Wacky currents and strong spatial variations in current magnitude and direction and in wave height and direction along Ocean Bea

1895
Rant - Wacky currents and strong spatial variations in current magnitude and direction and in wave height and direction along Ocean Bea

So this article gives a bit of science to the wacky currents and difference in wave size and characteristics along OB that we've always known about. The study seems to span at least the last five years, but I hadn't seen anything like it, although the actual report only looks to be a few months old.

I'll be sure to bring this up at the next Ocean Beach Master Plan meeting if I go. The guys operating the rockdropping backhoes zooside should probably be given a copy so they can better understand their placement decisions.

Source: Pressure‐gradient‐driven nearshore circulation on a beach influenced by a large inlet‐tidal shoal system
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rant by fatnewt
6 comments

paddleout
48 weeks ago

Just goes to show, a lot of times the current isn't driven by the ebb-flow of the tide, but the waves and how they refract.

3
fatnewt
48 weeks ago

@paddleout - Which is what left me a bit puzzled with the Ocean Beach Master Plan's visualization from a few days ago. Their take on it, IMHO, was oversimplified in some ways and seemed to ignore basic shore processes in others. I realize OB is special, but you can't negate science. The forces generated and the differential between wave energy delivered shows variation from one end to the other, though. Here's the MOST basic principle that should be kept in mind: armoring creates accretion/depletion (sometimes real fast). Pretty simple, although alternatives aren't as cheap or simple as dropping a bunch of rocks in a pile.

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Righteousdewd
48 weeks ago

Yep. If they were interested in science, they wouldn't attempt to protect coastal structures. Its cheaper to ignore and drop a bunch of rocks.

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Bobby G
48 weeks ago

I could use the "executive summry" for my short attention span please.

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fatnewt
48 weeks ago

@bobbyG - see how the depth lines kind of center around "Site 5" in the image above; depth controls wave energy, so the energy gets through as waves in the center orangey portion of the beach. the waves and the prevailing direction they hit the beach then drive currents; more energy, higher currents. that's probably oversimplified.

If someone knows more about the apparent canyon towards the ###### I'd love to hear more about it. Or is it just south of the sand deposition from the Bay and the mysterious N/S divergent currents?

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whoa
48 weeks ago

My take: the off-shore "shoal" (sand pushed out of the bay mouth by the retreating tide) "refracts" (changes the direction of) waves, so that certain parts of the beach (e.g., Slo@t) get bigger waves while other parts (K3lly's and Funston) get smaller ones. It's like seeing the sunlight that travels through a lens and hits the ground: there are brighter (S) and darker (K's and F) regions. Now you also need to understand "wave setup": when there are waves the average level of the water near shore is actually higher than off shore: it's like the waves "stack up" water at the coast. The bigger waves at S stack the water up higher than the littler waves at K's and F. If this was all that was going on (no tides, winds, etc.), then water would flow from the big-wave parts of the beach to the little-wave parts of the beach: you'd have south current south of S and north current north of S. Other stuff is going on, of course, but this article shows that the water-stacking effect on the currents is quite big & explains much of the current differences at different parts of the beach.

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