Our New England landscape was shaped over 10,000 years ago by glaciers that deposited sediments. On sunny summer days, people flock to relax on some of those glacial sediments — or beaches — up and down the New England coast.
Jon Woodruff, a professor in the geosciences department at UMass Amherst, recently published a paper in the journal Marine Biology that explains how the grain size of sand impacts the shape of those beaches.
Jon Woodruff, UMass Amherst: It all comes down to these glacial deposits along the coast. And so, along Cape Cod, we have essentially this big sand pile that was deposited during the last glaciation. And because of that, the beaches are sandy. They're sourced by these sandy bluffs that are very evident when you walk along the Outer Cape.
When you go to other places in the Northeast, the sediment that is creating them is sourced by very local deposits that could be a mixture of sand and cobble, something called till that was deposited underneath glaciers. And that's a much wider mixture of grain sizes and makes the beach cobbly and behave a bit differently.
Carrie Healy, NEPM: And this also has to do somehow with how steep the drop-off is and how much of this deposited material comes up and recedes?
Yes. So the two defining characteristics of a beach — to anyone who goes out on one, like you mentioned — is one, the grain size. People note if it’s cobble, or fine sand or coarse sand. And two, its slope, which plays into how wide it is, whether you can fit your beach chair out on the beach, and you can have a number of people, or you’re all sort of squeezed together in a very thin beach width that’s very steep.
For sandy beaches that, in the Northeast, they kind of behave the same way as throughout the globe. There's this relationship, as the beaches transition from fine sand to go to another location that has coarser sand, those coarser beaches are generally steeper, and there's a relationship that has been developed for those. Now for the Northeast, when you go to these systems that are a mixture of both sand and these cobbles, or coarser gravel, they've never really behaved with that relationship. Throughout the globe, there's been this mystery on why there isn't this strong relationship between grain size and slope for those types of beaches.
And what did we find?
It turns out that we have to sort of go back to the elementary school teachings of taking averages, and the issues with taking averages of a population.
So the analogy is that you could have a kindergarten class going into a nursing home, and you would have a lot of ... young children and older individuals. And if you took the average of that, it might be 45 years old, but there could ... be ... few people that are actually that age.
And so, for these mixtures of sand and cobble beaches, they're what we call bi-modal. They have a large population of sand, or kindergartners, and they also have another population of coarser grains, or these older individuals — using that analogy, and and very little in between. So, grains between 1 and 10 millimeters are fairly rare along beaches.
And so, when people were taking the averages of those, they were trying to relate beach slope to a grain size that really wasn't very representative of the beach. It was actually grains that were ... fairly rare.
So does that make them respond differently than beaches that have the rarer grain size?
Yeah, so it turns out that it's really one of those populations, the sand, that dictates the slope. And the slope of a beach is in part related to the permeability of the beach. And that's a fancy word that says if you're sitting on a beach and you look at those waves coming up, oftentimes you can see that the water rushes up and then some of that water actually gets absorbed into the sand. And so there's less water that's coming back down the beach, when the waves are returning. And that process can cause the beach to become steeper if there's more permeability.
And so, coarser grains allow more water to kind of percolate through the beach and make it a steeper beach face. And it doesn't take much sand to sort of really dictate how much water gets sort of absorbed into the beach on that up-rush of the wave. And so, the cobbles are pretty much along for the ride. They're there, but it's really the sand grains that are determining the characteristics: the steepness of the beach and the beach width.
Oh, interesting. So how can these coastlines withstand rising sea levels in the future?
So, the slope of a beach is directly related to how quickly it will erode landward. And so, a shallower sloping beach will erode landward a lot more with the same amount of sea level. And so knowing what the slope of a beach is, is very important.
And what we're showing is that it's actually a pretty intensive process, what we had to do, to get the full grain size distribution for these beaches. But really, you just have to sample the sand to relate that to this beach slope.
If we're talking about nourishing beaches, a mitigation technique to give beaches a little more lifespan, is to actually spray sediment onto the beach, to give it a little more material. And oftentimes you try to match it to the grain size of the beach itself. What we're showing is that it doesn't take much sand to make that beach very shallow in slope and require more material, but also make it sort of wider and more appealing to the general public. And you can add a lot of coarse cobble to it, potentially, and a little bit of sand, and still maintain that shallower slope.
So it has a lot to do with these types of mitigation techniques, as well as sort of determining how fast it will erode.
