Management of the Harbours at Glenelg and West Beach

• Background
• The Patawalonga
• Holdfast Shores Marina and the Patawalonga Entrance
• Adelaide Shores Boat Haven
• Future Management of the Harbours
  

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The harbours at West Beach (foreground) and Glenelg

Background

The most identifiable recent changes to the Adelaide coast have been at Glenelg with construction of the Holdfast Shores marina and extension of the Patawalonga breakwater, and at West Beach with construction of the Adelaide Shores boat haven. These developments are referred to generally as the Glenelg and West Beach harbours.

The Glenelg and West Beach harbours obstruct the northerly movement of sand along the coast, so that sand accumulates on their southern side. To date, in order to prevent subsequent erosion on the northern side of the structures, sand has been bypassed from the southern side to the northern side.

A paper called Monitoring Sand Management at the Glenelg and West Beach Harbors (100Kb PDF) outlines the original sand management arrangements for these facilities. This paper was presented at the 1999 Coastal Engineering Conference.

The Patawalonga

The Patawalonga has had an influence for change on the coast since European settlement. Attempts to train the entrance and reduce flooding in low-lying areas began in the 1880s. A weir built at the entrance in 1886 was destroyed by floods in 1887. The original timber floodgates were replaced in 1960 by the present floodgates and lock, and the southern breakwater was built in 1964; a sheet-piled breakwater to the north of the entrance was constructed soon after. The present configuration, an extension of the 1964 southern breakwater and replacement of the sheet piling with a more northerly located rock breakwater, was constructed in 1996 and 1997.

The breakwaters caused sand impoundment to the south (almost to the Broadway, Glenelg South) until sufficient sand accumulated to spill around the end of the breakwater to form the notorious 'Pat sandbar'. Accumulated dry sand was manually bypassed and carted to elsewhere on the coast by truck at rates of up to 60,000 cubic metres/year (on average around 30,000 cubic metres/year from 1973 to 1996) to reduce wind-blown sand drift in the area. In 1979 a dredging campaign aimed to move approximately 30,000 cubic metres of sand from the channel to Glenelg North beach and then excavate a channel into the clay and rock below. This was only partially completed and the channel concept was abandoned.

Sand bypassing at Holdfast Shores

Holdfast Shores Marina and the Patawalonga Entrance

The Holdfast Shores marina at Glenelg, constructed during 1995 and 1996, incorporated and built upon the existing Patawalonga entrance. Aspects of the development include the marina precinct, extensions to the training walls and construction of the offshore breakwater sand trap. The sand management parts of the marina are now a set of three rock breakwaters. The offshore breakwater just to the south of the harbour entrance, with a top level of +1 metre AHD, assists sand accumulation and protects the sand bypassing dredge from waves that could make its operation unsafe. The northern and southern rock breakwaters protect the marina from waves and trap sand to the north and south of the harbour entrance to minimise the amount of sand filling the boat channel.

Approval for the development of Holdfast Shores was provided on the basis that the State Government would be responsible for the certification, ownership and maintenance of the harbour facilities and sand management, and that adequate funds would be made available to cover the cost of these responsibilities in perpetuity from 1997–98.

Sand bypassing by dredge has been necessary to minimise the impact of the harbour on sand movement along the coast. During spring and summer, the northerly littoral drift in this region is at a maximum. The dredging program aimed to keep pace with sand arriving at the southern breakwater and yet avoid the main period of summer beach use. A small cutter suction dredge removed sand and seagrass trapped in the lee of the offshore breakwater. Sand and seagrass were also removed regularly from the entrance and inner harbour to maintain channel navigability.

Up to and including 2004–05, the sand bypassing program was undertaken by Transport SA under direction from the Minister for Transport, who was advised by the Minister for Environment and Conservation on the volumes of sand to be bypassed.

There were concerns about the sand bypassing operation during this period:

• At times, the dredge could not keep up with sand accumulation in the sand trap. The sand then spilled over into the channel. This also resulted in insufficient sand being bypassed to Glenelg North, exposing stones on the beach from time to time.
• Dead seagrass was also trapped at the sand trap, contaminating the sand. To minimise odours on the beach, the sand was discharged offshore at Glenelg North. If the sand had not been contaminated, it could have been discharged onshore to maintain the beach at Glenelg North.
• The actual volume of sand and seagrass dredged was not accurately known because mass flow measurement devices in common use cannot measure quantities of sand or seagrass accurately, particularly when the two are mixed.

Sand bypassing at Adelaide Shores

Adelaide Shores Boat Haven

The Adelaide Shores boat haven was constructed at West Beach in 1998 as part of the overall Holdfast Shores development. It is located about 1.5 kilometres north of the Holdfast Shores marina at Glenelg. It includes a 4-lane boat ramp sheltered by two rock breakwaters that enable small boats to launch in relatively calm conditions. An overpass was built over the beach to connect the breakwaters and boat ramp with the road and landward haven area, thus allowing uninterrupted public access along the beach. It had the added benefit of giving sand-carting trucks the same access.

Up to and including 2004–05, sand bypassing at Adelaide Shores was undertaken by Transport SA under direction from the Minister for Transport. The Minister for Environment and Conservation advised the Minister for Transport on sand volumes to be bypassed and is also responsible for the state of the beaches in the area, under provisions of the Local Government Act 1934 section 886BB – Coast Protection at West Beach.

Sand accumulating on the beach on the southern side of the boat haven and within the facility has been trucked along the beach just to the north in front of the West Beach dunes. Supplementary dredging of sand and seagrass to maintain navigable depths within the boat haven, and from the seaward side of the southern breakwater, has also taken place.

Concerns about the sand bypassing operation at the Adelaide Shores boat haven were similar to those at the Holdfast Shores marina:

• Sand and seagrass were dredged offshore, and the sand only slowly worked its way onshore over many years, which did little to protect the West Beach dunes.
• Seagrass from the dredge spoil area could have been drifting back into the boat launching area.
  

Future Management of the Harbours

The harbours at Glenelg and West Beach have required ongoing sand and seagrass bypassing and channel maintenance at a cost of $1.9 million per year (in 2004–05), which is about the same as the current cost of metropolitan beach replenishment. In 2004–05 and previous years, the bypassing work was undertaken by Transport SA. At Glenelg, sand was dredged from the Patawalonga channel and from the leeward side of the breakwater and pumped offshore at Glenelg North. At West Beach, sand was dredged from the harbour channel and pumped offshore immediately to the north, while sand south of the breakwater was removed by excavator and truck and carted north to the West Beach dunes.

The responsibility for sand management at the Glenelg and West Beach harbours was transferred to the Department for Environment and Heritage at the start of 2005–06. As part of Adelaide's Living Beaches: A Strategy for 2005–2025, sand bypassing at the harbours will be integrated with the management of the rest of the metropolitan beach system over the coming years. Sand building up at the harbours will be backpassed to replenish beaches to the south, which will result in more effective recycling of sand as well as reduced harbour management costs. Similarly, erosion north of the harbours will be prevented by backpassing sand from areas further north along the coast. In other words, sand building up at the Glenelg harbour will be backpassed to replenish Brighton and Seacliff, sand building up at the West Beach harbour will be backpassed to replenish Glenelg North, and sand building up at the Torrens Outlet will be backpassed to replenish the West Beach dunes.

For more information see Adelaide's Living Beaches: A Strategy for 2005–2025.