Ch 9 - Changing Coasts

beach erosion

beach erosion is especially significant on the coasts of barrier islands

spring tides

especially high tides resulting from the combination of the gravitational pull of the Sun and Moon on the Earth when the Earth, Sun, and Moon are all aligned.

neap tides

especially low tides resulting from when the Moon is at right angles to the alignment of the Sun and Earth.

surf zone

turbulent region along the coast created by breaking waves.

Saffir-Simpson Scale

used to further classify hurricanes by their wind speed and intensity. rates hurricanes in categories 1-5. the scale is based on the central pressure within the eye of the storm- the lower the pressure, the stronger the storm. the scale is also based on the maximum sustained winds and the potential for storm surge.

wave height (H)

vertical distance between a crest (peak) of the wave and an adjacent trough (lowest point).

wave celerity (C)

wave speed (C = L/T) - how quickly a particular wave moves through the water.

spilling breakers

waves developed when seafloor gradually becomes shallower. crest is frothy and turbulent, does not hook forward.

plunging breakers

waves formed on a more steeply inclined seafloor. frothing, hooked crest, hollow tube.

isostatic rebound

when ice accumulates in thick layers over land and then melts, the land isostatic rebounds to its original position. however, if isostatic rebound can't maintain pace with sea level from melting, the land may initially be flooded.

highest tides

when the oceans are directly facing the moon, these areas are experiencing the highest gravitational pull from the moon and therefore the highest tide.

sea cliff erosion

where the beach is wide, it provides a buffer against sea wave erosion & protects any sea cliffs present. where the beach is narrow, waves may erode the sea cliffs- undercutting them and causing their collapse into the sea. larger waves from storms cause the most damage by both eroding cliffs and driving air into existing cracks, causing the rocks to split.

removing sediment

wind and wave action remove sediment

nor'easters

winter storms that form on the Atlantic coast. nor'easters form when warm, moist air from the Gulf Stream rises over cold air masses in Canada. can include significant amounts of precipitation and hurricane-force winds.

hard stabilization

a common term for engineered structures designed to stop, prevent, or reverse coastal erosion. examples include seawalls, breakwaters, groins, and jetties.

active continental margin

active continental margins are coasts where subduction and volcanism occur. they are tectonically active - coastal mountains, volcanoes, and a narrow continental shelf are common. unlike passive margins, they lack a continental rise and abyssal plain- instead, the continental slope ends in an oceanic trench, beyond which is hilly & irregular topography dotted with volcanic seamounts. ex: the U.S. Pacific Coast

wave period (T)

amount of time it takes two successive crests to pass by a fixed point.

gradual sea level change

gradual movements are typically tied to the loading and unloading of weight on Earth's crust - glaciers.

Bering Sea

has been isolated from direct plate boundary influence for 40-50 mil years. broad continental shelf, coastline with barrier islands, many long beaches. sea has carved out headlands and sea cliffs where mountains extend to shoreline. sediment is brought to the sea by rivers.

Atlantic hurricanes

often start out as thunderstorms over West Africa and the Caribbean that have drifted westward over the tropical Atlantic Ocean.

estuaries

semi-enclosed coastal bays where freshwater from rivers mixes with seawater. shallow, marshy, and partially protected from oceans. evolved from drowned river valleys - ex: Chesapeake Bay, largest estuary in the U.S. formed from rising sea level.

groins

shore-perpendicular barriers designed to capture longshore drift. their purpose is to slow beach erosion and/or create a sediment budget somewhere to build out a beach. constructed from stone blocks, concrete, steel, or timber.

barrier splits

similar ridges that are connected to the mainland. also help buffer mainland from powerful storms.

bulkheads

smaller than seawalls, designed for lower energy environments. shore-parallel walls constructed of timber, concrete, or steel. 1) on beaches, they're placed at the start of the berm and backfilled to create flat surface- remains dry most of time. 2) in ports, they're used to line the inner harbour area and the base is located at low-tide mark- remains wet most of time.

wind/pressure components of storm surge

storm surge is mainly caused by 1. low pressure within the storm (as the storm approaches the coast, the low atmospheric pressure allows the sea surface to bulge upward) and 2. strong winds (as strong winds approach the coast, they push the sea ahead of them) --> as the water reaches shallow depths, it has nowhere to go but up, causing extreme coastal flooding.

coastal storm hazards

storms bring many hazards to coastlines, such as high winds, high waves, inland flooding, landslides, and storm surge. while wind can cause extensive damage and danger- storm surge, sedimentation and erosion associated with storms have the greatest impact on coastlines.

wave amplitude (A)

the height of the wave above mean sea level.

rip currents

this backwash may sometimes be organized into plume-shaped rip currents that carry the water from breaking waves back through the surf zone.

longshore

longshore bars are present just beyond the edge of the nearshore environment.

rapid sea level change

rapid movements are typically tied to plate tectonics - earthquakes

tidal currents

the resulting currents formed where tides flow through restricted channels, such as between islands or the mouths of bays.

coastline shape and tidal range

the shape of the coast and seafloor influences tidal range --> ex: when the tide moves into restricted areas, tidal range increases, because the only direction the water can go is up.

cyclones

the winds surrounding a hurricane that revolve counterclockwise around the center of low pressure.

dunes

dunes are formed behind a beach when dry sand on the beach is blown by the wind. wet sand is oscillated back and forth by the waves. longshore drift transports beach sediment along coastline.

hurricane formation

1) warm surface water heats the overlying atmosphere 2) the storm gains strength as the warming air rises 3) the rising air causes the atmosphere pressure in the center of the disturbance to fall.

determining wave size

3 factors determine size of waves generated on open water: 1) wind strength, 2) amount of time that the wind blows in a consistent direction over the surface of the water, 3) fetch

Arctic Ocean

Arctic coastline is on a passive continental margin that formed from seafloor spreading that opened the Arctic Ocean. characterized by a wide coastal plain, barrier islands, and deltas at river mouths.

mangrove trees

Mangrove trees grow along the shoreline of sheltered bays in brackish water. they trap sediment and gradually build out the shoreline, and also provide a buffer against coastal storms.

tropical disturbance

a tropical weather system of organized convection

backwash

after turbulent uprush, water pauses briefly, then flows backward as backwash.

The Perfect Storm

also known as the Halloween Storm. occurred in October of 1991- Hurricane Grace collided with an extratropical cyclone that had formed off the Northeast coast. caused extreme winds and waves in the northeast.

diurnal tide cycle

an area has a diurnal tidal cycle if it experiences one high and one low tide every lunar day. ex: Gulf of Mexico

mixed semidiurnal tide cycle

an area has a mixed semidiurnal tidal cycle if it experiences two high and two low tides of different size every lunar day. ex: North America western coast.

semidiurnal tide cycle

an area has a semidiurnal tide cycle if it experiences two high and two low tides of approximately equal size every lunar day. ex: North America eastern coast

uprush

as breaking waves encounter the shore, the water flows up the beach in turbulent uprush.

dredging

b/c energy is diminished behind the barrier in breakwaters, sediment is deposited, requiring dredging to remove it. beaches may build out, preventing erosion, but sometimes they are built too far out and peninsulas may extend all the way to the breakwater- this can disrupt longshore drift and create sediment deficit elsewhere.

refraction

b/c waves approach the shoreline at an angle, part of the wave will encounter shallow water before the rest of it does - the part reaching shallower water slows down while the rest of the wave continues at original velocity. this change in velocity along the length of the wave causes it to bend - undergo refraction- as different parts of the wave "feel bottom" before others.

eye wall (hurricane)

band of thunderstorms that surround the eye. as warm, moist air spirals inward toward the eye wall and rises, it begins to cool and its moisture condenses into rain.

breakwaters

barriers built parallel to coast out in water in higher energy environments. the barrier causes waves to break offshore, reducing the energy felt onshore and creating a calmer environment behind them. made from massive stone blocks or concrete cells.

berm

beach area in front of dunes and behind waves. distinctive parts develop on beaches as waves and winds come ashore under different conditions- the stronger waves and winds during storms are very effective at developing berms and dunes.

beaches

beaches are accumulations of unconsolidated water-transported material deposited on the edge of a body of water- typically a gentle sloping shore washed by waves or tides. includes sand, pebbles, mud, rocks, and shell fragments. not static- constantly in motion and during storms they can completely disappear.

Pacific Coast

both convergent (Oregon/Washington) and transform (California) tectonics occur along the Pacific coast. the continental shelf is narrow, rocky cliffs & pocket beaches are typical. the Pacific surf is extremely active, with sea arches, sea stacks, and sea caves throughout. further north exist longer beaches where rivers deliver sediment eroded from the mountains down to the coast.

relative sea level changes

changes in sea level change due to climate (shifts from glacial to interglacial periods --> rise and fall in sea level as water becomes tied up and subsequently freed from ice) or due to plate tectonics (land rises and falls).

littoral cells

coastlines are segmented into different littoral cells- individual self-contained systems where sediment enters/exits.

highest storm surges

coastlines with shallow, gently sloping continental shelves typically develop the highest storm surges. barrier islands and spits are particularly vulnerable to storm surge due to their low elevation and high population density.

disadvantages of seawalls

costly, periodic maintenance needed, continued wave attack scours base, contribute to beach erosion- when waves strike the wall they're reflected back, eroding the beach in front of the wall

hurricane

cyclones with sustained winds reaching 118 km/hour.

tropical storm

cyclones with winds exceeding 62 km/hour. convection is more concentrated near the center, and outer rainfall organizes into distinct bands.

tropical depressions

cyclones with winds less than 62 km/hour

fetch

distance over which the wind blows

sediment budget

each littoral cell has a sediment budget consisting of credits (sand entering cell) and debits (sand leaving cell).

coastal erosion and sedimentation

erosion on one part of coast = sedimentation in another. while beach erosion isn't necessarily a hazard in itself, expensive waterfront homes can be subject to major damage.

pollution of estuaries

estuaries become very polluted due to their being heavily populated. pollution may come from industrial waste discharges, untreated sewage release, runoff from urbanized areas or farmlands, and from inappropriate garbage disposal.

Alaska Coast

extremely long coastline, very diverse- includes glaciers and isolated islands formed from stratovolcanoes. contains convergent plate boundary in Southern Alaska that has one of the largest subduction zones in the world. active tectonism exists.

headlands

headlands interrupt the longshore currents that might otherwise bring in sand from other cells.

wavelength (L)

horizontal distance between two successive crests.

hurricanes

hurricanes are vast, rotating storms which gain their strength by drawing energy and moisture from warm ocean waters in the tropics.

surging breaker

if seafloor is very steep, wave may begin to crest but then just rush up the beach as a surging breaker.

mitigating coastal storm hazards

improved forecasting/tracking has helped reduce human fatalities from hurricanes. to reduce structural damage, rigorous building codes have been adopted- storm-proofing a house by fastening the roof with special nails that will hold the roof in place during extreme winds, and houses are also built on stilts to protect against storm surge.

Atlantic hurricane season

in the summer, from beginning of June to end of November. activity peaks in September, averaging two major hurricanes in a year. periods of above or below normal hurricane frequency tend to be extended- 25-40 years.

localized sea level change

local changes in sea level may not be global in scale but can still affect an entire coastline- land may move up or down, gradually or rapidly.

points/headlands

local seaward extensions of the coast in which wave refraction is pronounced. refracted waves become directed toward all sides of headland, which focuses energy into the headland --> higher coastal erosion rates.

barrier islands

long, low-lying ridges of sand and gravel isolated from the mainland by shallow lagoons, bays, or marshes. help buffer mainland from powerful storms. barrier islands formed off Atlantic Coast in past 6000-7000 yrs at current sea level.

longshore currents

longshore currents transport sediment along coastlines. waves that approach the shore obliquely (slanted) create longshore currents that are parallel to the beach- can transport sediment along coast and move sand grains in zigzag motion.

delivering sediment

longshore drift, rivers, and cliff erosion deliver sediment

revetments

made for even lower energy environments. provide a protective covering on embankments or beaches to resist erosion from light waves/nearshore currents. also shore-parallel, and vertical. constructed from blocks of broken rock, stone blocks, or poured concrete waves.

jetties - 1

manmade barriers that extend out from the shore. jetties deflect longshore currents seaward, sending the longshore sediment seaward as well. over time, beaches can shrink as a result as their sediment supply gets cut off.

seawalls

massive vertical shore-parallel structures designed to prevent upland erosion and storm surge flooding. they are generally massive concrete structures placed along a considerable stretch of shoreline at urban beaches. designed to withstand the full force of storm waves.

humans vs. sea

modifying the coastal system often has negative consequences- some structures meant to prevent erosion can make it worse. Ex: seawall in Galveston that protects the shore from storms but causes the beach in front of it to erode.

availability of sediment

most beach sediment is delivered to the coasts by rivers, and beaches respond to the availability of sediment by growing wider or being eroded.

wave motion

most waves are born where the atmosphere's wind blows across a body of water, setting the water's surface in motion - transfer of energy from wind to water. water acts as passive medium through which the energy (wave) moves.

coastal sedimentation

occurs when the influence of waves and currents are diminished. most commonly occurs in deeper water offshore, and in protected waters inshore. also occurs when supply > transport capacity- too much for waves/currents to move.

swells

ocean waves generated from distant storms. swells can travel great distances across the open ocean, and therefore have very long wavelengths.

lowest tides

oceans 90 degrees from the gravitational pull of the Moon and inertia experience the lowest tide.

deep submarine canyons

on the narrow continental shelf (of active continental margins), deep submarine canyons lie close to shore and act as a chute through which sediment is carried far from shore by turbidity currents.

Gulf of Mexico Coast

passive continental margin that formed as a result of seafloor spreading between the Yucatan Peninsula and the U.S. Southern coast. includes the barrier island of Galveston. stretches of beach, coastal swamps, bogs, lagoons, and estuaries are interspersed among the barrier islands.

passive continental margin

passive continental margins are coasts adjacent to where seafloor spreading occurs. ex: the atlantic coast, formed when rifting split North America from Europe, is a passive continental margin - it subsides as sediment from the continent accumulates there (not from tectonics). small changes in sea level can cause shorelines to advance (transgress) or recede significant distances on passive continental margins.

sand components

quartz is common due to its resistance to weathering. in tropical settings, beach sediment is primarily broken seashells and coral. in volcanic settings, beach sediment is mainly weathered basalt grains. where sea cliffs are pounded by sea waves, beach may be very gravelly.

drowned river valleys

river valleys that became drowned and land that became submerged as the sea level rose with the retreat of the glaciers.

ex: Santa Monica littoral cell

sediment that enters the littoral cell is carried along the shoreline and eventually lost to deep water through submarine canyons that mark the boundaries of littoral cells.

Florida Keys

the Florida Keys, a chain of islands that extend southwestward around the tip of Florida, are the exposed tops of ancient coral reefs.

storm surge

the abnormal rise of water generated by a storm, over and above the predicted astronomical tides.

tidal range

the amount of tidal rise and fall during a cycle.

Mississippi River Delta shrinkage

the delta is shrinking as a result of reduced sedimentation due to 1) management of the Mississippi River upstream (dam constructed has decreased sediment flow downstream) and 2) much of remaining sediment is deposited onto basin floor and isn't available to maintain delta). result: major loss in wetlands.

eye (hurricane)

the low-pressure area of warm, calm air in the center of a hurricane.

longshore drift

the movement of sediment along coastline by longshore currents

nearshore

the nearshore environment extends out for 20 meters to where waves begin to exert force on moving sediments.

coastline size and tides

the size of the coastline affects local tide --> larger and longer coastlines will experience larger tidal ranges, while smaller islands experience smaller tidal ranges.

tides

tides are a rhythmic rise and fall of the surface of the Earth's oceans. sea level fluctuates between 2 high and 2 low tides each day on most coasts.

causes of tides

tides are caused by gravitational interactions mainly between the Earth and the Moon, but also between the Earth and the Sun.