I
think a good place to start this blog is to consider coastline dynamics, to
get a sense of the scale of changes that occur commonly in sedimentary systems.
In the modern era, developed coastlines, such as in New Jersey, are predominantly
engineered systems. We stabilize beaches through sand
nourishment (literally, piping in sand from offshore sources), as well as construction
of groin fields—rock jetties that capture sand moving in shore-parallel
currents. Without such interventions, however, the coastline is a highly
dynamic environment, especially where barrier islands are present.
In New Jersey, rapid shoreline changes were abundantly evident in the past, and in some places can still be seen today. One of the most dynamic portions of the modern coast is Sandy Hook, a spit at the north end of the Sea Bright barrier in Monmouth County. Here, sand is transported northward by littoral currents and deposited at the tip of the spit year-after-year, gradually extending Sandy Hook into Lower New York Bay.
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| Sandy Hook seen in aerial images in Google Earth |
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| Sandy Hook seen from the Twin Lights of Navesink, looking towards New York City |
Using
a progression of satellite imagery, we can visualize changes in Sandy Hook with
time, capturing the transport and deposition of sand around the northern point
of the spit. Below, I produced a timelapse of Landsat photos taken from 1984 to
the present, using Google Earth Engine. For those unfamiliar, Google Earth
Engine (not to be confused with the Google Earth software that lets you view a map of the world) is a database of many satellite images, which
allows a user to perform detailed geospatial analyses using scripts—more on
that in a later post.
If
we want to go further back in time and see changes in Sandy Hook that have occurred
over the last hundred years, we have to seek alternatives to the satellite
image record, which ends in the 1970s. Luckily, in New Jersey, we have aerial
photos of the coast which extend all the way back to 1920. A great catalog of images can be accessed
courtesy of the United State Army Corps of Engineers, which digitized sets of
photos from 1920, 1933, 1944, and 1962: http://rsm.usace.army.mil/shore/
Using
Google Earth Pro, I georeferenced a 1920 aerial photo of the tip on Sandy Hook
and overlaid it on the modern aerial imagery. In the image below, I’ve
highlighted the 1920 shoreline in yellow, and the modern shoreline in pink. The
spit has grown northward about a kilometer!
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| 1920 photo of Sandy Hook overlaid on modern imagery |
More
surprising, however, is that at least half of this growth has actually been
in the last 20 years, as revealed when overlaying the modern shoreline on an
aerial image from 1995. If the 1995
shoreline is traced (orange), the area deposited along the north part of the spit since
that time can be calculated: 358,000 square meters.
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| Change in area at north end of Sandy Hook from 1995 to 2017 |
Let’s
assume for a moment that the average depth of beach sand at Sandy Hook is a relatively
conservative 3 meters (this is in the right ballpark given some data that was
recently acquired). If this is true, then the volume change at the north end of
the spit since 1995 is +1,074,000 cubic meters. To put that in perspective, the
volume of the Empire State Building is 1,047,723 cubic meters.
Such
large changes occur elsewhere on the New Jersey coast, especially where
other spits are present. A great example of this is the Holgate spit, at the
southern end of Long Beach Island, which I constructed another timelapse of,
starting in 1984.
Note: Vegetation colored red. I made this video using false-color imagery.
As
with Sandy Hook, we can examine changes further back in time, and here, we find
something extraordinary. The modern-day version of the spit is at least the
second incarnation of this feature.
Early in the 20th century, Beach Haven Inlet (now closed)
severed the spit from Long Beach Island, which resulted in runaway erosion of
the original pre-20th century spit, culminating in its complete
destruction. Nearly the entire southern end of Long Beach Island, a more than 2
square kilometer area comprising modern Forsythe National Wildlife Refuge, has regrown
since the 1920s.
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| Southern Long Beach Island / Little Egg Inlet. The Holgate spit is in the center of the image and extends for ~5 km, fronting the ocean. |
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| Aerial photos showing the destruction of the original spit and growth of the modern spit. Growth from right to left. |
In
future posts, changes in coastal systems around the U.S. east coast will be considered. I’ve talked a little about barrier islands, and I’ll go into much more
detail about these in the next installment, examining even greater changes than
I’ve shown here…
