Short and long term changes in estuary capacity

Abstract

Changes in capacity have been examined in the Lune, Ribble, Mersey and Humber estuaries to assess the role played by land-derived sediments in coastal accretion. The engineering concept of dynamic equilibrium is introduced and shown to imply that land-derived sediments should contribute exclusively to coastal accretion. The various factors responsible for changing estuary capacity are also introduced. Capacity changes over the last 100 years show that all the estuaries considered have suffered large scale siltation, with the majority of the sediment being derived from coastal and offshore sources. In the case of the Lune, Ribble and Mersey estuaries, engineering works have been responsible for large permanent changes in capacity. The situation in the Humber is less clear and siltation is probably a result of natural infilling; the estuary having been over-deepened by glacial and fluvial action in the past. A detailed study of the Mersey suggests that the estuary was also over-deepened and over-widened and has only been tidal for the last 9000 years or so. Calculations suggest that a new equilibrium will be achieved in about 250 years time and that the estuary may then be in a state of dynamic equilibrium. The much smaller Lune estuary may already have achieved this state, although detailed evidence is scarce.

Long-term morphological change and its causes in the Mersey Estuary, NW England

“…A similar pattern of net landward transfer is also evident in the neighbouring Dee and Ribble estuaries (Pye, 1996;van der Wal et al, 2002). The fluvial supply of sediment to the Mersey has always been small compared to the supply from offshore sources (O'Connor, 1987), and the amount of sediment reaching the estuary was reduced further following construction of the Manchester Ship Canal in the early 1890s (Comber et al, 1993).…”

“…These patterns were also identified by Price and Kendrick (1963), and are reflected in the migration pattern of sedimentary bedforms in the eastern Irish Sea (Sly, 1966;Belderson and Stride, 1969). The long-term net transport of sediment into Liverpool Bay has resulted in a reduction in average depth (Pye, 1977;Neal, 1993) and has contributed to a net drift of sand into the Inner Estuary, identified by O'Connor (1987). A similar pattern of net landward transfer is also evident in the neighbouring Dee and Ribble estuaries (Pye, 1996;van der Wal et al, 2002).…”

“…Although there was slow net accretion of sediment in the Outer Estuary and Liverpool Bay, and significant coastal progradation around Formby Point, net changes in the Inner Estuary appear to have been fairly limited, with the capacity of the estuary being held more or less constant as a result of the periodic migrations of the channels (McDowell and O 'Connor, 1977;O'Connor, 1987). This situation evidently began to change significantly in the later part of the 19th century, and particularly in the first half of the 20th century.…”

“…However, these reclamations have had only a small effect on the tidal prism of the Mersey Estuary. O'Connor (1987) calculated that the direct loss of capacity of the estuary due to land reclamation since 1861 was about 12 × 10 6 m 3 .…”

Cross‐shore sediment transport and the equilibrium morphology of mudflats under tidal currents

“…However, both empirical evidence and the results of simulations suggest that muddy shores respond to changes over a period of decades, so to compare predicted with actual change would require an observational programme on this timescale, including concurrent records of the bathymetry of a flat, the surface properties of the sediment and the offshore levels of suspended sediment, and records of extreme events such as large storms which might exert a lasting effect on the local morphology. (We note in passing the additional difficulty of judging whether estuarine environments are in an equilibrium state [O'Connor, 1987]. )…”