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.

A good day for a walk

I hope those that braved the walk out around Point Symthe area had a great day. One of the highlights was seeing a pair of hooded plovers possibly at a new nesting site and some oyster catchers resting on the beach.

The experiments with trying to directly observe sand drift, where not totally conclusive (steve may have photos) but overall seemed to show more movement to the north west than south east. This corresponds with the direction obtained by reading the sand bars, which where attached to the beach at the south eastern end.

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

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.