The dynamics of a megaripple field in northern Spencer Gulf, South Australia

Marine Geology, 61 (1984) 249--263

249

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE DYNAMICS OF A MEGARIPPLE FIELD IN NORTHERN SPENCER GULF, SOUTH AUSTRALIA

S.A. S H E P H E R D

and J.R. H A I L S *

Department of Fisheries, 135 Pirie Street, Adelaide, S.A. 5000 (Australia) Centre for Environmental Studies, University of Adelaide, Adelaide, S.A. 5000 (Australia) (Accepted for publication March 19, 1984)

ABSTRACT Shepherd, S.A. and Hails, J.R., 1984. The dynamics of a megaripple field in northern Spencer Gulf, South Australia. In: J.R. Hails and V.A. Gostin (Editors), The Spencer Gulf Region. Mar. Geol., 61: 249--263. The composition, form and migration of megaripples in a megaripple field in a tidal channel of northern Spencer Gulf, South Australia, were examined over a year. Migration rates of megaripples ranged from about 2 m yr -I for sandwaves 3 m high to about 8 m yr-I for megaripples 0.4 m high. Migration of the wave form was regular at some sites but irregular at others, where differential migration and erosion gave rise to branching or coalescence of megaripples. Winter migration rates were greater than s u m m e r rates at most sites. Estimates of sediment transport rates at two cross-sections of the field suggest only slow and minor net flood transport of sediment of 40--70 m 3 yr-I. Sediments are predominantly medium-grained sand well-sorted at the megaripple crests (and elsewhere on fast-moving megaripples) and poorly sorted in troughs between megaripples. The predominantly negative skewness values of the sediments support the conclusion that tidal currents selectively rework the sediments with littleinput of new material. INTRODUCTION

Sandwaves and megaripples are c o m m o n bedforms in estuaries and seas throughout the world where currents in the range 0.6--1.3 m s-I are the dominant form of water movement (Kenyon and Stride, 1970; Langhorne, 1978). However, there have been few attempts to measure their rate of movement and to evaluate the hydrodynamic parameters which generate changes in the form and position of these features {Jones et al., 1965; Langhorne, 1981). Northern Spencer Gulf (Fig.l) is a narrow hypersaline estuary with a channel 15--20 m deep, flanked on either side by intertidal sands and mudflats, and sublittoral terraces colonised by seagrasses. The bed of the channel is partly floored by megaripples and partly by a thin veneer of sediment overlying stiffalluvial clay {Hails et al., 1980; Gostin et al.,1984, *Present address: Environmental Services, C.S.R. Ltd., G.P.O. Box 483, Sydney, N.S.W., 2001, Australia. 0025-3227/84/$03.00

© 1984 Elsevier Science Publishers B.V.

250

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Fig.1. Map of northern Spencer Gulf showing principal features referred to in the text. The megaripple field described is marked A. The extent of Middle Bank is shown by the 10 m contour.

this volume). Middle Bank is an isolated sandbank with seagrass cover. The sediments have a biogenic carbonate c o m p o n e n t (Burne and Colwell, 1982) and a terrigenous c o m p o n e n t derived largely from scour of the seafloor; very little fluvial sediment enters northern Spencer Gulf. The benthic assemblages of the region are described by Shepherd (1983a, b). A semidiurnal tide, with a m a x i m u m amplitude of about 3.5 m at Middle Bank, causes relatively strong, tidal currents of 1--1.5 m s-1 in the main Flinders Channel. However, pulsatile wave-induced water motion on the seabed is negligible because the northern Gulf is land-locked on all sides except to the south where a narrow entrance channel and flanking spit limit the wave fetch. Following an initial survey of the bathymetry of the Flinders Channel east of Middle Bank, and the mapping of the megaripple field shown in Fig.l, this study was undertaken in order to determine the rate of sediment m o v e m e n t and to assess the stability of the seabed and the suitability of the channel adjacent to Middle Bank for deep-draught vessels.

251

In this paper, the composition, form and migration of megaripples* in different parts of the megaripple field are described. Sediment transport rates have been calculated and inferences drawn about the net direction of sediment m o v e m e n t in the megaripple field. MATERIALS AND METHODS

The survey o f the megaripple field was completed in August 1980 by the Survey Branch o f the Department o f Marine and Harbours (DMH). Survey tracks were 50 m apart. Decca navigational equipment was used to obtain horizontal positional control with an accuracy of + 3 m. Depth measurements were made by using a portable R a t h y e o n echo-sounder transducer and recorder. Six sites were selected for detailed study over a period of one year (Fig.2), and stakes were used to define a 30 m crest length within a train of megaripples. On 17 November 1980 six reference stakes (2 m X 20 mm) were hammered at 6 m intervals by divers into the crest of a selected megaripple at each site. On May 13, 1981, a second line o f 6 stakes, at 3 m intervals, perpendicular to the first line, i.e. transverse to the wave crest, was hammered into the substrate at each site. The sites were visited on January 13, May 13, August 11 and November 5, 1981, during neap tides when virtually no tidal variation occurs. During each visit, the angle of repose of stoss and lee slopes was measured with an inclinometer at the mid-position of the respective slopes. The wave length and height, and the position of the wave crest in relation to each stake were measured with a graduated 2 m stick. The respective distances between the stakes and the crest were averaged and divided by the period since last measurement in order to determine the mean daily rate o f migration for each site. The ridge crests were sufficiently sharp so t h a t measurement error did not exceed 10 cm. In order to show the seasonal variation in the migration o f the megaripples, the data for the first two periods (November 1980--May 1981) and the last two periods (May 1981--November 1981) were combined. Mean migration rates were then calculated for the summer and winter periods, each of 176 days. The rate o f volume transport of sediment was obtained by multiplying the cross-sectional area of a megaripple calculated from the measurem e n t data by its mean migration rate (averaged over a year) per centimetre length o f crestline. A total of 86 samples were collected by divers from the megaripple crests, mid-stoss slopes, and troughs at each site shown in Fig.2. In the laboratory, the samples were split, washed free of salt, dried, resplit and then mechanically sieved through 0.25 ¢ sieve intervals. Statistical parameters *The term megaripple refers to features less than 1.5 m in height, and the term sandwaves to larger features (Langhorne, 1978).

252

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400

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500 r

Fig.2. Spatial diagram of megaripple field compiled from August 1980 hydrographic survey. Vertical to horizontal scale ratio of megaripples 50:1. The location of study sites is shown. Arrows indicate direction of asymmetry of megaripples. Inset shows doublecrested megaripples at the extremity o f the field.

253 (mean, standard deviation and skewness) were computed, by using the m e t h o d of m o m e n t s (Friedman, 1961). Special attention is given to the description and interpretation of the mean grain-size and sorting data in terms of sediment transport. RESULTS

Megaripple field The megaripples are mostly 0.5--0.7 m high, although higher ones occur in the northeastern part of the field; a few large sandwaves, 3 m high (e.g. Site 1), occur at its southern extremity. The smallest megaripples (0.2--0.4 m high) occur in the northwestern section of the field (Table I). The symmetry index of the megaripples, defined as the ratio of the planar projection of the length of stoss to lee slopes (Reineck and Singh, 1980) varied from 1.2 to 3.0, indicating only slight megaripple asymmetry in the field. The direction of asymmetry (i.e. toward the lee slope) of the megaripples in the northeastern part of the field is to the northwest. On the southwestern side o f the field, the t e n d e n c y is reversed, and the direction o f asymmetry is to t h e southeast (Fig.2). The megaripples are double crested in one part of the field (Fig.2}, and m a y indicate the local absence of net tidal transport of sediment.

Megaripple dynamics Morphometric data, s y m m e t r y index values and direction of asymmetry for megaripples at t h e six sites are summarized in Table I. Repeated observations during the year at these sites did not reveal any appreciable change in wave-form, or reversal of asymmetry. The migration of megaripples proved to be regular in space and over time at some sites and irregular at others. For example, at Sites 1 and 5, the crests remained nearly parallel and straight throughout the study, while some differential migration occurred at Sites 6 and 7, with the slowest moving portion of the megaripple eroding after 6--9 months. Sites 3 and 4 showed the greatest differential migration with some shift in crest orientation as part of the megaripple stabilised, eroded, or coalesced with a newly formed megaripple. The position of the megaripple crests at Sites 3 and 4, during successive visits, is shown in Fig.3. Here the branching or coalescing of megaripples appears to be common, and unusual features such as the circular depression which developed at Site 3 (Fig.3) m a y also occur. The mean annual migration rates, the ratio o f summer to winter migration rates, and the mean daily volume transport of sediment for each site are given in Table I. The sites on the northeastern side of the field (Sites 4, 6, 7) show a net migration o f megaripples northward, while at the southwesterly sites (3, 5), there is a net migration southward. The direction of migration of the sand-

Depth (m)

16.8 10.8 13.6 10.0 9.5 12.0

Site

1 3 4 5 6 7

3 0.4 0.6 0.2 0.7 1.0

Megaripple height (m)

18 7 7.5 5 8 10

Wavelength (m)

NW SE NW SE NW NW

Direction of asymmetry

1.2 3.0 2.0 1.3 1.9 2.5

± 0.1 _+ 0.2 ± 0.1 ± 0.1 ± 0.2 ± 0.1

Symmetry index (± 1 s.e. )

0.17 1.26 0.16 1.02 0.30 0.58

Summer : Winter migration ratio

2.34 3.17 3.77 8.19 2.65 2.18

Annual migration rate ( m yr -1 )

43.9 4.0 6.6 3.1 6.8 9.6

Mean volume transport rate (cm 3 cm -1 day -~)

M o r p h o m e t r i c data, d i r e c t i o n o f a s s y m e t r y , r a t i o of s u m m e r to w i n t e r m i g r a t i o n rate, a n n u a l m i g r a t i o n rate a n d m e a n daily v o l u m e t r a n s p o r t o f s e d i m e n t for m e g a r i p p l e s a n d s a n d w a v e s at six sites

TABLE I

bO ¢91

255

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SITE3

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Fig.3• Position o f crests of megaripples at Sites 3 and 4 plotted during successive visits. Dates of m e a s u r e m e n t are (1): 5-11-80 ;(2): 13-1-81 ; (3) : 13-5-81 ;(4 ): 11-7-81; (5): 5-11-81. Arrows indicate the direction of migration o f wave crests•

wave at Site I is anomalous. However this sandwave and those in its vicinity form an isolated group in deeper water, some 200 m distant from the rest of the megaripple field. Here the direction o f asymmetry of sandwaves (Fig.2) is n o t consistent and the s y m m e t r y index is low. This part of the field was therefore excluded from consideration in estimating rates of sediment transport described below. The opposing direction of wave a s y m m e t r y on the two sides of the megaripple field (Fig.2), and the data on migration at the various sites, taken together, show t h a t the two sides of the megaripple field are influenced differentially by the flood and ebb tides. The field was therefore divided longitudinally into northward and southward moving components, whose c o m m o n b o u n d a r y was established from echo-trace data showing direction of wave asymmetry. The band width of the two components of the field was estimated at cross-sections in the vicinity of Sites 5--7, and Sites 3 and 4. The estimates o f total sediment transport (Table I) are in fair agreement at the cross-sections and suggest t h a t net accretion to the southern extremity of Middle Bank is occurring at a rate of 40--70 m 3 yr -1 (Table II). Examination o f the seasonal differences within sites (Table I) shows that the northerly migration occurs predominantly in winter, with winter rates

256 TABLE II Estimates of total annual northerly and southerly sediment transport at two cross-sections of the megaripple field Cross-section Southward moving

Sites 3--4 Sites 5--7

Northward moving

Width of field (m)

Sediment Total Width transport (m 3 yr-1) of rate field (cms cm-1 day -1) (m)

Sediment Total transport (m 3 yr -~) rate (cm 3 cm-1 day-1)

200 250

4.0 3.1

6.6 8.2

29.2 28.5

300 350

72.3 104.8

two to six times greater than those of summer, whereas southerly migration (Sites 3, 5) occurs at equal, or only slightly higher, rates in summer. The within-site seasonal differences are significant at the 5% level (Mann-Whitney U test) for sites with northward, but n o t southward, migration.

Grain-size parameters The mean grain size o f t he sediments from the megaripple crests, mid-stoss slopes and troughs ranges, respectively, from 0.15 to 2.00 ¢, --0.33 t o 2.12 ¢, to --0.07 to 2.07 ~, or from very coarse to fine sand (>1 mm < 2 5 mm). (Table III). However, as illustrated in Figs. 4--6 most sediments are pred o m i n a n t l y medium-grained sand, except for Sites 3 and 5 with markedly finer material. Coarse-grained sediments comprise about 9% of the megaripple samples. Well to m o d e r a t e l y well sorted samples predominate at Sites 3 and 5 (Figs. 4--8), while the standard deviation of sediments from the crests, mid-stoss slopes, and troughs ranges, respectively, from 0.51 to 1.08 ~, 0.42 to 1.47 ¢, and 0.46 to 1.53 ~, or from well-sorted t o poorl y sorted values. The best sorted sand (0.42 ¢) is from t he mid-stoss slope of Site 5, while th e most poor l y sorted material occurs in the megaripple trough of Site 6. Sorting is inversely related to mean grain size, particularly for sediments f r o m crests (Fig.5). Carbonate c o n t e n t varies from 25 to 95% by weight. The majority of samples reflect p o o r e r sorting with increasing carbonate c o n t e n t (Fig.7). Skewness, or t he third m o m e n t measure, is the degree of a s y m m e t r y o f a grain-size distribution. Positive skewness values indicate a tail with a b u n d a n t fines, whereas negative values show a p r e d o m i n a n t l y coarse-grained tail. Most o f t h e megaripple samples are negatively skewed; only 17% o f the samples show a positive value (Fi~,-~ DISCUSSION This study represents one of the first attempts to record the dynamics of megaripples over an e x t e n d e d period by direct measurements and demon-

0.44 0.15 0.20 0.28 0.66

0.58 0.51 0.55 0.60 0.51

--1.59 --2.72 --1.80 --1.36 --1.75

November 17, 1980 January 13, 1981 May 13, 1981 August 11, 1981 November 5, 1981

November 7, 1980 January 13, 1981 May 13, 1981 August 11, 1981 November 5, 1981

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Ripple crests (all sites; ~)

November 17, 1980 January 13, 1981 May 13, 1981 August 11, 1981 November 5, 1981

Sampling date

Summary of statistical grain-size parameters

TABLE III

--2.07 --1.87 --1.98 --1.98 --2.38

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to to to to to

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+0.37 +0.76 +0.11 +0.33 +0.70

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1.83 2.01 1.96 1.99 2.12

Mid-stoss slopes (all sites; ¢)

--1.68 --1.86 --3.06 --2.34 --2.20

0.48 0.46 0.49 0.51 0.50

0.71 0.10 --0.07 --0.07 0.02 1.23 1.53 1.32 1.49 1.29

1.89 2.00 2.07 1.97 2.05

to--0.10 to +0.09 to +0.52 to +0.40 to +0.23

to to to to to

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Third m o m e n t measure (skewness)

or sorting measure

Standard deviation

Mean grain size

Statistical parameters (range of values)

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Fig.4. Plot of phi mean grain size against phi standard deviation of sediment samples from troughs between megaripples from 5 sites from 17-11-1980 to 5-11-1981. strates the value of series of stakes used as bench marks to measure with precision the spatial and temporal variation in sandwave migration. We confirm two suggestions previously inferred largely from indirect evidence, namely that sandwaves are to be considered as three-dimensional features in which uniform movement is exceptional (Stride, 1970; Langhorne, 1973, 1978) and that the asymmetrical form of sandwaves can be used to indicate the net direction of sediment transport (Jones et al., 1965; McCave, 1971; Terwindt, 1971; Burton, 1977; Langhorne, 1978). Most studies in tidal regions have recorded a relatively slow rate of sandwave migration, between 18 and 36 m yr-' (Stride and Cartwright, 1958; Jones et al., 1965; Langeraar, 1966), while a few workers have found no measurable rate at all (Terwindt, 1971; Burton, 1977). The small displacements recorded here of mostly 2--4 m yr -~ indicate that this megaripple field is relatively stable. However, the faster passage of the smallest megaripples, moving up to a 8 m yr-', suggests that these features, which elsewhere are often superimposed on larger sandwaves, may be an important mechanism by which sediment is carried over the larger bedforms {Terwindt, 1971; Stride, 1973 ; Langhorne, 1977). The differences between the summer and winter rates of sediment transport could be attributed to a variety of factors. The study area undergoes significant winter to summer changes in sea temperature (affecting viscosity),

259

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1.10

Fig.5. Plot of phi mean grain size against phi standard deviation of sediment samples from crests of megaripples at 5 sites from 17-11-1980 to 5-11-1981.

salinity (affecting density), mean sea level and synoptic wind field. As the overall water circulation pattern of the northern Gulf is still poorly understood, any explanation for the seasonally changing rates of sediment transport must be speculative. The relationship between megaripple height and the rate of migration is of especial interest. Whereas the migration rate of megaripples decreases monotonically (but non-linearIy) with increasing megaripple height in accordance with the spectral models of Jain and Kennedy (1971), the volume transport of sediment rate is positively correlated with megaripple height (r = 0.99 ;P < 0.001), as predicted by the equation of Kennedy (1969). Observed and measured current velocities in the study area are commonly greater than 0.5 m s-~. According to Sundborg (1967), the mean grain-size/ mean velocity relationship indicates that at velocities less than 0.7 m s-1 fine-grained' sand is transported both as suspended and bedload, whereas medium sand is transported only as bedload. Velocities greater than 0.7 m s-1 suspend all fine sand and transport medium sand by both mechanisms. Figures 4--8 show that the mean grain size and sorting values of samples from Sites 3 and 5 differ from those of samples from the other sites. However,

260

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this difference appears to be related to the percentage calcium carbonate content (skeletal material; Fig.7), rather than to subtleties in the current velocities and general water movement at these two sites. It is possible that the sediments at these sites originate from Middle Bank immediately to the north, as suggested by the quantitative sediment transport calculations. The statistical parameters supplement the survey data on megaripple wave-form and migration. Available evidence, particularly the sorting and negative skewness values (positive skewness reflects a predominant unidirectional flow pattern as with the formation of dunes by prevailing winds), suggests that the sand and carbonate material comprising the megaripples are transported within the field by tidal currents. While the volume sediment transport data suggest a net northerly movement of sediment (which may have contributed to the formation of Middle Bank), the extent to which new material is added to the existing reservoir of sand, from outside the boundary of the existing megaripples, must still remain a matter of conjecture until either tracer or continuous quantitative studies are conducted by divers in conjunction with detailed surveys. For the present, it is concluded from the available evidence t h a t the same material is selectively reworked, and little new material is transported to the existing field during megaripple migration.

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ACKNOWLEDGEMENTS

The cost of this study was borne by the South Australian Department of Marine and Harbors. The first author is grateful for the assistance of Mr. G. Heppner who provided survey control, and to Messrs. M. Bagnall and A. Robinson who provided the data from which the spatial diagram was compiled. Messrs. K.L. Branden, L. Gray and G. Wright gave diving assistance, Mr. F. Gorostiaga analysed the sediments and Mrs. S. Proferes drew the figures. Mr. Doug Reilly kindly provided accommodation and facilities at Chinaman Creek. This paper was improved by the comments of Dr. A.P. Belperio.

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Megonpple crest (C) Mud-stoss slope + Megarnpple f rough(T) } Colcium carbonale removed POORLY SORTED 1 I I I 1 I i z

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Fig.8. Plot of skewness against phi standard deviation of sediment samples from crests, mid-stoss slopes and troughs of megaripples at 6 sites. REFERENCES Burne, R.V. and Colwell, J.B., 1982. Temperate carbonate sediments of northern Spencer Gulf, South Australia: a high salinity "foramol" province. Sedimentology, 29 : 223--238. Burton, B.W., 1977. An investigation of a sandwave field at the southwestern end of Sandetti6 Bank, Dover Strait. Int. Hydrogr. Rev., 54 : 45--59. Friedman, G.M., 1961. Distinction between dune, beach, and river sands from their textural characteristics. J. Sediment. Petrol., 31: 514--529. Gostin, V.A., Hails, J.R. and Belperio, A.P., 1984. The sedimentary framework of northern Spencer Gulf, South Australia. In: J.R. Hails and V.A. Gostin (Editors), The Spencer Gulf Region. Mar. Geol., 6 1 : 1 1 1 - - 1 3 8 (this volume). Hails, J.R., Gostin, V.A. and Sargent, G.E., 1980. The significance of the submarine geology of upper Spencer Gulf, South Australia, to environmental decision-making. Search, 11: 115--116. Jain, S.C. and Kennedy, J.F., 1971. The growth of sandwaves. In: Proc. 1st Int. Symposium on Stochastic Hydraulics. University of Pittsburgh, Pittsburgh, Pa~, pp.449--471. Jones, N.S., Kain, J.M. and Stride, A.H., 1965. The movement of sandwaves on Warts Bank, Isle of Man. Mar. Geol., 3: 329--336. Kennedy, J.F., 1969. The formation of sediment ripples, dunes and antidune& Ann. Rev. Fluid Mech., 1 : 147--168. Kenyon, N.H. and Stride, A.H., 1970. The tide-swept continental shelf sediments between the Shetland Isles and France. Sedimentology, 14 : 1 5 9 - 1 7 3 . Langeraar, W., 1966. Sandwaves in the North Sea. Hydrogr. Newslett., 1 : 243--246. Langhorne, D.N., 1973. A sandwave field in the outer Thames Estuary, Great Britain. Mar. Geol., 14: 129--143. Langhorne, D.N., 1977. Consideration of meteorological conditions, when determining the navigational water depth over a sandwave field. Int. Hydrogr. Rev., 54: 17--30.

263 Langhorne, D.N., 1978. Offshore engineering and navigational problems -- the relevance of sandwave research. Institute o f Oceanographic Sciences, London, 20 pp. Langhorne, D.N., 1981. An evaluation of Bagnold's dimensionless coefficient of proportionality using measurements o f sandwave movement. Mar. Geol., 43 : 49--64. McCave, I.N., 1971. Sandwaves in the North Sea off the coast of Holland. Mar. Geol., 10 : 199--225. Reineck, H.E. and Singh, I.B., 1980. Depositional Sedimentary Environments. Springer, Berlin, 549 pp. Shepherd, S.A., 1983a. Benthic communities of upper Spencer Gulf, South Australia. Trans. R. Soc. S. Aust., 107: 69--85. Shepherd, S.A., 1983b. Epifauna o f sandwaves, species adaptations and population responses to disturbance. Aust. J. Ecol., 8 : 3--8. Stride, A.H., 1970. Shape and size trends for sandwaves in a depositional zone of the North Sea. Geol. Mag., 107 : 469--477. Stride, A.H., 1973. Sediment transport by the North S e a In: E.D. Goldberg (Editor), North Sea Science. MIT Press, Cambridge, Mass., pp.101--130. Stride, A.H. and Cartwright, D.E., 1958. Sand transport at the southern end of the North Sea. Dock Harbour Auth., 39: 323--324. Sundborg, ~., 1967. Some aspects of fluvial sediments and fluvial morphology. Geogr. Ann., 49: 333--343. Terwindt, J.H.J., 1971. Sandwaves in the southern bight of the North Sea. Mar. Geol., 10: 51--67.