Case study: Adelaide metropolitan beach, South Australia
The Adelaide metropolitan beach is located on the eastern shore of the sheltered Gulf St Vincent (figure 2.26). The beach and narrow dune system extend from Seacliff to Outer Harbor.
Wave, wind, and tide
Tidal range is small (microtidal or mesotidal), with a spring tide range of 2.1 m. The highest recorded storm surge was 2.0 metres, in 1981.
Wind records show a dominance of south-westerly winds from the Southern Ocean. These are reinforced in summer by the late afternoon sea breeze, which exceeds 20 km/h in favourable pressure conditions.
Figure 2.26 Locations and sand movement at the Adelaide metropolitan beach
Source: DENR Reference Group 1997
Wave records at Seacliff indicate a medium- to low-energy shore. Most wave heights are below 1.5 m, and very few are above 2.5 m; wave periods are commonly 4 or 5 seconds. A south-westerly ocean swell dominates, although swell energy is much reduced by attenuation in the shallow water of Investigator Strait, and within the Gulf, north of Point Malcolm there is a fall in wave energy.
While little interannual difference of sea level has been recorded, there is seasonal variation with pressure changes. There is a local long-term sea level rise at Port Adelaide (in addition to global rise) because of local neotectonic sinking due to groundwater abstraction and wetland reclamation (Belperio 1993).
Beach and dune materials
The beach is composed of fine to medium grade sands made up predominantly of quartz, with up to 10% shell. The bulk of these sands were laid down at the end of the postglacial marine transgression (see figure 2.27), 7500-5500 BP (Bowman & Harvey 1986). These sands were moved onshore at the end of the transgression, forming a dune barrier 300-1000 m wide fronting a low swampy coastal plain. The major sediment pulse of 7500-5500 BP produced rapid northward progradation of a sand spit, extending northwards from Semaphore Park. After 5500 BP a reduced sediment supply and a change in coastline orientation contributed to spit recurvature and a flared beach ridge orientation.
Figure 2.27 The growth of the LeFevre Peninsula
Source: Bowman & Harvey 1986
This contrasts with barrier development in New South Wales where shore- normal development of barriers is common in the swash-aligned situation. Here, the LeFevre Peninsula can be described as 'drift-aligned' (Davies 1972), with the barrier extending along the shore.
An intermediate beach morphology (see figure 2.28) with rapidly changing surf zone and beach face conditions are common along the Adelaide foreshore, whereas at the northern end of the system dissipative multiple bars have developed in a situation of abundant fine sand accumulation. This latter morphology is stable, and beaches north of Point Malcolm are not subject to the storm damage evident further south.
Most of the Adelaide beaches, south of Point Malcolm, show clear offshore-onshore movement with a variation in wave energy. Beaches erode rapidly (in a matter of hours) in heavy weather, and the sand builds a nearshore bar which serves to further protect the beach (i.e. a negative feedback loop). Fine weather and low waves slowly (over days to weeks) rebuild the beach. The low waves drive the bar onto the foreshore, where it is pushed up the beach face to build a berm and wider backshore. At what point is there a changeover? Observation suggests the following:
• All storm seas and sea breezes with wave heights of more than 0.5-0.6 m will build bars, and subaerial beaches will fall.
• Swell waves (at the tail of a storm) of period 6-8 seconds and height < 0.6-0.7 m will restore beaches.
• Short-period waves will restore beaches only when waves are lower than 0.4-0.5 m.
There is no clear seasonal pattern, since storms can occur in any season, but they are usually more common in winter.
One pattern is clear: the south-to-north direction of net littoral drift dominates the Adelaide beaches. In the past it dominated the drift-aligned construction of the narrow sand barrier; today it dominates the pattern of change on the beach.
Slow littoral drift due to wave action gives a net northerly movement of sand along the Adelaide metropolitan beach. There is very little sand supply from the south past Marino, and no sand is delivered by the urban creeks (stormwater channels) to the beach. Thus, sand that has been eroded from the southern and central beaches and moved north is, on average, not fully replaced during fine weather, so these beaches erode. Sand is lost from the central and southern beaches, but accumulates with the fall in energy at the northern end of the system, in the shallow water of Largs Bay and off Outer Harbor.
Figure 2.28 The beach model of Wright and Short (1983). Profile form and surf zone circulation are shown for six beach states, a-f. Beach characteristics, including wave energy, tide, sand grain size, and prior change, determine beach state
Source: Wright & Short 1984
It is clear that there have been recent complications to this pattern. Pollution of the Gulf has led to seagrass loss off Adelaide (450 ha since 1970); the landward edge of the sea-grass beds has retreated up to 1 km. Surveys show that where the seagrass has been lost the sandy substrate has been eroded to a depth of30-50 cm. Over such a large area this is an enormous volume of sand – a million cubic metres or more. Beach and nearshore monitoring at Adelaide suggests that this has moved onshore and is accumulating at the northern end of the system.