UNIVERSITY of Newcastle Professors Ron Boyd explains the science behind the erosion at Stockton Beach and gives his opinion on the best way to solve it.
Stockton Beach – a long history of erosion
Stockton, on the north side of Newcastle Harbour, is one of the largest beaches in NSW but surprisingly has a serious coastal erosion problem. This problem has been an ongoing and unresolved issue for more than 60 years. But Stockton did not always have a beach erosion problem. In fact quite the opposite.
Early maps of Stockton and the Hunter River entrance from 1866 and 1887 show a typical NSW tidal inlet with 6m deep, 90m wide channels, and extensive sand bars on the northern side. These bars contained large volumes of sand, deposited in the ocean by waves and the falling tide as it flowed out the entrance channel. The sand bar on the Stockton side had so much sand, that at low water a tall person standing on it could have had their head above water around 1000m seaward of todays War Memorial on the beach at south Stockton.
The Oyster Bank opposite Nobbys was less than 1 metre deep. Another historical indication of the sand once present on the beach is the old mine workings near the Mission Australia Early Learning Centre. The mine shaft that was exposed in the surf during September 2017, was originally 110m behind the shoreline in 1895. The 50m-wide North Stockton fourth sewage pond situated next to the ocean was lost after 1974.
A steady progression of dredging and construction occurred since 1866, with dramatic results for Stockton Beach and the offshore bars. The most obvious construction has been the breakwaters. Originally, Nobbys was an island, and there was no beach at Nobbys. Governor Macquarie commenced a breakwater to link it to the mainland in 1818. This was completed beyond Nobbys in 1891. The smaller Stockton breakwater was built in 1886, and the longer 1136 m Stockton breakwater completed in 1912. BHP required a channel depth of 25 feet (7.6 m) be maintained from 1915. This depth was increased to 8.4 m by 1950; 11.6 m by 1964; 15.2 m by 1983, and now 18 m deep. The current artificial channel is now more than twice as deep as it was in the 19th century, and at 150 m, is much wider too.
Beach erosion has taken a cyclic history over the past century. Older Stockton residents remember the worst erosion of the late 1940s to early 1950s when parts of Dalby Oval were lost. Surfers remember the beach growth of the 1960s when a beach backed by wide dunes and vegetation existed seaward of Mitchell Street between the War Memorial and Corroba Oval. Good nearshore surfing banks were common during that period. More serious erosion occurred from the 1970s to 1980s. Older residents will remember the extension of Mitchell Street north in front of the Mission Australia Centre that joined up to Griffith Avenue, the circular road around the beach side of the War Memorial, and the fourth sewage pond that all disappeared during this period.
The ongoing beach erosion since the 1980s has seen the construction of the seawall from Pembroke to Stone Street in 1989, and the sandbag wall at the surf club in 1996. In 2017 a new seawall was installed in front of the surf club, and in 2018 a short extension of the seawall north towards the Mission Australia day care is underway. The latest 2018 erosion events have become more critical. The erosion scarp is now less than one metre from the Mission Australia day care playground, and has already eaten into the old council rubbish dump on Hunter Water and Crown land to the north. While the erosion is cyclic, the cycles are superimposed on a downward linear trend with little sand volume now available for natural rebuilding.
During the last half century, innumerable studies of the Stockton Beach erosion problem have been undertaken. Some of the most interesting facts we have learned from these are the large build-up of sand on Nobbys Beach, and the establishment of a 4.4 km wide sand lobe off Nobbys containing 32 million cubic metres of sand. On the Stockton side, we have learned that there has been an astonishing loss of over 10 million cubic metres of sand since 1816 at an average rate of 41,000 cubic metres per year. The seabed has lowered by 4-12 m. Stockton surfers learnt that their nearshore sand banks lost 2-3 m of sand height just between 1957 and 1995.
Understanding the causes of beach erosion
Understanding and combating beach erosion can be a complex issue. In simple terms, beach erosion is the loss of sand from a beach. This can be thought of as a budget, and similar to a bank account. In an area such as Stockton, deposits are made when more sand comes in than goes out, and withdrawals occur when you lose more sand than you gain. There are two main ways to make deposits and withdrawals.
The first is onshore/offshore transport, when sand either erodes seaward off the beach to deeper water, or deposits from deeper water back onto the beach. Seaward movement results from big waves such as in the Sygna or Pasha Bulker storms combined with high tides.
Sand is normally transported seaward in rip currents and erosion is made worse if several large storms like east coast lows follow each other during the winter. Deposits are made if we get longer periods of fine weather with small waves and lower tides. Sand then moves onto the beach and accumulates, typically in summer with more northeast winds and waves. A lot of onshore/offshore movement is seasonal or cyclical and is driven by climate.
The second way for sand to move is alongshore drift. This occurs when waves approach the beach at an angle and transport the sand in the same direction that the wave is travelling. At Stockton, the dominant direction of approaching waves is from the southeast (about 80%) and the rest from the northeast (20 per cent).
These dominant southeast waves then move sand up the beach to the north towards Port Stephens. Lots of summer northeast waves can also send sand southward temporarily, but this is normally only a short term effect. Net northward movement is the case for all of the NSW coast. As a result, there is not much sand on the south coast around Batemans Bay, but lots in the north in places like Fraser Island.
Previous studies at Stockton have shown that the northward movement of sand is around 30,000 m3 per year, starting around the seawall and increasing to the north. Ideally on coasts like NSW, any sand that is moved alongshore to the north is replaced in the budget by new sand coming in from the south.
Beach erosion occurs if we get more big storm events than periods of fine weather, and if more sand moves north than is replaced from the south. This northward movement is the main reason for Stocktons beach erosion. Periods of storms and fine weather tend to cancel each other out over the long term, but if we interrupt the flow of longshore drift, there is no new sand coming in from the south to replace that being lost to the north.
This is the case in many places in NSW such as the Tweed River mouth, where the northward flow of sand was interrupted by construction of breakwaters in 1965 and the beaches of the Gold Coast then all eroded in the late 1960s. The same thing is happening at Stockton where the Newcastle breakwaters stop the flow of sand from Nobbys Beach around to Stockton.
Solving beach erosion with hard structures
There have been a range of solutions to the Stockton beach erosion problem proposed, and many of these include hard structures. The most obvious solution, used extensively by Newcastle Council is building seawalls. The first built was the 1989 version from Pembroke to Stone Street, followed by the 2017 version in front of the surf club.
A scientific article written for the NSW Coastal Conference by engineer Lex Nielsen in 2017 showed that the construction of the harbour breakwaters changed the refraction shape of waves coming in to Stockton Beach, resulting in building out the beach in front of the caravan park but concentrating erosion between Pembroke and Stone Streets. This concentrated zone of erosion is now the location of the larger seawall.
The advantage of a seawall is clear, it protects the properties immediately behind the seawall in the short and medium term. But like all hard structures, there are serious disadvantages. Seawalls are much less visually attractive than the dunes and beach they replace. A seawall stops dune growth behind it, and increases wave reflection in front of it, raising the energy level of the beach and preventing accumulation of sand there.
This can be clearly seen along the length of the current seawall with the result that the beach and dunes are lost in that location. Secondly, seawalls have to end somewhere. Wherever they end, erosion is concentrated and accelerated. This occurs because sand behind the seawall cannot be accessed during an erosion event and has to be sourced from the beach adjacent to the seawall.
Again this is seen at Stockton where the worst erosion is north of the seawall in front of the Mission Australia Centre. A typical temporary solution for this is to dump more rocks at the end of the wall, but this just moves the problem further north along the beach. At Stockton this would move the problem north to Griffith Avenue and up to Corroba Oval. Another potential erosion hotspot may develop north of the shorter surf club seawall at Dalby Oval. But thats not the end of the disadvantages of seawalls. Once the beach is lost in front of a seawall, it becomes dangerous to access what is left of the beach.
At Stockton this results in stairs to the beach left hanging in mid-air and unusable. The seawall requires ongoing maintenance to prevent it being damaged (estimate for 2018-19 work is around $3.5m), and the lifespan of the current seawall will eventually end. Once it becomes undermined and steepened, as at the northern end of the current seawall, it becomes a safety hazard.
One option to solve the Stockton beach erosion problem is to extend the seawalls until they eventually stretch from the breakwater to Corroba Oval. This appears to be the default current solution adopted by Newcastle Council and is presented as part of Newcastle Councils 2014 Management Plan.
The Council does not have enough funding to fix the whole problem but they do have enough money to keep adding to the seawall each time a crisis occurs. We have seen this already in the addition of the short seawall from Lexies Café to the surf club, and the recent extension at the northern end off Stone Street. It seems likely that a further extension in front of the Mission Australia Centre may be proposed as a solution to the latest crises, and this has been suggested at public meetings.
It should be clear from the list of disadvantages above, that extending seawalls is not a viable solution to retain a beach that can actually be used. Hard structures remove the amenity of the beach and do not protect the beach itself. The general trend in coastal management is to move away from hard structures and armouring of the shoreline as a solution, and at least four US states have banned their use. Seawalls are only a temporary fix, they are not a long term solution.
Other hard structure solutions that have been proposed for Stockton include groynes, offshore breakwaters and artificial reefs. Groynes are rock structures placed perpendicular to the beach to trap the longshore flow of sand. Sand builds up on the updrift side of a groyne, but erosion occurs on the downdrift side. Groynes are not a suitable solution for Stockton Beach in its present condition as there is not enough drifting sand to trap, and no sand source to the south. Groynes dont accumulate sand, they just rearrange its location and some areas are eroded even more. Offshore breakwaters are rock structures that are placed parallel to the beach and a few hundred metres offshore.
They work by making waves break offshore, reducing the energy on the beach behind them. Sand also builds up behind them but erosion occurs between them, so there is no net gain. Because the waves break offshore there are none near the beach, so fishermen and surfers would not benefit from this option.
Artificial reefs are shallow banks made of rocks, bags or tyres that are constructed in the surf zone. Like breakwaters, they cause the waves to break further offshore and may protect the beach behind them. They may also provide a better recreational surfing resource. However, the only one in eastern Australia is on the Gold Coast, and since 1999 that reef has not yet produced any good surf, and more importantly for Stockton, has not accumulated any sand. The problem with all these hard structure solutions is that they do not create any extra sand, they just redistribute it or accelerate its loss.
A final alternative, Planned Retreat is not an option for Stockton with infrastructure like the Newcastle Council caravan park, Lexies Cafe, the surf club buildings, tennis courts, Mission Australia Centre, houses along Mitchell Street, Barrie Crescent, Eames Avenue and the road into Stockton (Fullerton Street) requiring preservation.
What is the best solution?
The only alternative left to save the beach, and the one with the best chance of success is sand nourishment. This option works by adding more sand to reduce the wave energy by friction before it reaches the beach. With this option, Stockton gets a beach instead of just a rock wall, and it gets protection in the short to medium term. Sand nourishment requires adding new sand to the beach from onshore or offshore sources, equal to or coarser than the existing sand. The 410,000m3 required could be scraped from Stockton Beach north from the Hunter Water lands, or dredged offshore from the sand lobe off Nobbys Beach or the entrance to Newcastle Harbour.
The northern source is sand that has already been transported from south Stockton, while the offshore lobe and harbour entrance has sand that should have been transported to Stockton if the breakwaters did not prevent it. Offshore sand nourishment is currently being successfully used to prevent erosion on Queenslands Gold Coast. Sand nourishment is not a permanent solution as eventually the sand will move on from south Stockton, where it is not being naturally replaced. It will need to be replenished occasionally, just as the harbour requires maintenance dredging.
Newcastle Council consultants have previously recommended the sand nourishment option, but have also combined it with other hard structures such as seawalls or headlands. The headland option located north near Fort Wallace was preferred as it would place a barrier to sand moving north and retain the new sand for a longer period.
The headland would act as a single large groyne extending 270 m offshore to depths of over 9 m to prevent sand moving around it. Any sand accumulating on the southern side could be periodically removed back to south Stockton, but erosion would likely occur on the northern side. From all these options, nourishment is the best solution to the problem put forward so far, if Stockton is to have a beach that can actually be used. The downside is the price. Initial nourishment has been costed at between $5.2 and $14.5 million. This is beyond the budget of Newcastle Council alone.
While these costs are large in absolute terms, they are small in relative terms. All consultant reports and informed opinion indicate that Newcastle Harbour works, including breakwaters and dredging, are the source of the Stockton erosion problem. The NSW Government has recently (2014) sold the Port of Newcastle for $1.75 billion and spent $44 million on consultants to assist that sale. Coal exports through the Port of Newcastle totalled $15.3 billion value in 2016 generating windfall royalty payments for the NSW Government, and the Port continues to make a healthy profit.
The essence and signature feature of the Stockton community is its beach and the community deserve it to be returned to its former amenity. The NCC is currently preparing a new Coastal Zone Management Plan, due for consideration by Council in July 2018. This plan unfortunately contains no short to medium term solution for the Stockton erosion problem, recommending yet more studies, and decisions postponed until later completion of a second Coastal Management Plan.
Continuing delay in fixing the problem is unacceptable. This problem is specific to Newcastle. It is not a regional NSW beach erosion or climate change issue. It is a direct result of NSW Government actions whose scale and value far exceed those in other locations. A faster, more innovative solution is required outside the current box, one that reflects the origin of the problem and the agencies that both benefit from the port, and have a shared social contract with the Stockton community.
These agencies include the NSW Government, the Federal Government, Newcastle Port leaseholders and users, and NCC. For too long these agencies have ignored the problem while generating major income, and leaving it to the NCC to solve without adequate funding.
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