Turbidity Maximum in the Macrotidal, Highly Turbid Humber Estuary, UK: Flocs, Fluid Mud, Stationary Suspensions and Tidal Bores

Estuarine, Coastal and Shelf Science 67 (2006) 30e52 www.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 and most turbid in the British Isles. This paper presents detailed spatial and temporal data on the estuarine turbidity maximum (ETM) within the Ouse Estuary of the Humber system during quiescent summer conditions, when freshwater runoff was very low and approximately steady. Under these 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 the tidal limit of the Ouse and the upper Humber. Tidal-cycle stations were worked between the upper Ouse and the coastal zone. In situ median floc sizes were measured at some stations. Tidal water levels were very asymmetric and currents were flood dominant in the upper estuary, especially at spring tides. Frictional drag on the currents was approximately balanced by water-level slope forcing, which led to a large reduction in tidal amplitude 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 cause suspension of fine sediment at locations up-estuary of the ETM. Generally, salinity was fairly well mixed vertically, despite strong SPM stratification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions (w90 g l1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g l1, extended over a longitudinal distance of 35 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 was much 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 small neap tides, when fluid mud layers and stationary suspensions formed in the tail region of the ETM, maximum near-bed SPM concentrations (w50 g l1) occurred close to the nose in the upper core region. The ETM was displaced down-estuary by ca. 12 km during the transition from spring to neap tides. It also was displaced down-estuary between HW and LW. Floc settling led to pronounced SPM stratification over the HW, HW-slack and early ebb period. Estimates of settling velocity, corrected for hindered settling, ranged from 1.2 to 2.1 mm s1.At HW slack, hindered settling prevented appreciable deposition of flocs to the bed in the main channel of the nose and upper core regions and at 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 sizes typically were 300e500 mm, whereas sizes during other states of the tide were in the range 70e300 mm. A strong, negative correlation existed between depth-averaged median floc size and bulk vertical current shear in the water column for flocs that were greater than a station-dependent size. 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. Ó 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.

1. Introduction The ETM is a very strong feature of the Humber, Ouse and Trent estuarine systems (Fig. 1). More than an order of mag- The estuarine turbidity maximum (ETM) is a feature of nitude increase in SPM concentrations can occur between many estuaries. It encompasses a huge range of suspended par- the Humber and the upper reaches of the Ouse and Trent Es- ticulate matter (SPM) concentrations (Uncles et al., 2002) that tuaries (Mitchell et al., 1998, 2003a,b; Uncles et al., 1998a,b,c, vary from less than 0.1 g l1, e.g. the Kennebec Estuary, USA, 1999; Mitchell, 2005). These strong horizontal gradients have for which an ETM only occurs during moderate or low fresh- been observed using airborne remote sensing and compared water flow conditions (Kistner and Pettigrew, 2001), to greater with simultaneous sea-truth measurements (Uncles et al., than 200 g l1, e.g. the Severn Estuary, UK (Kirby and Parker, 2001). The ETM is not a static feature and SPM concentra- 1983), where fluid mud layers and stationary suspensions oc- tions, observed over seasonal and annual time-scales, consis- cur. These very turbid systems are dynamically complex and tently exhibit pronounced seasonal variability in the low difficult to model, largely because of the strong interactions salinity, upper reaches of the system (Mitchell et al., 1998, that occur between their hydrodynamics and the high concen- 2003a; Uncles et al., 1998a, in press-a). Practical consequences tration suspensions within them, and the rheological behaviour from this variability arise throughout the Humber system, such of the suspensions themselves (e.g. Mehta, 1991; Winterwerp, as channel navigability and dredging requirements (Townend 1999; Dyer et al., 2004). The purpose of this paper is to pres- and Whitehead, 2003). Pontee et al. (2004) demonstrated both ent detailed spatial and temporal data on an ETM within the a relationship between siltation rates in the lower Humber upper Humber Estuary, UK, during conditions for which and freshwater runoff and the influence of the ETM on silta- SPM concentrations can exceed 90 g l1. tion and estuarine channel-switching.

Kingston Boothferry upon Hull Bridge

Spurn Head G10 G23 Immingham G13 Keadby N Bridge G24

10km

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 tidal 2.2. The HumbereOuse ETM limit of the Ouse (Naburn Weir, Fig. 1) are much less than those in the ETM. For example, freshwater concentrations dur- Mitchell (2005) presented data on SPM concentrations in ing 1994e1996 were <0.3 g l1 compared with >10 g l1 the Ouse that had been measured at a fixed height above the within the ETM (Uncles and Stephens, 1999). Measurements bed during July to December 1997 (at Drax, Fig. 1). Similar in the upper Humber and Ouse during late spring to early sum- measurements were made in the Trent during May 1997 to mer of 1994 showed that tidal advection of SPM generally was February 1998 (Mitchell et al., 2003a). These observations il- the dominant SPM flux mechanism and that pronounced lustrate both the very high concentrations and the pronounced floodeebb asymmetry in the tidal currents was reflected in seasonal and tidal variability that can occur in the upper Ouse these fluxes (Uncles and Stephens, 1999). In addition, the tid- and Trent. A summary of some tidal-cycle measurements ally averaged, up-estuary transport of sediment during spring made by us at stations between the coastal zone of the Humber tides was equivalent to about three months of average SPM in- and the tidal limit of the Ouse during low runoff, summer con- flows at Naburn Weir. This strong, tidally averaged, up-estuary ditions of 1995 and 1996 puts Mitchell’s (2005) data into an flux of SPM at spring tides in the late spring to early summer estuary-wide context. Our data demonstrate the very sharp period of 1994 indicated the potential for an accumulation of and very large increase in depth-averaged SPM concentration large amounts of fine sediment and an exceptionally turbid that occurs with decreasing depth-averaged salinity, progress- ETM in the upper estuary during low runoff, summer condi- ing from the North Sea to the upper Humber and Ouse tions. The following summer of 1995 was a period of pro- (Fig. 2A, B). SPM concentrations in the coastal zone and tidal longed, very low freshwater inflows and therefore provided river typically were <10 mg l1, whereas they peaked at ca. ideal conditions for the study of a highly turbid ETM. This pa- 30 g l1 in the upper Ouse when salinity was ca. 1. per presents the results of observations of the ETM at that time, focusing on the upper Humber and Ouse. 2.3. Nature of the SPM and sediment

SPM within the ETM consists largely of fine sediment 2. Background (silt and clay) that exists as microfloc and macrofloc aggregates and individual, primary particles (Uncles et al., 1998b, 2.1. Geographical setting in press-b). At both spring and neap tides, primary sediment particles are very fine-grained and at HW comprise ca. 20e The total catchment area of the Humber Estuary system is 30% clay-sized platelets that are dominated by chlorite and roughly 26 000 km2, which is approximately 20% of the area illite clay mineralogy. The specific surface area, SSA, of of England. It comprises the Ouse, Trent and Humber and their SPM (Holtz and Kovacs, 1981) within the ETM typically various tributaries, and is the largest estuarine system in the is 24 m2 g1. The loss on ignition (LOI) of SPM in the British Isles. The combined freshwater inflows have a temporal most turbid reaches of the estuary is ca. 10%, which indi- average of ca. 250 m3 s1. The confluence of the tidal Ouse, cates that the ETM largely comprises inorganic (mineral) Trent and Humber is located at Trent Falls (the Apex, fine sediment, consistent with direct measurements of partic- Fig. 1) which is approximately 60 km up-channel from where ulate organic carbon content (Uncles et al., 2000). During the Humber meets the North Sea at the sand spit of Spurn low freshwater inflow, summer conditions there is a pro- Head (Fig. 1). nounced increase in the size of primary particles progressing Tidal ranges referred to in this paper are predicted ranges up-estuary from the ETM into the tidal river, as well as from Immingham (inset in Fig. 1), which has mean spring a marked increase of LOI in the near-surface SPM, which and mean neap tidal ranges of 6.4 and 3.2 m, respectively, reflects less clay and a greater amount of organic matter as- and a mean tidal range of 4.8 m (ATT, 2005). The Ouse is tidal sociated with SPM in the low-turbidity waters there to Naburn Weir, located ca. 62 km up-channel from the Apex. (Fig. 2B). Mean spring and neap tidal ranges near the Apex at Blacktoft, The combined silt and clay percentage of total surficial station BTJ, are estimated to be 5.9 and 3.6 m (ATT, 2005), bed-sediment mass along the main channel of the upper Hum- whereas corresponding tidal ranges at Naburn Weir are re- ber and Ouse varies strongly as a function of longitudinal po- ported to be less than half of these values (RMBC, 1986). sition (Uncles et al., 1998c). During low freshwater inflow High water (HW) at the Apex is about 1 h after HW at Im- conditions, the relatively small silt and clay percentage of mingham (ATT, 2005). At Selby and Naburn, HW is reported main-channel, surficial bed-sediment mass within the most to be more than ca. 2 h and 3.5 h after HW at Immingham, re- turbid region of the ETM (ca. 10e35%) is in marked contrast spectively (RMBC, 1986). Although these estimates are to the overlying SPM, which largely comprises silt and clay. strongly dependent on the springeneap tidal state and other This illustrates that the sediment particle ‘mix’ within the environmental factors, they at least demonstrate the long prop- ETM is distinct from that within the underlying bed, which agation times and damping of the tide through the upper Hum- also is reflected in the low SSA of surficial bed sediment. ber and Ouse. A tidal bore can develop within the Ouse at Therefore, in terms of particle size distribution and organic spring tides (the ‘aegir’, RMBC, 1986), when it signals low content, SPM within the ETM during low freshwater inflow, water (LW) and the start of the flood. summer conditions is distinct both from that further up-estuary R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 33

(A) (B) Naburn Cawood 10 30 Selby ) ) -1

-1 1 20 Naburn Cawood 0.1 Selby SPM (g l SPM (g l Drax UW 10 G23 G10 0.01 G24 G13 0 0.1 110 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Salinity 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 bed salinity and turbidity were moored in the upper reaches in sediment. order to complement some of the short-term tidal-cycle data.

4. Results 3. Methods 4.1. Runoff into the Ouse A fast, inflatable boat was used to survey 19 stations along the main channel of the Upper Humber and Ouse between the The daily-averaged rate of freshwater input (runoff) to the tidal limit at Naburn Weir (0 km) and station UW in the upper Ouse across its tidal limit at Naburn Weir, which typically is Humber, 67 km down-estuary from the tidal limit and 5 km ca. 40% of the total inflow to the Ouse, generally decreased down-estuary from the Apex (Fig. 1). The boat was brought from the beginning of April until early September 1995. to each station and then allowed to drift with the current while Drought conditions occurred throughout the summer, from vertical profiles of salinity, temperature and turbidity were ob- the beginning of June 1995 to the beginning of September tained, starting at 0.1 m above the bed and continuing to the 1995, when runoff at Naburn decreased from approximately 17 to 4 m3 s1. These freshwater flows were much smaller surface. Survey measurements were made at either approxi- 3 1 mately local HW or local LW. This was feasible because of than the mean and maximum of 47 and 475 m s over the e the long tidal propagation times within the Ouse. Currents three-year period, 1994 1996. The maximum flow occurred were still flooding at local HW and ebbing at local LW. during the winter of late 1994 to early 1995. e Pumped samples were obtained from surface and near-bed at The data presented here were obtained largely during 10 most stations in order to determine SPM concentrations and 27 August 1995, when daily-averaged runoff at Naburn was approximately steady and very low, with a mean and standard to calibrate turbidity meters, as well as for subsequent analysis 3 1 of SPM properties. The methods used to determine SPM con- deviation of 4.1 0.2 m s . Other measurements presented here also were made under very low runoff conditions: centrations and properties have previously been described 3 1 3 1 (Uncles et al., in press-b). 6m s on 24 September 1995 and 7 m s on 17 July 1995. Tidal-cycle stations (shown in Fig. 1) also were worked at Cawood (from a bridge), Selby, Drax and station UW (from 4.2. ETM behaviour: longitudinal transects a boat at anchor) and station BTJ (using an extended-reach crane from the jetty). Measurements included vertical profiling Longitudinal and vertical measurements of salinity and tur- for salinity, temperature, SPM concentrations and currents, us- bidity, made at approximately local HW of spring and neap ing direct-reading current meters. Anchored ships were used to tides during August 1995, showed a pronounced ETM in the work tidal-cycle stations further down-estuary and in the upper Humber and Ouse (Fig. 3). For definiteness, the main coastal zone (stations G10, G13, G23 and G24 in Fig. 1). At ‘core’ of the ETM is defined arbitrarily as the longitudinal re- some of the tidal-cycle stations, in situ floc-size distributions gion where near-bed SPM concentrations exceed 16 g l1 and of SPM were measured using a Lasentec P-100 laser-reflec- the ‘nose’ and ‘tail’ of the ETM as the longitudinal regions up- tance particle size instrument (Law et al., 1997; Law, 1998). estuary and down-estuary of the core, respectively, between Self-recording instrument packages that measured water level, the near-bed 4 and 16 g l1 SPM concentrations. 34 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

32 16 -5 8 4 2 1 25 47

-10 20 (A) 11 Aug 95, Springs HW (7.5 m), SPM, g/l -15 15 0 102030405060 0 10 Sal.

1 5

-5 10 5

-10 1 (B) 11 Aug 95, Springs HW (7.5 m), Salinity -15 0 0 102030405060

Depth (m) 0 8 2 2 2 16 16 0.5 32 1 16 4 128 -5 8 45 64 8 -10 32 18 Aug 95, Neaps HW (6.1 m), SPM, g/l (C) 16 -15 0 10 20 30 40 50 60 8 0 g/l 1 4 5 10

15 20 -5 2 1 -10 0.5 18 Aug 95, Neaps HW (6.1 m), Salinity (D) 0 -15 0 102030405060 Distance from Tidal Limit (km)

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 distance decreasing salinity, up-estuary of the maximum SPM concen- of 35 km (Fig. 3A, C). The spring-tide core was located be- tration (Fig. 2B). tween 3 and 38 km from the tidal limit (Fig. 3A) and the Despite the strong SPM stratification, salinity was fairly neap-tide core 12 km further down estuary (Fig. 3C). This dis- well mixed vertically both for the spring and neap-tide surveys placement also was reflected in the locations of the maximum, (Fig. 3B, D). Slight inverse salinity stratification occurred at near-bed spring and neap-tide SPM concentrations. These had some stations, but was stabilized by density increases due to magnitudes of 47 and 45 g l1 and were positioned 12 and large SPM loads. Salinity distributions were very similar. 22 km from the tidal limit, respectively. The neap-tide stratifi- The salinity-20 isohaline was located in the upper Humber, cation of SPM in the ETM core was greater than that for the close to the Apex, and the 1 and 5 isohalines were located ap- spring tide. For both the spring and neap tide the nose was proximately 20 and 38 km from the tidal limit, near Selby and much sharper than the tail and extended over a horizontal dis- BFB, respectively (Fig. 1). The core of the spring-tide ETM tance of less than 3 km, compared with 12 km. SPM concen- was located up-estuary of the 5 isohaline, whereas it was located trations fell sharply progressing down-estuary of the ETM up-estuary of the 15 isohaline during the neap tide. The max- tail, approximately exponentially as a function of increasing imum SPM concentration was located 5 km up-estuary of the salinity, and were less than ca. 10 mg l1 in the coastal 1 isohaline at the spring tide and 2 km down-estuary at the zone, where salinity was ca. 34 (illustrated for low runoff, neap tide. The ETM nose was located 15 km up-estuary of summer conditions in Fig. 2A). SPM concentrations also the 1 isohaline at the spring tide and 5 km up-estuary at the rapidly decreased, approximately linearly with respect to neap tide. R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 35

A pronounced ETM also was a feature of the SPM distribu- flood of a small neap tide (the day following the HW neap- tion measured at approximately local LW of a neap-tide survey tide survey shown in Fig. 3C, D). The duration of the flood (the day preceding the HW neap-tide survey shown in Fig. 3C, tide in this part of the estuary is short compared with the D). The ETM core extended over a longitudinal distance of ebb, only 2e3 h compared with approximately 10 h (see later), 42 km and was located between 24 and 66 km from the tidal and insufficient time was available to obtain both good tempo- limit (Fig. 4A). The maximum near-bed SPM concentration ral and spatial resolution. A net up-estuary displacement of the was 58 g l1, located 32 km from the tidal limit. Therefore, ETM occurred between local LW and HW þ 1.9 h (Fig. 5AeE). the ETM nose and tail were located 9 and 16 km, respectively, Although the ETM nose was not well resolved for the transects further down-estuary at LW compared with the HW distribu- at LW and LW þ 1.4 h, it is apparent that the nose moved up- tion of the following day (Fig. 3C) and the maximum SPM estuary a minimum of 4 km and a maximum of 9 km between concentration was located 10 km further down-estuary. The LW and HW þ 1.9 h, whilst the 1 isohaline contour (location LW isohalines were located approximately 15 km further indicated by the vertical arrow in Fig. 5) moved up-estuary down-estuary than at HW (Fig. 4B). a distance of 12 km. Greatest stratification again occurred The down-estuary displacements of the ETM nose and at LW and following HW slack, during the early ebb maximum SPM concentration between HW and LW therefore (HW þ 1.9 h), when the ETM nose became very sharp (also were substantially less than those of the isohalines, which see Figs. 3C, 4A). were the same as the displacement of the ETM tail. This Measurements throughout the ebb portion of a spring tide spreading of the ETM core was accompanied by strong SPM were made during September 1995, when runoff across the tidal stratification and the formation of a very high concentration, limit was again very low (6 m3 s1). Greatest observed SPM near-bed layer in the vicinity of the upper core region of high- stratification and up-estuary intrusion of the ETM nose oc- est SPM concentrations (Fig. 4A). curred following local HW on the early ebb, at HW þ 1h, A more detailed view of the intratidal movements of the when the nose exhibited a very sharp longitudinal gradient ETM was obtained by making measurements throughout the in SPM (Fig. 6A). Lowering water levels during the later 128 64 32 16 8 4 2 1 0.5 0 SPM, g/l 0 0.5 8 4 2 16 16 32 32 8 58 -5

-10

(A) 17 Aug 95, Neaps LW (1.8 m), SPM, g/l -15 0 102030405060 25 20 15 10 5 1 0 Sal. Depth (m) 0

-5

-10

(B) 17 Aug 95, Neaps LW (1.8 m), Salinity -15 0 102030405060 Distance from Tidal Limit (km)

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

60 (A) 30 LW 0

60 (B) 30 LW + 1.4 h 0

60 )

-1 (C) 19 Aug 95, Neap Tide, HW (5.7 m), LW (2.7 m) 30 Depth-Averaged SPM HW - 1 h Near-Bed SPM

SPM (g l 0

60 (D) 30 HW + 0.3 h

60 (E) 30 HW + 1.9 h

15 20 25 30 35 40 Distance from Tidal Limit (km)

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); LW þ 1.4 h (B); HW 1 h (C); HW þ 0.3 h (D); and HW þ 1.9 h (E). ebb were associated with comparable depth-averaged and SPM concentrations increased over the 1.8-h period of the near-bed SPM concentrations, together with down-estuary flooding tide and reached a near-bed maximum immediately movement of the ETM nose and core between HW þ 4.4 h following HW slack (Fig. 7A). The nose of the ETM reached and LW 2.2 h (Fig. 6BeD). The ETM core moved down-es- the site ca. 0.8 h before HW and withdrew ca. 2.6 h after HW, tuary 11 km between HW þ 1 h and LW 2.2 h, whilst the 1 consistent with both the location of the station, 8 km from the isohaline contour moved down-estuary a distance of 13 km tidal limit, and the relative position of the ETM (Fig. 3A). The (Fig. 6A, D). core of the ETM reached the site immediately prior to HW slack and withdrew ca. 0.9 h later. Thereafter, SPM concentra- 4.3. ETM behaviour: tidal cycles tions decreased over the observed 6 h period of ebb currents. In addition to advection of SPM in the ETM, SPM also settled 4.3.1. The ETM nose region through the water column over the HW, HW-slack and early Profiling observations were made during a spring tide at ebb period, which led to a reduction in SPM concentrations Cawood (Fig. 1), 8 km from the tidal limit and in the up-estu- near the surface and an increase nearer to the bed (Fig. 7A). ary region of the ETM (Fig. 7). The tide was slightly smaller This increased stratification was consistent with longitudinal- than that during the spring-tide longitudinal transect (Fig. 3A, transect observations of the ETM nose region close to HW B). The salinity maximised at ca. 0.6, which corresponded to and on the early ebb (Figs. 3A, C, 5E, 6A). Current speeds HW slack at approximately 0.5 h after HW (Fig. 7B, C). were strongly asymmetric. Fastest flood currents were A rapid rise in water level followed the up-estuary passage 1.9 m s1, compared with 1.1 m s1 on the ebb (Fig. 7C). of a small tidal bore at LW (Fig. 7D), whereas the ebb was Faster currents and greater near-bed shears on the flood associated with a prolonged and gradual fall in water level (Fig. 7C) were associated with much less SPM stratification following HW. The tidal range was 2.0 m compared with compared with the ebb (Fig. 7A). 6.2 m at Immingham (i.e. 32% of the Humber mouth value) The tidal bore that signaled LW and the end of the ebb at and LW and HW lagged Immingham by 7.4 h and 3.4 h, re- Cawood had a height of 0.13 m and a rise time of 9 s. The spectively. The tidal rise, LW to HW, required 1.8 h compared crest of the bore was followed by an oscillation of ca. with 10.6 h for the tidal fall. 0.04 m range and 9 s duration, which was followed thereafter R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 37

90 (A) 60

30 HW + 1 h 0

90 (B) 24 Sep 95, Spring Tide, HW (7.1 m), LW (1.1 m) 60 Depth-Averaged SPM 30 HW + 4.4 h Near-Bed SPM )

-1 0

9 SPM (g l (C) 60

30 LW - 3.0 h 0

90 (D) 60

30 LW - 2.2 h 0 0 102030405060 Distance from Tidal Limit (km)

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 HW þ 1 h (A); HW þ 4.4 h (B); LW 3 h (C); and LW 2.2 h (D). by smaller oscillations as longer-term water level steadily after the nose and withdrew ca. 2 h before the nose. SPM con- rose (Fig. 7D). The consequence of the bore’s passage on centrations continued to decrease on the ebb until LW. SPM SPM concentrations was slight. SPM concentrations, was strongly mixed through the water column on the flood measured at a fixed height above the bed, were relatively and partially settled-out over the HW, HW-slack and early low (<0.3 g l1) and SPM variations over the 160-s ‘event’ ebb period, which led to enhanced stratification and greater period (plotted in Fig. 7D) were strongly, negatively correlated near-bed SPM concentrations (also observed in the longitudi- with temperature variations (mean: 20.26 C; SD: 0.01 C). nal transects, Figs. 5E and 6A). Current speeds were asymmetric, with fastest flood and ebb current speeds of 1.35 and 1 4.3.2. The ETM nose and upper core regions 1.19 m s , respectively (Fig. 8C), and fastest depth-averaged 1 Profiling observations were made during a small spring tide flood and ebb speeds of 1.2 and 0.9 m s , respectively. at Selby (Fig. 1), 20 km from the tidal limit and in the central The tidal bore had a height of 0.18 m and a rise time of 16 s or upper core region of the ETM at HW, dependent on tidal (Fig. 8D). The bore crest was followed by small oscillations in range (Fig. 3A, C). The tide was intermediate between those water level, which were less than 0.04 m in range and had a typ- pertaining to the spring and neap-tide longitudinal surveys ical duration of 6 s. The consequence of the bore’s passage on (Fig. 3). Salinity remained low throughout the tide and maxi- SPM concentrations was again slight. SPM concentrations 1 mised at ca. 1 around HW slack (Fig. 8B). The rapid rise in were relatively low (<1.7 g l ) and variations over the 160-s water level again followed the up-estuary passage of a small ‘event’ period (Fig. 8D) were strongly, positively correlated tidal bore at LW. Tidal range was 2.9 m compared with with temperature variations (mean: 20.118 C; SD: 0.002 C). 4.9 m at Immingham (i.e. 59% of the mouth value) and LW and HW lagged Immingham by 6.6 h and 3.0 h, respectively. 4.3.3. The ETM tail and lower core region The tidal rise time, LWeHW, was 2.4 h compared with A spring tide was worked at station BTJ (Fig. 1), 59 km 10.1 h for the tidal fall. from the tidal limit and down-estuary of the ETM tail at SPM concentrations increased over the 2.4-h duration of HW (Fig. 3A). The tide was slightly smaller than that observed the flooding tide and reached an observed near-bed maximum during the spring-tide longitudinal transect (Fig. 3A). The sa- of 66 g l1, 0.5 h after HW slack (Fig. 8A). The ETM nose linity reached a maximum of 17.6 at HW slack and a minimum reached the station ca. 2 h before HW and withdrew ca. 8 h of 4 at LW (Fig. 9B). The tidal range was 6.4 m, the same as after HW. The core of the ETM reached the station ca. 0.5 h Immingham, and LW and HW lagged Immingham by 3.2 h 38 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

3 3 (A)15 Aug. 95, Springs, HW (7.3 m), SPM, g/l (B) 15 Aug. 95, Springs, Salinity

2 2

4 .5

0.55 5

1 4 1 2 0.5

2 Height Above Bed (m)

1 0

0.45 .5

8 5 .4

6 0 0 1 0 10 12 14 16 18 20 10 12 14 16 18 20 Time in Day (hours) Time in Day (hours)

3 1.8 0.32 (C)15 Aug. 95, Springs, Speed, m/s (D) 15 Aug. 95, Springs, Bore

2 .8 1.7 0.26 0 Depth

1.6 SPM g/l

0.8 1 1 1.6 0.20 Sensor Depth (m) Height Above Bed (m)

0.4

Height Above Bed (m) .4

0 .4 SPM 0 1.5 0.14 10 12 14 16 18 20 0 20 40 60 80 100 120 140 160 Time in Day (hours) Time, Seconds

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 maximised 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, was The temporal resolution of the profiling and moored instru- 4.6 h compared with 8.1 h for the tidal fall. mentation was too coarse to resolve the transition from ebb to In contrast to the ETM nose region, SPM concentrations in- flood at LW, although at spring tides the transition was again creased during the ebb and reached a near-bed maximum of very sharp (illustrated for a similar spring tide in Fig. 9D). 39 g l1 at 0.5 h after LW, during the very early flood The displacement of the ETM over the LW period and the ef- (Fig. 9A). The ETM tail and core reached the station on the fects of settling and vertical mixing for this tide were evident ebb between 2.6 h and 0.7 h before LW, respectively, and from SPM concentrations recorded in the surface layers at moved back up-estuary on the flood between 0.8 h and 2.2 h a fixed height above the bed (Fig. 9D). after LW. This tidal displacement of the ETM also was illus- Neap-tide observations at station BTJ illustrated very high trated by the inversion of SPM concentrations through the wa- concentrations of near-bed SPM in the ETM tail region at, and ter column at mid ebb, in response to tidal ‘straining’ of the following, LW (Fig. 10A). The neap tide was very small, 2.9 m longitudinal gradient in SPM. The resultant decrease of SPM tidal range at Immingham, compared with 3.9 and 4.6 m dur- concentration with depth, with its associated density inversion, ing the HW and LW neap-tide longitudinal transects, respec- was stabilized by increased salinity deeper in the water col- tively (Figs. 3C, D, 4). Salinity reached a maximum of 14 at umn (Fig. 9B). HW slack and a minimum of 5 at LW slack (Fig. 10B). The Current speeds were asymmetric. The fastest flood current vertical salinity gradient was strongly inverted during the pe- was ca. 1.3 m s1, compared with 1.1 m s1 on the ebb riod from approximately mid flood to mid ebb. The tidal range (Fig. 9C). Settling of SPM occurred in the slow currents at station BTJ was 3.2 m (i.e. 110% of the mouth value) and near LW, when depth-averaged speeds of less than 0.2 m s1 LW and HW lagged Immingham by 3.0 h and 1.4 h, respec- persisted for ca. 0.5 h, which resulted in lower near-surface tively. The tidal rise time, LWeHW, was 5.1 h compared SPM concentrations (Fig. 9A). The down-estuary displace- with 7.6 h for the tidal fall. ment of the ETM between HW and LW, and the enhanced SPM concentrations increased during the ebb, especially stratification of SPM at LW due to settling, were consistent close to the bed (Fig. 10A). The ETM core reached the station with the observed longitudinal distributions of SPM at HW ca. 2.3 h before LW and SPM concentrations continued to in- and LW (Figs. 3C, 4A, 5A). crease rapidly until LW slack. Longitudinal advection and R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 39

4 4 (A) 24 Aug. 95, Spring, HW (6.6 m), SPM, g/l (B) 24 Aug. 95, Spring Tide, Salinity

3 3

6 1

2 4 2

8 0.9

1 16 32 1 Height Above Bed (m) Height Above Bed (m) 3 8 0.9 2 4 0.5

2 0.7 0.7 0 0 10 12 14 16 18 20 10 12 14 16 18 20 Hours Hours

4 0.7 1.7 (C) 24 Aug. 95, Spring Tide, Speed, m/s (D) 24 Aug. 95, Spring Tide, Bore

3 0.6 1.6

1.2 2 1.5 0.8 0.5

1 SPM g/l .2 0.4 Sensor Depth (m) 1 0.4 Depth 1.4 Height Above Bed (m) 0.8 SPM .4 0.8 0.8 0 0 0.3 1.3 10 12 14 16 18 20 0 20 40 60 80 100 120 140 160 Hours Time, Seconds

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 rangeat Immingham was 4.9 m. Maximum SPM occurred around HW and 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, high A tidal bore did not occur at LW during this small neap tide concentration layer of sediment, in which concentrations in- (Fig. 10D) and the transition from ebb to flood was smooth. creased to 92 g l1 on the flooding tide, 2.4 h after LW Near-surface SPM concentrations showed a decreasing trend (Fig. 10A). Current speeds within this layer were negligible toward LW, due to floc settling, with a minimum that coincided (Fig. 10C), although some entrainment of sediment occurred with minimum salinity at slack water, 0.4 h after LW. from the layer surface, especially during the fastest current speeds at mid flood (14:30 hours, Fig. 10A, C). These data 4.4. Floc sizes in the ETM are qualitatively consistent with the LW neap-tide transect, which exhibited high concentration layers in the station BTJ In situ floc sizes were measured during the spring tide at region of the upper Humber and lower Ouse, with SPM con- Selby (Fig. 1) in the upper core region of the ETM at HW centrations greater than 30 g l1 (Fig. 4A). The LW transect (Figs. 3A, 8A). Profiling observations showed that median di- survey (Fig. 4) had a tidal range of 4.6 m, substantially greater ameters exceeded 110 mm but were less than 310 mm than the 2.9 m (Immingham) tide profiled at station BTJ (Fig. 11A). Median floc size was 19 mm in the fluvial waters (Fig. 10). In addition, the station was worked on a tide that fol- immediately up-river of the tidal limit during this period lowed several days of small neap tides, which also would have (Law, 1998). Larger floc sizes at Selby generally were associ- assisted the formation of these muddy layers. ated with higher water levels. However, depth-averaged sizes The salinity inversion (Fig. 10B) was such that at, e.g. in excess of 120 mm also were negatively correlated with 18:15 hours, salinity decreased from 12.8 to 8.3 between 7 bulk indicators of tidal energy dissipation (U3=h, where h is and 8 m beneath the surface, which in isolation would have depth and U is depth-averaged current speed) and, more produced an unstable density difference of <4gl1 over this strongly, current shear (U=h). For example, when bulk shear section of the lower water column (a density conversion is given was less than 1 s1, which corresponded to sizes greater later, in Eq. (5)). However, SPM concentration increased from than 120 mm, then median floc size increased approximately 0.5 to 16 g l1 over this depth (Fig. 10A), i.e. a stable density linearly with decreasing shear. difference of greater than 9 g l1, so that the lower water col- The largest flocs occurred near mid-depth at HW slack and umn was overall stable. on the early ebb (up to 310 mm, Fig. 11A) and were associated 40 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

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

10 0 10

.5 6

1 14

12 4 2

10 5 2 5

4 8

9 Height Above Bed (m) Height Above Bed (m) 8 16 0 0 8 101214161820 8 101214161820 Hours Hours

0.4 SPM

0.8 4 4

5 .2 1

0.8 SPM, g/l

0.8 2 2 Sensor Depth (m) Height Above Bed (m) 0.4 0.4 Depth

0 0 0 8 101214161820 0 100 200 300 Hours Time, Minutes

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 rangeat Immingham was 6.4 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 (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- the neap tide (Fig. 11B). The ETM was located somewhat fur- face layers (between ca. 09:00 and 11:00 hours in Fig. 11A, ther down-estuary during September so that SPM concentra- C). The period from HW through to the early ebb also was tion contours were very similar to, but greater than, those for a time of very low current shears in the upper water column the August spring tide at station BTJ (Fig. 9A). Floc sizes (Fig. 8C). A spring tide was worked at this site in September, were vertically homogeneous for most of the tide but reached when the ETM was somewhat further down-estuary (not a maximum (ca. 500 mm) near mid-depth at HW slack. This shown). The data displayed very similar floc-size behaviour was associated with decreasing SPM concentrations in the sur- but with sizes between 170 and 360 mm. When depth-averaged face layers, due to floc settling, similar to the spring tide at sizes were greater than 180 mm there again was a negative cor- Selby over the HW-slack period (Fig. 11A, C). Sizes decreased relation between size and both bulk energy dissipation and during the ebb, reaching ca. 100 mm at LW slack and became bulk shear, in this case for shear less than 0.8 s1. smaller (ca. 70 mm) on the faster flooding currents before in- At station BTJ in the lower Ouse, the tail and core of the creasing again toward HW. There were strong negative corre- ETM water column had a wide range of median floc diameters lations between depth-averaged, median floc size and during the small neap tide, from less than 100 to more than indicators both of bulk energy dissipation and bulk current 300 mm(Fig. 11B, D). Maximum median sizes exceeded shear throughout the tidal cycle and for the whole range of ob- 1 mm in the stagnant, high concentration layer that remained served median floc size. near the bed at station BTJ throughout much of the tide (after A transect through part of the ETM core was undertaken 12:00 hours, Fig. 11B, D). Local current shear was high near close to HW conditions during a spring tide in July 1995. the top of the layer, which led to strong entrainment at peak The transect comprised five profiling stations progressing flood currents (ca. 14:30 hours, Figs. 10C and 11D) and a min- down-estuary between Selby and BFB (Fig. 1) and started ima in floc size just above the entrained layer, indicative of ca. 1 h before local HW at Selby and ended ca. 2 h after local shear-induced floc breakage (Fig. 11B). HW at BFB. The transect showed the settling of SPM from The observed floc sizes at station BTJ during a spring tide in near-surface waters following HW slack in the lower core September 1995 (not shown) were very different from those at region of the ETM (Fig. 12A) and the location of the largest R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 41

(A)21 Aug. 95, Neaps, HW (5.7 m), SPM, g/l (B) 21 Aug. 95, Neaps, Salinity 10 10

5 5 Height Above Bed (m) Height Above Bed (m)

16 32 32 16 64 0 0 10 12 14 16 18 20 10 12 14 16 18 20 Hours Hours

1 1.4 (C) 21 Aug. 95, Neaps, Speed, m/s (D) 21 Aug. 95, Neaps, LW 10 0.8 1.2

0.6 1.0 SPM

5 0.4 0.8 SPM, g/l Sensor Depth (m)

Height Above Bed (m) 0.6 0.2 Depth

0 0 0.4 10 12 14 16 18 20 020406080 Hours Time, Minutes

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, sediment budget for the upper 1.5 m layer of water column where salinity was <4(Fig. 12BeD). Floc sizes were smaller gave an effective settling velocity of 0.3 mm s1 at the base (ca. 150 mm) nearer the bed in this region, presumably due of the layer, when averaged over the 0.5-h period of observa- to floc disruption resulting from tidal shear at the bed. Floc tions that spanned HW slack. The effective settling velocity sizes also were smaller (ca. 100 mm) further up-estuary in was less than the actual floc settling velocity because of the the ETM core, but increased again close to the surface (ca. influence of mixing, which tended to maintain SPM in suspen- 200 mm). sion and reduce WES according to:

4.5. Settling of flocs within the ETM vP W P ¼ W P K ð1Þ ES S z vz Floc settling velocity, WS, is the sinking velocity of a floc in still water, which may be reduced by the presence of other In this equation, P is SPM concentration, z is depth beneath flocs that cause hindered settling (Winterwerp, 1999, 2001, the surface and Kz is the vertical eddy diffusivity for SPM. 2002). The effective floc settling velocity, WES, is the velocity If conditions were steady and longitudinal advection were neg- with which a suspension of flocs sinks through the in situ wa- ligible, then the effective settling velocity would be zero and ter column. Direct observations of WS were not made in this either Eq. (1) (Fugate and Friedrichs, 2002), or the Rouse work, although it was possible to make direct estimates of equation (Rouse, 1937; Chester, 1999; Orton and Kineke, WES and indirect estimates of WS because of the pronounced 2001) could be used to estimate the floc settling velocity. settling of SPM that occurred from the upper water column However, conditions generally were not steady, especially over the HW-slack period. around HW slack when the current reversed, although the in- In the ETM nose region, settling occurred from the upper fluence of longitudinal advection was likely to have been min- 1.5 m of water column over HW slack and led to substantially imal then because of slow current speeds. The classical eddy higher SPM concentrations nearer the bed (Fig. 7A). A viscosity relationship was used for water depth, h, and reduced 42 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

4 (A) 24 Aug. 95, Mean Tide, Sizes, microns (B) 21 Aug. 95, Neaps, Sizes, microns 10 Selby BTJ 3

2 250 5 250

1 2 5 Height Above Bed (m) Height Above Bed (m) 0 0 0 250 3 800 800 0 0 10 12 14 16 18 20 10 12 14 16 18 20 Hours Hours

4 (C) 24 Aug. 95, Mean Tide, SPM, g/l (D) 21 Aug. 95, Neaps, SPM, g/l 10 Selby BTJ 3

6 1 2 5

32 1 16 Height Above Bed (m) 32 Height Above Bed (m) 16 32 64 32 0 0 16 10 12 14 16 18 20 10 12 14 16 18 20 Hours Hours

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 In the ETM tail region, the whole water column underwent (e.g. Wolanski et al., 1988, 1992): partial sediment deposition to the bed during the spring-tide, HW-slack period (Fig. 9A). The effective settling velocity, Kz ¼ 0:4hUð1 z=hÞðz=hÞ f ðRiÞð2Þ equivalent here to a deposition velocity to the bed, was 1.3 mm s1 over the 1-h period of observations that spanned The gradient Richardson number, Ri, was calculated at the HW slack. The mean, near-bed SPM concentration and esti- e base of the near-surface 1.5 m layer and used in the Munk mated turbulent shear stress were 1 g l1 and 0.05 Pa. The Anderson relationship for f ðRiÞ (Munk and Anderson, 1948; mixing ‘correction’ was 0.2 mm s1 and Eq. (3) gave an esti- Wolanski et al., 1988). The friction velocity, U, was derived mated settling velocity of 1.5 mm s1. from the quadratic drag law (see later). Eq. (2) gave a vertical 2 1 In the ETM tail region, the upper 8 m of water column under- eddy diffusivity coefficient of 8 cm s at the bottom of the went partial sediment deposition onto the underlying high concen- 1.5 m surface layer when averaged over the 0.5-h period. tration layer during the neap-tide, HW-slack period (Fig. 10A). The mean SPM concentration and estimated turbulent shear The effective settling velocity was 0.6 mm s1, averaged over stress at the bottom of the 1.5 m layer, over the HW-slack pe- the 0.75-h period of observations that spanned HW slack. The riod, were 3.6 g l1 and 0.3 Pa. The mixing ‘correction’ was 1 1 mean SPM concentration and estimated turbulent shear stress at 0.7 mm s and an estimate of 1.0 mm s for the settling ve- the bottom of the 8 m upper layer of water column were locity was derived from: 1.3 g l1 and 0.7 Pa. The mixing ‘correction’ was 1.5 mm s1 and Eq. (3) gave a floc settling velocity of 2.1 mm s1. vP W ¼ W þ K P1 ð3Þ S ES z vz 5. Discussion In the ETM nose and core regions, the effective settling velocity from the near-surface 1-m layer was 0.5 mm s1 over HW slack The prolonged period of very low and nearly constant (Fig. 8A). The mean SPM concentration and estimated turbulent freshwater runoff and quiescent meteorological conditions shear stress at the bottom of the layer were 19.5 g l1 and 0.3 Pa. that preceded and persisted throughout these measurements The mixing ‘correction’ was 0.2 mm s1 and Eq. (3) gave an ensured that: (a) the ETM was located close to the tidal limit estimated floc settling velocity of 0.7 mm s1. at HW; (b) it was characterized by very high SPM R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 43

0 0

0 1 24 -2 6 -2

32 -4 -4 64

-6 -6

(A) 17 Jul. 95, Springs HW (7.1 m), SPM, g/l (B) 17 Jul. 95, Springs HW (7.1 m), Sizes, microns -8 -8 25 30 35 25 30 35

Depth (m) 0 0

-2 -2

-4 -4

-6 -6

(C) 17 Jul. 95, Springs HW (7.1 m), Salinity (D) 17 Jul. 95, Springs HW (7.1 m), Sizes + SPM -8 -8 25 30 35 25 30 35 Distance from Tidal Limit (km)

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 was where S is salinity, P is SPM concentration (g l1) and the largely driven by intratidal and springeneap tidal forcing. units of r are kg m3. Using salinity data from the Tees and Test Estuaries, UK, 5.1. Salinity and the ETM Dyer and New (1986) were able to define three parametric regions: when RiL > 20 a stable density interface may exist in Generally, salinity was fairly well mixed vertically, al- the water column that displays internal waves but no signifi- though both slight stable and unstable vertical salinity gra- cant tidal (bed shear) mixing; when 20 > RiL > 2 the interface dients sometimes occurred due to tidal ‘straining’, as well as is modified by tidal mixing; and when RiL < 2 then strong tidal pronounced salinity inversions that were stabilized by large, mixing occurs. Applying Eqs. (4) and (5) to the observed stable, vertical gradients in SPM concentration. The existence spring-tide tidal cycles, in which SPM-induced density differ- of these stable SPM gradients demonstrated the importance of ences dominated those due to salinity, implies that strong tidal particle (floc) settling to the temporal development of SPM mixing generally occurred throughout the tide. Exceptions to concentration and transport within the ETM, even though ver- this were periods of HW slack in the ETM nose and core re- tical mixing generally was sufficiently strong to ensure that sa- gions, as well as LW and HW slack in the ETM tail region, linity was approximately well mixed. The latter can be during which the stratifying influences of advection and tidal understood with the use of a bulk Richardson number that pri- straining were, in any case, minimal. Therefore, tidal mixing marily characterizes the influences of freshwater-induced is anticipated to result in approximate vertical homogeneity buoyancy on mixing (Dyer and New, 1986): for salinity. SPM concentrations in the ETM nose were, by definition, in 2 1 RiL ¼ ghDr= rU ð4Þ the range 4e16 g l , whereas salinity was less than 1, so that the SPM contribution to density there was at least three times In this definition, h is water depth, r is density and Dr is the (and generally many times) greater than that due to salinity. surface-to-bed density difference. Ignoring temperature varia- More importantly, the near-bed, longitudinal density gradient tions, an approximate formula for the bulk density of estuarine in the ETM nose was approximately 2.5 kg m3 km1, where- waters, r,is(Odd, 1988): as that due to salinity was two orders of magnitude smaller (0.02 kg m3 km1). Therefore, salinity was insignificant to r ¼ 1000 þ 0:76S þ 0:62P ð5Þ the dynamics of ETM movement in the nose and upper core 44 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 regions of highest SPM concentrations. There, the tendency and Daly estuaries (Simpson et al., 2004; Wolanski et al., would have been for near-bed, SPM-induced density currents 2004, respectively). The small undular bore observed in the to flow towards the tidal limit. In the tail region, the near- upper ETM region of the Ouse typically was 0.1e0.2 m in bed longitudinal density gradient due to SPM was approxi- height, although in this region it has been reported to be mately 0.6 kg m3 km1 and that due to salinity also was much higher on occasions, greater than 0.5 m at the largest 0.6 kg m3 km1, but oppositely directed. It follows that up- spring tides, and to travel at speeds from ca. 2 to 3 m s1 estuary-directed, salinity-induced density currents near the (RMBC, 1986). During the passage of the bore through the bed would have tended to oppose down-estuary-directed, nose and upper core stations there were strong correlations be- SPM-induced density currents and therefore would have con- tween SPM and temperature variations. Therefore, patches of tributed to the retention of fine sediment in the ETM. more fluvial, lower turbidity waters, together with patches of Although slight inverse salinity stratification sometimes oc- more estuarine, higher turbidity waters were transported curred on the flood due to tidal straining, and this was stabi- through the stations and SPM fluctuations were ‘conservative’, lized by vertical SPM gradients, much stronger inverse with no indication of bed-sediment erosion due to the passage salinity stratification occurred from mid flood to mid ebb dur- of the bore. ing the neap-tide tidal-cycle station in the tail region. This in- The Ouse bore, like the bore observed in the Daly Estuary, version was associated with a near-bed, high concentration was not associated with suspension of SPM from the bed, even layer and implies that the layer was derived from further up- though this might have been anticipated in view of the currents estuary, in the vicinity of maximum SPM concentrations, generated by the bore’s passage (Wolanski et al., 2004). The and that lower salinity waters were retained within it during reason for this lack of observed SPM suspension appears to its down-estuary advection on the ebb. be due to the locations of the nose and core measuring stations. At local LW (the time at which the bore travelled 5.2. Hydrodynamics and the ETM through the stations) the ETM and its sediment stock were located down-estuary of the tidal-cycle stations and only ap- A strong reduction in tidal water-level amplitude occurred peared at these stations on the flood, after the bore had progressing up-estuary from the ETM tail to the ETM nose travelled further up-estuary. For typical LW depths of 1 m in (a factor of ca. 3 at spring tides) due to the influences of fric- the upper estuary, and for small bore heights, the up-estuary tional energy losses, opposing freshwater flow and rising bed propagation speed of the bore would have approximated the 1 levels. The tide was strongly asymmetric, with a rapid and rel- shallow-water wave speed of ca. 3 m s relative to the fresh- 1 atively short-lived flood that followed the up-estuary passage water flow or, typically, faster than ca. 2 m s relative to the of a small tidal bore that signaled LW and the end of the pro- estuary’s bed. This speed is comparable with, or exceeds, the longed ebb. fastest observed flood currents, so that any SPM suspended by Ignoring advection, the depth-averaged balance of forces in the passage of the bore through the ETM (located further the momentum equation is: down-estuary at LW) and carried by flood currents into the upper estuary, would have been ‘outrun’ by the bore. 1 vU=vt ¼gv2=vx DUjUj=h gðh=rÞvr=vx ð6Þ 2 5.3. Mobility of the ETM

In Eq. (6), the water acceleration (left hand side) is equated to At both spring and neap tides the very turbid ETM core was the sum of accelerations or decelerations due to surface-slope separated longitudinally from much lower turbidity waters by forcing (first term, right hand side), quadratic frictional drag nose and tail regions that were located up-estuary and down- (second term, right hand side), which defines the friction ve- estuary of the core, respectively. Following the spring tide 1=2 locity in Eq. (2) to be U ¼ D jUj, and longitudinal density there was a down-estuary displacement of the ETM during gradient forcing (third term, right hand side). A value of the smaller tides that led into the neap tide of the second sur- 0.0022 was used for D (e.g. Orton and Kineke, 2001). vey. The displacement occurred with respect to both distance Application of Eq. (6) to the observed, spring-tide, tidal- from the tidal limit and the salinity distribution. The ETM cycle data showed that, apart from slack-water periods, the fric- nose extended a considerable distance into the tidal river dur- tional drag on tidal currents in the ETM region was approxi- ing the spring tide, which indicates that the various influences mately balanced by surface water-level (z) forcing. Density of tidal currents on SPM transport dominated those due to sa- forcing due to salinity and SPM were comparatively small. linity. The shape of the ETM nose was qualitatively similar to However, in the ETM tail region, density influences were a sig- that observed in gravity currents, e.g. the arrested salt wedge nificant component of the momentum balance during LW slack. (Simpson, 1997), so that the additional density of these As anticipated, water accelerations were greater than could be SPM-laden nose waters may have been a factor affecting their explained by frictional drag and density effects during the LW incursion into the tidal river during the late flood. However, passage of the bore in the ETM nose and core regions and there was no evidence to indicate that a two-layer density must therefore have been driven by surface-slope forcing. flow developed at HW slack and the momentum balance indi- Tidal bores are a feature of some strongly tidal estuaries cates that the density influence was much smaller than that due and their physics have been described in detail for the Dee to the tides. R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 45

Longitudinal displacements of the ETM also occurred dur- estuary. In the tail region, shear transport worked in the oppo- ing a tidal cycle, as well as over a springeneap cycle, and both site direction to that in the nose region and promoted down-es- longitudinal surveys and tidal-cycle measurements illustrated tuary transport, both at spring and neap tides, thereby acting as the up-estuary movement of the ETM’s nose and core during a counter to the strong, advective, up-estuary flood transport. the flooding tide and their subsequent down-estuary withdrawal Maximum ebb-directed rate of shear transport there exceeded during the ebb. The down-estuary movements between HW 75% of the maximum, flood-directed rate of advective and LW, both of the ETM nose and the location of the maxi- transport. mum SPM concentration, were less than those of the isohalines, whereas the displacement of the ETM tail was 5.4. Fluid mud in the ETM approximately the same as that of the isohalines. This indicates that fine sediment may have been partially retained in Winterwerp (1999) defined high-concentrated mud suspen- the upper estuary by shear dispersion mechanisms during the sions to be those with concentrations from a few hundred ebb portion of the tide. milligrams per litre to a few grams per litre, and fluid mud The possible importance of shear dispersion is consistent as suspensions several 10s of g l1 to 100s of g l1 at, or be- with the occurrences both of a very sharp ETM nose and of yond the gelling point (see later). The near-bed, high concen- a pronounced SPM stratification in the nose and core regions tration SPM suspension that occurred at the ETM tail station at HW slack and during the early ebb. SPM stratification throughout much of the neap tide had concentrations that ex- was maintained during the ebb by down-estuary advection of ceeded 90 g l1 in the base of its stagnant, approximately 1-m lower turbidity, near-surface waters (tidal straining of SPM), thick layer. These concentrations, and also those that occurred together with the tendency for flocs to settle. Reduced vertical in the up-estuary core region of the ETM around slack water, mixing due to SPM-induced density stratification and stabili- therefore correspond to fluid mud. In the Severn Estuary, zation also may have played a role in maintaining and enhanc- Kirby and Parker (1983) identified near-bed, mobile mud sus- ing vertical gradients (e.g. Wolanski et al., 1988). SPM pensions with mass concentrations between 3 and 15 g l1 stratification was very strong at HW slack for spring and (subsequently as great as ca. 150 g l1; Kirby, 1986) and sta- neap tides and at LW slack during neap tides, because of tionary, near-bed suspensions that were up to 1.5 m thick reduced vertical mixing and floc settling. SPM was much and had concentrations that reached 200 g l1 (Kirby and less stratified at LW of spring tides in the ETM nose and Parker, 1983; Kirby, 1986). These stationary suspensions core regions, due to faster currents and shallower depths dur- were located in the deeper parts of channels and were settling ing the ebb and the occurrence of a tidal bore, rather than slack but not moving. They were only persistent and widespread at water, at LW. neap tides in the Severn. Nevertheless, the Severn has a 6.5-m The importance of shear dispersion to the longitudinal sed- tidal range and currents of 1.8e2ms1 during neap tides iment budget within the ETM can be quantified using meas- (Kirby and Parker, 1983), which are similar to spring tides urements at the tidal-cycle stations. The rate of longitudinal in the Ouse. suspended sediment transport per unit width at a fixed station Such high concentration stationary suspensions will ulti- can be split into two components: one due to vertical shear, FS, mately form a consolidating bed under quiescent conditions, and the other due to longitudinal advection, FA. The rate of if given sufficient time. Delo (1988) refers to approximately transport per unit width of estuary is then (Uncles and Ste- 80 g l1 as the concentration at which a bed begins to form phens, 1999): in a laboratory settling column, whereas a range of 50e Z Z 100 g l1 is given by Whitehouse et al. (2000). As the concen- h h tration of the settling stationary suspension increases, the F ¼ UP dz ¼ hU P þ ðU UÞðP PÞdz ¼ FA þ FS 0 0 effective settling velocity of the constituent flocs progressively ð7Þ decreases, due to hindered settling. The velocity eventually be- comes zero at the suspension gel concentration, which is the The tidal-mean rate of longitudinal SPM transport in Eq. (7) is SPM concentration at which a space-filling network of fine therefore the tidally averaged sum of advective transport and sediment is formed though the flocculation mechanism (Win- shear transport. Ebb transport and flood transport of SPM terwerp, 1999, 2001, 2002). A considerable time, several hours were in approximate balance over the spring tidal cycles at to days (Delo, 1988), may therefore be required for these sus- each station within the ETM, so that the tide-averaged trans- pensions to consolidate. port was very small compared with the maximum instanta- The near-bed SPM layer observed in the ETM tail region neous transport. As anticipated, shear transport during the station had characteristics both of mobile fluid mud and ebb was directed up-estuary in the nose and upper core regions stationary suspensions. An increasingly thick and increasingly of the ETM and therefore acted to oppose down-estuary ad- more turbid layer of fluid mud from the ETM tail was advective transport of SPM. This maximum, flood-directed rate vected through the station on the late ebb. The speed, concen- of shear transport was ca. 20e30% of the maximum, down- tration and thickness of the mobile mud suspension (Kirby and estuary rate of advective transport and therefore contributed Parker, 1983; Kirby, 1986), or fluid mud layer (Winterwerp, substantially (ca. 10% of the total flood transport) to the 1999), typically ranged between approximately 0.1e0.5 m s1, maintenance of fine sediment in the upper region of the 15e40 g l1 and 0.5e3 m, respectively. It is known that 46 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 interactions between turbulent flow and a sediment suspension layers. Also, the period from HW through to the early ebb was can substantially affect mixing, vertical current profiles and a time of very low near-surface current shears and, by impli- other properties of the hydrodynamics (Wolanski et al., cation, low mixing in the upper water column that could 1988, 1989; Mehta, 1991; Shi, 1998; Winterwerp, 1999, have promoted floc growth and settling, despite increasing 2001, 2002; Li and Gust, 2000; Dyer et al., 2004). The ob- ebb current speeds as water level fell. These largest water- served, highest concentration of mobile SPM in the fluid column flocs had maximum median diameters of 310 mmin mud layer, 40 g l1, moved at speeds less than 0.1 m s1.At the ETM nose and core regions during a small spring tide LW slack, the layer concentration increased from more than and 360 mm during a larger spring tide the following month. 40 g l1 near the layer surface to 80 g l1 near its base. The Sizes were smaller during the mid-to-late ebb and during the base concentration increased to more than 90 g l1 later in flood, but exceeded 110 mm on the small spring tide and the tide. The velocities in this layer generally were zero for 170 mm on the larger spring tide. Similar results were observed the rest of the tide that followed LW slack, corresponding to during a spring tidal cycle in the ETM tail region. There, me- the stationary suspensions observed by Kirby and Parker dian floc sizes reached ca. 500 mm near mid-depth at HW (1983) and Kirby (1986) in the deeper parts of the Severn’s slack, but decreased to ca. 100 mm during the ebb and ca. channels. The ETM tail station was located in a deeper section 70 mm on the flood. These smaller, flood-tide floc sizes are of the Ouse’s main channel, which may have made it a more comparable with those observed in King Sound, Australia favourable location for the formation of these stationary (Wolanski and Spagnol, 2003). suspensions. Depth-averaged data illustrate a strong tendency for median Whitehouse et al. (2000) defined a bulk Richardson Num- floc sizes that were greater than a station-dependent minimum ber for fluid mud layers as: size to increase approximately linearly with decreasing bulk shear and, to a weaker extent, with decreasing tidal energy dis- 2 Rib ¼ ghm Dr= rðDUÞ ð8Þ sipation rate. The largest observed floc sizes in the Ouse were in excess of 1 mm and occurred in the near-bed stationary sus- where hm is the layer thickness and DU and Dr are the velocity pensions during the neap-tide ETM tail station. When strong and density differences between the fluid mud layer and over- entrainment occurred from the upper interface of this layer, lying water, respectively. Entrainment from the layer into the the associated SPM just above the layer had greatly reduced water is anticipated to occur if Rib, Eq. (8), is less than about floc sizes (<200 mm), which is indicative of shear-induced 10 (Whitehouse et al., 2000). According to this criterion, peri- floc breakage. In the York River Estuary, USA, Fugate and ods of entrainment would be expected to occur at the ETM tail Friedrichs (2003) also found that intense turbulent shear re- station on the early to late flood and for much of the ebb fol- duced floc sizes, which typically were in the range 50e lowing HW slack. This is consistent with the observed suspen- 300 mm, whereas inter-particle collisions during differential sion of SPM into the middle and upper parts of the water settling increased floc size. These finding are consistent with column from the high concentration layer. LW slack and those for the Ouse, despite the difference in SPM concentra- HW slack are predicted to be times of layer stability and tions for the two systems (<0.8 g l1 in the York River Estuary also growth, due to settling of SPM, which is consistent experiments): floc sizes are similar for the two estuaries; with observations of increased layer thickness and reduced increasing turbulent shear reduces floc size; and differential SPM concentrations in the overlying waters at those times. settling, which is likely to occur over HW slack in the HumbereOuse, is associated with increased floc sizes. 5.5. Flocculation in the ETM Measurements of floc sizes in surface and bottom waters of the Elbe Estuary, Germany, similarly showed that floc sizes The existence of these mobile suspensions, leading to fluid during HW slack were greater than those that occurred during mud layers, stationary suspensions and possible consolidation maximum flood and ebb currents (Eisma et al., 1994). Sizes in and bed formation, relies on the existence of particle floccula- the surface and bottom waters of the Elbe ranged from 100 to tion and associated enhanced settling of clay and silt-sized 250 mm and from 150 to 350 mm, respectively, which are sim- SPM from the bulk of the water column. Observed in situ ilar to the size ranges observed in the Ouse ETM, although floc sizes throughout the Humber and within the Humbere SPM concentrations were much less in the Elbe (<1gl1). Ouse ETM were much greater than that measured in the in- The occurrence of large flocs over the HW period also is a fea- flowing freshwater runoff (Law, 1998), although this increase ture of SPM data from the ETM of the Tamar Estuary, UK in floc size from river to estuary was unlikely to have been (Fennessy et al., 1994a,b). Bale et al. (2002) used a laboratory caused by salinity increases from fresh to brackish waters flume to show that marked flocculation of SPM occurred at (Eisma, 1991; Thill et al., 2001; Bale et al., 2002). specific times within a (simulated) tidal cycle when velocity The largest observed flocs in the ETM water column, not was decreasing and SPM concentration was high. When nom- within any underlying stationary suspensions, generally oc- inal SPM concentrations in the flume were ca. 1 and 4 g l1, curred near mid-depth at HW slack. This implies that slow median sizes typically were 80 mm for a flume water speed currents at slack water resulted in increased flocculation, of 0.45 m s1. Sizes increased to 180 mm when speeds had de- greater sizes and enhanced settling, which left smaller and celerated to 0.15 m s1, thereby imitating the approach to HW slower flocs and lower SPM concentrations in the near-surface slack. R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 47

5.6. Floc settling in the ETM Gironde (France) and Dollard (Netherlands) and is valid for t in the range 0.04e0.7 Pa and for P less than 8.5 g l1. It pre- Although floc size is an important SPM characteristic, floc dicts maximum settling velocities of the order of 10 mm s1. settling velocity is of more significance to engineering calcula- Settling velocities at relatively low SPM concentrations, e.g. tions. Measurements by Voulgaris and Meyers (2004) in a tidal 0.1 g l1, range between 0.7 and 2.2 mm s1, depending on creek that drained a salt marsh recorded mean floc sizes of 25e turbulent shear stress. Eq. (9) estimates that flocs with an 75 mm, settling velocities between 0.02 and 0.2 mm s1 and ESD diameter of 160 mm settle at 0.8 mm s1 and flocs greater SPM concentrations that were less than 80 mg l1. In the Pearl than 500 mm in excess of 3.5 mm s1. River Estuary, China, SPM concentrations typically were less Manning (2004) does not give a formula for the settling ve- than 100 mg l1, median floc sizes varied between 10 and locity of ‘microflocs’, which he defines as flocs with diameters 96 mm and settling velocities, which varied between 0.01 and less than 160 mm, but a Lorentz function fit to Manning’s 0.2 mm s1, increased with floc size (Xia et al., 2004). Much (2004) smoothed plot of settling velocity (mm s1) versus larger flocs were measured in the lower turbidity (15 mg l1) the logarithm of turbulent shear stress (Pa) gives a reasonable Po River prodelta (Fox et al., 2004). Sizes ranged from ca. 60 representation of his data: to 800 mm and settling velocities were of the order of 1 mm s1. Fox et al. (2004) gave an equation for the floc settling velocity: z 2 Ws 0:12 þ 3:5= p 4ðlog10ðtÞþ0:44Þ þ1:3 ð13Þ 4 1:33 WS ¼ 9:1 10 ðESDÞ ð9Þ Eq. (13) correlates settling velocity with turbulent shear stress 1 In Eq. (9), settling velocity has units of mm s and the floc only; it has a peak of just less than 1 mm s1 at a stress of ca. equivalent spherical diameter, ESD, has units of mm. Accord- 0.4 Pa and velocities in the range 0.2e0.3 mm s1 at very low ing to this relationship, flocs with ESD diameters of 96 and and high stresses of ca. 0.01 and 10 Pa. 75 mm settle at 0.4 and 0.3 mm s1, respectively, which are These various results for WS can be compared with indirect similar to the results (0.2 mm s1) given by Xia et al. (2004) estimates for the Ouse ETM. In the tail region, WS was and Voulgaris and Meyers (2004) for their largest observed set- 1.5 mm s1 for the spring tide (2.6 mm s1 from Eq. (12)) tling velocities. Eq. (9) estimates that flocs with an ESD diam- and 2.1 mm s1 for the neap tide (1.8 mm s1 from Eq. eter of 300 mm, similar to those in the Ouse ETM at HW slack, (12)). Corresponding SPM concentrations over HW slack 1 settle at 1.8 mm s . Sternberg et al. (1996) observed settling were ca. 1 g l1 and median floc sizes typically were ca. flocs on the northern Californian continental margin that had 200e300 mm, for which Eq. (9) predicts settling velocities sizes from 100 mm to greater than 500 mm with settling veloc- from 1.0 to 1.8 mm s1. These are similar to those simulated e 1 ities of 0.1 2.2 mm s . More recent work gave nominal ellip- using the Rouse equation for the Hudson River Estuary, e tical diameters (NED) of flocs in the range 130 740 mm and USA (2.2 mm s1; Orton and Kineke, 2001), where SPM con- 1 settling velocities between 0.09 and 8.1 mm s (Sternberg centrations typically were <2gl1, and for Chesapeake Bay, et al., 1999). A settling velocity equation was derived: USA (of order 1 mm s1; Fugate and Friedrichs, 2002), where 1 4 1:54 SPM concentrations typically were <80 mg l . WS ¼ 2:0 10 ðNEDÞ ð10Þ In the Ouse ETM nose and core regions, an indirect esti- 1 Settling velocities measured in the North Sea by Mikkelsen mate of WS was 1.0 mm s in the nose region at Cawood and Pejrup (2001) yielded: (5.5 mm s1 from Eq. (12)) and 0.7 mm s1 in the ETM core region at Selby (outside the range of application of Eq. : 4 1:53 WS ¼ 2 6 10 ðESDÞ ð11Þ (12)). Corresponding SPM concentrations at Cawood and 1 Eqs. (9)e(11) show a marked quantitative consistency with Selby over HW slack were ca. 4 and 19 g l , respectively. each other for sizes greater than ca. 150 mm, i.e. macroflocs. These indirect estimates of settling velocities are approximately Moreover, settling velocity versus floc-size data measured by half of those derived for the ETM tail, and much slower than Sternberg et al. (1996, 1999) fall well within the scatter of values from Eq. (12), which indicates that hindered settling data measured in the Tamar Estuary by Fennessy et al. may have affected the settling of flocs (Winterwerp, 1999, (1994b), which may indicate the relevance of these low-turbid- 2001, 2002). Winterwerp (2002) and Winterwerp and van ity coastal measurements to high-turbidity estuarine systems. Kesteren (2004) show that the settling velocity of a single A very wide range of floc sizes, from 20 to several hundred mi- floc of fine-grained sediment in still water is reduced by a fac- P crometers, have been observed in the Tamar Estuary ETM dur- tor, g, of approximately (for y 0): ing neap and spring tides (Fennessy, 1994a,b; Manning, 2004). 1 According to Manning (2004), the settling velocity (mm s ) g ¼ð1 fÞ 1 fr ð1 þ 2:5fÞð14Þ of ‘macroflocs’, which he defines as flocs with diameters in excess of 160 mm, is given by: In which f ¼ P=Pgel is the SPM volume concentration of the 2 muddy suspension, which is related to the mass and mass- Ws ¼ 0:718 þ 8:33t 12t þ 0:938P ð12Þ gelling concentrations, P and Pgel, respectively. In Eq. (14), Turbulent shear stress is denoted by t (units of Pa) in Eq. (12), fr ¼ P=rS, with rS the density of primary sediment particles which is based on data from estuaries of the Tamar (UK), (2700 g l1, taken as an average for the silt and clay mixture 48 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 within the SPM and based on data tabulated by Winterwerp turbulence and reduced mixing through the water column dur- and van Kesteren, 2004). ing the ensuing early ebb, which also was apparent from the Winterwerp (2002) estimated the gelling concentration of low current shears that occurred in the upper water column settling mud for three experimental situations and found that and which further facilitated floc settling. Wolanski et al. 1 Pgel was 40, 80 and 120 g l for the three cases. Settling-col- (1988) utilized a local, 1D vertical model of the water column umn data for the Ouse, when applied to Eq. (14), indicate that to demonstrate that sediment-induced buoyancy effects inhibit 1 Pgel is 64 g l (Uncles et al., in press-b). Eq. (14) shows that vertical mixing of sediment. In the upper core and nose region, the settling velocity is reduced to ca. 0.9 of the unhindered set- where salinity effects were negligible, tidal straining led to tling velocity at an SPM concentration of 2 g l1, to ca. 0.8 at even greater SPM stratification on the ebb. In the lower core 4gl1, 0.5 at 16 g l1, 0.2 at 32 g l1 and 0 at 64 g l1.Large and tail region, tidal straining would have tended to reduce reductions in effective settling velocity of SPM are therefore or invert SPM stratification on the ebb, although the tendency anticipated to result from hindered settling in the ETM nose to generate salinity stabilization would have partially compen- and core regions and in near-bed fluid mud layers that form sated this effect. The enhanced ebb sediment transport in the during neap tides and over slack-water periods. In the core re- ETM tail may have led to it being somewhat diffuse (typically gion at Selby, the estimated WS would be increased from 0.7 to extending over 12 km), whereas the nose (typically extending 1.7 mm s1 in the absence of hindered settling and in the nose over less than 3 km) remained strongly stratified and sharp as region at Cawood, WS would be increased from 1.0 to the high concentration suspensions moved back down-estuary 1.2 mm s1. on the relatively slower, near-bed ebb currents. Shallower water depths and faster currents on the mid-to-late ebb led to en- 5.7. Formation of the ETM trainment of sediment from near-bed suspensions and more effective mixing of SPM. These processes are reflected in The transition from ebb to flood was rapid at LW of spring the calculation of SPM fluxes, which illustrate the importance tides in the Ouse and was associated with a tidal bore in the of vertical shear and SPM stratification in reducing the down- ETM nose and core regions. Therefore, very little time was estuary sediment transport during the ebb in the nose and core available for settling of flocs. On the relatively short and ener- regions and which therefore act to accumulate sediment in the getic flood, tidal straining acted to reduce vertical SPM gra- upper estuary. dients in the upper core and nose regions because SPM The same transport mechanisms occurred at neap tides, al- concentrations decreased up-estuary. Salinity effects were neg- though the transition from ebb to flood was much gentler and ligible there. Straining, together with fast currents, tended to the longer LW and HW-slack periods enabled floc settling to generate strong near-bed shears, vertical mixing and vertical occur, which aided the development of fluid mud layers, espe- distributions of SPM that exhibited continuously increasing cially in the deeper parts of the main channel and over subse- concentrations from surface to bed, in which the greater, near- quent, decreasing neap tides. bed concentrations were of the same order of magnitude as Tidal and sedimentological processes in the Trent Estuary those at the surface. As a result, very strong, up-estuary advec- of the upper Humber (Fig. 1) are very similar to those de- tive transport of SPM occurred. SPM concentrations increased scribed here for the Ouse. Mitchell et al. (1998) show that up-estuary in the ETM tail and lower core region, so that tidal the Trent has a strong ETM in which concentrations can ex- straining tended to increase vertical SPM gradients on the flood, ceed 35 g l1. They conclude that the ETM is generated by although this stabilizing effect was partially compensated by the the combined influences of tidal asymmetry and SPM-induced tendency to destabilize salinity profiles. Therefore, vertical vertical density gradients. In the upper Trent, faster flood mixing and longitudinal sediment transport were expected to speeds, compared with the ebb, lead to tidal pumping of sed- be somewhat weaker than in the upper core and nose. iment and large, vertical SPM gradients also potentially inhibit At the end of the flood, over the HW-slack period, a fraction vertical mixing. They believe that non-Newtonian behaviour of the SPM settled into the lower water column, where con- of high concentration SPM suspensions may also enhance centrations in a near-bed layer typically were greater than the retention of fine sediment within the ETM. Other fluid 30 g l1 and could exceed 60 g l1. The HW-slack period en- mud estuaries, such as the Changjiang, China (Shi and Kirby, abled larger flocs to form and settle, although the effects of 2003), Fly River, Papua New Guinea (Wolanski et al., 1995) hindered settling, which were likely to have reduced settling and the South Alligator River, Australia (Wolanski et al., velocity from ca. 1 to ca. 0.1 mm s1 (4 to 0.4 m h1), pre- 1988) similarly exhibit these features. vented significant deposition to the bed and instead led to Uncles and Stephens (1993) described ETM formation the formation of high concentration, near-bed suspensions. mechanisms in the much less turbid Tamar Estuary. These This behaviour was reflected in an essentially linear, i.e. ‘con- mechanisms included floodeebb speed and vertical mixing servative’, scatter plot of depth-averaged SPM concentration asymmetries and different slack-water durations at HW and versus depth-averaged salinity for tidal-cycle data in the LW (cumulatively known as tidal pumping) as well as buoyancy nose region and at the tidal limit, with no evidence of erosion effects due to salinity (gravitational, or baroclinic, circulation) peaks or deposition troughs (Fig. 2B). and SPM-induced density gradients. Therefore, tidal influences The high concentration suspensions that formed over HW are thought to be important to ETM formation not only within slack would have led to the damping of bed-generated highly turbid estuaries, but also in medium and lower turbidity R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 49 estuaries and systems such as the Tamar (Uncles and Stephens, of spring tides, whereas erosive ebb stresses at springs con- 1993), King Sound (w3gl1, Wolanski and Spagnol, 2003), trolled the lower banks. It is likely, therefore, that waves the Hudson (w5gl1, Geyer et al., 2001) and the Pearl River will modify the behaviour of the Ouse ETM: (a) through their (w0.3 g l1, Wai et al., 2004). Even in the low-turbidity estuary influence on high concentration suspensions (Mehta, 1991); of Chesapeake Bay (w0.1 g l1), the ETM either would not (b) by supplying additional sediment to the ETM via erosion exist or would be greatly reduced without tidal suspension of mudbanks; and (c) by preventing or reducing deposition and transport of bed sediments (Sanford et al., 2001). onto the upper mudbanks. Wind waves are thought to be im- Gravitational circulation also appears to play a role in the portant to the entrainment of fluid mud in the Fly River Estu- accumulation of fine sediment within the Ouse’s ETM. Uncles ary (Wolanski et al., 1995) and in the King Sound (Wolanski et al. (1998a) showed that the ETM tended to occur at higher and Spagnol, 2003), possibly through wave pumping rather salinities (but <11) during winter months, when it was located than wave velocities. The Tana Estuary, Kenya (Kitheka in the upper Humber or the lower Ouse. Gravitational circula- et al., 2005), has large sediment inputs from its river and an tion will be stronger in the deeper, higher salinity regions of ETM (w5gl1) that is partly generated during spring tides the upper Humber and may contribute both to the ‘trapping’ by wave stirring of bed sediments and trapping of SPM in and to the maintenance of fine sediment within the ETM dur- the low salinity reaches. ing high-inflow winter months and to its up-estuary transport Grabemann and Krause (2001) found evidence that the during the late spring. Higher HW salinity establishes itself ETM (w1gl1) in the Weser Estuary, Germany, was pushed fairly quickly within the Ouse, whereas the mobile pool of up-estuary during times of higher mean water levels due to fine sediment lags about one month behind salinity (Uncles storms and conclude that conditions in the seaward region et al., 1998a). Mitchell (2005) suggests that this fairly rapid are as important as those in the river. In the Humber, Ouse response of the SPM to inflow (approximately one month) and Trent it is known that the ETM moves further into the up- is indicative of the highly canalized nature of the Ouse and per reaches with reduced river flow, and that SPM concentra- Trent. tions and bed-sediment distributions are strongly related to The complexity of the processes that determine the ETM freshwater flow (Uncles et al., 1998a,c, in press-a; Mitchell, has meant an increased use of models to understand and 2005). then predict its formation. Lin and Kuo (2003) used a 3D model to simulate the ETM (w0.1 g l1) in the York River 6. Conclusions Estuary. They found that the location of the ETM was associated with the null point of bottom gravitational circulation and An extremely turbid ETM exists in the upper reaches of the that bottom residual flow and tidal asymmetry contributed to Ouse during quiescent, approximately steady, low runoff sum- the formation of a smaller, secondary SPM concentration mer conditions. At these times, physical behaviour is dominated maximum. Burchard and Baumert (1998) used a 2D verti- by tidal and freshwater-induced currents. Tidal water levels calelongitudinal model of a hypothetical macrotidal estuary are very asymmetric in the up-estuary region, especially at to show that it is necessary to have gravitational circulation spring tides, when a rapid LW to HW rise (ca. 2e3 h) is fol- and tidal velocity asymmetry to generate an ETM lowed by a much longer HW to LW fall (ca. 10 h) and peak (w0.3 g l1) and that whereas tidal mixing asymmetry influ- flood current speeds (ca. 2 m s1) exceed peak ebb speeds. ences the magnitude of the ETM, it does not appear to be nec- The shallow waters and fast currents lead to strong frictional essary for its establishment. Le Hir et al. (2001) used a 3D drag at the bed, which is approximately balanced by forcing model to simulate fine sediment transport in the Seine. They due to surface water-level slopes. This drag leads to a large re- found that the ETM (>1gl1) was largely generated by tidal duction in tidal amplitude (ca. 0.3 of the mouth value close to pumping. Models of ETM formation that take into account the tidal limit) as the tide propagates into the estuary. A tidal suspended and deposited mud rheology (e.g. Mehta, 1991) bore, of height 0.1e0.2 m, signals local LW as it travels up-es- do not appear to have been attempted yet. tuary at spring tides in the upper reaches of the estuary. How- ever, the bore does not cause suspension of fine sediment at 5.8. Meteorological effects on the ETM locations up-estuary of the ETM and its associated pool of silt and clay-sized sediment. Salinity is fairly well mixed This paper has been concerned with the behaviour of the throughout the water column, despite the existence of strong low runoff ETM under quiescent conditions. Meteorological SPM stratification in the ETM region. Slight stable and unsta- influences, such as wind and wind-driven waves and variations ble salinity stratification due to tidal straining occurs on the in freshwater flow play a role at other times. Couperthwaite ebb and flood, respectively, but any unstable stratification is et al. (1998), Lawler et al. (2001) and Mitchell et al. stabilized by SPM stratification. Much larger salinity inver- (2003b) measured erosion and deposition on the upper mud- sions can occur in the presence of underlying stationary sedi- banks in the Trent and lower Ouse. They showed that deposi- ment suspensions in the lower Ouse, which implies that these tion was more likely to occur on the upper banks at reduced suspensions are derived from further up-estuary, in the upper wind speeds and that large ‘benches’ formed in the mudbanks core region of the ETM. after storms, indicating the importance of wave action. Sedi- The ETM core region, in which near-bed SPM concentra- ment deposition occurred on the upper banks near the peak tions exceed 16 g l1, extends over a longitudinal distance of 50 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

35 km at HW, both at spring and neap tides. It is separated by exists between depth-averaged median floc size and bulk ver- nose and tail regions from much lower turbidity waters, up-es- tical current shear in the water column for flocs that are greater tuary in the tidal river and down-estuary in the Humber and than a station-dependent size. This size threshold is likely to coastal zone. The nose region is very sharp, less than 3 km be a result of the SPM comprising primary particles and robust in length, and at spring tides is located 15 km into the tidal riv- microflocs, which are resistant to turbulent shear, and large, er, where salinity is less than 1, whereas the tail is much more fragile macroflocs that break into smaller flocs at high shears. diffuse (ca. 12 km in length). Except at very small neap tides, The largest median floc sizes occur within the near-bed sta- when fluid mud layers and stationary suspensions (w90 g l1) tionary suspensions at neap tides, where floc sizes can exceed can form in the tail region of the ETM, maximum near-bed 1 mm and concentrations 90 g l1. Entrainment of these flocs SPM concentrations (w50 g l1) occur close to the nose in leads to floc breakage, which reduces their median sizes to the upper core region of the ETM. less than 200 mm. The ETM is a mobile feature. It is displaced down-estuary by ca. 12 km during the 7-day period from spring to neap Acknowledgements tides. It also is displaced down-estuary, and spreads out, between HW and LW, such that the tail moves approximately We are grateful to Mrs. Carolyn Harris for her assistance the same distance as the isohalines during the ebb, whilst with particle-sizing analyses and Mr. Norman Bowley (PML the nose moves significantly less. Advection of SPM is the Technician and Coxswain, retired) for invaluable assistance dominant sediment-transport mechanism in the nose and upper and support during the fieldwork. The analyses of these core regions and there is an approximate equality between ad- data were undertaken with the aid of a grant from the Est- vective transport over the short and fast flood, and over the Proc research programme (www.estproc.net). EstProc is long but slower ebb. The effect of vertical-shear transport dur- funded within the joint DEFRA/EA (UK) Flood and Coastal ing the ebb, which is ca. 10% of the ebb advective transport, is Defence R&D Programme of Fluvial, Estuarine and Coastal to reduce down-estuary sediment transport and aid accumula- Processes. tion of sediment in the upper reaches. At spring tides, in the ETM tail region, vertical-shear transport is negligible during the ebb but acts in the down-estuary direction during the flood References to reduce the large, flood advective transport (by ca. 20%), in order to maintain an approximate sediment balance over the ATT, 2005. In: Admiralty Tide Tables, vol. 1. UK Hydrographic Office, Taun- tidal cycle. ton, UK, 354 pp. Bale, A.J., Uncles, R.J., Widdows, J., Brinsley, M., Barrett, C.D., 2002. Direct Floc settling leads to pronounced SPM stratification over observation of the formation and break-up of aggregates in an annular the HW, HW-slack and early ebb period. Estimates of settling flume using laser reflectance particle sizing. In: Winterwerp, J.C., velocity, corrected for hindered settling, range from Kranenburg, K.C. (Eds.), Fine Sediment Dynamics in the Marine Environ- 1.2 mm s1 in the nose and core regions to 2.1 mm s1 in ment. Elsevier Science BV, pp. 189e201. the tail region. Hindered settling is important in the nose Burchard, H., Baumert, H., 1998. The formation of estuarine turbidity maxima due to density effects in the salt wedge. A hydrodynamic process study. and upper core regions because it prevents appreciable deposi- Journal of Physical Oceanography 28, 309e321. tion to the bed in the main channel at HW slack. Instead, high Chester, M.A., 1999. The relationship between the biochemical oxygen de- concentration suspensions remain close to the bed and travel mand and total suspended solids concentration in the tidal River Ouse. relatively slowly down-estuary on the ebb until decreasing wa- PhD thesis, Department of Civil and Structural Engineering, University ter depths, due to falling water levels, and increasing current of Sheffield, UK, 216 pp. Couperthwaite, J.S., Mitchell, S.B., West, J.R., Lawler, D.M., 1998. Cohesive speeds result in increased vertical current shear, mixing and sediment dynamics on an inter-tidal bank on the Tidal Trent, UK. Marine entrainment. Complete entrainment may not occur during Pollution Bulletin 37, 144e154. small and decreasing neap tides, leading to strong stratification Delo, E.A., 1988. Estuarine Muds Manual. Report SR 164. Hydraulics Re- of SPM, fluid mud layers and stationary suspensions near the search, Wallingford, U.K., p. 31 þ figures and tables. bed. At LW of spring tides there is little time for deposition of Dyer, K.R., New, A.L., 1986. Intermittency in estuarine mixing. In: Wolfe, D.A. (Ed.), Estuarine Variability. Academic Press, Orlando, Flor- flocs in the nose and upper core regions. Strong tidal straining ida, USA, pp. 321e339. on the flood then generates large near-bed shears and vertical Dyer, K.R., Christie, M.C., Manning, A.J., 2004. The effects of suspended sed- mixing that maintains a more uniform vertical profile of SPM, iment on turbulence within an estuarine turbidity maximum. Estuarine, which ensures very strong up-estuary transport of SPM. Coastal and Shelf Science 59, 237e248. SPM within the ETM comprises silt and clay (w20e30% Eisma, D., 1991. Particle size of suspended matter in estuaries. Geo-Marine Letters 11, 147e153. clay by volume) and the relatively rapid settling that occurs Eisma, D., Chen, S., Li, A., 1994. Tidal variations in suspended matter floc during slack-water periods is a consequence of flocculation. size in the Elbe and Dollard Estuaries. Netherlands Journal of Aquatic In the water column, in situ floc sizes maximise at HW slack Ecology 28, 267e274. around mid-depth, so that slack water and low current shears Fennessy, M.J., Dyer, K.R., Huntley, D.A., 1994a. Size and settling velocity lead to increased flocculation, greater sizes and enhanced set- distributions of flocs in the Tamar Estuary during a tidal cycle. Netherlands e Journal of Aquatic Ecology 28, 275e282. tling. These maximum median sizes typically are 300 Fennessy, M.J., Dyer, K.R., Huntley, D.A., 1994b. INSSEV: an instrument to 500 mm, whereas sizes typically are in the range 70e300 mm measure the size and settling velocity of flocs in situ. Marine Geology 117, during other states of the tide. A strong, negative correlation 107e117. R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 51

Fox, J.M., Hill, P.S., Milligan, T.G., Ogston, A.S., Boldrin, A., 2004. Floc frac- Orton, P.M., Kineke, G.C., 2001. Comparing calculated and observed vertical tion in the waters of the Po River prodelta. Continental Shelf Research 24, suspended-sediment distributions from a Hudson River Estuary turbidity 1699e1715. maximum. Estuarine, Coastal and Shelf Science 52, 401e410. Fugate, D.C., Friedrichs, C.T., 2002. Determining concentration and fall veloc- Pontee, N.I., Whitehead, P.A., Hayes, C.M., 2004. The effect of freshwater ity of estuarine particle populations using ADV, OBS and LISST. Conti- flow on siltation in the Humber Estuary, north east UK. Estuarine, Coastal nental Shelf Research 22, 1867e1886. and Shelf Science 60, 241e249. Fugate, D.C., Friedrichs, C.T., 2003. Controls on suspended aggregate size in RMBC, 1986. Cruising Guide to the North East Waterways. Ripon Motor Boat partially mixed estuaries. Estuarine, Coastal and Shelf Science 58, 389e404. Club (Contact: Mr. E.A. Taylor þ44(0)1642 475922). www.waterwaysgui- Grabemann, I., Krause, G., 2001. On different times scales of suspended mat- des.co.uk/reviews/noreast.htm, 120 pp. ter dynamics in the Weser Estuary. Estuaries 24, 688e698. Rouse, H., 1937. Modern conceptions of the mechanics of fluid turbulence. Geyer, W.R., Woodruff, J.D., Traykovski, P., 2001. Sediment transport and Transactions of the American Society of Civil Engineers 102, 463e541. trapping in the Hudson River Estuary. Estuaries 24, 670e679. Sanford, L.P., Suttles, S.E., Halka, J.P., 2001. Reconsidering the physics of the Holtz, R.D., Kovacs, W.D., 1981. An Introduction to Geotechnical Engineer- Chesapeake Bay estuarine turbidity maximum. Estuaries 24, 655e669. ing. Prentice Hall, ISBN 0-13-484394-0, 733 pp. Shi, Z., 1998. Acoustic observations of fluid mud and interfacial waves, Hang- Kirby, R., Parker, W.R., 1983. Distribution and behaviour of fine sediment in zhou Bay, China. Journal of Coastal Research 14, 1348e1353. the Severn Estuary and Inner Bristol Channel, UK. Canadian Journal of Shi, Z., Kirby, R., 2003. Observations of fine suspended sediment processes in Fisheries and Aquatic Sciences 40 (Suppl. 1), 83e95. the turbidity maximum at the north passage of the Changjiang Estuary, Kirby, R., 1986. Suspended fine cohesive sediment in the Severn Estuary and in- China. Journal of Coastal Research 19, 529e540. ner Bristol Channel, UK. Report No. ETSU e STP e 4042 to the UK Atomic Simpson, J.E., 1997. Gravity Currents in the Environment and the Laboratory, Energy Authority, Ravensrodd Consultants Ltd., Taunton, UK, 243 pp. second ed. Cambridge University Press, Cambridge, 244 pp. Kistner, D.A., Pettigrew, N.R., 2001. A variable turbidity maximum in the Simpson, J.H., Fisher, N.R., Wiles, P., 2004. Reynolds stress and TKE produc- Kennebec Estuary. Estuaries 24, 680e687. tion in an estuary with a tidal bore. Estuarine, Coastal and Shelf Science Kitheka, J.U., Obiero, M., Nthenge, P., 2005. River discharge, sediment trans- 60, 619e627. port and exchange in the Tana Estuary, Kenya. Estuarine, Coastal and Shelf Sternberg, R.W., Ogston, A., Johnson, R., 1996. A video system for in situ Science 63, 455e468. measurement of size and settling velocity of suspended particulates. Jour- Law, D.J., Bale, A.J., Jones, S.E., 1997. Adaptation of focused beam reflec- nal of Sea Research 36, 127e130. tance measurement to in-situ particle sizing in estuaries and coastal waters. Sternberg, R.W., Berhane, I., Ogston, A.S., 1999. Measurement of size and set- Marine Geology 140, 47e59. tling velocity of suspended aggregates on the northern California continen- Law, D.J., 1998. Optical characterization of suspended particulate matter in es- tal shelf. Marine Geology 154, 43e53. tuaries and near coastal waters. PhD thesis, University of Wales, Bangor, Thill, A., Moustier, S., Garnier, J.-M., Estournel, C., Naudin, J.-J., Bottero, J.- UK, 169 pp. Y., 2001. Evolution of particle size and concentration in the Rhoˆne river Lawler, D.M., West, J.R., Couperthwaite, J.S., Mitchell, S.B., 2001. Applica- mixing zone: influence of salt flocculation. Continental Shelf Research tion of a novel automatic erosion and deposition monitoring system at 21, 2127e2140. a channel bank site on the tidal River Trent, UK. Estuarine, Coastal and Townend, I., Whitehead, P., 2003. A preliminary sediment budget for the Shelf Science 53, 237e247. Humber Estuary. Science of the Total Environment 314e316, 755e767. Le Hir, P.,Ficht, A., Jacinto, R.S., Lesueur, P.,Dupont, J.-P., Lafite, R., Brenon, I., Uncles, R.J., Stephens, J.A., 1993. Nature of the turbidity maximum in the Thouvenin, B., Cugier, P., 2001. Fine sediment transport and accumulations Tamar Estuary, UK. Estuarine, Coastal and Shelf Science 36, 413e431. at the mouth of the Seine Estuary (France). Estuaries 24, 950e963. Uncles, R.J., Easton, A.E., Griffiths, M.L., Harris, C., Howland, R.J.M., King, R.S., Li, M.Z., Gust, G., 2000. Boundary layer dynamics and drag reduction Morris, A.W., Plummer, D.H., 1998a. Seasonality of the turbidity maximum in in flows of high cohesive sediment suspensions. Sedimentology 47, 71e86. the HumbereOuse Estuary. Marine Pollution Bulletin 37, 206e215. Lin, J., Kuo, A.Y., 2003. A model study of turbidity maxima in the York River Uncles, R.J., Stephens, J.A., Plummer, D.H., 1998b. Sediment transport in the Estuary, Virginia. Estuaries 26, 1269e1280. HumbereOuse Estuary, UK, during May 1994. In: Dronkers, J., Manning, A.J., 2004. The observed effects of turbulence on estuarine floccu- Scheffers, M. (Eds.), Physics of Estuaries and Coastal Seas. Balkema, Rot- lation. Journal of Coastal Research, Special Issue 41, 90e104. terdam, ISBN 9054109653, pp. 73e82. Mehta, A.J., 1991. Understanding fluid mud in a dynamic environment. Geo- Uncles, R.J., Stephens, J.A., Harris, C., 1998c. Seasonal variability of subtidal Marine Letters 11, 113e118. and intertidal sediment distributions in a muddy, macrotidal estuary: the Mikkelsen, O., Pejrup, M., 2001. The use of a LISST-100 in-situ laser particle HumbereOuse, UK. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), sizer for in-situ estimates of floc size, density and settling velocity. Geo- Sedimentary Processes in the Intertidal Zone. Geological Society, London, Marine Letters 20, 187e195. pp. 211e219 (Special Publications 139). Mitchell, S.B., West, J.R., Arundale, A.M.W., Guymer, I., Couperthwaite, J.S., Uncles, R.J., Stephens, J.A., 1999. Suspended sediment fluxes in the tidal 1998. Dynamics of the turbidity maxima in the Upper Humber Estuary Ouse, UK. Journal of Hydrological Processes 13, 1167e1179. System, UK. Marine Pollution Bulletin 37, 190e205. Uncles, R.J., Frickers, P.E., Easton, A.E., Griffiths, M.L., Harris, C., Mitchell, S.B., Lawler, D.M., West, J.R., Couperthwaite, J.S., 2003a. Use of Howland, R.J.M., King, R.S., Morris, A.W., Plummer, D.H., continuous turbidity sensor in the prediction of fine sediment transport Tappin, A.D., 2000. Concentrations of suspended particulate organic car- in the turbidity maximum of the Trent Estuary, UK. Estuarine, Coastal bon in the tidal Yorkshire Ouse River and Humber Estuary. Science of and Shelf Science 58, 645e652. the Total Environment 251/252, 233e242. Mitchell, S.B., Couperthwaite, J.S., West, J.R., Lawler, D.M., 2003b. Measur- Uncles, R.J., Lavender, S.J., Stephens, J.A., 2001. Remotely sensed observa- ing sediment exchange rates on an intertidal bank at Blacktoft, Humber Es- tions of the turbidity maximum in the highly turbid Humber estuary, tuary, UK. Science of the Total Environment 314e316, 535e549. UK. Estuaries 24, 745e755. Mitchell, S.B., 2005. Discussion on ‘The effect of fresh water flow on siltation Uncles, R.J., Stephens, J.A., Smith, R.E., 2002. The dependence of estuarine in the Humber estuary, Northeast UK’ by Pontee, N.I., Whitehead, P.A., turbidity on tidal intrusion length, tidal range and residence time. Conti- and Hayes, C.M. (ECSS vol. 60, 241e249). Estuarine, Coastal and Shelf nental Shelf Research 22, 1835e1856. Science 62, 725e729. Uncles, R.J., Stephens, J.A., Harris, C. Runoff and tidal influences on the es- Munk, W.R., Anderson, E.R., 1948. Notes on a theory of the thermocline. tuarine turbidity maximum of a highly turbid system: the upper Humbere Journal of Marine Research 7, 276e295. and Ouse Estuary, UK. Marine Geology, in press-a. Odd, N.V.M., 1988. Mathematical modelling of mud transport in estuaries. In: Uncles, R.J., Stephens, J.A., Harris, C. Properties of suspended sediment in the Dronkers, J., van Leussen, W. (Eds.), Physical Processes in Estuaries: An estuarine turbidity maximum of the highly turbid Humber Estuary System, Introduction. Springer Verlag, Berlin, pp. 503e531. UK. Ocean Dynamics, in press-b. 52 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52

Voulgaris, G., Meyers, S.T., 2004. Temporal variability of hydrodynamics, Wolanski, E., Chappell, J., Ridd, P., Vertessy, R., 1988. Fluidization of mud in sediment concentration and sediment settling velocity in a tidal creek. estuaries. Journal of Geophysical Research 93, 2351e2361. Continental Shelf Research 24, 1659e1683. Wolanski, E., Asaeda, T., Imberger, J., 1989. Mixing across a lutocline. Lim- Wai, O.W.H., Wang, C.H., Li, Y.S., Li, X.D., 2004. The formation mechanisms nology and Oceanography 34, 931e938. of turbidity maximum in the Pearl River estuary, China. Marine Pollution Wolanski, E., Gibbs, R., Ridd, P., Mehta, A., 1992. Settling of ocean-dumped Bulletin 48, 441e448. dredged material, Townsville, Australia. Estuarine, Coastal and Shelf Sci- Whitehouse, R., Soulsby, R., Roberts, W., Mitchener, H., 2000. Dynamics of ence 35, 473e489. Estuarine Muds: A Manual for Practical Applications. Thomas Telford, Wolanski, E., King, B., Galloway, D., 1995. Dynamics of the turbidity maxi- London, 210 pp. mum in the Fly River Estuary, Papua New Guinea. Estuarine, Coastal and Winterwerp, J.C., 1999. On the dynamics of high concentrated mud suspen- Shelf Science 40, 321e337. sions. PhD thesis, Delft University of Technology, Delft, The Netherlands, Wolanski, E., Spagnol, S., 2003. Dynamics of the turbidity maximum in King 171 pp. Sound, tropical Western Australia. Estuarine, Coastal and Shelf Science Winterwerp, J.C., 2001. Stratiﬁcation effects by cohesive and noncohesive sed- 56, 877e890. iment. Journal of Geophysical Research 106 (C10), 22559e22574. Wolanski, E., Williams, D., Spagnol, S., Chanson, H., 2004. Undular tidal bore Winterwerp, J.C., 2002. On the ﬂocculation and settling velocity of estuarine dynamics in the Daly Estuary, Northern Australia. Estuarine, Coastal and mud. Continental Shelf Research 22, 1339e1360. Shelf Science 60, 629e636. Winterwerp, J.C., van Kesteren, W.G.M., 2004. Introduction to the physics of Xia, X.M., Li, Y., Yang, H., Wu, C.Y., Sing, T.H., Pong, H.K., 2004. Observa- cohesive sediment in the marine environment. In: Van Loon, T. (Ed.), De- tions on the size and settling velocity distributions of suspended sediment in velopments in Sedimentology, vol. 56. Elsevier, Amsterdam, 466 pp. the Pearl River Estuary, China. Continental Shelf Research 24, 1809e1826.