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Primary Causes of Erosion

Beach sand being directed and forced offshore by federal harbor


Aside from Army Corps personnel, most engineers and geologists generally agree that dredged and jettied channels cause widespread beach erosion. However, the reason why channels do this may be defined differently by the two disciplines. Engineers assume that jettied inlets interrupt the "river of sand," presumably the only source of sand for other beaches down the line. They consequently design sand bypass plants (fixed dredges) which are intended to transfer sand across the channelized interruption. For several reasons, such plants have seen little success in solving unnatural erosion. A primary reason for the ineffectiveness is incorrect modeling of natural sedimentary behavior.

From certain geological perspectives, jettied navigation channels do not interrupt a "river of sand," per se, rather they disrupt large onshore/offshore circulation cells. The resulting erosion causes a steepening of nearshore beach profiles in the vicinity of the channel. As profiles steepen with erosion, nearshore turbulence increases. Eventually this turbulence creates a kind of barrier, preventing the beach from accepting the onshore movement of sand from historic offshore reserves. More sand leaves than enters the beach system because of both channelized diversions and, of equal or greater significance, profile anomalies (channel-induced) that lose rather than gain sand from available sources. Geologists call this condition a "profile of deficit."

When engineers place sand bypass plants at dredged channels, the benefits are generally felt over only a short stretch of coast (generally 1%-2% of the total shoreline impacted by the channel). Eroding shorelines must therefore be further treated - usually by dredging up offshore sand and placing it on the beach. These projects attempt to install a so-called "equilibrium profile," a new beach profile whose shape will, according to theory, remain stable (in equilibrium) and unable to move offshore (depth of closure hypothesis). Dredged beaches wash away in relatively short time spans, however, as waves and currents work on the unconsolidated and randomly sorted artificial fill.

Beach profiles are sensitive to distribution of sediment sizes (the location of specific grain sizes on the profile). Sediments of varying size "find" a location on the profile where it is most stable. This is called a particle's "equilibrium position." Where the profile is relatively steep, generally nearest shore, larger sediments are more stable. As the profile shape becomes less steep (offshore, in deeper water) finer sediments become stable. Dredged beaches wash away in part because grain size distribution across the man made profile is random and improperly sequenced. Waves and currents "work" the artificial fill. In the reordering process, most of the fill is lost offshore, unable to return naturally because of turbulence on the re-steepening profile.

An alternate approach to neutralizing the effects of dredged channels is to plant special (Undercurrent Stabilizer) structures in the nearshore which mimic the type of geology which causes natural beach profile elevation. Sand accretion on this type of "induced" profile is correctly sequenced with respect to grain size distribution (because natural processes rather than dredges guide the resedimentation process). Though inlets in the vicinity of treated beaches still jet sand offshore, less and less sand travels to the inlet "loss point" as the beach grows and becomes progressively more efficient at holding sand arriving from historic sources.

Undercurrent Stabilizer Technology has proven to neutralize the effects dredged channels have on shorelines. The system has caused natural beach growth in many locations, including severely eroded areas immediately downdrift of large navigation channels, without relying on continuous beach dredging programs or sand bypass plants.

GENERAL DISCUSSION

Since engineering theory stipulates that the only source of sand for beaches is other beaches, engineers refrain from slowing the (quickening) movement of sand along the modern coast. In fact, engineers have two primary goals, again based on theory, 1) keep sand moving speedily along the coast (because longshore drift is presumably the only source of sand for other beaches down the line). The second goal is to keep sand on beaches. To the extent that each goal works against the other, engineering programs have become continuous, high maintenance projects. Symptoms are repeatedly treated. Underlying problems remain.

Today, the public is generally aware of only two schools of thought regarding coastline management. On the one hand, engineers make the case that shorelines are worth maintaining with traditional methods, despite the costs, because abandoning coastal infrastructure is far more costly. On the other hand, the environmental position advocates a retreat from the coast, letting nature take its course. The leading spokesman for the environmental position is Orrin Pilkey, a sedimentologist at Duke University
(See Dr. Pilkey vs. the Army Corps of Engineers).

Each school of thought seems to make reasonable points, though when push comes to shove and erosion threatens buildings, traditional beach maintenance programs are usually adopted. A great deal of Federal money has been available for these programs in the past. Recently, however, many beach communities have come to realize that the federal government has begun curtailing funds for unending maintenance programs. It is becoming more a local responsibility.

A third school of thought, not well-known outside Florida and the shores of Lake Michigan, is proposed by marine inventor Dick Holmberg, who believes shorelines should be restored with structures that mimic natural geologic substrate known to benefit sandy beaches.

Coastal engineers say there is no mechanism whereby Holmberg's geologically based ideas could work (coastal engineers generally do not acknowledge basic findings of coastal scientists). Environmentalists, on the other hand, generally led by Pilkey, contend that all structural approaches to coastal erosion must be counterproductive.

Holmberg counters that coastal engineers are vested in construction techniques which require them to disacknowledge basic findings of sedimentologists. The inventor faults Pilkey and his environmental following for assuming that it is impossible to work pro actively with natural sedimentary processes.

Holmberg also believes, as do most coastal professionals, that the modern, engineered shoreline would not be in a natural state if buildings were simply moved back from the coast, as Pilkey claims. This is because dredged navigation channels, not buildings, are the primary insult to natural shoreline mechanics (by a wide margin over other factors).

It's interesting to note that Pilkey believes most beaches and barrier islands have naturally moved landward in recent history (the late holocene) despite a geological record which demonstrates the opposite.

Harbor Pictures

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