{"id":793,"date":"2019-06-11T15:07:03","date_gmt":"2019-06-11T15:07:03","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/17-5-human-interference-with-shorelines\/"},"modified":"2021-12-08T23:23:02","modified_gmt":"2021-12-08T23:23:02","slug":"17-5-human-interference-with-shorelines","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/17-5-human-interference-with-shorelines\/","title":{"raw":"17.5 Human Interference with Shorelines","rendered":"17.5 Human Interference with Shorelines"},"content":{"raw":"There are various modifications that we make in an attempt to influence shoreline processes for our own purposes. Sometimes these changes are effective, and may appear to be beneficial, although in most cases there are unintended negative consequences that we don\u2019t recognize until much later.\r\n\r\nAn example is at the beach near Comox (described above), which has been armoured with rip-rap and concrete blocks in an attempt to limit the natural erosion that is threatening the properties at the top of the cliff (Figure 17.3.4). As already noted, the unintended effect of this installation will be to starve Goose Spit of sediment. As long as the armour remains in place, which might be several decades, there is a significant risk that the spit will start to erode, which will affect many of the organisms that use that area as their habitat, and many of the people who go there for recreation.\r\n\r\n[caption id=\"attachment_788\" align=\"alignright\" width=\"600\"]\"\"<\/a> Figure 17.5.1 The seawall at Stanley Park, Vancouver.[\/caption]\r\n\r\nSeawalls, like the one around Vancouver\u2019s Stanley Park (Figure 17.5.1), also help to limit erosion and can be very pleasant amenities for the public, but they have geological and ecological costs. When a shoreline is \u201chardened\u201d in this way, important marine habitat is lost and sediment production is reduced, and that can affect beaches elsewhere. Seawalls also affect the behaviour of waves and longshore currents, sometimes with negative results.\r\n\r\n[caption id=\"attachment_789\" align=\"alignright\" width=\"400\"]\"Breakwaters<\/a> Figure 17.5.2 Map of the impact of breakwaters (or groynes) on beach formation at Sunset Beach, Vancouver.[\/caption]\r\n\r\nAnother example is at Sunset Beach in Vancouver. As shown in Figure 17.5.2, a series of rip-rap [pb_glossary id=\"1928\"]breakwaters[\/pb_glossary]<\/strong> (structures parallel to the shore) were built in the 1990s and sand has accumulated behind them to form the beach. The breakwaters have acted as islands and the sand has been deposited in the low-energy water behind them, in the same way that a tombolo forms. This can be seen from a photograph taken from the Burrard Bridge in 2015 (Figure 17.5.3). The two benefits of this project are that a pleasant beach has been created, and some of the sediment that previously would have been moved into False Creek, and could have blocked its entrance, has been trapped in English Bay. The negative impacts are probably not well understood, but have likely involved loss of marine animal habitat.\r\n\r\n[caption id=\"attachment_790\" align=\"aligncenter\" width=\"900\"]\"\"<\/a> Figure 17.5.3 Photograph of the impact of breakwaters on beach development at Sunset Beach, Vancouver.[\/caption]\r\n\r\n[pb_glossary id=\"1929\"]Groynes[\/pb_glossary]<\/strong> (or groins in the U.S.) have an effect that is similar to that of breakwaters, although groynes are constructed perpendicular to the beach (Figure 17.5.4), and they trap sediment by slowing the longshore current.\r\n\r\n[caption id=\"attachment_791\" align=\"aligncenter\" width=\"900\"]\"\"<\/a> Figure 17.5.4 A groyne at Crescent Beach, Surrey, B.C.[\/caption]\r\n\r\nMost of the sediment that forms beaches along our coasts comes from rivers, so if we want to take care of beaches, we have to take care of rivers. When a river is dammed, its sediment load is deposited in the resulting reservoir, and for the century or two while the reservoir is filling up, that sediment cannot get to the sea. During that time, beaches (including spits, baymouth bars, and tombolos) within tens of kilometres of the river\u2019s mouth (or more in some cases) are at risk of erosion.\r\n
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Exercise 17.5 Crescent beach groynes<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nThis diagram shows the same area illustrated in Figure 17.5.4 at Crescent Beach in Surrey, B.C. Based on information that you can find on the Internet about the function of groynes, determine which way the prevailing longshore current is moving at this location.\r\n\r\n[caption id=\"attachment_792\" align=\"aligncenter\" width=\"945\"]\"\"<\/a> Figure 17.5.5 Beach and groyne locations at Crescent Beach, Surrey.[\/caption]\r\n\r\n \r\n\r\nSee Appendix 3 for Exercise 17.5 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n

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