If you have any questions not covered by the information we have provided, please email your question to Janell_Keegan@pittwater.nsw.gov.au

We will then publish responses to the key themes generated by residents questions.

Was the community consultation process beneficial to the success of the project?

Community input and feedback is an important part of the Pittwater Council's decision making process. Any foreshore restoration works at Rowland Reserve would affect the community directly therefore potential works were required to meet the needs of the local community while also managing the erosion problem. Pittwater Council involved the community by holding a public meeting at council offices to discuss the options available for restoring the foreshore, demonstrating onsite the works to be carried out, keeping the community notice board at the Reserve updated with the progress of works and erecting two education signs at the Reserve. Pittwater Council received positive feedback from the community and Rowland Reserve users were very interested and seemed to be much in favour of what Council were providing for the environment and the park users.

How were the seawall design options for Rowland Reserve selected?

The design principles for new seawalls in the Environmentally Friendly Seawalls Guide (DECCW 2009) suggests incorporating a number of design principles to optimise the environmental and habitat benefits. The key principles the guide outlines are:

• Decide whether a seawall is needed, or whether more environmentally favourable options could be used for example native vegetation and temporary wave barriers.
• Maximise the use of native riparian and estuarine vegetation into the structure.
• Maximise habitat diversity and complexity by incorporating microhabitats such as pools, crevices, boulders and ledges and by maximising surface roughness and texture.
• Create low-sloping seawalls or incorporate changes of slope to maximise habitat surface area

Once it was established that a sewall was required at Rowland Reserve, the next step was to understand the physical coastal and estuarine processes driving the sediment movement of the shoreline at the Reserve. The following image depicts the adopted plan of the protection arrangement for the Reserve. In order to reach this final design, analysis of various forms of data and information was required including:

historical otho-rectified aerial photography; photogrammetry; hydro-surveys; wind information; wave information; and tidal information.

Following this research, a series of calculations were made to quantify rates of erosion and sediment movement volumes for past, current and future behaviour. These values were used to develop a conceptual model of sediment movement. Several feasible management options were developed. The advantages an disadvantages of each management option was considered in conjunction with community feedback and support. The final design incorporated the following four protection options. Click on the links to view the design sketches.

• Conventional rock revetment - 35% of the final design
• Beach nourishment - 15% of the final design
• Benched revetment with Mangroves - 25% of the final design
• Benched revetment with Saltgrass - 25% of the final design

Should we be interfering with natural processes such as foreshore recession and coastal erosion?

Losing 1 metre of foreshore per year was not good for the long term future of Rowland Reserve as a recreational reserve. The current intervention arrested that problem in addition to providing additional habitat opportunities for marine, intertidal and terrestrial uses. The mini beach provides an additional focal point to get close to the water's edge with the mini headlands and central 'boulder stack' assisting with wave attenuation and sand retention.

While seawalls act as buffers against shoreline erosion, their construction means that intertidal vegetation is removed or will eventually die off through prevention of tidal inundation. The ability to encourage sediment deposition and to filter catchment runoff is lost and flow patterns alter. Also, when a hard structure is built where there is potential for wave action or strong currents, erosion is generally exacerbated at the toe or ends of the structure. These changes can impact on adjacent seagrasses through burial, increased scour and increased turbidity. In addition, a change in sediment composition from fine-grained to coarse could alter benthic invertebrate and fish community composition of the area (Batton, 2007).

Seawalls also act as a barrier between terrestrial and aquatic environments as they can stop the movement of natural wrack onto the shore where it would gradually break down. Instead, floating wrack can build up and form mats in front of seawalls. Under these conditions, underlying seagrasses can be smothered or shaded, eventually dying off. In addition, sediments can become anoxic from the breakdown of decomposing wrack resulting in the mortality of benthic fauna (Cummins et al., 2004). One study of benthic assemblages associated with seawalls in Lake Macquarie found that species richness and abundance were significantly reduced in sediments in front of seawalls compared with naturally sloping slatmarsh

Should all seawalls be designed using hard engineering principles as they are more robust?

Traditional seawalls are hard (typically rock or concrete), homogenous and vertical. They generally offer very little variety of habitat types because their profile differs so greatly from the natural intertidal habitats and natural rocky intertidal shores.

Natural rocky weathered intertidal shores consist of crevices, pools and overhangs that provide different habitat and complexity. The natural estuary profiles such as mudflats, beaches, mangroves, saltmarsh and swamp forests are 'softer' with greater diversity of substrate, sediment and vegetation.

The design of the seawall at Rowland Reserve follows the principles of the Environmentally Friendly Seawall Guidelines (DECCW, 2009). It incorporates both riparian vegetation and estuarine vegetation (mangroves and saltmarsh) and provides habitat diversity and complexity by using different sized and shaped boulders and adopting low sloping and changing gradients of slopes. The design process involved detailed research and calculation to form a structure that would resist the forces of nature. The resultant seawall can therefore withstand the conditions of its environment and promote habitat diversity and complexity by:

• using large boulders of various sizes and shapes;
• providing gaps and crevices between the boulders;
• increasing surface area to maximised the amount of intertidal area;
• promoting diversity of species.

Are very flat sloping seawalls better than steep sloping seawalls? 

Changing the natural foreshore slope from near-horizontal to near-vertical greatly reduces the amount of available intertidal habitat on seawalls. As natural intertidal shores can be tens of metres in width, the insertion of vertical seawalls reduces this to the tidal range bandwidth of the seawall (the area between low and high tide), up to 2m in Sydney (Chapman and Bulleri, 2003). The size of a patch of intertidal habitat shows a positive relationship with the abundance and diversity of species living in it (McGuinness, 1984): in general, less habitat means fewer species and lower abundance.

In addition to the reduction in available soft sediment habitat, the reduction of habitat area can increase local species densities and force species that might naturally live metres apart to occupy the same patch (Chapman, 2006). This crowding into smaller areas increases competition, reducing organism size, density and breeding success (Moreira et al., 2006). In addition, many intertidal plants and animals have been shown to be strongly influenced by the slope of the substrate: species type, abundance and behaviour can differ between vertical and horizontal shores (Chapman, 2007).  - Environmentally Friendly Seawall Guidelines (DECCW, 2009)

The seawall design at Rowland Reserve uses a low sloping revetment with benches. The low slope maximises the amount of intertidal area resulting in greater habitat variety and abundance and the change of slope from bench to bench caters for a wider variety or organisms.

What ongoing maintenance is occuring at Rowland Reserve since the works were completed in 2009?

Council gardeners conduct regular maintenance of the garden areas to ensure weeds are kept to a minimum. Gardeners have been instructed to carry out replanting in areas where gaps have formed including the saltmarsh and mangrove areas as well as the gardens. Following completion of the project, Council carried out herbicide spraying to the turf areas to remove "Parramatta Grass" and allow the turf areas to establish. Also the timber viewing deck is on a regular program to be treated with "Lanolin" to protect the timber from the weather.

Have there been other successful examples of foreshore stabilisation works? Hwo did they differ from the design at Rowland Reserve? 

Kogarah Council's Kogarah Bay, on the Georges River, adopted a step seawall incorporating saltmarsh. It was constructed to replace an existing degraded sewall that had low biodiversity.

Before the Seawall was constructed                After the Seawall was constructed

Mosman Council's Quakers Hat Bay seawall project involved restoring an old, collapsing dry laid sandstone seawall and was conducted in partnership with the University of Sydney's Centre for Research on the Ecological Impacts of Coastal Cities. Natural rocky shores created a range of habitat for intertidal animals including flat horizontal surfaces, overhangs, rock-pools and crevices. Mosman Council redesigned and redbuilt the seawall using a boulder field design and creating more crevices and horizontal surfaces for aquatic organisms to inhabit. Mosman Council has continued to monitor the seawall since its completion

The project involved the construction of steps to provide access into the water for boat users (similarly to Rowland Reserve), and the reconstruction of a dilapidated stormwater channel. Thanks to a separate grant, dinghy racks were built so that boat users are able to store their dignhy's without harming vegetation in the foreshore bushland.

Before the new seawall constructed                After the seawall was constructed                      Quakers Hat Bay seawall habitat  

Page Updated: 30 Jun 2011