- Email: [email protected]
Severn Estuary
Volume 15/Number 2/February 1984 at two stations in the Severn Estuary. Estuar. cstl mar. Sci., 9, 287-302. Uncles, R. J. & Jordan, M. B. (1980). A one-dimensional representation of residual currents in the Severn Estuary and associated observations. Estuar. cstl mar. Sci., 10, 39-60. Uncles, R. J. & Radford, P. J. (1980). Seasonal and spring-neap tidal dependence of axial dispersion coefficients in the S e v e r n - a wide, vertically mixed estuary. J. Fluid Mech., 98, 703-726. Uncles, R. J. & Joint, I. R. (1983). Vertical mixing and its effects on
phytoplankton growth in a turbid estuary. Can. J. Fish. aquat. Sci. (Suppl. 1), 40, 221-228. Van Veen, J. (1938). Water movements in the Straits of Dover. J. Cons. int. Explor. Met, 13, 7-36. Warwick, R. M. & Uncles, R. J. (1980). Distribution of benthic macrofauna associations in the Bristol Channel in relation to tidal stress. Mar. Ecol. Prog. Ser., 3, 97-103. Zimmerman, J. T. F. (1976). Mixing and flushing of tidal embayments in the western Dutch Wadden Sea - II. Neth. J. Sea Res. 10, 397-493.
0025-326X/84$3.00+ 0.00 © 1984PergamonPressLtd.
MarinePollutionBulletin, Vol. 15, No. 2, pp. 53-57. 1984 Printed in Great Britain
Sedimentation Processes in the Bristol Channel/Severn Estuary K. R. DYER
Natural Environment Research Council, Institute of Oceanographic Sciences, Taunton, Somerset, UK The Bristol Channel and Severn Estuary together form a wide, shallow estuary with a large tidal range and strong currents. At spring tide the range at Avonmouth exceeds 12 m and the currents over most of the area exceed 1.5 m s - ~. On neap tides the range is 6 m and the current velocity is generally greater than 0.75 m s-i. Consequently, sediment up to sand grade is likely to be mobile over most of the lunar cycle. The large tidal currents create a 10-15 km excursion of water at spring tides and, in common with most estuaries, this provides a mechanism for sorting the sediments moving in suspension from those that travel as bedload; thus there are fairly sharp divisions between muddy and sandy areas on the seabed, despite ubiquitous turbidity of the water. The lunar variation also causes very large differences in the amounts of sediment moving at spring and at neap tides. The residual current flow in the Bristol Channel has been modelled using a vertically integrated scheme by Uncles (1982); this indicates a general westerly residual current, with a clockwise circulation in Swansea Bay and west of Nash Point. There is also evidence of possible
circulation in Bridgwater Bay. Field observations, though generally in agreement, show large variations between surface and seabed (Uncles & Jordan, 1979) and it is not clear how well these circulations would be reflected in the distribution and transport of sediments. In common with other estuaries, the Severn contains more sediment in motion that the visible annual inputs, and there is a continual exchange of material from areas of erosion to areas of deposition through the turbidity maximum. Understanding this exchange is crucial in any investigation of pollution since fine suspended sediment scavenges pollutants, such as heavy metals, from the water and its deposition may result in their incorporation into the seabed.
Sediment Distribution The bed of the Bristol Channel and Severn Estuary comprises a wide range of lithologies (Fig. 1). Off the North Devon coast the sea floor is largely bare rock, with occasional sand and gravel ribbons. To the north and west
Fig. 1 Sediment distribution in the Bristol Channel and Severn Estuary. From various authors.
53
Marine Pollution Bulletin
this sediment cover becomes more widespread and gives way to extensive zones of sand ribbons and sand waves (Warwick & Davis, 1977; Murray et al., 1980; Warwick & Uncles, 1980), although there are frequent patches of gravel and rock. Off the Welsh coast the sand depositis are occasionally thicker in a series of linear sandbanks which are directed westwards from headlands; Nash Sand from Nash Point, Scarweather Sand from Porthcawl Point and Helwick Sand from Porteynon Point. Within Swansea Bay an isolated muddy zone occurs (Collins et aL, 1979; Heathershaw & Hammond, 1979). The heavy mineral suites in the outer Bristol Channel have been described by Barrie (1980). The sediment distribution and sediment budgets in the Inner Bristol Channel and Severn Estuary have been reviewed by Parker & Kirby (1982a). In the Inner Bristol Channel, eastwards of Nash Point, large areas of rock and thin lag gravel, with occasional sand ribbons and sand waves, separate two banks: Culver Sand and Holm Sand. In Bridgwater Bay there is an extensive area of muddy subtidal and intertidal sediment. East of the Holm Islands, in the Severn Estuary, the shallow margins of the estuary are rocky platforms with a thin sediment cover, predominantly sandy on the English side. On the Welsh side they are more muddy, for example in Peterston and Wentlooge Flats. There are two major sandy areas in the Severn Estuary. The Cardiff Grounds and Monkstone area is a relatively thin sand bank resting on a rocky platform, whereas Middle Ground and Denny Shoal fills a deep valley cut into bedrock (Evans, 1982). Within the surface sands of Middle Ground many mud layers exist, illustrating a complex depositional environment. The Holms Channel, Bristol Deep and King Road have a thin cover of gravelly sand, or sand with some mud patches, whereas in Newport Deep a thick mud deposit occurs. All of the tributary estuaries are muddy. In general the distribution of sediment correlates well with the maximum bed shear stresses calculated from the results of mathematical modelling (Warwick & Uncles, 1980; Miles, 1982). The benthic macrofaunal communities also correlate with the tidal mean bed stress (Warwick & Uncles, 1980) and with the sediment distribution in intertidal areas (Boyden & Little, 1973).
~ct~
~or,r ~y . . . . .
~
0 I
Sandy Sediment There appears to be no major modern source for sand. Belderson & Stride (1966) have postulated a bedload parting for sand to the south of Nash Point. This area has particularly high tidal currents, though the sea floor consists largely of Jurassic limestones and mudstones. It is possible that the sand was a residue from the Pleistocene glaciation, and then it was carried both inland and offshore during the Flandrian Transgression, the various grain size fractions being well sorted by the energetic environment. In the Outer Bristol Channel there is a dominant westward sand transport near the Welsh coast at a rate of the order of 2 t m-1 day-1 (Collins etal., 1979; Heathershaw & Hammond, 1979). Clockwise sand circulation around the three main sandbanks has been deduced from the facing directions of sand waves, as well as current meter data. Westward sand movement has also been postulated on the Devon side of the Channel. In the inner Bristol Channel and Severn Estuary, Parker & Kirby (1982a) have suggested, from the fining sequence in the sand banks, that there may be sand transport from Holm Sand via Cardiff Ground to Middle Grounds. From there some sand may travel via Denny Shoal to Sheperdine Sands, in the upper estuary, and some may return via the English Grounds to Culver Sand. There has been movement of Culver Sand towards the west by 2 km between 1929 and 1964, which raises the possibility of potential recirculation of sand. Uncles (1981) has shown, by modelling, that the tidal asymmetry causes a landward tidally averaged stress to the east of Nash Point, especially at spring tides. Further west there is a westward directed stress. This confirms the location of the bedload parting. In the vicinity of the line Nash Point to Minehead, Uncles (1982) demonstrates strong cross-channel residual flows which may cause recirculation between Culver and Holm Sands, and may limit the eventual westward extent of Culver Sand. Davies (1980) has postulated from sand wave facing directions that sand circulates in a clockwise direction around Holm Sand at a rate of the order of 500 m a m-i yr- ~, and that there is a progressive fining within the bank towards the crest. A similar circulation occurs around the Culver Sand and, from the dominance of flood flow in
5 10 I km I
15 t
-~
_ J ~ ~ . ~ e
Culve -B r i d g w a t e r ~
Fig. 2 Transport paths of sand in the Bristol Channel and Severn Estuary.
54
n'sv/ f enny
Volume 15/Number 2/February 1984
Newport Deep and ebb flow in Bristol Deep, a clockwise circulation of sand seems likely around Middle Ground. Denny Shoal also shows an indication of circulation in the same sense; the position of the shoal varies considerably, as does the depth of both King Road and the flood channel separating the shoal from the Welsh Grounds which was at one time the main navigational channel to Avonmouth. Hawkins & Sebbage (1972) have demonstrated the tidal reversal of sandwaves on Denny Shoal. The possible circulation pattern of sand in the estuary is summarized in Fig. 2.
Muddy Sediment There is a considerable volume of mobile or potentially mobile mud in the Severn Estuary. Approximately 10 x 106 t have been estimated to be in suspension at spring tides, and there is in excess of 270 x l06 t on the bed subtidally within Bridgwater Bay, in addition to the thick deposit in Newport Deep. Although there are no data on sources of mud, it appears that the majority of new material comes from the rivers. The mobile population is about 20 times the annual river supply. The settled mud deposits, which have surface densities of 1.3-1.5 t m -a, a r e generally soft, pale brown or grey silty clays, with occasional fine sandy layers and lenses. Seismic profiling has shown thickness of up to l0 m in Bridgwater Bay with considerable acoustic layering within the mud. Since the layers mirror the underlying rock surface in many areas, it is possible that they are indicative of
0
5
density or seismic velocity changes resulting from consolidation effects, rather than any depositional events. Examination of cores from Bridgwater Bay using radiochemical techniques and geotechnical and palaeomagnetic analysis, have revealed (Kirby & Parker, 1980a) zones of stability and deposition within the settled mud area of erosion. Deposition was occurring at up to 130 cm in 21 years in one area. Rates of erosion have not been quantified, but losses of 1 m were estimated from measurements of overconsolidation. In the 30 km 2 of subtidal mud about 10 km 2 is thought to be accretionary and 7.5 km 2 erosional. Intertidally, the salt marsh between Stert and Hinkley accumulated mud at about 6 × 104 t yr-t between 1962 and 1964 (Ranwell, 1964). Newport Deep and other intertidal mud areas are likewise probably areas of both erosion and deposition. The exchange of sediment between the areas of erosion and deposition obviously takes place via the turbidity maximum. This area, where the water has a higher suspended sediment concentration than the river or the sea, occupies the whole of the estuary landward of Bridgwater Bay. A combination of vertical profiling and horizontal traverses using optical turbidity meters has shown that the highest concentrations occur on the English side (Fig. 3). There is a relatively sharp front roughly down the middle of the estuary separating the very turbid water from clearer water on the Welsh side. This appears to be a persistent feature on flood and ebb, as well as spring and neap tides (Kirby & Parker, 1982a). In the cross-section the tidally and depth-averaged turbidity produces a density gradient equivalent to about 1%o salinity (Parker et aL, 1981) which is sufficient to have an important effect on the dynamics of water circulation. Though the tidally averaged observations show a fairly regular distribution (Fig. 3), satellite photographs reveal considerable small-scale surface patchiness and variability that may relate to advective processes and to settling, or entrainment from the seabed. Vertical profiling of turbidity has shown that the suspended sediment concentration varies drastically throughout the tidal cycle and the neap-spring cycle (Fig. 4). At maximum current at spring tide, the concentration is uniform from surface to bed, with values typically of 5000 ppm (mg l- t). As the current strength diminishes towards slack water, the turbulence is insufficient to maintain the suspension and the particles settle. This causes 'steps' in the concentration profile. These steps
10
i
O < 0.5g I r
.........
~
1.0 - 3.0g I I
Concentration /~21
5m c o n t o u r
Fig. 3 Distribution of tidally averaged suspended solids concentration for a spring tide: (A) surface, (B) bed. After Parker & Kirby (1982a).
SPRING TIDE MAX. CURRENT
SPRING TIDE SLACK WATER NEAP TIDE MAX. CURRENT
NEAP TIDE SLACK WATER
Fig. 4 Schematic vertical profiles of suspended sediment concentration during spring and neap tides.
55
Marine Pollution Bulletin
have been termed lutoclines by Kirby & Parker (1982b). Initially two steps are likely; one near the surface, and the other near the bed. That near the surface separates almost clear water from turbid water beneath, but it is likely to be marked only when surface wave activity is slight. Near the bed the downward flux of particles is so rapid that a dense layer with concentrations up to 200 g 1-1 can form and grow upwards from the bed. Ideally the two steps eventually come together and a single sharp lutocline is formed between clear water and the high concentration layer; in laboratory experiments this takes about six hours (Parker & Kirby, 1982b). Within the settling suspensions many subsidiary steps are often observed, but it is not clear whether these are caused by particle size variations of the settling flocs or result from advected concentration effects. The high concentration near-bed layer, which may be termed a 'lutoid' layer, can sometimes be detected on echo-sounders, at which time it is generally known as 'fluid mud'. It is not clear, however, at what stage in its development a lutoid layer becomes acoustically detectable. At slack water on spring tides in the Severn the lutoid layer may be up to about 1 m thick and at some stage during the accelerating current the layer becomes re-entrained. One model of the process is that towards neap tides the layers persist long enough over slack water for consolidation to begin at the base; not all of the layer becomes re-eroded during the succeeding tide, the base of the layer survives and it increases in thickness from tide to tide. Though density layering in fluid mud has been detected, and confirmed using radioactive densimeters (Kirby & Parker, 1974), no direct correlation has been proved between them and the consolidation sequence suggested above, or layering in the settled mud. At neap tides fluid muds up to 5 m thick have been detected, possibly containing up to 70°70 of the sediment mobile at spring tides. There is thus a sequence of evolution from mobile dispersed suspensions, to mobile layered suspensions, to stationary suspensions and to settled mud. But there is some doubt about the stages at which the high concentration layers cease to move, become stationary and begin to consolidate, and the conditions governing re-entrainment. Laboratory measurements have shown that at high concentrations the suspensions are likely to be pseudo-plastic (Bryant et al., 1980), showing shear thinning properties. This means that the viscosity of the suspension will vary with the shear, having values at low shear rates perhaps an order of magnitude higher than clear water. The reason for this effect is that the flocculated character of the particles is in balance with the shear stress. As the shear stress decreases, the floc sizes increase and the increasing interaction between them causes a rise in effective viscosity. In the Severn Estuary the critical concentration above which this effect is important is about 10 g 1-'. Thus the lutoid layers are likely to be non-Newtonian. This possibility raises interesting and important consequences for modelling and predicting movement of fine-grained suspensions in estuaries because it implies that the lutoid layers are entirely eroded on each tide, until towards neap tide the complete layer survives. For intermediate tides, fluxes of the order of 105 t per half tide have been measured in Newport Deep and the Holm Channel (Parker & Kirby, 1982a).
56
Consequences of Sediment Movement The presence of such high suspended sediment concentrations is likely to have some fairly fundamental effects on organisms and chemical processes. These have been considered by Kirby & Parker (1978) and Parker & Lee (1981). Hamilton et al. (1979) show that levels of dissolved cadmium, zinc and iron were related to suspended sediment concentration in Bridgwater Bay, and that there was slight enrichment in methane in bottom waters. The turbidity of the water obviously reduces light penetration, plants cannot photosynthesize as effectively, and productivity is generally low. Within the high concentration layers, which have organic carbon contents of up to 7o70, the BOD is high (Sylvester & Ware, 1976). Consequently the dissolved oxygen content falls rapidly and the layers can become anaerobic within a few tides after formation. Redispersion of the layers towards spring tide results in an appreciable dissolved oxygen sag, which is pronounced in the lower reaches in the vicinity of the tributary estuaries. Hamilton et al. (1979) suggest that the flocs include a reduced environment even in welloxygenated water. Additionally, in those areas frequently covered by the turbid layers, there is likely to be a paucity of filter-feeding animals. In the Severn, most of the subtidal settled mud areas are barren and the extreme mobility of the substrate at spring tides probably also restricts the colonization of sandy areas. The consequences of construction of a Severn Barrage have been considered by Kirby & Parker (1980b) and Miles (1982). The generally decreased velocities in the upper estuary would cause virtual cessation of movement of sand within the barrage, and settling of large quantities of the suspended sediment. This would deposit preferentially in the quieter water areas, and in the tributary estuaries. The clearer water would increase primary production as well as the diversity of bottom fauna. Outside the barrage, sedimentation of mud would be likely at the coast close to the barrage. In Bridgwater Bay the slightly decreased tidal range would make the intertidal zone narrower and might cause localized erosion. However, since the currents would be reduced, except locally near the sluices and turbines, deposition should predominate (Miles, 1982).
Conclusions In view of the small subtidal prism in the estuary above the Holms, it is possible that the system is more or less in balance and that any further sediment input would be deposited outside the Holms. The major sediment supply seems to be of fine-grained material from the rivers, but because of the very high tidal energy, the existing sediments are continually being reworked and redistributed. Any major alteration of the topography or the tidal regime would disrupt this regime. Because of the large quantity of fine sediment in motion and its affinity for pollutants, it is important to understand the processes by which the high concentrations settle, erode and ~ix. I would like to thank my colleagues Drs R. Kirby and W. R. Parker for many fruitful discussions about the Severn Estuary, and for their assistance in preparing this synopsis.
Volume 15/Number 2/February 1984 Barrie, J. V. (1980). Heavy mineral distribution in bottom sediment of the Bristol Channel, U.K. Estuar. cstlmar. Sci., 11,369-381. Belderson, R. H. & Stride, A. H. 0966). Tidal current fashioning of a basal bed. Mar. Geol., 4, 237-257. Boyden, C. R. & Little, C. (1973). Faunal distributions in soft sediments of the Severn Estuary. Estuar. cstl mar. Sci., 1,203-223. Bryant, R., James, A. E. & Williams, D. J. A. (1980). Rheology of cohesive suspensions. In Industrialised Embayments and their Environmental Problems (M. B. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 279-287. Pergamon Press, Oxford. Collins, M. B., Pattiarachi, C. B., Banner, F. T. & Ferentinos, G. K. (1979). The supply of sand to Swansea Bay. In Industrialised Embayments and their Environmental Problems (M. C. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 193-213. Pergamon Press, Oxford. Davies, C. M. (1980). Evidence for the formation and age of a commercial sand deposit in the Bristol Channel. Estuar cstl mar. Sci., 11, 83-99. Evans, C. D. R. (1982). The geology and superficial sediments of the inner Bristol Channel and Severn Estuary. In Severn Barrage, pp. 35-42. Thomas Telford, London. Hamilton, E. I., Watson, P. G., Cleary, J. R. & Clifton, R. J. (1979). The geochemistry of recent sediments of the Bristol Channel-Severn Estuary system. Mar. Geol., 31, 139-182. Hawkins, A. B. & Sebbage, M. J. (1972). The reversal of sand waves in the Bristol Channel. Mar. Geol., 12, M7-M9. Heathershaw, A. D. & Hammond, F. D. C. (1979). Transport and deposition of non-cohesive sediments in Swansea Bay. In Industrialised Embayments and their Environmental Problems (M. C. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 215-248. Pergamon Press, Oxford. Kirby, R. & Parker, W. R. (1974). Seabed density measurements related to echo-sounder records. Dock Harb. Auth., 54, 423-424. Kirby, R. & Parker, W. R. (1978). Ecological impact of cohesive sediment suspensions. In Hydraulic Engineering for Improved Water Quality Management, pp. 854-857. Proc. 17th Cong. IAHR. Kirby, R. & Parker, W. R. (1980a). Settled mud deposits in Bridgwater Bay, Bristol Channel. Institute of Oceanographic Sciences Report, 107, 65 pp. Kirby, R. & Parker, W. R. (1980b). Sediment Dynamics in the Severn Estuary. In An Environmental Appraisal of Tidal Power Stations: with Particular Reference to the Severn Barrage (T. L. Shaw, ed), pp. 47-62. Pitman, London.
Kirby, R. & Parker, W. R. (1982a). A suspended sediment front in the Severn Estuary. Nature, Lond., 295, 396-399. Kirby, R. & Parker, W. R. (1982b). The distribution and behaviour of fine sediment in the Severn Estuary and Inner Bristol Channel, U.K. Special Volume "Dynamics of turbid coastal environments", Can. J. Fish aquat. Sci. 40, 83-95. Miles, G. V. (1982). Impact on currents and transport processes. In Severn Barrage, pp. 59-64. Thomas Telford, London. Murray, L. A., Norton, M. G., Nunny, R. S. & Rolfe, M. S. (1980). The field assessment of effects of dumping wastes at sea: 7. Sewage sludge and industrial waste disposal in the Bristol Channel. Fisheries Research Tech. Rep., 59. MAFF, Lowestoft. Parker, W. R. & Kirby R. (1982a). Sources and transport patterns of sediment in the Inner Bristol Channel and Severn Estuary. In Severn Barrage, pp. 181-194. Thomas Telford, London. Parker, W. R. & Kirby, R. (1982b). Time dependent properties of cohesive sediment relevant to sedimentation management - European experience. In Estuarine Comparisons (V. Kennedy, ed), pp. 573-589. Academic Press, London. Parker, W. R. & Lee, K. (1981). The behaviour of fine sediment relevant to the dispersal of pollutants. Rapp. P-v. Reun. Cons. int. Explor. Mer, 181, 28-34. Parker, W. R., Smith, T. J. & Kirby, R. (1981). Observation of density stratification due to suspended fine sediment. 2nd IAHR Int. Symp., Stratified Flows. Trondheim, pp. 955-966. Ranwell, D. S. (1964). Spartina salt marshes in Southern England. II. Rate and seasonal pattern of sediment accumulation. J. Ecol., 52, 79-94. Sylvester, A. J. & Ware, G. C. (1976). Anaerobiosis of fluid mud. Nature, Lond., 264, 655. Uncles, R. J. (1981). A note on tidal asymmetry in the Severn Estuary. EstuarcstlShelfSci., 13, 419-432. Uncles, R. J. (1982). Computed and observed residual currents in the Bristol Channel. Ocean. Acta, 5, 11-20. Uncles, R. J. & Jordan, M. B. (1979). Residual fluxes of water and salt at two stations in the Severn Estuary. Estuar cstl mar. Sci., 9, 287-302. Warwick, R. M. & Uncles, R. J. (1980). Distribution of benthic macrofauna associations in the Bristol Channel in relation to tidal stress. Mar. Ecol. Prog. Ser., 3, 97-103. Warwick, R. M. & Davies, J. R. (1977). The distribution of sublittoral macrofauna communities in the Bristol Channel in relation to the substrate. Estuar. cstl mar. Sci., 5,267-288.
Marine Pollution Bulletin, Vol. 15, No. 2, pp. 57-61,1984 Printed in Great Britain
0025-326X/84 $3.00 + 0.00 Pergamon Press Ltd.
The Chemistry of the Severn Estuary and the Bristol Channel A. W. MORRIS N a t u r a l E n v i r o n m e n t R e s e a r c h Council, Institute f o r M a r i n e E n v i r o n m e n t a l Research, P r o s p e c t Place, T h e H o e , P l y m o u t h P L 1 3 D H , U K I n a n a p p r a i s a l o f w a t e r q u a l i t y i n t h e S e v e r n E s t u a r y , in this issue o f M a r i n e P o l l u t i o n Bulletin, O w e n s p r o v i d e s information about the concentrations of nutrients, metals a n d o t h e r d e t e r m i n a n d s . M y i n t e n t i o n h e r e is t o m a k e s o m e b r i e f c o m m e n t s o n a few a s p e c t s o f c h e m i c a l p r o cesses a n d t o s u g g e s t a f e w o f t h e t o p i c s w h i c h r e q u i r e further research. Oxygen O w e n s d r a w s a t t e n t i o n t o t h e o x y g e n sag w h i c h is f o u n d in t h e u p p e r r e a c h e s o f t h e e s t u a r y , especially u n d e r
summer conditions of low flow and high temperature. The s o u r c e s a n d m e c h a n i s m s c o n t r o l l i n g t h e e x t e n t a n d locat i o n o f t h e o x y g e n d e m a n d i n this p a r t o f t h e e s t u a r y h a v e n o t b e e n fully c h a r a c t e r i z e d . O x i d i z a b l e i n o r g a n i c species (nitrite, a m m o n i a , d i v a l e n t m a n g a n e s e ) w h i c h a r e c a r r i e d i n t o t h e e s t u a r y b y t h e S e v e r n R i v e r a r e m o s t l y lost w i t h i n the low salinity zone, but their potential oxygen demand accounts for only a very small proportion of the observed o x y g e n u t i l i z a t i o n . F u r t h e r m o r e , t h e r i v e r i n e flux o f d i s s o l v e d o r g a n i c c a r b o n is d i s t r i b u t e d c o n s e r v a t i v e l y w i t h i n t h e e s t u a r y ( M a n t o u r a & W o o d w a r d , 1983). It is
57
phytoplankton growth in a turbid estuary. Can. J. Fish. aquat. Sci. (Suppl. 1), 40, 221-228. Van Veen, J. (1938). Water movements in the Straits of Dover. J. Cons. int. Explor. Met, 13, 7-36. Warwick, R. M. & Uncles, R. J. (1980). Distribution of benthic macrofauna associations in the Bristol Channel in relation to tidal stress. Mar. Ecol. Prog. Ser., 3, 97-103. Zimmerman, J. T. F. (1976). Mixing and flushing of tidal embayments in the western Dutch Wadden Sea - II. Neth. J. Sea Res. 10, 397-493.
0025-326X/84$3.00+ 0.00 © 1984PergamonPressLtd.
MarinePollutionBulletin, Vol. 15, No. 2, pp. 53-57. 1984 Printed in Great Britain
Sedimentation Processes in the Bristol Channel/Severn Estuary K. R. DYER
Natural Environment Research Council, Institute of Oceanographic Sciences, Taunton, Somerset, UK The Bristol Channel and Severn Estuary together form a wide, shallow estuary with a large tidal range and strong currents. At spring tide the range at Avonmouth exceeds 12 m and the currents over most of the area exceed 1.5 m s - ~. On neap tides the range is 6 m and the current velocity is generally greater than 0.75 m s-i. Consequently, sediment up to sand grade is likely to be mobile over most of the lunar cycle. The large tidal currents create a 10-15 km excursion of water at spring tides and, in common with most estuaries, this provides a mechanism for sorting the sediments moving in suspension from those that travel as bedload; thus there are fairly sharp divisions between muddy and sandy areas on the seabed, despite ubiquitous turbidity of the water. The lunar variation also causes very large differences in the amounts of sediment moving at spring and at neap tides. The residual current flow in the Bristol Channel has been modelled using a vertically integrated scheme by Uncles (1982); this indicates a general westerly residual current, with a clockwise circulation in Swansea Bay and west of Nash Point. There is also evidence of possible
circulation in Bridgwater Bay. Field observations, though generally in agreement, show large variations between surface and seabed (Uncles & Jordan, 1979) and it is not clear how well these circulations would be reflected in the distribution and transport of sediments. In common with other estuaries, the Severn contains more sediment in motion that the visible annual inputs, and there is a continual exchange of material from areas of erosion to areas of deposition through the turbidity maximum. Understanding this exchange is crucial in any investigation of pollution since fine suspended sediment scavenges pollutants, such as heavy metals, from the water and its deposition may result in their incorporation into the seabed.
Sediment Distribution The bed of the Bristol Channel and Severn Estuary comprises a wide range of lithologies (Fig. 1). Off the North Devon coast the sea floor is largely bare rock, with occasional sand and gravel ribbons. To the north and west
Fig. 1 Sediment distribution in the Bristol Channel and Severn Estuary. From various authors.
53
Marine Pollution Bulletin
this sediment cover becomes more widespread and gives way to extensive zones of sand ribbons and sand waves (Warwick & Davis, 1977; Murray et al., 1980; Warwick & Uncles, 1980), although there are frequent patches of gravel and rock. Off the Welsh coast the sand depositis are occasionally thicker in a series of linear sandbanks which are directed westwards from headlands; Nash Sand from Nash Point, Scarweather Sand from Porthcawl Point and Helwick Sand from Porteynon Point. Within Swansea Bay an isolated muddy zone occurs (Collins et aL, 1979; Heathershaw & Hammond, 1979). The heavy mineral suites in the outer Bristol Channel have been described by Barrie (1980). The sediment distribution and sediment budgets in the Inner Bristol Channel and Severn Estuary have been reviewed by Parker & Kirby (1982a). In the Inner Bristol Channel, eastwards of Nash Point, large areas of rock and thin lag gravel, with occasional sand ribbons and sand waves, separate two banks: Culver Sand and Holm Sand. In Bridgwater Bay there is an extensive area of muddy subtidal and intertidal sediment. East of the Holm Islands, in the Severn Estuary, the shallow margins of the estuary are rocky platforms with a thin sediment cover, predominantly sandy on the English side. On the Welsh side they are more muddy, for example in Peterston and Wentlooge Flats. There are two major sandy areas in the Severn Estuary. The Cardiff Grounds and Monkstone area is a relatively thin sand bank resting on a rocky platform, whereas Middle Ground and Denny Shoal fills a deep valley cut into bedrock (Evans, 1982). Within the surface sands of Middle Ground many mud layers exist, illustrating a complex depositional environment. The Holms Channel, Bristol Deep and King Road have a thin cover of gravelly sand, or sand with some mud patches, whereas in Newport Deep a thick mud deposit occurs. All of the tributary estuaries are muddy. In general the distribution of sediment correlates well with the maximum bed shear stresses calculated from the results of mathematical modelling (Warwick & Uncles, 1980; Miles, 1982). The benthic macrofaunal communities also correlate with the tidal mean bed stress (Warwick & Uncles, 1980) and with the sediment distribution in intertidal areas (Boyden & Little, 1973).
~ct~
~or,r ~y . . . . .
~
0 I
Sandy Sediment There appears to be no major modern source for sand. Belderson & Stride (1966) have postulated a bedload parting for sand to the south of Nash Point. This area has particularly high tidal currents, though the sea floor consists largely of Jurassic limestones and mudstones. It is possible that the sand was a residue from the Pleistocene glaciation, and then it was carried both inland and offshore during the Flandrian Transgression, the various grain size fractions being well sorted by the energetic environment. In the Outer Bristol Channel there is a dominant westward sand transport near the Welsh coast at a rate of the order of 2 t m-1 day-1 (Collins etal., 1979; Heathershaw & Hammond, 1979). Clockwise sand circulation around the three main sandbanks has been deduced from the facing directions of sand waves, as well as current meter data. Westward sand movement has also been postulated on the Devon side of the Channel. In the inner Bristol Channel and Severn Estuary, Parker & Kirby (1982a) have suggested, from the fining sequence in the sand banks, that there may be sand transport from Holm Sand via Cardiff Ground to Middle Grounds. From there some sand may travel via Denny Shoal to Sheperdine Sands, in the upper estuary, and some may return via the English Grounds to Culver Sand. There has been movement of Culver Sand towards the west by 2 km between 1929 and 1964, which raises the possibility of potential recirculation of sand. Uncles (1981) has shown, by modelling, that the tidal asymmetry causes a landward tidally averaged stress to the east of Nash Point, especially at spring tides. Further west there is a westward directed stress. This confirms the location of the bedload parting. In the vicinity of the line Nash Point to Minehead, Uncles (1982) demonstrates strong cross-channel residual flows which may cause recirculation between Culver and Holm Sands, and may limit the eventual westward extent of Culver Sand. Davies (1980) has postulated from sand wave facing directions that sand circulates in a clockwise direction around Holm Sand at a rate of the order of 500 m a m-i yr- ~, and that there is a progressive fining within the bank towards the crest. A similar circulation occurs around the Culver Sand and, from the dominance of flood flow in
5 10 I km I
15 t
-~
_ J ~ ~ . ~ e
Culve -B r i d g w a t e r ~
Fig. 2 Transport paths of sand in the Bristol Channel and Severn Estuary.
54
n'sv/ f enny
Volume 15/Number 2/February 1984
Newport Deep and ebb flow in Bristol Deep, a clockwise circulation of sand seems likely around Middle Ground. Denny Shoal also shows an indication of circulation in the same sense; the position of the shoal varies considerably, as does the depth of both King Road and the flood channel separating the shoal from the Welsh Grounds which was at one time the main navigational channel to Avonmouth. Hawkins & Sebbage (1972) have demonstrated the tidal reversal of sandwaves on Denny Shoal. The possible circulation pattern of sand in the estuary is summarized in Fig. 2.
Muddy Sediment There is a considerable volume of mobile or potentially mobile mud in the Severn Estuary. Approximately 10 x 106 t have been estimated to be in suspension at spring tides, and there is in excess of 270 x l06 t on the bed subtidally within Bridgwater Bay, in addition to the thick deposit in Newport Deep. Although there are no data on sources of mud, it appears that the majority of new material comes from the rivers. The mobile population is about 20 times the annual river supply. The settled mud deposits, which have surface densities of 1.3-1.5 t m -a, a r e generally soft, pale brown or grey silty clays, with occasional fine sandy layers and lenses. Seismic profiling has shown thickness of up to l0 m in Bridgwater Bay with considerable acoustic layering within the mud. Since the layers mirror the underlying rock surface in many areas, it is possible that they are indicative of
0
5
density or seismic velocity changes resulting from consolidation effects, rather than any depositional events. Examination of cores from Bridgwater Bay using radiochemical techniques and geotechnical and palaeomagnetic analysis, have revealed (Kirby & Parker, 1980a) zones of stability and deposition within the settled mud area of erosion. Deposition was occurring at up to 130 cm in 21 years in one area. Rates of erosion have not been quantified, but losses of 1 m were estimated from measurements of overconsolidation. In the 30 km 2 of subtidal mud about 10 km 2 is thought to be accretionary and 7.5 km 2 erosional. Intertidally, the salt marsh between Stert and Hinkley accumulated mud at about 6 × 104 t yr-t between 1962 and 1964 (Ranwell, 1964). Newport Deep and other intertidal mud areas are likewise probably areas of both erosion and deposition. The exchange of sediment between the areas of erosion and deposition obviously takes place via the turbidity maximum. This area, where the water has a higher suspended sediment concentration than the river or the sea, occupies the whole of the estuary landward of Bridgwater Bay. A combination of vertical profiling and horizontal traverses using optical turbidity meters has shown that the highest concentrations occur on the English side (Fig. 3). There is a relatively sharp front roughly down the middle of the estuary separating the very turbid water from clearer water on the Welsh side. This appears to be a persistent feature on flood and ebb, as well as spring and neap tides (Kirby & Parker, 1982a). In the cross-section the tidally and depth-averaged turbidity produces a density gradient equivalent to about 1%o salinity (Parker et aL, 1981) which is sufficient to have an important effect on the dynamics of water circulation. Though the tidally averaged observations show a fairly regular distribution (Fig. 3), satellite photographs reveal considerable small-scale surface patchiness and variability that may relate to advective processes and to settling, or entrainment from the seabed. Vertical profiling of turbidity has shown that the suspended sediment concentration varies drastically throughout the tidal cycle and the neap-spring cycle (Fig. 4). At maximum current at spring tide, the concentration is uniform from surface to bed, with values typically of 5000 ppm (mg l- t). As the current strength diminishes towards slack water, the turbulence is insufficient to maintain the suspension and the particles settle. This causes 'steps' in the concentration profile. These steps
10
i
O < 0.5g I r
.........
~
1.0 - 3.0g I I
Concentration /~21
5m c o n t o u r
Fig. 3 Distribution of tidally averaged suspended solids concentration for a spring tide: (A) surface, (B) bed. After Parker & Kirby (1982a).
SPRING TIDE MAX. CURRENT
SPRING TIDE SLACK WATER NEAP TIDE MAX. CURRENT
NEAP TIDE SLACK WATER
Fig. 4 Schematic vertical profiles of suspended sediment concentration during spring and neap tides.
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Marine Pollution Bulletin
have been termed lutoclines by Kirby & Parker (1982b). Initially two steps are likely; one near the surface, and the other near the bed. That near the surface separates almost clear water from turbid water beneath, but it is likely to be marked only when surface wave activity is slight. Near the bed the downward flux of particles is so rapid that a dense layer with concentrations up to 200 g 1-1 can form and grow upwards from the bed. Ideally the two steps eventually come together and a single sharp lutocline is formed between clear water and the high concentration layer; in laboratory experiments this takes about six hours (Parker & Kirby, 1982b). Within the settling suspensions many subsidiary steps are often observed, but it is not clear whether these are caused by particle size variations of the settling flocs or result from advected concentration effects. The high concentration near-bed layer, which may be termed a 'lutoid' layer, can sometimes be detected on echo-sounders, at which time it is generally known as 'fluid mud'. It is not clear, however, at what stage in its development a lutoid layer becomes acoustically detectable. At slack water on spring tides in the Severn the lutoid layer may be up to about 1 m thick and at some stage during the accelerating current the layer becomes re-entrained. One model of the process is that towards neap tides the layers persist long enough over slack water for consolidation to begin at the base; not all of the layer becomes re-eroded during the succeeding tide, the base of the layer survives and it increases in thickness from tide to tide. Though density layering in fluid mud has been detected, and confirmed using radioactive densimeters (Kirby & Parker, 1974), no direct correlation has been proved between them and the consolidation sequence suggested above, or layering in the settled mud. At neap tides fluid muds up to 5 m thick have been detected, possibly containing up to 70°70 of the sediment mobile at spring tides. There is thus a sequence of evolution from mobile dispersed suspensions, to mobile layered suspensions, to stationary suspensions and to settled mud. But there is some doubt about the stages at which the high concentration layers cease to move, become stationary and begin to consolidate, and the conditions governing re-entrainment. Laboratory measurements have shown that at high concentrations the suspensions are likely to be pseudo-plastic (Bryant et al., 1980), showing shear thinning properties. This means that the viscosity of the suspension will vary with the shear, having values at low shear rates perhaps an order of magnitude higher than clear water. The reason for this effect is that the flocculated character of the particles is in balance with the shear stress. As the shear stress decreases, the floc sizes increase and the increasing interaction between them causes a rise in effective viscosity. In the Severn Estuary the critical concentration above which this effect is important is about 10 g 1-'. Thus the lutoid layers are likely to be non-Newtonian. This possibility raises interesting and important consequences for modelling and predicting movement of fine-grained suspensions in estuaries because it implies that the lutoid layers are entirely eroded on each tide, until towards neap tide the complete layer survives. For intermediate tides, fluxes of the order of 105 t per half tide have been measured in Newport Deep and the Holm Channel (Parker & Kirby, 1982a).
56
Consequences of Sediment Movement The presence of such high suspended sediment concentrations is likely to have some fairly fundamental effects on organisms and chemical processes. These have been considered by Kirby & Parker (1978) and Parker & Lee (1981). Hamilton et al. (1979) show that levels of dissolved cadmium, zinc and iron were related to suspended sediment concentration in Bridgwater Bay, and that there was slight enrichment in methane in bottom waters. The turbidity of the water obviously reduces light penetration, plants cannot photosynthesize as effectively, and productivity is generally low. Within the high concentration layers, which have organic carbon contents of up to 7o70, the BOD is high (Sylvester & Ware, 1976). Consequently the dissolved oxygen content falls rapidly and the layers can become anaerobic within a few tides after formation. Redispersion of the layers towards spring tide results in an appreciable dissolved oxygen sag, which is pronounced in the lower reaches in the vicinity of the tributary estuaries. Hamilton et al. (1979) suggest that the flocs include a reduced environment even in welloxygenated water. Additionally, in those areas frequently covered by the turbid layers, there is likely to be a paucity of filter-feeding animals. In the Severn, most of the subtidal settled mud areas are barren and the extreme mobility of the substrate at spring tides probably also restricts the colonization of sandy areas. The consequences of construction of a Severn Barrage have been considered by Kirby & Parker (1980b) and Miles (1982). The generally decreased velocities in the upper estuary would cause virtual cessation of movement of sand within the barrage, and settling of large quantities of the suspended sediment. This would deposit preferentially in the quieter water areas, and in the tributary estuaries. The clearer water would increase primary production as well as the diversity of bottom fauna. Outside the barrage, sedimentation of mud would be likely at the coast close to the barrage. In Bridgwater Bay the slightly decreased tidal range would make the intertidal zone narrower and might cause localized erosion. However, since the currents would be reduced, except locally near the sluices and turbines, deposition should predominate (Miles, 1982).
Conclusions In view of the small subtidal prism in the estuary above the Holms, it is possible that the system is more or less in balance and that any further sediment input would be deposited outside the Holms. The major sediment supply seems to be of fine-grained material from the rivers, but because of the very high tidal energy, the existing sediments are continually being reworked and redistributed. Any major alteration of the topography or the tidal regime would disrupt this regime. Because of the large quantity of fine sediment in motion and its affinity for pollutants, it is important to understand the processes by which the high concentrations settle, erode and ~ix. I would like to thank my colleagues Drs R. Kirby and W. R. Parker for many fruitful discussions about the Severn Estuary, and for their assistance in preparing this synopsis.
Volume 15/Number 2/February 1984 Barrie, J. V. (1980). Heavy mineral distribution in bottom sediment of the Bristol Channel, U.K. Estuar. cstlmar. Sci., 11,369-381. Belderson, R. H. & Stride, A. H. 0966). Tidal current fashioning of a basal bed. Mar. Geol., 4, 237-257. Boyden, C. R. & Little, C. (1973). Faunal distributions in soft sediments of the Severn Estuary. Estuar. cstl mar. Sci., 1,203-223. Bryant, R., James, A. E. & Williams, D. J. A. (1980). Rheology of cohesive suspensions. In Industrialised Embayments and their Environmental Problems (M. B. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 279-287. Pergamon Press, Oxford. Collins, M. B., Pattiarachi, C. B., Banner, F. T. & Ferentinos, G. K. (1979). The supply of sand to Swansea Bay. In Industrialised Embayments and their Environmental Problems (M. C. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 193-213. Pergamon Press, Oxford. Davies, C. M. (1980). Evidence for the formation and age of a commercial sand deposit in the Bristol Channel. Estuar cstl mar. Sci., 11, 83-99. Evans, C. D. R. (1982). The geology and superficial sediments of the inner Bristol Channel and Severn Estuary. In Severn Barrage, pp. 35-42. Thomas Telford, London. Hamilton, E. I., Watson, P. G., Cleary, J. R. & Clifton, R. J. (1979). The geochemistry of recent sediments of the Bristol Channel-Severn Estuary system. Mar. Geol., 31, 139-182. Hawkins, A. B. & Sebbage, M. J. (1972). The reversal of sand waves in the Bristol Channel. Mar. Geol., 12, M7-M9. Heathershaw, A. D. & Hammond, F. D. C. (1979). Transport and deposition of non-cohesive sediments in Swansea Bay. In Industrialised Embayments and their Environmental Problems (M. C. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 215-248. Pergamon Press, Oxford. Kirby, R. & Parker, W. R. (1974). Seabed density measurements related to echo-sounder records. Dock Harb. Auth., 54, 423-424. Kirby, R. & Parker, W. R. (1978). Ecological impact of cohesive sediment suspensions. In Hydraulic Engineering for Improved Water Quality Management, pp. 854-857. Proc. 17th Cong. IAHR. Kirby, R. & Parker, W. R. (1980a). Settled mud deposits in Bridgwater Bay, Bristol Channel. Institute of Oceanographic Sciences Report, 107, 65 pp. Kirby, R. & Parker, W. R. (1980b). Sediment Dynamics in the Severn Estuary. In An Environmental Appraisal of Tidal Power Stations: with Particular Reference to the Severn Barrage (T. L. Shaw, ed), pp. 47-62. Pitman, London.
Kirby, R. & Parker, W. R. (1982a). A suspended sediment front in the Severn Estuary. Nature, Lond., 295, 396-399. Kirby, R. & Parker, W. R. (1982b). The distribution and behaviour of fine sediment in the Severn Estuary and Inner Bristol Channel, U.K. Special Volume "Dynamics of turbid coastal environments", Can. J. Fish aquat. Sci. 40, 83-95. Miles, G. V. (1982). Impact on currents and transport processes. In Severn Barrage, pp. 59-64. Thomas Telford, London. Murray, L. A., Norton, M. G., Nunny, R. S. & Rolfe, M. S. (1980). The field assessment of effects of dumping wastes at sea: 7. Sewage sludge and industrial waste disposal in the Bristol Channel. Fisheries Research Tech. Rep., 59. MAFF, Lowestoft. Parker, W. R. & Kirby R. (1982a). Sources and transport patterns of sediment in the Inner Bristol Channel and Severn Estuary. In Severn Barrage, pp. 181-194. Thomas Telford, London. Parker, W. R. & Kirby, R. (1982b). Time dependent properties of cohesive sediment relevant to sedimentation management - European experience. In Estuarine Comparisons (V. Kennedy, ed), pp. 573-589. Academic Press, London. Parker, W. R. & Lee, K. (1981). The behaviour of fine sediment relevant to the dispersal of pollutants. Rapp. P-v. Reun. Cons. int. Explor. Mer, 181, 28-34. Parker, W. R., Smith, T. J. & Kirby, R. (1981). Observation of density stratification due to suspended fine sediment. 2nd IAHR Int. Symp., Stratified Flows. Trondheim, pp. 955-966. Ranwell, D. S. (1964). Spartina salt marshes in Southern England. II. Rate and seasonal pattern of sediment accumulation. J. Ecol., 52, 79-94. Sylvester, A. J. & Ware, G. C. (1976). Anaerobiosis of fluid mud. Nature, Lond., 264, 655. Uncles, R. J. (1981). A note on tidal asymmetry in the Severn Estuary. EstuarcstlShelfSci., 13, 419-432. Uncles, R. J. (1982). Computed and observed residual currents in the Bristol Channel. Ocean. Acta, 5, 11-20. Uncles, R. J. & Jordan, M. B. (1979). Residual fluxes of water and salt at two stations in the Severn Estuary. Estuar cstl mar. Sci., 9, 287-302. Warwick, R. M. & Uncles, R. J. (1980). Distribution of benthic macrofauna associations in the Bristol Channel in relation to tidal stress. Mar. Ecol. Prog. Ser., 3, 97-103. Warwick, R. M. & Davies, J. R. (1977). The distribution of sublittoral macrofauna communities in the Bristol Channel in relation to the substrate. Estuar. cstl mar. Sci., 5,267-288.
Marine Pollution Bulletin, Vol. 15, No. 2, pp. 57-61,1984 Printed in Great Britain
0025-326X/84 $3.00 + 0.00 Pergamon Press Ltd.
The Chemistry of the Severn Estuary and the Bristol Channel A. W. MORRIS N a t u r a l E n v i r o n m e n t R e s e a r c h Council, Institute f o r M a r i n e E n v i r o n m e n t a l Research, P r o s p e c t Place, T h e H o e , P l y m o u t h P L 1 3 D H , U K I n a n a p p r a i s a l o f w a t e r q u a l i t y i n t h e S e v e r n E s t u a r y , in this issue o f M a r i n e P o l l u t i o n Bulletin, O w e n s p r o v i d e s information about the concentrations of nutrients, metals a n d o t h e r d e t e r m i n a n d s . M y i n t e n t i o n h e r e is t o m a k e s o m e b r i e f c o m m e n t s o n a few a s p e c t s o f c h e m i c a l p r o cesses a n d t o s u g g e s t a f e w o f t h e t o p i c s w h i c h r e q u i r e further research. Oxygen O w e n s d r a w s a t t e n t i o n t o t h e o x y g e n sag w h i c h is f o u n d in t h e u p p e r r e a c h e s o f t h e e s t u a r y , especially u n d e r
summer conditions of low flow and high temperature. The s o u r c e s a n d m e c h a n i s m s c o n t r o l l i n g t h e e x t e n t a n d locat i o n o f t h e o x y g e n d e m a n d i n this p a r t o f t h e e s t u a r y h a v e n o t b e e n fully c h a r a c t e r i z e d . O x i d i z a b l e i n o r g a n i c species (nitrite, a m m o n i a , d i v a l e n t m a n g a n e s e ) w h i c h a r e c a r r i e d i n t o t h e e s t u a r y b y t h e S e v e r n R i v e r a r e m o s t l y lost w i t h i n the low salinity zone, but their potential oxygen demand accounts for only a very small proportion of the observed o x y g e n u t i l i z a t i o n . F u r t h e r m o r e , t h e r i v e r i n e flux o f d i s s o l v e d o r g a n i c c a r b o n is d i s t r i b u t e d c o n s e r v a t i v e l y w i t h i n t h e e s t u a r y ( M a n t o u r a & W o o d w a r d , 1983). It is
57