Coastal Systems Energy & Landforms PDF

Coastal Systems Energy & Landforms “The shore is an ancient world, for as long as Dr. Christopher Smith there has been an Earth and sea there has been this place of the meeting of land and [email protected] water.” –Rachel Carson OCSB...

Coastal Systems Energy & Landforms “The shore is an ancient world, for as long as Dr. Christopher Smith there has been an Earth and sea there has been this place of the meeting of land and [email protected] water.” –Rachel Carson OCSB 344 Slide Pack Organization Part 1: Basic Processes 1. Coastal Hydrodynamics: waves, tides, long-shore currents 2. Sediment Budgets & Sea Level Part 2: Systems 3. Beaches: development, sand movement, consequences of human interference 4. Deltas 5. Estuaries 6. Beach Barriers 7. Lagoons 2  What is the coast? Coast: all the land near the sea, including the beach and land just inland from the beach can be rocky, mountainous and cliffs, or broad, gentle plains Coasts  1. Coastal Hydrodynamics: WAVES Waves: created by wind blowing over the surface of the water. Wave height is controlled by: 1. wind speed 2. time of wind blowing 3. Fetch: distance over which the wind blows when waves strike coastlines, wind energy is transferred to the rocks and sediments on beaches energy is available to erode coastlines and transport sediments beach erosion during storms increases greatly, and can undermine structures Coasts  1. Coastal Hydrodynamics: WAVES Waves: Height of water waves determined by wind speed, length of time that wind blows, and distance wind blows over the water (fetch) Wave height: the vertical distance between the crest (top) and trough (bottom) of a wave Wavelength: the horizontal distance between two wave crests Waves break along the shore as surf, spending most of their energy moving sand along the beach Coasts  1. Coastal Hydrodynamics: WAVES Orbits: As the water shallows, orbital motion will eventually impact the sea bottom, causing waves to pile up and topple over, forming breakers in the surf zone in deep water, energy advances with the wave, but the water does not orbital motion in waves decreases with depth until it is essentially gone at a depth of half the wavelength as water shallows, orbital motion will eventually impact the sea bottom, causing waves to pile up and topple over, forming breakers in the surf zone Interaction of waves with bottom is an important erosional force Coasts  1. Coastal Hydrodynamics: WAVES Wave refraction: waves hitting the shoreline at an angle bend and change direction to become more nearly parallel to the shoreline Longshore Currents: refracted waves hitting the coastline at a slight angle, which pushes water parallel to the coastline Coasts  1. Coastal Hydrodynamics: WAVES Longshore Drift: movement of sediment parallel to shoreline under the force of longshore currents some sediment is transported along the shoreline as waves wash up on the beach face most sediment is transported by longshore current in the more energetic surf zone Coasts  1. Coastal Hydrodynamics: WAVES Longshore drift can produce: Spits: build out into the open water off a point of land Baymouth bars: ridges of sediments that cut bays off from the ocean Tombolo: a bar of sediment connecting a former island (generally bedrock) to the shore Coasts  1. Coastal Hydrodynamics: WAVES Wave action ultimately cause coastlines to mature over time (Davis, 1896). Some diagrams illustrating coastal maturation: See slide 18, ‘Deserts’. Questions: 1) Where is erosion, deposition, transport? 2) Label spits, bars, tombolos. Coasts  1. Coastal Hydrodynamics: TIDES Tides: Bulges in the ocean caused by the gravitational attraction of the moon and the sun on Earth’s ocean, bulges remained aligned with Moon. Spring Tides: Highest tides produced by the alignment of the Earth, sun, & moon. Tides are another important force Neap Tides: At the first capable of re- and third quarter of the working, moon, sun and moon are transporting, at right angles to each eroding, etc. other, sediment in the coastal zone. Coasts  2. Sediment Budgets & Sea Level Sediment Budgets: The result of all the sedimentary inputs and outputs into a sedimentary environments. Inputs: terrestrial supply, marine sediments from offshore, riverine sources Outputs: transport to dunes, export to deep ocean. Coasts 2. Sediment Budgets & Sea Level Positive Sediment Budget: Sedimentary inputs greater than output Creates aggregating coastal systems Sediment begins to build-up & accumulate along the coastline forming a variety of geomorphological features: tombolo, expansive deltas & barriers, etc. Negative Sediment Budget: Sedimentary outputs greater than input Creates erosional coastal systems Waves and tides begin to “steal” sediment from the coastal system, collecting eroding away coastal The beaches of Galveston Island systems have a negative sediment budget. 13  2. Sediment Budgets & Sea Level Coasts  2. Sediment Budgets & Sea Level Erosional Coasts Common where bays are separated by irregular rocky headlands jutting out into the ocean Coastal straightening will occur, with wave erosion of headlands and wave deposition of sediments in bays, causing Coastal maturation Mature coast Note: does not imply a length time Coasts  2. Sediment Budgets & Sea Level Depositional coasts: typically exhibit gently sloping plains showing few effects of erosion shaped primarily by sediment deposition, particularly by longshore drift Example Barrier Islands: ridges of sand that parallel the shore, are common on depositional coasts protected lagoons separate barrier islands from the mainland barrier islands are dynamic, with rapid erosion and deposition in various areas heavy population on some barrier islands has led to property loss from rapid, localized erosion Coasts  2. Sediment Budgets & Sea Level Sea level: the height, relative to some datum, of the surface of the sea (ocean) as measured at a given time and place. Not constant through time or space As introduced during our Sediments Lecture, transgression is global sea- level rise, and regression is global sea- level fall. Coastal systems, are by definition, in dynamic equilibrium with sea level. This is an In situ coral head on Great Inagua Island, The Bahamas that is in life position, several meters above current If sea-level changes, coastal geologic, sea-level position. How did it get there? ecologic, and environmental systems must also migrate with sea-level change. Coasts  2. Sediment Budgets & Sea Level Sea level: the height, relative to some datum, of the surface of the sea (ocean) as measured at a given time and place. Example: Gulf of Mexico sea-level change over the last 12,000 years (Milliken et al., 2008) Curves are generated using samples that you know only form at sea level (e.g., salt marshes) and then date them (e.g., radiocarbon, U/Th). Coasts  2. Sediment Budgets & Sea Level Sea level: the height, relative to some datum, of the surface of the sea (ocean) as measured at a given time and place. Global sea-level rise over the last 24,000 years Significant change in sea level is related to the melting of global glaciers from the last Ice Age When sea level has been formally discussed in this course: 1st time: Sediments – basics of transgressions and regressions 2nd time: Coasts – how sea-level effects coastlines 3rd time: Climate – how climate and sea level are closely related Coasts  2. Sediment Budgets & Sea Level Drowned (submergent) coasts: Result from sea- level rising and flooding landscapes Common along coastlines today because sea level has been rising for 15,000 years since end of last ice age. Quiet waters of estuaries rich in marine life Cities and factories have badly polluted some estuaries 1. Example Estuaries: drowned river mouths 2. Example Fjords: drowned glacially-cut valleys Coasts  2. Sediment Budgets & Sea Level Emergent coasts: caused by landscapes that are elevated by tectonic forces, such as at convergent plate boundaries uplift has occurred more rapidly than rise in sea level uplifted marine terraces (originally formed just offshore from the beach face) are exposed along the tectonically active western coast of North America Although the land is moving up, you would perceive the landscape as a relative sea- level fall or drop Coasts  3. BEACH 4. ESTUARIES 5. BEACH BARRIERS carbonate siliciclastic Coastal Systems These are all Depositional 7. LAGOONS Environments that produce unique geologic deposits (see Sediments lecture) Most have both carbonate and siliciclastic analogs 6. REEF Flood-tidal delta Classic delta 4. DELTAS Ebb-tidal delta Bayhead delta Coasts  3. Beaches Beach: a strip of sediment (usually sand or gravel) from the low-water line inland to a cliff or zone of permanent vegetation Sand is from several sources: local rock erosion, re-working of sand stored seaward of the surf zone, carbonate remains of shelled marine organisms, and rivers Both siliciclastic or carbonate sand beaches exist, depending on local climate & sediment supply (Bahamas vs. Galveston), occur along many other coastal systems. Dominated by wave processes Coasts  3. Beaches Beaches: can be divided into very specific zones, morphology controlled by wave energy and sediment supply (budget, source) Backshore: above high-tide, inundated during storms. Berm Crest: highest point of normal wave activity. Foreshore: intertidal zone marked by swash processes. Inshore: wider sub-tidal area that extends just beyond breakers, limit of troughs and bars, includes the surf zone, breaker zone and swash zone. Sea level backshore foreshore Coasts  3. Beaches Beach Modification Anthropogenic changes have varying degrees of success, depends where you live Residential and political for expanding If negative sediment supply (Galveston), beaches at the mercy of the sea Some engineering structures used to control beaches: Jetties: rock walls designed to prevent the entrance of a harbor from filling with sand Groins: short walls perpendicular to shore built to trap sand and widen a beach Breakwaters: offshore structures, typically parallel to the shoreline, built to absorb the force of large breakers and provide quiet water near shore Coasts  4. Estuaries Estuaries: semi-enclosed coastal bodies of seawater that are freely connected to the ocean, and within which seawater is measurably diluted with freshwater from the land (Pritchard, 1967). Many developed by stream action incising a fluvial valley, which has become subsequently flooded by sea-level rise (Nelson, 1972), E.g., Fjords-Glacial lecture From the Latin word aestus, meaning boiling or tide Seawater mixing and circulation controlled by tides and fluvial inputs Net sediment sinks primarily from rivers Estuarine sedimentation dominated by WAVES & TIDAL variability Coasts  4. Estuaries Estuaries: semi-enclosed coastal bodies of seawater that are freely connected to the ocean, and within which seawater is measurably diluted with freshwater from the land (Pritchard, 1967). Many developed by stream action incising a fluvial valley, which has become subsequently flooded by sea-level rise (Nelson, 1972), E.g., Fjords-Glacial lecture From the Latin word aestus, meaning boiling or tide Seawater mixing and circulation controlled by tides and fluvial inputs Net sediment sinks primarily from rivers Estuarine sedimentation dominated by WAVES & TIDAL variability Coasts Example: Delaware Bay  4. Estuaries Estuaries: semi-enclosed coastal bodies of seawater that are freely connected to the ocean, and within which seawater is measurably diluted with freshwater from the land (Pritchard, 1967). Wave-dominated estuary Tide-dominated estuary Coasts  4. Deltas Deltas: A body of sediment deposited when a river or river-like current flows into a standing water body, like a lake or sea. Develop when a river carrying sediment fills its incised valley and associated estuary Word First used by Herodotus in 5th c. BC, applied to Nile River Delta Form when a river carries more sediment to the sea than can be re-worked by long- shore currents Delta morphology is controlled by rate of sediment supply, waves, tides. River- dominated Example 1: Classic deltaic morphology Wave- dominated Tide- dominated Coasts 4. Deltas Deltas: A body of sediment deposited when a river or river-like current flows into a standing water body, like a lake or sea. Develop when a river carrying sediment fills its incised valley and associated estuary Word First used by Herodotus in 5th c. BC, applied to Nile River Delta Form when a river carries more sediment to the sea than can be re-worked by long- shore currents Delta morphology is controlled by rate of sediment supply, waves, tides. Example 2: Bayhead delta Example 3: Flood-tidal & Ebb-tidal deltas Ebb-tidal delta Flood-tidal delta  5. Barrier Islands Barrier Island Complexes Long semi-straight features that parallel the coastline and separated from mainland by lagoons, bays and other bodies of water. Can occur in freshwater lakes (Great Lakes) and seas (Galveston Island) Longest in World: Padre Island in Texas, 200 km long, up to 8 km wide Are broken at intervals by tidal inlets May be just above high tide (longshore bar) or a comprehensive Barrier Island with own beach, and called a spit if connected to headlands Frenchman’s Bay, Lake Ontario Spit anchored to antecedent geology Coasts  5. Barrier Islands Barrier Island Complexes Long semi-straight features that parallel the coastline and separated from mainland by lagoons, bays and other bodies of water. Cross-section through a Beach Barrier Island. Note the position of beaches, dunes, marshes, lagoons with respect to the whole island complex. Large system of sediment mobilization, inherently linked to local sediment budgets. Coasts  6. Lagoons Lagoons: are shallow marine environments separated from the open sea by barrier-island complexes. On carbonate coastlines, barrier complex is typically the reef front On siliciclastic coastlines, barrier complex is typically a bay-front bar, beach-barrier spit, etc. with quartz sand Low energy (quiescent) environments, sediments primarily fine grained Clays and silts deposited in siliciclastic lagoons, carbonate silts and sands in carbonate lagoons, evaporites deposited in hypersaline lagoons. Atoll Reef lagoon lagoon lagoon Motus Frenchman’s Bay, Lake Ontario Coasts  Learning expectations: 1. What is a coast? How do waves form and re-work sediment in the coastal zone? How can a coastal mature over time? 2. Sediment Budgets: what happens during a negative vs. positive sediment budget? Sea level: what happens to coastal landforms during sea-level rise or fall? What is a spit, tombolo, barrier? How do they form? 3. What are 7 common coastal geomorphologic features, and how do they form? What physical processes dominate each environment? What were the sub- categories of each feature? 4. What kinds of rock types would each coastal system produce? What types of sediment structures do they produce? What happens to coastal environments when sediment supply or sea level changes? Coasts 34