[WPRESTO]

Thanks WPRESTO - Not a place for surfing at this precise spot - the other side of the harbour at Porthleven is where the surfing is normally done. This is regarded as a dangerous beach and because the water is deep close to shore the waves don't so much break as fling themselves onto the beach. Whenever there are widespread storms in the UK, it is often photos from this stretch of beach that hit the newspaper front pages.

---------- Post added 16th Dec 2018 at 07:31 AM ----------

Wave breaking at Lambeth Walk, St Ives, Cornwall

on Flickr

FYI, in case you did not know, some elements from a standard lecture to geology students about the dynamics of waves.

1) Ordinary waves (=not tsunami) cause circular disturbance below the surface down to a depth equal to half the distance between wave crests (= one half the wave length); the size of the circular motion gets smaller and smaller until you reach this depth, which incidentally is called the wave base.
2) When a wave enters water that is shallower than one-half the wave length, the wave begins to "feel bottom" and several changes take place:
A, the wave starts to slow down
B. the wave starts to lift up higher
C. the wave length gets shorter (because the waves closer to the beach are moving slower than those further out, so the ones further out start to "catch up"
D. some wave energy is lost dragging along and stirring up the bottom, part of this lost energy expresses itself as the slower forward motion of the wave.
3) in a sense, you can think of the wave as "trying" to maintain the relationship between wavelength and depth of the wave base in water that is too shallow, and the wave does this by making the wavelength shorter and lifting upward (literally pulling the wave base up) but the wave can only rise up an insignificant part of what is needed to maintain the deep-water relationship between wavelength and wave base, so the wavelength must get much shorter to compensate.

SO: if the water remains deep close to shore, a wave will hit the shore with far more energy than if there is shallow water nearshore. If there is a very gradual increase in depth that extends far off shore, wave energy will be depleted as the wave drags along the bottom and only a small wave will reach the beach.

IF THIS DYNAMIC IS UNDERSTOOD, then it is easy to understand why a bay or inlet is a safe harbor. As a wave enters a bay, it feels bottom along the sides or ends much sooner than in the middle, consequently the sides of the wave running along the shore slow down, while at the center of the bay the wave is still moving fast. The causes the wave to change from being straight (as seen from above) to being curved in toward the head of the bay. This has the effect of stretching wave = making it longer, which stretches out its energy which means as the wave moves into a bay, it gets weaker/smaller. The farther into a bay a wave travels the weaker it gets, so a deep embayment, even though it is open to the sea, can be a safe harbor.

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Last edited by WPRESTO; 12-26-2018 at 04:17 PM.

[nicolpa47]

WPRESTO, that's interesting and it explains why I see different behaviours in the places we visit in Cornwall - both "safe haven" bays, the deep water beach that is the Loe Bar where the photo was taken, and other wider bays where the effect of the bay sides is presumably less. I would imagine that these dynamics also help explain rip tide patterns which occur notably in one or two places we visit.

[WPRESTO]

WPRESTO, that's interesting and it explains why I see different behaviours in the places we visit in Cornwall - both "safe haven" bays, the deep water beach that is the Loe Bar where the photo was taken, and other wider bays where the effect of the bay sides is presumably less. I would imagine that these dynamics also help explain rip tide patterns which occur notably in one or two places we visit.

Tides are a little different, but here's another insight into wave behavior. Out at sea, seen from above, a wave front is approximately a straight line over modest distances (not tens of miles/kilometers). Except in exceedingly rare cases, waves approach a beach at an angle, not parallel. Consequently if you're on a beach watching incoming waves, they break progressively along a beach, commonly described as rolling down the length of a beach. Because of this, the waves are pushing water down the length of the beach creating a long shore current. The circular motion below the surface stirs up sediment, typically sand, which once tossed up into the long shore current is carried by it down the length of the beach. Where waves breaks on a sandy beach, they pick up sand and transport it or throw it up on the beach at an angle, moving it down the beach in the same direction as the long shore current out in deeper water. But when the swash of the wave becomes backwash, it flows straight down the beach face into the sea not back along the angled path it followed transporting sand up onto the beach. Therefore offshore because of the long shore current, and in the breaker zone of the beach face because waves run up at at angle but flow back straight, sand is constantly being moved down the length of a beach. Where there are extensive sandy beaches and vigorous wave action, the annual transport of sand can be prodigious. Where there is a sudden change in the line of the beach, for example bay or inlet, the river of sand will tend to accumulate as a spit. If it is the mouth of a river, the river of sand will attempt to block the river and cause it to turn parallel to the beach (=the river of sand builds a spit and the river instead of flowing directly into the sea turns and flows behind the length of the spit in the same direction that the long shore current is moving offshore). If humans construct a jetty to keep the river of sand from blocking the entry to a river or harbor or inlet, the river of sand will accumulate on one side of it eventually creating a wide roughly triangular beach, which if it grows long enough will allow the river of sand to flow around the end of the jetty, where it will construct a spit. Sometimes people construct groins = rock walls perpendicular to a beach = a jetty where there is no inlet or river mouth etc. On one side the river of sand will accumulate creating a wide beach (more-or-less triangular seen from above). On the other side, wave action will pick up sand, put it in motion down the beach and thereby gradually erode away the beach on the "downstream" side of the groin. From the air, a series of groins will convert an originally straight or gently curved beach into a saw-tooth pattern. Parts of the south shore of Cape Cod (the "Upper Cape") have precisely this appearance in aerial photos.

[nicolpa47]

Thanks again, WPRESTO. Some of what you have described I was aware of - but in general terms only. Porthmeor Beach at St Ives, regularly(at least annually), is bulldozed to push sand away from the cottages and other buildings at the back of the beach. I have usually seen it done in March/April after winter storms have largely passed, but in recent years it has been necessary much earlier. In fact the first such occasion this winter was about 3 weeks ago.

During WWII and after, the sand movement was not carried out, and there is a published photograph dating from 1952 where the sand build up extended to roof level of the buildings. At first sight I thought it was snow, but it was sand. There is certainly a large amount of sand regularly removed from and/or deposited on the beaches around Cornwall every year.

[WPRESTO]

While I'm on beaches: on some beaches the sand undergoes a summer-winter cycle. In winter the waves are generally larger, this tends to erode the beaches moving sand outward underwater under the influence of the circular motion below the surface caused by the waves. Eventually the outbound sand gets to such a depth that the circular motion of the water is insufficient to move it, and it starts to accumulate as an offshore underwater bar that parallels the shoreline.The bar itself acts as a barrier to block the outbound sand which causes the bar to grow larger, so once established, the bar is self perpetuating and continues to get bigger. As it gets bigger, it interferes with the waves passing over it, eventually causing them to "beak" over the top of the bar well offshore before they reach the beach. This puts an end to erosion of the beach. In spring, when gentler waves are rolling in, they break over this bar then cross a temporary lagoon between the bar and the shore proper. But these gentler waves have a much shallower wave base, and the underwater action tends to erode sand from the top of the bar and begin to move it back toward the beach. Eventually the bar will be destroyed and all the sand piled back up on the beach. So in winter, some beaches become quite narrow with a shallow lagoon separating the beach from an offshore bar where the waves break (the bar and lagoon may only be apparent during quiescent waves in the Spring), but in summer the beaches are wide again and gentler waves break directly against the beach. Also because of how the waves either erode sand from or pile sand back onto a beach, in winter there's commonly a fairly gentle slope down the narrow beach and out under the water of the temporary lagoon, but in summer the beach is higher, sometimes with a distinct berm built just at the high-tide line, with a steep "beach face" going down to the breaking waves which, of course, only come up to the base of the berm at high tide. This onshore-offshore movement of sand is in addition to the continuous longshore transport described above. It would be far easier to explain if I could use my power-point illustrations.

[nicolpa47]

A wave on the rocks at Poldhu Cove on The Lizard Peninsula, Cornwall
on Flickr

[WPRESTO]

Spectacular wave splash in the preceding image. A little splash in the image below.

Attached Images

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[taks]

beautiful sunset, but too cold for me to try paddle boarding

[mattb123]

Down at the river park this morning. Just a little exposed water left but it feels like it's starting to shift into being a little warmer as we head to early spring. It was even above zero this morning! (x-post).

[aaacb]

both shot on superia 400, and i converted the first to b/w

[WPRESTO]

Pretty little fountain in the "garden court" of the Frick Collection = his former private residence.

[nicolpa47]

Sunset over water, on Porthmeor Beach, St. Ives, Cornwall


on Flickr

[WPRESTO]

Sunset over water, on Porthmeor Beach, St. Ives, Cornwall on Flickr

That a wonderful image - fit to fill a wall up over the living room sofa.

[WPRESTO]

Here's another little fountain, struggling to keep flowing in a New England winter

[nicolpa47]

That a wonderful image - fit to fill a wall up over the living room sofa.

Many thanks WPRESTO, that is much appreciated although, as I say all the time, nature did the hard work.