Potential gain and loss of sand by some sand banks in the Southern Bight of the North Sea

Potential gain and loss of sand by some sand banks in the Southern Bight of the North Sea

Marine Geology, 41 (1981) 239--250 239 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands P O T E N T I A L GAIN AND L...

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Marine Geology, 41 (1981) 239--250

239

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

P O T E N T I A L GAIN AND LOSS OF SAND BY SOME SAND BANKS IN THE S O U T H E R N BIGHT OF THE N O R T H SEA

G.F. CASTON

Institute of Oceanographic Sciences, Wormley, Godalming, Surrey (U.K.) (Received June 30, 1980; revised and accepted September 9, 1980)

ABSTRACT Caston, G.F., 1981. Potential gain and loss of sand by some sand banks in the Southern Bight of the North Sea. Mar. Geol., 41: 239--250. A detailed echo-sounding and side-scan sonar survey has shown that sand banks in the Southern Bight of the North Sea have rounded heads in the approximate " u p s t r e a m " direction and tapered tails in the approximate " d o w n s t r e a m " direction of net regional sand transport. Thus, the plan view o f sand banks can be used as an indicator o f the approximate direction of net sand transport. Sand-wave aprons occur in front of the heads and " d o w n s t r e a m " of the tails o f the sand banks. The asymmetry of the sand waves implies that sand is being added to sand-wave aprons at the heads and to the " u p s t r e a m " flanks of these sand banks, but is shed from the " d o w n s t r e a m " sand-wave aprons. There is no evidence for a closed sand circulation on the banks. INTRODUCTION

Sand banks (tidal current ridges o f some authors) c o m m o n l y occur in the mouths of estuaries and in shallow tidal seas (Off, 1963). The spacing and size of sand banks with sand waves on them shows some tendency to increase with water depth, implying a dynamic relationship with their environm e n t (Off, 1963; Allen, 1968). The majority of modern sand banks are asymmetrical in transverse cross section and, over the long term, the banks migrate in the direction faced by their steeper sides (Houbolt, 1968; Cloet, 1963; Caston, 1972). The axes of sand banks are c o m m o n l y offset by up to 20 ° with respect to the direction of the net regional sand transport and the dominant peak tidal current (Smith, 1969; Huthnance, 1973; McCave, 1979; K e n y o n et al., in press). The axes of the majority of sand banks are offset anticlockwise b u t a minority are offset clockwise (Kenyon et al., in press). In both cases the migration o~ the steeper side of the sand bank is deduced to progress obliquely in the direction of the net regional sand transport. On the steeper parts of sand banks, sand waves move up both flanks towards the crest of the sand bank. The crests of the sand waves swing around progressively until they are more nearly parallel to the crest of the sand bank (Caston and Stride, 1970; Caston, 1972).

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The obvious disparity in shape of the opposite ends of some sand banks has led to the use of the terms head and tail on navigational charts such as Hydrographic Department Chart 1610. Caston (1972) has noted that the blunt shape of the southerly heads of some of the Norfolk banks in the North Sea contrasts with their narrow northerly tails in a region of net sand transport to the north. A comparison of the 1851 and 1967 Admiralty Surveys has demonstrated the elongation of one isobath (12 m) at the tail of the Leman Bank, Norfolk banks (Caston, 1972). However, this change could represent a very small increment in height on the bank's tail. The present study has made use of recent detailed Hydrographic Department echo-sounder surveys to examine a number of sand banks in the Southern Bight of the North Sea, paying particular attention to their ends. The northern ends of the North and South Falls banks have been designated "heads" and the southern ends "tails" in a region of southerly sand transport (Hydrographic Department Chart 1610). In this paper the terms head and tail are also applied to the north and south ends of the other sand banks in the locality. The heads of the Galloper, North Falls and Sandettie banks, the tails of the Inner Gabbard, Outer Gabbard and Galloper banks and most of the South Falls Bank have been examined (Fig.l). The head of the Galloper Bank and the tail of the Outer Gabbard Bank have been selected to demonstrate the morphological characteristics observed to be typical, but recognisable to a lesser extent in the other sand banks. DATA

Closely spaced echo-sounding lines, at 120 m intervals, were run in 1971 by H.M.S. " F o x " and H.M.S. " F a w n " of the Hydrographic Department, Ministry of Defence (Survey K6235). The survey lines trend approximately parallel to the regional net direction of sand transport. This is not in the optimum direction for defining sand bank shape, b u t is good for judging sand-wave asymmetry on the echograms. The sounding sheets, with closely spaced soundings, were used to determine crest orientations of the larger sand waves. Sonographs from an E.G. and G. 110 kHz side-scan sonar were available for the tracks over the southern part of the Inner Gabbard Bank and the northern part of the South Falls Bank. A network of sample stations was occupied and the samples were described qualitatively by Hydrographic Department surveyors. REGIONAL AND LOCAL DIRECTIONS OF SAND TRANSPORT

The net regional direction of sand transport is to the south-southwest, towards a convergence zone in the Straits of Dover (Stride, 1973). In the vicinity of the sand banks, the peak tidal current direction varies from 204 ° to 217 ° (Hydrographic Department Chart 1610} with maximum strengths varying from 90 to 115 cm/sec. Abundant bedform evidence for ground west

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Fig.1. S a n d b a n k s a n d fields o f s a n d waves w i t h i n t h e s t u d y area. A t h i c k line o u t l i n e s t h e d e t a i l e d s u r v e y area. A r e a s i n c l u d e d in Figs. 2a a n d 2 b are s h o w n . T h e 20 m i s o b a t h ( f r o m C h a r t 1 6 1 0 ) o u t l i n e s t h e sand b a n k s . T h i c k a r r o w s i n d i c a t e t h e d i r e c t i o n of n e t r e g i o n a l sand t r a n s p o r t . T h e s h a d e d areas are fields o f s a n d waves.

242 o f 1 ° 55'E, provided by sand-wave m o r p h o l o g y and numerous wreck marks, has d e m o n s t r a t e d that the prevailing sand transport to the south-southwest is interrupted only by the steeper western flanks of the Inner Gabbard and South Falls sand banks, where it is reversed (Caston, 1979). Local variations in the direction of elongation of wreck marks vary from 180 ° to 222 ° , with a mean o f 204 ° . South of 51°45'N and east of 2° 04'E, large fields of sand waves (Fig.l) again indicate t hat net sand transport is consistently towards the south-southwest, e x c e p t for the western flank of the N ort h Falls Bank where it is reversed. There is no bedform evidence on the sea floor immediately adjacent to the Outer Gabbard and Galloper banks, but sand-wave morphology has been used to infer the local patterns of net sand transport that predominate over these sand banks. SANDBANK MORPHOLOGY The o u t e r limit of a sand bank is taken at the break in slope between the flanks and the adjacent sea floor. The base o f slope also delineates the head and tail o f each sand bank so that the terms head and tail are used for the opposite ends of the bank proper. T h e y do not include any local areas of sand waves on the sea floor surrounding the end of a sand bank, for which the term " a p r o n " is now used. F o r the n o r t h e r n part of the Galloper Bank and for the Inner Gabbard and O u ter Gabbard banks, the base o f slope is apparent on the echograms and is largely coincident with the limits of sand waves (Fig.2). U nfort unat el y, large fields of sand waves merge with the heads of the N ort h Falls, South Falls and Sandettie banks and the tails of the Galloper and N ort h Falls banks (Fig.l). Accordingly, the slope breaks forming the outlines of these banks must be based on the isobaths alone. This arbitrary depth level may include some of the large sand waves on the adjacent floor. In plan-view, all of the banks examined have narrow, elongate tails. Although the tips of the North and South Falls tails are outside the survey coverage, in both cases the isobaths show southward tapering of the tails. The tail of the Galloper Bank narrows more sharply than shown on Chart 1610 (Fig.l). The charted isobath, which delineates the Inner Gabbard Bank for most of its length, includes local highs of coarse material in the vicinity of the tail. In reality, the tail of the bank tapers southwards and shows a similar m o r p h o l o g y and a similar pattern of sand-wave distribution to that of the tail of the Outer Gabbard Bank (Fig.2b). The North Falls head is broader and more r o u n d e d than its tail. The South Falls, very straight and narrow for most of its length, broadens at the edge of the survey coverage, towards its head. The head of Sandettie is considerably broader than the higher part of the bank and its tapered tail, which is included in a survey [~y Burton (1977). The difference between the shape of the head and tail of the Galloper Bank is less p r o n o u n c e d , possibly due to the lack of a d o m i n a n t direction of peak tidal current flow in the immediate vicinity. A tidal current measurement near the tail of the bank gives m a x i m u m values of 85 cm/sec to both 211 ° and 39 ° (Chart 1610).

243 All o f the banks examined in detail have rounded heads in the " u p s t r e a m " direction and narrow tails in the " d o w n s t r e a m " direction of the net sand transport, although this may n o t be demonstrated by the isobaths on Fig.l, for reasons stated above. The true plan-view shape of sand banks is thus of value as one more line of evidence t h a t may be used to indicate the approximate direction of net regional sand transport, although the bank axis may be offset by as much as 20 ° . The sand banks presently considered are all asymmetrical in cross section, though to varying degrees. They are offset in an anticlockwise sense to the regional direction of sand transport, except for the Sandettie Bank, which is offset in a clockwise direction. Thus Sandettie has its steep slope on its southeastern side, whereas the other sand banks have steeper western sides. Figure 3 shows a number of transects of each bank and the m a x i m u m gradients of each slope along them. In general the slopes are very gentle. The m a x i m u m gradients encountered on the central parts of the sand banks are 3.4 ° and 3.3 ° for the western slopes of the Galloper and South Falls banks, respectively. The m a x i m u m slope angles over the Outer Gabbard tail of 4.5 ° (west side) and 5 ° (east side) are anomalously high, probably because the measurements were made at the tip of the tail and represent the slope angles of individual large sand waves rather than the overall slope of the tail. The head and tail of each bank are lower and more rounded in transverse cross section than the main part of the bank. The head and tail may be symmetrical or, at some points, the asymmetry may be the reverse of the main part of the bank. At the heads of the Sandettie Bank, South Falls Bank, Galloper Bank and the tail of the Outer Gabbard, the asymmetry of the banks is reversed (Fig.3). The tail of the Galloper Bank is largely symmetrical in cross section. The tails of the Inner Gabbard and North Falls banks remain steeper on their west sides, but the asymmetry is much less than over the central part of these sand banks. The head of the Galloper Bank and the tail of the Outer Gabbard are here chosen as models to demonstrate the detailed sand-wave morphology at the ends of sand banks and as a guide to sand bank evolution.

Head and apron o f the Galloper Bank The Galloper Bank rises to a least water depth of 2.4 m, but within the area of Fig.2a the least depth is 7.5 m. The bank is covered with sand waves. For the most part the sand-wave boundary coincides with the base of slope of the bank. This is at a shallower depth on the west side (by approximately 4 m) than along the eastern flank. Sand-wave asymmetry implies their migration to the south-southwest on the eastern flank, and to the northnortheast on the steeper western flank. The crest line of the sand bank curves around to the northwest at the closure of the 20 m isobath, with a slope down to the north comprising the bank head.

244

An apron of sand waves extends for more than 2 km nort h of the bank head. Th e apron is divided into three zones. The eastern zone, the largest in area, is occupied by asymmetrical sand waves, implying migration towards the south-southwest. Successive sand waves increase in height in the direction o f migration, to a m a x i m u m of 6--7 m. The decreasing depths to the troughs between the sand waves indicate either a thickening of sand or a slope up to the south-southwest beneath these sand waves. T h e western zone, which narrows and pinches o u t only 1.5 km nort h of the bank head, consists of sand waves which are steep to the north-northeast. These sand waves spread northwards f r om the west flank of the Galloper Bank o n t o a flat sea floor. The sand waves increase in height from 4 to 6 m I,

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247 at the o u t e r edge o f the apron, t o 8 to 9 m at the b o u n d a r y of the central zone. Between the eastern and western zones there is a narrow zone of symmetrical sand waves which stretches northwards from the crest of the bank. T h e symmetrical sand waves are up t o 5 m in height and have a needle-like appearance on the echograms, quite distinct from the asymmetrical sand waves, although subject to the same vertical exaggeration. Similar symmetrical sand waves have been observed at a bed-load convergence zone on Kwinte Bank, Flemish Banks (A. Bastin, pers. commun.). Their wavelengths are markedly less t han those o f the asymmetrical sand waves and their troughs are at a high level above the adjacent sea floor, which suggests thicker sand accumulation in this zone. If the sand waves over the apron change their orientation in a m anner similar t o the sand waves over the bank proper, then the crest lines of the symmetrical sand waves may be almost parallel to the crest o f the bank. The symmetrical sand waves would be almost longitudinal features and the zone would one o f bed-load convergence and incipient bank development. However, it awaits a comprehensive side-scan sonar survey of a sand-wave apron to reveal the true trends o f these sand-wave crests. T o the n o r t h and east of the apron, isolated sand waves up to 5 m in height are found, all with steep faces on the south-southwest side. The s o u t h e r n m o s t sand waves lie u p o n the eastern flank of the head of the Galloper Bank. No isolated sand waves occur f u r t h e r to the west or east o f those shown in Fig.2.

Tail and apron o f the Outer Gabbard Bank Th e Ou ter Gabbard Bank rises to a least water dept h of 5.2 m, but the least depth included in the area of Fig.2b is 13 m. Only the tail of the bank lies within the present survey area. Sand waves, asymmetrical in opposing directions, m e e t at the sharp crestline of the tail. The crest terminates in a bend to the southeast. The d o w n w a r d slope to the south and west o f the bend o f the crest is occupied by mostly symmetrical sand waves. An apron o f sand waves extends over the sea floor for a f u r t h e r 8 km south o f the taft, initially broader than the sand bank tail but tapering southwards. T h e eastern part o f the tail apron is occupied by sand waves with a s y m m e t r y implying m o v e m e n t in the net regional direction of sand transport. These sand waves progressively increase in height f rom the o u t e r edge o f the field to crests 8--10 m high along the western boundary. T he symmetrical sand waves are mo s tly 4--5 m high, but diminish to 2 m at the western edge of the apron. The spacing of the symmetrical sand waves is much less than for the asymmetrical sand waves, even f or sand waves of an equivalent height. The overall size range o f the sand waves diminishes southwards, and the zone o f symmetrical waves narrows southwards, so t hat small (2--4 m) sand waves implying m o v e m e n t t o the south-southwest o c c u p y the whole o f the tip of the tail apron. The b r o a d e r part o f the apron has a sinuous outline and sinuous divide between the two zones of sand waves, but the distal part is more streamlined and almost exactly parallel to the regional direction o f net sand transport.

248 Isolated sand waves, from 2 to 5 m high and mostly steep to the southsouthwest, lie between the tail apron of the Outer Gabbard Bank and the apron at the head of the Galloper Bank (Figs.2a and 2b). The peak tidal current velocities prevalent in the region are stronger than are normally associated with sand waves, and these sand waves are strictly limited in occurrence to the area between the two sand banks. SAND TRANSPORT AND SAND BANK EVOLUTION Sand waves with a profile implying their migration in the direction of net regional sand transport occupy the larger part of the sand-wave apron of the Galloper Bank head and isolated examples occur upstream of the apron. Therefore, sand is probably being added to the bank on its " u p s t r e a m " side, and this contributes to the blunt, rounded shape of the head. If the supply of sand is plentiful, the Galloper Bank should grow headwards. A similar pattern exists on the other sand banks. The sand supply to the North Falls head is not at present restricted because a field of sand waves with steep slopes facing in the regional direction of net sand transport merges with the head and east flank of the bank. Sand waves with asymmetry implying net transport to the south.southwest, cover the low rounded head of the Sandettie Bank. There is also a tendency for sand waves steep to the southsouthwest to occupy a greater part of the head of South Falls Bank, at the northern edge of the survey coverage, than over the narrow central part of the bank. The predominance of sand waves steep to the south-southwest over the tail apron of the Outer Gabbard Bank implies sand loss at the tail of this sand bank. Sand is shed from the distal end of the apron and the train of isolated sand waves farther south may represent a routeway for sand transport from the tail apron to the nearby head of the Galloper Bank. If this is so the sand "stream", which is parallel to the direction of net regional sand transport and supplies the head, should " f i x " its position and limit growth of the bank headwards (Fig.l). Sand waves migrating in the regional sand transport direction also occupy the greater part of the Inner Gabbard tail apron, the whole of the North Falls tail, and most of the area south of the Galloper tail. There is no apparent link between the Galloper tail and the North Falls head. The long tongue of sand waves pointing in a south-southwesterly direction from the Galloper-North Falls gap implies sand transport through this gap and from the Galloper tail (Fig.l). The presence of aprons of active sand waves beyond the tails of the South Falls and Sandettie banks is confirmed by SEASAT radar pictures detecting surface turbulence over the sand waves during peak tidal flow to the south (Kenyon, in press). However, neither the asymmetry of the sand waves nor the direction of net sand transport can be determined from the radar pictures. The tail apron of Sandettie consists of sand waves facing in opposing directions, separated by symmetrical sand waves, but the relative importance of the zones of asymmetrical sand waves is not clear at the outer end of the sand-wave field (Burton, 1977).

249 The apparent difference in depth between the f o o t of the west flank and that of the east flank of the Galloper Bank suggests the presence of a step or slope in the sea floor underlying the bank. A high point in the L o n d o n Clay underlying the axis of the Galloper sand bank has been observed on B o o m e r records (J. Redding, pers. commun.). Such an immobile feature may provide a " k e y " to initiate bank development and may also limit its migration. A difference in level also occurs at the South Falls Bank and local highs occur on one side of the Inner Gabbard Bank tail. The presence of fields of large sand waves obscures the level of the base of the North Falls Bank and that of Sandettie Bank. At both the head apron and the tail apron described in detail, the sand waves increase in size in the south-southwesterly direction of migration and smaller, symmetrical sand waves are found in the lee of the largest sand waves. The overall sizes of the sand waves in the apron increase with proximity to the bank head or tail. Sand waves steep to the north-northeast extend farther o n t o the flat ground of the apron of the Galloper Bank head than in any other case. Commonly, these sand waves are restricted to the bank proper, as in the Outer Gabbard taft. Thus, sand waves implying movem e n t in the reverse direction to the net regional sand transport direction are more usually found where the sand bank has considerable relief. Where sand waves facing in opposing directions occur on opposite sides of a bank, the bank sides are straighter. In all cases, the bank height rapidly increases with distance from the head or tail, wth sand being "swept u p " from both sides. The distribution of sand waves raises some important implications with respect to the sand transport and to the water m o v e m e n t around the sand banks. The obliquity of the sand banks with respect to the dominant peak tidal current direction (SSW) inevitably results in cross bank transport (Stewart and Jordan, 1964) but water is also deflected along the sand bank, transporting sand along the bank and thence d o w n the tail apron, in the direction of regional sand transport. The tail apron of sand waves is similar to the wreck mark from a wreck lyingsub-parallel to the regional sand transport direction. Sand in transit is slowed by the obstruction, swept along the wreck and then passes on into a plume of sand waves from its downstream end. During the o p p o s e d flow of the weaker peak tidal currents to the northnortheast, there is again an important c o m p o n e n t of flow across the bank. However, some is deflected northwards along the bank and its head and apron, transporting sand along the train of sand waves steep to the northnortheast on the west side of the head apron. The pattern of growth of a sand bank, whether it is being built up in height and then width, or extended along its length, must depend upon the ratio of cross bank to deflected c o m p o n e n t s of flow, in relation to the total sand supply. Contrary to the widely supported hypothesis, there is no evidence for a closed sand circulation over the sand banks.

250

The actual growth of each sand bank will depend on a greater supply of sand being added to the head and along its length than is lost at its tail. A quantitative study of the net gain and loss of sand at the edge of the sand banks would throw light on the problem of origin of the sand banks. Budgetary considerations would determine (a) whether an individual sand bank owes its existence to an excess of sand supply in a strong current area, or (b) whether it is a remnant of a much larger sand deposit being whittled away by the present-day tidal currents, or (c) whether an equilibrium exists and a bank is bein.g merely maintained within an active sand transport path, perhaps tied to a pre-existing sea bed feature. ACKNOWLEDGEMENTS

I am indebted to the Hydrographer of the Navy for permission to study much of the survey data for the Southern Bight of the North Sea and to Dr. A.H. Stride and Mr. R.H. Belderson for valuable discussion. REFERENCES Allen, J.R.L., 1968. Current Ripples. North-Holland, Amsterdam, 434 pp. Burton, B.W., 1977. An investigation of a sand wave field at the southwestern end of the Sandettie Bank, Dover Strait, Int. Hydrogr. Rev., Monaco, LIV (2), pp. 45--59. Caston, G.F., 1979. Wreck marks: indicators of net sand transport. Mar. Geol., 33: 193--204. Caston, V.N.D., 1972. Linear sand banks in the southern North Sea. Sedimentology, 18: 63--78. Caston, V.N.D. and Stride, A.H., 1970. Tidal sand movement between some linear sand banks in the North Sea off northeast Norfolk. Mar. Geol., 9: M38--M42. Cloet, R.L., 1963. Hydrographic analysis of the sand banks in the approaches to Lowestoft Harbour. Admit. Mar. Sci. Publ., 6 : 1 5 pp. Houbolt, J.J.H.C., 1968. Recent sediments in the Southern Bight of the North Sea. Geol. Mijnbouw, 47: 245--273. Huthnance, J.M., 1973. Tidal current asymmetries over the Norfolk sand banks. Estuarine Coastal Mar. Sci., 1: 89--99. Kenyon, N.H., Belderson, R.H., Stride, A.H. and Johnson, M.A., in press. Offshore tidal sand banks as indicators of net sand transport and as potential deposits. Int. Assoc. Sedimentol. Spec. Publ., Texel 1979. McCave, I.N., 1979. Tidal currents at the North Hinder Lightship, southern North Sea: flow directions and turbulence in relation to maintenance of sand banks. Mar. Geol., 31: 101--114. Off, T., 1963. Rhythmic linear sand bodies caused by tidal currents. Bull. Am. Assoc. Pet. Geol., 47: 324--341. Smith, J.D., 1969. Geomorphology of a sand ridge. J. Geol., 77: 39--55. Stewart, H.B. and Jordan, G.F., 1964. Underwater sand ridges on Georges Shoal. In: R.L. Miller (Editor), Papers in Marine Geology. MacMillan, New York, N.Y., pp.102--114. Stride~ A.H., 1973. Sediment transport by the North Sea. In: E.D. Goldsmith (Editor), North Sea Science. M.I.T. Press, Cambridge, Mass., pp.101--130.