The Gold Coast is like Mecca for the tourists. They flock here from all over the world to enjoy a relaxing day on the beach sunbathing, fishing or surfing. Many of them visit the Southport Seaworld and learn tons about Australian marine life and then visit the Gold Coast Seaway in hopes of seeing a dolphin in the wild. Yet very few will learn about the geological challenge in creating the Gold Coast Seaway. The forever battle between man and longshore drift.
The Gold Coast Seaway separates the Southport Broadwater from the Pacific Ocean. It is also the mouth of the Nerang river, which until the formation of the permanent Southport spit, was continually drifting North due to longshore drift.
Longshore drift, also known as longshore transport or littoral drift, is the transportation of sediments (clay, silt, sand and shingle) when waves, or swash, meet a coastline at an oblique angle and then backwash perpendicular to the shore, thus transporting sediment in a zigzag pattern through the water alongside the beach. Therefore, longshore drift is dependent on the strength and direction of prevailing winds, which drive the force and direction of the swash, and the degree of backwash, which erodes the shoreline and moves sediment down the beach due to gravity.
The impact of longshore drift on the alteration of shorelines is dependent on five factors:
- Geological changes – e.g. the formation of headlands can limit the effect of longshore drift
- Changes in hydrodynamic forces – e.g. change in wave diffraction in headland and offshore bank environments
- Change to hydrodynamic influences
- Alterations of the sediment budget – e.g. how much sediment can be deposited
- The intervention of humans – e.g. cliff protection and the formation of seaways limit the effect of longshore drift.
The Gold Coast is world famous for its soft sandy beaches and huge surf. This soft sand is largely affected by the oscillatory force of the swash and backwash, a.k.a. breaking waves or surf, and the longshore currents that that affect the movement of sediment. With winds predominately coming from the Southeast and no barriers between the groyne at Kirra Beach and the Southport seaway there is no stopping the strong longshore currents from moving the soft sand North and into the mouth of the Nerang river, thus producing the Southport Spit.
A spit or sandspit is a deposition landform in the formation of a bar or beach which forms when longshore drift reaches a point where the direction of the shore inland re-enters or changes direction more than 30 degrees, such as at the mouth of the Nerang river. At this point the longshore current dissipates and can no longer carry the full load. Thus the sediment is dropped or deposited. As longshore drift continues the amount of sediment deposited increases and a sandspit is formed. This process will continue until the water pressure (such as a river) becomes too strong and does not allow sand to deposit. The length and height of the spit is dependent on the strength of wave driven current, the wave angle, and the height of incoming waves. A spit has two important features: (1) the proximal end (or head) is attached to the land and may form a barrier between the sea and estuary, and (2) the distal end (down-drift end or tail) may become long and curved due to the influence of varying wave direction.
Longshore drift has constantly been a problem on the Gold Coast. It created the long wide sandspit between Southport Broadwater and the Pacific Ocean. For years the steady migrating Nerang river mouth varied greatly in depth and width, creating hazardous navigational conditions and resulting in a significant number of boating accidents. This is very similar to the constantly changing channel between North Stradbroke Island and Moreton Island. In fact, in 1840 the mouth of the Nerang river was located at Surfers Paradise, but by 1924 it entered the ocean just south of SeaWorld’s present location. After major erosion in 1967 left the Gold Coast beaches vulnerable to further wave attacks a study, known as the Delft Report, recognised that the movement of sand within the nearshore zone (the area between the low water level seaward to beyond the breaking waves) of the Gold Coast beaches was greatly influenced by longshore drift. In order to mitigate the effects of longshore drift the Gold Coast Seaway was created.
The construction of the Gold Coast Seaway cost $50 Million AUD, took six years (from releasing of the plans to completion), and involved six key components: (1) training walls on either side of the seaway, (2) dredging 4.5 million cubic meters, (3) closure of the old entrance, (4) sand bypassing system, (5) revegetation of 70 hectares on South Stradbroke Island and Wavebreak Island, and (6) sewage outfall. Wavebreak Island was formed from the sand collected during the dredging and protects the Southport foreshore from strong storm waves and erosion. The Gold Coast Seaway is unique in that it was the first entrance in the world to include the construction and operation of a permanent sand bypassing system at the time that the entrance was created.
This Earthcache is designed to teach you about the Southport spit and longshore drift mitigation. It can be done by visiting all three waypoints and reporting your observations. Not all the answers to the questions can be found on signs. For these we are interested in your observations and what you can deduce from the information given. Keep in mind this cache is about longshore drift. Please note: the Seaway Kiosk housing the information at waypoint 1 is only open from 7:00 AM weekends (7:30 AM weekdays) until 11:00 PM.
Waypoint 1: Seaway Kiosk contains signs detailing longshore drift mitigation
- How fast and in what direction was the Nerang river mouth moving?
- Longshore drift has been a major problem to the Gold Coast Seaway. How much sand drifts North each year? What problems would this have on the Gold Coast Seaway?
- Training Walls
- What is the purpose of the training walls?
- What are the lengths of the two training walls? Why do you think the south training wall is longer than the north training wall?
- The training walls are constructed from 4,500 concrete cubes cast onsite. Each cube weighs 2 to 15 tonnes (1 million tons in total weight). In the event of erosion what happens to these concrete cubes?
- The Sand Bypass System plays a huge role in longshore drift mitigation.
- How far South of the training wall is it situated? Why is it situated so far South?
- The sand bypass system pumps 250-400 cubic meters of sand per hour using varying combinations of 3-7 pumps. Where does the pumped sand end up (name the beach and island)?
- What problems could exist from moving so much sand to this location?
Waypoint 2: A plaque commemorates the official opening of the Gold Coast Seaway. What are the official names of the north and south training walls?
- At what bearing (in degrees) are the training walls oriented? Why are they oriented on this angle?
- What evidence do you see that longshore drift exists?
- If the Gold Coast Seaway was not constructed what would happen to the Southport spit? Would this be good or bad? Why?
Feel free to take a photo of yourself on the Southport Spit and post it with your log.
Please e-mail us the answers to the questions, and include your e-mail address in the message so we can easily contact you. Please do not post any of your answers in your log as it will ruin the educational experience for other geocachers. No problem making this find in a group, but could all finders send through their answers. Any finders who do not e-mail through the answer and/or any logs with the answers will be deleted without warning. Once you have e-mailed us the answers feel free to log your find. If there are any problems we will be in contact.
Hope you enjoyed learning about longshore drift and the Gold Coast Seaway. There are thousands of international tourists who visit the beauty of the Gold Coast Seaway, but when it comes to longshore drift most of them don’t give a Spit.