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Sediment Budget

Sediment budgetting

The definition of sediment budgets for estuaries are challenging, particularly for large estuaries such as the Thames, with many (and generally difficult to quantify) anthropogenic (particularly historic dredging and disposal) forcing factors. Only one sediment budget has been published for the Thames Estuary (Institute of Estuarine and Coastal Studies, 1993). This budget suggests that the estuary exists in a balance between sediment deposition and erosion over a number of tidal cycles or seasons, and there is neither loss nor gain of sediment from the estuary. This assertion, if proven, suggests the system is in dynamic equilibrium and this would include the influence of dredging activities. It should be recognised that such equilibrium may be disrupted by new anthropogenic factors, or by accelerated sea-level rise, or by change in components of the budget such as discharge of sewage effluent (HR Wallingford, 2004, Royal Haskoning, 2004).

The table below shows a proposed sediment budget for the Thames Estuary using data from 1940 to 1998 (figures in cubic metres per year) published by the Institute of Estuarine and Coastal Studies in 1993. One of the main difficulties in being definitive about the sediment budget is the small amount of net sediment movement during a tidal cycle compared to the very large volumes of water and sediment moved. For example, at Southend-on-Sea, the instantaneous discharge on a spring tide is around 42,500 m3s-1 and the background suspended sediment concentration is around 50

(Thorn and Burt, 1978). Therefore, around 2 tonnes of sediment are transported across a given section of the estuary every second, which equates to 46,000 tonnes on a typical spring tide. Of this amount, it is estimated (using the budget shown in the table below) that less than 300 tonnes (less than 0.7%) is retained in the estuary on the flood tide, the rest is returned to the sea on the ebb tide.

Location

Source

Sink

Saltmarsh

65,000

 

Mudflat

50,000

98,600

Subtidal (dredged)

 

250,000

Fluvial

27,000

 

Marine Source

206,100

 

Total

348,600

348,600

Subtidal budget

The average annual volume of sediment dredged from the estuary channel since 1961 is around 225,000 m3. Assuming a dry density for this sediment in the hoppers and barges is around 500 kgm-3 (bulk density of 1300 kgm-3) then the approximate mass of sediment presently extracted from the estuary (sink) is 113,000 tonnes per year and hence this is re-supplied to the system from the various sediment sources.

Blyth Sand & Yantlet Flats

HR Wallingford (2002a) showed that the volume of sediment on the intertidal flats of Blyth Sands and Yantlet Flats increased by 2.39 x 106 m3 between 1970 and 1998. Using a dry density of 1000 kgm-3 for soft silts, this equates to an annual mass increase (sink) of 86,000 tonnes per year.

Mucking Flats

HR Wallingford (2002a) showed that the volume of sediment on the intertidal flats of Mucking Flats increased by 8.55 x 105 m3 between 1970 and 1998. This equates to an annual volume increase (sink) of 31,000 m3 or approximately 31,000 tonnes per year.

Saltmarsh Sediments

Burd (1992) calculated that between 1973 and 1988, 0.98 x 106 m2 of saltmarsh were eroded in the Inner Thames Estuary (between Higham Marshes and Shoebury Ness). Assuming that the surface of the saltmarsh is elevated 1 m above the level of the mudflat, and the erosion rate has continued to the present day then 65,000 m3 (65,000 tonnes) of eroded saltmarsh is yielded every year. In terms of a sediment sink, if it is assumed that accretion of sediment on the saltmarsh surface averages around 3.0 mm per year (1.9-3.9 mm per year at Higham and 3.4 mm per year at Benfleet) then using the 1988 area of saltmarsh of 3.2 x 106 m2 equates to a maximum sink of 9600 m3 per year or approximately 9600 tonnes per year.

Fluvial Sediments

Institute of Estuarine and Coastal Studies (1993) estimated an annual fluvial input of suspended solids of 28,000 m3. Using a dry density of 1000 kgm-3 for the sediment, this equates to a mass of 28,000 tonnes per year.

The Environment Agency and its predecessors have dredged the River Thames upstream of the tidal limit at Teddington for many years with records going back to the 1930s. These schemes now present the Agency with an ongoing liability to maintain the design channel profiles and this means that over the last 10 years, on average, about 40,000 tonnes are dredged annually (Kirby, 2000).

Dredgings are currently disposed of at a licensed waste site known as Penton Hook Pit, a former gravel pit adjacent to the Thames and thus are removed from being available to the system.

Response to historical sea level rise

The data suggest that vertical accretion on the intertidal areas of Blyth Sands/Yantlet Flats and Mucking Flats and the saltmarshes downstream of Gravesend has exceeded sea level rise over the past 30 years (Royal Haskoning, 2004). However, some areas of intertidal flat along the northern shore (e.g. Southend Flat) appear to have eroded over this time period. Overall, Blyth Sands/Yantlet Flats have vertically accreted at a rate of 4-5 mm per year between 1970 and 1998 (HR Wallingford, 2002a). Similarly, Mucking Flats has accreted by around 11 mm per year during the same time period (HR Wallingford, 2002e). Vertical accretion rates on saltmarsh surfaces have been measured up to 3.9 mm per year at Higham and 3.4 mm per year at Benfleet. Historic relative sea-level rise has varied between 1.22 and 2.14 mm per year (Woodworth et al., 1999) and as such, sediment availability to the estuary downstream of Gravesend is adequate to meet demand under current environmental conditions.

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