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DISCUSSION, DEPTH OF CLOSURE HYPOTHESIS
A central canon of coastal engineering theory is known as "depth of closure."Engineers divide the submerged beach profile into two discrete zones. The nearshore zone is sedimentologically active in this model, while the offshore zone is inactive with regard to beach dynamics. The hypothetical point dividing the two zones, usually at about 15 feet of depth, is called the depth of closure point. In the model, sand deeper than this point is unable to move any closer to shore. Likewise, sand that is shallower, within the closure zone, is unable to move offshore beyond closure depth. This "two-zone" theory is popularly known as the "river of sand" model. It is at the core of modern engineering practice and, consequently, regulatory policy.
The river of sand model thus implies that beaches can not be fed sand from offshore reserves. Nor can nearshore sand move out of the beach system. The only natural source of sand for beaches, therefore, must come directly from other beaches, moving along the nearshore in a discrete "river."
Data gathered by coastal geologists conflict with this engineering model. For many of the world's beaches, science has determined that offshore sand (beyond closure depth) has been the primary natural source of sand for beaches. Bottom sediments are moved landward in response to overhead waves. In the Gulf of Mexico, for example, a one meter wave with along period (wave length) moves bottom sediment as deep as thirty feet. Oceanic storms have little trouble moving deposits that are deeply submerged on the continental margins. In other words, geologists have demonstrated that the entire beach profile is active. There is no inactive sedimentary zone as seen in the engineering model.
Geologists note that a longshore drift of sand is apparent in nature, but these nearshore sand flows are actually part of larger onshore/offshore circulation cells.
Engineers are unable to supply actual data to support the river of sand model. The theory is based on the observation that at certain nearshore depths, bottom elevation remains fairly constant. If bottom elevations change little, the reasoning goes, there must be no sand moving at that point (closure).
One way to view the engineering model is to consider the movement of sand on riverbeds. Bottom elevations could be measured at a point on the Mississippi River, for example. Overtime, as the bottom contour changed little, one may conclude that little sediment moves along the river bed, an incorrect assumption.
Another view of the river of sand theory is afforded by a look at the Atlantic shoreline in Florida. The theory can only produce one of two outcomes. Either all erodings and migrates south, over the ages, to a point near Miami, where mountains of sand would eventually accumulate - or the sand has moved a short distance offshore (stopping at closure depth) where a large sand ridge would have formed along the length of the state. Neither has happened.
Engineers generally defend their theories against the weight of opposing geological data (the importance of offshore sand sources) by asserting that what happens in geologic time is of little consequence to the time frames engineers must contend with, typically 20- 50 years. Geologists may counter by noting that beaches were generally accreting at about two feet per year until recently. Since this yearly beach growth was caused largely by offshore sand (beyond closure) moving into the nearshore (cross shelf drift) these processes do indeed have relevance in "engineering" time. Today, significant amounts of sand both enter and leave the nearshore, passing right through the hypothesized sediment barrier (depth of closure) in response to storms. In general, coastal engineering programs are short - lived or counter-productive because engineers fail to acknowledge important sedimentary processes.
The cardinal rule of coastal regulation is to not disrupt the natural flow of sand as it flows to the beach. From a geological perspective, however, most engineering programs do just this. When dredges create offshore "borrow pits," the excavation creates huge depressions known as sediment "sinks." These sinks interfere with the cross shelf drift of sediment to the beach. Nevertheless, engineers define offshore dredging as benign in that it occurs on an inactive part of the beach profile, beyond closure depth. When the dredged beach consequently erodes with alarming speed, engineering theory also assures us that the sand is still in the nearshore system "by definition," unable to move seaward beyond closure depth. Geologists, however, find that dredged fill returns to the continental shelf, well beyond the axiomatic point of closure.
Another aspect of engineering theory, now that it has become institutionalized, is to limit the application of beach restoration systems based on alternate understandings of coastal dynamics. In many jurisdictions, only those methods that conform to engineering theory may be readily deployed in the coastal zone.
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