Students take a break for journaling during our canoe trip on the Current River.
It was not all dark and stormy on our Outdoor Education canoe trip. The first afternoon was warm and bright; the first splashes of fall color spicing up the deep, textured greens of the lush, natural vegetation. It was so nice that, in the middle of the afternoon, we took a short break, just shy of half an hour, to reflect and journal.
A time an a place for reflection.
Our guides chose to park our boats at a beautiful bend in the river. Most of my students chose to sit in the canoes or on the sandy point-bar on the inside of the meander, but a few to be ferried across the river to a limestone cliff on the cut-bank of the curve. An enormous, flat-topped boulder had fallen into the water to make a wonderfully picturesque site for quiet reflection for two students. A third student chose to sit in a round alcove sculpted by the solution weathering of the carbonate rock itself.
A shady place to stop and think.
The cut-bank of a river’s meander tends to be deeper than the inside of the curve, because the water is forced to flow faster on the outside of the bend where it has more distance to travel. This proved to be quite convenient for my students, because it meant that the stream-bed around their boulder was deep enough that they could jump into the water after the hot work of writing while sitting in the sun. And they did.
The western end of Deer Island extends a white, sandy, artificial, spit that partially covers the first of a series of riprap breakwaters that protect the waterfront development of the city of Biloxi. Although we’d landed there to pick up garbage as part of our coastal science camp, the beautifully developed beach profile was worth a few minutes.
Figure 2: Narrow beach typical of the east-west trending shorelines that are not exposed to the direct force of the waves.
The spit curves just ever so slightly northward, so it feels more of the direct force of waves blown all the way along the length of Biloxi Bay. The combination of unvegetated sand and stronger waves makes the beach along the spit looks very different from the beaches that parallel the shore. While the parallel beaches on Deer Island are covered in grass almost to the water’s edge (Fig. 2), the spit has a much wider beach, with a nicely developed sandbar protecting a shallow, flat-bottomed, water-saturated trough behind it (Fig. 1).
While the white beaches are pretty (that’s why they imported this sand after all), there are a number of fascinating features in the trough.
Figure 3: On our Natchez Trace hike we found it quite easy to stick fingers into the red precipitate at the bottom of the stream.
The first, and most obvious question is, why the reddish-orange color in the fine grained sediment at the bottom of the trough? A microscope and a little geochemical analysis would be useful here, however, lacking this equipment, we can try drawing parallels with some of our experiences in the past. In fact, we should remember seeing the same color in some of the streambeds when we were hiking in Natchez Trace State Park in Tennessee (Fig. 3). My best guess at that time was that the red was from iron in the groundwater being oxidized when it reached the surface.
Figure 4: The rich black of decaying organic matter, sits just beneath the rusty-orange surface sediement.Figure 5: Green, organic matter, freshly deposited at the edge of the trough. If it decays while saturated with water it will turn black. Note also the splay of white sand at the top of the picture.
This is probably not a bad guess for the red in the trough as well, since there is some fresh groundwater discharge from the shallow watertable on the island. However, I suspect that the story is a bit more complex, because the rich black color of the organic matter just beneath the surface (Fig. 4) suggest that the shallow water and surface sediment in the trough is lacking in oxygen. On the other hand, it’s not uncommon to have steep geochemical gradients in boundary environments like this one.
The physical and geochemical gradients extend horizontally as well as vertically. At the edges of the trough the organic matter just beneath the surface is green, not black (Fig. 5), because this is the color of the undecayed algae.
At the seaward side of the beach, the waves of Biloxi Bay lap against the sand bar. When the tide rises, and the wind picks up, these waves wash over the crest of the sand bar pushing water and sediment over the top into trough. When the sand washes evenly over the top it creates thin layers (possibly one layer with each high tide). If you cut into these layers you’ll see little the laminations in profile, which, because the layering is close to horizontal, look like the lines of topography on a map (Fig. 6). When the waves wash over small gaps in the sandbar the sediment it transports is deposited in a more concentrated area – these are called sand splays – that overlap and cover some of the fine-grained, orange sediment at the edge of the trough. These are both two of the small ways that the sand bar moves, slowly pushing inland.
Figure 6: Sand splay and laminations on the landward side of the sand bar. The laminations are created by even overwash of the sandbar, while the splay is the result of more concentrated flow.
Bioturbation
The features on the bottom of the trough are a quite interesting because of the observable effects of bioturbation (disturbance by organisms) (Figs. 7, 8 & 9).
Figure 7: In close-up, the holes of the crabs and the mixture of colors looks like an arid, volcanic landscape photographed from space.Figure 8. Digging deep beneath the orange surface sediment, small crabs create mounds of white sand. Figure 9. Footprints of predators. Paleontologists use features like these that are preserved in rocks to discover interpret what the relationships between organisms was like in the past.
A groin strains to hold back the longshore drift. It is, as always, only partially successful.
It was about 1.5 kilometers from the Research Lab to the estuary where we spent our first morning sampling (overview of the trip is here).
Elevated beach house.
Walking along the beach to get there, we could see the beach houses to the right of us, across the narrow road of East Beach Drive, standing tall on columns to keep them above the reach of the storms. According to Stephanie, our guide, the storm surge from Hurricane Katrina in 2005, reached awfully close to the tops of the columns. The research lab, which is not elevated, lost an entire building to that hurricane. Indeed, much of the coast is still recovering from Katrina’s damage.
The white sandy beach, on the other hand, looked beautiful, which was a bit odd. After all, how did it survive the storm? Furthermore, when you think about it, this beach is located behind a string of barrier islands, which protect the coast from the full force of the waves coming out of the Gulf of Mexico, so how come there is enough wave energy to maintain a sandy beach. The relatively calm waters should allow finer grained sediment, like clay and silt, to settle out, and this area really should be a marsh. The answer, it seems, is that this is an artificial beach. Every few years, thousands of tons of sand are dumped along the coast to “replenish” the beachs.
Without beach replenishment the beaches would revert to salt marshes like this one.
This coastline really should be a tidal marsh, like the one we found when we got to the estuary. These Gulf-coast salt-marshes are fronted by a relatively short version of smooth cordgrass (spartina alterniflora), backed up by the taller, and more common black needlerush (Juncus roemerianus Scheele) .
Longshore Drift
Now, if this is a low-energy environment that allows silt and clay can settle out of the water column, where does the sand go so that it has to be replenished every so often? It is gradually moved along the coast by longshore drift.
Longshore drift moves sand along the coast in the direction of the wind. Image via the USGS.
Waves hit the beach at an angle. As they break, the turbulent swash pushes sand up the beach at the same angle as the movement of the waves. As the wave retreats, the backwash, drawn by gravity, pulls sand perpendicularly down towards the water. The net effect, is that sand gradually moves down the coastline with each swash and backwash of the waves.
Since dumping tons of sand is expensive, engineers try other things to prevent the sand from running off down the beach. Someone, a very long time ago, had the great idea to build a wall sticking out from the beach to impede the sand in its unwanted migration. This type of wall is called a groin (or sometimes a groyne in polite company), and it does stop the sand. In fact, the sand builds up on the upwind side of the groin. Unfortunately, it does not stop the longshore drift on the downwind side, and that results in the erosion of a bay on that side.
A groin impedes longshore drift. Note that the waves approach the beach at an oblique angle.
Pufferfish
Beaches are also great places to find random things washing up. We lucked upon an unusually large pufferfish (family: tetraodontidae). It was quite puffed up. It was also quite dead.
Pufferfish.
Pufferfish are famous for being extremely poisonous. According to the National Geographic page on pufferfish, their tetrodotoxin over a thousand times more poisonous than cyanide, and there is no known antidote.
I’ve always had a predilection for texture photography, despite the fact that I’ve only just now come to realize that there is actually a term for it. The changes in perspective you get from looking at things at different scales continues to fascinate. Texture photography focuses on the small but looking at big things from far away can have a similar effect.
Oil slick in the Gulf of Mexico on April 25th, 2010. Image from NASA.
The scale of the disaster caused by the oil leaking into the Gulf of Mexico from the damaged oil rig is increasing day by day. We are preparing to go on the end-of-year adventure trip soon, but I’m wondering if students might be interested in heading down to the Gulf coast to volunteer in the clean-up.
NASA’s Earth Observatory has some amazing imagery on its page on the oil leak. Many of the images also show the mouth of the Mississippi and its delta, which tie directly into our observations in the sandbox. The impact of the oil spill also brings up the topic of density differences in fluids, something we’ve seen in the making bread jars, but applied to a much larger scale.
The sandbox was built to be a wave tank so we could look at interference patterns and wave properties. But if you tilt it a little, and put in a few holes on the lower end, you can get sandbox to look at the formation of streams, deltas and the sedimentary layering that traps oil and natural gas.
Using the holes at the bottom end the students started with a low “sea-level”, raised it and lowered it. At the end of the run, they drained all the water and sliced the tank to see the depositional layers in cross-section.
We added red and green sand to try to make marker beds before each change in base level. The marker beds worked reasonably well, but it would have been better to have sand with different densities that could be sorted by the stream flow and depositional environment. It also helps to get the colored sand wet, to make a slurry, otherwise the grains will float on the water.
The shifting lobes of the delta showed up very well (see the animation) and some nice river features showed up as well. What I want to do sometime is to have students build coastlines and have waves erode them away creating typical coastal features.
My students were even able to demonstrate the tank for their presentation, because it really only takes half an hour to get all the features if you know what you’re aiming for.
