Testing the sand drift

On the summer by the sea walk we tried a simple experiment. By quickly digging a trench in front of a wave, then we watched carefully as the backwash from the wave refilled the trench.

Thanks to steve for the video
If you watch, you will see the first and main backwash is from the top right (south eastern side), but then material also washes in from the top left (north western side). So the waves are not washing back down the beach in a uniform direction and maybe even our trench is influencing the backwash a little. Still we repeated the test three times with much the same result. So we must assume an inconclusive result that the sand is mostly washing to the north west (ie coming from the south east) which is consistent with the form of the sand bars, which joins to the beach at the southern end in this location.

Reading the sandbars

Most of the action on the beach at the moment is occurring down at the low tide line. If you look there is extensive development and changes in the sand bars. You can read the direction of the longshore drift of the sand by analysing the sand bars.

Look to find which end the bar is connected to the beach. If this is at the southern end of the sand bar the net longshore sand drift will be going to the north. If connected at the northern end the net longshore sand drift will the going south.

Note: Diagrammatic only & not to scale

The mechanism of longshore drift on a long sweeping beach like Venus Bay is fairly easy to explain. The wave hit the beach slightly obliquely (this depends of ocean currents and nearby coast and sea floor topography) and carry some sand particles up the beach in the same direction that the wave was travelling as it hit the shore. Then the wave receded but now the water runs down the gradient of the beach (ie at right angles to the beach) so some sand is again washed back but following this new flow direction. so the sand particles zig zag up and then down the beach moving them along the shore a little as each wave carries them up the beach.

Very much related to the development of sand bars along the Venus Bay beach are Rip currents. These are feed with water but the lagoons and depressions that form behind the sand bars water coming over the bars has to run along this hollow to get back out to sea. Sometimes it quickly erodes deep gutters (a favourite pace for surf fisherman at higher tides) or pot holes. Often the sand near the deeper pots holes is very loose and unconsolidated because it is recently transported there.

Over the past few years the longshore drift has been dominantly to the south but this year It would appear that the general sand drift has turned to the north. It would be nice if all those interested observed the sandbars and posted their observations of sand drift  as comments here or as messages  on Twitter using the either hashtag #VBOP or to @VBOP

Eg for twitter

@VBOP beach one sand bars show sand drift to north

What is a Spit?

The geomorhpic definition of Spit is a sandy bar projecting from the mainland into open water. Spits are formed by deposition of sediments moved by longshore drift. (here is a wikipedia link on spits, if you want more details)

So the Big question; Is Point Symthe a Spit?

Well it is widely described as a spit in tourist literature guide books, Often as Victroia's largest spit. However there may be a few problems calling it a true spit

1) The Andersen inlet estuary is no longer "open water" and there is a very strong tidal inflow and outflow cutting across the longshore drift

2) Observations in this project suggest that the longshore drift does change directions and has been larger going southwards for the past few years

3) It doesn't have the characteristic hooked end.

Yet physically it is a very large continuous section of sand and the present situation may represent a very late stage of spit development. Where by the entrance to Andersen inlet may close over with a continous sand bar from Point Symthe to Inverloch.

However as Andersen inlet gradually fills with sediment (not sure if anyone is accurately measuring this) The effects of the tidal flows (it is the same amount of tidal change but in a progressively shallower inlet) mean that there is now considerable erosion cause along the main chanels giving outlet to the sea. These infact are behaving like a meandering streams thrashing back and forth across the inlet floor and heavily eroding the coast & beaches but just at very specific locations (where the chanels curve most).

Who will win? Longshore drift or the tidal flow from Andersen Inlet.

Drifting North Again

This panorama was taken at the beach 5 car park and shows how the sandbars are attached to the beach at the southern end (see this photo full size). You can also see the ruts developing into rip channels also point to a return to genrally northerly longshore drify

Understanding Longshore Drift

When Waves arrive at the beach at an angle to the shore line, sand will be pushed up the beach in that direction however in the backwash the sand will usually moved directly down the beach, perpendicular to the shore line. So the sand follows a zigzag pattern. The result is a net movement of sand along the beach. An enormous amount of sand can moved in this manner.

By looking at new sand bars you can determine the longshore drift. If the sand bar is attached to the beach at its northern end the longshore drift is to the south. Conversely if the sandbar is attached to the beach at its southern end the longshore drift is to the north.

Longshore drift can fluctuate from day to day, and may vary locally as the beach tries to readjust its shape to be in better equilibrium with the waves and currents.