Changes in the morphology of the estuary through the removal or redistribution of sediment within the system
Removal of sediment from the system can create an artificial sink for sediment which may modify the fine sediment regime reducing supply to other nearby areas (Royal Haskoning, 2004). In the Thames Estuary this has the potential to occur where material is disposed of onshore.
However, over half of the maintenance dredging on the Thames Estuary is now undertaken using Water Injection Dredging (WID) techniques. This agitation technique, which retains fine sediment in the estuary, is used to remove about 105,000 m3 per year at the berths at Coryton and approximately 85,000 m3 per year at Tilbury Docks. The quantity removed by conventional dredging methods is up to 50,000m3 per year (HR Wallingford, 2002e) with this material being placed at Rainham Marshes and Cliffe Pools. It is worth noting that the sediment budget that has been published for the Thames Estuary (Institute of Estuarine and Coastal Studies, 1993) suggested that there was a net sediment shortfall in the budget of some 200,000m3 per annum, met through material entering the system from the sea.
Since 1998, the volume of material taken out of the system to land (Cliffe and Rainham) has reduced, and there is currently some 200,000m3 per annum of dredging undertaken by WID. Based on the earlier assertion by IECS, the change in dredging practice over the past 15 years balances the budget and suggests the system is in dynamic equilibrium, including the influence of dredging activities. Therefore it is regarded that the annual removal of 50,000-150,000 m3 of sediment from the estuary is already accounted for in the sediment budget and hence morphology of the study area.
The North Kent CHaMP (Coastal Habitat Management Plan) identifies that under various sea level rise scenarios (2mm and 6mm rises); sediment budgets within the estuaries of the embayment could become increasingly depleted over the next 50 years and go into deficit over the next 50-100 years. It is possible that additional fluvial sediment input may arise, but this is unlikely to amount to a significant additional contribution to make up for shortfalls. There is therefore a clear need to ensure that mudflat and saltmarsh health are monitored and an ongoing evaluation of sediment input is maintained. If the effects of shortfalls are identified, there will be a need to find additional ways of retaining sediment within the system through techniques such as Water Injection Dredging and sediment placement at suitable locations.
It is anticipated that the Baseline Document will evolve over time to incorporate and respond to new information on sediment regimes as it becomes available (i.e. through key projects and initiatives such as TE2100 and CHaMPs). There is some evidence for change in bathymetry within the entrance to Holehaven Creek and to Mucking Flats, indicating changing morphologic conditions.
The redistribution of sediment within the system as a result of Water Injection Dredging, has been considered in studies by HR Wallingford (EX2648, September 1992) and Environmental Tracing Systems (RE40981, November 2003). HR Wallingford concluded that the material arising from WID should not cause significant damaging changes to either the general estuary regime or the maintenance dredging commitments of local river users, and that the impact was no different to that arising from a cessation of dredging at the location of the WID exercise. HR Wallingford also concluded that WID should not be used in higher reaches of the estuary (above Tilbury). However, as noted above, the annual remobilised volume relative to the sediment budget in the outer estuary is not significant.
The current trend for accretion has been identified from survey data predating the implementation of WID techniques. The current deposition pattern, while additionally influenced by particular climatic events in noted years, does not appear to have significantly accelerated or altered this underlying trend. The Institute of Esturine and Coastal Studies analysis of Coastal Processes and Conservation in the Thames Estuary (October 1993) concluded that "the estuary has reached dynamic equilibrium and that dredging at the rate practiced over the 100 years does not appear to have any deleterious effect on the intertidal morphology of the estuary. Dredge spoil dumping within the estuary system until 1961 merely resulted in a rapid return of the sediment to the dredge sites. Mudflats within the estuary appear to have responded not only to sea level changes, but to have kept pace with the increase in the rate of such sea level rise which appears to have taken place over the past 40 years".