Figures to be added
June 11; The last week of wild weather has stress tested beaches and coastal management strategies along the east Australia coast line. As per wave buoy plot from wave rider off Gold Coast, maximum wave heights reached almost 10m in the peak of the storm.
It is not surprising that developed areas with long term pro-active coastal management strategies to protect beaches and foreshore assets have weathered the test with very little impacts and developed coastlines with “do nothing” or ad hoc strategies have fared badly.
Examples of pro-active coastal management plans are Gold Coast and Noosa. Both use a combination of terminal seawalls combined with beach nourishment and coastal control structures such as groynes to widen the beaches seaward of the walls. In extreme events, the walls are an important element of the plan as they limit the extent of erosion to protect beachfront public and private property.
Seawalls are a very common type of structure used to protect beachfront assets from erosion damage and are often the first works implemented. Once beachfront assets are protected, there is a more stable political and economic environment to plan and fund broader coastal management works.
There is a very simplistic and sweeping statement often heard that building seawalls [rock or other] = No Beach. Like most things in nature, particularly on a high energy coastline like the east coast of Australia it is not that simple. In reality, seawalls don’t cause erosion problems – they are constructed along beaches with erosion problems. The impact of seawalls on the beaches depends on how far seaward they are located. Certainly walls that are badly designed and located too far seaward into the surfzone can exacerbate the erosion in front of the wall and downdrift.
Walls as part of an effective and integrated coastal management strategy should be located sufficiently landward to have no significant adverse effects on the beaches. Such “terminal” or asymptotic walls located landward of general wave action are dormant most of the time but reduce the extent of erosion during severe storms and eliminate the zone of reduced foundation capacity (Figure 1).
Figure 1 Cross sections of typical terminal seawall for fine weather and storm erosion event conditions
It is my observation over some 40 years of working along the coast that:
- Terminal walls that are landward of the surf zone, except for limited periods in severe storms, have little impact on the beach processes and beach recovery after storms.
- Seawalls that are located in the surf zone that act as long headlands and do have a significant impact adversely.
Two examples, which I am very familiar with, are the Gold Coast, and Noosa where terminal seawalls are part of the Coastal Management Plans. These engineered walls are located on an alignment as far landward as practical and are generally buried under dunes to protect public and private beachfront infrastructure in severe erosion events. Both these councils benefit financially from the high rateable values of the protected beachfront properties.
The Gold Coast has over 20km of terminal rock walls that are mostly buried in the dunes (Figure 2). The only exception is Palm Beach where there are walls that effectively protected beachfront properties over the last week but beach widening is still to be fully implemented. Walls such as shown in Figure 2 may only be exposed in an event greater than a 1 in 25 year event and the recent event did very little damage. The City of Gold Coast is progressively protecting public area and the private walls are a requirement for any beachfront development and must be funded by owners to Council’s standard. The beach and dunes in front of these walls are managed in a variety of ways by the Council. They have been doing this for over forty years and the walls are effective in protecting properties against severe erosion events. The walls are part of the Town Planning Scheme and private properties are required to be protected by a seawall to the approved design at the owner’s expense. These walls effectively “draw a line in the sand” as to the location of the wall to prevent ad hoc works too far seaward and emergency works in dangerous conditions.
Figure 2 Top of buried terminal seawall at Tugun on the Gold Coast. This wall was a critical line of defence exposed to wave attack in storms before coastal management works were implemented in the 1980s and 1990s by City of Gold Coast. (Photo ICM 6/5/16)
During the event, the nourished beaches absorbed the wave energy and the artificial submerged reef breakwater off Narrowneck not only protected the beach but produced great conditions for tow in surfing (https://www.youtube.com/watch?v=V6UxtSzN70U).
The Noosa embayment is downdrift of a large natural headland and suffers from erosion shadows at times (similar to Byron Bay). Walls that were constructed in the 1970’s to protect the beachfront properties along Hastings Street now provide a line of last defence since coastal management works were implemented to protect the beaches. These works include groynes and beach nourishment (Figure 3).
Figure 3 Left; Seawalls in 1970s exposed to waves before coastal management works. (From ICM photo archives) Right: Present beach with walls buried (GoogleEarth).
The wave heights were similar but the erosion impacts were more severe in the Sydney area. The media concentrated on Collaroy where a section without seawalls suffered severe property damage while adjacent areas with seawalls, including the Council street ends, were protected satisfactorily (Figure 4).
Figure 4 Collary - WRL photo (published www.news.com.au)
At Byron Bay the recently constructed terminal seawall at Manfred Street protected the public street end, and private property to the north (Figure 5). Without this wall it is likely that the dunes would have breached causing large scale damage to the roads, public utilities, properties and wetlands behind the dune.
Figure 5 Manfred St wall Byron Bay
SUMMARY: The recent event showed weaknesses along many sections of the east coast of Australia and many areas failed nature’s stress test. Not surprisingly, developed areas with protection works in place performed the best.