GEOSC Rockon8 - Most U.S beaches are shrinking or...

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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. Causes include: A) Global sea level fell as the ice-age ice sheets grew, and this caused rivers to deliver much sediment to the coast. B) 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. C) Global sea level fell as the ice-age ice sheets grew, exposing sand to the attack of large waves during winter storms, so beaches are now being removed. D) Local regions are rising as fluid injection wells used for waste disposal bulge up the land in many places. E) Dams on rivers have increased sediment delivery to the beaches. Feedback: Beaches lose sediment to deep water, and will shrink if the sediment is not replaced. Much sediment is now being trapped in the heads of bays such as the Chesapeake Bay, which formed from the end-of-ice-age sea-level rise flooding what had been the lower part of the Susquehanna valley. The sediment from the Susquehanna thus does not reach the Atlantic beaches, so as sediment is lost from them to deep water, they narrow, and waves cross the beaches during storms to erode the land behind to get sediment. Global sea level fell way back in time (from about 110,000 years ago to about 20,000 years ago), but that isn’t having much effect on coasts any more. And if the ground were rising from injection wells, then the land would be getting bigger, not smaller. Table for Individual Question Feedback Points Earned: 1.0/1.0 Correct Answer(s): B 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 longshore drift to add to the beach; this almost always works well. B) 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. C) 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. D) The landowners are trying to catch sediment from the onshore/offshore drift to add to the beach; this almost always works well. E) The landowners are trying to make a nice photographic platform from which to take pictures of their houses falling apart during the next storm. Feedback: 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
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problem is real and often dominates. If you wanted to trap sand going in and out, you
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