMouth, and 392 twas transported seawards throughMouth. Duringthe following four tidal cycles with no rain, 16 t of sediment wasimported from the Central Waitemata Harbour, 36 t was erodedfrom the reach between Confluence and Mouth, and 52 t wastransported landward of station Confluence, where it wasdeposited.

4.3. Calm and dry

Looking specifically at calm and dry weather provides insightinto the “background” sediment transport driven by tidal currentsbetween rainstorms. Five tides during calm and dry weather wereidentified for analysis, these corresponding to spring, neap andaverage tide ranges. These are shown in Fig. 3, denoted as “Neap(1)”, “Neap (2)”, “Average”, “Spring (1)” and “Spring (2)”.

During calm and dry weather, QTIDE at both Mouth andConfluence was small (relative to transport during and in theaftermath of rain) under spring, neap and average tides (Table 2).Sediment was typically carried into the tidal creek from the CentralWaitemata Harbour (QTIDE at station Mouth was negative). DQTIDEwas typically negative (with one exception), indicating erosion ofsediment from the reach between Mouth and Confluence. Sedi-ment was transported and deposited landward of station Conflu-ence (QTIDE at Confluence was negative).

All components of the sediment transport (QEBB,QFLOOD andQTIDE) were larger for spring tides than for neap and average tides.

Fig. 6 shows how the net transport arose. At both stations (i.e.,Mouth and Confluence), the variation in water depth over the tidalcycle closely approximated the shape of a sine wave. Likewise, andcompared to the suspended-sediment concentration, variation in Uover the tidal cycle closely approximated the shape of a sine wave,


with no obviousmagnitude or duration asymmetry. In contrast, C atboth stations was larger on the flood tide compared to on the ebbtide which, combined with the symmetrical variations in currentand water depth, resulted in landward net transport over the tidalcycle at both stations (see Table 2). The difference betweensuspended-sediment concentrations during the flooding tidecompared to during the ebbing tide was greater at Confluence thanat Mouth. This caused greater net landward transport at Conflu-ence, indicating erosion of sediment in the reach between Mouthand Confluence and deposition landward of Confluence. DQTIDE waslarger for spring tides than for neap and average tides.

4.4. Windy and dry

When waves were present, suspended-sediment concentrationat station Sandflat in the Central Waitemata Harbour was raisedcompared to whenwaves were not present (Fig. 3), which has been

1

2

3

4

h (m

)

8

12

16

20

24

C (m

g/L)

-40

0

40

U (c

m/s

)

-0.002

-0.001

0

0.001

0.002

q (t/

s)

Ensemble averages

Confluence - sp - av - neMouth - spring - average - neap

One tidal cycle

a

b

c

d

Landward

Seaward

Fig. 6. Ensemble averages of the tidal cycles denoted in Fig. 3 as “Neap (1)” and “Neap (2)”,Weather calm and dry. (a) Water depth h. (b) Current speed U (negative denotes along-chatransport q. (e) Current speed plotted against water depth. (f) Suspended-sediment concentrwater depth.

attributed in other New Zealand estuaries to resuspension of bedsediments by orbital motions under locally generated wind waves(e.g., Green and Coco, 2007; Green, 2011). Six tides during windyand dry weather were selected for further analysis to investigatethe effect of raised suspended-sediment concentration in CentralWaitemata Harbour on sediment transport in Henderson Creek.The six tides selected are shown in Fig. 3, denoted by “Wind (1)”,“Wind (2)”, “Wind (3)”, “Wind (4)”, “Wind (5)” and “Wind (6)”.

Fig. 7(a) shows the variation in suspended-sediment concen-tration over the tidal cycle at station Sandflat during these six tides.C was largest just before the optical backscatter sensor emergedfrom the water on the ebbing tide, and just after the sensor becamesubmerged on the flooding tide. This pattern of suspension isconsistent with the existence of awave-generated “turbid fringe” asdescribed by, for instance, Green et al. (1997). Cwas typically largerduring the flooding tide compared to during the ebbing tide. Thisobservation is discussed further in the Discussion.

8 12 16 20 24C (mg/L)

1

2

3

4

5

h (m

)

-40 0 40U (cm/s)

1

2

3

4

5

h (m

)

-0.002 -0.001 0 0.001 0.002q (t/s)

1

2

3

4

5

h (m

)

ringerageap g

f

e

Landward Seaward

“Spring (1)” and “Spring (2)”. Also shown is the single tidal cycle denoted as “Average”.nnel landward). (c) Suspended-sediment concentration C. (d) Instantaneous sedimentation plotted against water depth. (g) Instantaneous sediment transport plotted against

Table 3Instantaneous sediment transport q integrated over the ebb tide (QEBB), the subse-quent flood tide (QFLOOD), and the whole tidal cycle (QTIDE) during periods when itwas windy and dry. DQTIDE ¼ QTIDE at station Confluence minus QTIDE at stationMouth, where negative denotes sediment erosion from the bed in the reach betweenMouth and Confluence. All units are tonnes. Periods for which transport is given areshown in Fig. 3. “e” denotes no data.

Windy and dry

Mouth Confluence DQTIDE Tide range


Wind (1) 11.6 �22.6 �11.0 e e e e AverageWind (2) 15.1 �37.8 �22.7 e e e e AverageWind (3) 19.7 �55.2 �35.4 34.9 �54.1 �19.1 16.3 SpringWind (4) 9.4 �12.3 �2.9 e e e e NeapWind (5) 20.9 �35.9 �15.0 e e e e SpringWind (6) 18.2 �23.2 �5.0 e e e e Neap


The landward-directed instantaneous sediment transport q atstation Mouth was greater when it was windy compared to when itwas not windy, but seaward-directed q when it was windy wassimilar to seaward-directed q when it was not windy (Fig. 7, andcompare QEBB and QFLOOD in Table 3 with QEBB and QFLOOD under thesame tide range in Table 2). This disparity between QEBB and QFLOODtranslated into a larger net transport into Henderson Creek throughthe mouth when it was windy compared to when it was not windy(Fig. 7, and compare QTIDE in Table 3 with QTIDE under the same tiderange in Table 2).

Data at station Confluence are available for only one windyperiod: Wind (3). As was the case at station Mouth, landward-directed instantaneous sediment transport q at station Conflu-ence was greater when it was windy compared to when it was notwindy (Fig. 7, and compare QFLOOD in Table 3 with QFLOOD under thesame tide range in Table 2). However, unlike at Station Mouth,seaward-directed instantaneous sediment transport q at stationConfluence was larger when it was windy compared towhen it wasnot windy (Fig. 7, and compare QEBB in Table 3 with QEBB under thesame tide range in Table 2). Again, the disparity between QFLOODand QEBB resulted in QTIDE directed landwards at Confluence.

-0.004 -0.002 0 0.002q (t/s)

1

2

3

4

h (m

)

0

0.5

1

1.5

2

2.5

h (m

)

0 100 200 300C (mg/L)

0 100C (mg

Sandflat

MouthLandward Seaward

Ebb Flood

Optical Backscatter sensor out of water

Tide ebbs this way

Tide flo

a

b c

Fig. 7. Data from tidal cycles when it was windy and dry (see Fig. 3). (a) Suspended-sedimenHarbour, outside the mouth of Henderson Creek. The left panel shows the change in C when(b) Instantaneous sediment transport plotted against water depth at station Mouth. Data adifferent tide ranges (neap, average and spring) when it was calm and dry (see Fig. 6g). (c) Inare shown for just the one tidal cycle when it was windy (no other Confluence data are avaiwhen it was calm and dry (see Fig. 6g).

Looking at the one period when there were data available atboth Mouth and Confluence, Wind (3), DQTIDE was positive, indi-cating deposition of sediment in the reach between Mouth andConfluence. In contrast, during calm and dry weather, DQTIDE wastypically negative, indicating erosion in the same reach.

Wind (1)Wind (2)Wind (3)Wind (4)Wind (5)Wind (6)Calm and dry - neap tideCalm and dry - average tideCalm and dry - spring tide

-0.004 -0.002 0 0.002q (t/s)

1

2

3

4

5

h (m

)

200 300/L)

Landward Seaward

Confluence

ods this way

t concentration plotted against water depth at station Sandflat in the Central Waitematathe tide was ebbing; the right panel shows the change in C when the tide was flooding.re shown for the 6 tidal cycles when it was windy, as well as ensemble averages forstantaneous sediment transport plotted against water depth at station Confluence. Datalable) as well as ensemble averages for different tide ranges (neap, average and spring)


4.5. Vertical and horizontal structure in the water column

To assess the applicability of Eq. (1) used to estimate sedimenttransport, the vertical and horizontal distributions of U and C needto be examined. Recall that C used in Eq. (1) is a point measurementmade by an OBSmoored in the centre of the channel and positionedat a fixed elevation above the bed; and U is a depth-averaged valuecalculated from an upwards-looking ADCP, also moored in thecentre of the channel.

Fig. 8a and b show vertical profiles of suspended-sedimentconcentration and current, respectively, at three locations (centreof channel, near true right bank, near true left bank; see Fig. 2) atstation Confluence over a tidal cycle during a rainstorm. Fig. 9a andb likewise show profile data at Mouth. The period when theseprofile measurements were taken is denoted by “Profiles” in Fig. 3.Around the period the profiles were measured, the water level atOratia (the freshwater section of Henderson Creek) rose to aboutthe same level as it did during the first rainstorm analysed in theevent (see “Rainstorm 1” in the “Event” in Fig. 3), but the profilemeasurements did not correspond exactly to the peak Oratia waterlevel. Nevertheless, analysis of the profiles will shed some light onwater-column structure when freshwater runoff was high, whichwill help with assessing the applicability of Eq. (1) during the event.

The suspended-sediment concentration profiles at stationConfluence (Fig. 8a) indicate the existence of a turbid surface layer(TSL) which appeared to maintain a virtually constant thickness of

0 40 80 120

mg/L

4

3

2

1

0

m b

elow

sur

face

Confluence

-0.4 0 0.4

m/s

m a

bove

bed

2

1

0

Fig. 8. Profile data at Station Confluence during the period denoted by “Profile” in Fig. 3. (surface. The interval between groups of profiles (i.e., true right bank, centre, true left bank)the bed of the moored optical backscatter sensor (OBS) is also shown, although there are no Othe same horizontal scale (as shown), but graph origins march across the page to the right, assurface, which are unreliable. (b) Current profiles, plotted against height above the seabed30 min. The black line is the northesouth component (positive ¼ north-flowing) and the gesouth is approximately along-channel. Each profile is plotted against the same horizontal sgrey dashed lines.

about 2 m and which was approximately uniform across the tidalcreek. (The latter is a little difficult to judge, since suspended-sediment concentration in the TSL did evolve in time and theprofiles across the creek are separated from each other by about15 min; see caption to Fig. 8a.)

When the water depth was less than 2 m (early and late in thetide), the TSL occupied the entirewater column and the suspended-sediment concentration profiles were virtually uniform. At thesetimes, the moored OBS (which was used to measure C, which inturn was used in Eq. (1) to estimate sediment transport) wasenveloped in the TSL and Cwould be a good estimate of the depth-and section-averaged suspended-sediment concentration.

When the water depth was greater than 2 m, the TSL wasunderlain by a less turbid layer. CTD data also obtained at the time(not shown) showed that salinity was low in the TSL compared to inthe underlying layer, indicating that the TSL contained turbidfreshwater runoff. During the 2 h before high tide, suspended-sediment concentration increased towards the bed in the under-lying layer in the middle of the channel (but not at the edges of thesection), which is consistent with resuspension of bottom sedi-ments in the channel. At all other times, suspended-sedimentconcentration in the underlying layer was virtually uniform withheight above the bed and across the section.

The current profiles at Confluence (Fig. 8b), although noisycompared to at Mouth (Fig. 9b), show no sign of flow reversal atdepth associated with the TSL.

True right bankCentreTrue left bank

Elevation of moored OBS

Approximate base of turbid surface layer

N-S component (approx. along-channel)E-W component

a) Suspended-sediment concentration profiles, plotted against depth below the wateris 60 min. The interval between profiles in each group is w15 min. The elevation aboveBS data available at Confluence for this particular period. Each profile is plotted againstshown by the grey dashed lines. The stippled box at the top denotes data from near the, where the blanking distance of the ADCP is 0.44 m. The interval between profiles isrey line is the eastewest component (positive ¼ east-flowing) of the current. North-cale (as shown), but graph origins march across the page to the right, as shown by the

0 40 80 120mg/L

4

3

2

1

0m

bel

ow s

urfa

ceMouth

SSC, moored OBSTrue right bankCentreTrue left bank

Approximate base of turbid surface layer

-0.4 0 0.4m/s

m a

bove

bed

N-S component (approx. along-channel)E-W component2

1

0

Fig. 9. Profile data at Station Mouth during the period denoted by “Profile” in Fig. 3. Legend as Fig. 8, except suspended-sediment concentration from the moored OBS is shown inpanel (a) at the depth below the water surface at which the measurement was made.


We infer from the Confluence profile data that when themooredOBS was in the underlying layer C would underestimate the truedepth-averaged suspended-sediment concentration at all locationsacross the section. q calculated by Eq. (1) should therefore under-estimate the true seaward sediment transport through the sectionat station Confluence around high tide when the TSL was underlainby a less turbid layer. At all other times, measured C should bea reasonable estimate of the true depth- and section-averagedsuspended-sediment concentration, and therefore q should alsobe a reasonable estimate. It is difficult to extrapolate this conclusionto conditions when freshwater runoff was at its peak. On the onehand, the TSL may have been thicker, thus enveloping the mooredOBS for more of the time, which would increase confidence in theestimate of q. On the other hand, the TSL may have carried pro-portionately more sediment than the underlying layer, thusdecreasing confidence in the estimate of q.

The suspended-sediment concentration profiles were uniformat all times across the section at station Mouth, and there is alsoevidence of a TSL (Fig. 9a). Here, the TSL was thinner (w1.5 m)compared to at Confluence (w2 m), which is consistent with theTSL spreading as the tidal creek widens to seaward. Late in theebbing tide the suspended-sediment concentration profiles wereuniform, indicating that the TSL occupied the entire water columnat that time. The TSL was not present late in the flooding tide,presumably because the surface freshwater layer had been dis-placed back up-creek beyond the measurement station. As atConfluence, the current profiles at Mouth showed no sign of flowreversal at depth associated with the TSL (Fig. 9b).

With uniform sections and less vertical structure in the watercolumn at Mouth, C from the moored OBS is likely to be a betterestimate of the true depth- and section-averaged suspended-sediment concentration compared to at Confluence, and the esti-mates of q at Mouth are therefore likely to be reasonable.

Profiles of current and suspended-sediment concentrationweremeasured for 12 h at Confluence and Mouth on two other occa-sions, both during freshwater baseflow. All profiles were unre-markable in that suspended-sediment concentration was uniformin the vertical and across the two sections, and the current profilesapproximately conformed to the law of the wall. Estimates of q atboth locations based on C and U are therefore likely to bereasonable.

5. Discussion

We know from studies of larger systems (e.g., Tamar, tributariesof Chesapeake Bay, Hudson, Weser, Seine, Mekong, Fly, Changjiang)that, in very general terms, gravitational circulation and tidaldynamics interact with bed exchange and particle transformations(flocculation) to establish transport patterns, which often featurean estuarine turbidity maximum, and that these patterns can bemodulated by springeneap variation in tide range, be influenced bysuppression of mixing by stratification, and be perturbed by windand freshwater runoff (e.g., Geyer, 1993; Uncles et al., 1994;Wolanski et al., 1995, 1998; Grabemann et al., 1997; Li and Zhang,1998; Geyer et al., 2001; Woodruff et al., 2001; Le Hir et al., 2001;Dickhudt et al., 2009).

However, tidal creeks are smaller, and they differ in importantrespects to these larger systems. For one thing, they are veryshallow at low tide, which will affect mixing and limit, but notnecessarily eliminate, baroclinic processes. For example, Mitchellet al. (2004) showed how episodic freshwater discharge througha tidal flap gate at the tidal limits of a semi-enclosed lagoonenhanced net landward sediment transport by increasing localvertical density gradients in a very short-lived (30 min) salt wedgeat the turn of the tide. Also, Ralston and Stacey (2007) showed thatsuspended-sediment concentration and flux were closely linked to


advection and dispersion of a tidal salinity front, and that changesin buoyancy due to freshwater runoff affected dynamics of thefront. Tidal creeks are also likely to be purged entirely of saltwaterby freshwater spates (or freshets), at least at low tide, which willaffect the control of buoyancy forces on the fate of terrestrialsediment carried in freshwater off the catchment (see Geyer et al.,2004; for a review). Fenster et al. (2001) described the importanceof spates to the sediment dynamics of the Kennebec River estuary,which is a high-latitude, rock-bound estuary with a large tidalprism and highly variable freshwater inputs. They drew a distinc-tion between this kind of estuary and the low-latitude coastal plainestuaries that generally trap sediment from both riverine andoffshore sources by way of an estuarine turbidity maximum. In thecase of the Kennebec River estuary, freshwater spates were shownto supplant a substantial part of the flood-tidal prism, thusproducing ebb-dominant bedload sediment transport. Fenster et al.concluded that episodic freshets, superimposed on ebb-directedtides, control the net, long-term (decades to centuries) sediment-transport trends in the Kennebec River estuary. Similarly, Horneand Patton (1989) showed that sediment transport in the micro-tidal Connecticut River estuary is ebb oriented throughout theentire estuary during periods of high freshwater discharge.

The data presented here show that Henderson Creek tidal creekvariously imports, exports and deposits sediment sourced fromlandward and from seaward, depending on the wind and fresh-water runoff, and modulated by the tide.

During the first rainstorm of the event, net 580 t of sedimentwas transported seaward through station Confluence during onetidal cycle (highelowehigh); net 392 t of sediment was depositedinside the tidal creek between Confluence andMouth; and net 187 tof sediment was transported seaward through station Mouth intothewider CentralWaitemata Harbour (Table 1). Sincewe know thatthe flux of water over this period was nearly exclusively seaward atboth Confluence and Mouth (Fig. 4f) it is possible to makeconclusions from these numbers about sediment sources andsediment retention in/export from the tidal creek, as follows.

Firstly, it is reasonable to conclude that most of the 580 t ofsediment that was transported seaward through station Confluencewas sourced from landward of Confluence. Sediment sources mayhave included sediment eroded from the bed and banks of Hen-derson Creek and Swanson Creek landward of their confluence, aswell as sediment eroded from the catchment. To provide context forthis number, we note that simulations by the GLEAMS catchmentsediment erosion model predicted 1000e2000 t of sediment runofffrom the catchment under 80 mm of rainfall that falls during oneday (i.e., 24 h) (Fig. 4.3 in Green, 2008b). The GLEAMSmodel (Kniseland Davis, 2000) predicts sheetwash and rill erosion in the catch-ment, but does not include in-stream dynamics or mass movement.The GLEAMS prediction is 2e4 times higher than the measuredsediment runoff of 580 t during aw12-h period (i.e., one tidal cycle,highelowehigh) and under 80 mm of rainfall. However, during thesecond tidal cycle (highelowehigh) in the event, a further net 437 tof sediment was transported seawards through Confluence(Table 1) which, added to the net flux of 580 t during the first tidalcycle, is a total of about 1000 t of sediment that was transportedseawards through Confluence, which is closer to the GLEAMSprediction. However, the flux of water through Confluence duringthis second tidal cycle was both seawards (during ebb) and land-wards (during flood), so it is less clear that all of this 1000 t ofsediment was sourced from landward of Confluence. Hence, it isalso less clear that comparing this number to a GLEAMS predictionis a valid exercise.

Secondly, any amount between 580 t to 392 t of the sedimentsourced from landwards of Confluence could have deposited in thetidal creek between Confluence and Mouth. (No less than 392 t

could have been deposited since that would have resulted in morethan the observed net 187 t being transported through Mouth.)Hence, the retention in Henderson Creek of sediment sourced fromlandwards of Confluence must have been between 100% (in whichcase 187 t of sediment was eroded from the reach betweenConfluence and Mouth and exported through Mouth) and 67% (inwhich case no sediment was eroded from the reach betweenConfluence and Mouth). Accordingly, the export of sedimentsourced from landwards of Confluence must have been between 0%and 33%.

Green (2008a) predicted that, with a freshwater discharge thatis associated with 80 mm of rain that falls in one day, 65% of 40-mmsediment injected into Henderson Creek landwards of the conflu-ence with Swanson Creek is exported through Henderson Creekand out into thewider CentralWaitemata Harbour. For these modelsimulations, a unit load of sediment was injected at a single point,continuously over 24 h, and the model was allowed to run toequilibrium, which was deemed to be achieved when 99% of theinjected sediment had settled somewhere in the model domain,which included the whole of the Central Waitemata Harbour. Asalready argued, Table 1 shows that a maximum of 33% of the sus-pended sediment (10e30 mm particle size) that was sourcedlandwards of Confluence was exported through Henderson Creekinto the wider Central Waitemata Harbour over one tidal cycle(highelowehigh), which is about half the predicted export.

By way of comparison, and applying the same reasoning asabove, a maximum of 86% of the suspended sediment (10e30 mmparticle size) that passed seawards through Confluence duringthe second rainstorm, when only 30 mm of rain fell in one day, wasexported through Henderson Creek. (Note, however, that the netsediment load that passed seawards through Confluence was notnecessarily all sourced landwards of Confluence, since the flux ofwater was both landwards and seawards during the second rain-storm.) Green (2008a) predicted that export should increase withan increase in rainfall, not decrease. However, the water level atOratia, which is indicative of freshwater discharge, was in facthigher during the second rainstorm than during the first and it wasdelivered over a shorter period, and the rise in water level beganearlier in the tidal cycle. These factors could all influence the exportof material through the tidal creek.

Between rainstorms when it was calm, sediment was trans-ported landwards through Mouth, eroded from the reach of thetidal creek between Mouth and Confluence, and deposited land-wards of Confluence.

The net landward transport at bothMouth and Confluence arosefrom C at both stations being larger on the flood tide compared toon the ebb tide. This, on the face of it, could be attributed tocurrent-velocity asymmetry (e.g., Dyer, 1995), but Fig. 6 reveals nosuch asymmetry. It could also be due to tidal-period asymmetry invertical mixing (alternating periods of straining and “over-strain-ing”; e.g., Simpson et al., 1990; Chant and Stoner, 2001), althoughthe Henderson Creek profile data, which show limited verticalstructure in the water column during times when such structurewould most likely be evident, would not seem to support thispossibility.

Fig.10, which plotsU against C on the ebbing and flooding tide atboth stations, sheds light on the process driving net transport whenit is calm and dry. Fig. 10 shows an asymmetrical response ofsuspended-sediment concentration to current speed (as opposed toasymmetrical currents causing an asymmetrical variation insuspended-sediment concentration over the tidal cycle). Specifi-cally, C tends to be smaller during the ebb tide compared to duringthe flood tide for the same values of U in excess of 30e40 cm/s.Furthermore, the asymmetrical response is greater at Confluencethan it is at Mouth. Ralston and Stacey (2007) also observed

-40 0 40U (cm/s)

-40 0 40U (cm/s)

0

10

20

30

C (m

g/L)

Mouth ConfluenceEbb EbbFlood Flood Spring

AverageNeap

Fig. 10. Current speed U (negative denotes along-channel landward) plotted againstsuspended-sediment concentration C. Calm and dry weather.


suspended-sediment concentrations during flood flows that weremuch greater than during ebb flows despite similar current speeds.By calculating sediment fluxes, they were able to rule out deposi-tion on the surrounding intertidal mudflats as removing anysignificant volume of suspended sediment (compared to thevolume in suspension) from the system at high water. Instead, theyattributed the observations to dispersion of a transition zonebetween fresh and salty water during each tidal inundation, whichstretches salinity and concentration gradients and results inreduced concentrations in the (by then more dispersed) transitionzone as it transits the channel during the next ebb flow. In contrast,at Henderson Creek, there is a net landwards sediment transport,and a plausible explanation for the asymmetrical response can befound by taking a Lagrangian view of the situation: sediment that istransported landwards of Confluence on a flooding tide may bedeposited in sheltered regions fromwhere it may not be able to beresuspended, thus in effect removing that sediment from thesystem. In this way, the system is effectively source-limited on theebbing tide, but not so on the flooding tide, when sediment is beingtransported into the tidal creek from the wider Central WaitemataHarbour. Eulerian measurements of current speed and suspended-sediment concentration in a system that was source-limited onlyduring the ebbing tide would show an asymmetrical response ofsuspended-sediment concentration to current in just the wayshown in Fig.10. This mechanism is a type of settling lag, whichwas

0 100 200 300 400C (mg/L), Sandflat

-60

-40

-20

0

20

QE

BB, Q

FLO

OD (

t), M

outh

QTI

DE

(t), M

outh

Calm and dry Windy and dry

Ebb

Flood

a b

Fig. 11. (a) QEBB and QFLOOD at station Mouth plotted against suspended-sediment concentrgenerate waves which in turn resuspend sediments at Sandflat). The particular value of C use(Table 1) and fromwindy and dry periods (Table 3), classified by tide range (spring, average aat Sandflat. Transport is plotted from calm and dry periods (Table 1) and fromwindy and dryare sketched to draw the eye to trends.

described by van Straaten and Kuenen (1958) and recentlymodelled by de Swart and Zimmerman (2009), who found ananalytical expression for the residual sediment flux due to settlinglag. Sediment that is being transported landwards on the flood tidebegins to settle before high slackwater (depending on particle sizeand density) but, because particles take a finite time to reach thebed, they are carried landwards while settling and finally depositedin areas where ebbing currents are tooweak to resuspend them andmake them available for subsequent seaward transport. The resultis a pumping of sediment landward, which can occur even undersymmetrical tidal currents. Dyer et al. (2000) confirmed the prin-ciples of settling lag, at least for enclosed and semi-enclosedsystems under relatively small waves (Shi and Chen, 1996), andfound that, by increasing the sediment settling rate at high waterand the subsequent critical bed shear stress for sediment entrain-ment, flocculation can enhance the resulting net landward trans-port to the point where it is actually counter to the barotropic tidalasymmetry.

Compared to when it was calm and suspended-sedimentconcentration along the intertidal flats beyond the mouth of Hen-derson Creek was zero, more sediment was transported into thetidal creek from the wider Central Waitemata Harbour when it waswindy and suspended-sediment concentration on the externalintertidal flats was raised above zero. Furthermore, sediment wasdeposited inside the tidal creek between Mouth and Confluencewhen it was windy, whereas sediment was eroded from the reachbetween Mouth and Confluence when it was calm. During bothwindy weather and calm weather, sediment was transportedlandwards of Confluence and deposited. Fig. 11 clearly demon-strates these differences, showing how net landwards transportthrough Mouth is increased by wind for each tide range (spring,neap, average), and how the increased net transport is driven onlyby an increase in QFLOOD, with QEBB being virtually insensitive to thewind. A plausible explanation of these observations is that thewider Central Waitemata Harbour acts as a source of sediments toHenderson Creek when it is windy. Specifically, the sedimentsource is sediment that is resuspended on intertidal flats by localwind-generated waves. Furthermore, the more sediment insuspension in the Central Waitemata Harbour, the greater thesource, and the larger the net transport of sediment is into the tidalcreek.

0 100 200 300 400C (mg/L), Sandflat

-60

-40

-20

0

20

Landward

SpringAverageNeap

Seaward

ation C at Sandflat, which is intended as a surrogate for “windiness” (given that windsd in the plot is the tidal-cycle maximum. Transport is plotted from calm and dry periodsnd neap). (b) QTIDE at station Mouth plotted against suspended-sediment concentrationperiods (Table 3), classified by tide range (spring, average and neap). The dashed lines

30 20 10 0

0

100

200

300

C (m

g/L)

0 10 20 30

EbbFlood

Wind (1)Wind (2)Wind (3)Wind (4)Wind (5)Wind (6)

Usig,b (cm/s)

Sandflat

Fig. 12. Wave-orbital speed at the bed Usig, b plotted against suspended-sediment concentration C for the windy and dry tidal cycles (Fig. 3) at station Sandflat in the CentralWaitemata Harbour, outside the mouth of Henderson Creek. Note that the horizontal axis in the left-hand panel descends, not ascends, from left to right.


Fig. 12 shows suspended-sediment concentration C plottedagainst wave-orbital speed at the bed Usig, b at the Sandflat sitewhen it was windy. The data are separated by flood and ebb,mimicking the way data are plotted in Fig. 10 (note that the hori-zontal axis in the left-hand panel of Fig. 12 descends, not ascends,from left to right). As in Fig. 10, there appears to be an asymmetricalresponse of C to current speed, as opposed to asymmetricalcurrents causing an asymmetrical variation in suspended-sedimentconcentration over the tidal cycle (note that in Fig. 12 the currentspeed being referred to is Usig, b, not U). Again as in Fig.10, C tends tobe smaller during the ebb tide compared to during the flood tide forthe same current speeds (i.e., Usig, b). As was also argued for Fig. 10,deposition of fine sediment inside Henderson Creek at high tidecausing the system to become effectively source-limited on theebbing tide may explain the asymmetrical response of C to Usig, b atSandflat.

Many studies have noted the link between wind waves andresuspension of bed sediments in estuaries (e.g., Pejrup, 1988;Shoelhamer,1995; de Jonge and van Beusekom,1995; Shi and Chen,1996; Talke and Stacey, 2008). There seems to be a consensus thatwaves drive a net loss of sediment from intertidal flats to offshore,which reverses a net accumulation of sediment on intertidal flatsduring calm weather (e.g., Chen et al., 2010). For example, Dyeret al. (2000) found that a net landward sediment flux duringcalmweather on mudflats in the Dollard estuary (Netherlands) wasreversed during windy weather when waves resuspended bedsediments. Similarly, Christie et al. (1999) found that waves causeda net seaward flux of sediment from mudflats in the Humberestuary (UK), reversing a net landward flux during fair weather. Incontrast, waves on the intertidal flats in the Central WaitemataHarbour initiate net landward transport of sediment from theintertidal flats into Henderson Creek, although seaward fluxes offthe flats into the interior of the wider estuary may also occur at thesame time.

6. Conclusions

A tidal creek that connects to the freshwater stream network ismore than a simple conduit of terrestrial sediment to the widerestuary beyond the base of the creek. At different times, dependingon the freshwater runoff and the wind, Henderson Creek imports,exports and deposits fine sediment sourced from both landward

and seaward. Sediment is also redistributed within the tidal creekon a regular basis by tidal currents.

During freshwater spates, saltwater was largely displaced fromHenderson Creek at low tide, and sediment sourced from the landwas deposited inside the tidal creek and exported beyond the creekto the wider estuary. During periods between spates, sediment wasreturned from the wider estuary by tidal currents (but not neces-sarily the same sediment that was exported during the spates).Sediment was also transported through and eroded from themiddle reaches of the tidal creek, and deposited in the upper rea-ches of the tidal creek. The up-estuary deposition can be explainedin Lagrangian terms as a type of settling lag. The return of sedimentto the tidal creek was greatly enhanced by wind waves thatresuspended sediments from the intertidal flats of the widerestuary, with that sediment being transported by tidal currents intothe tidal creek where it was deposited, largely in the middle rea-ches. That sediment will then be available for relatively slowerremoval on a regular basis to the upper reaches of the tidal creek bytidal currents. Ralston and Stacey (2007) arrived at a similarconclusion regarding the way tidal and episodic forcing cyclessediment between a mudflat and the region immediately offshoreof the mudflat.

The analysis of waves at the Sandflat site has demonstrated inbroad terms that there is a connection between wave resuspensionof intertidal-flat sediments in the main body of the Central Wai-temata Harbour and the transport of sediment into HendersonCreek tidal creek. To further resolve the transport processes andpathways between the intertidal flats of the wider estuary and thetidal creeks that drain into the wider estuary, data from more thanjust the one site (that is, more than just Sandflat, in this case) will berequired. This could be the subject of future work.

The long-term fate of the tidal creek and sediment that is erodedfrom the land will depend on the balance that is establishedbetween export of sediment during spates and return of sedimentbetween spates. Extrapolation from the Henderson Creekmeasurements or a model simulation that captures the kinds ofprocesses described herein could be used to construct a tidal-creeksediment budget that sheds light on how any such balance playsout in the long term.

It is clear that terrestrial sediments that are brought down byfreshwater spates can pass through tidal creeks and into the widerestuary, where they may cause ecological, biological and physical


effects of the sort described in the Introduction. In addition, wavescan be seen as “cleansing” the wider estuary of fine sediments,since they initiate net landward transport of sediment off theintertidal flats. Over time, it is likely that the balance of processeswill change as the tidal creek infills with sediment. Climate change,by changing sea level, wind climate and rainfall patterns, is alsolikely to have an effect over time on the functioning of tidal creeksand the fate of terrestrial sediments in the coastal marine area.

Acknowledgements

The work was funded by Auckland Council, NIWA and theMinistry for Science and Innovation (contract C01X1005, Manage-ment of Cumulative Effects of Stressors on Aquatic Ecosystems). Weare very grateful for the hardwork of the field team, which includedRod Budd, Scott Edhouse, Jennifer Gadd, Shane Grayling, Cliff Hart,Christian Hyde, Andy Miller, Rohan Wells and Aslan Wright-Stow.Special thanks to Judy-Ann Ansen, Matthew Davis and HaydenEaston (Auckland Council) for their support and interest in theproject. We thank two anonymous reviewers of the manuscript fortheir thoughtful and constructive comments.

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