I flew out of Sydney recently and on the ascent noticed the ripples of waves across Botany Bay. At Kurnell the pattern of the waves was affected by some rock walls. A bit of research later and hey presto! an interesting subject for an EarthCache.
Kicked in the Groynes
Botany Bay in Sydney is the site of the first landing of Lieutenant James Cook in Australia, way back in 1770. Since that time there have been many developments in and around the bay, including Australia’s busiest airport which has two runways that run into the Bay on reclaimed land.
The shoreline of Botany Bay has been increased by nearly 20 per cent by adding these extra runways and additional port structures to the original shoreline, but the resultant impact on the ecosystem has been devastating. Dredging the bay has significantly altered wave direction, causing ongoing severe erosion along the western (Brighton) shore of the Bay and along the southern shore of Kurnell Peninsula. Protective groynes were built to absorb the altered, and now more powerful, wave energy impacting on Lady Robinsons Beach to the west and Kurnell's Silver Beach to the south. They have reduced but not stopped the erosion, and more than 50 per cent of the natural Botany Bay shoreline has had to be reconstructed.
Silver Beach at Kurnell is today characterised by 14 rockwall groynes which project into Botany Bay. The first eight of these groynes were constructed in 1969-70 in response to severe beach erosion which was concentrated on the eastern end.
Lady Robinsons beach (where you are standing) had groynes installed in two stages in 1997 and 2005. There has been some debate as to their effectiveness including such articles as this one:
Above is a bird’s eye view of the groynes at Silver beach, Kurnell.
Geological land form evolution
A groyne (groin in the United States) is a rigid hydraulic structure built from an ocean shore (in coastal engineering) or from a bank (in rivers) that interrupts water flow and limits the movement of sediment. In the ocean, groynes create beaches, or prevent them being washed away by longshore drift. In a river, groynes prevent erosion and ice-jamming, which in turn aids navigation. Ocean groynes run generally perpendicular to the shore, extending from the upper foreshore or beach into the water. All of a groyne may be under water, in which case it is a submerged groyne. The areas between groups of groynes are groyne fields. Groynes are generally made of wood, concrete, or rock piles, and placed in groups. They are often used in tandem with seawalls. Groynes, however, may cause a shoreline to be perceived as unnatural. .
Groynes can be distinguished by how they are constructed, whether they are submerged, their effect on stream flow or by shape.
By construction method
Groynes can be permeable, allowing the water to flow through at reduced velocities, or impermeable, blocking and deflecting the current.
- Permeable groynes are large rocks, bamboo or timber
- Impermeable groynes (solid groynes or rock armour groynes) are constructed using rock, gravel, gabions.
By whether they are submerged
Groynes can be submerged or not under normal conditions. Usually impermeable groynes are non-submerged, since flow over the top of solid groynes may cause severe erosion along the shanks. Submerged groynes, on the other hand, may be permeable depending on the degree of flow disturbance needed.
By their effect on stream flow
Groynes can be attracting, deflecting or repelling.
- Attracting groynes point downstream, serving to attract the stream flow toward themselves and not repel the flow toward the opposite bank. They tend to maintain deep current close to the bank.
- Deflecting groynes change the direction of flow without repelling it. They are generally short and used for limited, local protection.
- Repelling groynes point upstream; they force the flow away from themselves. A single groyne may have one section, for example, attracting, and another section deflecting.
By shape
Groynes can be built with different planview shapes. Some examples include straight groynes, T head, L head, hockey stick, inverted hockey stick groynes, straight groynes with pier head, wing, and tail groynes.
The Earthcache
In order to log this Earthcache, you'll need to answer the following questions and email your answers. Feel free to log your find in anticipation- I'll let you know if there are any issues. Logs without the emailed answers may be deleted. It is also preferable that you upload a picture of you/ your group and your GPSr at the EarthCache with your log.
1. Based on the information above and your observations do you think these Groynes are permeable or impermeable?
2. Based on the information above and your observations what type of Groynes are these? Attracting, deflecting or repelling?
3. Based on the information above and your observations what type of shape are these Groynes?
4. Take a look at the sand around the groynes. Does it look like they have had any impact on erosion? Describe what you see. Is there any difference between each side of the groynes?
5. (Optional) Walk down to the next Groyne to the south (photo waypoint) and take a photo from the handrail with the Groyne in the background.
References:
http://en.wikipedia.org/wiki/Groyne
http://www.ssec.org.au/our_environment/our_bioregion/kurnell.htm
