Case study: Applying ASMITA to UK estuaries
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There are over 170 estuaries with a variety of physical characteristics, spatial extents and management issues dissect the coastline of the United Kingdom . Many have some form of nature protection designation and intertidal areas are particularly important for numerous species, including migrating birds. Intertidal areas provide important natural coastal defences, protecting the low lying land surround estuaries from flooding. Estuaries may also be used for recreational activities such as sailing, fishing and walking and are economically important as ports, fishing grounds and for aggregate extraction . The diverse uses and morphologies of estuaries can lead to complex and sometimes conflicting management demands. In order to manage estuaries effectively it is important to be able to predict how they are likely to change in the future, both to natural and anthropogenic forcing. This article looks at historical morphological development of four UK estuaries and uses a model (ASMITA) to predict the maximum rate of sea-level rise each estuary can undergo before intertidal areas are lost completely.
Sea-level rise is predicted to accelerate over the 21st Century, with a global-mean rise of 9 to 88 cm, with the largest relative sea-level rise in the UK in the south . As sea-level rises, the water volume of the estuarine channels increases, while the intertidal sediment volume decreases (Fig.1). With high rates of sea-level rise major changes in the morphology of the estuary may occur, including loss of intertidal areas, leading to habitat loss, shoreline erosion and flooding of low lying areas around estuaries.
ASMITA (Aggregated Scale Morphological Interaction between Inlets and Adjacent coast , represents the estuary as a series of morphological elements (Fig.2). Each element evolves towards an empirically derived equilibrium volume and interacts with adjacent elements by sediment exchange. In the ASMITA model, sediment exchange between adjacent elements is assumed to depend on the difference in suspended sediment concentration, which in turn is assumed to depend on the difference in the amplitude of the tidal velocity. This diffusion type sediment transport model contrasts with other models for net sediment transport where advective processes play a dominant role, either through estuarine circulation or through tidal asymmetry effects (see e.g. Tidal asymmetry and tidal basin morphodynamics). ASMITA was calibrated to reproduce historic estuary evolution for four UK estuaries. Calibrated models were used to predict the maximum rate of sea-level rise (SLRCRIT) each estuary can undergo before intertidal areas are lost.
- High sediment supply from Holderness cliff erosion
- High capacity for sediment transport
- Relatively large intertidal area
- Morphology is in equilibrium with sea-level rise
- Morphology dominated by hard rock geology of area
- Limited sediment supply
- Small areas of intertidal mudflats and salt marshes
- In equilibrium with sea-level rise and sediment supply on time-scale of interest
- Fine sediment transported into harbour on flood tides, coarse sediment moves seaward to form ebb-tidal delta
- Extensive salt marshes and mud flats
- Element volumes varied over the study period, but tend towards equilibrium
- Sediment regime similar to Langstone Harbour
- Extensive intertidal areas
- Ebb-tidal delta is dredged for gravel
- Element volumes tend towards equilibrium
ASMITA was able to reproduce the general evolution of each study estuary to a satisfactory level. Allowing for uncertainty in the data and model parameter values, a range of maximum sea-level rise rates was produced for each estuary (Fig.3). Maximum rates of sea-level rise varied between estuaries. The Humber has the largest maximum rates, suggesting it will be resilient to morphological change driven by sea-level rise. The Dart estuary has the smallest maximum rates, indicating it is sensitive to sea-level rise. The Dart estuary, Langstone Harbour and Chichester Harbour all have the lower limits of the SLRCRIT range within predicted future rates of sea-level rise.
The maximum rate of sea-level rise an estuary can undergo before losing all intertidal area varies between estuaries. Estuaries with a large sediment supply and capacity to transport sediment can withstand greater rates of sea-level rise. The results suggest that some UK estuaries may experience rates of sea-level rise within their SLRCRIT range by 2080 (Fig. 3). If intertidal areas are to be maintained in these estuaries, estuary management must focus on increasing the sediment supply to the intertidal zone. Dredging or managed realignment may exacerbate the problem by increasing the sediment demand of the estuary. Future work will examine this in more detail.
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