Regional variation of coastal morphology in southwestern Australia: a synthesis

Geomorphology 34 Ž2000. 73–88 www.elsevier.nlrlocatergeomorph

Regional variation of coastal morphology in southwestern Australia: a synthesis P.G. Sanderson a,) , I. Eliot b,1, B. Hegge c,2 , S. Maxwell b,1 a

Department of Geography, National UniÕersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Department of Geography, UniÕersity of Western Australia, Nedlands 6907, Western Australia, Australia c DA Lord and Associates, 97 Broadway, PO Box 3172, LPO Broadway, Nedlands 6009, West Australia, Australia Received 12 April 1999; received in revised form 26 November 1999; accepted 1 December 1999

Abstract The morphology of landforms on the south coast of Western Australia is determined predominantly by wave refraction around discrete headlands and islands. Wherever offshore structures protect the shore from the direct effects of swell, sheltered sandy beaches have developed in association with cuspate forelands and tombolos. In contrast to this open coast setting, the nearshore waters of the west coast are protected by semi-continuous reef systems, which significantly modify the morphology of large-scale accretionary landforms, beaches and foredune sequences. On the south coast, foredune plains occur primarily as fill in sheltered embayments and storm built ridges do not occur. Foredunes on the west-coast include washover ridges and low aeolian dunes on the backshore of embayment and inset beaches. The form of high wave-energy beaches of the south coast fluctuates between reflective and dissipative morphodynamic states, and most commonly between transitional and dissipative states. Sediments are mainly fine grained siliceous sands. In contrast, the low-energy west-coast beaches are composed of medium to coarse grained, calcareous sands. The beaches are planar in section, characterised by lines of debris deposited by tidal and longer-term fluctuations in sea-level and their form does not alter with short-term changes in the wave regime. Despite the very low energy micro-tidal conditions experienced by the coasts of southwestern Australia, systematic variation in the morphology of coastal landforms does occur. As protection to the coast increases from the open-fetch south-coast environment to the reef-protected west-coast setting, swell energy decreases, there is an increase in the relative importance of locally generated wind waves, wave set-up and tidal forcing of currents, and forelands become increasingly asymmetric due to the strength of longshore sediment transport. q 2000 Elsevier Science B.V. All rights reserved. Keywords: coastal environment; lagoonal environment; fringing reefs, coastal dunes; coastal landform evolution

)

Corresponding author. Tel.: q65-874-6101; fax: q65-7773091. E-mail addresses: [email protected] ŽP.G. Sanderson., [email protected] ŽI. Eliot., [email protected] ŽB. Hegge.. 1 Tel.: q61-8-9380-2706; fax: q61-8-9380-1054. 2 Tel.: q61-8-9389-9669; fax: q61-8-9389-9660.

1. Introduction Variation in landform pattern and process underpins regional differences in coastal management practice. This is particularly true in Western Australia, however such descriptions are not complete in

0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 9 . 0 0 1 3 2 - 4

74

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

a scientific context. Extension and integration of the knowledge of coastal morphodynamics can help in the development of an understanding of large-scale coastal behaviour. Carter and Woodroffe Ž1994. and List and Terwindt Ž1995. noted that consideration of the behaviour of large-scale coastal systems may lead to greater confidence in prediction of changes occurring within systems for which there is an integrated understanding of the underlying geology, sediment supply and nearshore geomorphologic processes. This paper contributes to the knowledge of Western Australian coastal systems through a review of recent investigations of large-scale accretionary landforms, foredune sequences and sandy beach morphology. It examines the association of particular accretionary landforms with low energy environments along the south and west coasts. The morphodynamic approach to the study of coastal environments ŽWright and Thom, 1977; Cowell and Thom, 1994. fits within the broad framework of littoral cell-scale research. It relies on predictability along certain environmental gradients, such as tidal range, wave exposure and sediment type and provides an integrative approach to understanding coastal behaviour at a broad scale. It facilitates the development of coastal ‘models’ that link observed geomorphology to a variety of physical or biological phenomena. Some morphodynamic models link morphology on sandy beaches with coastal process dynamics dominated by moderate to high wave energy ŽWright and Short, 1984; Sunamura, 1989; Lippman and Holman, 1990. and on beaches with high tidal ranges ŽWright et al., 1982; Short, 1991; Masselink and Short, 1993.. The beach stage models of Wright and Short Ž1984. provide a link between beach morphodynamics and regional environmental conditions. Their research has focused, however, on wave-dominated beaches where energy levels are high. Nordstrom Ž1977, 1992., Owens Ž1977. and Nordstrom and Jackson Ž1992. have undertaken detailed research on the morphodynamics of low-wave energy, sheltered coastal environments and Jackson et al. Žin review. have reviewed the current state of knowledge of low-energy beaches. Their observations demonstrate that beaches that experience low-wave energy conditions respond very differently to changing wave conditions from beaches exposed to high-wave energy. Modes of foredune or

beach-ridge plain formation have been examined by a great many researchers since the seminal work of Johnson Ž1919., and particularly since the 1950s. More recent reviews include those of Davies Ž1957., McKenzie Ž1958., Psuty Ž1967., Bird Ž1969; 1976., Hesp Ž1984a,b., and Taylor and Stone Ž1996.. Much of the Western Australian literature dealing with broad scale coastal evolution, including morphological and process studies, focuses on individual landforms Že.g., Semeniuk and Johnson, 1982; Semeniuk et al., 1988; Woods, 1983; Searle and Semeniuk, 1985; Searle et al., 1988., small regions ŽSearle, 1984; Semeniuk, 1997; Semeniuk and Searle, 1986; Collins, 1988; Hamilton and Collins, 1988., or the distribution of particular landforms. More recent investigations of these distributions include identification of sandy beach morphotypes from sheltered coastal environments ŽHegge, 1994; Hegge et al., 1996., description of foredune and beach ridge sequences on the southwestern coast of Western Australia ŽMaxwell, 1995; Sanderson et al., 1998., and examination of regional variability in depositional landform type and associated nearshore hydrodynamics on the central-west and Ningaloo coasts of Western Australia ŽSanderson, 1997; Sanderson and Eliot, 1996.. Sandy beaches along much of the coast of southwestern Australia experience micro-tidal conditions Ž- 2 m. and low-wave energy Ž- 1.0 m significant wave height..

2. Regional environment The coast of southwestern Australia ŽFig. 1. extending from Cape Arid on the southern coast to North West Cape on the west coast has been divided into a number of regions, based on similarities in geological structure, sediment supply, exposure to waves, tidal influence and resultant nearshore wave and current processes. Sanderson and Eliot Ž1996. provided a preliminary classification of the southwestern Australian coast on the basis of coastal sedimentary landform occurrence and dimensions. They concluded that the south coast, from Cape Arid to Cape Leeuwin, was significantly different from the central-west coast ŽMandurah to Geraldton. and the Ningaloo coast ŽGnarraloo Bay to North West Cape.. Relationships between the morphometry of

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

75

Fig. 1. The location of the south, central-west and Ningaloo coastal regions of Western Australian.

the sedimentary landforms and the size and distance offshore of obstacles, such as islands and reef plat-

forms behind which cuspate forelands have developed, vary. On the south coast, accumulation forms

76

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

develop behind granitic outcrops due to the diffraction of swell. However, the reef chain flanking the central-west and Ningaloo coasts leads to complex wave interaction patterns inside the lagoon basins and a less clear relationship between landform geometry and the degree of reef protection. Sanderson Ž1997. extended this regional differentiation by showing that the central-west coast and Ningaloo coast were also significantly different in terms of nearshore processes, tidal influence and landform development although both are flanked by an offshore protective reef chain. 2.1. Physical setting From Cape Leeuwin to Cape Arid, the coast is open to the Southern Ocean. The continental shelf is part of the Great Australian Bight shelf and is approximately 25–30 km in width ŽHarris et al., 1991.. There are few offshore islands or submerged features that interact with swell waves. Sediments can be transported, almost unimpeded, to and from the coast. An exception to this is near Esperance, where the Archipelago of the Recherche offers some protection to the coast. In general, the shoreline is characterised by rocky headlands, mainland coastal sand barriers and transgressive onshore dune systems, while rock platforms occur irregularly along the exposed sections of embayments ŽWoods et al., 1985.. Granitic headlands control much of the landform configuration, particularly in southeasterly facing crenulate-shaped bays. Beaches of the region are largely composed of quartzose sand, which tends to become increasingly finer grained towards the east ŽHodgkin and Clarke, 1990.. The southern part of the continental shelf of the west coast includes the Rottnest Shelf, which varies in width between 43 and 93 km. The coast is substantially protected by offshore reefs and island chains. The reef system extends along the centralwest coast from Cape Naturaliste to south of the Houtman Abrolhos. It is comprised of a seaward sloping platform from which submerged ridges and lithified barrier islands rise to a maximum height of 60 m ŽCollins, 1988.. The shore parallel ridges, including the Pleistocene-age Direction and Pelseart Banks ŽHarris et al., 1991. occur semi-continuously over its entire length but are undergoing erosion and

collapse as a result of continued wave activity. The rapid sea-level transgression between 17,000 and 6000 years BP led to the mobilization and removal of floodplain deposits, which had been delivered to the subaerially exposed shelf by fluvial processes ŽCollins, 1988.. Sediment supply to the coast is now restricted to nearshore areas where seagrass banks up to 10-m thick occur, and along coastlines protected by the submerged reefs. In the northern part of the west coast, the continental shelf is extremely varied in width. It broadens to a maximum width of 170 km offshore from Shark Bay. Further north, it is very narrow, particularly in the vicinity of the North West Cape where the 1000 m isobath is 20 km offshore Žan average slope of 2.88.. North West Cape is flanked by the Ningaloo Reef; a modern fringing coral reef. Sediment transport along the continental shelf adjacent to the North West Cape is minimal and sediment supply to the coastline is dominated by the contribution of biogenic material from the reef. The reef encloses a shallow sedimentary lagoon that contains patch and nearshore reef platforms. The shoreline is characterised by Pleistocene limestone outcrops interspersed with areas of Holocene sediment accumulation. These occur mainly as deposits of calcareous sands and gravels ŽWyrwoll, 1990.. 2.2. Climate and coastal processes Three oceanographic processes are of importance in controlling the morphology of depositional landforms, sandy beaches and foredune plains in Southwestern Australia. These are tides, waves and lowfrequency fluctuations in sea-level. The importance of the ‘relative’ rather than the ‘absolute’ amplitudes of these processes has been the focus of recent research into nearshore and coastal morphology ŽDavis, 1991; Davis and Hayes, 1984; Masselink and Short, 1993; Hegge et al., 1996; Jackson et al., in review.. Differences in the relative contribution of each of these processes underlie regional differences in the development of similar landforms on the coast of Western Australia. Prevailing winds on the south coast in summer are southwesterly to southeasterly with wind speeds of over 30 km hy1 being experienced 30% of the time. On the central-west coast, the winds are dominated

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

in summer by southerly to southwesterly conditions with speeds of over 30 km hy1 being experienced 37% of the time. During winter, strong southerly winds on the south coast and rapidly shifting northwesterly to westerly winds on the central-west coast are associated with mid-latitude low-pressure systems. The Ningaloo coast experiences quiescent weather conditions in early winter, but in spring and summer, strong easterly to southwesterly winds are experienced. Winds of over 30 km hy1 occur on over 70% of days. Strong sea breezes, which are frequently ) 10 msy1 ŽMasselink, 1996., are experienced in summer on the south and central-west coasts and in spring and summer on the Ningaloo coast. Occasional tropical cyclones bring gale force winds to the Ningaloo coast region. From 1907 to 1993, 79 tropical cyclones passed through the 58 latituderlongitude grid including Exmouth. In contrast, tropical cyclones occur only once every 10 years on the central-west coast and have not been recorded on the south coast. The offshore wave climate is dominated by a persistent, low to moderate-energy wave regime, characterised by south to southwesterly swell. The mean annual deep-water wave height is 2–3 m, with a period of 10–14 seconds ŽLemm et al., 1999.. Superimposed on the offshore swell regime are waves generated locally by sea breezes, mid-latitude depressions and occasional tropical cyclones. Closer to shore, and particularly leeward of the reef chains of the central-west and Ningaloo coasts, the inshore wave energy is attenuated by up to 90% through shoaling, breaking, refraction, diffraction and reflection of incident waves ŽHegge et al., 1996.. Breaking wave heights may be as little as 10 cm ŽHegge, 1994.. Following tidal nomenclature described by Davies Ž1980., the central-west and south coasts are microtidal and experience mixed, mainly diurnal tides ŽDepartment of Defence, 1996.. Spring tidal range ŽMLLW to MHHW. from Geraldton to Albany is less than 0.5 m and rises to 0.7 m at Esperance in the east. Other processes contributing to sea-level variations may equal or exceed tidal and wave amplitudes. For example, barometric effects result in water levels ranging over 0.5 m with the successive passage of anticyclonic and mid-latitude cyclonic conditions. In some localities, storm surge is amplified by

77

local topography and may be in the order of 1–2 m. These non-tidal sea-level fluctuations often play the most significant role in shaping the beach morphology along much of the southwestern coast of Australia. Tides on the Ningaloo Reef are also micro-tidal however the area is transitional between the small, predominantly diurnal tides of the central-west coast, and the large semi-diurnal tides of the North West Shelf. The spring tidal range at Exmouth is 1.8 m. Currents regulated by wave pumping across the reef crest and tidal variability are significant within the Ningaloo reef lagoon ŽSanderson, 1997..

3. Landform diversity Depositional landforms of the southwestern Australian coast have been described in a review of the distribution, type and geometry of cuspate forelands and tombolos on the south, central-west and Ningaloo coasts ŽSanderson and Eliot, 1996.. There is significant disparity in morphology between the landforms of the south and west coasts. On the south coast where little protection is offered to the shoreline, the development of accumulation forms is directly linked to the occurrence of offshore granitic islands, and changes in nearshore bathymetry. Forelands and tombolos form where the refraction and diffraction of incident swell around the offshore islands result in opposing longshore currents and thus sediment deposition. The geometric relationships between the size and form of the foreland or tombolo, and the size and distance offshore of the island or obstacle, are similar to those described by Silvester and Hsu Ž1993.. In contrast to the south coast, protection offered by reef chains on the west coast results in complex interaction of swell waves, wind waves and longshore currents. This causes sediments to accumulate in areas that are not directly landward of the offshore reef but skewed to the direction of longshore transport or tidal flow, as is the case at Ningaloo. On the central-west coast, cuspate forelands and tombolos typically form behind offshore islands, patch reefs, and limestone headlands. Some of the accumulation forms of the central-west coast have dimensions comparable to those on the south coast. Many cuspate forelands have basal widths of ) 200 m and

78

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

extend offshore up to 100 m. On the Ningaloo coast ŽFig. 1., some very large cuspate forelands have formed at Jurabi Point Ž4 km basal width., Turquoise Bay Ž800 m basal width. and Winderabandi Point Ž2 km basal width. ŽSanderson, 1997.. At these places, a reduction of lagoonal current energy occurs due to a balance of wave diffraction through gaps in the reef crest, tidal currents and wave-pumped flow of water exiting the lagoon. The past and future development of the forelands on the west coast may, however, be considered in the context of their reef settings, and this provides some insight into the longer-term constraints on morphological evolution. The central-west coast has a degrading reef system and in the past it is likely to have been more extensive, with fewer gaps and higher elevation. As a result of ongoing wave attack and erosion in the future, the reef system will become less extensive, with more gaps and of lower elevation. Shoreline configuration and zones of sediment deposition respond quite rapidly to changing inshore wave regimes as a result of alteration in the reef topography. Variability in weather patterns and long-term changes in storminess are also important in the consideration of factors driving coastal evolution on the central-west coast. On the Ningaloo coast, the fringing reef is alive and growing. In the past, the reef would have been less extensive and in the future the reef will continue to provide substantial shelter to the coastline from the offshore swell. As such, variability in shoreline configuration may be more closely related to the impact of extreme events and the geological setting at each site, rather than to any change in wave regime. The growth of the reef system along the Ningaloo coast over the Holocene and the associated increase in mobile marine sediments is also related directly to the size of forelands found in this region.

4. Foredune sequences Foredune plain sequences are commonly found on the coast of Western Australia ŽMaxwell, 1995. both in association with tombolos and cuspate forelands, as well as in more isolated embayments and insets. Many of the foredune plains have continued to accrete to the present, which is apparently unusual in

Australian coastal environments ŽThom, 1964; Hesp, 1984a,b.. The highest proportion of foredune plains is found on the central-west coast between Lancelin and Dongara ŽFig. 2., and progradation of the coast over the past 40–50 years has occurred at up to 4.5 m yeary1 . This reach of coast apparently is a depositional site for sediments being transported northwards along the west coast of Western Australia. Depositional foredune sequences also are found on the south coast however they are less common. Generally, foredune plains are established in three main geographical locations ŽTable 1.: 1. embayments — where sediment is transported by longshore drift and accumulates in a bay, 2. insets — these are minor coastal sediment accumulations Žtwo to five dune ridges., and are prevalent along the west coast where there is adequate sediment supply, but reduced wind and wave activity, and 3. cuspate forelands — foredune ridges may be found on both prominent and smaller forelands that are influenced by diffraction of waves around an offshore islands or shallow reef outcrop. Present day erosion of the foredune sequences is restricted to the south facing coasts of forelands and similar geographic settings where the coast is exposed to stronger currents, winds and wave activity. The location of the foredune plains is closely related to nearshore and coastal geomorphology as well as sediment supply and coastal processes such as wave and longshore current activity. On the south coast, foredune sequences are most commonly found in embayments, and two small fordune plain insets are identified. They are not found on the flanks of forelands or tombolos. Most commonly, foredune plains on the south coast are stable and progradation over the period for which aerial photographs are available is recorded in only two embayments. Sediment supply, essential to the development of these landforms, is generally limited to erosion of coastal cliffs and the eastward transport of littoral sediments. On the central-west coast, foredune plain insets have formed along essentially linear coast in the lee of islands or offshore rock outcrops. In some cases, such as at Lancelin, Cervantes and Jurien ŽFig. 1., foredune sequences provide an accurate depositional

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

79

Fig. 2. Location of foredune plain sequences and beach morphotypes in southwestern Australia.

history of large-scale cuspate foreland development. Prograding foredune plains are commonly found on north-facing flanks of cuspate forelands and as small insets. Sediments, which are transported northwards on the oceanward side of the offshore reef chain, are propelled landward through breaks in the reef crest. They accumulate in areas protected from the prevailing southwesterly swell and wind wave activity. As such, eroding foredune plains generally occur only on south facing flanks of forelands on the west coast. It is noted that the focus for sedimentation at the coast and the subsequent development of foredune

sequences responds rapidly to changes in the direction of wave approach resulting from collapse or erosion of the offshore reef. For example, it was reported ŽSanderson, 1997. that rapid accretion of the foredune plain on the northern flanks of the large cuspate foreland at Jurien took place following collapse of a section of offshore reef during storm conditions in 1978. Foredune plains are also found on the Ningaloo coast, although they have not been studied at the regional scale. Sanderson Ž1997. reported that foredune sequences could be found in the north-facing

80

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

Table 1 The occurrence of the various types of foredune plain settings and associated shoreline dynamics Location and stability of foredune plains

Forelands

Embayments

Insets

Prograding foredune plains Ž) 1 m year.; Central West Coast Prograding foredune plains Ž) 1 m year.; South Coast Stable foredune plains; Central West Coast Stable foredune plains; South Coast Eroding foredune plains Ž) 1 m year.; Central West Coast Eroding foredune plains Ž) 1 m year.; South Coast

Common Že.g., Cervantes, Rockingham, Jurien. Absent Absent

Limited occurrence Že.g., Coolimba. Two foredune plains reported Common Že.g., Green Head.

Absent

Common Že.g., Albany.

Common on south facing flanks Že.g., Cervantes, Jurien. Absent

Absent

Common Že.g., Illawong, Gum Tree Bay. One foredune plain reported Common Že.g., North of Jurien, Alkimos. One foredune plain reported Absent

embayments of large cuspate forelands such as Winderabandi Point and Turquoise Bay. Their mode of formation has not been determined, but it is anticipated that raised sea-levels during tropical cyclones are likely to play a significant role in the deposition of sedimentary material above spring high tide levels, and thus in the initiation of dune building. The role of sediment supply and inherited geologic structure is reflected in the rate of progradation of foredune plains and in their location. Where sediment supply is plentiful and protection is provided to the coast by reef chains, headlands or islands, sediments may accumulate and be transported onshore. This takes place during storm surge activity on the central-west coast. It leads to formation of 2 to 3 m high washover ridges such as those described from Coolimba and Cervantes ŽMaxwell, 1995; Sanderson et al., 1998. being built as composite features by storm wave action. On the south coast, aeolian transport of beachface sediments contributes to formation of high foredune structures while storm-built ridges do not occur.

5. Sandy beaches In high-energy coastal environments, sandy beaches undergo rapid erosion and accretion in re-

One foredune plain reported

Absent

sponse to the onset and passage of storm conditions. This quasi-cyclic pattern of change may take place over periods of days to weeks and this phenomenon may be observed on some of the more exposed areas of the southern coast of Western Australian ŽEliot and Clarke, 1986.. In the low-energy environments that characterise much of the west coast and protected areas of the south coast, the response of beach morphology to changing energy inputs is of much lower frequency. The modal wave conditions prevailing on sheltered beaches are commonly insufficient to facilitate full beach recovery between periods of storm activity. Relict or inherited morphology is therefore often an important feature of low-energy environments, reflecting the importance of antecedent conditions in the evolution of the beach face. Hegge Ž1994. made an empirical classification of beach geometry using data describing 24 variables associated with 39 beaches from Southwestern Australia ŽFig. 2.. Canonical variate analysis techniques were used to distinguish a range of low-energy beach types on the basis of sedimentological characteristics, morphology and slope, as well as nearshore wave height and longshore currents. Beach types were most strongly differentiated by morphology and sediment characteristics ŽTable 2.. There was a notable lack of a strong relationship between beach morphology and nearshore dynamics suggesting that factors other than modal energy conditions may be more important in shaping the beach face.

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

81

Table 2 Location and characteristics of the six beach morphotypes around the coast of southwestern Australia Beach morphotypes

Profile characteristics

Sediment characteristics

Location

Concave

steep foreshore and swash zone, relatively flat inshore zone, uniform decrease in curvature with distance offshore, beaches are small, narrow beachfaces Ž- 10 m., swash width - 5 m, moderate size step may be present steep and linear beachface slope, steep inshore zone, large overall dimensions, distance from berm crest to primary breakers) 40 m, large overall dimensions

wide range of sediment charac teristics, poorly sorted to very well sorted, mean grain-size 0.26 mm, permeability 0.007 cm3 sy1

significant degree of protection, located either on the West Coast and sheltered by offshore reef chains, or on the east facing leeward side of Rottnest and Garden Islands

sands are moderately well sorted, mean grain-size 0.56 mm, permeability 0.018 cm3 sy1 — Žmost permeable.

generally found on the West Coast, where beaches have a westerly aspect ŽFishery Beach and Salmon Beach are found on the South Coast., more exposed than the concave beaches southeasterly aspect and are commonly located on the South Coast, level of protection is generally low Žalthough this is not always the case.

Steep

Flat

Moderately concave

Moderately steep

Stepped

broad, flat nearshore zone, wide foreshore, swash and surf zone, flattest swash and inshore zones, generally uniform profile, no stepped beaches, large dimensions similar to concave beaches– nearshore slope and concavity less however, nearshore dimensions small, swash zones - 10 m, surf zone widths- 15 m steep linear nearshore zones, wide beach face Ž15–25 m., considerably high berm, wide swash zone Žaverage 10 m. very narrow nearshore profiles, relatively steep beachfaces, presence of a very large sub-tidal step beyond the beachface

Overall, the morphologic associations between the six beach morphotypes may be conceptualized in terms of their dimension and slope ŽFig. 3.. The flat and steep beaches had comparatively large dimensions but very different slopes. In all cases, these two morphotypes consistent with high-energy beach classifications were distinguished from the lower energy forms and their dynamics were dissimilar from the other beaches. The flat and steep beaches, respectively, correspond in form with dissipative and reflective morphotypes identified from high-energy environments by Wright and Short Ž1984.. The four remaining groups ŽFig. 3. are all low energy forms. The concave and moderately concave beaches have similar profile dimensions but may be distinguished

finest sediments, least permeable sediments, very well sorted, mean grain-size 0.18 mm, permeability 0.005 cm3 sy1

moderately well sorted, homogenous sediments, mean grain-size 0.26 mm, permeability 0.005 cm3 sy1 moderately well sorted, mean grain-size 0.35 mm, permeability 0.01 cm3 sy1 well sorted sediments, mean grain-size 0.36 mm, permeability 0.014 cm3 sy1

predominantly located on the West Coast or on the leeward side of Garden and Rottnest Islands Žexceptions Cheyne and Hopetoun beaches. found on the West Coast, have westerly aspects and are more exposed than the concave beaches these are found in locations where there is a moderate level of protection, regardless of aspect

by their steepness. The dimensions of the moderately steep and stepped beaches are only slightly smaller than the concave and moderately concave beaches. Sediments are coarsest on steep beaches and finest on flat beaches. The beach locations represent a range of aspects and exposures ŽFig. 4.. This information was determined from hydrographic charts. Aspect was denoted by the direction a beach faced. Due to the orientation of the southwestern Australian coast, westerly facing beaches were most commonly sampled. The level of protection offered to a beach was denoted by a rating of 0 to 4, ranging from fully protected to fully exposed conditions. The beaches may thus be considered as fetch-limited or open-fetch settings and

82

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

Fig. 3. Characteristic profiles of the beach morphotypes identified for the sheltered coastal environment of southwestern Australia.

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

83

Fig. 4. Beach shelter attributed to aspect and degree of protection in southwestern Australia. Protected beaches are located towards the centre of the shelter plot. Aspect is indicated by compass bearing. Symbols denote the morphotypes of each beach.

their morphologies and dynamics in this context. While relationships between beach morphotypes and the aspect of beaches are not clear, the significance of the degree of protection is more apparent. The flat, energy dissipative beaches are found primarily on south-facing coasts and are offered little or no protection Žopen-fetch conditions.. Leighton is the only flat beach found on the west coast and notably it is exposed to higher than average incident

wave energy due to the diffraction of swell through an adjacent break in the offshore reef system. Steep beaches may be found in both fetch-limited Žhigh protection. and open-fetch Žlow protection. settings. All of the steep beaches are west facing and are open to wave approach through breaks in the west coast reefs or through diffraction of wave energy around headlands along the south coast. The low energy beach morphotypes characterized by concave, mod-

84

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

erately concave and moderately steep profiles are generally afforded high to moderate protection Žfetch-limited. and most are west facing. Concave beaches are found along the low-energy east coast of Garden Island and in areas where reef protection dampens incident swell energy. Except for Cheyne Beach on the south coast, which is northeasterly facing and so protected from southwesterly swell and southeasterly wind wave activity, moderately concave beaches are found in fetch-limited settings of Geographe Bay and the Perth coastline. Moderately steep beaches have a northwesterly aspect. They are thus exposed periodically to the northwesterly storm waves generated by the winds associated with the passage of mid-latitude depressions and intercyclonic fronts. Stepped beaches are found around Rottnest Island where the local reef configuration is complex. Protection is high to moderate and incident wave energy is significantly attenuated.

6. Discussion The diversity of structural settings, wave climates and current patterns on the west coast of Western Australia, exemplified by the study of Sanderson Ž1997. of cuspate foreland development on the central-west and Ningaloo coasts suggests a relationship between geologic setting, degree of protection and structure of offshore reef, significance of locally generated sea, modal surface current velocities, and residual currents ŽFig. 5.. The calcarenite coral reefs and topographic irregularities of the inner continental shelf on the west coast provided the framework for sedimentation during the Holocene transgression, and the principal differences between the morphology of large-scale cuspate forelands is attributed to the dimensions of inshore lagoons, the porosity of the offshore reef structure to swell, the wave climate and hydrodynamics within the lagoons. In fringing reef settings in other parts of the world, wave set-up driven currents are suggested to be most significant in determining water levels on coral reefs. The flow, which is induced in the reef lagoon as a result of the wave set-up across the reef crest, is the main driving force of circulation in Caribbean reefs ŽGourlay, 1996; Kirugara et al., 1998; Yamano et al., 1998.. However, where tidal

range is large and where lagoon widths are greater, tides are thought to be the main driving force of circulation ŽYamano et al., 1998.. Additionally, where there is little restriction to flow in and out of the lagoon, tidal forcing will be a significant moderator of water levels within the lagoon. On the centralwest coast of Western Australia, the deeper reef crest, wider and deeper lagoon, and perforate nature of the offshore barrier means that wave pumping across the reef crest would not be significant. Tide ranges are small Ž- 0.5 m. and wind-driven circulation becomes important in this context. Wind has not been given much consideration in some environments ŽYamano et al., 1998. as the reefs are found in trade wind areas where wind conditions and induced ocean swells are generally constant. However, where wind conditions vary seasonally, the relative importance of tidal, wave set-up and wind-driven forces of circulation will also vary. Flow in the Ningaloo reef lagoon, where entrance size is restricted, is driven by a combination of tidal, wave set-up and wind forcing ŽHearn and Parker, 1988.. Wave pumping across the main reef drives non-tidal currents, but current speeds are strongly tidally modulated. However, under high swell conditions, the modulation is not total and the outflowing current rarely reverses. In the wider lagoon areas or under very low swell conditions, Hearn and Parker Ž1988. report that true tidal flushing may be identified. Wind forcing may act to enhance the flushing of the nearshore region and produce mixing between lagoonal systems. Thus, as the lagoonal basins landward of the west coast reef systems become narrower and shallower and the degree of protection increases, there is a diminution in swell wave energy, an increase in the significance of residual currents, and an increase in the relative importance of wind waves, particularly those generated by southwesterly sea breezes. Wave set-up and tidal forcing of flow in the lagoons is increasingly significant. Landforms such as cuspate forelands and tombolos become systematically asymmetric due to the strength of longshore sediment transport and prevailing southwesterly wind conditions. The foredune sequences found on the central-west and south coasts have morphologies, which are closely related to the sediment availability, variation

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

Fig. 5. Variability in nearshore geometry and hydrodynamic processes on the central west and Ningaloo coasts of Western Australia.

85

86

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

in local wind wave and swell patterns, and the presence of colonising vegetation. Sediment supply and accumulation must be abundant for formation of foredune ridges. On the central-west coast, sediment is transported to the sink areas Žembayments, sheltered locations. via the northerly longshore drift. This is driven along both the shore and the offshore reef chains by the southwesterly swell. Sediment moves onshore and becomes trapped in embayments, in small coastal incisions, or in the lee of islands or reef structures. On the central-west coast, the sediment is then transported to the back of the beach through washover processes during storm events, where subsequent vegetation colonisation occurs, assisting the formation of successive ridges. On the south coast, sediments trapped in embayments are transported onshore during quiescent conditions Žbeach berm formation. and then blown landward. Aeolian processes are apparently more significant in the transport of sediment and formation of foredune ridges on the south coast than on the central-west coast. Local variation in wave conditions and nearshore currents may be linked with progradation and accretion of the various foredune sequences. The alignment of a considerable portion of the foredune plains found on both the central-west and south coasts reflects the local swell regime. There is a wide spectrum of beach morphotypes in the region reflecting aspects of coastal sheltering and the effects of modal low wave energies. Beaches in the areas exposed to moderate to high levels of open-ocean wave activity are similar to morphotypes described by Wright and Short Ž1984.. Beaches in the lee of offshore reefs are closer in form to estuarine beaches described by Nordstrom Ž1992. and Nordstrom and Jackson Ž1992.. These beaches are narrow, planar in beachface cross-section, marked by lines of debris stranded by sea-level fluctuations and in some instances, stepped at the transition from the beachface to the inshore zone. Profile morphology is significantly narrower than that on exposed, open-ocean coasts in the region. For example, the width of the active shoreface is less than 50 m in sheltered areas, commonly found on the central-west and Ningaloo coasts, compared with the 500-m wide active zones in exposed waters on the south coast. Low tide terraces do not form on the beaches of the west coast region.

The sheltered beach types from the low energy sheltered environments exhibit a wide range of profile shapes and concavities that cannot be adequately described by a single, reflective beach state Žcf. Wright and Short, 1984.. Instead, the low energy beaches may be considered in terms of their aspect and the degree of protection offered to the shore by submerged reefs, rocky platforms and outcropping headlands and islands. The open-fetch south coast environment is characterized by flat, dissipative beaches while low-energy beach morphotypes are found in fetch-limited west coast settings. The lack of a strong association of low-energy beach type with nearshore dynamics indicates the modal low-energy processes that prevail on sheltered beaches do not markedly affect the overall profile. The beach morphology is related more closely to significant storm events and sea breeze effects that modify the coastline, leaving a morphology that is inherited and persists through the more quiescent wave and tidal conditions at a wide range of temporal scales, from a few hours ŽHegge, 1994. to several thousand years ŽShepherd and Eliot, 1995; Sanderson et al., 1998.. 7. Conclusion The summary of coastal morphologies from southwestern Australia indicates that processes such as storm surge, wave diffraction through breaks in the offshore reef chain on the west coast, and longperiod water level variations play significant roles in the morphological evolution of sandy beaches and depositional landforms. The south coast is characterised by markedly different depositional sequences and beach morphologies to those found on the sheltered west coast. The form of accretionary forelands and tombolos on the coast between Cape Leeuwin and Cape Arid is predominantly determined by wave refraction and diffraction processes, with little contribution from nearshore currents; beaches are similar to those described by Wright and Short Ž1984. from wave-dominated environments; and foredune plains that occur along the coast result mainly from aeolian deposition and transport. This contrasts markedly with the reef-protected west coast. There, the form of forelands varies with the degree of reef protection; with the form of cuspate forelands in narrow, shallow lagoonal waters determined by a combination of

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

wave diffraction and tidal, wave set-up and wind driven current processes. The beaches are low-energy forms, similar to those described from estuarine and bay beaches by Nordstrom Ž1992., and beach ridges formed by deposition during washover events are found in the most sheltered environments.

Acknowledgements The Department of Geography at the University of Western Australia supported much of the research presented here. The first and third authors, respectively, examined landform variability and sandy beach morphotypes as part of their doctoral research. Their separate studies were funded by an Australian Postgraduate Research Award and a University of Western Australia University Research Studentship. A large number of people have provided technical assistance, support in the field and cartographic work throughout the various phases of the research presented. We would like to thank all concerned. Mrs. Chong Mui Gek and Mrs. Lee Li Kheng completed the cartography.

References Bird, E.C.F., 1969. Coasts. Australian National University Press, Canberra. Bird, E.C.F., 1976. Shoreline Changes during the Past Century. I.G.U. Working Group on the Dynamics of Shoreline Erosion, Department of Geography, University of Melbourne, Melbourne, Australia. Carter, R.W.G., Woodroffe, C.D., 1994. Coastal evolution: an introduction. In: Carter, R.W.G., Woodroffe, C.D. ŽEds.., Coastal Evolution: Late Quaternary Shoreline Morphodynamics. Cambridge Univ. Press, Cambridge, pp. 1–31. Collins, L.B., 1988. Sediments and history of the Rottnest Shelf, Southwestern Australia: a swell-dominated, non-tropical carbonate margin. Sedimentary Petrology 60, 15–49. Cowell, P.J., Thom, B.G., 1994. Morphodynamics of coastal evolution. In: Carter, R.W.G., Woodroffe, C.D. ŽEds.., Coastal Evolution: Late Quaternary Shoreline Morphodynamics. Cambridge Univ. Press, Cambridge, pp. 33–86. Davies, J.L., 1957. The importance of cut and fill in the development of sand beach ridges. Australian Journal of Science 20, 105–111. Davies, J.L., 1980. Geographical Variation in Coastal Development. 2nd edn. Longman, London. Davis, R.A. Jr., 1991. Morphodynamics of wave and tide-

87

dominated coasts. In: Proceedings of the 10th Australasian Conference on Coastal and Ocean Engineering, Auckland ŽNew Zealand.. pp. 19–22. Davis, R.A., Hayes, M.O., 1984. What is a wave dominated coast? Marine Geology 60, 313–329. Department of Defence, 1996. Australian national tide tables 1996, Australian Hydrographic Publication 11. Department of Defence, Commonwealth of Australia. Eliot, I.G., Clarke, D.J., 1986. Minor storm impact on the beachface of a sheltered sandy beach. Marine Geology 73, 61–83. Gourlay, M.R., 1996. Wave set-up on coral reefs: 1. Set-up and wave-generated flow on an idealized two dimensional horizontal reef. Coastal Engineering 27, 161–193. Hamilton, N.T.M., Collins, L.B., 1988. Placer formation in a Holocene barrier system, southwestern Australia. Journal of Coastal Research 14 Ž1., 240–255. Harris, P.T., Baker, E.K., Cole, A.R., 1991. Physical sedimentation of the Australian continental shelf: with emphasis on late quaternary deposits in major shipping channels, port approaches and choke points. Ocean Sciences Institute Report 51, The University of Sydney. Hearn, C.J., Parker, I.N., 1988. Hydrodynamic processes on the Ningaloo coral reef, Western Australia. In: Proceedings of the 6th International Coral Reef Symposium, Australia Vol. 2pp. 497–502. Hegge, B.J., 1994. Low-energy sandy beaches of southwestern Australia: two-dimensional morphology, sediments and dynamics. PhD thesis, Department of Geography, The University of Western Australia. Hegge, B., Eliot, I., Hsu, J., 1996. Sheltered sandy beaches of southwestern Australia. Journal of Coastal Research 12 Ž3., 748–760. Hesp, P.A., 1984a. Foredune formation in Southeast Australia. In: Thom, B.G. ŽEd.., Coastal Geomorphology in Australia. Academic Press, Sydney, pp. 69–97. Hesp, P.A., 1984b. Formation of sand ‘beach ridges’ and foredunes. Search 15, 289–291. Hodgkin, E.P., Clarke, R., 1990. In: Estuaries of the Shire of Albany. Estuarine Study Series No. 8, November. Environmental Protection Authority, Western Australia. Jackson, N.L., Nordstrom, K.F., Eliot, I., Masselink, G., in review. Low energy marine and estuarine beaches — a review. Geomorphology. Johnson, D.W., 1919. Shore Processes and Shoreline Development. Wiley, New York, 584 pp. Kirugara, D., Cederlof, U., Rydberg, L., 1998. Wave-induced net circulation in a reef fringed lagoon: Bamburi, Kenya. Ambio 27 Ž8., 752–757. Lemm, A.J., Hegge, B.J., Masselink, G., 1999. Offshore wave climate, Perth ŽWestern Australia., 1994–96. Marine and Freshwater Research 50, 90–102. Lippman, T.C., Holman, R.A., 1990. The spatial and temporal variability of sand bar morphology. Journal of Geophysical Research 95 ŽC7., 11575–11590. List, J.H., Terwindt, J.H.J., 1995. Large-scale coastal behaviour. Marine Geology 126, 1–3. Masselink, G., 1996. Sea breeze activity and its effect on coastal

88

P.G. Sanderson et al.r Geomorphology 34 (2000) 73–88

processes near Perth, Western Australia. Journal of the Royal Society of Western Australia 79, 199–205. Masselink, G., Short, A.D., 1993. The effect of tide range on beach morphodynamics and morphology: a conceptual beach model. Journal of Coastal Research 9 Ž3., 785–800. Maxwell, S., 1995. Foredune plains: variation in plant community structure and composition on foredune plains of the west coast of Western Australia. BSc ŽHonours. thesis, Department of Geography, The University of Western Australia. McKenzie, P., 1958. The development of beach sand ridges. Australian Journal of Science 20, 213–214. Nordstrom, K.F., 1977. Bayside beach dynamics: Implications for simulation modelling on eroding sheltered tidal beaches. Marine Geology 25, 333–342. Nordstrom, K.F., 1992. Estuarine Beaches: An Introduction to the Physical and Human Factors Affecting Use and Management of Beaches in Estuaries, Lagoons, Bays and Fjords. Elsevier, London. Nordstrom, K.F., Jackson, N.L., 1992. Two-dimensional change on sandy beaches in meso-tidal estuaries. Zeitschrift fur Geomorphologie 36, 465–478. Owens, E.H., 1977. Temporal variations in beach and nearshore dynamics. Journal of Sedimentary Petrology 47 Ž1., 168–190. Psuty, N.P., 1967. The geomorphology of beach ridges in Tabasco, Mexico. Technical Report 30, Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana. Sanderson, P.G., 1997. Cuspate forelands on the west coast of Western Australia. PhD thesis, Department of Geography, The University of Western Australia. Sanderson, P.G., Eliot, I., 1996. Shoreline salients, cuspate forelands and tombolos on the coast of Western Australia. Journal of Coastal Research 12 Ž3., 761–773. Sanderson, P.G., Eliot, I., Fuller, M., 1998. Historical development of a foredune plain at Desperate Bay, Western Australia. Journal of Coastal Research 14 Ž4., 1187–1201. Searle, D.J., 1984. Sediment transport system, Perth sector Rottnest Shelf Western Australia. PhD thesis, Department of Geology, The University of Western Australia. Searle, D.J., Semeniuk, V., 1985. The natural sectors of the inner Rottnest Shelf coast adjoining the Swan Coastal Plain. Journal of the Royal Society of Western Australia 67, 116–136. Searle, D.J., Semeniuk, V., Woods, P.J., 1988. Geomorphology, stratigraphy and Holocene history of the Rockingham-Becher Plain, South-western Australia. Journal of the Royal Society of Western Australia 70 Ž4., 89–100. Semeniuk, V., 1997. Pleistocene coastal palaeogeography in Southwestern Australia — carbonate and quartz sand sedimentation in cuspate forelands, barriers and ribbon shoreline deposits. Journal of Coastal Research 13 Ž2., 468–489.

Semeniuk, V., Johnson, D.P., 1982. Recent and Pleistocene beachrdune sequences, Western Australia. Sedimentary Geology 32, 301–328. Semeniuk, V., Searle, D.J., 1986. The Whitfords Cusp — its geomorphology, stratigraphy and age structure. Journal of the Royal Society of Western Australia 68 Ž2., 29–36. Semeniuk, V., Searle, D.J., Woods, P.J., 1988. The sedimentology and stratigraphy of a cuspate foreland, southwestern Australia. Journal of Coastal Research 4 Ž4., 551–564. Shepherd, M.J., Eliot, I.G., 1995. Major phases of coastal erosion ca. 6700–6000 and ca. 3000–2000 BP between Cervantes and Dongara, Western Australia. Quaternary International 26, 125–130. Short, A.D., 1991. Macro–meso tidal beach morphodynamics — an overview. Journal of Coastal Research 7 Ž2., 417–436. Silvester, R., Hsu, J.R.C., 1993. Coastal Stabilization: Innovative Concepts. Prentice-Hall, Englewood Cliffs, New Jersey. Sunamura, T., 1989. Sandy beach geomorphology elucidated by laboratory modelling. In: Lakan, V.C., Trenhaile, A.S. ŽEds.., Applications in Coastal Modelling. Elsevier, Amsterdam, pp. 159–213, Oceanography Series. Taylor, M., Stone, G.W., 1996. Beach-ridges: a review. Journal of Coastal Research 12 Ž3., 612–621. Thom, B.G., 1964. Origin of sand beach ridges. Australian Journal of Science 26, 351–352. Woods, P.J., 1983. Evolution of, and soil development on, Holocene beach ridge sequences, west coast, Western Australia. PhD thesis, Department of Soil Science and Plant Nutrition, The University of Western Australia. Woods, P.J., Webb, M.J., Eliot, I.G., 1985. Western Australia. In: Bird, E.C.F., Schwartz, M.L. ŽEds.., The World’s Coastline. Van Nostrand-Reinhold, New York, pp. 929–947. Wright, L.D., Nielsen, P., Short, A.D., Green, M.O., 1982. Morphodynamics of a macrotidal beach. Marine Geology 50, 97–128. Wright, L.D., Short, A.D., 1984. Morphodynamic variability of surf zones and beaches: a synthesis. Marine Geology 56, 93–118. Wright, L.D., Thom, B.G., 1977. Coastal depositional landforms: a morphodynamic approach. Progress in Physical Geography 1, 412–459. Wyrwoll, K.-H., 1990. The Geomorphology and Geomorphological Evolution of the Coast of the Cape Range–Exmouth Gulf Region, unpub. report. Department of Planning and Urban Development, Western Australia. Yamano, H., Kayanne, H., Yonekura, N., Nakamura, H., Kudo, K., 1998. Water circulation in a fringing reef located in a monsoon area: Kabira Reef, Ishigaki Island, southwest Japan. Coral Reefs 17, 89–99.