Low-frequency changes of sediment volume on the beachface at Warilla Beach, New South Wales, 1975–1985

Marine Geology, 79 (1988) 189-211

189

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

LOW-FREQUENCY CHANGES OF SEDIMENT VOLUME ON THE BEACHFACE AT WARILLA BEACH, NEW SOUTH WALES, 1975-1985 D.J. CLARKE

1 and I.G. ELIOT 2

1Mathematics Department, University of Wollongong, P.O. Box 1144, Wollongong, N.S.W. 2500 (Australia) 2Geography Department, University of Western Australia, Nedlands, W.A. 6009 (Australia) (Received February 25, 1986; revised and accepted August 25, 1987)

Abstract Clarke, D.J. and Eliot, I.G., 1988. Low-frequency changes of sediment volume on the beachface at Warilla Beach, New South Wales, 1975-1985. Mar. Geol., 79: 189-211. Characteristic patterns of shore-parallel and shore-normal movements of sediment on a sandy beach are described in this study. The sediment movements are apparent as low-frequency variations in the volume of sediment stored above mean low water spring tide (MLWST) level on Warilla Beach, N.S.W. They were determined by a n empirical orthogonal-function (EOF) analysis of beach profile records. The profiles were surveyed from eighteen stations along a 2 km long beach at least monthly, generally fortnightly, for over 10 years, from July 1975 until August 1985. Results from the EOF analysis confirm and substantially extend a n earlier study of Warilla Beach when the first 5 years of the beach survey record was examined*. The importance of low-frequency beachface change identified in the earlier, shorter study was confirmed from the extended data set; zones of maximum variability, linked with rip-current activity, are consistent in both studies, and similar patterns of alongshore sediment movement in the swash zone were determined from both analyses. The principal differences between the two sets of results highlight beachface responses to processes operating in different time scales. Amplitude spectra from time series describing the characteristic modes of sediment movement along and across the beachface have periodicities of 2.0, 1.7 and 1.0 years. Beach changes with periods close to 3 years are apparent in the record, but c a n n o t be resolved by standard spectral analysis techniques. Each period is associated with a characteristic p a t t e r n of beach change; the triennial and 1.7 year cycles with alongshore sediment transport, and the biennial cycle with onshore-offshore sediment exchange. Eigenvectors from the EOF analysis identify the characteristic patterns of beach change from survey data specified to highlight the shore-parallel sediment movement along 0.5 m deep, horizontal segments of beach, and shore-normal sediment movement along the beach profiles. In the analysis of shore-parallel variation in beach profile configuration, the fundamental eigenvectors explain between 70 and 80% of beachface variance in each instance, and different patterns of sediment movement were identified for each horizontal slice. On the upper part of the beachface, all eigenvectors identified a zone of low variability along the shoreline skirting a rockwall. Below berm crest, in the swash and intertidal zones of the beach, and away from the rockwall, the n o r t h e r n half of the beach was highly variable. Second mode eigenvectors (cellular-flux modes) explain a further 5 to 14~o of the variance and describe sediment exchanges between discrete zones in each horizontal segment of the beachface. Zones of greatest variability occurred landward of places where rip currents frequently develop. Conversely, zones of low variability are coincident with areas of beachface in the lee of places where inshore bars frequently form. Analysis of sediment movement along each profile is consistent with the beach changes described. The fundamental eigenvectors account for 91 to 99°/0 of the variance on each profile. They describe an onshore-offshore movement of sediment t h r o u g h the intertidal zone of the beach.

*Eliot and Clarke (1982a) and Clarke and Eliot (1982, 1983a). 0025-3227/88]$03.50

© 1988 Elsevier Science Publishers B.V.

190 Introduction

Context A primary objective of recent studies of shoreline change reported by Fox and Davis (1976), Dolan et al. (1977, 1979a, 1979b, 1983), Aubrey (1979, 1983), Clarke et al. (1980), Bowman (1981), Clarke and Eliot (1982, 1983a), Eliot and Clarke (1982a, 1982b), and Miller (1983) has been to develop analytic procedures that quantitatively describe and predict shorenormal and shore-parallel movement of sediment. To this end, systematic variations in beach morphology have been examined by using least squares, spectral analysis and empirical orthogonal-function (EOF) techniques. Characteristic patterns of temporal variation can be identified from beach change data, including beach width (Dolan et al., 1979a, 1983; Clarke and Eliot, 1983b) and profile survey information (Aubrey, 1979, 1983; Eliot and Clarke, 1982a; Clarke and Eliot, 1983a, 1983b; Miller, 1983). These occur at time scales described by the secular trends of least-squares analyses, the results of the spectral analyses, or as time series associated with eigenfunction (EOF) modes. The results yield a context for process studies t h a t are necessarily tied to explanation of topographically controlled, regional and local beach responses. Results from an EOF analysis of 10 years of beach survey records from Warilla Beach, N.S.W. (Fig.l), are reported in this study. These are compared with descriptions of beach variation at Warilla previously reported by Clarke and Eliot (1982, 1983a) for the first 5 years of the survey, from 1975 to 1979. The comparison is intended to establish the significance of low-frequency beach changes identified in the first study, as well as to extend that analysis, through the inclusion of survey information gathered up to August 1985. Additionally, the further analysis describes patterns of shore-parallel and shore-normal sediment movement that are characteristic of the beachface during the 10 years of the survey.

Lake ( Windang b Windang/5 Warilta Beach ,Shellharbour

~0

~ 5Bass km

I l, 7 ~TomboloBar L~ Waritla __~ Embayment !.~ ~

LitHe Lake Lagoon Barrack

[E~ Exlstrng Urban Areas Rockwall

Ori..an%,

0.,5 1~,.

Fig.1. Regional setting of Warilla Beach.

The low-frequency domain Eliot and Clarke (1982a) compared the lowfrequency beach changes at Warilla with corresponding fluctuations in sea-level from records at Fort Denison, Sydney, N.S.W. The peak phase of the biennial component in the sea-level record led by ] month the most depleted state of the sub-aerial beach at Warilla. Here, the subaerial beach is arbitrarily defined as the sediment body seaward of a survey datum located in the backshore zone of the beach (usually in the frontal dunes) and above mean low water spring tide (MLWST) level (Fig.2). The strength of the result reported by Eliot and Clarke (1982a) was unexpected because the biennial component had not been identified in the sea-level record prior to the Warilla study and because other processes markedly contribute to beach change. Waveregime variations and rip-current activity during storm-wave conditions occur aperiodically,

191

Fig.2. Beach profile stations and baseline locations on Warilla Beach.

in response to unpredictable storms, and these are likely to affect the amount of sediment held on the beach at any time. Additionally, Lanyon et al. (1982) and Eliot and Clarke (1982b) have pointed out that long-term changes in beach groundwater levels, associated with regional precipitation, sea-level variation, and stormwater discharge are all likely to trigger sediment exchanges between the subaerial beach and inshore zones. Despite these factors, the unequivocal strength of the low frequency signals from Warilla and other beaches, including Scarborough in W.A. (Clarke and Eliot, 1983b), Moruya (Eliot and Clarke, 1982a) and Stanwell Park (Bryant, 1983) require further examination.

Explanation for the low-frequency beach changes at Warilla is being sought through an examination of synoptic weather systems, barometric-pressure anomalies, and variations in precipitation, onshore wind activity, sea-level fluctuation and sea-surface temperature (Eliot and Clarke, 1982b; Clarke and Eliot, 1982). The shoreline changes previously described from Warilla are close to those described for a range of low-frequency, planetary-scale atmospheric phenomena including the Southern Oscillation (SO) and associated El Nifio effects, with periods of 3 to 6 years (Chen, 1982; Pazan and Meyers, 1982; Rasmussen and Carpenter, 1982); the stratospheric Quasi-biennial Oscillation (QBO), which has a period of 26 to 27 months (Quiroz, 1981; Labitzke, 1982; Van Loon et al., 1982); and tropospheric Biennial Oscillations (BO) with periods similar to that of the QBO (Angell and Korshover, 1974; Trenberth, 1980). These phenomena have been linked to mesoscale oceanographic processes through their effects on sea-level air pressures, sea-surface temperatures, and global winds (Bjerknes, 1969; Rochford, 1973; Brier, 1978; Covey and Hastenrath, 1978; Julian and Chervin, 1978; Wyrtki, 1979). However, significant and reliable correlations among indices identifying the meso-scale atmospheric phenomena and beach change have yet to be established with any degree of reliability. Few, if any, beach-survey records from Australia match the detail of available meteorologic information. The reliability of sediment-budget analyses, such as those described here, is very much dependent on the sampling density, which is a function of the time lapse between successive surveys, the number of profile stations located along the beach, and the length of time over which the surveys have been repeated. Several problems are apparent in extant beach-survey records. Many of the primary studies, including those reviewed by Thom et al. (1973), span only 12 months, and are too brief for determination of low-frequency beach change. However, Thorn et al. (1973) concluded from the studies of Short (1967), Stone

192

(1967), Baxter (1969) and Stockwell (1969) that high-frequency beach changes could be of higher amplitude than the seasonal variability. They noted that there is a need for more direct information on the nature of the East Coast wave climate, not only in deep water but also nearshore, over a period of many years; and that these records should have associated with them long-term records of beach response. Longer beach-survey records, such as those from Moruya (Thom and Bowman, 1980) and Scarborough (Eliot et al., 1982), are taken from small segments of long sandy beaches. Eliot and Clarke (1982a), Clarke and Eliot (1982) and Bryant (1983) have demonstrated that spatially restricted surveys do not yield reliable representation of total beach change. Survey records from isolated profiles or groups of profiles have a low signal-to-noise ratio. They contain subaerial-beach responses to localised foreshore and nearshore processes which may exceed the total beach variance. In this respect the Australian observations confirm similar findings from the U.S.A., reported by Dolan et al., (1979a, 1979b, 1983). In other respects, the longer records purportedly describing lowfrequency shoreline changes are either too brief, or the sampling density too sparse, to enable reliable resolution of periodic constituents in the 2 to 5 year band. An example of a brief record is provided by the field surveys previously reported from Warilla Beach. Eliot and Clarke (1982b, p.206) pointed out that their results could not be used to establish a firm linkage between low-frequency atmospheric phenomena, such as the Southern Oscillation, and 2 to 5 year variations in subaerial-beach sediment volume. A survey with a low sampling density was reported by Bryant (1983). He sought to establish correlations between the Southern Oscillation index and measurements of beach width made, at best, once or twice per year at Stanwell Park, between 1933 and 1983. Further advances in our understanding of low-frequency beach changes are therefore dependent on the acquisition of longer, more

detailed, and geographically more extensive data sets than hitherto have been available. These may be obtained with passage of time after large-scale beach monitoring programmes have been initiated and maintained. The 10 year record of beach change reported here describes an ongoing field survey of Warilla Beach that was initiated in J u l y 1975. Since then variation in beachface geometry has been monitored by monthly, often fortnightly, surveys of 18 profiles spaced at 100 m intervals along the beach. As described by Clarke et al. (1980) and Eliot and Clarke (1982a), each profile was measured from a fixed benchmark. The benchmarks were tied along a common baseline located in the beach backshore zone (Fig.2), and related to a local geodetic datum. Profile lines were extended seaward to approximately 1.5 m below Australian Height Datum (Fig.4) under all but extreme high energy wave conditions. The surveys provide a comprehensive record of three-dimensional beach change. The spatial sampling density used to resolve characteristic patterns of sediment movement on those parts of the N.S.W. coast, including Warilla Beach, where a broad range of morphodynamic conditions occurs over time, is crucial to the outcome of the analysis. The suite of morphodynamic patterns incorporates a hierarchy of nearshore water-circulation patterns ranging from small, transitory rip currents, through larger systems that migrate along the beach, to large quasi-stationary circulations tied to the geometry of the beach embayment (McKenzie, 1958; Chappell and Eliot, 1979). The spatial arrangement of survey stations employed in any beach survey establishes a bias within this hierarchy. In this respect, survey procedure used at Warilla Beach was chosen to describe the effects of large-scale, standing circulations, that is nearshore cells that tend to recur in the same location, but not short-lived, migratory rip currents. The largescale effects were selected for study because they are most significant in the low-frequency domain.

193 Warilla B e a c h Warilla Beach has been comprehensively described by Clarke et al. (1980) and Eliot and Clarke (1982a). It is a 2 km-long, open-ocean, quartzose sandy beach on the Illawarra Coast of N.S.W. (Fig.l). The beach experiences a wave climate similar to t h a t for other parts of the Central and South Coast of N.S.W., as described by Thom et al. (1973), Lawson and Abernethy (1975), Wright (1976) and Short and Wright (1981). A highly variable wind-wave climate is superimposed on a persistent, high energy, south to southeasterly swell regime. A wind-wave climatology spanning the beach survey period, 1975 to 1984, has been compiled for the Illawarra Coast by Clarke and Eliot (1985a, 1985b). Major onshore wind events, capable of generating deepwater waves with amplitudes equal to or greater than 3.0 m, are indicated in Fig.3. Tides of the region are semidiurnal, with mean neap tides of 0.9 m and mean spring tides of 1.3m. The lowest to highest astronomical tidal range (LAT to HAT)

is 2 m (Anonymous, 1984). Variation in tidal range may be masked by combinations of storm surge or of shelf-wave activity, such as t h a t reported by Clarke (1974, 1979), Bryant and Kidd (1975), Foster et al. (1975) and Louis and Clarke (1986). The inshore and foreshore morphology is also highly variable, (Clarke and Eliot, 1983b) and the beach exhibits the full range of morphologic states described for other beaches of N.S.W. by Chappell and Eliot (1979), Wright et al. (1979) and Short and Wright (1981). Complex arrangements of transverse and longshore bars are common at the northern end of the beach, whereas broad, terrace shoals occur more commonly along the southern section. R e d u c t i o n and a n a l y s i s o f s u r v e y data

Data analysis The survey data for each profile were reduced to volumetric information for 0.5m thick, horizontal slices or segments of beach

4 3

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Fig.3. Major onshore windevents at Port Kembla.a. Number of onshore wind events capable of generatingdeepwaterwaves with a significantheight of 3 m or greater, b. Maximumwind gust recorded each month, c. Duration of storms for which the maximum gust velocity was recorded.

194 SURVEY BENCHMARK BEACH SATURATION ZONES (Pollock and Hummon, 1971) 'X~ SUBMARINE B E A C H

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PROFILE 14

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|

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Sediment volumes for the ulR)er beach mgment include oll moterlol above the 2.0 m contour and saaward of the survey benchmark.

~ 1975 '

UPPER INI"Lq~rlDAL LOWER SWAIH MID-SWASH UPPER BEACH

0 1976

' 1977. YeQrs

' 1978

Stort July 1975

Fig.4. Data presentation and analysis. Volumes for each slice are treated separately for calculation of secular trends, cyclic fluctuations and EOF analysis. Encircled numbers indicate slices used in Figs.5 and 6. Beach segment data from Clarke and Eliot (1983a).

related to Australia Height Datum (A.H.D.). Seven sets of volumes were determined for each set of profile data (Fig.4). Over repeated surveys these constitute the time series illustrated in Fig.5. The horizontal slices approximately correspond with tidal and high-tide,

swash-process zones identified by Duncan (1964), Schiffman (1965) and Pollock and Hummon (1971). The berm slice incorporates a dry or drying sand zone, which is influenced by swash action during occasional spring high tides or by high seas. Swash inundation occurs

195 b 600 1

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more frequently in the upper-swash zone, particularly during high-tide conditions. The zone retains moisture for much of the tide cycle. Groundwater resurgence characterises the intertidal zone, and intensive water flux occurs throughout the tide cycle. The lower intertidal zone is saturated under most conditions of tide and swash. Separation of the horizontal slices according to the predominant processes operating at each level assists data interpretation. Analysis of the time series for the seven beach segments provides a detailed, three-dimensional account of shore-parallel and shore-normal sediment movements. The analytic techniques used in this study involved a standard application of empirical orthogonal-function (EOF) analysis. The methodology has been described by Morrison (1976), Joreskog et al. (1976), and Halliwell and Mooers (1979). It has been applied to beach data by Dolan et al. (1979b, 1983), Aubrey (1979, 1983), Bowman (1981), Clarke and Eliot (1982, 1983a), Eliot and Clarke (1982a), Miller (1983) and Clarke et al. (1984). The manner in which data is specified for EOF analysis is crucial to the outcome of the calculations. In the present analysis the means were subtracted from time series describing variation in sediment volume for each slice on each profile, so that the EOF modes represent variation from the mean beach state. The time series for the beach segments of each profile station were reconstituted for EOF analysis in two ways (Fig.6): (1) The time series were specified on a profile-by-

Mark Profile 2

Bench

Profile 2

b)

Bench Mark

Profile I

2 5 4 5 6 7

UPPER BEACH MID-SWASH LOWER SWASH UPPER INTERTIDAL MID-TIDAL LOWER MID-TIDAL LOWER INTERTIDAL

Fig.6. Specification of volumetric data for EOF analysis, a. Alongshore transfers. Profile slices are compared laterally, along the beach, for each horizontal slice, b. Shore-normal transfers. Profiles are analysed separately. The slices are compared between successive horizontal levels.

198

profile basis for each horizontal beach segment. The new time series, now describing volumetric changes for particular beach slices, were analysed to establish patterns of sediment movement that are characteristic of each level above mean low water spring tide. This manner of specifying the data provided a direct description of shore-parallel sediment movements in each slice for the duration of the study. (2) The time series were reconstituted on a slice-byslice basis for each profile, to determine patterns of sediment movement along each profile. The EOF analysis of each time series separates the spatial structure into empirical modes, each of which represents a spatially distinct pattern of sediment movement. Associated with each mode is a modal-amplitude time series which describes how t h a t mode varies in time. In the present analysis the mean shoreline configuration in each horizontal slice has been removed and the EOF technique is applied to data from the 18 beach profiles. An 18×18 autocovariance matrix was generated from the time series for the 18 profiles in each beach segment above MLWST to determine characteristic patterns of shore-parallel sediment movement. Each matrix was analysed to yield 18 eigenvalues and associated eigenfunctions. Similarly, subtraction of the means of the time series for each slice on each profile, resulted in removal of the mean profile configuration so t h a t shore-normal sediment movements could be determined from a 7 × 7 autocovariance matrix for each of the 18 profiles. This second analysis yielded seven eigenvalues and associated eigenfunctions for each of the 18 profiles. In both instances the eigenfunctions represent the empirical modes or characteristic patterns of sediment movement. The associated modal time series is calculated from these. Also the ratio of an eigenvalue to the sum of the eigenvalues is expressed as the percentage of variance attributed to t h a t mode. In the standard application of the analysis most of the variance is contained in the first few modes.

Terminology In this paper, the terms shore-parallel and shore-normal are used to identify data organisation. The data analysis identifies characteristic patterns of beach variability separately for shore-parallel slices of beach and shorenormal profile transects. In each instance, sediment movements described by the patterns may be in onshore-offshore or alongshore directions. The direction is resolved by the analysis. Results Sediment movement for the shore-parallel and shore-normal components of beach change at Warilla have been determined by EOF methods where nodes in the eigenfunctions represent relatively stable, pivotal points through which sediment is transferred bidirectionally, whereas the antinodes identify places of high beachface mobility. Onshore-offshore sediment movement is identified by eigenfunctions without distinct nodes. Conversely eigenfunctions with marked nodes and antinodes represent sediment exchanges that occur laterally, between discrete zones (cells) on the beachface. The eigenfunctions, together with their associated time series and amplitude spectra, are now described.

Shore-parallel sediment movement The first three EOF's for each of the seven beachface segments below the berm crest on Warilla Beach contain 85 and 95% of the total variance for each segment (Table 1).

The fundamental beach response The dominant pattern of sediment movement, i.e., the fundamental beach response, is represented by the first eigenfunction mode. It contains more than 72% of the variance, ranging from 72.1% on the upper beach to 77.4% in the mid-tidal zone (Table 1). Variation in the volume of sediment stored in each horizontal segment is largest along the

199 TABLE 1 The percentage of variance explained by each eigenfunction mode describing alongshore sediment movement Percentage of variance for each eigenfunction mode

Segment

No.

Zone

Elevation

(m-A.H.D.) 1 2 3 4 5 6 7

Lower intertidal Lower mid-tidal Mid tidal Upper intertidal Lower swash Mid swash Upper beach

-1.0 -0.5 0.0 0.5 1.0 1.5 > 2.0

FIRST MODE I I I I I I

== 6EACHFRONT

Vo f i a n c e

ZONES

-0.5 0.0 0.5 1.0 1.5 2.0

MODE

SECOND ,

i

,

;

.

,o

i

2

3

4

74.7 77.0 77.4 77.2 75.8 74.1 72.1

6.3 6.8 6.8 6.4 5.7 8.1 13.8

4.8 4.1 4.1 4.5 5.4 4.6 3.8

3.5 3.2 3.5 3.9 4.5 3.8 3.2

,. ,. ,...

I

I

Variance

741%

ol [ ~ V ° r i o n c e

75'8%

Mid- S w a s h

THIRD MODE

: ; ; : ',~ ,~ ,:,;

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3"8%

6,.A

46%

721%

-i

(~

to to to to to to

1

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5"4%

6...

[ [

i Variance

oo., Intertidal

77,2%

o

Variance

(~

Mid- T i d a l

77'4%

° ~

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4' 5% r

4"1% /

~

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Variance

77'0%

V o r i o nee

74'7%

6"8%

4"1%

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4"6%

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2

4

G

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le

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i|

Profile

Fig.7. Eigenfunction modes describing shore-parallel sediment movement, Warilla Beach, 1975-1985.

J4

16

FO

200 northern half of the beach, away from the rockwall, and least in the vicinity of profile stations 9 and 11 (Fig.7). The general pattern is most apparent for the mid-swash and upper beach segments; that is for the beachface above 1.5 m A.H.D. where beach changes are most affected by the presence or absence of the rockwall. Below this level, the distinction between the northern and southern sections of the beach is increasingly less well defined in a downslope direction. It is negligible in the lower intertidal zone. The eigenfunction of the first mode for each beachface segment (Fig.7a) has no zero crossings (nodal points). It thus describes an onshore-offshore movement of sediment into and out of each segment, which is consistent with switching between berm and bar dominated profile configurations. Such switching is well known and was first observed on the western coast of the United States of America according to Komar (1976, p.289). It has been described for beaches in N.S.W. by Wright and Short (1984) and for Warilla Beach by Clarke and Eliot (1982). Whereas descriptions of switching between berm and bar morphologies generally refer to changes at seasonal and subseasonal time scales (e.g. Aubrey, 1979; Miller, 1983) the profile changes at Warilla Beach are dominated by switching that occurs at lower frquencies. The amplitude spectra of time series associated with the fundamental beach response at Warilla consistently register peak periods in spectral bands with periods close to 2.0, 1.7 and 1.0 years (Fig.7a). Beach responses with periods close to 3.0 years are also apparent in the record. These have been isolated by least-squares techniques, to confirm their presence. However, they cannot be satisfactorily described from the analyses reported here. Transfers of sediment occurring in the 2.0, 1.7 and 1.0 year bands may be considered as quasi-cyclic in recurrence but are necessarily treated as being cyclic in the analysis. The amplitude and sequence of beach change is different for each time scale (domain) represented by the spectral peaks (Table 2).

The phase angle of each peak in the amplitude spectra (Table 2) identifies the time at which the maximum volume of sediment is stored in the particular beachface segment being considered. Hence, the sequence of beachface change for the spectral band under consideration can be established by comparing the phase angles for successive segments. In this manner, three sequences of beach change have been determined for the biennial, 1.7 year and annual components of beach change. The major sequence of beach change has a biennial period. During the biennial oscillation the pattern of beach change involves accretion on the lower beachface with subsequent infilling of the mid-tidal zone. The upper beach (segment 7) fills first. Peak volumes then occur for successively lower beach segments down to the upper intertidal zone (segment 3) over an interval of approximately 40 days. In the lower intertidal zone (segment 1) the peak sediment volume leads that in the mid-swash zone by approximately 6 days. During the annual movement of sediment on and off the beachface the maximum sediment volume occurs earliest in the mid-tidal zones (segments 2 and 3). The phase angles indicate that there is little delay in the spread of sediment up the beach to the mid-swash zone (segment 6) and down beach to the lower intertidal zone. Filling of the lower mid-tidal zone (segment 2) is completed first and leads the maximum volume in the upper beach zone (segment 7) by approximately 123 days. The third sequence of beachface change determined from the analysis has a period of approximately 1.7 years. This sequence involves completion of the aggradational phase of sediment transfer, first in the upper beach zone (segment 7) then successively downslope to the lower intertidal zone (segment 1). Peak deposition in the upper beach zone leads the lower intertidal zone by approximately 139 days.

Cellular-flux modes The second and subsequent eigenfunction modes (Figs.7b and 7c) describe alongshore sediment transfers within each horizontal seg-

201 TABLE 2 Amplitudes and phase angles of the dominant peaks in amplitude spectra of time series associated with the first three eigenfunction modes describing alongshore sediment transport

Segment

Period of dominant peaks

No. Zone

2.0 year

1.7 year

1.0 year

Ampl.

Phase

Ampl.

Phase

Ampl.

Phase

55 46 36 27 19 11 4

162 163 165 163 159 153 145

22 16 13 10 8 7 4

211 202 196 184 170 153 130

38 31 24 27 10 5 5

247 244 245 250 262 300 7

(b) EOF Mode 2 1 Lower intertidal 2 Low mid-tidal 3 Mid-tidal 4 Upper intertidal 5 Lower swash 6 Upper swash 7 Upper beach

19 17 14 11 5

140 141 142 139 136

16 16 13 11 8

97 90 88 86 74

7 4 4 3 2

207 197 188 173 101

(c) EOF Mode 3 1 Lower intertidal 2 Lower mid-tidal 3 Mid-tidal 4 Upper intertidal 5 Lower swash 6 Upper swash 7 Upper beach f

7 4 4 4 6

11 11 8 5 4

221 217 218 229 226

(m 3) (a) EOF Mode 1 1 Lower intertidal 2 Lower mid-tidal 3 Mid-tidal 4 Upper intertidal 5 Lower swash 6 Upper swash 7 Upper beach

(o)

(m 3)

(o)

(m 3)

(o)

Low amplitude and unreliable

122 123 132 137 146

10 13 12 10 6

231 236 236 236 227

LOw amplitude and unreliable

ment. Antinodes in the eigenfunctions identify cells of high variability in the volume of sediment stored on the beachface in that cell, whereas nodes indicate places of low shoreline variability through which sediment is transferred bidirectionally along the beach. Clarke and Eliot (1982) referred to the second and higher order eigenfunction modes as cellularflux modes. They attributed the sediment transfers represented by the eigenfunctions to an effect of the distribution of rips and nearshore bars along the beach. The zones of maximum variability are coincidental with beachface locations landward of rip channels, whereas the zones of low variability occur

landward of places where bars frequently form and are tied to the shoreline. In the present study, the variance contained by the second, third and fourth eigenfunctions respectively ranges from 5.7 to 13.8%, 3.8 to 5.4% and 3.2 to 4.5~/o. This indicates that the volume of sediment moved to-and-fro along each segment is significantly less than that involved in the onshore-offshore transport. Never-the-less, several characteristic patterns of alongshore sediment movement have been identified in the analysis. Only those represented by the second and third eigenfunction modes are described here (Fig.7b) because the amount of variance attributable to subsequent

202 modes is v e r y small. The p r i m a r y cellular-flux mode is r e p r e s e n t e d by the second eigenfunction. It prevails in the lower i n t e r t i d a l (segm e n t 1) to lower swash (segment 5) zones. A s e c o n d a r y cellular-flux mode describes sedim e n t e x c h a n g e s at all levels on the beachface. It is r e p r e s e n t e d by the second e i g e n f u n c t i o n mode for the mid-swash and u p p e r b e a c h zones, as well as by the third e i g e n f u n c t i o n mode for all levels below the mid-swash zone (Fig.7c). The p r i m a r y cellular-flux mode involves a sediment e x c h a n g e b e t w e e n the b e a c h skirting the r o c k w a l l and the n o r t h e r n h a l f of the beach. P i v o t a l points in this e x c h a n g e o c c u r in the v i c i n i t y of profile 14 at all levels on the beachface, and n e a r profile 11 in the lower i n t e r t i d a l to mid-tidal zones. T h e s e c o n d a r y cellular-flux mode encompasses a more complex e x c h a n g e of sediment, p a r t i c u l a r l y n o r t h of the rockwall. Along the beach, cells of m a x i m u m v a r i a b i l i t y o c c u r b e t w e e n profiles 2 to 4 and 10 to 12; n e a r profiles 13 and 16; and at the b e a c h ends (profiles 1 and 18). These are s e p a r a t e d by areas of low v a r i a b i l i t y or pivotal points (represented by zero crossings in the e i g e n f u n c t i o n p a t t e r n s ) b e t w e e n profiles 1 and 2, at profile 4 and b e t w e e n profiles 7 to 10, as well as at profiles 13, 15 and 17. Amplitude spectra from the time series associated with each of the eigenfunctions describing the cellular-flux modes register peak periods in the 2.0, 1.7 and 1.0 y e a r bands. However, the cellular-flux modes are dominated by sediment exchanges with periods of 1.7 years whereas the fundamental beach response was dominated by the biennial oscillation. Additionally, cellular flux modes for different levels on the beachface are dominated by changes o c c u r r i n g in different time scales. Biennial sediment transfers are strongest in the inter-tidal zone (segments 1 to 4) for the primary cellular-flux mode, but are less i m p o r t a n t on the upper part of the beachface (segments 5 to 7) where the secondary cellular-flux mode prevails. The s e c o n d a r y mode is dominated by sediment exchanges over a period of 1.7 years and, to a lesser extent, by the a n n u a l cycle. As with the f u n d a m e n t a l b e a c h response, the

TABLE 3 The percentage of variance explained by each eigenfunction mode describing onshore-offshore sediment movement Profile

(South)

(North)

Percentage variance for eigenfunction mode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1

2

3

4

96.7 99.2 98.9 97.9 98.1 97.7 98.1 96.1 96.6 97.4 91.9 94.9 96.3 95.0 96.7 97.1 96.8 97.9

2.9 0.68 0.89 1.7 1.7 1.8 1.5 3.3 2.9 2.3 7.0 4.3 3.1 4.1 2.3 2.4 2.6 1.7

0.28 0.10 0.11 0.30 0.16 0.38 0.29 0.04 0.35 0.17 0.78 0.59 0.52 0.66 0.60 0.43 0.50 0.38

0.09 0.03 0.03 0.17 0.06 0.07 0.06 0.10 0.05 0.05 0.15 0.08 0.05 0.10 0.06 0.05 0.06 0.05

sediment e x c h a n g e s are not s y n c h r o n o u s from level to level. T h e y o c c u r in sequences similar to those described for the o n s h o r e - o f f s h o r e t r a n s p o r t . Again, the phase angles of peak periods in amplitude spectra of time series associated with the e i g e n f u n c t i o n s h a v e been used to identify the s e q u e n c e of c h a n g e up and down the beachface. Shore-normal sediment movement

The shore-normal c o m p o n e n t s of sediment m o v e m e n t are described by e i g e n f u n c t i o n s s e p a r a t e l y d e t e r m i n e d for e a c h of the e i g h t e e n profiles. T h e first two E O F ' s c o n t a i n o v e r 98.9% and up to 99.9~/o of the t o t a l v a r i a n c e for all profiles (Table 3). In e a c h i n s t a n c e the largest p e r c e n t a g e of v a r i a n c e belongs to the first mode. T h e e i g e n f u n c t i o n of t h a t mode (Fig.8a), the f u n d a m e n t a l profile response, has no zero crossings (nodal points) and represents a n o n s h o r e - o f f s h o r e sediment m o v e m e n t

203

I. SEGMENT 3 5 7

a

"Vori~.... 2 °/o

[

99'2%

o -I

I 3 5 7 i i = i i J i

-I I .Vari ~ e.... /o 0

0.1%

I

0 -I

I

iI

Mode

I = ~ ; i i J i

I i i i i i = i

iI

3. SEGMENT

2. SEGMENT 3 5 7

Mode

Mode

0.9%

98-9% -I

u_

a.

979=/°

I

0.3%

1.7%

® -I

-I

0-2%

98,1% II

®

1.7=,,,, ~e

0

0

-I

-I 0.4"/,

97-7%

®

0

-I = i i i [ i i I 3 5 7

SEGMENT

I

I

I

I

I

1 3 5 7

I

I

SEGMENT

= i i i i i J ~

3

5

7

SEGMENT

Fig.8.a. (See caption, p. 205).

involving the whole profile from the upper beach to the lower intertidal zone. The second mode, referred to as the swash-function mode (Clarke and Eliot, 1983a), describes sediment exchange between the lower intertidal and swash zones via

a pivotal (nodal) point located in the vicinity of the mid-tidal zone (Fig.8b). Subsequent, higher modes account for a very low percentage of variance and therefore, they have been disregarded in the description of beachface change.

204

Mode I. SEGMENT r ~r i i i I

Mode 2 SEGMENT 1357 fllllll

Vorionce98'1%

Variance 1"5%

I

3

S

7

®

Mode 3.

SEGMENT I i

~

3 i

i

5 i

7 i

i

l[ Vori.... 03/~/e 0

-I

-I

I

96-1%

3"3%

-I

I

96.6%

®°/ -J

-I

:[

I

ft. 2.3%

®

:I

-I

91-9%

®

o\ -I

® il

94"9%

OI

4-~%

-I i

I

J

,

5

J

,

5

i

i

7

SEGMENT

i=1111t 1557 SEGMENT

i

I

i

,

S

i

i

5

i

1

7

SEGMENT

Fig.8.b. (See caption, p. 205).

The fundamental profile response explains greater than 90% of the variance for all profiles, ranging from 91.9% on profile 11 to 99.2% on profile 3. The explained variance

ranges from 96.1 to 99.2% along the rockwall and from 91.9 to 97.9% on the open beach. The amplitude of profile change increases with distance downslope, so that the greatest varia-

205

Mode

Mode

I.

Mode

2.

SEGMENT

SEGMENT I 3 5 7 i i i i i I i

I 3 5 7 = , ; = i i Voriance

3"1=/o

I 4'i%

o\

®

3.

s EGMENT f :5 5 7 = = i i i v =

I [ 0 " 7 " / o

0

-I

-I

I

i[

0.6=/.

2.3%

96.7%

\ -I

,'7 -I

I

(1.

97,1%

0.4%

2'4%

-,,,, -I I ~-

il

@

0.5%

I

I-7%

97.9%

I[

0'4°1o

® -I i

L

i

i

i

I

5 5 SEGMENT

i

i

7

L i i i i i i I 3 5 7 SEGMENT

i i i i i ~ i I

3 5 7 SEGMENT

Fig.8. a-c. Eigenfunction modes describing shore-normal sediment movement, Warilla Beach, 1975-1985.

tion occurs in the lower intertidal zone and the least in the upper beach segment. Amplitude spectra from time series associated with the eigenfunctions indicate that the central portion of the beach (profiles 7-10) is dominated

by an annual fluctuation in sediment volume; whereas the northern and southern thirds of the beach (profiles 1-6, and 12-18) experience biennial oscillations with amplitudes in excess of those for the annual cycle.

206 D i s c u s s i o n and c o n c l u s i o n s

Low-frequency movement of sediment The results of the EOF analysis facilitated description of characteristic patterns of shoreparallel and shore-normal movement of sediment on the beachface at Warilla Beach. The dominant patterns of sediment movement have been described separately, from this analysis. They were delineated by the eigenfunction modes, the amount of variance explained by each mode, and the peak periods in amplitude spectra from the series associated with each eigenfunction. However, beach changes also may be specified for each of the dominant lowfrequency periods determined in the analysis. In this respect several patterns of sediment movement (represented by the eigenfunction modes) combine to determine beach change over a particular period. At Warilla Beach the largest amplitude changes in sediment volume occur as biennial oscillations both along and across the beach. Sediment movement associated with the biennial period is largely explained by combining the fundamental beach response and the primary cellular-flux modes. The combination of the two eigenfunction modes, weighted to allow for the difference in their amplitudes, describes an onshore-offshore sediment transfer. The amplitude of the primary cellular-flux mode is approximately 35% of that for the fundamental beach response. Also the primary cellular flux mode leads the fundamental beach response by approximately 7 weeks (22°). Hence, there is a larger variance for the northern end of the beach than on the beach skirting the rockwall, and the variance increases with distance north of the rockwall. The annual cycle of beach change is approximated by a combination of the fundamental beach response and 30% of the secondary cellular-flux mode amplitude. Sediment exchanges indicated by the secondary cellularflux mode essentially involve an exchange of sediment between adjacent profiles on the

extreme northern and, separately, southern ends of the beach. Although the secondary cellular-flux mode leads the fundamental beach response by approximately 4 weeks (26°), it is a small amplitude fluctuation and is overwhelmed by the fundamental beach response. The net result is that the annual cycle is principally determined by the onshore-offshore movement of sediment, but with some lateral displacement at the beach ends. The sequence of net sediment movement involved in the 1.7 year cycle is mostly explained by the amplitude of the fundamental beach response plus 90~/o of the primary and 75% of the secondary cellular-flux mode amplitudes; with changes in the primary cellularflux mode leading and the secondary cellularflux mode lagging the fundamental beach response by approximately 26 weeks (108 ° ) and 9 weeks (37 °) respectively.

Low-frequency processes Reasons why the characteristic patterns of alongshore sediment movement vary down the beachface, and why they operate in different time scales is subject to ongoing investigation. Intuitively, beach changes may be explained by variation in onshore wind, including storm activity, and through this to variation in the wind-wave climate, storm surge and sea-level fluctuation. Major onshore wind events (storms), those capable of generating deepwater waves with amplitudes equal to or greater than 3.0m (Fig.3), correlate with known occurrences of beach erosion. Major erosional episodes in the beach-survey record are sometimes linked to high-intensity storms with low recurrence interval so that seasons of maximum beach erosion can coincide with times of low storm frequency. On other occasions, sequences of moderate-intensity storms are associated with substantial beach erosion. In contrast to the major erosional events, beach aggradation may continue through periods of frequent, low-intensity storm activity, such as that which occurred during 1976 and 1977, and again in 1980 and 1981 (Clarke

207

seasonal values of the I1/3 intensities for the period were spectrally analysed to identify periodic variations with recurrence intervals corresponding to those in the frequency band shared by the beach changes at Warilla. The dominant periodic components identified from the time series (Fig.9), include 3 years, 27 months and 21.6 months (Fig.10). The triennial and biennial components combine to account for approximately 32% of variance in the time series. Additionally, the 11/3 onshore-windintensity index is in phase with the monthly mean-sea-level record from Fort Denison, Syd-

and Eliot, 1985a, 1985b). Hence storm-frequency counts do not yield a reliable method to hindcast shoreline changes. A preliminary study of onshore-wind activity at Port Kembla, for the period 1965 to 1984, has been completed for the purposes of this study. Onshore-wind events listed by Clarke and Eliot (1985a, 1985b, 1985c) were grouped by seasons and the intensity of the upper-tercile (Ii/3) of the seasonal anomalies utilised to represent the record for that season. This has the effect of ignoring extremely large storms and the many low-intensity storms. The time series of

II 60-

Spring 1972

0

Autumn 1979

50,

403020. K) 0 -iO -20 -40, -50. -60. Autumn 1965 •

. 67

• 1965' 6 6

, 68

69

, 70

, 71

, 72

, 73

, 74

. 75

.

. 77

76

. 78

. 79

.

.

. 81

80

83

84

Yeors

40.

b

30. 20'

~o o - tO

-20 -30.

f

- 4 0 - Spring 1965

,

,

196.5

A u t u m n 198:3

,

66

,

67

,

68

,

69

,

70

,

71

,

72

,

73

,

74

,

75

,

76

i,

77

,

76

,

79

,

80

81

82

83

Years

Fig.9. Seasonal anomalies in the intensity of onshore wind events recorded at Port Kembla, 1965-1984. a The original time series, b Filtered time series, smoothed to remove high-frequency oscillations from the original time series. The series was derived from application of an A~A 3 filter (Godin, 1971) to the original time series.

208

9

~

8

~" p

~. 3. 2 I

Frequency Length of data = 216 months

Fig.10. A m p l i t u d e s p e c t r u m f r o m a F a s t F o u r i e r T r a n s f o r m

analysis of the A~A~ filtered time series.

ney, over corresponding periods. This, in turn, leads the net sediment movement on Warilla Beach by 1 month. Several processes are interposed between the atmospheric driving mechanisms and the beach responses. These include processes associated with storm surge, sea-level variation, large-scale, standing nearshore water circulation systems, and change in beach saturation associated with groundwater recharge from subaerial drainage. Not all of these processes are thoroughly understood because time-series matching the weather and beach-change records are not available for all process variables. Low-frequency beach changes at Warilla have been linked to corresponding fluctuations in coastal rainfall at Mount Keira and Wollongong, close to South Beach (Eliot and Clarke, 1982b) as well as with monthly mean-sea-level variations monitored at Fort Denison, Sydney (Eliot and Clarke, 1982a, 1982b). However, other processes are also considered to contribute markedly to the low-frequency beach changes. Wave-regime variations, which include highenergy swell as well as wind waves, and ripcurrent activity also contribute to sediment movement on the beachface. Time-series records describing wave-regime and rip-current activity have not been available for this study. However, the importance of rip currents is indicated by the eigenvector patterns describing sediment movement alongshore on the

beachface. Zones of high variability identified from the eigenvectors occur on the beachface landward of places where rips recurrently form (Eliot and Clarke, 1982a; Clarke and Eliot, 1983a). Two types of rip currents could contribute to the low-frequency beach changes. Large rip-currents, such as those described for storm conditions by Wright et al. (1978), and which occupy topographically fixed positions on the beach, may well account for seasonal and longer period beach changes by moving sediment into deep water; whereas smaller ripcurrents would have similar but lower amplitude effects on the lower beachface. In addition to the effects of waves and rip-currents, Lanyon et al. (1982) and Eliot and Clarke (1982a) have pointed out that long-term changes in beach groundwater levels, associated with regional precipitation, monthly mean-sea-level variation, and stormwater discharge from the hinterland, are also likely to trigger sediment exchanges between the subaerial beach and inshore zone in the same frequency band as the beach changes described from Warilla. Regardless of the driving mechanisms, the strength of the low-frequency signals from the survey record at Warilla is unequivocal. Amplitude spectra from time series associated with eigenfunctions describing the characteristic modes of sediment movement along and across the beachface consistently have periodicities of 2.0, 1.7 and 1.0 years. These beach changes are consistent with dominant period components identified from an independent analysis of onshore wind events affecting Warilla. Beach changes associated with the biennial oscillation dominate the beach-survey record. The strongest of the biennial changes involves an onshore-offshore movement of sediment. Its amplitude is generally 50% greater than the annual variation in beach sediment volume.

Concluding remarks Results reported from the EOF analysis of the 10-year survey record are consistent with those from the 5-year study reported by Eliot and

209 C l a r k e (1982a) a n d C l a r k e a n d E l i o t (1982, 1983a) although with some disparities. The importance of low-frequency beach changes identified in the earlier study was confirmed and extended in this study and similar patterns of alongshore and onshore-offshore movement of sediment were determined from both analyses. Differences between the two sets of results highlight beach responses that may be due to beach processes operating at different frequencies. I n p a r t i c u l a r , t h e p r o p o r t i o n o f v a r i a n c e explained by the fundamental beach response has increased significantly for the later study. In the 5-year study, the variance for this mode r a n g e d f r o m 52 t o 61~/o c o m p a r e d w i t h a r a n g e f r o m 74 t o 77%. T h e d i s p a r i t y w a s m o s t m a r k e d for the upper beach segment, where it altered f r o m 52 t o 7 4 % . I n b o t h i n s t a n c e s , t h e g r e a t e s t variation in the volume of sediment stored in the segment occurred on the northern half of the beach, away from the rockwall. The difference between the two sets of results indicate that short-term changes in beachface configuration are more closely tied to the alongshore movement of sediment than are the long-term changes. The long-term, low-frequency changes are largely described by an onshore-offshore sediment exchange between the beachface and the inshore zone.

Acknowledgements We are pleased to thank Shellharbour Mun i c i p a l C o u n c i l ; t h e E n g i n e e r , M r . A. Cluff; and the Surveyor, Mr. Phil Kendrick for their continued maintenance of the beach surveys s i n c e 1979. O u r a p p r e c i a t i o n is a l s o e x t e n d e d t o Denise Richards for assistance with data p r e p a r a t i o n a n d a n a l y s i s , a n d t o M r s . M. Bekle, for the cartography. We are grateful to the Coastal Council of N.S.W. for funding this phase of the data analysis.

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