Living on the edge
Real estate values in coastal towns and cities show that, after the CBD, the very edge of the beach, the cliff or the quay is the most desirable place to own property: the harbour view, the varied seascape, the ambience of the tranquil waters of the coastal lagoon, can be a significant multiplier of the cost of residential, commercial and office property. This powerful desire for the water view, or simply proximity to the beach, has led to the invasion and destruction of numerous dynamic and fragile coastal environments. Consequent flood and erosion protection costs often prove expensive.
Adelaide is examined as a case study of long-term management effort to contain the protection problem of building to the foredune, while maintaining a recreation beach.
Case study: Adelaide
Metropolitan Adelaide extends for 30 km along the eastern shore of the Gulf St Vincent, from Seacliff in the south to Outer Harbor in the north (see figure 2.26, page 68). The entire coastal plain, including sand dunes up to 500 m wide and extensive coastal wetlands, has been settled right to the edge of the beach. While some centres such as Glenelg were established in the 19th century, most beach suburbs were built between 1930 and 1970. The suburbs were constructed across the dunes, including the foredune, with an expectation of both protection from storm and the maintenance of a recreational beach. These two conditions are maintained today only through beach replenishment to offset the natural trend of recession along most of the beach. The erosive trend is largely of geological origin, but is exacerbated by ecological changes brought about by pollution of the Gulf.
Adelaide has a protection problem, not only because development involving many thousands of people has encroached into the active beach-dune zone, but also because this particular coast is actively receding. Relative sea level rise, derived from uncorrected tide gauge data at Port Adelaide, is 2.08 mm/year (refer to 'Recent sea level change' in chapter 2). While this rise is significant, the most important factor leading to recession is a deficit sand budget. The beach and dune sands were deposited along the metropolitan coast from about 7000 BP, by the advancing postglacial marine transgression. Predominant winds in the Gulf and south-westerly swell established a net northerly littoral drift, which led to the formation of the LeFevre Peninsula between 7000 and 3000 BP (Bowman & Harvey 1986; see figure 2.27, p. 69). This northerly drift continues: today it is of an order of 40 000-50 000 m3/year, leading to beach accretion north of Point Malcolm and in shallow water north of Outer Harbor. However, there is only minimal supply of sand alongshore from the rocky shore to the south of Seacliff and so the drift to the north can be supplied only by erosion, particularly at the southern end of the metropolitan beach.
The erosion issue became apparent as a result of storm damage in the 1950s and 1960s. At that time the metropolitan Adelaide coastal councils were responsible for coastal protection, and after a series of severe storms the Metropolitan Seaside Councils Committee asked the University of Adelaide Engineering Department to study the causes of the problem and to recommend solutions. The Culver Report of 1970 (Culver 1970) proposed a sand nourishment strategy, with rock revetment as reserve protection. The strategy, which was commenced in 1972, has succeeded in maintaining a recreation beach, and storm damage has been confined to seawalls and jetties. It is not possible to verify that storm damage plus the cost of nourishment have been less than storm damage would have been had nothing been done, but estimates of the value of the beach are much larger than the costs of maintaining it. Had nothing been done, some parts of the beach would have been lost. Three reviews of the strategy, in 1984, 1992, and 1996, have not proposed any major change of direction, and it appears that the majority of the populace support the strategy, in spite of the seasonal presence of heavy machinery on the beaches.
The beach replenishment strategy at first involved simply trucking sand from points of accumulation to depleted beaches; for example, from the Torrens Outlet to North Glenelg. It took some while to establish the scheme, as sand placed on the beach disappeared quickly offshore to fill the whole active profile, but by the early 1980s some recovery was apparent. At Brighton, where the beach had been very narrow, it was even possible to re-establish dunes in front of the esplanade. However, West Beach and North Glenelg did not improve until large quantities were barged from Torrens Island (187 000 m3) and offshore Outer Harbor (100 000 m3) in 1989 and 1990.
From 1991 to 1997 sand was dredged (600 000m3) off Port Stanvac (10 km south) and barged to Brighton, but this supply is no longer viable and new sources of sand are needed to replace what had been locked up under the beach suburbs. At present, only the long-term accumulation immediately offshore at Outer Harbor has been located as a possible major source. More distant sources are constrained by cost.
Physical evaluation of the scheme has been possible by a system of monitoring. Profiles from the dune to approximately 1000 m offshore, at approximately 500 m intervals have been measured annually (or more frequently) since 1977. Measurement has been by land survey to wading depth and echo sounding offshore; repeated analysis has shown the accuracy to be ±3 cm on land and ±10 cm at sea. Checks on survey accuracy are given by rods fixed in the seabed along 16 of the survey lines. For key areas, more detailed information has been added through more closely spaced survey and remote sensing. Spatial information system software has placed this detailed information into digital terrain models of the beach and nearshore zone (Noyce 1993). Comparison of successive models of the same areas shows gain and loss in 'surface difference maps', as well as allowing calculation of sand volume change. In this way it has been possible to judge the vulnerability of certain beaches coming into the winter storm season, the response to replenishment, and to infer the movement of major sand bodies.
The results have yielded some surprising insights. For example, it appears that seagrass loss in the zone 500-1000 m offshore, which occurred mainly in the 1970s (as a result of wastewater pollution), has led to the mobilisation of seafloor sands (to a depth of <0.3 m). This sand appears to have moved inshore in the late 1980s (DENR 1997). For example, the sediment budget for the inner seabed from North Glenelg to the Torrens River outlet shows, after allowance for replenishment and littoral drift, an unexplained annual gain of 25 000 m3/km from 1989 to 1996. The seagrass beds immediately offshore have receded and the surface lowered, but the link is not proven. A 'once-off' offshore source for the excess sand means that the level of replenishment to date has been less successful than was previously thought, implying larger quantities of sand may be needed to maintain the beaches in the future. At the far north end of the metropolitan beach, a very large accumulation of sand and sea- grass fibre (of 150 000 m3/year) cannot be explained by drift and replenishment activity, but could be accounted for by loss from adjacent and updrift former seagrass meadows.
There is no doubt, however, that the increased water depth in the offshore profile and the removal of the damping effect of seagrasses has increased wave energy reaching the beaches, accelerated littoral drift and increased storm erosion. This surprising change would not have been detected but for the ongoing monitoring scheme, and underlines the importance of keeping a record of environmental management actions.