Sunday, November 30, 2008

A steeper beach profile

If you look at the beach profile at the moment it shows both erosion at top of the beach (backshore) and also in the areas exposed at low tide (foreshore)

This is not quite what the theory suggests might happen with erosion due to rising sea level. When the top of the beach erodes the lower section should be filled with the sand eroded, in other worlds the whole beach profile should lift. This is described in what is know as the brunn rule. However the beach at venus bay often has this form in winter

This photo was taken at beach one on 2/11/2008 at 11:17 am

If you notice new erosion scarps at the top of the beach (or at the base of the dunes). Take a photo, note the location and time and uploaded it. Just into the photos on Flickr (with the tag VBOP) and/or make a blog post here about it.

At low tide take photos of what is exposed.

So here is a summary list of what you may be able to include include

1) Take a photo, if you can. of the erosion
2) Record the place, data and time you took it

Any of the following information is likely to be helpful, so record that as well if you can

3) Look up a tide table and record the last high tide
4) Note the air pressure, and also if it is rising or falling
5) Describe the weather, and wind
6) Describe the waves (and swell)

Friday, November 28, 2008

Undertanding some names for parts of the beach

Before getting too involved with recording changes in the beach it is a good idea to become familiar with a few of the technical terms used to describe parts of the beach (or maybe more technically the coastal zones)

Note: this diagram is not to scale and is also vertically exagerated

There are two commonly refered to lines along the beach. The first is the coast line and it is the line that appears on maps. It marks the foot of cliffs or dunes, and it is usually marked by a distinct change of slope. This is the most important area to measure and observe coastal erosion (often called coastal retreat). The second is the shore line, which is very hard to see and define. It is conventionally described as the conceptual intersection between the high watermark and the shore (beach) and is usually only loosely indicated on navigation charts or nautical maps. This line is really difficult to define in practice beacuse the high water line (MHWL, mean high waterline) is a fleeting event and as we will see changes over time.

The Beach (you may see it called shore in american texts) actually extends past the shoreline out to the low tide point (MLWL, Mean Low waterline). After that the sandy slope and water above it is know at the Littoral Zone. This extends out to the closure depth, the conceptual depth beyond which there is no longshore or cross shore transportation of sand. Usually the breaker zone extends out towards to this depth.

Sunday, November 23, 2008

What is happening to the beach?

The beach along Venus Bay has been eroding significantly this year, with many high scarps left around beaches 1, 2 & 3, So what is going on? Superficially it looks like it might be evidence of sea level rise, perhaps strong evidence of global warning. However if that were the case the beach should be higher and narrower, whereas the beach is wider than normal for this time of year and lower.

The erosion has been occurring at specific location associated with specific high tide events. The king tides (higher spring tides) and some storm tides (mainly due to strong onshore winds & swells rather than storms) have been responsible for specific stretches of erosion rather that a continuous event along the beach. At the same time the beach profile has deepened exposing shell beds in many locations and is clearly lower at most locations,

I suspect that many of these changes started last spring with the king tides, when a few erosion scarps occurred in only a couple of places along the beach, At the same time the normal change of the profile from its winter shape to summer shape did not occur along much of the beach at all last summer. The winter berm, (the beach’s winter shape) has a distinct parabolic shape with a steeper gradient at the back of the beach. The same winter profile appears to be continuing this year. The summer berm, (the beach’s summer shape) has a flat upper dry section at the back of the beach and long flat foreshore with extensive development of sandbars. This flatter profile in the tidal zone now seems to be occurring at many places but under an obvious winter berm at the top of the beach.

I have been trying to work out the source of all the sand that builds the venus bay spit and direction of sand drift for a few years now. I have also noticed that last year the dominant sand drift direction has been fluctuating locally from a general northern drift over previous years. My current thoughts are that the beach is not washing away, the sand is just being moved up and down the coast but that the beach profile has been rotated down a little, I suspect that the shape of the coast is trying to adjust to a subtle change in the equilibrium of the forces that shape it, currents, swells, tides, storms and climate generally.

This blog is a place for everyone to recorded their observations.