D) Water, oil and gas are being pumped into the ground in some places, causing the land to rise. E) Sea-level rise as the last ice age drained river valleys to get rid of bays, and sediment now is transported to the coast rather than being trapped in bays. Feedback: 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. Points Earned: 0.0/1.0 Correct Answer(s): 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 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 problem is real and often dominates. If you wanted to trap sand going in and out, you would build walls or dams that are perpendicular to that motion, and thus parallel to the beach. And groins are not the best places on which to stand during storms, nor do many landowners actually plan ahead to get good pictures of their houses falling apart in the waves. Points Earned: 1.0/1.0 Correct Answer(s): B 3. Beaches change size with every storm, but if you average over a few decades, the size of a typical sandy beach is usually controlled by: A) The balance between gain of quartz sand from weathering of granite bluffs just behind the beach, and loss of sand blown away to make sand dunes. B) The balance between removal of sand from the beach by smaller summertime waves, and gain of sand from deep water by bigger wintertime waves. C) The balance between sand loss to deep water, and sand supply from rivers or from coastal erosion. D) The balance between sand loss to the wind, and sand supply from glaciers.
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- Spring '12
- The Land, Correct Answer, Death Valley, geologist