The fine sediment regime of the Thames Estuary – A clearer understanding

Estuaries are well known to be ecologically important in themselves with their high biological productivity and as habitat areas of wider value, for example, for over wintering water birds. Fine sediments have an important role in estuarine functioning both as settled material forming intertidal areas (mudflats and salt marsh) and in suspension. Hence the fine sediment regime of an estuary is a key consideration in understanding and managing an estuarine system.A reappraisal of the fine sediment regime of the Thames Estuary has been undertaken with reference to new observations of suspended sediment concentrations and collated morphological change data.The new observations of suspended sediments made in 2004 showed a very similar form of the longitudinal variability of suspended sediment concentrations compared to the last estuary-wide survey undertaken in 1969 with the turbidity maximum centred 50 km landward of Southend.The assessment of historic morphological change indicated that over the 20th Century the Thames has experienced differing types of morphological change (anthropogenic and natural) in the upper estuary landward of London Bridge and in the lower estuary seaward of Barking. In the upper estuary the sub-tidal channel has deepened and widened and has been accompanied a loss of intertidal area. In the lower estuary the sub-tidal channel has deepened and narrowed accompanied by a gain in intertidal area. The recent regime of morphological change in the estuary is characterised by intertidal accretion with some sub-tidal erosion.The inclusion of morphological change into the fine sediment budget has removed the discrepancy between sediment inputs and outputs to the Thames Estuary that was hitherto assumed to be proof of large amounts of sediment coming into the Thames from marine sources. Whilst the possibility of some sediment sourced from the seaward end of the estuary, for example from cliff erosion on the Isle of Sheppey or dredging in Sea Reach cannot be completely excluded, the sediment budget does not require a large marine input in order to present a coherent picture of the fine sediment regime of the estuary.

► Observations of suspended sediments in 2004 were very similar to those measured in 1969.
► Since 1900, the upper Thames Estuary has deepened and widened, reducing intertidal area.
► The lower Thames Estuary has deepened and narrowed, increasing intertidal area.
► Recent morphological change in the estuary is intertidal accretion with some sub-tidal erosion.
► Including morphological change broadly balances the sediment budget of the Thames Estuary.