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GEOLOGICAL EVALUATION OF TRADITIONAL COASTAL ENGINEERING


The central proposition of coastal engineering theory is the so-called "River of Sand" model. This model states that the only source of sand for beaches is other beaches; offshore reserves are thus disregarded as an active part of the beach system.Geologic findings, on the other hand, demonstrate that an important, often primary source of sand for most of the world's beaches has been the offshore shelf. According to geologists, though a longshore drift is apparent in nature, beach sedimentation is largelygoverned by onshore/offshore circulation cells (which only appear to be a discrete"river of sand" to the nearshore observer) (Tanner, Stapor).

A number of leading geologists have recently published scholarly revues of traditionalengineering theory and practice.

The National Park Service, Department of the Interior, commissioned reports from independent geologists to study engineering programs for erosion control. The geologists reported that traditional coastal engineering practices were "not only ineffective and inefficient, they were actually harmful to the natural health and equilibrium of the barrier islands that the Service is responsible to protect."

Elsewhere, coastal scientists who co-authored the book, Living with the East Florida Shore, state that "shoreline engineering destroys the beach it was designed to save."

A recent Congressional Office of Technology Assessment report concludes that "so little is known about these natural processes (shoreline erosion) that public works improvements often are undertaken with insufficient understanding to ensure structural longevity."

A Donner Foundation-Duke University study examined hundreds of beach dredging projects undertaken in the U.S. since 1948. The conclusion: "virtually all projects have been dramatically underestimated and have experienced significant cost overruns-some beaches are unreplenishable within any reasonable economic framework." The study noted that beach dredging increases the local rate of erosion, and that coastal engineers "are using design parameters that do not work."

In recent papers authored by a number of leading coastal scientists, the engineering model of beach dynamics is criticized for: 1) assuming the beach is a two dimensional system, 2) assuming that underlying geology plays no role in the configuration of shoreface profile, 3) inappropriate use of a hypothetical depth of closure sediment barrier in the nearshore, and 4) using linear equations to describe non-linear phenomena.

Along similar lines, W.F. Tanner, senior geologist at Florida State University, produced a scholarly paper, "The Beach, Where is the 'River of Sand'?" in which he demonstrates that the central hypothesis of coastal engineering theory is incorrect when applied to most shorelines.

When coastal engineers design a beach dredging project, formulas are used to determine construction parameters for the shape of the beach profile being installed. Engineering formulas are based on several theoretic assumptions. A newly dredged beach presumably settles into a more or less permanent shape, called an "equilibrium profile." The permanency of the newly dredged equilibrium profile is further assured by another engineering theory, called "depth of closure." This theory states that there is a nearshore depth (about 15 feet) beyond which nearshore sand can not migrate. Engineering equations assume that the depth of closure point is a kind of dam holding sand securely in the nearshore (and also preventing offshore sand from entering the nearshore). In practice, however, dredged beach profiles do not behave as theory predicts. Most dredged beaches are lost long before their intended design life. Engineers occasionally defend the early losses of dredged beaches by blaming unexpected storm activity. Geologists have retorted that storm activity should not be unexpected.

Florida's coastal policies, like those of many states in the U.S., are governed by the coastal engineering community, not coastal geologists. Florida's State Geologist, in a memo to the state coastal engineer, notes that "Engineering solutions to coastal problems are only as good as the fundamental science they are based on... too often, the real world does not respond as the (coastal engineering) textbook formulas imply...geologic parameters are not included or even understood by the engineering community."

Another Florida geologist, James Balsille, wrote a paper for a recent Geological Society of America Conference. At the time, Balsille was employed by the bureaucracy which directs coastal work in the state. Balsille wrote, "One may be surprised to learn that in the U.S., the collective field of coastal science and engineering was begun by geologists at the turn of the century who felt it a responsibility to engage the participation of engineers. Since that time, engineers have gradually come to dominate the field in terms of influencing applications, many of which have reached paradigmatic proportions. (The engineering community) needs to encourage greater receptiveness to learn, if the work of the geologist is to be of value." Balsille's agency, The Division of Beaches and Coastal Systems, canceled the presentation of the paper and removed Balsille from his position in the agency.

Another Florida geologist (Donogue, FSU) notes that coastal engineering equations have a window of uncertainty that is an order of magnitude wide (x10). When engineers measure sand movement along a coast, a calculation of 1 million cubic yards could actually be 100,000 or 10 million cubic yards.

ENGINEERING TIME VS. GEOLOGIC TIME

When faced with geological findings that conflict with engineering theory, engineers have attempted to down play the contradictions by observing that geologists and engineers are concerned with different time frames. Coastal engineering time frames are generally considered to be from five to fifty years, where as geological time frames extend to thousands of years.

Engineers postulate that what happens very slowly over geologic time, such as the processes responsible for the general growth of beaches worldwide during the past several thousand years, has little relevance to treating beach erosion today. This argument is generally forwarded by state and federal bureaucracies when defending an engineering paradigm that disregards coastal processes which drive coastal change over extended timespans.

This argument makes sense for structural engineers, for example, who need not consider glacially slow processes driving the uplift of mountains when boring tunnels, or who may safely ignore the effects of wind erosion on a concrete building. The argument is inappropriately applied to shorelines, however, where natural change is robust on a yearly basis.

The steady expansion of most beaches over the past several thousand years is analyzed by telltale markers, called beach ridges, that are left behind as shorelines prograde seaward.

The rate of historic beach expansion is determined by simple math. The width of the beach ridge plain, from point of origin to seaward terminus, is divided by the number of years it took the multiple ridges to form and prograde seaward. In Florida, for example,beaches grew at the rate of a foot or two annually over the past several thousand years - prior to reversals caused by the widespread introduction of harbor projects.

Coastal engineers normally consider shoreline change of a foot per year significant. An erosion rate of two feet per year is defined by coastal engineers as nearing"critical." Since the natural processes driving shoreline change in geologic time express similar annual rates, they are indeed relevant to engineering time. These processes are inappropriately ignored by traditional engineering practice.


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