Summary of changes and trends

• The North Sea has both southerly and northerly offshore sediment transport - northerly transport in the central eastern regions of the southern North Sea, southerly transport nearer the UK coast and several areas of variation in the northern North Sea.
  The nearshore transport is predominantly southerly on N-S orientated sections and westerly on E-W orientated sections.
• The English Channel has both westerly and easterly offshore sediment transport. The nearshore transport is predominantly easterly, with some reversals in the lee of headlands.
• The Celtic Sea has a variable offshore sediment transport. The nearshore transport is predominantly northerly on the N-S orientated coasts and easterly on E-W orientated coasts.
• The Irish Sea has southerly and south-westerly offshore sediment transport on the North Wales coast south of the Lleyn Peninsula and northerly and north easterly offshore transport north of the Lleyn Penisula (Bardsey Sound). The nearshore transport is predominantly northerly on the N-S orientated sections of the coast and easterly on E-W orientated sections.
• Turbidity (water clarity) in the Menai Straits (Irish Sea) deteriorated from the mid 1960s to the late 1980s.
• There was no overall trend turbidity in the Irish Sea between 1987 and 1997.

1. Introduction

This chapter deals with the transport of sediment formed from mineral particles and does not consider the other two components of ‘suspended particulate matter’ (SPM): living plankton and phyto-detritus. (Plankton are considered in the Sector Report on Marine Fisheries). However, ‘turbidity’ is included and is a measure of the amount of SPM in the water, including organic and inorganic material, which results in the scattering and absorption of light rays and hence affects water ‘clarity’. Light is scattered mainly by mineral suspended solids, whereas light is absorbed by mineral suspended solids, chlorophyll and the water itself (Bowers et al., 2002).

Major sources of data and information for this chapter are the Futurecoast report and the Southern North Sea Sediment Transport Study Phase 2 (SNS2).

The Futurecoast study was commissioned by Defra and carried out by a team led by Halcrow Group Ltd. The study provides predictions of coastal evolutionary tendencies over the next century, based on the use of data sets, information and experience of coastal systems. The output from the study is available on an interactive CD (Defra, 2002) and includes reports, guidance, data and mapping at various scales.

The SNS2 (HR, 2002) was designed to provide the broad appreciation and detailed understanding of sediment transport between Flamborough Head and the North Foreland. The study was undertaken between 2000 and 2002 by a consortium comprising of HR Wallingford, CEFAS Lowestoft Laboratory and UEA Norwich, Posford Haskoning and independent consultant Dr Brian D'Olier. It built on the earlier Phase 1 study completed in 1996 (ABP 1996).

Other useful review sources are the Strategic Environmental Assessment documents and reports produced by the Department of Trade and Industry (DTI, 2004) for parts of the North Sea, English Channel/Celtic Sea, Irish Sea and the Western Approaches.

Sedimentary processes affect the coastal and marine environment in a variety of ways:

Turbidity also has an effect on the marine ecosystem - highly turbid water may inhibit growth by reducing both the amount of light available for photosynthesis (Dennison, 1987) and, in shallow waters, the amount of dissolved oxygen, because the water becomes warmer due to the absorption of heat from sunlight by the suspended particles.

The concentration of suspended sediment in UK waters depends upon a wide range of physical processes, and the presence or absence of fine sediment. Suspended sediment concentration (SSC) is the net result of the ‘erosion-transport-deposition (ETD) cycle’. Sediment transport is initiated by the shear stress imposed upon the bed by waves and currents, and transport is predominantly brought about by advection due to a combination of surge-, tide-, wave- and density-induced currents. SSC is thus highly variable and at any one place can vary by more than one order of magnitude over periods of minutes and hours. Further, because in many cases the suspended sediment is derived from the seabed, SSC can vary greatly vertically in the water column, and spatially. Time-series data are thus essential to the effective description and understanding of SSC and its variations, and associated measurements of physical processes are vital in order to correctly ascribe cause and effect.

Sedimentary processes are primarily influenced by the sediment type and availability, and by the nature of currents and waves. In a wave-dominated environment (such as an exposed coast or in shallow water), waves will generally be the dominant mechanism for resuspending sediment into the water column, and the sediment is then transported by the current. In a current-dominated environment (typically offshore deeper water or in an estuary) the currents and waves may both be important stirring mechanisms, and again the current transports the mobilised sediment.

Most types and sizes of sediment grains are moved only when the waves and/or currents are strong enough to exceed its threshold of motion. For very fine grains, they will tend to be transported at the same rate as the transporting fluid. In contrast, for sands and gravel, once in motion under the action of currents, the rate of transport is generally proportional to the third or fourth power of ‘excess’ current speed or wave height (‘excess’ being the magnitude above the grain threshold of motion). Hence, the residual pathways of sediment transport may be very different to the distribution of the residual current (Soulsby, 1997).

The transport of suspended sediment is highly dependent on grain settling velocity. Generally, the transport of slowly settling particles is controlled by the residual circulation, but the transport of faster settling particles occurs via a series of episodes of resuspension from and settlement upon the sea bed, and therefore is controlled by specific tidal, wind and wave conditions (Prandle et al., 1993). For example, large bottom shear stresses induced by tidal currents can drive the transport of larger particles along the seabed (bedload) and induce the regular resuspension and transport of the fine sediments in the water column. Asymmetry between ebb and flood tidal currents can induce net sediment transport (‘Stokes drift’) (Simpson, 2001).

Very fine sediments (clay-size grains) will generally remain suspended in areas of strong tidal currents or significant wave action, but there are some regions around the UK where they can be deposited (such as the head of some estuaries). In contrast, silt-size particles may be periodically deposited and resuspended over semi-diurnal and monthly tidal cycles. In some cases, sand-sized particles may only be transported over a small part of the semi-diurnal tidal cycle (Prandle et al., 1993). Exceptions to these general rules apply in near-coastal, near-frontal or highly stratified areas where physical and biological processes can produce significant variability on small temporal and spatial scales, e.g. the persistent residual gyre observed in the Dover Strait (Prandle and Player, 1993; Prandle et al., 1993).

The measurement of suspended sediment and turbidity in situ involve various techniques. The primary method of measurements is using a device to measure optical back-scatter (OBS, often referred to as a nephelometer), using either instantaneous hand-held instruments of those which are pre-programmed to take readings and are deployed on moorings or beneath buoys. In these instruments, light is transmitted form the instrument into the water column and some light is reflected back from suspended particles and measured. Other instruments measure the degree of transmittance of light between a transmitter and receiver (a ‘Transmissometer’), assuming that most of the loss en-route represent obscuration by particles.

Other instruments use the backscatter of sound (Acoustic backscatter instrument and Acoustic Doppler Current Profiler) to measure sediment and turbidity. All these instruments require calibration to obtain a measure of suspended sediment concentration (units are typically mg/l), which can be a complex process containing many uncertainties. For example, for the optical sensors, a number of factors have a significant impact on instrument response. A change in grain size from medium sands to fine silts may lead to a x100 increase in instrument response; flocculation of fine particles may decrease instrument response by x2; and the presence of plankton in suspension may lead to poor instrument calibrations of SPM concentration (Bunt et al., 1999).

Older-style traditional methods such as ‘Secchi Disks’, whilst easy to use, are of very limited use in comparing datasets or in deriving information about sedimentary processes, but may give some qualitative information on water clarity.

Some measurements from remote sensing sources involve multi-channel optical and near-infrared radiometer observations (e.g. the satellite–borne Advanced Very High Resolution Radiometer or the aircraft-borne Compact Airborne Spectrographic Imager). In particular, there is a relation between the concentration of near-surface SPM and the brightness or reflectance of the sea, provided that allowance is made for changes in absorption due to the concentration of phytoplankton species (Bowers et al., 2002).

Descriptions of the monitoring networks that regularly measure sediments and/or turbidity are given in the chapter on Monitoring Networks, together with details of how to access archived and near real-time data.

Click here for a list of links to monitoring networks and data sets.

The relative importance of the factors influencing sediment concentration and transport (sediment type and availability, currents and waves, see section 1.2) varies across the UK continental shelf. Generally speaking, currents tend to be more important offshore (>10m water depth), whilst waves tend to be the more dominant force in shallower nearshore areas (<10m water depth). The character of the shelf seabed is also highly variable, depending upon modern tidal and current patterns, and on the inherited nature of the seafloor (i.e. past patterns of sediment accumulation). Relatively shallow areas off southeast England give way to deeper areas to the north of Flamborough Head and west of the Isle of Wight. Sandy or gravelly areas occur in regions of strong tidal currents in the Bristol Channel and the southern North Sea. Finer sediments lie in areas where tidal currents are weaker, such as off Plymouth and in parts of the Irish Sea. Mobile sediments may be absent, and thus a rocky seabed occurs, in many nearshore areas and in areas swept by strong currents like the English Channel and near promontories of the west coast of England and Wales (Defra, 2002).

Click here to see a map of the broad-scale seabed sediment distribution in UK waters, based on the BGS product DigSBS250.

Refer also to: http://www.bgs.ac.uk/products/digitalmaps/digsbs250.html.

At a scale of tens of kilometres, the key feature of the offshore sediment transport in UK waters is that the sediment transport pathways tend to follow the large-scale orientation of the coast. There are a number of areas of divergence of bed-sediment transport paths (‘bedload partings’) and other areas of convergence (Stride, 1982, Stride and Belderson, 1991). The locations of these zones are subject to some temporal variation, especially when tidal currents are modified by storm-generated currents. Divergences occur where peak tidal currents are orientated in opposing directions, where tidal current velocities (and hence shear stresses) also tend to be high (Pingree and Griffiths, 1979). Zones of convergence, where sediment transport pathways meet, occur in between zones of divergence.

In contrast, the key features of nearshore sediment transport paths in UK waters are governed more by the predominant wave directions, which is from the northeast on the east coast and from the southwest on the west coast. Sediment divergences or convergences occur where there are significant changes in coastal orientation (e.g. headlands or embayments) and onshore/offshore movements in localised areas (especially major estuaries).

In some areas, where tides and wave forces are orientated in a similar direction, nearshore and offshore sediment transport pathways are aligned. In other areas, where tidal and wave forces are orientated in different directions, the nearshore and offshore pathways may run in opposite directions to each other.

Generally, mapped distributions of suspended sediment show highest concentrations in coastal zones, related to sediment resuspension from the seabed, river discharge and coastal erosion. In the coastal zone, near-bed suspended sediment concentrations can be several orders of magnitude larger during winter storms, compared with calm summer conditions, because the associated surge- and wave-currents enhance resuspension. These processes require ample availability of fine sediments, and in some areas prevailing there is relatively little fine sediment available to be resuspended during high energy events, so that any increased turbidity results from temporary resuspension of sand, and may be confined to the lower parts of the water column. Exceptions to these general conclusions apply in near coastal, near-frontal or highly stratified areas where physical and biological processes can produce significant variability in turbidity on small temporal and spatial scales; e.g. in persistent residual gyres or coastally trapped river water (Charnock et al., 1994), but it is important to separate the biogenic from the physical causes of turbidity.

Within an estuary, the sediments are a complex mixture of particles that have been brought down by the river or in from the sea. The driving forces for sediment transport are variable on a number of time-scales, ranging from the semi-diurnal tidal period through to the spring-neap cycle, and to the seasonal, with additional episodic transport events associated with storm surges and river floods. On time periods of decades, some UK estuaries are relatively close to a long-term equilibrium, with a large amount of sediment transported on each tidal cycle, but relatively little of it either imported (and accumulated) or exported to the shelf.

Click here to see a figure of the broad-scale seabed sediment distribution in the North Sea. Link to figure 5.2 in
http://www.offshore-sea.org.uk/sea/dev/html_file/sea2_consult.cgi?sectionID=33

More details are provided in a series of coastal maps produced in the Futurecoast report (Defra, 2002), including seabed sediments and an indication of the direction of movement of offshore sediment (both suspended and bedload).

The North Sea has both southerly and northerly offshore transport of sediment - northerly transport in the central eastern regions of the southern North Sea, southerly transport nearer the UK coast and several areas of variation in the northern North Sea. Bedload divergences occur due to the interaction of the different amphidromic points, with convergences in between. There are relatively few published field datasets of turbidity in the North Sea (exceptions include Jago and Bull, 2000). Historically, as might be expected, there are low average Secchi depth values (and hence high turbidity) in the southern North Sea (Aarup, 2002), probably due to sediment resuspension by the strong tidal currents (the data is available from http://www.ices.dk/ocean/project/secchi/).

The nearshore sediment transport pathways are predominantly southerly on the N-S orientated sections and westerly on E-W orientated sections (e.g. North Norfolk). Coastal and offshore transport pathways are aligned where the tides and wave forces are orientated in similar directions, i.e. southerly transport from St. Abbs Head to Flamborough Head and around East Anglia on N-S orientated coasts. From Flamborough Head to the Wash, the coastal and offshore pathways run in opposite directions because tidal and wave forces are orientated in different directions.

The main nearshore source of sediments are the eroding cliffs on the East Anglian coastline and sediment transport divergences occur at the mouth of the River Tees and Tyne, Sheringham (North Norfolk), Clacton and at the North Foreland. The main offshore divergences occur at North Sunderland, Cromer, within the Thames Estuary and along a line from Dunwich to the Hoek of Holland. The main nearshore convergences for sediment are the Tees estuary, Humber estuary, the Wash, Stour and Orwell estuaries, the Thames and south of Flamborough Head. The main offshore convergence is off Flamborough Head. Onshore/offshore exchange occurs off North Norfolk.

A map showing seabed sediment transport indicators within the SNS2 study area is available by selecting the link to Appendix 15 at http://www.sns2.org/projects-outputs.html .

Click here to see a figure of the broad-scale seabed sediment distribution. Link to figure 5.2 in
http://www.offshore-sea.org.uk/sea/dev/html_file/sea2_consult.cgi?sectionID=33

More details are provided in a series of coastal maps produced in the Futurecoast report (Defra, 2002), including seabed sediments and an indication of the direction of movement of offshore sediment (both suspended and bedload).

The English Channel has both westerly and easterly offshore transport of sediment. Bedload divergences occur near to the amphidromic point off the Solent. The nearshore sediment transport is predominantly easterly, with some reversals in the lee of headlands. Coastal and offshore transport pathways are aligned where the tides and wave forces are orientated in the same direction, i.e. easterly transport in the eastern part of the Channel and westerly transport in the very western part. In the central part of the Channel, the coastal and offshore pathways run in opposite directions because tidal and wave forces are orientated in different directions.

The main nearshore divergences occur at Selsey Bill, Portsmouth Harbour, the Needles and Dungeness. The main offshore divergence occurs from the south of the Isle of Wight to the Contentin Penisula. The main nearshore convergences for sediment are near Ramsgate, Dungeness, Ryde and the Isle of Wight. Additionally, minor divergences and convergences are associated with each bay headland unit. The main offshore convergence is along a line from Hythe/Dungeness to Boulogne.

2.2.2 Celtic Sea including Bristol Channel

Details of seabed sediments and an indication of the direction of movement of offshore sediment (both suspended and bedload) are provided in a series of coastal maps produced in the Futurecoast report (Defra, 2002).

The Celtic Sea has a variable offshore transport of sediment. The nearshore sediment transport is predominantly northerly on the N-S orientated coasts and easterly on E-W orientated coasts. Off the northern coast of Devon and Cornwall, coastal and offshore transport pathways run in opposite directions because tidal and wave forces are orientated in different directions. Sand moves westwards into the Celtic Sea in the outer and central Bristol Channel (Harris, 1988).

Nearshore, the main nearshore divergences of sediment are in Barnstaple Bay, along a line from Lavernock Point to Sand Bay (in the Bristol Channel) and in Carmarthen Bay. Onshore/offshore exchange occurs in the central portion of the Bristol Channel.

With respect to sea-bed sediments, the Irish Sea has southerly and south-westerly offshore transport of sediment on the North Wales coast south of the Lleyn Peninsula, but a northerly and north-easterly offshore transport path north of the Lleyn Penisula (Bardsey Sound). Bedload divergence occurs near to the amphidromic point in the eastern Irish Sea.

The nearshore sediment transport pathway is predominantly northerly on the N-S orientated sections of the coast and easterly on E-W orientated sections. Coastal and offshore transport pathways are aligned where the tides and wave forces are orientated in the same direction, i.e. northerly transport to the north of North Wales. Off the south and mid-Wales coast, the coastal and offshore pathways run in opposite directions because tidal and wave forces are orientated in different directions.

The main nearshore divergences of sediments are off the Lleyn Penisula (Bardsey Sound), off Anglesey, Formby and Bispham (Blackpool). The main offshore divergences occur from Ireland to Bardsey Sound and from Northern Ireland to Scotland. The main offshore convergences are west and east/southeast of the Isle of Man, Liverpool Bay and Morecambe Bay (DEFRA, 2002).

There is evidence that turbidity water clarity in the Irish Sea deteriorated between the mid 1960s and the late 1980s - the mean annual Secchi depth in the Menai Straits decreased from around 2.3m to below 1.5m during that period (Lumb, 1990).

Using satellite reflectance imagery, White et al. (2003) have shown that there was no overall trend in near-surface turbidity between 1987 and 1997 in the Irish Sea, but that year-to-year variability was positively correlated with changes in the mean annual regional wind strength, controlled by the north-south atmospheric pressure gradients and related to the NAO Index.

References

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Bowers, D.G., S. Gaffney, M. White and P. Bowyer (2002). Turbidity in the southern Irish Sea. Continental Shelf Research, 22: 2115-2126.

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Available via http://www.offshore-sea.org.uk/sea/dev/html_file/library.php

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