Sea Stack Oceanography Chapter 12 Coasts pt. 2 Coasts are temporary structures, often subject to rapid change. The shape of a coast is a product of many processes: uplift and subsidence, the wearing down of land by erosion, and the redistribution of material by sediment transport and deposition. Sea Stack 1970 1990

Depositional coasts, especially along subsiding continental margins, often exhibit characteristic large-scale features. Sea Arch 1890 1910 1920

Coasts Are Shaped by Marine and Terrestrial Processes (top-left) The southeastern coast of the United States looked much different 18,000 years ago, during the last ice age. Because of lower sea level, the position of the gently sloping southeastern coast has been as much as 200 kilometers (125 miles) seaward from the present shoreline, leaving much of the continental shelf exposed. (bottom-left) In the distant future, if the ocean were to expand and the polar ice caps were to melt because of global warming, sea level could rise perhaps 60 meters (200 feet), driving the coast inland as much as 250 kilometers (160 miles)

Summary of Sea Level Changes 13,000 to 11,000 years ago sea level fell 400 feet (20 ft/century). 11,000 years ago sea level was at its lowest, 175 feet below present. 10,000 years ago to present sea level has risen 175 feet. 5,000 years ago sea level was 13 feet below present. 3,000 years ago sea level was 6 feet below present. 3,000 years ago the rate of sea-level rise slowed to 0.2 ft/century. Since 1912 sea level has risen at a rate of 0.6 ft/century. The current rate of sea-level rise is the fastest in the last 3000 years. Source: IPCC Technical Summary of Working Group I Report, Fig. 24, p. 74.

Erosional Coasts Often Have Complex Features Features of an erosional coast at low tide. Wave erosion of a sea cliff produces a shelflike wave-cut platform visible at low tide.

Shorelines Can Be Straightened by: Selective Erosion Wave energy converges on headlands and diverges in the adjoining bays. The accumulation of sediment derived from the headland in the tranquil bays eventually smoothes the contours of the shore. Marine erosion is usually most rapid on high-energy coasts, areas frequently battered by large waves. Low-energy coasts are only infrequently attacked by large waves. (Quiet beaches, bays)

Beaches Often Have a Distinct Profile A beach is a zone of loose particles that covers a shore. What are the features of a beach? Berm Berm crest Backshore Foreshore Beach scarp Longshore trough Longshore bars

Waves Transport Sediment on Beaches A longshore current – moves sediment along the shoreline between the surf zone and the upper limit of wave action.

Sand Input and Outflow Are Balanced in Coastal Cells Coastal sediment transport cells. A) Sand is introduced by rivers, transported southward by the longshore drift, and trapped within the nearshore heads of submarine canyons.

(B) Example of a sand budget. 1. If sediment gains and losses are approximately equal, the nearshore system is in equilibrium. This is a coastal cell. 2. If losses exceed gains, as shown here, the beaches within the cell will shrink and possibly disappear.

Large-Scale Features Accumulate on Depositional Coasts sand spit -forms where the longshore current slows as it clears a headland and approaches a quiet bay. bay mouth bar forms when a sand spit closes off a bay by attaching to a headland adjacent to the bay. Depositional coasts can also develop narrow, exposed sandbars that are parallel to but separated from land - known as barrier islands. A long, shallow body of seawater isolated from the ocean is known as a lagoon. (above) composite diagram of the large-scale features of an imaginary depositional coast. Not all these features would be found in such close proximity on a real coast.

The Characteristics of U.S. Coasts The Pacific Coast - An actively rising margin where indications of recent tectonic activity can be observed. The Atlantic Coast - A passive margin on the trailing position of the North American plate. The Gulf Coast - Smaller wave size and a smaller tidal range characterize the Gulf Coast.

Humans Have Interfered in Coastal Processes What are some ways that humans try to influence coastal processes? Groins Seawalls Importing sand (left) A few of the many types of measures taken to slow and prevent beach erosion. In many cases these methods help serve as a reminder that shorelines and beaches are constantly changing, and are not under human control.

(a) Groin Groins are structures that extend from the beach into the water. They help counter erosion by trapping sand from the current. Groins accumulate sand on their updrift side, but erosion is worse on the downdrift side, which is deprived of sand. Current (b) Seawall Seawalls protect property temporarily, but they also increase beach erosion by deflecting wave energy onto the sand in front of and beside them. High waves can wash over seawalls and destroy them and property. (c) Importing sand Importing sand to a beach is considered the best response to erosion. The new sand often is dredged from offshore, can cost tens of millions of dollars, and can disturb aquatic biodiversity. Because it is often finer than beach sand, dredged sand erodes more quickly. Figure 12.36: Some measures taken to slow beach erosion. Stepped Art Fig. 12-36, p. 342

Humans Have Interfered in Coastal Processes A map showing shore erosion by region. One example of shore erosion is the lighthouse on Cape Hatteras, which was moved during 1998 and 1999 to protect it from destruction. It was threatened by rising sea levels and a changing shoreline.

Maine Beaches and the Processes that Shape Them 4th Annual Maine Coast Natural History Seminar June 5, 2004 Peter Slovinsky, Coastal Geologist Department of Conservation Maine Geological Survey Intro

Maine’s Diverse Shoreline Photo by Ian Britton MGS File Photo Where’s Waldo??? From rocky shorelines… …to highly developed long sandy beaches…. …to undeveloped pocket beaches. Sandy beaches only comprise 2% of the Maine coastline! About 70 miles! Beaches and dunes are a VALUABLE RESOURCE.

Maine Beaches: Major Factors on Morphology Underlying Geology/Sediment Supply Sea Level Rise Waves, Currents, Tides, Wind Shoreline Stabilization/Development Recreational Usage: Colonial Ordinance

Underlying Geology From Kelley (1987)

Underlying Geology Beaches are limited: 2% of coast Only 8% of nearshore is considered “beach quality” sand Most are “pocket” beaches or enclosed littoral cells (bound by headlands) Most have limited sand supply Beach shapes heavily influenced by the underlying geology

Sea Level Change: The Last 13,000 Yrs The curve shows the fall and rise in the ocean over the last 13,000 years. In the last 3,000 years sea level has been relatively stable (a stillstand). From Dickson (1999) and based on Barnhardt (1994) and Belknap et al., (1987).

Shoreline response to Sea Level Change Map showing the inland-most shoreline about 14,000 years ago, based on marine deltas and shelly marine sediment. The offshore shoreline position was about 11,000 years ago and defined by sediments in cores, radiocarbon age dates, and geophysical surveys of submerged strata. MGS/UMaine graphic based on Barnhardt (1994).

Sea Level at Portland

Intergovernmental Panel on Climate Change Sea Level Forecast Source: IPCC Technical Summary of Working Group I Report, Fig. 24, p. 74.

Summary of Sea Level Changes 13,000 to 11,000 years ago sea level fell 400 feet (20 ft/century). 11,000 years ago sea level was at its lowest, 175 feet below present. 10,000 years ago to present sea level has risen 175 feet. 5,000 years ago sea level was 13 feet below present. 3,000 years ago sea level was 6 feet below present. 3,000 years ago the rate of sea-level rise slowed to 0.2 ft/century. Since 1912 sea level has risen at a rate of 0.6 ft/century. The current rate of sea-level rise is the fastest in the last 3000 years. Source: IPCC Technical Summary of Working Group I Report, Fig. 24, p. 74.

Waves, Currents, Tides, and Wind These are the “everyday” forces that move sand and cobble, building beaches and eroding them. The majority of beach response in Maine is due to waves in conjunction with currents and tides.

Barrier Beach Migration with Rising Sea Level and Storm Waves J. T. Wells (1995, after Dubois,1992). J. T. Wells (1995, after Everts,1987). Sand Loss Offshore Sand Overwash Ocean Rise Dune Retreat Barrier Beach Migration with Rising Sea Level and Storm Waves

Processes in response to storm waves Frontal dune ridge in Cape Elizabeth. About half of the ridge was eroded and subsequently rebuilt with American beach grass and sand. MGS File Photo by S. M. Dickson, 1986. US Army Corps of Engineers (1984)

Seasonal Beach Change -1 -2 -3 -4 -5 -6 Elevation (meters) -7 -1 -2 -3 -4 -5 -6 -7 0 20 40 60 80 100 120 140 160 180 200 220 Distance (meters) Elevation (meters) May 2001 October 2001 March 2001 July 2001 September 2001 October 2000 November 2000 January 2001 August 2001 February 2001 April 2001 December 2000 June 2001 Ogunquit Beach Profile #2

Dune Scarp Formation December 1992 Northeaster, Seawall Beach, Phippsburg A 4-day northeaster in December 1992 resulted in a coastal storm surge (left) of less than 2 feet produced an erosional cut in the dunes. Northeasters commonly produce frontal dune erosion and scarp formation (right). Dune scarps from major storms can take years to return to the pre-storm condition. MGS File Photo by S. M. Dickson, 12/13/1992 (left) and 1/24/1992 (right)

Frontal Dune Overwash October 1991 “Perfect Storm,” Saco Bay Beaches The fontal dune ridge is built primarily from wave action. In storms, waves deposit sand on the crest of the dune ridge. Most frontal dune ridges are built by the process of overwash to the elevation of active wave run-up and sand transport in a 100-year storm. Lesser storms add sand to low dune areas where wind and foot traffic have lowered the ground elevation. American beach grass grows through the new sand and helps hold it in place. MGS File Photos of Pine Point in Scarborough (left) and Ferry Beach State Park, Saco (right) by S. M. Dickson, 1991.

Signs of Dune Migration Tree stumps (in situ) on a beach in Kennebunk were drowned by the rising sea (left). Salt marsh peat from the back barrier marsh is exposed on Laudholm Beach in Wells (right). In both locations, the barrier beach and dunes have migrated inland over forest and marsh environments due to sea-level rise of about 6 feet in the last 3,000 years. MGS File Photos by S. M. Dickson. Drowned Forest, Kennebunk Drowned Salt Marsh, Wells

Shoreline Stabilization and Development Seawalls result in wave reflection that causes enhanced beach scour in front of the wall. Over time the beach will lower and expose more of the wall. Continued erosion in front of the wall leads to a narrower beach and the need to reinforce or enlarge the wall to increase its resistance to wave action. MGS File Photos (Scarborough, 2002 (left) by P.A. Slovinsky, and Saco, 1986 by S. M. Dickson.

Popham Beach Property Damage 1976 beach erosion at Hunnewell Beach, Phippsubrg L. K. Fink, 1976 J. T. Kelley, 1976 Erosion undermines homes at Hunnewell Beach, Phippsburg

Recommended Development Strategy Development located landward of a natural frontal dune ridge Back dune development behind a wide frontal dune ridge and elevated on posts in Wells. MGS File Photo by S. M. Dickson, 1986.

So where is the worst beach erosion in Maine? Camp Ellis Beach, Saco

Saco Bay Saco Bay NORTH Pine Point Grand Beach Prouts Neck Surfside Fletcher Neck Biddeford Pool Saco River Goosefare Brook Scarborough River Ferry Beach Camp Ellis Beach Hills Beach Ocean Park Old Orchard Beach Kinney Shores Pine Point Surfside Grand Beach Saco Bay From Kelley (1987)

Saco Bay Sand Budget About 4,000,000 cy of beach sand moved north (1859-1955), from the vicinity the Saco River jetty to the Scarborough River tidal inlet. This is about 3x the pre-jetty transport rate and 3x the sand resupply rate of the river. From Kelley et al., 1995.