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.