catherine96821:
what is surge in scientific terms? a backwash effect of the swell?
Scroll down to the first diagram in the the link in my first post and you'll see some diagrams showing the round-and-round motion of water beneath a wave. It shows both deep water action and shallow water action. That's the back and forth you feel as surge.
What gets really tricky both above and below water is what happens when a swell comes into shallow water, and particularly what happens when it hits a hard abrupt depth change -- a wall being the most extreme. On the surface you'll get a strong reflected wave that can cause some nasty lumpiness in the water as the incoming and outgoing swells meet each other ---- essentially doubling the wave height. You will also get this additive effect if you have two different swells coming in from different directions, or if you have both swells and wind waves.
Underneath, a sloping bottom will turn the normally horizontal surge into a nasty combination of both sideways and an up/down movement which is rough on divers. My worst ever subclinical DCS was shortly after getting certified. I played around, repeatedly riding the surge from 65' up to 50' for about 30 minutes out at Molokini. A real nasty, repeated sawtooth dive profile. Dumb. Dumb. Dumb. Felt really beat up and tired for the next 24 hours.
That article doesn't touch on the effect of sloping bottoms much, but it does have some nice diagrams of reflected and diffracted wave patterns near the bottom of the article Perhaps you have read stuff about the non-instrument navigational skills of the early Polynesian explorers. Reading the effect of islands on swell patterns was an important part of their skillset.
So, break down swell, wave, surge for me.
Surge is just water movement caused by swells, and to a lesser extent, waves. In the marine forecasts you will often see a forecast for both swells and wind waves. Swells are very smooth (sinusoidal is the 25 cent word) long period waves. They are coming from far away storms.
Wind waves are typically shorter period, have sharper faces to the waves, and are generated by winds closer in. Wind waves are dependent upon the recent winds, and typically will have just a short (1/2 hour to a couple hour) delay from the winds that generate them. In other words, when the trades kick up, the wind waves will soon follow and will be pretty closely aligned with the wind. OTOH, the swell is coming from far away and its direction is pretty much independent of the local winds. We know exactly what the swells will be and their arrival time because we have seen them pass by the various buoys that surround Hawaii. That's why the surf forecasts for the surfing crowd are so accurate.
