What a Geologist Sees - Part 34 - Urban Beaches

Revisiting an old post.  Some aspects of the contemporary "quirks" of the system still remain, such as the size and placements of the photos (which it won't let me change, apparently).

The photo above, from near the Portside Towers in Jersey City, NJ is not intended to represent the entirety of "urban beaches", but rather to consider what makes up a particular sand and where does all that stuff come from? The darkness of the sand suggests a significant heavy mineral percentage, however, it is likely that as the tide was receding, the last waves moved the lighter quartz downslope, exposing a thin veneer of heavy minerals. Under the microscope, heavy minerals are not the dominant mineral-type.  As in most sands, quartz is the dominant mineral.

To get a wider perspective of the location, the large building in the left foreground is the Goldman Sachs building. In the right background is a portion of downtown Manhattan, with the Hudson River between the two. To the left of the beach is a narrow spit of land adjacent to a portion of an abandoned, historic canal. 

In an effort to prevent erosion of the narrow spit, a wide variety of materials have been piled in the upper beach area, as well as miscellaneous old tires as well as other items of human flotsam and jetsam. 

  

The remainder of the photos illustrates different perspectives on the beach materials that contribute to the sand chemistry combined with the never-ending wave action, tides, and variations in river discharge rates.

Under the microscope a brief examination of the sand - without concern to relative percentages - the constituents of the sampled beach sands include: 1) Angular quartz; 2) Well-rounded quartz; 3) Brick; 4) Glass; 5) Coal fragments; 6) Greenish Fe-Mg minerals; 7) Garnet; 8) Rutile; 9) Magnetite; 10) Ilmenite; 11) Slag,... Among the particles, there are some odd, spherical grains that seem slightly magnetic, a quick guess might be magnetite, slag, or (least likely) Fe-Ni micro-meteorites. There will be some more examinations of the sand and the spheres in particular.

What a Geologist Thinks About...[Original Post Date 9/15/07]

often while reading or hearing a news report on a "man vs. nature" issue, is - "Why is this happening?". From The Seattle Times, by way of WND - there is a story on serious coastal erosion problems on the northern side of Willapa Bay along the southwest Washington State Pacific Coastline (see inset map associated with the article). Though there are some explanations within the article, we realize that there may be more than one reason why this is happening.

From the Seattle Times article:

..."This two miles of shoreline at the northern confluence of the Pacific Ocean and Willapa Bay, 12 miles south of Westport, is believed to be the fastest-eroding beach on the Pacific Coast. It has lost about 65 feet a year to the sea since the late 1800s. More than 100 homes, including the entire town of North Cove, have already disappeared, many of them in the past 20 years."...

Shorelines are unquestionably active - deposition takes place in some areas, while erosion takes place in other areas. The first question would be - "Is there something humans have done to make this natural process worse?" Can we estimate how long this rate of erosion has been taking place?

Sometimes along shorelines, erosion control efforts, e.g., jetties, groins, seawalls,...may slow erosion in their intended area, while making it worse somewhere else. And it may take decades to see the results. We have to look carefully before spending additional millions of taxpayer dollars, when any "fix" may be only temporary.

One of my cousins rents a beach house every summer at St. Simons Island, GA. It didn't used to be a beach house. Up until the late 1950s/early 1960s, there used to be another row of cottages along the shoreline. The loss of this row of cottages was not the result of ongoing regional shoreline migration, but the effects of a couple of hurricanes. [Yes, before 1970, there was another "active hurricane cycle", which may have started in the 1940s.]

As for the Pacific Coast in SW Washington State:

..."George Kaminsky, a coastal engineer for the state Department of Ecology who has studied the erosion since 1993, thinks it has become "self-feeding."

Sand from the Columbia River built up a sandbar at the mouth of Willapa Bay, channeling the water flowing out of the bay straight into Cape Shoalwater. And as the cape eroded, the sand built up the sandbar even more.

Pinpointing the original trigger is difficult, Kaminsky said. But man-made jetties likely have halted a natural sand migration that could reverse the erosion."...

This local interruption of sand migration is a form of "sand starvation", i.e., a normal supply of sand might have protected the shoreline.

And though this Seattle Times article doesn't address this possibility, there could be a larger "sand starvation" issue along the coastline. This Wikipedia map shows more than 2 dozen dams in the Columbia River drainage basin (watershed). Prior to the construction of these dams, the down-river perpetual movement of sand, as "bedload" and as "suspended load" (during storms), kept an ample supply of loose sand available for distribution at the "whims" of longshore currents.

Each one of the dams on this map disrupts the normal river transport of sand, ultimately resulting in there being less sand reaching the Pacific Ocean than there was prior to the construction of the first dam. The cumulative effects of the existence of these dams (for decades) is that there is "sand starvation" along portions of the coastline.

This is a problem that takes decades to reveal itself and other than the "deep ecologist" dream of dynamiting everyone one of these dams, there is probably not an easy solution. The dams are a source of relatively clean energy and the environmental disruptions associated with building the dams has already taken place. To remove the dams (or open them permanently) would again represent a series of environmental disruptions. Besides, it would take decades for the sand supply to "resume" it pre-dam transport pattern.