The display I run is called The Wave Tank.
The Wave tank was a shallow pan with about 1-2 inches of water in the bottom. At one end is a shop fan, the type that can move a lot of air in a small space. In the middle of the pan is an island, built from playground sand, and green and brown colored sand from Micheals. The colored sand helps define the island surface and show the erosion as the model runs. I usually build the island like a dog's toy bone, thick and high on the ends and thin and low in the middle. On a real island the sand piles would represent dunes. On top of the dunes I place houses built from small wooden cubes, some with nails in them to represent pilings, others without nails. They are stuck into the sand at various levels and places. It represents a typical barrier island, one you can find along much of the East and Gulf coasts of the US. At the back of the pan there is another pile of sand, which represents the mainland behind the barrier island. It's a surprisingly simple representation of a coastal system and I use it to show what can happen during a hurricane.
When I turn on the fan, the effect is almost immediate. This simple model begins to erode within minutes. The whole thing takes about 60-90 minutes to run and shows several import coastal processes that occur during a storm. First, a scarp forms on the front of the island, and the sand begins to move from the front of the island to the sides. This demonstrates the natural sand transport that is always occurring on the beach. A small flat shelf of sand develops in front of the island as the "dunes" erode.If you look close, you can actually see the sand grains moving along that shelf. Sometimes, when the houses aren't completely stuck into the sand, they'll blow over. That's OK, because damage to coastal structures isn't always caused by water.
When I build the island, I put a layer of brown sand inside the island. The neat thing about the brown sand is that, for some reason, it doesn't like to absorb the water. As it turns out, this is a good thing for the model. The brown sand traps air and behaves the same way a layer of muddy, marsh sediment would behave, acting at first like a resistant layer to erosion from waves. It makes the dune scarp more pronounced and for a short time protects the island from eroding.
As the storm continues to blow more sand is removed from the front of the island and transported to the sides, where spits form. In the center, where the surface is lower and narrower, the island becomes dangerously thin.
On the ends, the brown sand has been exposed and the overlying layer sides off into the water, taking the houses with it. This sand becomes an additional source of sediment for the growing spits. The houses on top of the sand, with pilings beneath them might tip or fall into the water.This year, as an added twist, I built a seawall of small stones for a few of the runs. Behind the seawall, things are better, but only for a while. Waves refract around the seawall and begin to erode the dunes from the sides. In fact, in one run during the open house,
the side of the island with the sea wall actually eroded faster then the side without at first, due to the action of the waves around the seawall. Eventually, the unprotected side lost it's battle while some of the island with the seawall remained protected. It showed that the seawall wasn't the solution to the problem of erosion and in some places made the erosion worse.
The model also shows that just because it had pilings didn't mean a house will survive. If they aren't blown over by the initial gusts of wind from the fan, they still had to deal with the erosion, overwash and undermining of their supports. If the pilings were long enough to push all the way through the sand to the bottom of the pan they might survive. Sometimes, a house that started at the top of the dune ended up standing in water, with no land around it anymore. If the pilings were too short, then it was only a matter of time before the sand would erode away, and the house would fall over.
Eventually, the center of the island breaks through, forming a breach. Sand from the two now disconnected islands is transported through the gap and an overwash deposit is formed. My once tall pile of sand has flattened out and changed shape.
I reset the model three times during open house. Sometimes with a seawall, once with a breakwater in front of the island, but not touching it. No two runs are ever the same. Sometimes the island erodes quicker, and sometimes slower. If the island is closer to the fan, it erodes faster, just as if the hurricane were a stronger storm.
In our model we see scarps form and underlying "muds" exposed, dunes washed away and structures fail, we see islands breached and islands overwashed. All these things we see in nature, all these things we see on real islands in real storms, on real coastlines.The first time I ran this model was in Reston, VA, for the USGS Open House at the National Center. I wasn't completely sure at the time if it would work. Oh boy, did it work! The kids not only loved it... they got it. Would you like to live on a barrier island?... "No..." Why? "Because it's going to go away." Where would you like to live? "Back here" they say pointing to the sand at the back. A father looked at me and said "Now I get it." One boy was particularly excited. He explained what he thought was going to happen to his Mom. And he was right. Then for the next hour, he returned about every 10 minutes, Mom in tow, to check the process. With each visit he pointed out what had changed, and which houses were closer to falling over, and which ones had. This year, in St Petersburg there was girl who returned again and again to check the progress of "her" house, the one she'd picked at the start of the run, to see if it was still standing.
It's a simple model, but it works. Over the course of an hour, I can show someone what might happen to a house built on a beach. I can show them what might happen to the island itself. A beach, or a barrier island, is a pile of sand. It is, by it's very nature, designed to move. Putting seawalls and riprap in front of a structure to stabilize it only delays the inevitable. Water moves sand. Wind moves water. In a hurricane the sand is going to move. A barrier island not a place you want to be during a storm. It's not hard to understand that. And maybe, just maybe, we really should be considering if we should be building out there in the first place.Island Animation - Click to play (660kb)
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