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

Changes in the morphology of the Mersey Estuary, and their possible causes, have been investigated using Historical Trend Analysis and Expert Geomorphological Assessment. Historical bathymetric charts were digitised and analysed within a GIS to provide quantitative estimates of changes in areas and sediment volumes above and below selected elevation planes. The results show that the estuary experienced major changes over the last 150 years, notably between the late 19th century and ca. 1950. An analysis of data relating to possible natural and human factors which could have influenced these changes suggests that training wall construction and dredging in the Outer Estuary and Liverpool Bay was the most significant factor contributing to change during this period. These activities modified the hydrodynamic and sediment transport regime in a way which enhanced an existing natural tendency for movement of sediment from Liverpool Bay and the Outer Estuary into the Inner Estuary. Changes in other factors, including sea level, tidal range, wind/wave climate, freshwater flow and embanking/land reclamation, were of relatively minor importance. Between 1950 and 1977 the rate of sediment accumulation in the Inner Estuary declined as the estuary approached a new condition of dynamic equilibrium, and since 1977 there has been a slight net loss of sediment. Under these conditions the changes in natural forcing factors, such as sea level and storminess, are likely to have a relatively greater effect on the estuary than in the past.