Where in the hell do our beaches come from? Sand on the brain ...

View of the Homochitto River.
Source: Google Earth, Imagery Date: 4/9/2013

I was flying across Mississippi last week, looked down on a river, and was struck by the white sand bars shining brilliant through the green forests. Going on Google Earth it took a few minutes to find out what river I was looking at, but I found it. The image here shows the tortuous path of the Homochitto Rive. The white sand pops through the green-forested terrain outlining its path as it flows eventually to the Mississippi River.  To me, an engineer/scientist who has been looking at water bodies for most of my career, it truly floored me the amount of sand in this river.  Certainly, this river has no problems there!

Sand is an important, sometimes critical resource, in coastal communities.

Anyone living near the coast can likely relate to a local beach or coastal region where sand or a lack thereof, is a problem for local communities. Sand is an important, sometimes critical resource, in coastal communities. It can provide a resource in the form of a thriving tourist industry to lay it’s blanket down and support a local economy. It also provides an important protective barrier in regions ravaged by periodic hurricanes. Sand is akin to a band-aid protecting the tender shoreline from the ravages of storm waves.

The high value of sand, and its management throughout the world, had me reflecting on the importance of what I saw abundantly in the Homochitto River from above and what we desire so deeply the coasts. If we take a brief journey from the hillsides, fields, and mountainsides where this sand originates, we can follow it through the river, down to the coast, and eventually to the deep ocean. 

The river itself provides the true punch of the mighty force to deliver this precious gold (figuratively and literally) from the mountains to the coast. Most of us know, or can certainly surmise, that most of the sand and mud we see flowing in a river comes from upstream. What we well understand is that scouring, erosion, and removal of soil and rocks from upstream bring sediment (sand and mud) downstream. Leonardo Da Vinci pondered these problems in the 15^th and 16^th century while watching the complex course of water flowing through local rivers (Graf, 1984).  We'll hear a bit more from him in a bit.

All this “stuff” transports downstream through the river like a conveyor belt. Of course, some of this material rains out in deposits along the river’s course during this journey. Where does it rain out? Rivers twist and turn, and like a freeway zooming with cars, the water speeds up and slows down. Slowing down along the inside of turns and speeding up on the outside of turns. Sediment drops out in the slower flow and is picked up and moved through the faster flow. Where the sediment drops out and builds up on the inside of the turns “point bars” are formed.  Where sediment picks up and the water cuts away at the riverbanks are features called “cut banks”.

Point bars form on the inside of river bends where the
water slows down and drops out sediment.

An interesting little rule of thumb as true today as in Da Vinci’s time is an observation he noted (Simons and Sentruk, 1992):

Where water has the least movement, the bottom will be of the finest mud or sand, where the water has a stronger current the shingle is larger.

What he is saying is that where the water is moving slow the sand and mud drop out of the river flow, and where it is fast the “shingle” (or material the river bottom is made up of), is larger. We see this in rivers that have cobblestones and boulders where it is fastest flowing. Pretty cool physics in action!

What struck me looking at this river from the airplane is no just that there is sand, there is a TON of sand on the point bars visible from 30,000 ft in the air.

Why do we care about this and why are you laboring through my long-winded explanation? If we go back to that picture of the Homochitto River we see sand. What struck me looking at this river from the airplane is no just that there is sand, there is a TON of sand on the point bars visible from 30,000 ft in the air. The blatant display of sand by this river is unique. It’s not easy to go to Google Earth and look for lots of rivers around that appear as brilliant with sand. To me, that means there is TONS and TONS more sand making it downstream that we don't see. Downstream to the coast!

What happens to all of this sandy gold when it hits the coast? It can hit an estuary, think San Francisco or Chesapeake Bay, it can form a delta like the Mississippi River, or it can tumble full bore into the ocean. The interaction of the fresh water and salt water in these zones form many fields of science, but it is sufficient to say that the sand makes it to the coast eventually.

If we think about that sand going from a pretty swift flowing river to the big deep slow moving coastal current, we can imagine all that sand dropping out like it did on the point bars. At this point (no pun intended), the waves start to take over. Similar to the high and low speeds in the river, big waves pick up the sand and move it around, while smaller waves allow that sand to drop out. We all know that these waves vary in size daily, monthly, yearly, and by location and depth of water. The movement and fate of sand along the coast is amazing topic I am going to save for our next article titled “Now I am at the beach, where do I go?” or something equally satisfying!

Sign up for our newsletter and keep up with our comings and goings on this page and at The WaveClock. Order your very own WaveClock there to see the ocean in real-time and really know how that sand is moving. Email us anytime at contact@thewaveclock.com

All the best!
Craig Jones

References

U.S. EPA (2014, February 5^th) WATERS Data. Retrieved from http://water.epa.gov/scitech/datait/tools/waters/tools/waters_kmz.cfm

Graf, W. (1984) Hydraulics of Sediment Transport. Water Resource Publications.

Simons, D. and F. Sentruk (1992) Sediment Transport Technology: Water and Sediment Dynamics. Water Resource Publications.

How to know what ocean wave measurements really show?

You are sitting out on the pier one day watching the waves roll by (not quite a song lyric, but you have the image). Each wave slowly rolls under your feet. You see the water rise and fall along the barnacle covered piling. You definitely have a sense of which direction that wave is moving. The top of the wave is a few feet above the bottom of the wave.  That's your wave height. It takes about ten seconds between the peak of two waves. So, that's your wave period. Wave height is how high the wave is from crest to trough and wave period is the time between those troughs and crests. Simple right?

Description of ocean waves from the Coastal Data Information Program (CDIP).

But wait ... it took 10 seconds for the first one to pass, 12 seconds for the next one, which was a foot higher, and then a bunch of smaller waves that are moving faster. This is not quite as straightforward as the single height and period we see coming from NOAA or other information sources.

The reason, as anyone who watches the ocean for any time knows, is that the surface is a mix of waves of different lengths, heights, and directions all mixing together. An average condition, sometimes referred to as the sea state, does emerge, but all those waves jumble together. So, what measurements are the buoys giving us? The buoys measure the vertical motion of the sea surface and compile all of those height measurements into what's called an energy spectrum. The energy spectrum is the amount of wave energy (which is the height of a wave squared) split up into frequencies (frequency = 1/period). There are many reasons energy spectra are used to describe ocean waves, but primarily they provide scientists and engineers an easy way to compile complex information resulting from measuring the sea surface.

It helps to break down a single spectrum to better comprehend it. The figure below shows a spectral measurement from a NOAA wave buoy. The spectra is essentially the "raw" data coming out of a wave measurement buoy. It is compilation of the wave data over 20 minutes. The plot shows the energy (that's roughly our wave height squared) on the Y-axis, frequency on the bottom X, and period on the top X.

Image of the Monterey Bay NOAA NDBC buoy 46042 spectral wave energy. 

The wave spectra has a dominant or peak period that we see from the NOAA measurements. That's the easiest part to pick out. It's just the peak along that little spectrum and is about 10 seconds or at a frequency of 0.1 Hertz on this plot. Another one we see from the NOAA buoys is the average period. It is the average period of all of the waves over a time period. However, in my opinion the dominant period usually tells us what we are interested in, but it can miss longer period swells like a good south swell in Hawaii or California. That's where we may look for a second peak in the higher periods (lower frequencies) in the spectra.

Then there is also the height, but that expresses as energy in the spectral plots. How do we get a wave height from this? Fortunately, we can easily calculate a significant wave height form the spectra. It's the average of the highest one third (highest 30%) of the waves measured in a 20 minute time period. The way we do this mathematically is we sum up the area under the spectra, take the square root, and multiply it by 4.

The wave heights and periods are all essentially based on the vertical motion of the buoy. If you are more interested in the math definitely dig in here: NOAA Wave Measurement Description

Wave directions are determined from either the horizontal moving or tilting of the buoy.  Think of an object floating on the sea surface.  As a wave moves past, the object moves forward then backward as the wave moves past. Also, its going to tilt forward and backward along the face and back of the wave. The measurements of these motions are also compiled into what's called a directional spectrum. It is similar to an energy spectrum, but it tells us which direction the waves are coming from over a 20 minutes period. AND .. if there are two swells in the water, it will measure both of the directions and be able to discern the height spectra of each.

Below is a directional spectrum from a CDIP buoy in Monterey Bay. Think of these plots like a compass with each direction representing where wave energy is coming from. Now instead of a line spectrum like the previous graph, we have colors to represent our higher (red) and lower (blue) wave energy. The plot shows the most wave energy (red) coming from about 300 degrees (west-northwest). We can also see some wave energy from the southerly direction. We have a little bit of south swell in the water.

Directional spectra from CDIP buoy in Monterey Bay.

Amazing stuff and maybe a little more complicated than most people thought, but now that you're in the know, share the knowledge!

Check out the WaveClock for up to date wave information right on your desk, living room, bedside, or anywhere you have a good WiFi connection! Sign up for our newsletter for more information. Latest designs available on Etsy.

What is the WaveClock?

Think of a classic nautical clock.  We get time, temperature, barometric pressure, and humidity in a classic analog format that's been in use for generations.

But many of us from surfers, ocean enthusiasts, fishermen, boaters, and anyone interested in the coast want to know what the waves and tides are doing and more importantly what the conditions are right now!

Lucky for us, NOAA maintains thousands of stations throughout the United States and outer coastal waters that measure the real-time wave and tide conditions continuously. All of that information is made available through internet connected data bases.

Welcome in the WaveClock! By connecting through your WiFi, the WaveClock is easily programmed through the smartphone app to the nearest NOAA wave buoy and tide station to display that data on an intuitive analog interface.

So what? 

We can look all this up on our smartphones, computers, tablets, or whatever and see what's going on. The big difference is that an analog interface is easier to read! It updates all by itself every 15 minutes and shows you exactly what's happening out there without messing around with any other devices. First thing in the morning, when you get home for a lunch break, after work or school, whenever you walk into your house! Also, we're working hard to make the coolest designs and even offer custom clocks just for you so it just looks plain awesome in your room.

So, check out the WaveClock. See our website www.thewaveclock.com and our Etsy store with our current stock ready to ship.

We've designed and developed the newest device to show you the ocean now. Pleasure Point Design has worked hard to perfect the design and displays. Everything is manufactured in the USA and the WaveClock is built right here in Santa Cruz, California.

Drop us an email with any questions or suggestions support@thewaveclock.com.

Thanks!
Craig Jones and The WaveClock Team