Understanding Dive Visibility

Understanding Dive Visibility

There's nothing more frustrating than going through all the preparation for a dive, only to get there and have awful visibility. Visibility is the biggest thing that makes or breaks a dive, and yet it remains a mystery. What is it? How does it work? And how do we know when it will be good or bad? We're going to cover all of this, but we're going to start with the basics. Hope you're ready to talk nerdy.

What is visibility?

For us as divers, it means how far away we can discern an object. 

When we see something, light is being reflected off of it and into our little eyeballs. When we see something underwater, that light typically is coming from the sun, down to the object, and then is reflected back to us. 

I would say the simplest equation for visibility would be:

Light entering the water + water clarity = visibility

*if night diving, this equation would essentially boil down to just water clarity, but for the purpose of this article we're going to focus on day time diving

Basics of Bad Viz

Visibility will be reduced by anything that blocks, scatters, or dims light. Both the light that gets to the water, the light that gets through the surface of the water, and then the light that gets down to the object, and from the object to our eyes. The first two are easier to understand.

Light Getting to the Water (Light Source)

If it's dark out, or if it's cloudy, there will be less light that gets down to the surface of the water, and therefore less light that can illuminate an object and reflect back to our eyes. What this means is that visibility is better when it's a clear sunny day and worse during a cloudy day.

Light Getting Through the Surface (Surface Scatter)

When light hits a flat ocean surface it will have a much easier time getting down to an object than if the surface is choppy. A choppy surface, usually due to wind, will scatter that light and decrease visibility. 

Beyond this we start getting into the really tricky part, and the real mystery, which is water clarity. This is by far the biggest and most impactful factor regarding visibility, and there are a ton of components that affect it. There is a principle that we have to understand first, which is called suspended sediment. This means any particulates that are in the water column that are blocking light. They can come from a myriad of sources (which makes visibility so hard to predict) but for the most part these particulates are the things on the ocean floor that are getting churned up for some reason.

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Importance of Ocean Floor Composition

First you have to start with an understanding of your bottom type, or what your ocean floor is made of...usually rock, sand, or silt. The smaller the particulates on the bottom (silt), the easier they are to suspend and the longer they take to fall back down through the water column and settle on the bottom. Imagine being underwater with your fins on. If you kick downwards on a rocky bottom, not much is going to happen. If you kick downwards on a sandy bottom, you're going to kick up a big cloud of sand on the bottom. If you do this in a silty area, you're going to kick up so much sediment that it's going to cause a blackout.

What Affects Water Clarity

Ok now that we understand bottom type, and we know that some areas will be much more volatile than others, we're going to talk about the forces that act on the ocean floor and kick up the sediment that blocks out the light. I'm going to do my best to break them down to the best of my understanding, and I'm organizing them in the order of what I check when determining if I will dive or not in San Diego. 

Waves

We get a lot of waves in San Diego, and it's one of the reasons I love living here! While great for surfing, big waves don't help with visibility. 

Waves 101: How they work

They say a picture is worth 1000 words, so I'm going to let this one do some talking.

When the wave energy comes into contact with the bottom, the waves start to steepen and eventually form breaking waves. Keeping visibility in mind, when the lower portion of the wave comes in contact with the bottom it starts to stir it up, but only a little because there isn't much energy there. As the wave gets closer to shore there is more energy coming into contact with the bottom, which means it's stirring up more sediment and creating worse visibility. 

Waves 201: Swell height, period, and direction

As we saw in the first picture, waves reach further down into the water column than we realize. But how far? It's all based on the wave length. The wave length is the distance between two crests, and this picture shows that.

While wavelength can be measured as a distance it is more often measured by the time it takes two wave crests to pass the same point. This is called period, and is measured in seconds. The longer the wave length (or period), the further down the waves energy will reach. What this means for us is that one 3 foot swell with a long period can have a much greater impact on visibility (and surge) than a short period swell. It may look the same from the surface, but the longer period swell will create more surge and worse visibility.

So what about wave height?

While wave length is massively important because of how far down the wave reaches, wave height (the thing we see) definitely plays a huge roll. I would attribute this simply to impact. A larger wave is going to have more water that crashes into the bottom when it breaks, which means it will suspend a large amount of whatever is on the bottom.

Wave direction is probably the easiest. Think about it like wind. If you're in Chicago (not that you would ever want to be) and it's crazy windy, you could stand behind a building and be protected from that wind. Or you could stand in front of the building and feel the full force. I would recommend not going to Chicago, but if you're there the best bet is to hide behind the building. Same thing with waves. If we have a steep south swell in San Diego, La Jolla Point is going to stick out and block a lot of those waves from hitting the cove or the shores. 

Tides

Just like waves, tides are moving water that stirs up sediment. Here are a few tips regarding tides and why

1) Dive on an incoming tide

If you dive on an outgoing tide, all the water is moving from shore out to sea. This means that the murky water that has been churned up by breaking waves is getting pulled out, bringing all the light blocking sediment with it. If you dive on an incoming tide, clear deep ocean water is moving towards shore. That's what you want, but you want to catch it at the right time

2) Tide intensity

When tides are at their peak (high or low) there is no water moving. At the midpoint between peak tides, the water is moving at its fastest. The faster water is moving the more sediment it's going to kick up. Also, the bigger the difference between the high and low tides, the faster the water will move. During full and new moons the tides are more extreme, which means they will move more water in between the highs and lows. Best bet is to dive an hour or two before the high tide. Worst bet is diving halfway between a high to low tide on a full or new moon.

Wind

Wind is what creates waves. Let's take a look

Sometimes we get what is called a wind swell, which means the waves are more in the chop category than the fully developed sea swells, or ground swells. While these waves don't have the same period as a ground swell and don't reach as deep, they still have a big impact on visibility for a couple reasons. First off, they have a really high frequency. This means that while they aren't as powerful, there are a lot of them and they come in non stop. This causes non stop churn...aka suspended sediment. In addition to this the surface scatter is much more intense with wind chop than with a ground swell. In the grand scheme of things this isn't nearly as important as the suspended sediment, but it's something to think about.

Rain

Rain itself isn't a bad thing, but rain creates runoff which can be a bad thing. Remember the more things that get in the water column to block light, the worse the viz is. So if there's a bunch of dirt, grime, and pollution pouring off of the streets and into the ocean, the viz will be worse.

So those are the main things, but there are some other factors that are either more anomalies or are much harder to predict.

Algae Blooms

Algae blooms are the type of thing that only happen once or twice a year, but when they do happen, they can shut down everything. They are usually our fault (something we've dumped in the ocean starts growing), and NOAA is spending a ton of money trying to figure out how to predict them. For me the best way to stay on top of this is to listen to the local dive reports and wait until the bloom dives off.

Longshore Sediment Transport

Now we're getting really nerdy. Basically there are these big longshore currents that can take someone else's murk to your own beaches. In order to stay on top of this you have to know where your currents are coming from, and know what has been going on in that area (aka a big storm, a polluted river, etc)

 

 


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