Waves, suspended particles and turbidity
Waves affect transport, fishing, offshore industry and coastal communities; they can cause coastal erosion and structural damage, which contribute to flood risk. They influence the stratification of surface layers and the rate at which gases pass between the atmosphere and the ocean surface. In shallow waters, waves cause strong currents within a few centimetres of the seabed, affecting habitats and suspending sediment.
In turn, suspended particulate matter (SPM) influences nearshore and benthic habitats; it affects marine communities including plankton, benthic invertebrates and fish, by carrying pollutants and blocking sunlight, so inhibiting photosynthesis. SPM also includes plankton and so forms part of the marine ecosystem. Hence studying SPM can help us understand the transportation of pollutants and nutrients, primary production and its fate – how much falls to the seabed or contributes to the water-column food web – and perhaps also eutrophication. SPM also affects bathing water quality. Its transport, for example longshore drift, is a factor in coastal erosion and morphology. SPM is driven directly by seabed currents from tides, wind and waves, and so varies greatly with water depth. It also depends on sediment availability, which can be affected by dredging and land use, and varies locally with rainfall and flooding around the coast.
For waves, this assessment uses data from satellite altimetry, wave sensors on moored buoys and lightships, offshore and many nearshore sites. We have also used modelling for wave prediction, forecasts and state estimation, which is well-developed.
In the west (especially the north-west) and the Irish Sea, winter wave heights correlate significantly with the North Atlantic Oscillation Index, which is a measure of the strength of westerly winds at UK latitudes. They increased through the 1970s and 1980s west of the UK and in the North Sea from the relatively calm conditions experienced during the 1960s. However, recent trends are not clear, with some measurement sets appearing to show a decrease in winter wave heights. Year-to-year variability is such that there is no clear longer-term trend and no clear change since Charting Progress was published in 2005.
In very shallow waters, for example near coasts, trends in wave heights are less marked because the water depth limits the height of the waves as they break. However, as rising sea levels increase nearshore depths, larger waves may approach the shore, enhance erosion and steepen intertidal profiles.
For SPM, we used data from traditional assessment methodologies such as measuring the depth over which a white disk can be seen suspended in the water. However, more sophisticated optical techniques such as back scatter from light beams are increasingly available for particle size as well as concentration. This has increased our understanding of SPM dynamics and processes in shelf seas, especially the tidal stirring of sediments. Remote sensing measurements of ocean colour provide time series for studying variability of SPM, phytoplankton pigments and coloured dissolved material. However, these techniques can be hampered by clouds and by insufficient understanding of optics in turbid coastal and shelf waters.
There is much ongoing research on SPM and turbidity in coastal regions of the UK and Europe but we still need to understand more about nutrient binding and the breakdown of particulate matter. The data currently available show that SPM concentrations, and therefore turbidity for UK waters, are very variable depending on currents, biological influence on sediment properties and seabed characteristics. However, we have no evidence for any changes in the general state of SPM around the UK since Charting Progress.