OC103 Lesson #19: The Dynamic Shoreline The Problems • Increasing population pressure – people like to live and vacation near the ocean. There is $3 trillion in coastal property in U.S. alone! • Steady rise of sea level – Sea level is currently rising by 3-4 mm/yr. At that rate (which is increasing), sea level could be up to 1 foot higher by the time you want to retire to a beachfront home in 2050. That does not sound too worrisome if your beachfront home is on top of a 50-foot cliff in Oregon, but is much more of a concern in places closer to sea level (<5 ft), such as Florida or Bangladesh. In some island nations, the entire country is within a few feet of sea level. These countries are deeply concerned about their future prospects. The maximum possible sea level rise is ~70 m if all of the glaciers melted (unlikely anytime soon, but see the image below right for what that would mean for New York City). • Increasing storm activity – Climate change and periodic phenomenon such as El Niño bring with them more and larger unusual events such as storms and floods. • Increasingly common and expensive erosion and damage – Government subsidies currently make it affordable to build, rebuild, and insure coastal structures, but as erosion and property damage becomes more common, it will become politically and financially more difficult to justify and afford these government subsidies. If these subsidies go away, people will be much less likely to build in damage-prone coastal areas. • We need to understand how beaches work to better predict how they will change over time (both slowly and with catastrophic storms). Until recently, we were able to observe but not predict how beaches change over time. Sophisticated instruments and computer models are now making it possible to predict how beaches will change. Many of the world’s beaches are dynamic places of pounding surf, shifting sand, and potential flooding from storm surges and rising sea level. This lesson covers some of the ocean processes and hazards along our coastlines, and what we can do to understand and live with them.Our Changing Beaches The shapes of beaches often change with the seasons. Sand disappears from the beach when the backwash of the larger waves of winter moves the sand just offshore onto sandbars, leaving behind smaller, steeper beaches. The sand then reappears during the summer, when small waves pick the sand up from the sand bars and push it back onto the beach, depositing it as a wide, gradual, smooth slope (one of your Lab exercises covers some of these changes). Go to http://coastalchange.ucsd.edu/st3_basics/beaches.html and scroll down to the “Seasonal Movements of Nearshore Sand” figure and click on the different seasonal buttons to see how sand moves around and the beach profile changes from summer to winter. See how the larger waves of winter move the sand off the beach and onto the sand bar, and then the smaller waves of summer move it back. Also look at the photos just below that figure, which show two examples of summer vs. winter views of beaches. Only the largest waves, those associated with significant storms or waves during unusually high tides for example, get high enough to erode at the land above the beach (such as a cliff face or dunes), causing long-term changes, such as cliff collapse and property damage (see image below). But as sea level rises, these previously unusual high-water events become more commonplace. Humans try to prevent the erosional events by building protective structures such as solid seawalls, piles of large boulders (called riprap) that deflect waves, or jetty structures that prevent sand from moving naturally along the shore. Some coastal communities periodically replenish their beaches by paying someone to dredge up sand from some- where offshore, but this is only a temporary solution because the replenished beach is just as vulnerable to future erosional events as the original beach was.Why Don’t All Beaches Look the Same? The shape, texture, and color of a beach depends on: • Types of waves Low wave energy (e.g., Gulf Coast U.S.) - gentle beach, fine sand High wave energy (e.g., West Coast U.S.) - steep beach, coarse sand or pebbles • Types of sediments - Not all sand is the same -- The type of sand that makes up a beach is determined by a combination of source and “aging” o Beaches on the West Coast of the U.S. tend to be constructed from local sources; that is, whatever sand was dumped into the ocean by a nearby river or eroded from nearby land by wave action. This is because the many rocky headlands along the West Coast block sand from traveling very far along the coast. Even though waves generated by storms in the North Pacific arrive at a slight angle from the north, and would otherwise tend to drive sand toward the south due to longshore drift, the rocky headlands prevent the sand from moving far. So the sand on most West Coast beaches is determined by the local geology. o In contrast, the beach sand on the East Coast of the U.S. varies systematically as you move north or south along the coast. Along the northern East Coast, the sand comes from local rivers and is dark-colored. As you go southward along the coast, the sand gets progressively lighter (see images below, courtesy of Rob Holman). This is because waves from storms in the North Atlantic hit the U.S. East Coast at a slight angle from the north, driving sand slowly southward along the coast. As the sand slowly makes its way southward it is continually battered and broken by the waves, so that only the sand particles most resistant to the pounding make it very far south without being pulverized. The less resistant minerals tend to be the darker ones, and the more resistant minerals tend to be the lighter-colored ones (mainly quartz). So as the sand moves southward the dark grains are destroyed and the sand becomes progressively lighter in color. Also, in the warmer waters toward the south there are more shells and coral that the waves can break into pieces, which contribute additional light-colored sand grains. So between the “aging” that tends to remove the darker mineral grains, and the addition of shell fragments, the sand becomes whiter and smoother, until you reach the wonderful sugar-like sands of southern Florida (sigh). New Jersey beach sand North Carolina beach sand Florida