Longshore movements: Burrum Coast National Park, Hervey Bay Q

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Comments (8)

ebi lutze on April 20, 2012

Hi Ian* From the group (A1- Unique Australia) the advantage of aerial photography, you clearly can see how the movement works. excellent capture **L Regards Ebi

Ian Stehbens on April 20, 2012

Yes indeed, Ebi. Let me have a go at describing the process seen here.

There's a big supply of sand available on the floor of the bay, supplemented by the reservoir of sand in the beach-dune system. The refracted waves moving across the surface of the bay approach the beach at an angle, resulting in their breaking at the northern end first. This sets up a longshore flow of water in the surfzone, moving southward. Sand, that is disturbed by breaking waves in the surfzone, zigzags its way southward as it is moved by the swash and backwash of the successive breaking waves. The net result is that the sand is migrating southwards. Thus the beach and foredune (or spit) grows southward diverting the outflow of groundwater from the backing dune system. This water forms a swamp or long lagoon behind the foredune/beach. It becomes a slow-moving stream heading south, until during storm or flood incidents its flow overflows the beach/dune barrier. This beach is also eroding during the storm (at the same time) and when the overflow begins the rush of out-flowing water scours an exit to the sea. Then the tanin-stained fresh or brackish water discharges freely into the sea. But if the discharge of water from the lagoon decreases, and as the sand is replenished, the flow of lagoon water may once again be blocked from emptying into the sea. Then water from the lagoon only reaches the sea by seeping through the beach. Until the next storm.

Hope that makes sense of this beach, Ebi.

Regards,

Ian

bdeh on April 22, 2012

Great shot and very nice area Ian. Greetings Berend

©junebug on April 23, 2012

Very impressive photo of this beautiful seascape, Ian! The explanations in your comment are very interesting and instructive! Thank you for that! It's always good to learn and at least partly understand correlations! Best regards, Anne

Ian Stehbens on April 25, 2012

It really is wilderness in the green sense of the word, Berend. Here there is little human impact on the system- beach, dune, lagoon, wallum forests. However, off-road vehicles of some recreational fishermen have driven along the foredune and beach and tracks are evident among the foredune vegetation (centre of picture).

Appreciatively,

Ian

Ian Stehbens on April 25, 2012

Dear Anne,

I would love to take you to such an area of unspoiled natural beauty, but as that is not likely, I enjoy sharing the scene with you through Pano. Thanks for your appreciative response. If I can give you even half as much delight in our landscapes as you have given to me from your many walks in your forests in the different seasons, then I am most satisfied.

Warmest regards,

Ian

Greg Swinfield on May 13, 2012

Hi Ian, thanks for the explanation you offered in the earlier comment. You have cleared up the shape of the creeks as they enter the ocean. It follows the same pattern on the NSW coast it seems. I am still a little confused. The sand is generally swept north to form the deposits like Fraser Is. Are there 2 flows of sand happening here? One near the shore and one in deeper water? Really like the shot Like

regards

Greg

Ian Stehbens on May 25, 2012

Hi Greg,

Your question is right on the ball!

If you take the GE image out 6 hits, you will see that this coast lies behind Fraser Island, and therefore the wave pattern is a refracted one, as the SE swell is refracted into Hervey Bay, or as the storm waves from the NE also are refracted into the Bay. This means that on the western side of the bay the waves are breaking at the northern or western end first so they are generating a sand flow southward on this coast, accumulating sand in the bay. Likewise on the western coast of Fraser Island, the sand flow is generally southward also, then the tidal currents suck and ebb the sand arranging it is ribbon islands and banks in Great Sandy Strait.

The southern shore of Hervey Bay, which is usually a low-energy coast, is very sandy, and shallow. This it is one of the sand sinks for your Hunter Valley sands, as is Moreton Bay. Yet most of that 300 000 tonnes of sand moving northward pa, ends up off the edge of the continental shelf just north of Sandy Cape and BreakSea Spit, for there the edge of the continental shelf changes direction to a SE/NW direction. One day the Hawkesbury Sandstone will have found its way into new sandstone beds east of Bundaberg!

Ian

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  • Uploaded on April 19, 2012
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    by Ian Stehbens

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