As consequências de barreiras costeiras, molhes e paredões

To Stabilize a Shoreline or Not - and Why (Not)

by Orrin Pilkey* | Aug 04, 2009

[postagem original em Green Gork]

Ocean shorelines are eroding virtually everywhere. But seawalls and artificial beaches — the main methods used to halt erosion — often make long-range problems worse by encouraging more development (read high rises). A bad idea at a time of rising seal level.

The coastal scientist’s role is to point out geological and biological environmental impacts related to buttressing a shoreline — not to decide whether to build a seawall or not. When all facts are in, engineering costs estimated, and environmental impacts understood, society can decide: to stabilize a shoreline or not.

But it seems this prescription to a rational decision about our coastline is often not followed. A prime example, which I discuss at length in my book The Corps and the Shore (1996), was the 1993 installation of 53 offshore breakwaters (large rock structures that parallel the shore) along the shoreline of Pennsylvania’s Presque Isle — a controversy that rose to national prominence in the late 1980s.

Coastal geologists, including myself, argued vehemently that such structures would immediately halt all transport of sand to the tip of the spit and that Presque Isle would begin to change dramatically to the detriment of recreational and wildlife activities. The U.S. Army Corps of Engineers ignored these warnings and went ahead with the breakwater project. After the structures were put in, Presque Isle began to change dramatically; recreational and wildlife activities declined. Rapid erosion at the cape’s tip continues to this day.

Denying Environmental Impacts of Coastal Engineering Is to the Detriment of Beaches

Similar examples exist in all coastal states, including North Carolina.

Last week a consulting engineer here suggested that a proposed small artificial beach project on Topsail Island should last eight to 15 years. I believe it is highly unlikely that the beach will last a single year based on published studies of beach nourishment (see here [pdf] for example) as well as local experience.

Four years ago, the same consulting company oversaw the mining of North Carolina’s Bogue Inlet to obtain sand for a nearby artificial beach, Emerald Isle. A number of scientists noted that such mining would cause a sand shortage (erosion) on adjacent islands.

The consultant denied this would happen and dredging proceeded. Some 2,000 years ago the Romans learned that channel excavation could cause erosion on adjacent shorelines and we will learn this again at Bogue Inlet.

You Say ‘Terminal Groin,’ I Say ‘Recipe for Disaster’

Today, terminal groins — they used to be called jetties — are the state’s most pressing beach concerns; eventually they’ll vex other states as well. Redefining the term by jetty proponents was a way to avoid the baggage of real-world evidence: jetties have caused widespread beach damage, a problem also understood by Roman engineers.

The proposed installation of shore-perpendicular walls adjacent to inlets (aka jetties or terminal groins) is meant to halt erosion at the north ends of several islands but in fact are likely to increase erosion. North Carolina’s coastal science community firmly disagrees with arguments that play down or deny the high erosion potential of terminal groins.

Like the Presque Isle breakwaters, the terminal groins are a recipe for disaster, and their proponents are waging a completely disingenuous battle against scientific truth.

For example, claims that a 1995 blue ribbon panel report on beach nourishment supports the use of terminal groins are faulty — the discussion of groins in that National Academy of Sciences report had nothing whatsoever to do with terminal groins (I was a member of that panel). The proponents and their consultants have widely quoted the technical literature that, when examined closely, has nothing to do with the proposed terminal groins. The misquotes include a 1990 peer-reviewed paper of mine. Proponents state that previous terminal groin projects in the state have successfully reduced erosion rates, but the opposite is true.

With a 350-mile long ocean shoreline and thousands of miles of estuarine shores, North Carolina has one of the nation’s strongest academic coastal science programs. Programs in marine geology, marine biology, physical and chemical oceanography and coastal management exist at five state universities plus Duke. So it is disconcerting when coastal management decisions fly in the face of the scientific principles and opinions espoused by the experts the state has brought together to train the next generation. Why bother?

*Orrin H. Pilkey is a James B. Duke professor emeritus at the Nicholas School of the Environment. His new book The Rising Sea (2009), co-authored by Rob Young, is due out at the end of the month.