I. Types of Coastlines A. Definition: 1. Shore = narrow zone where ocean meets land (e.g. beach) 2. Coast is a broad area where both ocean and land processes act a. Includes onshore marshes, dunes, sea cliffs, and off shore sand bars and barrier islands b. Processes acting to shape coasts include: i. Tectonic, erosion, waves, tides, storms etc. ii. Changes in sea level! - Emergent coasts = coast rising relative to sea level - Submergent coasts = coast sinking relative to sea level B. Factors influencing Sea level 1. Eustatic Changes (global changes in sea level) a. Glaciation locks water in ice, decreases amount of water in oceans i. Emergent coasts ii. Melting of glaciers -> rise in sea level = Submergent coasts b. Tectonics Change ocean basin volumes (rapid spreading = shallow ridges = low volume) c. Global Warming/Cooling warm oceans expand 2. Local Changes in sea level a. Tectonic & Isostatic adjustments change level of land b. Wind, currents, tides, ENSO 3. Sea level today is high a. sea level has varied by ~130 m over past 2 million years b. Maximum sea level is 6 m higher than today s sea level c. Recent Low i. 18,000 yrs ago at end of last glaciation ii. Coastlines are not yet in equilibrium with the current high sea level (they are eroding and being reshaped) 11/4/04 1
C. Classification of Coasts 1. Primary vs. Secondary Coasts a. Distinction based on dominant processes Terrestrial vs. Marine b. Primary Coasts i. Shaped by Terrestrial processes (tectonics, rivers, glaciers, volcanoes) - Remnant of the geologic processes that formed the land ii. Generally rough & irregular -Not significantly modified by marine processes c. Secondary Coasts: i. Shaped by Marine Processes ii. Little evidence remains of non-marine processes 2. Examples: Primary Coasts a. Land Erosion Coasts i. Eroded during low stand of last glacial maximum ii. Flooded as sea level increased during current interglacial period. b. Constructed Coasts - Built by deposition from rivers c. Volcanic Coasts d. Tectonic Coast 3. Examples: Secondary Coasts II. Beaches A. Beaches 1. Composed of Sand & gravel 2. Source: OFFSHORE (ultimately from land) 3. Constantly in motion / reshaped B. Beach Anatomy 1. Backshore all parts above high tide mark a. Berms i. Ridge (berm crest) - accumulation of sediment parallel to the beach front ii. Formed by waves moving sediment up on the beach b. Winter berm = storm berm 11/4/04 2
2. Foreshore all parts below high tide mark a. Beach scarp = wave cut (high tide) scarp b. Beach Face = sloped space between low and high tide c. Low tide terrace i. ~ Flat surface ii. Not exposed d. Cusps i. Evenly spaced ii. Arcs or crescent protrusions iii. Unknown cause 3. Near shore a. Effected by waves that touch bottom, but not exposed to atmosphere during low tide. b. Longshore bars indicate substantial sediment transport along shore 4. Offshore - Not part of beach, but lots of sand is moving as bars III. Beach Processes A. Erosion Processes & Features 1. Waves Strike shore and cause erosion a. Weight of crashing wave pounds shore b. Abrasion 2. Sea cliffs 3. Headland erosion - Results in straightening of shorelines a. Sea Stacks b. Sea arches etc. B. Sediment Transport along shorelines - Waves approach coastline from dominant direction (weather patterns) 1. Beach Drift a. Swash (waves running onto shore) move particles at oblique angle b. Backwash (waves running back to sea) fall perpendicular to shore c. Result of a. & b. is beach drift moves sand along shore 11/4/04 3
2. Longshore Current. a. Current produced by dominant angle of incidence b. Able to transport lots of sand great distances - Off shore bars moving 3. Seasonal Variations a. Summer i. Gentle winds and waves ii. Sand moved onto beach b. Winter i. Big storms and high winds ii. Sand moved offshore to bars iii. Transported down current iv. Back onto beach in summer 4. Coastal Circulation Cells a. Length of shoreline where sand supply = loss b. Source of sediment supplies beaches with sand - rivers and erosion of coastline c. Longshore transport and beach drift moves sediment down coast d. Cell is disrupted when i. Sediment is lost too canyons that funnel sediment off the shelf and away from beach forever ii. Longshore current is deflected by coast line, and sediment is transported well off-shore iii. Rivers are dammed and sediment supply is cut-off. C. Types of Beaches - Result from sediment transport 1. Spits a. Long shore transport toward bay b. Low E of bay = deposition of sand 2. Bay Mouth Bar - Spit closes bay 11/4/04 4
3. Tambolo sand connecting shore to island 4. Barrier islands a. What are they? b. Dunes from the last glacial maximum i. low sea level, dunes behind the former beach ii. Barrier Islands result when glacier melt Raise sea level and these former back-beach dunes are left as islands as the rising ocean flood the lagoons behind these islands. IV. Environmental Issues with Beaches: Trapping sediment robbing your neighbor A. Issue 1. People like beaches 2. Want to protect houses and towns (Miami) B. Rip Currents 1. Waves move water on-shore 2. That water must leave shore somewhere (through troughs or bays) 3. Narrow zones of rapid flow = rip currents - Turbid with sediment C. Problem: Build stabilizers to trap sediment 1. Groins and Jetties a. Trap sediment & grow beach b. Prevent longshore transport erosion down current 2. Seawalls built to protect shore face a. Increases wave energy at base of wall b. Erodes beach in front of wall (Miami) 11/4/04 5