Page 546 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

510 part III The earth–atmosphere interface
cliffs. Portions of the coasts of British Columbia and the Atlantic Provinces are classic examples. These coasts, composed of resistant rock, are scenically rugged and have few beaches.
A beach acts to stabilize a shoreline by absorbing wave energy, as is evident from the amount of material that is in almost constant motion (see “Sand movement” in Figure 16.11a). Some beaches are stable; others have seasonal cycles of deposition, erosion, and redeposition. Many beaches accumulate during the summer; are moved offshore by winter storm waves, forming a submerged bar; and are redeposited onshore the following summer. Protected areas along a coastline tend to accumulate sed- iment, which can lead to large coastal sand dunes. Pre- vailing winds and storms often drag such coastal dunes inland, sometimes burying trees, highways, and housing developments.
Beach Protection Changes in coastal sediment trans- port can disrupt human activities as beaches are lost, harbours are closed, and coastal highways and beach houses are inundated with sediment. Thus, people use various strategies to interrupt littoral drift (Figure 16.15). The goal is either to halt sand accumulation or to force a more desirable type of accumulation, through construc- tion of engineered structures, or “hard” shoreline protec- tion. Common approaches include the building of groins to slow drift action along the coast, jetties to block ma- terial from harbour entrances, and breakwaters to create zones of still water near the coastline. However, inter- rupting the littoral drift disrupts the natural beach re- plenishment process and may lead to unwanted changes in sediment distribution in areas nearby. Careful plan- ning and impact assessment should be part of any strat- egy for preserving or altering a beach.
(a)
Original shoreline
Erosion
Deposition
Jetty
Jetty
Erosion
Deposition
Groin
Shore-connected breakwater
Erosion Deposition
Groin
(b) Groins disrupt sediment movement along the coast of Lake Michigan, north of Chicago.
(c) A breakwater and jetties protect the entrance to Marina del Rey, California.
(d) The Five Sisters breakwaters in Winthrop, Massachusetts (near Boston). Coarse gravels and sand have accumulated in bars behind the breakwaters since their construction in the 1930s.
▲Figure 16.15 interfering with the littoral drift of sand. Breakwaters, jetties, and groins are constructions that attempt to control littoral drift and beach drift along a coast. [(b), (c), and (d) Bobbé Christopherson.]
Animation
Coastal Stabilization Structures
Breakwater
Harbour entrance
Littoral current
Sand movement