SeaBees, Groynes and Rips: reflections on Coastal Engineering field trip 2017
Story Written by Dr Mitch Harley
Professor Ian Turner and I led a group of 60 budding engineers on a field trip around various coastal engineering sites in Botany Bay and Cronulla. We run this field trip as part of the CVEN9640 Coastal Engineering class, which comprises a mix of fourth year undergraduate students as well as postgraduate students. I find it fascinating to see the students apply their theoretical knowledge that they have been learning over the past nine weeks of classes to real-life coastal engineering works.
First stop on the trip was the Banksmeadow revetment, built in the early 1970s to provide vital protection from destructive storm waves to Sydney’s Port Botany. The impressive structure has a bit of an odd look to it, being a mix of uniformly-placed tribar units (20 tonnes) interrupted by randomly-dumped dolosse. This was a result of challenges faced during construction. A March 1974 UNSW Water Research Lab report by Foster and Nagle describes this:
As a result of the high wave climate in the area, considerable difficulty has been experienced in placing these units, especially under water
Consequently, they abandoned the idea of carefully placing the individual tribar units with a crane and switched to the rough-and-ready technique of randomly dropping the dolos units into place.
Little did they know however, their construction difficulties were only to get much worse - and quickly. In May and June of that same year, the still-to-be-completed structure was subject to the worst storms that SE Australia has faced in the past 100 years. Another UNSW Water Research Lab study undertaken in 1975 describes what happened:
In May and June of 1974, the action of storm waves on the partially constructed Botany Bay revetment resulted in core material being washed into a section of placed tribar and dolosse units. As well as causing severe damage to the primary armour units, this section of the revetment was choked with core material, lowering the porosity of the armouring layer to an unacceptable extent. To avoid the difficult and time consuming job of removing and replacing the damaged units, alternative procedures were devised.
Talk about Murphy's Law! Fortunately, the structure and repairs were completed by 1975.
Our next stop on the trip was Foreshore Beach, arguably Sydney’s least natural beach (being wedged between Port Botany, Sydney Airport and major arterial roads). Still, the beach does provide welcome respite from the surrounding activity and it is also an important seagrass habitat. Three newly-completed (December 2016) groynes – built to prevent the stormwater pipes from blocking as well as to halt long-term beach recession – had to be particularly sensitive of these seagrass habitats. As a result, the groynes have an unusual design, consisting of rock groynes above mean sea level, but sheet piling for the 100m section extension below sea level. This sheet piling has a much smaller footprint to potentially impact the seagrasses – and supposedly reduces rip currents adjacent to the structure to boot. A Cardno study investigating the impacts of the construction on the adjacent seagrass habitats found that some small patches did in fact disappear following construction. With such limited data however (just two surveys), it is always very difficult to pinpoint this disappearance on the construction itself – particularly in such a modified environment such as this one.
The next few stops involved more groyne fields at Lady Robinsons Beach and Kurnell. With commercial airplanes flying immediately overhead (with their wheels extended ready to land), I was thinking that Kurnell is definitely one place where you would not like to fly a drone (and is in fact highly illegal unless you have specific CASA permission). So the UAV pilot in me shudders when I see drone videos like this one taken from that very site of kite surfers from above. I fear that a major incident between recreational drones and a commercial aircraft is just waiting to happen – and the whole drone industry is likely to pay the consequences.
Our final destination of the day was the Cronulla seawall. This “SeaBee” type seawall is named to not only reflect its honeycomb-like appearance, but also for the fact that its inventor (Chris Brown) has the initials CB (geddit?). The seawall protects a large bluff with significant development on it at North Cronulla from being undermined during large wave conditions. An interesting history of SeaBee-type seawalls written by the inventor can be found in the Coasts and Ports 2013 proceedings.
We also discovered that the sloped honeycombs also provide a fantastic auditorium to conduct coastal engineering lectures! While looking out along the 5 km-long Cronulla-Wanda embayment (the longest beach in Sydney), we were able to see all sorts of beach morphodynamics in action: transverse bars and rips, wave shoaling, spilling waves etc. We were also fortunate enough to have Mr. Adrian Turnbull from Northern Beaches Council give an impromptu lecture on the practical challenges of implementing coastal engineering solutions when multiple stakeholders are involved. Perhaps a future lecturing career, Adrian?
All in all, a fantastic day of coastal engineering!