Salinity, Mixing and the Turbidity Maximum
The relationship between tidal range and river discharge enables all estuaries to be classified between highly stratified estuaries at one end and well mixed estuaries at the other. The Thames Estuary is generally a well mixed estuary; this means that river flow is small compared with the volume of the tide, and the whole water mass migrates up and down the estuary with the flood and ebb tides.
A longitudinal salinity gradient also exists and mixing takes place at the interface between the river water and sea water; saline water is mixed upwards (being denser and thus freshwater moves above the saline water) and freshwater is mixed downwards. This mixing causes a weak density current to flow (in addition to the tidal currents), which is a natural mechanism for maintaining a balance of fresh and saline water.
This current flows upstream and is an important agent for the transportation of suspended sediment into the Thames Estuary. The near bed residual flows result in the formation of a null point where there is no net movement of water at the bed in either direction.
During summer freshwater discharges, the null point is generally located along the Gallions, Barking and Halfway Reaches but variations such as freshwater input will cause the location of the null point to move up- or down- estuary (Royal Haskoning, 2004).
Tide & Tidal Range
The Thames Estuary is macrotidal with a mean spring tide range of 5.2 m at Sheerness gradually increasing upstream to 5.9 m at Tilbury and 6.6 m at London Bridge (United Kingdom Hydrographic Office, 2003). The increasing tidal range upstream is due to the funneling effect of the estuary, which has gradually been magnified by the formation and subsequent land-claim of extensive areas of saltmarsh.
The Thames Estuary has historically experienced an increase in the elevation of high water levels. Rossiter (1969) showed that between 1934 and 1966 there were increases in mean high water (MHW) and mean low water (MLW) at both Southend-on-Sea and Tower Bridge. He found that superimposed on the 18.6 year (lunar) oscillation were other water level increases. The rate of increase of mean high water (MHW) and mean low water (MLW) at Southend-on-Sea and Tower Bridge between 1934 and 1966/69 is shown in the table below.
|Source||Water level||Southend-on-Sea mm/yr||Tower Bridge mm/yr|
Overall, the data shows that an increase in tidal range has taken place, which itself increased steadily with distance upstream from Southend-on-Sea. An increase in tidal range of around 1-1.1 mm per year is described for Southend-on-Sea and 6.4-6.8 mm per year for Tower Bridge, between 1934 and 1969. The increase in tidal range is probably due to a combination of anthropogenic and natural causes (Royal Haskoning, 2004). Bowen (1972) considered that a large part of the observed increase in tidal range is likely to be due to the effects of embanking the estuary. Before construction of flood defences much of the water entering the Thames spread laterally to cover mudflats and saltmarshes.
Flood defences have caused a loss of this water storage volume at high tide levels, thus increasing the height of high water contained within the banks through morphological effects. Other contributory artificial causes may include the historic dredging of deeper shipping channels, the damming of tidal creeks and changes to estuary morphology caused by waterside developments. Natural causes also have an influence on tidal range, but the main drivers are difficult to ascertain. The predominant causes of the observed increase in tidal range appear to be (although not definitively) anthropogenic in nature; for this reason a simple extrapolation of the observed rates into the future would not be appropriate (Littlewood and Crossman, 2003) and further analytical work is required to (Littlewood et al., 2003):
- Determine the causes of the rise in water levels at Tower Bridge and their relative importance.
- Examine whether a rise at Tower Bridge will continue into the future and if so whether it will continue to be greater than the rise at Southend-on-Sea.
The primary driver of flood risk along most of the Thames Estuary is tidal water level enhanced by a storm surge component. The incidence and magnitude of these surges depend on the air pressure and the severity of winds in the North Sea. Positive storm surges in the North Sea are generated by low air pressure combined with strong northerly winds. If the surge component peaks at the same time as high water (particularly spring tides) there will be a high risk of flooding unless the flood defences are able to cope with the increased elevation. Rossiter (1961) compiled surge data for Southend-on-Sea between 1928 and 1938 and showed a tendency for surges to be amplified by around 25% on the rising tide (over those at any other state), irrespective of whether the surges were negative or positive. Rossiter (1961) suggested that the propagation of the tide up the Thames Estuary is retarded (shifted back in time) by the presence of a negative surge and advanced (shifted forward in time) by a positive surge as a result of:
- The rate of progression is reduced by a reduction in water depth during a negative surge but increased by an increase in water depth during a positive surge.
- Bottom friction has the effect of retarding a wave, and as bottom friction is proportional to water depth a negative surge will increase frictional effects and a positive surge will decrease frictional effects.
Predicted tide levels in the Thames Estuary have been raised by as much as 2.5 m at high water, and up to 4 m on the rising tide by storm surges (Trafford, 1981; Horner, 1984). On the 1st February 1953, the storm surge increased the rising tide by 2.9 m and the high tide level at Tower Bridge by 1.9 m (Trafford, 1981).
Tidal & Residual Currents
Tidal currents in the Thames Estuary show an increasing degree of asymmetry in an upstream direction. With the exception of Sheerness and Southend-on-Sea, the tidal wave becomes increasingly flood-dominated in an upstream direction. Between Sheerness and Gravesend, maximum ebb current velocities are in excess of the flood, whereas upstream of Gravesend the flood current velocities are in excess of the ebb (Thorn and Burt, 1978). The switch of tidal dominance coincides with the narrowing of the channel into Long Reach; The Institute of Estuarine and Coastal Studies (1993) suggested that the ebb dominance at Sheerness was due to the exit of the large tidal prism held in the Medway Estuary, confluencing with the Thames Estuary through a constricted mouth at Sheerness.
Thorn and Burt (1978), using historical measured current velocity variation with depth in Halfway Reach (1968 and 1969), found that the velocities at all depths rose sharply after low water slack after which they decreased steadily to a smaller peak just before high water. During both the flood and the ebb tides, velocities generally increased with height above the bed.
The tidal current ebb- or flood-dominance has important implications for sediment transport in the Thames Estuary. Other things being equal, flood-dominance will tend to favour net movement of sediment into the estuary, whereas ebb-dominance will favour net export of sediment. However, this general scenario is complicated by the presence of upstream-directed density currents which enhance the flood tidal currents, and if increased river flows occur, they will enhance the ebb tidal currents (especially in the upper reaches). In addition, it has been shown by HR Wallingford (2002e) that the flow regime of the Thames Estuary downstream of Gravesend Reach has three-dimensionality. 3D modelling demonstrates that, although secondary currents are weak in comparison to the main tidal current flows, the flow field has a complex vertical structure in both lateral and longitudinal directions (HR Wallingford, 2002d). Greater detail is now available from ADCP measurements carried out in parts of the estuary by the Port of London Authority to support the investigation of various developments in the Thames Estuary. These datasets provide full river-width current velocity distributions at intervals through the tidal cycle and support this view of a complex flow field (Littlewood and Crossman, 2003).
HR Wallingford (2002b) modelled the wave regime of the Thames Estuary in Lower Gravesend Reach, Lower Hope Reach and Sea Reach. They found that wind action is the main wave generation process in this part of the Thames Estuary as waves generated offshore were dissipated over the Outer Estuary banks and wide intertidal flats. They modelled waves generated by winds from the east and those from 205o representing waves generated locally from the south across Lower Hope Reach. They found relatively short wave periods and since the fetch is longest for winds from the south-east and east, these winds generally result in the highest wave conditions in this part of the Thames Estuary. However, a lot of energy is dissipated by the extensive offshore bank and channel system before the waves reach Sea Reach leading to relatively small overall wave heights (HR Wallingford, 2004). Significant wave heights were predicted to be slightly greater than 1.5 m at Coryton for 1 in 50 year winds from all directions and under 0.7 m for 10 times a year winds (at all water levels). Another method of wave generation in the estuary is that created by the passage of vessels. Although individually of less energy than wind-generated waves, they may be the largest waves in areas that are protected from wind waves and the passage of large vessels may also influence flow direction and turbulence and hence sediment mobilisation and net direction of transport.
Freshwater input to the Estuary
The main freshwater input to the Thames Estuary is at Teddington; this has an average flow rate of 90 m3s-1 (Institute of Estuarine and Coastal Studies, 1993). A long record of flow exists with the highest flow estimated at 1059 m3s-1 in 1894. Other major fluvial events occurred in 1947 (714 m3s-1) and 2003 (461 m3s-1) (Littlewood and Crossman, 2003). Tributary inputs are relatively small (10-15% of the total flow) compared to the main input at Teddington. Average freshwater inputs are very small compared to tidal discharge in the estuary (Inglis and Allen, 1957). Using Acoustic Doppler Current Profilers (ADCP) measurements in July 2001, HR Wallingford (2002d) reported tidal discharges of up to 15,000 m3s-1 on both flood and ebb tides (in Lower Hope Reach).
Freshwater input also partly influences morphology through the salinity regime in terms of the position of the null point for sedimentation. Crooks (1994) analysed water level records over the last 100 years for locks upstream of Teddington, and found that there was a greater number of above average peaks before 1940 than after 1940. This work concluded that channel dredging in the upper parts of the Thames and flood prevention schemes have resulted in localised decline in peak flood levels and event duration (particularly since dredging of the main fresh watercourses took place in the 1930s and 1940s); this may have influenced sediment supply to the estuarine parts of the system.
Response to historic sea level rise
The most recent relative sea-level curve (Wilkinson and Sidell, 2000) shows that there is a general rise of sea level through time, with an initial rapid rise of 3.5 mm per year, slowing down around c. 6000 years ago to 0.7 mm per year. This is supported by a wider analysis of land-level and sea-level change around Britain (Shennan and Horton, 2002), which calculated a late Holocene (last 4000 years) relative sea-level rise of 0.74 mm per year for the Thames, 0.85 mm per year for Essex and 0.67 mm per year for Kent. This can be compared with those for 20th century sea-level changes published by Woodworth et al. (1999) using tide gauges. They showed relative sea-level rises of 1.22+/-0.24 mm per year at Southend-on-Sea, 1.58+/-0.91 mm per year at Tilbury and 2.14+/-0.15 mm per year at Sheerness. Overall, these figures suggest an additional rate of relative sea level change in the 20th century of around 1 mm per year, as compared to the Late Holocene. This is in general agreement with the view that global sea levels have increased by 100-200 mm over the last century.
Shennan and Horton (2002) suggest, however, that some deficiencies may be inherent in the 20th century dataset. These include the unequal distribution of measurements and the considerable amount of interannual (typically decadal) variability present in all tide gauge records. Littlewood and Crossman (2003) also questioned the degree of accuracy of the tide gauge data based on concerns that they may not have remained at the same level relative to Ordnance Datum throughout their period of deployment. They indicated that the gauges were levelled to Ordnance Datum over 40 years ago, and since that time differential ground subsidence may have caused their perceived level to be different to their actual level. Monitoring using GPS at the tide gauge locations at Richmond, Tower Pier, Silvertown, Erith, Tilbury and Southend-on-Sea has shown that between March 1997 and July 1999, the movement of ground levels at these locations was statistically insignificant.