Summary of Morphological Characteristics
Historic morphological change in the Thames Estuary can be divided into two periods. The first, from the middle of the 19th century until around 1970, was a period when many natural morphological changes were masked by extensive anthropogenic activity, such as flood defences, dredging programmes, waterside schemes and developments.
The second period, from 1970 to the present day, corresponds with the implementation of legislation to control waterside activities enabling the estuary to (commence a process to) establish a more natural regime (HR Wallingford, 2003, 2004, Royal Haskoning, 2004). Morphological change in the estuary post-1970 is therefore more representative as a baseline to future development than the long term development (pre- and post-1970s combined), which must be set within the context of large artificial changes to the estuary.
Estuarine channels experience natural periodic shifts in position, due to gradual meander migration or more sudden changes during periods of high river flow, extreme tides or strong winds. A lateral shift in the position of the subtidal channel may over time produce a change in the slope of the intertidal profile and may alter the wave and tidal energy impinging on the shoreline. Historic changes in the position and depth of the Thames Estuary subtidal channel are difficult to ascertain because of the influence of dredging and disposal activities. This is exemplified by two examples:
- A major programme of capital dredging took place between 1909 and 1928 to improve navigation in the Yantlet Dredged Channel. It was further deepened in 1965 and much of the dredged sediment was deposited in Leigh Channel to the north to encourage flow in the Yantlet Channel (HR Wallingford, 2002a).
- The navigation channel in Lower Gravesend Reach migrated to the south after dredging was undertaken in 1963/64. The dredged sediment may have been used to close the previous (more northerly) alignment of the navigation channel.
However, in broad terms, and from both natural and anthropogenic influences, between 1820 and 1988 the subtidal channel increased in width between Lower Hope Reach and the eastern tip of Canvey Island, and narrowed between Canvey Island and the Isle of Grain (Institute of Estuarine and Coastal Studies, 1993). These changes were due to advance or retreat of the adjacent intertidal areas. During a similar period (1834 to 1957/59), HR Wallingford (2003) found that in Lower Hope Reach and Upper Sea Reach, the deep water channel deepened. Between 1920 and 1957/59, this deepening was accompanied by a shallowing of the subtidal areas fronting Mucking Flats and Blyth Sands.
Institute of Estuarine and Coastal Studies (1993) showed that between 1820 and 1940 (using a comparison of the low water mark on Ordnance Survey maps) the area of intertidal flat downstream of Gravesend increased by 4.86 x 106 m2, but decreased by 1.26 x 106 m2 between 1940 and 1988 (shown in the table below). The decrease occurred mainly because a decrease on the north side of the Thames was greater than a continued increase on the south side. The increase mainly occurred at the low water mark (the low tide boundary of the mudflats encroaching into the subtidal channel) rather than at the boundary with the adjacent saltmarshes. The decrease on the north bank may be due to the increased land-claim and industrialisation that took place here during the mid 20th century.
|Location||Area (106m2) in 1820||Area (106m2) in 1940||Area (106m2) in 1988|
The extensive intertidal flats downstream of Gravesend can be divided into shorter sections for a more detailed appraisal of change. It should be recognised that these areas exhibit dynamic change on a short term basis but also some assessments have been made of longer term processes. The areas are:
on the south bank,
- Blyth Sands.
- Yantlet Flats/Grain Spit
and on the north bank,
- Mucking Flats
- Canvey Island (Chapman Sands, Marsh End Sand, Leigh Sand)
- Southend Flat.
According to Institute of Estuarine and Coastal Studies (1993) the northern edge of Blyth Sands fronting Cliffe and Halstow Marshes generally retreated between 1820 and 1940. Between 1940 and 1988 the edge generally advanced seaward, possibly responding to loss of mudflat at Coryton on the opposite bank (dredging of the channel close inshore, deepening the berths at the oil refinery jetties), allowing the main channel to maintain a constant width. This scenario is supported by the results of HR Wallingford (2002a) who showed more deep water adjacent to Coryton in 1970 compared to 1834. HR Wallingford (2002a, c) also showed that the surface of Blyth Sands lowered by up to 1 m over the period 1970-1998 (an average vertical erosion of 12 mm per year), even though accretion (up to 4 m, an average of around 100 mm per year) had taken place around the low water mark (Figure 6.3). These changes equate to an increase in area of around 4% (5.93 x 106 m2 to 6.19 x 106 m2) but a volume decrease of more than 11% (9.5 x 106 m3 to 8.4 x 106 m3). HR Wallingford (2002f) showed a continuation of general lowering of the Blyth Sands surface between 1998 and 2002, but at a higher rate. They also found a lower rate of general accretion around the lower water mark during this 4-year period compared to 1970-1998.
Yantlet Flats/Grain Spit
There has been significant general accretion along the low water mark of Yantlet Flats fronting St Mary’s and Allhallows Marshes between 1820 and 1988 (Institute of Estuarine and Coastal Studies, 1993; HR Wallingford, 2002a). HR Wallingford (2002a) also demonstrated that the main body of the flats has accreted vertically by around 0.5 m between 1970 and 1998, and the flats around the low water mark by up to 8 m. These changes equate to an increase in area of around 16% (11.22 x 106 m2 to 12.98 x 106 m2) and a volume increase of around 15% (22.9 x 106 m3 to 26.4 x 106 m3).
Between 1820 and 1988, the southern part of the mudflats at Mucking (opposite East Tilbury) generally advanced into the subtidal channel whilst the northern portion (Mucking) generally retreated, with most of the movement taking place between 1940 and 1988 (Institute of Estuarine and Coastal Studies, 1993). HR Wallingford (2002a) showed a similar trend around the low water mark between 1959 and 1998 and a general vertical accretion (0.5-2 m) of the main body of the flats over the same period. They found that the change had been fairly continuous over the period and was not influenced significantly by the anthropogenic activities that took place in the area in the 1960s. HR Wallingford (2002e) described an average vertical accretion rate of 11 mm per year for the entire intertidal area, between 1970 and 1998, although rates of 20-40 mm per year occurred at the northern end (HR Wallingford, 2002f) and some erosion took place at the southern end (towards low water mark). Area and volume comparisons of Mucking Flats between 1970 and 1998 revealed an overall decrease in area above Chart Datum of less than 1% (2.74 x 106 m2 to 2.72 x 106 m2) but a general volume increase of 12% (7.2 x 106 m3 to 8.1 x 106 m3) (HR Wallingford, 2002a). However, HR Wallingford (2002e, d) demonstrated that between 1998 and 2002, Mucking Flats generally lost volume. They postulated two reasons for this change, and the concurrent changes occurring on Blyth Sands (see above):
- Increased wind speeds (and hence increased wave activity) in the 1998-2002 period relative to those experienced 1970-1998.
- Unusually high freshwater flow experienced in winter 2000/2001 causing a change in salinity, tidal flows and hence suspended sediment concentrations.
The 1998-2002 bed level changes could be a short-term perturbation, and hence they may demonstrate the natural variation of change that can occur, within the longer-term trend described by the 1970-1998 data (HR Wallingford, 2002f). Alternatively this could be a more significant change in the processes that future measurement may be able to confirm.
The intertidal flats in front of Canvey Island have generally retreated between 1820 and 1988 (Institute of Estuarine and Coastal Studies, 1993). Prior to 1909, Chapman Sands and Marsh End Sand to the east of Canvey Island were receding westwards, but since 1909 they have generally extended eastwards. The movement of Marsh End Sand has resulted in a northward shift of Ray Gut and erosion of the mudflats (Leigh Sand) to its north-east.
Southend Flat has suffered erosion to the west of Southend-on-Sea Pier (due to the northward movement of Ray Gut), while the creek systems draining the flats have enlarged and migrated (Institute of Estuarine and Coastal Studies, 1993). HR Wallingford (2002a) also showed a general loss of intertidal flat volume at Canvey Island and Southend-on-Sea between 1970 and 1998.
These changes are also influenced by the large-scale historic ballast winning and disposal activities in Lower Hope Reach. A more complex pattern of change occurred between 1957/59 and 1970/71, related to realignment of the navigation channel in Lower Gravesend Reach in the 1960s. Overall, between 1920 and 2002, the deep water channel has deepened significantly in Lower Hope Reach and Upper Sea Reach and the subtidal areas adjacent to the intertidal flats have shallowed (HR Wallingford, 2003, 2004).
Maps of 1820 show that large areas of saltmarsh had already been enclosed (Institute of Estuarine and Coastal Studies, 1993). It is likely that early (14th-16th century) land-claims were piecemeal with construction of walls by landowners to protect their property. However, out of this piecemeal approach a fairly uniform sea wall fronting the Thames Estuary developed. Burd (1992) showed that between 1973 and 1988, 4.44 x 106 m2 of saltmarsh identified between Higham Marshes and Shoebury Ness in 1973 had been reduced to 3.21 x 106 m2 by 1988, a loss of 28%. Of the 1.23 x 106 m2 lost (1.06 on the north bank and 0.17 on the south bank), 0.98 x 106 m2 were lost through erosion and 0.25 x 106 m2 through land-claim.
The main mechanism of saltmarsh erosion over the last 30 years is considered to be increased wave energy at the seaward edge (Pye, 2000; Van der Wal and Pye, 2004). The periods 1970-73, 1976-79 and 1985-88 in particular, are characterised by stormy conditions with sustained periods of strong winds and waves from the south-east and east. The erosion is concentrated along the seaward edge of the saltmarshes which are exposed to wave action (e.g. Leigh Marsh and Canvey Point, Cooper et al., 2000), whereas the more sheltered saltmarshes, in Benfleet and Holehaven Creeks, have not experienced a retreat of their seaward edges.