Our website uses cookies so that we can provide a better service. Continue to use the site as normal if you're happy with this, or find out how to manage cookies.
X

Current Dredging Operations

There are three main types of dredging operations within the Thames. These are:

Berth operators are responsible for the maintenance of their berths and approaches under the regulation of the PLA and in accordance with the Maintenance Dredging Framework. As such, details of these operations can be found on the 'Other Dredging Activities' page.

 Back to Existing Dredging Regime

Water Injection Maintenance Dredging

The majority of dredged sites within the inner estuary of the Thames, by volume and frequency, are undertaken using water injection dredging (WID), rather than more conventional excavation processes. The technique involves the injection of high volumes of water into the recently deposited seabed sediments. This re-fluidises the silts and fine sands, which then flow by gravity or current from the dredge site. The water is injected at low pressures, ensuring the sediment material is re-energised as a density current at the bed, rather than being re-suspended into the full water column. To be effective, the technique requires a flow gradient away from the dredge site, so material is transported to locations from which it is subsequently re-distributed by natural currents. The technique therefore promotes relocation of material based on local dispersion rather than removal to licensed marine or terrestrial disposal grounds. Retention of sediments within the natural estuarine system is widely considered to be a potentially significant environmental benefit of the technique. In order to minimise the environmental effects, dredging is already required to be undertaken on an ebb tide to provide maximum dispersion and minimise sedimentation on the designated conservation sites. Where adjacent facilities are dredged (Coryton for example), the sequence in which berths are dredged is managed, when possible, to work downstream, thereby avoiding deposition within recently maintained areas.

Re-deposition rates vary depending on the grading of the dredged materials. Sand material will be re-deposited within close proximity of the dredge site whereas fine silts may remain in suspension for a period of days following dredging.

The following subsections identify the locations of water injection dredging operations within the study area. The locations of these areas are shown in Figure 1.1. Information is provided on the dredging frequency, estimated quantities, dredge material (where identified) and distances to the SPAs and SSSI. For WID, quantities are estimated based on pre and post dredge survey data undertaken for each campaign.

Port of Tilbury Bellmouth

The Bellmouth is located at the entrance to the main lock barrel leading to the Port of Tilbury. Dredging activity for the port is detailed below.

Client Organisation

Port of Tilbury London Limited

Contractor

Van Oord UK Limited

Maintained Depth

-8.5 m CD (with restriction to -7.2 m CD)

Dredging frequency

3 months - 20,500m3 in last campaign

Average annual dredge quantity

85,000 m3

Material type

soft silt

Need for Maintenance Dredging activity

To maintain access to the Port of Tilbury through the lock for all vessels at all states of the tide.

Historical context

While the adjacent berths are largely self maintaining, the lock entrance is a natural silt trap. Consequently, dredging has been necessary throughout the operational life of the dock system. Injection dredging on the ebb has been implemented over the past decade, with no obvious impact on the adjacent river berths (Northfleet Hope Container Terminal for example). WID commenced in early 1990s.

Contamination testing

Sediments are tested on a two yearly cycle, with the seven samples tested for TBT in October 2003, and two tested for a full suite of metal and organic compounds. TBT has been identified in deeper sediments in an area adjacent to the East Lead-in Jetty (test results dated October 2003). In consequence, and following extensive monitoring and review, the PLA dredging licence is now issued conditional on POTLL/ Van Oord ACZ demonstrating that a buffer layer is maintained with the dredge surface in excess of 500mm above the contaminated strata at all stages of the dredge process.

The sampling regime has identified no other contamination that precludes dredging and TBT has not been found at elevated levels in subsequent analysis cycles.

Distance to Thames Estuary and Marshes SPA

6.4 km

Distance to Benfleet and Southend Marshes SPA

20.4 km

Distance to Holehaven Creek SSSI

14.6 km

 

Robbins Wharf

Client Organisation

Foster Yeoman

Contractor

Van Oord UK Limited

Maintained Depth

-3.0 m CD

Dredging frequency

1 year

Average annual dredge quantity

1,000 m3

Material type

Silt

Need for Maintenance Dredging activity

Maintenance of the berth pocket to allow receipt and unloading of aggregate vessels at the jetty.

Historical context

Maintenance dredging operations have historically taken place using trailer equipment. However, WID techniques were introduced in early 1990s.

Contamination testing

2 samples tested for full suite of metal and organic compounds in October 2003.

Distance to Thames Estuary and Marshes SPA

7.0 km

Distance to Benfleet and Southend Marshes SPA

21.0 km

Distance to Holehaven Creek SSSI

15.2 km

 

Customs House Jetty

Client Organisation

HM Customs

Contractor

Van Oord UK Limited

Maintained Depth

-4.0 m CD

Dredging frequency

6 months

Average annual dredge quantity

5,000 m3

Material type

soft silt

Need for Maintenance Dredging activity

Maintenance of the berth pocket to permit all tide operations for the Customs vessels

Historical context

The jetty has been dredged regularly since construction, with the implementation of WID techniques in early 1990's. Testing is undertaken on a 2 year cycle, with the most recent testing dated January 2006.

Distance to Thames Estuary and Marshes SPA

3.8 km

Distance to Benfleet and Southend Marshes SPA

18.4 km

Distance to Holehaven Creek SSSI

12.8 km

 

Divers Shoal

Client Organisation

Port of London Authority

Contractor

Van Oord ACZ Limited & Westminster Dredging Limited

Ruling Depth

-9.1 m CD

Dredging frequency

3 months

Average annual dredge quantity

6,000 m3 for all dredging at the location (see also trailer dredging)

Material type

Fine sand and silt, rare gravel - some debris

Need for Maintenance Dredging activity

The shoal impinges on the deepwater channel, reducing the all tide ruling depth for the docks and jetties to the west. Maintenance of depth is undertaken by WID, but the PLA undertakes less frequent trailer campaigns to remove coarser materials and debris.

Historical context

Located upstream of Coalhouse Point, Divers Shoal has historically provided the limiting depth for the river. Training works implemented on the northern side of the channel in 1995 successfully generated higher currents in the channel itself, while allowing accretion on the northern foreshore. This has considerably reduced, although not eliminated, the shoal's maintenance dredging requirements.

The material was tested for metals and organic compounds in August 2005 with no abnormalities

Distance to Thames Estuary and Marshes SPA

1.0 km

Distance to Benfleet and Southend Marshes SPA

15.6 km

Distance to Holehaven Creek SSSI

10.2 km

 

Shell Bravo

Client Organisation

Shell

Contractor

Van Oord UK Limited

Maintained Depth

-14.6 m CD

Dredging frequency

3 months

Campaign Duration

15 to 20 hours

Average annual dredge quantity

60,000 m3

Material type

soft silt/ fine sand

Need for Maintenance Dredging activity

Maintenance of the berth pocket to permit all tide operations. The berth is the deepest of the jetties on this portion of the coast.

Historical context

The Shell jetty has been dredged regularly since construction. Frequent dredging is now a necessity to provide appropriate levels of access to the berths. Testing of sediments is undertaken on a 2 yearly cycle.

Maintenance of depth is undertaken by WID, but Shell undertakes infrequent trailer campaigns to remove coarser materials and debris.

Distance to Thames Estuary and Marshes SPA

1 km

Distance to Benfleet and Southend Marshes SPA

9.8 km

Distance to Holehaven Creek SSSI

3.6 km

 

Petroplus Coryton Berths

Client Organisation

Petroplus UK

Contractor

Van Oord ACZ Limited

Maintained Depth

Varies:

  1. -10.6 m CD
  2. -5.1 and -7.0 m CD
  3. -13.4 m CD
  4. -14.0 m CD
  5. -13.1 m CD

Dredging frequency

3 months

Campaign Duration

Up to 50 hours

Average annual dredge quantity

105,000 m3

Material type

Silt and fine/medium sand

Need for Maintenance Dredging activity

The Coryton facility is of national significance and, without regular dredging, would become unable to accommodate the delivery fleet.

Historical context

The site comprises a range of jetty structures serving differing vessel profiles. All have been dredged regularly since construction. However, analysis of survey data from 1970 to 1999 indicates that the deposition patterns in the area have been changing over the past 25 years. The grading of the sediments at Coryton is understood to be tending to coarser sand fractions.

Maintenance of depth is undertaken by WID, but BP undertakes infrequent trailer campaigns to remove coarser materials and debris. The last such campaign took place in January 2003, during which 42,850 m3 was removed and taken to South Falls (marine disposal).

Sediment testing is undertaken on a 2 year cycle, with the most recent results dated July 2005.

Distance to Thames Estuary and Marshes SPA

1.4 km

Distance to Benfleet and Southend Marshes SPA

5.8 km

Distance to Holehaven Creek SSSI

400 m

 

Oikos Terminal (Holehaven Wharf)

Client Organisation

Oikos

Contractor

Van Oord UK Limited

Maintained Depth

-10.5 m CD

Dredging frequency

3 months

Campaign Duration

Approx 15 hours

Average annual dredge quantity

60,000 m3

Material type

Silt and fine sand

Need for Maintenance Dredging activity

Maintenance of the berth pocket to permit all tide operations. The rate of accretion at the jetty appears to be accelerating, reflecting changing sedimentation patterns in the area.

Historical context

The Oikos jetty has been dredged regularly since construction.

Sediment testing is undertaken on a 2 yearly cycle, with the most recent set dated June 2004 and June 2005. No abnormalities have been recorded.

Distance to Thames Estuary and Marshes SPA

1.5 km

Distance to Benfleet and Southend Marshes SPA

5.0 km

Distance to Holehaven Creek SSSI

200 m

Back to Existing Dredging Regime Back to the top

Trailing or Trailing Suction Hopper Dredger Maintenance Dredging

 

Water injection dredging is not suitable for all locations and bed materials. Consequently, some areas are maintained using conventional trailing suction hopper dredging (TSHD) equipment.

Material is taken from the seabed and transported to disposal sites in hoppers. There is a one established marine disposal site licensed for material arising from the Thames, namely South Falls. This is located 110km east of Gravesend, within a polygon defined by:

CEFAS Site Code Name Degrees and Decimal Mins
    Latitude Longitude
TH070 SOUTH FALLS 51 35.000 N 01 58.000 E
    51 35.000 N 02 00.000 E
    51 35.000 N 02 00.000 E
    51 35.000 N 01 57.000 E
    51 35.000 N 01 58.000 E

TSHD Operation
 
TSHD operation

A further sand placement site has recently been characterised in the North Edinburgh Channel but this is yet to be used and is subject to a monitoring regime to validate the environmental assessment. As a consequence of the costs associated with transporting dredged materials over this distance, it is relatively unusual for sediment dredged from the Thames for maintenance to be taken to sea disposal. In preference, arisings from TSHD operations are typically taken to land-based site for disposal. Currently, the main land placement sites on the Thames Estuary are as follows:

  • Rainham Marshes, where the site is owned by Defence Estates and leased by RSPB, licensed to PLA and operated by Westminster Dredging. The site is to be managed under procedures agreed jointly by RSPB (Inner Thames Marshes SSSI) and PLA (Rainham Silt Lagoons)
  • Cliffe Pools, where the site is owned by RSPB and managed by Westminster Dredging.

Tilburyness Shoal

Tilburyness is located approximately 400 metres seaward of Tilbury Lock entrance.

Client Organisation

Port of London Authority

Contractor

Westminster Dredging

Ruling Depth

-9.1 m CD

Dredging frequency

3 years

Average annual dredge quantity

700 m3

Disposal Site

Rainham or Cliffe Pools

Material type

Sand

Need for Maintenance Dredging activity

The shoal impinges on the deepwater channel, reducing the all tide ruling depth for the docks and jetties to the west.

Historical context

Dredging quantities are variable, and are influenced by the presence of sand waves. Ploughing and WID has been tested in the area, but the coarseness of the sediment is such that mechanical excavation remains the most practical dredging method.

The sand sediment was tested for TBT, metals and organic compounds in October 2003. No abnormalities were found

Distance to Thames Estuary and Marshes SPA

5.6 km

Distance to Benfleet and Southend Marshes SPA

19.6 km

Distance to Holehaven Creek SSSI

13.8 km

 

Tilbury Power Station

Client Organisation

RWE NPower

Contractor

Westminster Dredging

Maintained Depth

-14.0 m CD

Dredging frequency

6 months

Average annual dredge quantity

40,000 m3

Disposal Site

Cliffe Pools

Material type

soft silt

Need for Maintenance Dredging activity

Maintenance of the berth pocket to permit all tide operations for the importation of coal for the power station

Historical context

Previously maintenance dredging was carried out on the older B Jetty. However since construction of a new berth (approximately 2 years ago), maintenance dredging has only been carried out on the new jetty and the older jetty has not been dredged.

Sediment testing is undertaken on a 2-yearly cycle, with the most recent set dated June 2004 and June 2005. No abnormalities have been recorded.

Distance to Thames Estuary and Marshes SPA

3.0 km

Distance to Benfleet and Southend Marshes SPA

18.4 km

Distance to Holehaven Creek SSSI

11.4 km

 

Divers Shoal

Client Organisation

Port of London Authority

Contractor

Westminster Dredging

Ruling Depth

-9.1 m CD

Dredging frequency

3 years, for trailing campaign

Average annual dredge quantity

6,000 m3 for all dredging at the location (see also WID).

Disposal Site

Rainham or Cliffe Pools

Material type

Fine sand and silt

Need for Maintenance Dredging activity

The shoal impinges on the deepwater channel, reducing the all tide ruling depth for the docks and jetties to the west.

Historical context

Located upstream of Coalhouse Point, Divers Shoal has historically provided the limiting depth for the river. Training works implemented on the northern side of the channel in 1995 successfully generated higher currents in the channel itself, while allowing accretion on the northern foreshore. This has reduced, although not eliminated, the shoal's maintenance dredging requirements. The shoal is regularly dredged by water injection dredging. However, it is found that debris accumulates in the bed and this is removed by mechanical plant. The excavated material comprises gravels, but with additional waste materials (tyres and steel debris). Testing undertaken on a 2 year cycle, with the last report dated August 2002.

Distance to Thames Estuary and Marshes SPA

1.0 km

Distance to Benfleet and Southend Marshes SPA

15.6 km

Distance to Holehaven Creek SSSI

10.2 km

 

Coalhouse Shoal

Client Organisation

Port of London Authority

Contractor

Westminster Dredging

Ruling Depth

-9.0 m CD

Dredging frequency

3 years

Average annual dredge quantity

1,000 m3

Disposal Site

Cliffe Pools

Material type

Sand and gravel

Need for Maintenance Dredging activity

The shoal impinges on the deepwater channel, reducing the all tide ruling depth for the docks and jetties to the west.

Historical context

Minor accretion of coarse sediments and debris has required infrequent maintenance, in tandem with works upstream at Divers Shoal.

Distance to Thames Estuary and Marshes SPA

26.0 m

Distance to Benfleet and Southend Marshes SPA

14.6 km

Distance to Holehaven Creek SSSI

9.2 km

 

Sea Reach

Client Organisation

Port of London Authority

Contractor

Westminster Dredging

Ruling Depth

-10.2 m CD

Dredging frequency

3 months

Average annual dredge quantity

4,000 m3

Material type

Sand

Need for Maintenance Dredging activity

The shoal impinges on the deepwater channel, reducing the all tide ruling depth for the docks and jetties to the west.

Historical context

Dredging takes place on the northern side of the main navigation channel. Dredging quantities are variable.

Distance to Thames Estuary and Marshes SPA

1.0 km

Distance to Benfleet and Southend Marshes SPA

6.0 km

Distance to Holehaven Creek SSSI

600 m

Back to Existing Dredging Regime  Back to the top

Plough Maintenance Dredging 

 

Some plough dredging is undertaken generally in tandem with other maintenance dredging techniques, but also as a stand alone technique.

Ploughing utilises a tug vessel equipped with a plough unit (a steel box suspended on cables/chains). The plough is lowered to predetermined levels and is used to drag sediment along the seabed. Because the vessels are small and maneuverable in comparison to, particularly, trailing dredgers, ploughing is utilised to move material from areas inaccessible to the main dredging plant. As with water injection dredging, ploughing should not lead to significant re-suspension of sediment but if the sediment ploughed is soft it may be sufficiently disturbed to rise in to suspension. Ploughing equipment has also been deployed in the Thames to level sand waves in the channel bed, but without significant success. Upstream of the study boundary at least two sites are licensed to use plough dredging on a frequent basis thus remobilising sediment on a little and often principle.

Back to Existing Dredging Regime  Back to the top