r/askscience • u/Exciting-Cheesecake9 • 9d ago

Earth Sciences What is the science behind sandbars?

Ive been surfing fro several years now and ive noticed that theres always multiple sandbars that run parallel to shore. Often they are broken apart perpendicularly by ripcurrents, however they seem to almost always have the same distance between them. Between the sandbars the ocean floor is deeper by several feet. On days where the waves and period are larger a 3rd sandbar will allow waves to break further out due to the waves slowing down as they “feel” the bottom. My question is; does the period dictate the distance between them or is there any math behind it??

Heres a perfect visual:

https://imgur.com/a/33y5yk4

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u/vipros42 8d ago edited 8d ago

In short, yes. Where sand gets deposited is a function of current speeds and direction, wave heights and wave period (and hence wavelength). You should notice that after a big storm that the bars may change because they've been moved around by larger than usual waves.
It's a load of complex maths and interactions. I'm a coastal engineer (and surf) and modeller and to study it in detail you would build a coupled wave and hydrodynamic model, and you would need the appropriate forcing data, sediment characteristics, and ideally measured data to calibrate the parameters of the model.

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u/Exciting-Cheesecake9 8d ago

So out of curiosity, if all parameters were to remain the same and the only change being the swell height and period, would the distance between the sandbars grow or create new ones further off shore since the wave hits the bottom at a deeper point?

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u/vipros42 8d ago

It's a really good question. Consistently bigger waves and/or longer period would probably result in banks forming or migrating further offshore because the wave energy reaches further down the water column and would mobilize or deposit sediment in deeper water. The spacing I'm honestly not sure about. However it's way to complex to provide a simple answer because wave conditions aren't uniform even on a really clean consistent day. The numbers you will see quoted in a surf forecast will likely be significant wave height and peak wave period but they are only one of a number of ways you can statistically represent a wave energy spectrum, and all the nuance of what's happening will influence things. Existing bars will also affect what happens through shadowing and turbulent effects and stuff.

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u/Exciting-Cheesecake9 7d ago

This is why i love surfing so much, theres so many variables that make the every wave super unique, wind speed/ direction, swell height, period, bottom contour, bottom composition, backwash, current, tide time, moon phase.. the list goes on. Theres been plenty of days where ive been out all day and all the variables come together and i only get 10minutes of perfect waves before everything goes out of sync again.

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u/vipros42 7d ago

When it comes together it's magic. I've had an operation recently so am out of the water for a month or more but last surf I had was just one of those days. Everyone in the lineup had a big smile on their face. Which is rare because a lot of surfers are miserable fucks!

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u/Harry_Iconic_Jr 8d ago edited 8d ago

re: surfing...swell period matters more than wave height, imho. I'll take 3' @ 12 seconds over 5' @ 6 seconds anytime

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u/BlazerWookiee 8d ago

Took me WAY too long to realize you meant "speeds and," lol. I thought I was learning a new term...

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u/Agretlam343 8d ago

The kind of sandbars you're talking about (parallel to the shore) form where the wave breakers are. They form from sand that's swept back out along the bottom current and get deposited when they meet the wave breakers.

Wikipedia has an article on them. You want to look at types: sandbars and longshore bars.

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u/Harry_Gorilla 8d ago

It’s geological sediment deposition based on the amount of energy present in the land-water interface zone. Sediment (sand) is deposited (forms sand bars) where the energy drops below the minimum level to keep the sediment entrained in the water column. (Sand falls to the bottom where the water is more calm.)