9/25/2018 OCN 201:Coastal processes Drakes Estero Estuary, CA Cannon Beach, OR Barter Island, Alaska Lulworth cove, England Question Oil and natural gas are: A. formed from the remains of dead organisms B. renewable marine resources C. formed mainly in the deep ocean D. none of the above 1 9/25/2018 The coastal zone Coasts are complex, diverse, and dynamic environments • More than half of the US population lives on the narrow coastal strip that covers 17% of the total land area of the country • Much of the population of other countries along the sea also lives within a small distance from the coast • The position of the coastline is not constant due to – Vertical movements of the crust due to tectonic activity (subsidence/uplift) – Eustatic sea level changes – Sedimentation and erosion Coasts are shaped by marine and terrestrial processes Classification of coasts • Erosional coasts: dominant processes remove coastal material • Depositional coasts: steady or growing due to the accumulation of sediment or the action of living organisms (e.g. corals) Sunset Cliffs, San Diego Bolonia Beach, Spain 2 9/25/2018 Erosional coasts • High energy coasts • Sea cliffs • Sea coves • Sea stacks • Wave-cut platforms Features of an erosional coast at low tide Wave erosion of a sea cliff produces a wave-cut platform visible at low tide Sea cliff and wave cut platform The result of wave action on the coast Cliffs Area of cliff recession Wave cut platform 3 9/25/2018 Sea stacks The result of wave action on the coast Over extended periods of time, shorelines are straightened by wave erosion Wave energy converges on headlands and diverges in the adjoining bays. Sediment eroded from headlines accumulates as beaches on relatively calm bays, eventually smoothing the contours of the shore. 4 9/25/2018 Depositional coasts and beaches 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 Longshore transport Result of waves hitting the coast at an angle • Wave-driven movement of sand along exposed beach • Current-driven movement of sand in the surf zone 5 9/25/2018 Rip currents Rip currents occur when water forced up the beach returns out to sea (these also provide a mechanism for moving sand offshore). The higher the surf, the greater the probability of rip currents Question What would you do if you are caught in a rip current and you are being carried offshore? A. Swim as fast as I can towards the shore B. Slowly swim parallel to shore before returning to the beach C. Stop swimming and wait for rescue D. Swim away from the shore 6 9/25/2018 Other large-scale features of depositional coasts • Sand spit: deposition bar that forms when the longshore current slows as it clears a headland and approaches a quiet bay. • Bay mouth bar: sand spit that closes off a bay Other large-scale features of depositional coasts • Barrier islands: narrow, exposed sandbars that are parallel to but separated from land • Lagoon: a long, shallow body of seawater isolated from the ocean • Sea islands: contain a firm central core that was part of the mainland when sea water was lower 7 9/25/2018 River deltas • Landform that forms from deposition of sediments carried by a river • They form at places where sediment-laden rivers enter enclosed or semi- enclosed seas Mississippi Delta Coast formed by biological activity Atolls • Fringing reef grows around the perimeter of rising or static volcanic island • Island starts to sink isostatically, barrier reef develops as corals grow upward, separated from the island by a lagoon • Eventually the volcanic island sinks completely beneath the surface, leaving an atoll where the barrier reef continues to grow upward 8 9/25/2018 Question Who was the principal proponent of the theory of atoll formation: A. Alfred Wegener B. Charles Darwin C. Arthur Holmes D. James Hutton Estuaries An estuary is a body of water where fresh river water mixes with ocean water Types of esturaries: • Drowned river mouths: coastal river valleys with gentle sloping bottom • Fjords: narrow, steep, glacially eroded • Bar-built: when sand bars build along the coastline. Shallow, limited tidal action • Tectonic: formed by faulting and local subsidence. 9 9/25/2018 Characteristics of estuaries Characteristics of estuaries are determined by: • shape of the estuary • volume of river flow at the head of the estuary • range of tides at the mouth According to their circulation patterns they can be classified as: • Salt wedge estuary: rapidly flowing river, low to moderate tidal range. Exciting freshwater holds back a wedge of intruding seawater • Well-mixed estuary: slower flow, tidal range moderate to high. No vertical gradient in salinity, salinity increases from river to ocean • Partially mixed estuary: deeper, tidal range moderate to high, high river flow • Fjord estuary: where glaciers have gouged steep U-shape valleys bellow sea level. River water flows at the surface, little mixing with seawater Erosion of Coastal Areas Erosion of coastal areas occur: • where a beach has an insufficient supply of sand to replenish loss from coastal processes • when large (seasonal) waves and storm surge remove sand, some of which is not returned when waves subside • when rising sea level causes a beach to migrate landward and erode the dune/land behind it • when humans interfere with the natural sand supply, often resulting in sand starvation of the beach All of the above drive a beach to seek new sand, usually by eroding the adjacent land 10 9/25/2018 Human interferences in coastal processes Groins cause differential erosion because of how they impact water flow and interrupt longshore transport of sand Human interferences in coastal processes Seawalls protect property but increase beach erosion by deflecting wave energy onto the sand in front and beside them 11 9/25/2018 Human interferences in coastal processes Importing sand in response to erosion. New sand often is dredged offshore. Expensive. Can disturb aquatic biodiversity. Imported sand is normally finer than beach sand it will erode faster Question Human interference with coastal processes has generally accelerated the erosion of coasts near inhabited areas: A. True B. False 12.