Turbidity Maximum in the Macrotidal, Highly Turbid Humber Estuary, UK: Flocs, Fluid Mud, Stationary Suspensions and Tidal Bores
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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 strat- ification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions (w90 g lÿ1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g lÿ1, 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 lÿ1) 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 sÿ1.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. 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.10.013 R.J. Uncles et al. / Estuarine, Coastal and Shelf Science 67 (2006) 30e52 31 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 lÿ1, 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 lÿ1, 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 lÿ1. tion and estuarine channel-switching. Kingston Boothferry upon Hull Bridge Spurn Head G10 G23 Immingham G13 Keadby N Bridge G24 10km UW 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 lÿ1 compared with >10 g lÿ1 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 lÿ1, whereas they peaked at ca. ideal conditions for the study of a highly turbid ETM. This pa- 30 g lÿ1 in the upper Ouse when salinity was ca.