BoMan wrote: Interesting! Does this happen at a beach break where smaller waves break on the inside sandbar and the set waves fire on the outer one? Also, do the sandbars move around enough that you have to check for changes before paddling out?
Waves tend to break at a depth equal to about 1.3 times their height.
If your beach has a gently sloping sea bed, then waves will break further out directly correlated to their height ie the bigger the wave the further out it breaks
Most sea beds are not consistently sloping but will have sandbanks ie deep then the bank builds up to be shallower then goes deep again (sand banks are not not upside down U shaped, they are more pyramidal shape, so usually they have a slope leading up to their peak).
A bigger wave will break toward the outside (seaside) of the sandbank, whereas a smaller wave might not break right until the peak (shallowest part) of the sandbank. Depending on the seabed depth variations, the bigger waves might break 3m further out or 20m further out. And, of course, if there are outer sandbanks that are deeper, then bigger waves may break there and smaller waves just go straight over the top
if you have somewhere that goes deep -> suddenly shallow (like a reef or a point), then the deep part might be more than 1.3X the height of all waves, so all of the waves break right at that point. Because of the sudden depth change, the speed of the bigger waves is still pretty high so the bottom of the wave is suddenly slowed (as it hits the reef) but the top of the wave is still going very fast. Result.... barrels. But even small waves will still break at that point. Which is why point (and to a lesser extent reef) waves are easier to surf, because the take off point is almost always within a small area.
note: formula is
db =1.28Hbwhere
db - depth where wave break
Hb - height of breaker
as to sandbars - sandbars move slowly if at all during normal weather. Sand is pretty hard when compacted. However, storms can wash sandbars away entirely and the new sandbars will then need to form. Most of the time they will form roughly similar to where they were before (since the beach, rocks, headlands etc are usually similar and so water - carrying the sand - travels in similar patterns) but not always, and of course it will take time to rebuild grain by grain.
I suppose the summary is - day to day (excluding storms), you rarely notice much difference due to sandbar movement (i mean, tide, winds, swell direction etc etc are far more important). But if you visit every 3 months you may notice differences; if you visit after a storm it may be completely different and remain completely different
for example, a break I surfed a lot 15 years ago - a nice peeling left hander starting off a point - had the sandbar/sand over the rocks washed away in a storm and has never been the same since. In fact, it doesnt break at all now. Whereas the beach down the road, you have been able to line up with 'the blue house' on shore since forever, and its still the takeoff spot despite the many storms in the intervening years
