How does the sand get to the beach?!

Now we get to the fun stuff ... the beach! In the last post we explored how the sand gets to the coast from the mountains. We all know it's rivers, but it's a great process to explore. Then the sand hits that crazy area where currents, waves, and tides all interact. These competing forces move the sand around and get it to the place we are most often thinking about ... the beach.

I am going to take a step back a little, because we haven't really talked about what sand is. The particles that make up sand have an important characteristic that makes them sand and that's the size of the particles. The most common definition of sand is any particle that is larger than 63 microns (0.063 mm) in diameter according to whats called the Wentworth size scale. Now that's pretty small!  More commonly what we think of as nice "beach sand" is larger than about 200 micron (0.2 mm) in diameter. So, beach sand is essentially quartz sediment larger that about 200 microns in particle size.

Great, that's the size, but what is sand made of? Most commonly, sand coming from the land is made up of the mineral quartz. Straight from the dictionary, quartz is:

 A hard white or colorless mineral consisting of silicon dioxide, found widely in igneous, metamorphic, and sedimentary rocks. It is often colored by impurities (as in amethyst, citrine, and cairngorm). (Google, 2015)

However, that definition gives us some good insight into the sources of most sand. Much of our land on this planet is made up of some type of igneous, metamorphic, or sedimentary rock. When rain erodes these rocks over time, a lot of the sand ground out is made up of quartz. The ground up rock washes down gullies, hillsides, and other streams to the rivers and so on, to the coast.

One other big source of sand, in areas such as the pacific islands, is coral reefs. The reefs are made up primarily of a material called carbonate (another mineral like quartz). Interestingly, fish are a common way some reefs get broken up into sand. Parrot fish cruise along munching on the reef and grind it up into sand.  One parrot fish can produce up to 200 lb of sand a year (Thurman and Webber, 1984). That's a lot of sand!

One parrot fish can produce up to 200 lb of sand a year!

Suffice to say there are a few different sources of the sand grains. Now let's get down to it and talk about how it get's where it is going! We have a pile of sand on the coast where a river has dumped it out, parrot fish has pooped it all over, a cliff has eroded, or some other source has dumped it there. Once on the coast, the waves can start pushing this stuff around. We'll save a full wave mechanics discussion for future articles, but we are going talk about some of the important things the waves do at shorelines to move the sand.

There are two ways our sand can be moved by the waves:

• along the shore, called longshore transport, and
• on or off the shore, called cross-shore transport.

Longshore transport moves the sand in pretty exciting and phenomenal ways. I'm sure you've sat on the shore watching the waves come in and have probably noticed that they hardly ever come straight into the shoreline. If you can imagine, this is because of that angle the waves are often peeling along the beach. Think of that peeling like a push along the beach from the waves. That push is a real force expressed by the waves and is felt by the sand out there under the waves. The force moves our sand down the shoreline in ... you guessed it ... longshore transport.

... here in Santa Cruz, waves can push enough sand in the harbor mouth to fill in up to 15 ft of channel overnight during a storm!

Figure from the Santa Cruz Harbor showing how sand is
moved from the river down the coast to the Harbor and
beyond due to longshore transport (also called littoral drift).
Source: http://www.santacruzharbor.org/dredgeWhy.html

Longshore transport is typically the biggest way sand is moved out of the river mouth or delta to other areas along the coast. In fact, here in Santa Cruz, waves can push enough sand in the harbor mouth to fill in a 15 ft deep channel overnight during a storm!

When you look at the figure below, imagine a river has dumped sand on the left side of the picture (upcoast direction). The waves coming in from offshore are also coming from the left slightly and moving to the right (blue arrows show the wave direction). The pushing from the waves moves the sand to the right (downcoast direction). The black arrows point in the direction of the steady march of sand due to longshore transport.

Waves coming into the coast from the upper left push to the right along the beach. Sand gets picked up by the waves and moved along that direction. The movement along the shore is called Longshore Transport.

If the sand is just pushed along the shoreline how does any sand get onto the beach? When the waves move to the coast they create another force pushing to the beach. Just like longshore transport, the force of the waves pushing into the beach creates a cross-shore transport of the sand.

In the summertime, when the waves are smaller, the sand piles up on the beach with each wave and slowly builds up. Over time that sand builds to a happy status quo called the equilibrium profile. The equilibrium profile typically has a nice berm with a flat beach behind it.  This is where we set our towels down, throw up some volley ball, and roast ourselves in the sun while enjoying a frosty cold beverage. Those gentle waves push the sand higher and higher up the beach with each tide.  The sand eventually builds us a summer beach profile as shown in the figure below. 

Cross-sections showing summertime and wintertime beach formation and sand movement. Smaller waves in the summer typically bring sand to the beach, while large winter waves move sand offshore into sand bars.

The whole sand dynamic on the beach does somersaults in the wintertime.

You're well aware that things liven up quite a bit in the wintertime. The weather gets cooler, the sky darkens, and the wind starts brewing in the ocean. The winds build up big waves that get bigger ... and bigger .. until we have big storm waves breaking along our nice calm summer beach. Those waves, as you'd expect, pack a much bigger punch when they slam onto the sand. They pick up the sand in all that turbulent mess and pull it off the beach. The assault pulls the sand out to deeper, somewhat calmer, water where it forms a new underwater sand bar. The figure above shows how our nice summer berm was cut away by the waves and moved to the winter sand bar. 

In Santa Cruz County at Manresa Beach, the winter sand bars cause the waves to break offshore and give us some fantastic surfing conditions when the conditions are right. That's what inspired the invention of the WaveClock, to see the waves and tide out there in real time. We've got a number of them up on Etsy ready to go now, or order a custom one on the website. Also, don't forget to sign up for our free newsletter on the left of this page to keep up with what's going on in the ocean world!

This article describes pretty ideal world stuff. Some beaches may have no sand in the summertime and lots of sand in the winter depending on the waves, sources of sand, and the type of coast (such as cliffs or offshore reefs). Also there are rip currents, offshore canyons, rocky points, and other coastal features that can modify the movement of all this sand. In the next articles we will explore sediment budgets (yeah sediment has a budget too!) and wave mechanics to develop a deeper understanding of how all these things play together on the coast.

Happy Exploring!
Craig Jones

Reference
Thurman, H.V; Webber, H.H. (1984). "Chapter 12, Benthos on the Continental Shelf". Marine Biology. Charles E. Merrill Publishing. pp. 303–313.