Sediment Sources & Transport
It is likely that the sediment is derived from multiple sources (Royal Haskoning, 2004). During the Holocene the estuary has partially filled with marine, estuarine and freshwater sediments derived from ‘natural’ sources such as the underlying bedrock and reworking of the Holocene sediments themselves. The modern sources of sediment include these sources together with other anthropogenic (industrial) sources not available to the system prior to human influence (HR Wallingford, 2004). Prentice (1972) suggested that the dominant source of muddy sediment during the Holocene was the London Clay under the lower reaches of the estuary. Other sources may include the London Clay cliffs of the Isle of Sheppey (Nicholls et al., 2000) and sediment transported down the East Anglian coast and from the Essex cliffs (Marsland, 1986). It may be that far-field sources supply suspended sediment into the western North Sea where it accumulates in the ‘Thames Embayment’ immediately offshore of the estuary (east of a line between Walton-on-the-Naze and North Foreland). This sediment slowly enters the estuary where it becomes trapped in the tidal oscillations. The origin of bedload (fine to medium-grained sand) is believed to be mainly from reworking of earlier Thames/Medway Pleistocene fluvial sediments, from cliff erosion in Kent, Essex and Suffolk and from Tertiary sea bed exposures.
Nature of Bed Sediments
The characteristics of the bed sediments of the Inner Thames Estuary vary across and along the estuary. British Geological Survey (1997) and HR Wallingford, 2002d, e, 2004) showed that between Erith and Canvey Island the main subtidal channel generally comprises sand and gravel. To the east of Canvey Island, these sediments are replaced by mainly sand. The Outer Thames Estuary intertidal flats are characterised by sediment with high sand content due to the winnowing action of waves generated locally and those that propagate into the estuary from the North Sea. Mean sediment particle size becomes markedly smaller up-river into the Inner Thames Estuary. Mucking Flats are typified by mud whereas Blyth Sands/Yantlet Flats are muddy towards the high water mark becoming sandy towards the low water mark with a transition zone between the two (British Geological Survey, 1997; HR Wallingford, 2002d, e). A thin strip of coarser sediment (gravel and conglomerates) is generally found at the base of the flood defences backing the intertidal flats. Information on bed characteristics upstream of Erith is limited.
Littlewood and Crossman (2003) divided the Inner Thames Estuary into four suspended sediment zones on spring tides (they suggested that little sediment is in suspension on neap tides).
From Teddington to Lower Pool the suspended load is low, there is little deposition on the bed and banks of the river, and much of the sediment passes through downstream.
The second zone, downstream to Erith Reach, includes the turbidity maximum which forms around the null point in Gallions, Barking and Halfway Reaches. This is a zone where large concentrations of suspended sediment accumulate (collectively known as the ‘Mud Reaches’) which coincides with the limit of saline water intrusion (Inglis and Allen, 1957). Turbulence and the high concentrations of sediment in this zone encourage flocculation, and deposition occurs. The exact position of the turbidity maximum is sensitive to tidal range, changes of sea level, and the seasonal variability of the freshwater flow and saline tidal flow (Kendrick, 1972; Littlewood and Crossman, 2003).
During periods of higher river discharge (winter flows), the saline water is pushed seawards and sediments are flushed out of the Mud Reaches and stored downriver in the Gravesend Reach area. During periods of lower river discharge (summer flows), there is a gradual upriver migration of the saline water, modifying residual flows and sediments gather and settle back in the Mud Reaches.
Littlewood and Crossman (2003) suggested that the upriver migration of sediment is a slow process (months) because the forces are weak. However, the first freshwater flow of sufficient strength will rapidly move the ‘summer’ load back to the position it occupied before. They suggested that the downriver movement takes the form of a high suspended sediment concentration close to the bed and in the deeper parts of the channel, with only a small percentage at higher levels in the water column. Inglis and Allen (1957) observed that a sustained increase in river flow of around 1-2 weeks caused the Barking Reaches channel to deepen by over 0.5 m in the shoal areas. They suggested three reasons for the change:
- The silt-laden water in the Mud Reaches is pushed downstream and replaced by relatively clear water which encourages re-suspension of the bed and hence scour.
- The high river flow appreciably increases the ebb discharge and thus physically scour the bed.
- The almost fresh upland water acts as a dispersing or deflocculating agent on the uppermost layers of consolidated mud thus reducing the effective particle size and bonding of particles and making them more readily transportable.
Once the suspended sediment enters the Inner Estuary system, material movement and accumulation is complex. Using measurements taken in 1953, Inglis and Allen (1957) showed a striking drop in suspended sediment concentration upstream of the Mud Reaches (Figure 6.4) with concentrations in Upper Pool and Bugsby’s Reach consistently below 200 ppm. The concentrations rise to a peak near the upper end of the Mud Reaches and gradually decrease seawards. They also described higher concentrations of suspended sediment on the ebb than on the flood. This may be a result of the differential re-suspension of sediment after low water slack and high water slack. On low water slack sediment settles out to form a high concentration (100,000- 150,000 ppm) fluid mud layer close to the bed. Some of the sediment at the base of this mud layer consolidates under its own weight raising the bed level, effectively removing it from re-suspension. At high water slack some suspended sediment again settles out but it is brought back into suspension by the ebb current. More thorough mixing takes place during the ebb, with consequently higher concentrations in the middle to surface layers.
As a result of suspended sediment monitoring, Thorn and Burt (1978) were able to propose several longitudinal areas of the estuary which act as temporary sediment stores releasing and accumulating sediment on a semi-diurnal and spring tide cycle. On the flood tide, sediment deposited on the previous low slack water is re-entrained and moved upstream in a series of ‘jumps’ corresponding to the 16 km tidal excursion and is then re-deposited at high slack water. On the following ebb tide almost all of this sediment is re-entrained and moved downstream once more where it is deposited close to the original source area at low slack water. Thus the temporary storage areas in the lower estuary supply sediment only on the flood and receive it again only on the ebb, whereas storage areas in the middle estuary, between Gravesend Reach and Blackwall Reach, both receive and supply during flood and ebb. In contrast, the most landward temporary store, in the Syon Reach, receives only on the flood and supplies only on the ebb.
A programme of water sampling at discrete points in the estuary downstream of Gravesend Reach was undertaken in July 2001 by HR Wallingford (2002e). They found a marked concentration gradient with spring tide near-bed levels up to 2000 mg/l in Lower Hope Reach decreasing to 1000 mg/l at Coryton to less than 100 mg/l at Southend-on-Sea. A similar pattern emerged from the neap tide measurements with highs of up to 500 mg/l in Lower Hope Reach and lows of less than 100 mg/l at Southend-on-Sea. They also showed vertical layers on both spring and neap tides; at high water bed concentrations were an order of magnitude greater than mid-depth concentrations and at other states of the tide were several times higher.
HR Wallingford (2002c) modelled fine sand transport (median diameter 0.1 mm) transport in the estuary downstream of Gravesend and found a net spring and neap tide sediment flux out of the estuary (i.e. export of sediment). Tidal currents transported a majority of the sediment with negligible wave influence. These results support the general conclusions that the estuary is ebb-dominated downstream of Gravesend and wave heights are relatively small and have less influence on the sediment movements.
More recently, it has been recognised that single point measurements in the estuary may not provide information on the full complexity of suspended sediment distribution. For example, the presence of wide meanders influences suspended sediment transport. The interaction of these meanders (and the secondary currents set up by them) with the adjacent intertidal mudflats gives rise to a complex suspended sediment regime with large fluxes of sediment moving on and off the mudflats, with subsequent morphological change (HR Wallingford, 2004, Royal Haskoning, 2004). Bed sediments can also change across the section from the outer to inner part of the meander: For example, the meander separating Gravesend Reach and Lower Hope Reach results in secondary currents that move near-bed sediment towards the inside of the meander increasing suspended sediment concentrations relative to the outside of the meander (HR Wallingford, 2002e).
Transport in bedload
On the south shore of the Outer Thames Estuary, longshore sediment transport is inclined to the west under the action of north-easterly waves although this is largely interrupted at the Isle of Sheppey by the River Swale and at the Isle of Grain by the outflow of the River Medway (Welsby and Motyka, 1987). The net transport of sediment decreases in magnitude upstream in the estuary and is generally less than 5000 m3 per year (Scott Wilson, 1998). Scott Wilson (1998) argued that Kentish Flats and Whitstable Flats (intertidal areas) attenuate the wave energy that would otherwise reach the Isle of Sheppey, and they may therefore influence the relatively low rates of sediment transport along this shoreline. These results indicate that movement of sediment as bedload is very small in comparison to the loads of suspended sediment that are carried into and out of the estuary.