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Anthropogenic Influences

Thames clean-up
 
Thames Clean-up

 

The human race has placed considerable demands on the Thames Estuary. Little control and poor recording of these activities have meant that it is difficult to relate changes in the morphology of the estuary to any particular impact (Royal Haskoning, 2004).

Since the 1960's/ and 1970's the large capital or maintenance dredging programmes, the discharge of pollutants and the construction of riverside developments have been subject to increasing legislation to ensure their impact on the hydrodynamic and morphological regimes of the estuary are acceptable.

 

 

West Thurrock Oil Jetty

Kendrick (1984) examined the impact on the estuary of the construction of West Thurrock oil jetty. A first jetty in place in 1873 caused a local 80m seaward movement of the low water mark. A second jetty built further downstream in 1966 caused further deposition and the low tide mark at the new jetty advanced around 50m due to reduced tidal current velocities near the bank. Prior to construction, sediment brought into the area on the flood tide was deposited around high water slack, but then re-entrained into the flow on the ebb tide, maintaining a balance. Once jetty construction was completed the reduction in current velocities provided a longer period for deposition, and the ebb current was less efficient in re-entraining sediment.

Woolwich Ferry Terminals

Kendrick (1984) found that cofferdams used in the construction of the Woolwich ferry terminals (starting 1964) created eddies in the current flow, particularly on the north bank, reducing current velocities leading to increased sediment deposition. Bed levels during construction were raised by over 3 m in places. Former bed levels were not re-established following the removal of the cofferdams because during construction, the silt had become compacted and post–construction the large number of piles supporting the terminals continued to impede flow. The zone of deposition extended beyond the terminals along the adjacent banks. This was attributed by Kendrick to a secular increase in tidal penetration causing the gradual upstream movement of the zone of main deposition (the Mud Reaches), which increased the quantity of suspended sediment in the area as a whole. There is a possibility that the process may have been enhanced by the cessation of dredging in the downstream Barking Reach between 1963 and 1966, allowing more suspended sediment to arrive in Woolwich Reach on the flood tide.

Rainham Creek Dam

To prevent tidal surges flooding the low lying Hornchurch and Rainham Marshes flanking Rainham Creek, a sheet pile dam was constructed in 1978/79 spanning the mouth of the creek (about 100 m wide) (Kendrick, 1984). An alternative outlet for the creek (Ingrebourne River) was provided by sluices further up-river. The result was extensive siltation in to the previous location of the low water channel in front of the dam.

Divers Shoal

HR Wallingford (2002a, f) compared 1970 and 1998 bathymetric charts and found large amounts of accretion (of the order of 1 x 106 m3) in the area now occupied by the Diver Shoal groynes along the northern shore of Gravesend Reach. They concluded that, as anticipated, the accumulation was almost completely due to this scheme which took place in 1995 and occurred over a 3-year period (1995-1998).

Dredging & Disposals

The Port of London Authority has a statutory duty to provide and maintain designated depths of water in the navigable channels, jetties and berths of the Thames Estuary. As a result of sedimentation it is therefore necessary to periodically undertake maintenance dredging. The importance of London as a port has resulted in a history of dredging, although, as elsewhere, few accurate records of dates of dredging and quantities removed exist. The records that do exist are difficult to use in a quantitative fashion because the units are not always compatible. They may also be approximations such as nominal values (provided as hopper tonnes, in situ volumes, paid volumes etc) assigned to barge loads.

Dredging can potentially have two effects on the processes of sediment exchange in the estuary. First, deepening may increase the proportion of total tidal discharge which takes place through the main channel reducing velocities adjacent to the channel. Second, dredging may create an artificial sink for sediment which may modify the fine sediment regime reducing supply to other nearby areas (Royal Haskoning, 2004).

Inglis and Allen (1957) described dredging activities in the estuary between 1928 and 1956. The average annual dredged volume taken from the estuary as a whole during this period was 1.86 x 106 m3 (Institute of Estuarine and Coastal Studies, 1993). Most of this sediment along with sewage sludge from London’s main sewage works at Barking and Cross Ness was disposed of in Black Deep in the Outer Estuary. Some was disposed of in Lower Hope Reach in front of north Mucking Flats. HR Wallingford (2002f) reported that a total around 0.58 x 106 m3 per year of material is estimated to have been disposed of in Lower Hope Reach between 1941 and 1967.

Inglis and Allen (1957) suggested that the disposed sediment at Black Deep was re-entrained and transported back into the estuary, adding to the rate of deposition in primary sources (Thorn and Burt, 1978). Following these results the disposal site was changed in 1961 (and still ongoing) to Rainham Marshes mid-way between London Docklands and Tilbury. This had the effect of removing the dredged sediment from the system. The route of transport of sediment back into the estuary from Black Deep may not be as direct a path as suggested by Inglis and Allen; it may be a more indirect contribution to the general sediment pool in the outer Thames Estuary.

In addition to the change of disposal site, the practice of regular maintenance dredging in the Mud Reaches and at Diver Shoal in Gravesend Reach was discontinued in the 1960s. Since this time dredging of the subtidal channel has been limited to local activities related to new jetties or deepening of existing riverside facilities (Kendrick, 1984) and the annual volume of sediment dredged appears to have fallen dramatically. The Port of London Authority estimates that the present annual requirement for maintenance dredging in the Thames Estuary, removed by conventional dredging methods, is 50,000-150,000 m3 (HR Wallingford, 2002e) with this material being placed at Rainham Marshes and Cliffe Pooles. Most of the dredging on the Thames Estuary is now undertaken using water injection dredging techniques. This agitation technique, which retains fine sediment in the estuary, is used to remove about 225,000 m3 per year at the berths at Shell Bravo, Coryton and Oikos, together with approximately 85,000m3 per year from the Port of Tilbury Bellmouth.

Operation of the Thames Barrier

 Thames Barrier
 
Thames Barrier

The operation of the Thames Barrier can influence hydrodynamics and sediment transport along the length of the estuary, although the type and magnitude of the influence is presently unclear. For example, Prandle (1975) simulated deployment of the Thames Barrier during the 1953 storm surge, and found that the amplitude of the reflected wave at Southend-on-Sea was negligible.

However, Littlewood and Crossman (2003) showed that closure of the Thames Barrier for prevention of fluvial flooding (without a surge component) could result in a reflected wave that may raise high water levels downstream of the barrier by around 0.5 m, depending upon the time of closure. A small negative wave (depression of water level) is generally recorded propagating upriver.

It is likely that barrier operations will increase in the future in response to climate change, and thus the influence on morphology will increase (Royal Haskoning, 2004). Due to a lack of data, the impacts of the Barrier on the Greater Thames Estuary (e.g. the Medway), are not currently fully understood.

 

 

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