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SAND BYPASSING AND COASTAL RESTORATION
An example of modern coastal engineering practice is found in the design principles applied to sand bypass plants. These plants are designed to mitigate beach erosion caused when dredged navigation channels disrupt nearshore sand movements. Bypass plants, or sand transfer plants as they are sometimes called, are fixed dredges placed on one side of navigational inlets. As sand moves along the shoreline, it is collected by the plant and transferred across harbor structures to downcoast beaches.
Studies demonstrate that dredged and jettied channels cause unnatural erosion along many miles of shoreline. Sand transfer plants, however, seldom benefit more than a mile of coastline adjacent to the navigational cut. Reasons for the performance shortfall are exemplified in the following example.
Assume a shoreline experiences annual longshore drift of sand equal to 1 million cubic yards - 600 thousand cubic yards of sediment moves north to south, driven by north winds, and 400 thousand cubic yards of sand moves in the opposite direction driven by winds from the south. This is a representative ratio of gross longshore drift for many shorelines.
Sand arriving at the navigation channel from both the north and the south descends into the dredged channel and is subsequently jetted out of the nearshore beach system. (Jetties bracketing navigation channels are so-called because they make water "jet". This flushing action helps maintain channel depth. Unfortunately, adjacent beaches are also flushed to sea.)
Most experts outside the U.S. Army Corps of Engineers agree that 80% or more of all beach sand arriving at dredged channels (from both the north and the south) is lost from the beach system to channel dynamics. In the example above, approximately 800k cubic yards or more (80% the 1 million cubic yards of gross annual drift) is lost to channel dynamics. Coastal engineers, however, have hypothesized that it is generally not necessary or practical to transfer the gross amount of sand arriving at navigation channels from shorelines on both sides of the channel. Instead, engineering theory determines that the net flow is all that must be bypassed to mitigate channel-induced beach erosion. The engineer consequently subtracts the sediment arriving at the channel from the south from that which arrives from the north. In the example above, the net annual longshore drift is 200k cubic yards (600k from the north less 400k from the south). The engineer consequently designs the plant to transfer 200k cubic yards of sand across the navigational cut.
Assuming the bypass plant works as intended - actually transferring 200k cubic yards of sand yearly across the inlet in one direction only - from north to south - about 40% of the sand transferred to the south of the channel (80k cubic yards) will return to the channel during longshore flow reversals (in response to southerly winds). Only about 120k cubic yards of sand is consequently transferred south of the channel - a fraction of the gross drift (800k cubic yards) that the channel exports annually from the nearshore beach system.
The U.S. Army Corps of Engineers, a primary constructor of both navigation channels and sand transfer plants, officially disagrees with independent experts that navigation projects cause 80% - 100% of current beach erosion. The Army Corps uses formulae which imply that their projects cause only about 1% of current beach erosion. (The Corps is directed to mitigate damage caused by its navigation projects under Section 111 of the River and Harbor Act. The Act also allows the Corps to police itself in determining the extent of damage done by Corps projects. This leads some experts to claim the fox is guarding the hen house. See Coastal Owners Rights, Section 111 and the Fifth Amendment.)
Another problem complicating design for sand bypass plants is engineering formulae used to determine the amount of sand actually in transport along shorelines. These equations contain margins of error equal to an order of magnitude "times ten". This means that a calculation of one million cubic yards of sand in longshore flow ranges anywhere from one hundred thousand cubic yards to ten million cubic yards.
Still another problem with bypass schemes is plant capacity - the hourly rate at which a plant can transfer arriving sand. Much sand movement during a 5-year period occurs in response to just a few of the most intense storms of the period. Transfer plants, however, with the capacity to move a few hundred to perhaps a thousand cubic yards of sediment per hour, will transfer only a fraction of the large sediment (pulse) arriving at channels during storms.
Design criteria for transfer plants are further confounded by engineers¹ use of the so-called "river of sand" model of coastal functioning. This model hypothesizes that the only source of sand for beaches is other beaches along the shoreline - nearshore sand is theoretically unable to move offshore, and offshore sand presumably does not move onshore. Engineers therefore assume that normal patterns of beach sedimentation are reestablished when bypass plants keep this "river of sand" flowing.
Although sand obviously flows along the nearshore, geologists have discovered that these nearshore sand flows are actually part of larger onshore/offshore circulation cells - something the casual observer does not notice. This means that a primary source of beach sand derives not from a discrete nearshore flow, as the "river of sand" theory predicts, but from the offshore shelf.
Geologists use various analytic techniques to arrive at these conclusions, but a simple thought experiment also supports the findings. If engineers were correct about a river of sand being the primary source of beach sand, then places like Miami, the necessary end-point of the hypothesized river of sand along the eastern U.S. seaboard, should have collected mountains of quartz sand over the ages. Such deposits have never existed.
Dredged navigation cuts cause erosion not simply because a "river of sand" has been interrupted,
but for somewhat more complex reasons.
In summary, sand transfer plants perform poorly because 1) inappropriate math is employed in plant design (using net drift when gross drift is the significant factor) 2) error-prone equations are used (order of magnitude uncertainty) to determine volumes of sand moving along the shore, and 3) a "river of sand" beach sedimentation model is used that applies poorly to a majority of world coastlines. Despite this, coastal engineers are able to sell sand transfer plants as state of the art inlet management practice.
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