There is a common belief that artificial opening of the channel, either by high-volume dredging or permanent opening procedures, would help to alleviate flood impacts (flood level and flood duration) while also improving water quality. However, coastal engineers and scientists have proven this to be wrong through a series of entrance morphodynamic studies (Patterson and Britton, 1994; Umwelt, 2011, Cardno 2013a, Cardno 2013b, Cardno 2015). This issue has become a 'hot topic' for The Entrance residents, often featuring in the media and during community meetings.
Scientific analysis of the lakes' entrance dates back to the 1990s. An independent study by Umwelt (2011) investigated sediment dynamics along North Entrance Beach and at the entrance. It concluded that: "Council should not construct training walls at the entrance (…) None of the investigations to date demonstrate that construction of training walls would benefit the lake or North Entrance Beach."
What's more, Umwelt's study didn't support high-volume dredging, removal of the berm, or alternative opening mechanisms to drive the lake entrance to a permanently wide-open condition. The researcher went on to say that "Enhanced wave penetration into the estuary and increased lake water levels, now and as sea level rises, both have significant risks for lake ecology and flooding." (Umwelt, 2011)
In 2011, based on a recommendation from this study, the NSW Government commissioned an engineering services company Cardno to develop a numerical model of the entrance channel and its surrounds. In 2013, Cardno was again commissioned by the state to undertake two additional studies, which looked at the morphodynamics of the entrance channel and investigated a range of engineering solutions proposed by the community. The latter study also provided the cost analysis for proposed solutions.
The first analysis used a numerical model to evaluate a range of popular management options, including training walls, break walls or breakwaters. The study concluded that the various training wall options "would not increase flood level (if spaced at 150m or more apart)", noting that placing them closer would have unacceptable, negative impacts on flooding. The authors also stated that these options "would lead to virtually no change in the lakes' water quality" and "would require that maintenance dredging of the type already undertaken by Council continues." (Cardno, 2013)
|Training Wall Configuration
|Impact on Flood Levels
|Impact on Flood Duration
|Impact on Lake Flushing
|Single Northern Training Wall
|Dual Training Wall - 200m Apart
|Dual Training Wall - 150m Apart
|Dual Training Wall - 100m Apart
The second Cardno study looked at ways to improve sand retention on the North and South Entrance beaches. It went on to list a range of options, one of which was building twin training walls that would cost around $46.9 million, with negligible benefit to the community or the estuary. The report led to the construction of the current breakwater at The Entrance Beach to assist with sand accumulation over time. The longer-term impacts of this structure on local morphodynamics are currently unknown.
In 2015, the former Estuary Floodplain & Coastal Management Committee ordered a third Cardno study. It revisited the modelling, adding a proposal to deepen the channel that is currently constrained by the rock shelf at the mouth of the estuary. As researchers concluded, following the deepening, the "dredged channels would begin to infill almost immediately from both the upstream and downstream ends" and "comparison of simulations undertaken with or without the training walls showed little difference about water quality and water levels in the lake system" (Cardno, 2015)
However, the study also cautioned that any significant increase in the tidal exchange might have additional impacts on the estuary, including:
- Decrease of the lake's water levels;
- Exposure of muddy shores;
- Reduced recreational value and fish catch;
- Reduced estuary navigation with limited access to jetties and boat ramps;
- Destabilised foundations of Entrance Bridge;
- Permanent changes to the shorelines around Terilbah.
Based on observations in other trained estuaries on the NSW coast, other changes could include:
- Loss of seagrass and saltmarsh (as seen in Lake Illawarra);
- Changes to the commercial fish population (same as above, S/A);
- Loss of prawns (S/A);
- Loss of endangered Little Tern habitat (S/A);
- Migration of the flood tide delta west of the bridge, making this area even shallower (as seen in Lake Macquarie);
- Exposure and oxidisation of acid sulphate soils (as seen in Tweed and Shoalhaven Rivers);
- Mangrove ingress to the intertidal zone (a significant increase in mangrove area has been seen in Lake Illawarra);
- Erosion of the entrance seawall along Memorial Park (as seen in Swansea Channel);
- Ongoing shoreline recession and erosion (as seen with the 2016 collapse of Milano's restaurant into Lake Macquarie);
- Bridge instability (as seen in Lake Illawarra where emergency works to stabilise Windang Bridge cost $2.5 million);
- Less protection from coastal storms and wave impacts in a changing climate.
These scientific studies confirm that managing the estuary and its entrance is complex and must be based upon the current best available information and according to relevant legislation. The Floodplain and Coastal Management Programs set out by the NSW Government seek to manage coastal waterways in an ecologically sustainable way, and with the social, cultural and economic well-being of the people of the Central Coast in mind (DPIE, 2020).