longshore transport and littoral drift – mammoth // building nothing out of something

longshore transport and littoral drift


[
Dauphin Island, Alabama, at the south opening of Mobile Bay.]

Continuing the theme of Sandy-inspired rumination on the risks and rewards of littoral urbanization, a pair of articles by Justin Gillis and Felicity Barringer at the New York Times utilize the example of Alabama’s Dauphin Island — a hurricane-battered barrier island near the port of Mobile, both illustrating a broader argument about the role of incentive structures set up by the federal government in encouraging littoral urbanization and, in a follow-up on the Green blog, describing in greater detail the erosive forces acting on Dauphin Island.

First, the illustration of the broader argument:

Dauphin Island is a case study in the way the federal subsidies have enabled repetitive risk taking. Orrin H. Pilkey, an emeritus professor at Duke University who is renowned for his research in costal zones, described the situation here as a “scandal.” The island, four miles off the Alabama coast, was for centuries the site of a small fishing and farming village reachable only by boat. But in the 1950s, the Chamber of Commerce in nearby Mobile decided to link it to the mainland by bridge and sell lots for vacation homes.

Then Hurricane Frederic struck in 1979, ravaging the island and destroying the bridge. President Jimmy Carter flew over to inspect the damage. Rex Rainer, the Alabama highway director at the time, recalled several years later that the president “told us to build everything back just like it was and send him the bill.”

The era of taxpayer largess toward Dauphin Island had begun. With $33 million of federal money, local leaders built a fancier, higher bridge that encouraged more development in the 1980s. Much of that construction occurred on the island’s western end, a long, narrow sand bar sitting only a few feet above the Gulf of Mexico.

Since 1988, federal figures show, Dauphin Island property owners have paid only $9.3 million in premiums to the national flood insurance program, but they have received $72.2 million in payments for their damaged homes. Figures from a federal contractor show that the average island resident pays less than $700 a year for flood insurance, though a few do pay as much as $3,000.

On Dauphin Island and in many other beachfront communities, the federal subsidies have helped people replace small beach shacks with larger, more valuable homes. That is a main reason the nation’s costs of storm recovery are roughly doubling every decade, even after adjusting for inflation.

As you’ll note if you read the full article, these are the two primary federal subsidies that encourage littoral urbanization: flood insurance that doesn’t accurately price local risk, and post-disaster infrastructure spending that helps damaged communities rebuild, but typically ignores broader questions about the suitability of the stricken terrain for settlement. There is nothing terribly new about these observations, but it is good to see them receive a wider hearing.


[Detail of central Dauphin Island, at the joint between the rapidly-eroding western end of the island and the relatively stable -- though hardly stable in an absolute sense -- eastern end.]

The second article, which I find the more interesting of the two, discusses in much greater depth the specific causes of erosion on Dauphin Island, and the reasons that erosion is unequally distributed — severe on the ocean side, but actually accreting on the landward side; producing extreme instability on the thin western end of the island, and relatively less instability on the bulkier eastern end. The first cause the article discusses is the interruption of longshore transport by dredging operations for the port of Mobile (though, it should be noted, the Corps disputes this account):

As we mentioned, local residents blame the Army Corps of Engineers for their erosion problems. In a role similar to the one it plays in many coastal regions, the Corps conducts frequent dredging operations in the nearby Mobile Ship Channel, to the east of Dauphin Island, so that oceangoing cargo vessels can make use of the Port of Mobile.

Why would that make any difference?

Many people imagine that beaches and barrier islands are just mountains of sand that sit unmoving at the edge of our shores. In reality, they are highly dynamic systems, constantly moving and adjusting to storms, currents and changes in sea level. Sand actually flows up and down our shorelines, by the billions of tons, and often there’s a net direction to this flow, known as “littoral drift” or “longshore transport.” That is to say, averaged over time, more sand flows one way than the other. The beaches we see above water are but a small part of this system. Far more sand lies offshore, and these unseen hills of sediment play a crucial role in the overall sand supply to beaches down the line. Along the Alabama and Mississippi stretch of the Gulf Coast, the net drift of sand is from east to west. When sand from further east falls into the Mobile Ship Channel, the Corps dredges it out to keep the channel clear – and then, Dauphin Island residents and some scientists contend, dumps it in spots far enough away that the sand is lost to the littoral drift. The Corps does so to save money, under a mandate from Congress to conduct its operations in the most cost-effective way.

In the context of the Dredge Cycle proposed by the Dredge Research Collaborative, what I find fascinating about this is that it is a landed instance of the cyclical feedback we have argued characterizes the Dredge Cycle, of the tendency of the Dredge Cycle to suck ever-increasing volumes of sediment into itself: dredging begets erosion, and further dredging is proposed to provide a source of sediment for beach nourishment to counteract that erosion.

The other cause I mentioned is the natural landward drift of the island itself — all land is, of course, unstable when considered at sufficiently long time-scales, and the “sufficiently long time-scale” for a barrier island is rather brief:

On the south side of the island, fronting the Gulf of Mexico, hundreds of feet of beach have eroded. Numerous lots that were platted and sold in the 1950’s are now inundated by the sea, and the houses that once stood there are gone, many of them knocked down by Hurricane Katrina. The gulf is now lapping at the pilings of surviving houses that used to be three rows back.

“That island is virtually migrating out from under those buildings,” Dr. Pilkey told me. “It’s just so amazing. There is no worse example of unsafe development on barrier islands than Dauphin Island — nothing else like it in North America.”

As the front erodes, the back of the island keeps growing, as storms carry sand over the top and deposit it at the rear. I saw boat houses and docks that had been marooned on dry land. One island resident whom I interviewed by phone, Jack L. Gaines II, has lived on the north side of the island since 1999. “I’ve watched the south beach erode and come toward us,” he said. “I’ve accreted 600 feet of property.”

This may sound familiar to the victims of Hurricane Sandy in New York and New Jersey, some of whom are still shoveling sand out of their living rooms. Scientists say it is no coincidence that the pattern is similar from Dauphin Island to Long Island.

The simple reality is that the nation’s barrier islands are attempting to move inland, a natural response to an unnatural situation. Scientists say that global warming caused by human emissions of greenhouse gases is causing the sea to rise. If left to their own devices, barrier islands would respond to that rise by migrating landward, and so would the marshes behind them. As storms wash the sand from front to back, the islands would essentially roll uphill, a classic process that scientists have dubbed island rollover.

The problem, of course, is that people have planted buildings on these shifting sands and declared that they can no longer be allowed to move. On the Jersey Shore as on Dauphin Island, Mother Nature seems to be telling us what she thinks of that proposition.

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