turbidity maximum in the macrotidal, highly turbid humber

Estuarine, Coastal and Shelf Science 67 (2006) 30e52www.elsevier.com/locate/ecss

Turbidity maximum in the macrotidal, highly turbid Humber Estuary,UK: Flocs, fluid mud, stationary suspensions and tidal bores

R.J. Uncles*, J.A. Stephens, D.J. Law 1

Estuarine and Coastal Function and Health, Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, Devon PL1 3DH, UK

Received 23 October 2005; accepted 29 October 2005

Available online 6 January 2006

Abstract

The macrotidal Humber system, comprising estuaries of the Humber, Trent and Ouse, and their various tributaries, is one of the largest andmost turbid in the British Isles. This paper presents detailed spatial and temporal data on the estuarine turbidity maximum (ETM) within theOuse Estuary of the Humber system during quiescent summer conditions, when freshwater runoff was very low and approximately steady. Underthese conditions, an extremely turbid ETM existed in the low salinity reaches of the upper Humber, within the Trent and Ouse Estuaries. Lon-gitudinal surveys of salinity, temperature and turbidity were obtained at approximately local high water (HW) or low water (LW) between thetidal limit of the Ouse and the upper Humber. Tidal-cycle stations were worked between the upper Ouse and the coastal zone. In situ median flocsizes were measured at some stations. Tidal water levels were very asymmetric and currents were flood dominant in the upper estuary, especiallyat spring tides. Frictional drag on the currents was approximately balanced by water-level slope forcing, which led to a large reduction in tidalamplitude as the tide propagated into the estuary. A tidal bore, 0.1e0.2 m high, formed at spring tides in the upper estuary, but did not causesuspension of fine sediment at locations up-estuary of the ETM. Generally, salinity was fairly well mixed vertically, despite strong SPM strat-ification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions(w90 g l�1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g l�1, extended over a longitudinal distance of35 km at HW, both at spring and at neap tides. It was separated by nose and tail regions from much lower turbidity waters. The nose wasmuch sharper than the tail and was located 15 km into the tidal river at spring tides, where salinity was less than 1. Except at very smallneap tides, when fluid mud layers and stationary suspensions formed in the tail region of the ETM, maximum near-bed SPM concentrations(w50 g l�1) occurred close to the nose in the upper core region. The ETM was displaced down-estuary by ca. 12 km during the transitionfrom spring to neap tides. It also was displaced down-estuary between HW and LW. Floc settling led to pronounced SPM stratification overthe HW, HW-slack and early ebb period. Estimates of settling velocity, corrected for hindered settling, ranged from 1.2 to 2.1 mm s�1. AtHW slack, hindered settling prevented appreciable deposition of flocs to the bed in the main channel of the nose and upper core regions andat LW of spring tides there was little time for deposition. In the water column, in situ floc sizes maximised around mid-depth at HW slack.Therefore, slack water and low current shears led to increased flocculation, greater sizes and enhanced settling. These maximum median sizestypically were 300e500 mm, whereas sizes during other states of the tide were in the range 70e300 mm. A strong, negative correlation existedbetween depth-averaged median floc size and bulk vertical current shear in the water column for flocs that were greater than a station-dependentsize. The largest median floc sizes occurred within the near-bed stationary suspensions at neap tides, where floc sizes could exceed 1 mm. En-trainment of these flocs led to floc breakage, which reduced their median sizes to less than 200 mm.� 2006 Elsevier Ltd. All rights reserved.

Keywords: turbidity maximum; sediment transport; tidal bore; floc size; settling velocity; Humber Estuary, UK; Ouse Estuary, UK

* Corresponding author.

E-mail address: [email protected] (R.J. Uncles).1 Current address: Data Technology Ltd, Plymouth PL21 9GB, UK.

0272-7714/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2005.10.013

mailto:[email protected]

http://www.elsevier.com/locate/ecss

31R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

1. Introduction

The estuarine turbidity maximum (ETM) is a feature ofmany estuaries. It encompasses a huge range of suspended par-ticulate matter (SPM) concentrations (Uncles et al., 2002) thatvary from less than 0.1 g l�1, e.g. the Kennebec Estuary, USA,for which an ETM only occurs during moderate or low fresh-water flow conditions (Kistner and Pettigrew, 2001), to greaterthan 200 g l�1, e.g. the Severn Estuary, UK (Kirby and Parker,1983), where fluid mud layers and stationary suspensions oc-cur. These very turbid systems are dynamically complex anddifficult to model, largely because of the strong interactionsthat occur between their hydrodynamics and the high concen-tration suspensions within them, and the rheological behaviourof the suspensions themselves (e.g. Mehta, 1991; Winterwerp,1999; Dyer et al., 2004). The purpose of this paper is to pres-ent detailed spatial and temporal data on an ETM within theupper Humber Estuary, UK, during conditions for whichSPM concentrations can exceed 90 g l�1.

The ETM is a very strong feature of the Humber, Ouse andTrent estuarine systems (Fig. 1). More than an order of mag-nitude increase in SPM concentrations can occur betweenthe Humber and the upper reaches of the Ouse and Trent Es-tuaries (Mitchell et al., 1998, 2003a,b; Uncles et al., 1998a,b,c,1999; Mitchell, 2005). These strong horizontal gradients havebeen observed using airborne remote sensing and comparedwith simultaneous sea-truth measurements (Uncles et al.,2001). The ETM is not a static feature and SPM concentra-tions, observed over seasonal and annual time-scales, consis-tently exhibit pronounced seasonal variability in the lowsalinity, upper reaches of the system (Mitchell et al., 1998,2003a; Uncles et al., 1998a, in press-a). Practical consequencesfrom this variability arise throughout the Humber system, suchas channel navigability and dredging requirements (Townendand Whitehead, 2003). Pontee et al. (2004) demonstrated botha relationship between siltation rates in the lower Humberand freshwater runoff and the influence of the ETM on silta-tion and estuarine channel-switching.

UW

N

Spurn HeadG10

G13

G24

G23

Kingston upon Hull

Immingham

BoothferryBridge

Bridge

10km

Keadby

Fig. 1. The Humber Estuary, showing the confluence of the Humber, Ouse and Trent at the Apex in the Humber’s upper reaches. Naburn is the tidal limit of the

Ouse Estuary and Naburn, Cawood, Selby, Drax, BTJ and UW refer to stations that were worked in the upper Humber and Ouse, some of which are referred to in

Fig. 2. Other stations referred to in Fig. 2 are G23 (between Spurn Head and Immingham) and G10, G13 and G24 in the Humber’s coastal zone, just seaward of

Spurn Head at the mouth of the Humber.

32 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

SPM concentrations in the inflowing freshwater at the tidallimit of the Ouse (Naburn Weir, Fig. 1) are much less thanthose in the ETM. For example, freshwater concentrations dur-ing 1994e1996 were <0.3 g l�1 compared with >10 g l�1

within the ETM (Uncles and Stephens, 1999). Measurementsin the upper Humber and Ouse during late spring to early sum-mer of 1994 showed that tidal advection of SPM generally wasthe dominant SPM flux mechanism and that pronouncedfloodeebb asymmetry in the tidal currents was reflected inthese fluxes (Uncles and Stephens, 1999). In addition, the tid-ally averaged, up-estuary transport of sediment during springtides was equivalent to about three months of average SPM in-flows at Naburn Weir. This strong, tidally averaged, up-estuaryflux of SPM at spring tides in the late spring to early summerperiod of 1994 indicated the potential for an accumulation oflarge amounts of fine sediment and an exceptionally turbidETM in the upper estuary during low runoff, summer condi-tions. The following summer of 1995 was a period of pro-longed, very low freshwater inflows and therefore providedideal conditions for the study of a highly turbid ETM. This pa-per presents the results of observations of the ETM at thattime, focusing on the upper Humber and Ouse.

2. Background

2.1. Geographical setting

The total catchment area of the Humber Estuary system isroughly 26 000 km2, which is approximately 20% of the areaof England. It comprises the Ouse, Trent and Humber and theirvarious tributaries, and is the largest estuarine system in theBritish Isles. The combined freshwater inflows have a temporalaverage of ca. 250 m3 s�1. The confluence of the tidal Ouse,Trent and Humber is located at Trent Falls (the Apex,Fig. 1) which is approximately 60 km up-channel from wherethe Humber meets the North Sea at the sand spit of SpurnHead (Fig. 1).

Tidal ranges referred to in this paper are predicted rangesfrom Immingham (inset in Fig. 1), which has mean springand mean neap tidal ranges of 6.4 and 3.2 m, respectively,and a mean tidal range of 4.8 m (ATT, 2005). The Ouse is tidalto Naburn Weir, located ca. 62 km up-channel from the Apex.Mean spring and neap tidal ranges near the Apex at Blacktoft,station BTJ, are estimated to be 5.9 and 3.6 m (ATT, 2005),whereas corresponding tidal ranges at Naburn Weir are re-ported to be less than half of these values (RMBC, 1986).High water (HW) at the Apex is about 1 h after HW at Im-mingham (ATT, 2005). At Selby and Naburn, HW is reportedto be more than ca. 2 h and 3.5 h after HW at Immingham, re-spectively (RMBC, 1986). Although these estimates arestrongly dependent on the springeneap tidal state and otherenvironmental factors, they at least demonstrate the long prop-agation times and damping of the tide through the upper Hum-ber and Ouse. A tidal bore can develop within the Ouse atspring tides (the ‘aegir’, RMBC, 1986), when it signals lowwater (LW) and the start of the flood.

2.2. The HumbereOuse ETM

Mitchell (2005) presented data on SPM concentrations inthe Ouse that had been measured at a fixed height above thebed during July to December 1997 (at Drax, Fig. 1). Similarmeasurements were made in the Trent during May 1997 toFebruary 1998 (Mitchell et al., 2003a). These observations il-lustrate both the very high concentrations and the pronouncedseasonal and tidal variability that can occur in the upper Ouseand Trent. A summary of some tidal-cycle measurementsmade by us at stations between the coastal zone of the Humberand the tidal limit of the Ouse during low runoff, summer con-ditions of 1995 and 1996 puts Mitchell’s (2005) data into anestuary-wide context. Our data demonstrate the very sharpand very large increase in depth-averaged SPM concentrationthat occurs with decreasing depth-averaged salinity, progress-ing from the North Sea to the upper Humber and Ouse(Fig. 2A, B). SPM concentrations in the coastal zone and tidalriver typically were <10 mg l�1, whereas they peaked at ca.30 g l�1 in the upper Ouse when salinity was ca. 1.

2.3. Nature of the SPM and sediment

SPM within the ETM consists largely of fine sediment(silt and clay) that exists as microfloc and macrofloc aggre-gates and individual, primary particles (Uncles et al., 1998b,in press-b). At both spring and neap tides, primary sedimentparticles are very fine-grained and at HW comprise ca. 20e30% clay-sized platelets that are dominated by chlorite andillite clay mineralogy. The specific surface area, SSA, ofSPM (Holtz and Kovacs, 1981) within the ETM typicallyis 24 m2 g�1. The loss on ignition (LOI) of SPM in themost turbid reaches of the estuary is ca. 10%, which indi-cates that the ETM largely comprises inorganic (mineral)fine sediment, consistent with direct measurements of partic-ulate organic carbon content (Uncles et al., 2000). Duringlow freshwater inflow, summer conditions there is a pro-nounced increase in the size of primary particles progressingup-estuary from the ETM into the tidal river, as well asa marked increase of LOI in the near-surface SPM, whichreflects less clay and a greater amount of organic matter as-sociated with SPM in the low-turbidity waters there(Fig. 2B).

The combined silt and clay percentage of total surficialbed-sediment mass along the main channel of the upper Hum-ber and Ouse varies strongly as a function of longitudinal po-sition (Uncles et al., 1998c). During low freshwater inflowconditions, the relatively small silt and clay percentage ofmain-channel, surficial bed-sediment mass within the mostturbid region of the ETM (ca. 10e35%) is in marked contrastto the overlying SPM, which largely comprises silt and clay.This illustrates that the sediment particle ‘mix’ within theETM is distinct from that within the underlying bed, whichalso is reflected in the low SSA of surficial bed sediment.Therefore, in terms of particle size distribution and organiccontent, SPM within the ETM during low freshwater inflow,summer conditions is distinct both from that further up-estuary


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(g l-1

)

NaburnCawoodSelby

Salinity

Fig. 2. Tidal-cycle data of depth-averaged SPM concentrations plotted against depth-averaged salinity at stations throughout the Ouse, Humber and the Humber

coastal zone that demonstrate the existence of a strong ETM at salinities less than approximately 3 (A). Apart from data at Drax, measured on 31 July 1996, all

these plotted data were measured during June through August 1995. Strong, non-linear variations of SPM with salinity occur at all but the very low salinity stations

and in the coastal zone (32e34 salinity), which demonstrate erosion and deposition of sediment. Despite very high SPM concentrations there is, approximately,

conservative mixing between fine sediment suspended in fluvial ‘fresh’ waters and that suspended in much more turbid, low salinity estuarine waters within the

ETM nose and upper core regions during August 1995 (B).

in the low-turbidity tidal river and in the underlying bedsediment.

3. Methods

A fast, inflatable boat was used to survey 19 stations alongthe main channel of the Upper Humber and Ouse between thetidal limit at Naburn Weir (0 km) and station UW in the upperHumber, 67 km down-estuary from the tidal limit and 5 kmdown-estuary from the Apex (Fig. 1). The boat was broughtto each station and then allowed to drift with the current whilevertical profiles of salinity, temperature and turbidity were ob-tained, starting at 0.1 m above the bed and continuing to thesurface. Survey measurements were made at either approxi-mately local HW or local LW. This was feasible because ofthe long tidal propagation times within the Ouse. Currentswere still flooding at local HW and ebbing at local LW.Pumped samples were obtained from surface and near-bed atmost stations in order to determine SPM concentrations andto calibrate turbidity meters, as well as for subsequent analysisof SPM properties. The methods used to determine SPM con-centrations and properties have previously been described(Uncles et al., in press-b).

Tidal-cycle stations (shown in Fig. 1) also were worked atCawood (from a bridge), Selby, Drax and station UW (froma boat at anchor) and station BTJ (using an extended-reachcrane from the jetty). Measurements included vertical profilingfor salinity, temperature, SPM concentrations and currents, us-ing direct-reading current meters. Anchored ships were used towork tidal-cycle stations further down-estuary and in thecoastal zone (stations G10, G13, G23 and G24 in Fig. 1). Atsome of the tidal-cycle stations, in situ floc-size distributionsof SPM were measured using a Lasentec P-100 laser-reflec-tance particle size instrument (Law et al., 1997; Law, 1998).Self-recording instrument packages that measured water level,

salinity and turbidity were moored in the upper reaches inorder to complement some of the short-term tidal-cycle data.

4. Results

4.1. Runoff into the Ouse

The daily-averaged rate of freshwater input (runoff) to theOuse across its tidal limit at Naburn Weir, which typically isca. 40% of the total inflow to the Ouse, generally decreasedfrom the beginning of April until early September 1995.Drought conditions occurred throughout the summer, fromthe beginning of June 1995 to the beginning of September1995, when runoff at Naburn decreased from approximately17 to 4 m3 s�1. These freshwater flows were much smallerthan the mean and maximum of 47 and 475 m3 s�1 over thethree-year period, 1994e1996. The maximum flow occurredduring the winter of late 1994 to early 1995.

The data presented here were obtained largely during 10e27 August 1995, when daily-averaged runoff at Naburn wasapproximately steady and very low, with a mean and standarddeviation of 4.1� 0.2 m3 s�1. Other measurements presentedhere also were made under very low runoff conditions:6 m3 s�1 on 24 September 1995 and 7 m3 s�1 on 17 July 1995.

4.2. ETM behaviour: longitudinal transects

Longitudinal and vertical measurements of salinity and tur-bidity, made at approximately local HW of spring and neaptides during August 1995, showed a pronounced ETM in theupper Humber and Ouse (Fig. 3). For definiteness, the main‘core’ of the ETM is defined arbitrarily as the longitudinal re-gion where near-bed SPM concentrations exceed 16 g l�1 andthe ‘nose’ and ‘tail’ of the ETM as the longitudinal regions up-estuary and down-estuary of the core, respectively, betweenthe near-bed 4 and 16 g l�1 SPM concentrations.


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11 Aug 95, Springs HW (7.5 m), SPM, g/l

11 Aug 95, Springs HW (7.5 m), Salinity

18 Aug 95, Neaps HW (6.1 m), Salinity

18 Aug 95, Neaps HW (6.1 m), SPM, g/l

(A)

(B)

(C)

(D)

Fig. 3. SPM and salinity distributions from longitudinal and vertical surveys of the upper Humber and Ouse, undertaken between station UW and the tidal limit at

Naburn Weir during approximately local HW of a spring tide (11 August 1995) and a neap tide (18 August 1995). The near-bed SPM concentration maxima are

highlighted. Spring-tide data are shown for SPM (A) and salinity (B). The tidal range at Immingham was 6.6 m. Neap-tide data are shown for SPM (C) and salinity

(D). The tidal range at Immingham was 3.9 m. The ETM nose was much sharper than the tail and the SPM stratification was much greater at the neap tide (A, C).

Salinity was fairly well mixed, but with some indication of stratification due to tidal straining (B, D).

The core of the ETM extended over a longitudinal distanceof 35 km (Fig. 3A, C). The spring-tide core was located be-tween 3 and 38 km from the tidal limit (Fig. 3A) and theneap-tide core 12 km further down estuary (Fig. 3C). This dis-placement also was reflected in the locations of the maximum,near-bed spring and neap-tide SPM concentrations. These hadmagnitudes of 47 and 45 g l�1 and were positioned 12 and22 km from the tidal limit, respectively. The neap-tide stratifi-cation of SPM in the ETM core was greater than that for thespring tide. For both the spring and neap tide the nose wasmuch sharper than the tail and extended over a horizontal dis-tance of less than 3 km, compared with 12 km. SPM concen-trations fell sharply progressing down-estuary of the ETMtail, approximately exponentially as a function of increasingsalinity, and were less than ca. 10 mg l�1 in the coastalzone, where salinity was ca. 34 (illustrated for low runoff,summer conditions in Fig. 2A). SPM concentrations alsorapidly decreased, approximately linearly with respect to

decreasing salinity, up-estuary of the maximum SPM concen-tration (Fig. 2B).

Despite the strong SPM stratification, salinity was fairlywell mixed vertically both for the spring and neap-tide surveys(Fig. 3B, D). Slight inverse salinity stratification occurred atsome stations, but was stabilized by density increases due tolarge SPM loads. Salinity distributions were very similar.The salinity-20 isohaline was located in the upper Humber,close to the Apex, and the 1 and 5 isohalines were located ap-proximately 20 and 38 km from the tidal limit, near Selby andBFB, respectively (Fig. 1). The core of the spring-tide ETMwas located up-estuary of the 5 isohaline, whereas it was locatedup-estuary of the 15 isohaline during the neap tide. The max-imum SPM concentration was located 5 km up-estuary of the1 isohaline at the spring tide and 2 km down-estuary at theneap tide. The ETM nose was located 15 km up-estuary ofthe 1 isohaline at the spring tide and 5 km up-estuary at theneap tide.


A pronounced ETM also was a feature of the SPM distribu-tion measured at approximately local LWof a neap-tide survey(the day preceding the HW neap-tide survey shown in Fig. 3C,D). The ETM core extended over a longitudinal distance of42 km and was located between 24 and 66 km from the tidallimit (Fig. 4A). The maximum near-bed SPM concentrationwas 58 g l�1, located 32 km from the tidal limit. Therefore,the ETM nose and tail were located 9 and 16 km, respectively,further down-estuary at LW compared with the HW distribu-tion of the following day (Fig. 3C) and the maximum SPMconcentration was located 10 km further down-estuary. TheLW isohalines were located approximately 15 km furtherdown-estuary than at HW (Fig. 4B).

The down-estuary displacements of the ETM nose andmaximum SPM concentration between HW and LW thereforewere substantially less than those of the isohalines, whichwere the same as the displacement of the ETM tail. Thisspreading of the ETM core was accompanied by strong SPMstratification and the formation of a very high concentration,near-bed layer in the vicinity of the upper core region of high-est SPM concentrations (Fig. 4A).

A more detailed view of the intratidal movements of theETM was obtained by making measurements throughout the

flood of a small neap tide (the day following the HW neap-tide survey shown in Fig. 3C, D). The duration of the floodtide in this part of the estuary is short compared with theebb, only 2e3 h compared with approximately 10 h (see later),and insufficient time was available to obtain both good tempo-ral and spatial resolution. A net up-estuary displacement of theETM occurred between local LWand HWþ 1.9 h (Fig. 5AeE).Although the ETM nose was not well resolved for the transectsat LW and LWþ 1.4 h, it is apparent that the nose moved up-estuary a minimum of 4 km and a maximum of 9 km betweenLW and HWþ 1.9 h, whilst the 1 isohaline contour (locationindicated by the vertical arrow in Fig. 5) moved up-estuarya distance of 12 km. Greatest stratification again occurredat LW and following HW slack, during the early ebb(HWþ 1.9 h), when the ETM nose became very sharp (alsosee Figs. 3C, 4A).

Measurements throughout the ebb portion of a spring tidewere made during September 1995, when runoff across the tidallimit was again very low (6 m3 s�1). Greatest observed SPMstratification and up-estuary intrusion of the ETM nose oc-curred following local HW on the early ebb, at HWþ 1 h,when the nose exhibited a very sharp longitudinal gradientin SPM (Fig. 6A). Lowering water levels during the later

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(A) 17 Aug 95, Neaps LW (1.8 m), SPM, g/l

32

32

16 16

SPM, g/l

(B) 17 Aug 95, Neaps LW (1.8 m), Salinity

Fig. 4. SPM and salinity distributions from a longitudinal and vertical survey of the upper Humber and Ouse, undertaken between station UW and the tidal limit at

Naburn Weir during approximately local LW of a large neap tide (17 August 1995). The near-bed SPM concentration maximum is highlighted. Data are shown for

SPM (A) and salinity (B). The tidal range at Immingham was 4.6 m. SPM stratification is strong throughout the estuary, but especially in the ETM nose region (A)

whilst salinity is well mixed (B).


15 20 25 30 35 40

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19 Aug 95, Neap Tide, HW (5.7 m), LW (2.7 m)Depth-Averaged SPMNear-Bed SPM

LW + 1.4 h

HW - 1 h

HW + 0.3 h

HW + 1.9 h

Fig. 5. Near-bed (B) and depth-averaged (-) SPM distributions from a longitudinal and vertical survey of the upper Humber and Ouse, which was undertaken

between BFB and mid-way between Selby and Cawood during a mainly flooding neap tide (19 August 1995). The tidal range at Immingham was 3.2 m. The

location of the near-bed salinity-1 isohaline is denoted by a vertical, downward-pointing arrow, if it lies within the surveyed reach. Distributions are shown for

times that are relative to local HW or LW at the nose and correspond to LW (A); LWþ 1.4 h (B); HW� 1 h (C); HWþ 0.3 h (D); and HWþ 1.9 h (E).

ebb were associated with comparable depth-averaged andnear-bed SPM concentrations, together with down-estuarymovement of the ETM nose and core between HWþ 4.4 hand LW� 2.2 h (Fig. 6BeD). The ETM core moved down-es-tuary 11 km between HWþ 1 h and LW� 2.2 h, whilst the 1isohaline contour moved down-estuary a distance of 13 km(Fig. 6A, D).

4.3. ETM behaviour: tidal cycles

4.3.1. The ETM nose regionProfiling observations were made during a spring tide at

Cawood (Fig. 1), 8 km from the tidal limit and in the up-estu-ary region of the ETM (Fig. 7). The tide was slightly smallerthan that during the spring-tide longitudinal transect (Fig. 3A,B). The salinity maximised at ca. 0.6, which corresponded toHW slack at approximately 0.5 h after HW (Fig. 7B, C).A rapid rise in water level followed the up-estuary passageof a small tidal bore at LW (Fig. 7D), whereas the ebb wasassociated with a prolonged and gradual fall in water levelfollowing HW. The tidal range was 2.0 m compared with6.2 m at Immingham (i.e. 32% of the Humber mouth value)and LW and HW lagged Immingham by 7.4 h and 3.4 h, re-spectively. The tidal rise, LW to HW, required 1.8 h comparedwith 10.6 h for the tidal fall.

SPM concentrations increased over the 1.8-h period of theflooding tide and reached a near-bed maximum immediatelyfollowing HW slack (Fig. 7A). The nose of the ETM reachedthe site ca. 0.8 h before HW and withdrew ca. 2.6 h after HW,consistent with both the location of the station, 8 km from thetidal limit, and the relative position of the ETM (Fig. 3A). Thecore of the ETM reached the site immediately prior to HWslack and withdrew ca. 0.9 h later. Thereafter, SPM concentra-tions decreased over the observed 6 h period of ebb currents.In addition to advection of SPM in the ETM, SPM also settledthrough the water column over the HW, HW-slack and earlyebb period, which led to a reduction in SPM concentrationsnear the surface and an increase nearer to the bed (Fig. 7A).This increased stratification was consistent with longitudinal-transect observations of the ETM nose region close to HWand on the early ebb (Figs. 3A, C, 5E, 6A). Current speedswere strongly asymmetric. Fastest flood currents were1.9 m s�1, compared with 1.1 m s�1 on the ebb (Fig. 7C).Faster currents and greater near-bed shears on the flood(Fig. 7C) were associated with much less SPM stratificationcompared with the ebb (Fig. 7A).

The tidal bore that signaled LW and the end of the ebb atCawood had a height of 0.13 m and a rise time of 9 s. Thecrest of the bore was followed by an oscillation of ca.0.04 m range and 9 s duration, which was followed thereafter


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HW + 1 h

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LW - 3.0 h

HW + 4.4 h

24 Sep 95, Spring Tide, HW (7.1 m), LW (1.1 m)

Depth-Averaged SPM

Near-Bed SPM

Fig. 6. Near-bed (B) and depth-averaged (-) SPM distributions from a longitudinal and vertical survey of the upper Humber and Ouse, which was undertaken

between BTJ and Cawood during an ebbing spring tide (24 September 1995). The tidal range at Immingham was 6.2 m. The location of the near-bed salinity-1

isohaline is denoted by a vertical, downward-pointing arrow, if it lies within the surveyed reach. Distributions are shown for times that are relative to local HW or

LW at the nose and correspond to HWþ 1 h (A); HWþ 4.4 h (B); LW� 3 h (C); and LW� 2.2 h (D).

by smaller oscillations as longer-term water level steadilyrose (Fig. 7D). The consequence of the bore’s passage onSPM concentrations was slight. SPM concentrations,measured at a fixed height above the bed, were relativelylow (<0.3 g l�1) and SPM variations over the 160-s ‘event’period (plotted in Fig. 7D) were strongly, negatively correlatedwith temperature variations (mean: 20.26 �C; SD: 0.01 �C).

4.3.2. The ETM nose and upper core regionsProfiling observations were made during a small spring tide

at Selby (Fig. 1), 20 km from the tidal limit and in the centralor upper core region of the ETM at HW, dependent on tidalrange (Fig. 3A, C). The tide was intermediate between thosepertaining to the spring and neap-tide longitudinal surveys(Fig. 3). Salinity remained low throughout the tide and maxi-mised at ca. 1 around HW slack (Fig. 8B). The rapid rise inwater level again followed the up-estuary passage of a smalltidal bore at LW. Tidal range was 2.9 m compared with4.9 m at Immingham (i.e. 59% of the mouth value) and LWand HW lagged Immingham by 6.6 h and 3.0 h, respectively.The tidal rise time, LWeHW, was 2.4 h compared with10.1 h for the tidal fall.

SPM concentrations increased over the 2.4-h duration ofthe flooding tide and reached an observed near-bed maximumof 66 g l�1, 0.5 h after HW slack (Fig. 8A). The ETM nosereached the station ca. 2 h before HW and withdrew ca. 8 hafter HW. The core of the ETM reached the station ca. 0.5 h

after the nose and withdrew ca. 2 h before the nose. SPM con-centrations continued to decrease on the ebb until LW. SPMwas strongly mixed through the water column on the floodand partially settled-out over the HW, HW-slack and earlyebb period, which led to enhanced stratification and greaternear-bed SPM concentrations (also observed in the longitudi-nal transects, Figs. 5E and 6A). Current speeds were asymmet-ric, with fastest flood and ebb current speeds of 1.35 and1.19 m s�1, respectively (Fig. 8C), and fastest depth-averagedflood and ebb speeds of 1.2 and 0.9 m s�1, respectively.

The tidal bore had a height of 0.18 m and a rise time of 16 s(Fig. 8D). The bore crest was followed by small oscillations inwater level, which were less than 0.04 m in range and had a typ-ical duration of 6 s. The consequence of the bore’s passage onSPM concentrations was again slight. SPM concentrationswere relatively low (<1.7 g l�1) and variations over the 160-s‘event’ period (Fig. 8D) were strongly, positively correlatedwith temperature variations (mean: 20.118 �C; SD: 0.002 �C).

4.3.3. The ETM tail and lower core regionA spring tide was worked at station BTJ (Fig. 1), 59 km

from the tidal limit and down-estuary of the ETM tail atHW (Fig. 3A). The tide was slightly smaller than that observedduring the spring-tide longitudinal transect (Fig. 3A). The sa-linity reached a maximum of 17.6 at HW slack and a minimumof 4 at LW (Fig. 9B). The tidal range was 6.4 m, the same asImmingham, and LW and HW lagged Immingham by 3.2 h


10 12 14 16 18 20Time in Day (hours)

0

1

2

3

10 12 14 16 18 200

1

2

3

10 12 14 16 18 200

1

2

3

Depth

SPM

15 Aug. 95, Springs, HW (7.3 m), SPM, g/l 15 Aug. 95, Springs, Salinity

15 Aug. 95, Springs, Speed, m/s 15 Aug. 95, Springs, Bore

0 20 40 60 80 100 120 140 160Time, Seconds

1.5

1.6

1.7

1.8

SPM

g/l

0.14

0.20

0.26

0.32

(A) (B)

(C) (D)

16

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Time in Day (hours)

Sens

or D

epth

(m)

Hei

ght A

bove

Bed

(m)

Time in Day (hours)

0.8

0.4

0.8

0.5

0.45

0.550.55

0.45

12

2

4

4

2

0.5

0.4

8

0.4

0.4

1.2

1.6

Fig. 7. A spring-tide tidal cycle of salinity, tidal current speed and SPM data measured at Cawood in the upper Ouse on 15 August 1995. The tidal range at Im-

mingham was 6.2 m. Maximum SPM occurred around HW, which illustrates that the ETM was located down-estuary (A); salinity was very low (<0.6) and maxi-

mised at HW slack (B); current speeds were flood dominant (C); a tidal bore occurred in the upper estuary at LW, leading to a rapid rise in water level (heavy line in

(D)) but with no pronounced increase in SPM concentrations (light line in (D)).

and 1.7 h, respectively. The tidal rise time, LWeHW, was4.6 h compared with 8.1 h for the tidal fall.

In contrast to the ETM nose region, SPM concentrations in-creased during the ebb and reached a near-bed maximum of39 g l�1 at 0.5 h after LW, during the very early flood(Fig. 9A). The ETM tail and core reached the station on theebb between 2.6 h and 0.7 h before LW, respectively, andmoved back up-estuary on the flood between 0.8 h and 2.2 hafter LW. This tidal displacement of the ETM also was illus-trated by the inversion of SPM concentrations through the wa-ter column at mid ebb, in response to tidal ‘straining’ of thelongitudinal gradient in SPM. The resultant decrease of SPMconcentration with depth, with its associated density inversion,was stabilized by increased salinity deeper in the water col-umn (Fig. 9B).

Current speeds were asymmetric. The fastest flood currentwas ca. 1.3 m s�1, compared with 1.1 m s�1 on the ebb(Fig. 9C). Settling of SPM occurred in the slow currentsnear LW, when depth-averaged speeds of less than 0.2 m s�1

persisted for ca. 0.5 h, which resulted in lower near-surfaceSPM concentrations (Fig. 9A). The down-estuary displace-ment of the ETM between HW and LW, and the enhancedstratification of SPM at LW due to settling, were consistentwith the observed longitudinal distributions of SPM at HWand LW (Figs. 3C, 4A, 5A).

The temporal resolution of the profiling and moored instru-mentation was too coarse to resolve the transition from ebb toflood at LW, although at spring tides the transition was againvery sharp (illustrated for a similar spring tide in Fig. 9D).The displacement of the ETM over the LW period and the ef-fects of settling and vertical mixing for this tide were evidentfrom SPM concentrations recorded in the surface layers ata fixed height above the bed (Fig. 9D).

Neap-tide observations at station BTJ illustrated very highconcentrations of near-bed SPM in the ETM tail region at, andfollowing, LW (Fig. 10A). The neap tide was very small, 2.9 mtidal range at Immingham, compared with 3.9 and 4.6 m dur-ing the HW and LW neap-tide longitudinal transects, respec-tively (Figs. 3C, D, 4). Salinity reached a maximum of 14 atHW slack and a minimum of 5 at LW slack (Fig. 10B). Thevertical salinity gradient was strongly inverted during the pe-riod from approximately mid flood to mid ebb. The tidal rangeat station BTJ was 3.2 m (i.e. 110% of the mouth value) andLW and HW lagged Immingham by 3.0 h and 1.4 h, respec-tively. The tidal rise time, LWeHW, was 5.1 h comparedwith 7.6 h for the tidal fall.

SPM concentrations increased during the ebb, especiallyclose to the bed (Fig. 10A). The ETM core reached the stationca. 2.3 h before LW and SPM concentrations continued to in-crease rapidly until LW slack. Longitudinal advection and


10 12 14 16 18 20Hours

0

1

2

3

4

10 12 14 16 18 20Hours

0

1

2

3

4

Hei

ght A

bove

Bed

(m)

10 12 14 16 18 20Hours

0

1

2

3

424 Aug. 95, Spring Tide, Salinity(A) (B)

(C)

0 20 40 60 80 100 120 140 160Time, Seconds

0.3

0.4

0.5

0.6

0.7

Sens

or D

epth

(m)

SPM

g/l

24 Aug. 95, Spring Tide, Bore

SPMDepth

1.7

1.6

1.5

1.4

1.3

(D)

32

16

32

16

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

24 Aug. 95, Spring, HW (6.6 m), SPM, g/l

24 Aug. 95, Spring Tide, Speed, m/s

0.9

0.70

.5

0.7

0.9

0.4

0.8

1.2

1.2

0.8

0.8

0.8

0.4

2

4

8

8

4

Fig. 8. A small spring-tide tidal cycle of salinity, tidal current speed and SPM data measured at Selby in the upper Ouse on 24 August 1995. The tidal range at

Immingham was 4.9 m. Maximum SPM occurred around HWand HW slack, which illustrates that the bulk of the ETM was located down-estuary (A); salinity was

very low (<1) and maximised at HW slack (B); current speeds were flood dominant (C); and a tidal bore occurred, leading to a rapid rise in water level (heavy line

in (D)) but with no pronounced increase in SPM concentrations (light line in (D)).

settling of SPM then led to the formation of a near-bed, highconcentration layer of sediment, in which concentrations in-creased to 92 g l�1 on the flooding tide, 2.4 h after LW(Fig. 10A). Current speeds within this layer were negligible(Fig. 10C), although some entrainment of sediment occurredfrom the layer surface, especially during the fastest currentspeeds at mid flood (14:30 hours, Fig. 10A, C). These dataare qualitatively consistent with the LW neap-tide transect,which exhibited high concentration layers in the station BTJregion of the upper Humber and lower Ouse, with SPM con-centrations greater than 30 g l�1 (Fig. 4A). The LW transectsurvey (Fig. 4) had a tidal range of 4.6 m, substantially greaterthan the 2.9 m (Immingham) tide profiled at station BTJ(Fig. 10). In addition, the station was worked on a tide that fol-lowed several days of small neap tides, which also would haveassisted the formation of these muddy layers.

The salinity inversion (Fig. 10B) was such that at, e.g.18:15 hours, salinity decreased from 12.8 to 8.3 between 7and 8 m beneath the surface, which in isolation would haveproduced an unstable density difference of <4 g l�1 over thissection of the lower water column (a density conversion is givenlater, in Eq. (5)). However, SPM concentration increased from0.5 to 16 g l�1 over this depth (Fig. 10A), i.e. a stable densitydifference of greater than 9 g l�1, so that the lower water col-umn was overall stable.

A tidal bore did not occur at LW during this small neap tide(Fig. 10D) and the transition from ebb to flood was smooth.Near-surface SPM concentrations showed a decreasing trendtoward LW, due to floc settling, with a minimum that coincidedwith minimum salinity at slack water, 0.4 h after LW.

4.4. Floc sizes in the ETM

In situ floc sizes were measured during the spring tide atSelby (Fig. 1) in the upper core region of the ETM at HW(Figs. 3A, 8A). Profiling observations showed that median di-ameters exceeded 110 mm but were less than 310 mm(Fig. 11A). Median floc size was 19 mm in the fluvial watersimmediately up-river of the tidal limit during this period(Law, 1998). Larger floc sizes at Selby generally were associ-ated with higher water levels. However, depth-averaged sizesin excess of 120 mm also were negatively correlated withbulk indicators of tidal energy dissipation (U3=h, where h isdepth and U is depth-averaged current speed) and, morestrongly, current shear (U=h). For example, when bulk shearwas less than 1 s�1, which corresponded to sizes greaterthan 120 mm, then median floc size increased approximatelylinearly with decreasing shear.

The largest flocs occurred near mid-depth at HW slack andon the early ebb (up to 310 mm, Fig. 11A) and were associated


8 10 12 14 16 18 20

Hours

0

5

10

8 10 12 14 16 18 20

Hours

0

5

1010 Aug. 95, Springs, HW (7.2 m), SPM ,g/l 10 Aug. 95, Springs, Salinity

8 10 12 14 16 18 20Hours

0

5

1010 Aug. 95, Springs, Speed, m/s

(A) (B)

(C)

0 100 200 300Time, Minutes

0

2

4

6

0

2

4

6

SPM

, g/l

Depth

SPM

27 Aug. 95, Springs, HW (7.3 m), LW(D)

16

16

16

0.5

2

1

4

8

0.5

4 2

10.4

0.8

1.2

0.8

0.4

0.4

0.8

14

12

10

8

9

9

10

14

12

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Sens

or D

epth

(m)

Fig. 9. A spring-tide tidal cycle of salinity, tidal current speed and SPM data measured at station BTJ in the lower Ouse on 10 August 1995. The tidal range at

Immingham was 6.4 m. Maximum SPM occurred around LWand LW slack, which illustrates that the bulk of the ETM was located up-estuary (A); salinity was >4

and maximised at HW slack (B); current speeds were flood dominant (C); and the transition from ebb to flood was abrupt, leading to a rapid rise in water level

(heavy line in (D)) although some floc settling occurred over the brief LW-slack period (light line in (D)). Flocs were vertically mixed on the flood, which led to an

increase in SPM followed by a decrease as the ETM moved up-estuary of the station.

with floc settling and reduced SPM concentrations in the sur-face layers (between ca. 09:00 and 11:00 hours in Fig. 11A,C). The period from HW through to the early ebb also wasa time of very low current shears in the upper water column(Fig. 8C). A spring tide was worked at this site in September,when the ETM was somewhat further down-estuary (notshown). The data displayed very similar floc-size behaviourbut with sizes between 170 and 360 mm. When depth-averagedsizes were greater than 180 mm there again was a negative cor-relation between size and both bulk energy dissipation andbulk shear, in this case for shear less than 0.8 s�1.

At station BTJ in the lower Ouse, the tail and core of theETM water column had a wide range of median floc diametersduring the small neap tide, from less than 100 to more than300 mm (Fig. 11B, D). Maximum median sizes exceeded1 mm in the stagnant, high concentration layer that remainednear the bed at station BTJ throughout much of the tide (after12:00 hours, Fig. 11B, D). Local current shear was high nearthe top of the layer, which led to strong entrainment at peakflood currents (ca. 14:30 hours, Figs. 10C and 11D) and a min-ima in floc size just above the entrained layer, indicative ofshear-induced floc breakage (Fig. 11B).

The observed floc sizes at station BTJ during a spring tide inSeptember 1995 (not shown) were very different from those at

the neap tide (Fig. 11B). The ETM was located somewhat fur-ther down-estuary during September so that SPM concentra-tion contours were very similar to, but greater than, those forthe August spring tide at station BTJ (Fig. 9A). Floc sizeswere vertically homogeneous for most of the tide but reacheda maximum (ca. 500 mm) near mid-depth at HW slack. Thiswas associated with decreasing SPM concentrations in the sur-face layers, due to floc settling, similar to the spring tide atSelby over the HW-slack period (Fig. 11A, C). Sizes decreasedduring the ebb, reaching ca. 100 mm at LW slack and becamesmaller (ca. 70 mm) on the faster flooding currents before in-creasing again toward HW. There were strong negative corre-lations between depth-averaged, median floc size andindicators both of bulk energy dissipation and bulk currentshear throughout the tidal cycle and for the whole range of ob-served median floc size.

A transect through part of the ETM core was undertakenclose to HW conditions during a spring tide in July 1995.The transect comprised five profiling stations progressingdown-estuary between Selby and BFB (Fig. 1) and startedca. 1 h before local HW at Selby and ended ca. 2 h after localHW at BFB. The transect showed the settling of SPM fromnear-surface waters following HW slack in the lower coreregion of the ETM (Fig. 12A) and the location of the largest


10 12 14 16 18 20Hours

0

5

10

10 12 14 16 18 20Hours

0

5

10

Hei

ght A

bove

Bed

(m)

21 Aug. 95, Neaps, HW (5.7 m), SPM, g/l 21 Aug. 95, Neaps, Salinity

10 12 14 16 18 20

Hours

0

5

1021 Aug. 95, Neaps, Speed, m/s

(A) (B)

(C)

0 20 40 60 80

Time, Minutes

0

0.2

0.4

0.6

0.8

1

Sens

or D

epth

(m)

SPM

, g/l

21 Aug. 95, Neaps, LW

SPM

Depth

0.4

0.6

0.8

1.0

1.2

1.4(D)

643216 32

16

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Fig. 10. A small neap-tide tidal cycle of salinity, tidal current speed and SPM data measured at station BTJ on 21 August 1995. The tidal range at Immingham was

2.9 m. Maximum SPM occurred around LW and LW slack, which illustrates that the bulk of the ETM was located up-estuary (A); salinity was >4 and maximised

at HW slack with a pronounced inversion in the near-bed muddy layer (B); current speeds were flood dominant (C); and the transition from ebb to flood was

smooth (heavy line in (D)) with floc settling from the upper layers over LW slack (light line in (D)).

flocs (ca. 330 mm) beneath the region of clearing waters,where salinity was <4 (Fig. 12BeD). Floc sizes were smaller(ca. 150 mm) nearer the bed in this region, presumably dueto floc disruption resulting from tidal shear at the bed. Flocsizes also were smaller (ca. 100 mm) further up-estuary inthe ETM core, but increased again close to the surface (ca.200 mm).

4.5. Settling of flocs within the ETM

Floc settling velocity,WS, is the sinking velocity of a floc instill water, which may be reduced by the presence of otherflocs that cause hindered settling (Winterwerp, 1999, 2001,2002). The effective floc settling velocity, WES, is the velocitywith which a suspension of flocs sinks through the in situ wa-ter column. Direct observations of WS were not made in thiswork, although it was possible to make direct estimates ofWES and indirect estimates of WS because of the pronouncedsettling of SPM that occurred from the upper water columnover the HW-slack period.

In the ETM nose region, settling occurred from the upper1.5 m of water column over HW slack and led to substantiallyhigher SPM concentrations nearer the bed (Fig. 7A). A

sediment budget for the upper 1.5 m layer of water columngave an effective settling velocity of 0.3 mm s�1 at the baseof the layer, when averaged over the 0.5-h period of observa-tions that spanned HW slack. The effective settling velocitywas less than the actual floc settling velocity because of theinfluence of mixing, which tended to maintain SPM in suspen-sion and reduce WES according to:

WESP¼WSP�Kz

vP

vzð1Þ

In this equation, P is SPM concentration, z is depth beneaththe surface and Kz is the vertical eddy diffusivity for SPM.If conditions were steady and longitudinal advection were neg-ligible, then the effective settling velocity would be zero andeither Eq. (1) (Fugate and Friedrichs, 2002), or the Rouseequation (Rouse, 1937; Chester, 1999; Orton and Kineke,2001) could be used to estimate the floc settling velocity.However, conditions generally were not steady, especiallyaround HW slack when the current reversed, although the in-fluence of longitudinal advection was likely to have been min-imal then because of slow current speeds. The classical eddyviscosity relationship was used for water depth, h, and reduced


10 12 14 16 18 20Hours

0

5

10

10 12 14 16 18 20Hours

0

5

10

21 Aug. 95, Neaps, SPM, g/l

21 Aug. 95, Neaps, Sizes, microns

10 12 14 16 18 20Hours

0

1

2

3

424 Aug. 95, Mean Tide, Sizes, microns

10 12 14 16 18 20Hours

0

1

2

3

424 Aug. 95, Mean Tide, SPM, g/l

Selby

Selby

BTJ

BTJ

(A) (B)

(C) (D)

300

250

250

250

250

800

800

32

16

6432

16

32

16

32

16

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Hei

ght A

bove

Bed

(m)

Fig. 11. Tidal cycles of in situ median floc sizes and SPM data measured for a small spring tide at Selby on 24 August 1995 (A, C), and a small neap tide at station

BTJ on 21 August 1995 (B, D). Floc sizes usually exceed 100 mm and maximise at more than 300 mm around slack water of HW springs (A), and more than

800 mm within the high concentration muddy layer at neaps (B).

by a factor that depended on the gradient Richardson number(e.g. Wolanski et al., 1988, 1992):

Kz ¼ 0:4hU�ð1� z=hÞðz=hÞ f ðRiÞ ð2Þ

The gradient Richardson number, Ri, was calculated at thebase of the near-surface 1.5 m layer and used in the MunkeAnderson relationship for f ðRiÞ (Munk and Anderson, 1948;Wolanski et al., 1988). The friction velocity, U�, was derivedfrom the quadratic drag law (see later). Eq. (2) gave a verticaleddy diffusivity coefficient of 8 cm2 s�1 at the bottom of the1.5 m surface layer when averaged over the 0.5-h period.The mean SPM concentration and estimated turbulent shearstress at the bottom of the 1.5 m layer, over the HW-slack pe-riod, were 3.6 g l�1 and 0.3 Pa. The mixing ‘correction’ was0.7 mm s�1 and an estimate of 1.0 mm s�1 for the settling ve-locity was derived from:

WS ¼WES þKzP�1vP

vzð3Þ

In the ETM nose and core regions, the effective settling velocityfrom the near-surface 1-m layer was 0.5 mm s�1 over HW slack(Fig. 8A). Themean SPMconcentration and estimated turbulentshear stress at the bottom of the layer were 19.5 g l�1 and 0.3 Pa.The mixing ‘correction’ was 0.2 mm s�1 and Eq. (3) gave anestimated floc settling velocity of 0.7 mm s�1.

In the ETM tail region, the whole water column underwentpartial sediment deposition to the bed during the spring-tide,HW-slack period (Fig. 9A). The effective settling velocity,equivalent here to a deposition velocity to the bed, was1.3 mm s�1 over the 1-h period of observations that spannedHW slack. The mean, near-bed SPM concentration and esti-mated turbulent shear stress were 1 g l�1 and 0.05 Pa. Themixing ‘correction’ was 0.2 mm s�1 and Eq. (3) gave an esti-mated settling velocity of 1.5 mm s�1.

In the ETM tail region, the upper 8 m of water column under-went partial sediment deposition onto the underlying high concen-tration layer during the neap-tide, HW-slack period (Fig. 10A).The effective settling velocity was 0.6 mm s�1, averaged overthe 0.75-h period of observations that spanned HW slack. Themean SPM concentration and estimated turbulent shear stress atthe bottom of the 8 m upper layer of water column were1.3 g l�1 and 0.7 Pa. The mixing ‘correction’ was 1.5 mm s�1

and Eq. (3) gave a floc settling velocity of 2.1 mm s�1.

5. Discussion

The prolonged period of very low and nearly constantfreshwater runoff and quiescent meteorological conditionsthat preceded and persisted throughout these measurementsensured that: (a) the ETM was located close to the tidal limitat HW; (b) it was characterized by very high SPM


25 30 35-8

-6

-4

-2

0

25 30 35-8

-6

-4

-2

0

25 30 35-8

-6

-4

-2

0

25 30 35-8

-6

-4

-2

0Dep

th (m

)


17 Jul. 95, Springs HW (7.1 m), Sizes + SPM

17 Jul. 95, Springs HW (7.1 m), Sizes, microns

17 Jul. 95, Springs HW (7.1 m), Salinity

(A) (B)

(C) (D)

6432

16 240

17 Jul. 95, Springs HW (7.1 m), SPM, g/l

Fig. 12. Longitudinal and vertical profiling measurements of in situ floc sizes, salinity and SPM concentrations are shown for a down-estuary transect between

Selby and BFB during HW and the early ebb of a spring tide on 17 July 1995. Tidal range at Immingham was 6.3 m. SPM (A); median floc sizes (B); salinity

(C); and median floc sizes superimposed on SPM concentration contours (D).

concentrations; and (c) the temporal variability in SPM waslargely driven by intratidal and springeneap tidal forcing.

5.1. Salinity and the ETM

Generally, salinity was fairly well mixed vertically, al-though both slight stable and unstable vertical salinity gra-dients sometimes occurred due to tidal ‘straining’, as well aspronounced salinity inversions that were stabilized by large,stable, vertical gradients in SPM concentration. The existenceof these stable SPM gradients demonstrated the importance ofparticle (floc) settling to the temporal development of SPMconcentration and transport within the ETM, even though ver-tical mixing generally was sufficiently strong to ensure that sa-linity was approximately well mixed. The latter can beunderstood with the use of a bulk Richardson number that pri-marily characterizes the influences of freshwater-inducedbuoyancy on mixing (Dyer and New, 1986):

RiL ¼ ghDr=�rU2

�ð4Þ

In this definition, h is water depth, r is density and Dr is thesurface-to-bed density difference. Ignoring temperature varia-tions, an approximate formula for the bulk density of estuarinewaters, r, is (Odd, 1988):

r¼ 1000þ 0:76Sþ 0:62P ð5Þ

where S is salinity, P is SPM concentration (g l�1) and theunits of r are kg m�3.

Using salinity data from the Tees and Test Estuaries, UK,Dyer and New (1986) were able to define three parametric re-gions: when RiL> 20 a stable density interface may exist inthe water column that displays internal waves but no signifi-cant tidal (bed shear) mixing; when 20> RiL> 2 the interfaceis modified by tidal mixing; and when RiL< 2 then strong tidalmixing occurs. Applying Eqs. (4) and (5) to the observedspring-tide tidal cycles, in which SPM-induced density differ-ences dominated those due to salinity, implies that strong tidalmixing generally occurred throughout the tide. Exceptions tothis were periods of HW slack in the ETM nose and core re-gions, as well as LW and HW slack in the ETM tail region,during which the stratifying influences of advection and tidalstraining were, in any case, minimal. Therefore, tidal mixingis anticipated to result in approximate vertical homogeneityfor salinity.

SPM concentrations in the ETM nose were, by definition, inthe range 4e16 g l�1, whereas salinity was less than 1, so thatthe SPM contribution to density there was at least three times(and generally many times) greater than that due to salinity.More importantly, the near-bed, longitudinal density gradientin the ETM nose was approximately 2.5 kg m�3 km�1, where-as that due to salinity was two orders of magnitude smaller(0.02 kg m�3 km�1). Therefore, salinity was insignificant tothe dynamics of ETM movement in the nose and upper core


regions of highest SPM concentrations. There, the tendencywould have been for near-bed, SPM-induced density currentsto flow towards the tidal limit. In the tail region, the near-bed longitudinal density gradient due to SPM was approxi-mately 0.6 kg m�3 km�1 and that due to salinity also was0.6 kg m�3 km�1, but oppositely directed. It follows that up-estuary-directed, salinity-induced density currents near thebed would have tended to oppose down-estuary-directed,SPM-induced density currents and therefore would have con-tributed to the retention of fine sediment in the ETM.

Although slight inverse salinity stratification sometimes oc-curred on the flood due to tidal straining, and this was stabi-lized by vertical SPM gradients, much stronger inversesalinity stratification occurred from mid flood to mid ebb dur-ing the neap-tide tidal-cycle station in the tail region. This in-version was associated with a near-bed, high concentrationlayer and implies that the layer was derived from further up-estuary, in the vicinity of maximum SPM concentrations,and that lower salinity waters were retained within it duringits down-estuary advection on the ebb.

5.2. Hydrodynamics and the ETM

A strong reduction in tidal water-level amplitude occurredprogressing up-estuary from the ETM tail to the ETM nose(a factor of ca. 3 at spring tides) due to the influences of fric-tional energy losses, opposing freshwater flow and rising bedlevels. The tide was strongly asymmetric, with a rapid and rel-atively short-lived flood that followed the up-estuary passageof a small tidal bore that signaled LW and the end of the pro-longed ebb.

Ignoring advection, the depth-averaged balance of forces inthe momentum equation is:

vU=vt ¼�gv2=vx�DUjUj=h� 1

2gðh=rÞvr=vx ð6Þ

In Eq. (6), the water acceleration (left hand side) is equated tothe sum of accelerations or decelerations due to surface-slopeforcing (first term, right hand side), quadratic frictional drag(second term, right hand side), which defines the friction ve-locity in Eq. (2) to be U� ¼ D1=2jUj, and longitudinal densitygradient forcing (third term, right hand side). A value of0.0022 was used for D (e.g. Orton and Kineke, 2001).

Application of Eq. (6) to the observed, spring-tide, tidal-cycle data showed that, apart from slack-water periods, the fric-tional drag on tidal currents in the ETM region was approxi-mately balanced by surface water-level (z) forcing. Densityforcing due to salinity and SPM were comparatively small.However, in the ETM tail region, density influences were a sig-nificant component of the momentum balance during LW slack.As anticipated, water accelerations were greater than could beexplained by frictional drag and density effects during the LWpassage of the bore in the ETM nose and core regions andmust therefore have been driven by surface-slope forcing.

Tidal bores are a feature of some strongly tidal estuariesand their physics have been described in detail for the Dee

and Daly estuaries (Simpson et al., 2004; Wolanski et al.,2004, respectively). The small undular bore observed in theupper ETM region of the Ouse typically was 0.1e0.2 m inheight, although in this region it has been reported to bemuch higher on occasions, greater than 0.5 m at the largestspring tides, and to travel at speeds from ca. 2 to 3 m s�1

(RMBC, 1986). During the passage of the bore through thenose and upper core stations there were strong correlations be-tween SPM and temperature variations. Therefore, patches ofmore fluvial, lower turbidity waters, together with patches ofmore estuarine, higher turbidity waters were transportedthrough the stations and SPM fluctuations were ‘conservative’,with no indication of bed-sediment erosion due to the passageof the bore.

The Ouse bore, like the bore observed in the Daly Estuary,was not associated with suspension of SPM from the bed, eventhough this might have been anticipated in view of the currentsgenerated by the bore’s passage (Wolanski et al., 2004). Thereason for this lack of observed SPM suspension appears tobe due to the locations of the nose and core measuring sta-tions. At local LW (the time at which the bore travelledthrough the stations) the ETM and its sediment stock were lo-cated down-estuary of the tidal-cycle stations and only ap-peared at these stations on the flood, after the bore hadtravelled further up-estuary. For typical LW depths of 1 m inthe upper estuary, and for small bore heights, the up-estuarypropagation speed of the bore would have approximated theshallow-water wave speed of ca. 3 m s�1 relative to the fresh-water flow or, typically, faster than ca. 2 m s�1 relative to theestuary’s bed. This speed is comparable with, or exceeds, thefastest observed flood currents, so that any SPM suspended bythe passage of the bore through the ETM (located furtherdown-estuary at LW) and carried by flood currents into the up-per estuary, would have been ‘outrun’ by the bore.

5.3. Mobility of the ETM

At both spring and neap tides the very turbid ETM core wasseparated longitudinally from much lower turbidity waters bynose and tail regions that were located up-estuary and down-estuary of the core, respectively. Following the spring tidethere was a down-estuary displacement of the ETM duringthe smaller tides that led into the neap tide of the second sur-vey. The displacement occurred with respect to both distancefrom the tidal limit and the salinity distribution. The ETMnose extended a considerable distance into the tidal river dur-ing the spring tide, which indicates that the various influencesof tidal currents on SPM transport dominated those due to sa-linity. The shape of the ETM nose was qualitatively similar tothat observed in gravity currents, e.g. the arrested salt wedge(Simpson, 1997), so that the additional density of theseSPM-laden nose waters may have been a factor affecting theirincursion into the tidal river during the late flood. However,there was no evidence to indicate that a two-layer densityflow developed at HW slack and the momentum balance indi-cates that the density influence was much smaller than that dueto the tides.


Longitudinal displacements of the ETM also occurred dur-ing a tidal cycle, as well as over a springeneap cycle, and bothlongitudinal surveys and tidal-cycle measurements illustratedthe up-estuary movement of the ETM’s nose and core duringthe flooding tide and their subsequent down-estuary withdrawalduring the ebb. The down-estuary movements between HWand LW, both of the ETM nose and the location of the maxi-mum SPM concentration, were less than those of the isoha-lines, whereas the displacement of the ETM tail wasapproximately the same as that of the isohalines. This indi-cates that fine sediment may have been partially retained inthe upper estuary by shear dispersion mechanisms during theebb portion of the tide.

The possible importance of shear dispersion is consistentwith the occurrences both of a very sharp ETM nose and ofa pronounced SPM stratification in the nose and core regionsat HW slack and during the early ebb. SPM stratificationwas maintained during the ebb by down-estuary advection oflower turbidity, near-surface waters (tidal straining of SPM),together with the tendency for flocs to settle. Reduced verticalmixing due to SPM-induced density stratification and stabili-zation also may have played a role in maintaining and enhanc-ing vertical gradients (e.g. Wolanski et al., 1988). SPMstratification was very strong at HW slack for spring andneap tides and at LW slack during neap tides, because ofreduced vertical mixing and floc settling. SPM was muchless stratified at LW of spring tides in the ETM nose andcore regions, due to faster currents and shallower depths dur-ing the ebb and the occurrence of a tidal bore, rather than slackwater, at LW.

The importance of shear dispersion to the longitudinal sed-iment budget within the ETM can be quantified using meas-urements at the tidal-cycle stations. The rate of longitudinalsuspended sediment transport per unit width at a fixed stationcan be split into two components: one due to vertical shear, FS,and the other due to longitudinal advection, FA. The rate oftransport per unit width of estuary is then (Uncles and Ste-phens, 1999):

F¼Z h

0

UP dz¼ hU PþZ h

0

ðU�UÞðP�PÞdz¼ FA þFS

ð7Þ

The tidal-mean rate of longitudinal SPM transport in Eq. (7) istherefore the tidally averaged sum of advective transport andshear transport. Ebb transport and flood transport of SPMwere in approximate balance over the spring tidal cycles ateach station within the ETM, so that the tide-averaged trans-port was very small compared with the maximum instanta-neous transport. As anticipated, shear transport during theebb was directed up-estuary in the nose and upper core regionsof the ETM and therefore acted to oppose down-estuary ad-vective transport of SPM. This maximum, flood-directed rateof shear transport was ca. 20e30% of the maximum, down-estuary rate of advective transport and therefore contributedsubstantially (ca. 10% of the total flood transport) to themaintenance of fine sediment in the upper region of the

estuary. In the tail region, shear transport worked in the oppo-site direction to that in the nose region and promoted down-es-tuary transport, both at spring and neap tides, thereby acting asa counter to the strong, advective, up-estuary flood transport.Maximum ebb-directed rate of shear transport there exceeded75% of the maximum, flood-directed rate of advectivetransport.

5.4. Fluid mud in the ETM

Winterwerp (1999) defined high-concentrated mud suspen-sions to be those with concentrations from a few hundredmilligrams per litre to a few grams per litre, and fluid mudas suspensions several 10s of g l�1 to 100s of g l�1 at, or be-yond the gelling point (see later). The near-bed, high concen-tration SPM suspension that occurred at the ETM tail stationthroughout much of the neap tide had concentrations that ex-ceeded 90 g l�1 in the base of its stagnant, approximately 1-mthick layer. These concentrations, and also those that occurredin the up-estuary core region of the ETM around slack water,therefore correspond to fluid mud. In the Severn Estuary,Kirby and Parker (1983) identified near-bed, mobile mud sus-pensions with mass concentrations between 3 and 15 g l�1

(subsequently as great as ca. 150 g l�1; Kirby, 1986) and sta-tionary, near-bed suspensions that were up to 1.5 m thickand had concentrations that reached 200 g l�1 (Kirby andParker, 1983; Kirby, 1986). These stationary suspensionswere located in the deeper parts of channels and were settlingbut not moving. They were only persistent and widespread atneap tides in the Severn. Nevertheless, the Severn has a 6.5-mtidal range and currents of 1.8e2 m s�1 during neap tides(Kirby and Parker, 1983), which are similar to spring tidesin the Ouse.

Such high concentration stationary suspensions will ulti-mately form a consolidating bed under quiescent conditions,if given sufficient time. Delo (1988) refers to approximately80 g l�1 as the concentration at which a bed begins to formin a laboratory settling column, whereas a range of 50e100 g l�1 is given by Whitehouse et al. (2000). As the concen-tration of the settling stationary suspension increases, theeffective settling velocity of the constituent flocs progressivelydecreases, due to hindered settling. The velocity eventually be-comes zero at the suspension gel concentration, which is theSPM concentration at which a space-filling network of finesediment is formed though the flocculation mechanism (Win-terwerp, 1999, 2001, 2002). A considerable time, several hoursto days (Delo, 1988), may therefore be required for these sus-pensions to consolidate.

The near-bed SPM layer observed in the ETM tail regionstation had characteristics both of mobile fluid mud andstationary suspensions. An increasingly thick and increasinglymore turbid layer of fluid mud from the ETM tail was ad-vected through the station on the late ebb. The speed, concen-tration and thickness of the mobile mud suspension (Kirby andParker, 1983; Kirby, 1986), or fluid mud layer (Winterwerp,1999), typically ranged between approximately 0.1e0.5 m s�1,15e40 g l�1 and 0.5e3 m, respectively. It is known that


interactions between turbulent flow and a sediment suspensioncan substantially affect mixing, vertical current profiles andother properties of the hydrodynamics (Wolanski et al.,1988, 1989; Mehta, 1991; Shi, 1998; Winterwerp, 1999,2001, 2002; Li and Gust, 2000; Dyer et al., 2004). The ob-served, highest concentration of mobile SPM in the fluidmud layer, 40 g l�1, moved at speeds less than 0.1 m s�1. AtLW slack, the layer concentration increased from more than40 g l�1 near the layer surface to 80 g l�1 near its base. Thebase concentration increased to more than 90 g l�1 later inthe tide. The velocities in this layer generally were zero forthe rest of the tide that followed LW slack, corresponding tothe stationary suspensions observed by Kirby and Parker(1983) and Kirby (1986) in the deeper parts of the Severn’schannels. The ETM tail station was located in a deeper sectionof the Ouse’s main channel, which may have made it a morefavourable location for the formation of these stationarysuspensions.

Whitehouse et al. (2000) defined a bulk Richardson Num-ber for fluid mud layers as:

Rib ¼ ghm Dr=�rðDUÞ2

�ð8Þ

where hm is the layer thickness and DU and Dr are the velocityand density differences between the fluid mud layer and over-lying water, respectively. Entrainment from the layer into thewater is anticipated to occur if Rib, Eq. (8), is less than about10 (Whitehouse et al., 2000). According to this criterion, peri-ods of entrainment would be expected to occur at the ETM tailstation on the early to late flood and for much of the ebb fol-lowing HW slack. This is consistent with the observed suspen-sion of SPM into the middle and upper parts of the watercolumn from the high concentration layer. LW slack andHW slack are predicted to be times of layer stability andalso growth, due to settling of SPM, which is consistentwith observations of increased layer thickness and reducedSPM concentrations in the overlying waters at those times.

5.5. Flocculation in the ETM

The existence of these mobile suspensions, leading to fluidmud layers, stationary suspensions and possible consolidationand bed formation, relies on the existence of particle floccula-tion and associated enhanced settling of clay and silt-sizedSPM from the bulk of the water column. Observed in situfloc sizes throughout the Humber and within the HumbereOuse ETM were much greater than that measured in the in-flowing freshwater runoff (Law, 1998), although this increasein floc size from river to estuary was unlikely to have beencaused by salinity increases from fresh to brackish waters(Eisma, 1991; Thill et al., 2001; Bale et al., 2002).

The largest observed flocs in the ETM water column, notwithin any underlying stationary suspensions, generally oc-curred near mid-depth at HW slack. This implies that slowcurrents at slack water resulted in increased flocculation,greater sizes and enhanced settling, which left smaller andslower flocs and lower SPM concentrations in the near-surface

layers. Also, the period from HW through to the early ebb wasa time of very low near-surface current shears and, by impli-cation, low mixing in the upper water column that couldhave promoted floc growth and settling, despite increasingebb current speeds as water level fell. These largest water-column flocs had maximum median diameters of 310 mm inthe ETM nose and core regions during a small spring tideand 360 mm during a larger spring tide the following month.Sizes were smaller during the mid-to-late ebb and during theflood, but exceeded 110 mm on the small spring tide and170 mm on the larger spring tide. Similar results were observedduring a spring tidal cycle in the ETM tail region. There, me-dian floc sizes reached ca. 500 mm near mid-depth at HWslack, but decreased to ca. 100 mm during the ebb and ca.70 mm on the flood. These smaller, flood-tide floc sizes arecomparable with those observed in King Sound, Australia(Wolanski and Spagnol, 2003).

Depth-averaged data illustrate a strong tendency for medianfloc sizes that were greater than a station-dependent minimumsize to increase approximately linearly with decreasing bulkshear and, to a weaker extent, with decreasing tidal energy dis-sipation rate. The largest observed floc sizes in the Ouse werein excess of 1 mm and occurred in the near-bed stationary sus-pensions during the neap-tide ETM tail station. When strongentrainment occurred from the upper interface of this layer,the associated SPM just above the layer had greatly reducedfloc sizes (<200 mm), which is indicative of shear-inducedfloc breakage. In the York River Estuary, USA, Fugate andFriedrichs (2003) also found that intense turbulent shear re-duced floc sizes, which typically were in the range 50e300 mm, whereas inter-particle collisions during differentialsettling increased floc size. These finding are consistent withthose for the Ouse, despite the difference in SPM concentra-tions for the two systems (<0.8 g l�1 in the York River Estuaryexperiments): floc sizes are similar for the two estuaries;increasing turbulent shear reduces floc size; and differentialsettling, which is likely to occur over HW slack in theHumbereOuse, is associated with increased floc sizes.

Measurements of floc sizes in surface and bottom waters ofthe Elbe Estuary, Germany, similarly showed that floc sizesduring HW slack were greater than those that occurred duringmaximum flood and ebb currents (Eisma et al., 1994). Sizes inthe surface and bottom waters of the Elbe ranged from 100 to250 mm and from 150 to 350 mm, respectively, which are sim-ilar to the size ranges observed in the Ouse ETM, althoughSPM concentrations were much less in the Elbe (<1 g l�1).The occurrence of large flocs over the HW period also is a fea-ture of SPM data from the ETM of the Tamar Estuary, UK(Fennessy et al., 1994a,b). Bale et al. (2002) used a laboratoryflume to show that marked flocculation of SPM occurred atspecific times within a (simulated) tidal cycle when velocitywas decreasing and SPM concentration was high. When nom-inal SPM concentrations in the flume were ca. 1 and 4 g l�1,median sizes typically were 80 mm for a flume water speedof 0.45 m s�1. Sizes increased to 180 mm when speeds had de-celerated to 0.15 m s�1, thereby imitating the approach to HWslack.


5.6. Floc settling in the ETM

Although floc size is an important SPM characteristic, flocsettling velocity is of more significance to engineering calcula-tions. Measurements by Voulgaris and Meyers (2004) in a tidalcreek that drained a salt marsh recorded mean floc sizes of 25e75 mm, settling velocities between 0.02 and 0.2 mm s�1 andSPM concentrations that were less than 80 mg l�1. In the PearlRiver Estuary, China, SPM concentrations typically were lessthan 100 mg l�1, median floc sizes varied between 10 and96 mm and settling velocities, which varied between 0.01 and0.2 mm s�1, increased with floc size (Xia et al., 2004). Muchlarger flocs were measured in the lower turbidity (15 mg l�1)Po River prodelta (Fox et al., 2004). Sizes ranged from ca. 60to 800 mm and settling velocities were of the order of 1 mm s�1.

Foxet al. (2004) gave an equation for the floc settling velocity:

WS ¼ 9:1� 10�4ðESDÞ1:33 ð9Þ

In Eq. (9), settling velocity has units of mm s�1 and the flocequivalent spherical diameter, ESD, has units of mm. Accord-ing to this relationship, flocs with ESD diameters of 96 and75 mm settle at 0.4 and 0.3 mm s�1, respectively, which aresimilar to the results (0.2 mm s�1) given by Xia et al. (2004)and Voulgaris and Meyers (2004) for their largest observed set-tling velocities. Eq. (9) estimates that flocs with an ESD diam-eter of 300 mm, similar to those in the Ouse ETM at HW slack,settle at 1.8 mm s�1. Sternberg et al. (1996) observed settlingflocs on the northern Californian continental margin that hadsizes from 100 mm to greater than 500 mm with settling veloc-ities of 0.1e2.2 mm s�1. More recent work gave nominal ellip-tical diameters (NED) of flocs in the range 130e740 mm andsettling velocities between 0.09 and 8.1 mm s�1 (Sternberget al., 1999). A settling velocity equation was derived:

WS ¼ 2:0� 10�4ðNEDÞ1:54 ð10Þ

Settling velocities measured in the North Sea by Mikkelsenand Pejrup (2001) yielded:

WS ¼ 2:6� 10�4ðESDÞ1:53 ð11Þ

Eqs. (9)e(11) show a marked quantitative consistency witheach other for sizes greater than ca. 150 mm, i.e. macroflocs.Moreover, settling velocity versus floc-size data measured bySternberg et al. (1996, 1999) fall well within the scatter ofdata measured in the Tamar Estuary by Fennessy et al.(1994b), which may indicate the relevance of these low-turbid-ity coastal measurements to high-turbidity estuarine systems.A very wide range of floc sizes, from 20 to several hundred mi-crometers, have been observed in the Tamar Estuary ETM dur-ing neap and spring tides (Fennessy, 1994a,b; Manning, 2004).According to Manning (2004), the settling velocity (mm s�1)of ‘macroflocs’, which he defines as flocs with diameters inexcess of 160 mm, is given by:

Ws ¼ 0:718þ 8:33t� 12t2 þ 0:938P ð12Þ

Turbulent shear stress is denoted by t (units of Pa) in Eq. (12),which is based on data from estuaries of the Tamar (UK),

Gironde (France) and Dollard (Netherlands) and is valid fort in the range 0.04e0.7 Pa and for P less than 8.5 g l�1. It pre-dicts maximum settling velocities of the order of 10 mm s�1.Settling velocities at relatively low SPM concentrations, e.g.0.1 g l�1, range between 0.7 and 2.2 mm s�1, depending onturbulent shear stress. Eq. (9) estimates that flocs with anESD diameter of 160 mm settle at 0.8 mm s�1 and flocs greaterthan 500 mm in excess of 3.5 mm s�1.

Manning (2004) does not give a formula for the settling ve-locity of ‘microflocs’, which he defines as flocs with diametersless than 160 mm, but a Lorentz function fit to Manning’s(2004) smoothed plot of settling velocity (mm s�1) versusthe logarithm of turbulent shear stress (Pa) gives a reasonablerepresentation of his data:

Wsz0:12þ 3:5=�p�4ðlog10ðtÞ þ 0:44Þ2þ1:3

��ð13Þ

Eq. (13) correlates settling velocity with turbulent shear stressonly; it has a peak of just less than 1 mm s�1 at a stress of ca.0.4 Pa and velocities in the range 0.2e0.3 mm s�1 at very lowand high stresses of ca. 0.01 and 10 Pa.

These various results for WS can be compared with indirectestimates for the Ouse ETM. In the tail region, WS was1.5 mm s�1 for the spring tide (2.6 mm s�1 from Eq. (12))and 2.1 mm s�1 for the neap tide (1.8 mm s�1 from Eq.(12)). Corresponding SPM concentrations over HW slackwere ca. 1 g l�1 and median floc sizes typically were ca.200e300 mm, for which Eq. (9) predicts settling velocitiesfrom 1.0 to 1.8 mm s�1. These are similar to those simulatedusing the Rouse equation for the Hudson River Estuary,USA (2.2 mm s�1; Orton and Kineke, 2001), where SPM con-centrations typically were <2 g l�1, and for Chesapeake Bay,USA (of order 1 mm s�1; Fugate and Friedrichs, 2002), whereSPM concentrations typically were <80 mg l�1.

In the Ouse ETM nose and core regions, an indirect esti-mate of WS was 1.0 mm s�1 in the nose region at Cawood(5.5 mm s�1 from Eq. (12)) and 0.7 mm s�1 in the ETM coreregion at Selby (outside the range of application of Eq.(12)). Corresponding SPM concentrations at Cawood andSelby over HW slack were ca. 4 and 19 g l�1, respectively.These indirect estimates of settling velocities are approximatelyhalf of those derived for the ETM tail, and much slower thanvalues from Eq. (12), which indicates that hindered settlingmay have affected the settling of flocs (Winterwerp, 1999,2001, 2002). Winterwerp (2002) and Winterwerp and vanKesteren (2004) show that the settling velocity of a singlefloc of fine-grained sediment in still water is reduced by a fac-tor, g, of approximately (for y P 0):

g¼ ð1�fÞ�1�fr

��ð1þ 2:5fÞ ð14Þ

In which f ¼ P=Pgel is the SPM volume concentration of themuddy suspension, which is related to the mass and mass-gelling concentrations, P and Pgel, respectively. In Eq. (14),fr ¼ P=rS, with rS the density of primary sediment particles(2700 g l�1, taken as an average for the silt and clay mixture


within the SPM and based on data tabulated by Winterwerpand van Kesteren, 2004).

Winterwerp (2002) estimated the gelling concentration ofsettling mud for three experimental situations and found thatPgel was 40, 80 and 120 g l�1 for the three cases. Settling-col-umn data for the Ouse, when applied to Eq. (14), indicate thatPgel is 64 g l�1 (Uncles et al., in press-b). Eq. (14) shows thatthe settling velocity is reduced to ca. 0.9 of the unhindered set-tling velocity at an SPM concentration of 2 g l�1, to ca. 0.8 at4 g l�1, 0.5 at 16 g l�1, 0.2 at 32 g l�1 and 0 at 64 g l�1. Largereductions in effective settling velocity of SPM are thereforeanticipated to result from hindered settling in the ETM noseand core regions and in near-bed fluid mud layers that formduring neap tides and over slack-water periods. In the core re-gion at Selby, the estimated WS would be increased from 0.7 to1.7 mm s�1 in the absence of hindered settling and in the noseregion at Cawood, WS would be increased from 1.0 to1.2 mm s�1.

5.7. Formation of the ETM

The transition from ebb to flood was rapid at LW of springtides in the Ouse and was associated with a tidal bore in theETM nose and core regions. Therefore, very little time wasavailable for settling of flocs. On the relatively short and ener-getic flood, tidal straining acted to reduce vertical SPM gra-dients in the upper core and nose regions because SPMconcentrations decreased up-estuary. Salinity effects were neg-ligible there. Straining, together with fast currents, tended togenerate strong near-bed shears, vertical mixing and verticaldistributions of SPM that exhibited continuously increasingconcentrations from surface to bed, in which the greater, near-bed concentrations were of the same order of magnitude asthose at the surface. As a result, very strong, up-estuary advec-tive transport of SPM occurred. SPM concentrations increasedup-estuary in the ETM tail and lower core region, so that tidalstraining tended to increase vertical SPM gradients on the flood,although this stabilizing effect was partially compensated by thetendency to destabilize salinity profiles. Therefore, verticalmixing and longitudinal sediment transport were expected tobe somewhat weaker than in the upper core and nose.

At the end of the flood, over the HW-slack period, a fractionof the SPM settled into the lower water column, where con-centrations in a near-bed layer typically were greater than30 g l�1 and could exceed 60 g l�1. The HW-slack period en-abled larger flocs to form and settle, although the effects ofhindered settling, which were likely to have reduced settlingvelocity from ca. 1 to ca. 0.1 mm s�1 (4 to 0.4 m h�1), pre-vented significant deposition to the bed and instead led tothe formation of high concentration, near-bed suspensions.This behaviour was reflected in an essentially linear, i.e. ‘con-servative’, scatter plot of depth-averaged SPM concentrationversus depth-averaged salinity for tidal-cycle data in thenose region and at the tidal limit, with no evidence of erosionpeaks or deposition troughs (Fig. 2B).

The high concentration suspensions that formed over HWslack would have led to the damping of bed-generated

turbulence and reduced mixing through the water column dur-ing the ensuing early ebb, which also was apparent from thelow current shears that occurred in the upper water columnand which further facilitated floc settling. Wolanski et al.(1988) utilized a local, 1D vertical model of the water columnto demonstrate that sediment-induced buoyancy effects inhibitvertical mixing of sediment. In the upper core and nose region,where salinity effects were negligible, tidal straining led toeven greater SPM stratification on the ebb. In the lower coreand tail region, tidal straining would have tended to reduceor invert SPM stratification on the ebb, although the tendencyto generate salinity stabilization would have partially compen-sated this effect. The enhanced ebb sediment transport in theETM tail may have led to it being somewhat diffuse (typicallyextending over 12 km), whereas the nose (typically extendingover less than 3 km) remained strongly stratified and sharp asthe high concentration suspensions moved back down-estuaryon the relatively slower, near-bed ebb currents. Shallower wa-ter depths and faster currents on the mid-to-late ebb led to en-trainment of sediment from near-bed suspensions and moreeffective mixing of SPM. These processes are reflected inthe calculation of SPM fluxes, which illustrate the importanceof vertical shear and SPM stratification in reducing the down-estuary sediment transport during the ebb in the nose and coreregions and which therefore act to accumulate sediment in theupper estuary.

The same transport mechanisms occurred at neap tides, al-though the transition from ebb to flood was much gentler andthe longer LW and HW-slack periods enabled floc settling tooccur, which aided the development of fluid mud layers, espe-cially in the deeper parts of the main channel and over subse-quent, decreasing neap tides.

Tidal and sedimentological processes in the Trent Estuaryof the upper Humber (Fig. 1) are very similar to those de-scribed here for the Ouse. Mitchell et al. (1998) show thatthe Trent has a strong ETM in which concentrations can ex-ceed 35 g l�1. They conclude that the ETM is generated bythe combined influences of tidal asymmetry and SPM-inducedvertical density gradients. In the upper Trent, faster floodspeeds, compared with the ebb, lead to tidal pumping of sed-iment and large, vertical SPM gradients also potentially inhibitvertical mixing. They believe that non-Newtonian behaviourof high concentration SPM suspensions may also enhancethe retention of fine sediment within the ETM. Other fluidmud estuaries, such as the Changjiang, China (Shi and Kirby,2003), Fly River, Papua New Guinea (Wolanski et al., 1995)and the South Alligator River, Australia (Wolanski et al.,1988) similarly exhibit these features.

Uncles and Stephens (1993) described ETM formationmechanisms in the much less turbid Tamar Estuary. Thesemechanisms included floodeebb speed and vertical mixingasymmetries and different slack-water durations at HW andLW (cumulatively known as tidal pumping) as well as buoyancyeffects due to salinity (gravitational, or baroclinic, circulation)and SPM-induced density gradients. Therefore, tidal influencesare thought to be important to ETM formation not only withinhighly turbid estuaries, but also in medium and lower turbidity


estuaries and systems such as the Tamar (Uncles and Stephens,1993), King Sound (w3 g l�1, Wolanski and Spagnol, 2003),the Hudson (w5 g l�1, Geyer et al., 2001) and the Pearl River(w0.3 g l�1, Wai et al., 2004). Even in the low-turbidity estuaryof Chesapeake Bay (w0.1 g l�1), the ETM either would notexist or would be greatly reduced without tidal suspensionand transport of bed sediments (Sanford et al., 2001).

Gravitational circulation also appears to play a role in theaccumulation of fine sediment within the Ouse’s ETM. Uncleset al. (1998a) showed that the ETM tended to occur at highersalinities (but <11) during winter months, when it was locatedin the upper Humber or the lower Ouse. Gravitational circula-tion will be stronger in the deeper, higher salinity regions ofthe upper Humber and may contribute both to the ‘trapping’and to the maintenance of fine sediment within the ETM dur-ing high-inflow winter months and to its up-estuary transportduring the late spring. Higher HW salinity establishes itselffairly quickly within the Ouse, whereas the mobile pool offine sediment lags about one month behind salinity (Uncleset al., 1998a). Mitchell (2005) suggests that this fairly rapidresponse of the SPM to inflow (approximately one month)is indicative of the highly canalized nature of the Ouse andTrent.

The complexity of the processes that determine the ETMhas meant an increased use of models to understand andthen predict its formation. Lin and Kuo (2003) used a 3Dmodel to simulate the ETM (w0.1 g l�1) in the York RiverEstuary. They found that the location of the ETM was associ-ated with the null point of bottom gravitational circulation andthat bottom residual flow and tidal asymmetry contributed tothe formation of a smaller, secondary SPM concentrationmaximum. Burchard and Baumert (1998) used a 2D verti-calelongitudinal model of a hypothetical macrotidal estuaryto show that it is necessary to have gravitational circulationand tidal velocity asymmetry to generate an ETM(w0.3 g l�1) and that whereas tidal mixing asymmetry influ-ences the magnitude of the ETM, it does not appear to be nec-essary for its establishment. Le Hir et al. (2001) used a 3Dmodel to simulate fine sediment transport in the Seine. Theyfound that the ETM (>1 g l�1) was largely generated by tidalpumping. Models of ETM formation that take into accountsuspended and deposited mud rheology (e.g. Mehta, 1991)do not appear to have been attempted yet.

5.8. Meteorological effects on the ETM

This paper has been concerned with the behaviour of thelow runoff ETM under quiescent conditions. Meteorologicalinfluences, such as wind and wind-driven waves and variationsin freshwater flow play a role at other times. Couperthwaiteet al. (1998), Lawler et al. (2001) and Mitchell et al.(2003b) measured erosion and deposition on the upper mud-banks in the Trent and lower Ouse. They showed that deposi-tion was more likely to occur on the upper banks at reducedwind speeds and that large ‘benches’ formed in the mudbanksafter storms, indicating the importance of wave action. Sedi-ment deposition occurred on the upper banks near the peak

of spring tides, whereas erosive ebb stresses at springs con-trolled the lower banks. It is likely, therefore, that waveswill modify the behaviour of the Ouse ETM: (a) through theirinfluence on high concentration suspensions (Mehta, 1991);(b) by supplying additional sediment to the ETM via erosionof mudbanks; and (c) by preventing or reducing depositiononto the upper mudbanks. Wind waves are thought to be im-portant to the entrainment of fluid mud in the Fly River Estu-ary (Wolanski et al., 1995) and in the King Sound (Wolanskiand Spagnol, 2003), possibly through wave pumping ratherthan wave velocities. The Tana Estuary, Kenya (Kithekaet al., 2005), has large sediment inputs from its river and anETM (w5 g l�1) that is partly generated during spring tidesby wave stirring of bed sediments and trapping of SPM inthe low salinity reaches.

Grabemann and Krause (2001) found evidence that theETM (w1 g l�1) in the Weser Estuary, Germany, was pushedup-estuary during times of higher mean water levels due tostorms and conclude that conditions in the seaward regionare as important as those in the river. In the Humber, Ouseand Trent it is known that the ETM moves further into the up-per reaches with reduced river flow, and that SPM concentra-tions and bed-sediment distributions are strongly related tofreshwater flow (Uncles et al., 1998a,c, in press-a; Mitchell,2005).

6. Conclusions

An extremely turbid ETM exists in the upper reaches of theOuse during quiescent, approximately steady, low runoff sum-mer conditions. At these times, physical behaviour is dominatedby tidal and freshwater-induced currents. Tidal water levelsare very asymmetric in the up-estuary region, especially atspring tides, when a rapid LW to HW rise (ca. 2e3 h) is fol-lowed by a much longer HW to LW fall (ca. 10 h) and peakflood current speeds (ca. 2 m s�1) exceed peak ebb speeds.The shallow waters and fast currents lead to strong frictionaldrag at the bed, which is approximately balanced by forcingdue to surface water-level slopes. This drag leads to a large re-duction in tidal amplitude (ca. 0.3 of the mouth value close tothe tidal limit) as the tide propagates into the estuary. A tidalbore, of height 0.1e0.2 m, signals local LW as it travels up-es-tuary at spring tides in the upper reaches of the estuary. How-ever, the bore does not cause suspension of fine sediment atlocations up-estuary of the ETM and its associated pool ofsilt and clay-sized sediment. Salinity is fairly well mixedthroughout the water column, despite the existence of strongSPM stratification in the ETM region. Slight stable and unsta-ble salinity stratification due to tidal straining occurs on theebb and flood, respectively, but any unstable stratification isstabilized by SPM stratification. Much larger salinity inver-sions can occur in the presence of underlying stationary sedi-ment suspensions in the lower Ouse, which implies that thesesuspensions are derived from further up-estuary, in the uppercore region of the ETM.

The ETM core region, in which near-bed SPM concentra-tions exceed 16 g l�1, extends over a longitudinal distance of


35 km at HW, both at spring and neap tides. It is separated bynose and tail regions from much lower turbidity waters, up-es-tuary in the tidal river and down-estuary in the Humber andcoastal zone. The nose region is very sharp, less than 3 kmin length, and at spring tides is located 15 km into the tidal riv-er, where salinity is less than 1, whereas the tail is much morediffuse (ca. 12 km in length). Except at very small neap tides,when fluid mud layers and stationary suspensions (w90 g l�1)can form in the tail region of the ETM, maximum near-bedSPM concentrations (w50 g l�1) occur close to the nose inthe upper core region of the ETM.

The ETM is a mobile feature. It is displaced down-estuaryby ca. 12 km during the 7-day period from spring to neaptides. It also is displaced down-estuary, and spreads out, be-tween HW and LW, such that the tail moves approximatelythe same distance as the isohalines during the ebb, whilstthe nose moves significantly less. Advection of SPM is thedominant sediment-transport mechanism in the nose and uppercore regions and there is an approximate equality between ad-vective transport over the short and fast flood, and over thelong but slower ebb. The effect of vertical-shear transport dur-ing the ebb, which is ca. 10% of the ebb advective transport, isto reduce down-estuary sediment transport and aid accumula-tion of sediment in the upper reaches. At spring tides, in theETM tail region, vertical-shear transport is negligible duringthe ebb but acts in the down-estuary direction during the floodto reduce the large, flood advective transport (by ca. 20%), inorder to maintain an approximate sediment balance over thetidal cycle.

Floc settling leads to pronounced SPM stratification overthe HW, HW-slack and early ebb period. Estimates of settlingvelocity, corrected for hindered settling, range from1.2 mm s�1 in the nose and core regions to 2.1 mm s�1 inthe tail region. Hindered settling is important in the noseand upper core regions because it prevents appreciable deposi-tion to the bed in the main channel at HW slack. Instead, highconcentration suspensions remain close to the bed and travelrelatively slowly down-estuary on the ebb until decreasing wa-ter depths, due to falling water levels, and increasing currentspeeds result in increased vertical current shear, mixing andentrainment. Complete entrainment may not occur duringsmall and decreasing neap tides, leading to strong stratificationof SPM, fluid mud layers and stationary suspensions near thebed. At LW of spring tides there is little time for deposition offlocs in the nose and upper core regions. Strong tidal strainingon the flood then generates large near-bed shears and verticalmixing that maintains a more uniform vertical profile of SPM,which ensures very strong up-estuary transport of SPM.

SPM within the ETM comprises silt and clay (w20e30%clay by volume) and the relatively rapid settling that occursduring slack-water periods is a consequence of flocculation.In the water column, in situ floc sizes maximise at HW slackaround mid-depth, so that slack water and low current shearslead to increased flocculation, greater sizes and enhanced set-tling. These maximum median sizes typically are 300e500 mm, whereas sizes typically are in the range 70e300 mmduring other states of the tide. A strong, negative correlation

exists between depth-averaged median floc size and bulk ver-tical current shear in the water column for flocs that are greaterthan a station-dependent size. This size threshold is likely tobe a result of the SPM comprising primary particles and robustmicroflocs, which are resistant to turbulent shear, and large,fragile macroflocs that break into smaller flocs at high shears.The largest median floc sizes occur within the near-bed sta-tionary suspensions at neap tides, where floc sizes can exceed1 mm and concentrations 90 g l�1. Entrainment of these flocsleads to floc breakage, which reduces their median sizes toless than 200 mm.

Acknowledgements

We are grateful to Mrs. Carolyn Harris for her assistancewith particle-sizing analyses and Mr. Norman Bowley (PMLTechnician and Coxswain, retired) for invaluable assistanceand support during the fieldwork. The analyses of thesedata were undertaken with the aid of a grant from the Est-Proc research programme (www.estproc.net). EstProc isfunded within the joint DEFRA/EA (UK) Flood and CoastalDefence R&D Programme of Fluvial, Estuarine and CoastalProcesses.

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turbidity maximum in the macrotidal, highly turbid humber

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