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geosc8 - 1 Most U.S beaches are shrinking or encroaching on...

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1. Most U.S. beaches are shrinking or encroaching on the land rather than growing or moving seaward, so the  land of the U.S. is getting smaller, not bigger. Which of the following is not a likely cause: A. Levees along rivers have blocked sediment supply to  deltas that feed longshore drift, forcing the sediment to  fall in deep water where waves cannot pick it up. B. Dams have increased sediment transport from the land  to the sea. C. Sea-level rise as the last ice age ended flooded river  valleys to form bays, and sediment now is deposited in  these bays rather than being delivered to beaches. D. Water, oil and gas are being pumped out of the ground  in some places, causing subsidence. E. Global sea level is rising, covering more land. As sea level rises, beaches are pushed landward unless something happens to offset this tendency. Dams  keep sediment away from beaches, as do the bays formed by post-glacial sea-level rise, and human-caused  subsidence of the land is an important problem. But land rising would make for more land, not less. Points Earned: 0/1 Correct Answer: B Your Response: A 2. Often, landowners along eroding beaches will build groins, which are walls or dams sticking out into the ocean  or lake from the beach. Why are these built, and what happens? A. The landowners are trying to catch sediment from the  onshore/offshore drift to add to the beach; this almost  always works well. B. The landowners are trying to catch sediment from the  onshore/offshore drift to add to the beach; this can  work, but often erosion on the “downstream” side of the  groin makes the neighbors mad. C. The landowners are trying to catch sediment from the 
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longshore drift to add to the beach; this almost always  works well. D. The landowners are trying to make a nice photographic  platform from which to take pictures of their houses  falling apart during the next storm. E. The landowners are trying to catch sediment from the  longshore drift to add to the beach; this can work, but  often erosion on the “downstream” side of the groin  makes the neighbors mad. The “river of sand” that is the longshore drift along the beach is similar to a river in many ways. “Damming” the  flow with a groin will trap sand upstream, on the side from which water and sand are coming, but that will allow  water with less sand to attack the downstream side, causing erosion there. Dense groin networks may actually  so roughen the coast that they hold sand overall, but the erode-the-downstream-neighbors problem is real and  often dominates. If you wanted to trap sand going in and out, you would build walls or dams that are 
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