Lecture 3: Tsunamis, Coastal Hazards

earthquake-triggered tsunami development: stage 2 (speed, height, and spacing of waves)

- in deep ocean, waves move rapidly (can reach 500 km/h), and spacing of wave crests is very large (over 100km) - height (amplitude ) is often small (less than 1 m)

what should be included in education on tsunamis?

- info on where to go and what to take - clear evacuation routes for mass exodus - people must be aware that tsunamis are a series of waves (not just one) - knowing the difference between watch (might happen) vs. warning (will happen)

sea level rise & Maldives - how high above sea level is it and what is being done to protect it?

- island nation of 300 000 people in Indian Ocean - about 80% of country is less than 1 m above sea level - seawalls have been built around many of its islands to protect from waves up to 2 m in height

Seismograph

- records ground movements caused by seismic waves - estimates earthquake magnitude

structural control

- regulations on buildings and structures E.g. Hawaii: flood proofing measures (basement window sealing, bolting homes to foundation); concrete levees (expensive); offshore barriers (only feasible outside cities with v large populations)

sand transport

- sand is v dynamic and constantly moved by wave action - littoral transport: sand movement parallel to the shore - consists of beach drift & longshore drift

natural service functions of erosion

- scenic bluffs & coastal landscapes are a direct result of erosion - beaches are maintained by a constant deposition of sediment - coastal areas are popular tourist attractions for recreation

tsunameters

- scientific instrument that can detect when a tsunami passes by measuring changes in water pressure - sensors electronically connected to buoys; resting on the seafloor

erosion (how does it occur and why)

- sea cliffs and bluffs erode due to wave action, running water, and landslides - most erosion occurs during storms that bring powerful wind

inundation maps

- show geographic area that can be potentially impacted by tsunamis; created to help plan for future events - many cities on Pacific coast of NA have these (high risk of tsunamis) - historical records, geologic data & aerial photography help to make these maps

landforms produced by littoral transport

- spits: ridges that extend parallel to the shore from a point of land on a coast - barrier islands: long, narrow islands separated from the mainland by a bay or lagoon

seawalls (hard stabilization)

- structures built parallel to shoreline - vertical design reflects waves and redirects energy to shore - promotes beach erosion

groins (hard stabilization)

- structures built perpendicular to shoreline usually in groups - trap sand carried by longshore drift - accumulation of sand contributes to wider beach but erosion still occurs in downdrift area

hard stabilization

- structures designed to protect the shoreline (e.g. seawalls, groins, breakwaters, jetties)

coastal zone management: - due to dynamic nature of coastlines, most structures in coastal zones should be considered ________ and _________ - is this concept with or against the attitude of developers?

- temporary; expendable - this goes against attitude of developers who believe coasts are too valuable not to develop

Why did countries bordering the Indian ocean have such high death tolls from 2004 tsunami?

- they didn't have the tsunami warning system like countries bordering the Pacific Ocean - many were unfamiliar w/ tsunamis and some were intrigued by approaching waves - most people in area were ignorant of early warning sign (receding sea)

distant tsunami

- travels thousands of kilometres across open ocean - on remote shorelines across the ocean, reduced energy lessens the impact (also called tele-tsunamis)

size of a wave depends on:

- velocity of the wind - duration of the wind - fetch (distance wind blows across water)

earthquakes can cause tsunamis in 2 ways:

1) vertical displacement of sea floor 2) triggering a landslide that enters water

tsunamis develop in a _____ stage process

4

generally, an earthquake must be a magnitude of at least ______ in order to trigger a tsunami

7.5

both types of drift occur where waves strike the coast at an angle other than ______

90 degrees

breaking wave

A wave whose top falls over the bottom due to coming in contact with land. (when you can start to see the white cap)

pacific ring of fire

Major earthquake zone that forms a ring around the pacific ocean

areas at greatest risk are the _________ Ocean and the __________ Sea

Pacific, Mediterranean

North American cities at greatest risk of sea level rise include

Vancouver, Miami, New Orleans, New York

longshore trough

a depression on the seafloor formed by wave action landward of the longshore bar

local tsunami

a tsunami that affects shorelines a few km to about 100 km from its source (e.g. source = underwater earthquake) because of this short distance, local tsunamis provide little warning

E-zone

area between the present shoreline and the respective E-line

breaker zone

area where incoming waves peak and break (plunging/spilling)

surf zone

area where waves move toward the shore after they break

earthquake-triggered tsunami development: stage 3

as tsunami approaches land, the water depth decreases --> water piles up and causes these effects: - decrase in speed - decrease in spacing of waves - increase in wave amplitude (height)

beaches: diagram

beaches consist of loose material that has accumulated by wave action at the shoreline

in the open ocean, wave motion is ________

circular

circles __________ in diameter with depth because energy __________ with depth

decrease; decreases

wavelength

distance between successive wave crests

most tsunami deaths are from

drowning

sea cliff (bluff)

erosional landform that marks the landward boundary of the beach

secondary effects of tsunamis

fires, contaminated water supplies, water-borne disease (e.g. cholera) *generally occur after event is over*

spilling breaker waves: gentle slope/steep beach? more/less erosive?

gentle slope; less erosive e.g. Great Lakes

tsunami is the japanese word for

harbour wave

- irregularities in topography cause variations in wave ___________ as they approach the shore

height

those located in coastal areas during an earthquake should leave for _________ _________. if sea level noticeably recedes, leave the ________

higher ground; coast

erosion on great lakes increases during times of higher ______ ________

lake levels

irregular coastlines have headlands that the wave reach first; resultant waves are typically _________ and have ________ energy

large; high

2 types of tsunamis

local and distant

longshore bar

low ridge on the seafloor in the breaker zone

run-up

maximum vertical distance that the largest wave of a tsunami reaches as it travels inland essentially: how high the tsunami got on land

berm

onshore portion of the beach that is *generally flat* and formed by deposition of sediment (through wave action)

beach face

onshore portion of the beach that slopes seaward. it lies within the *swash zone* (where waves repeatedly rush up and then back)

2 types of breaking waves

plunging and spilling

coastal hazards

rip currents, erosion, sea level rise, storm surges, hurricanes, tsunamis (landscape where continents meet oceans -- dynamic and capable of rapid change)

coastal topography is influenced by the type of _______ as well as the _________ deposited from rivers or glaciers

rock; sediment

beach drift

sand moving in zig-zag pattern in the swash zone

during some tsunamis, the water first recedes from the shore and exposes the _________

seafloor

tsunami event

series of large waves reaching shore (can last for several hours)

plunging breaker waves: gentle slope / steep beach? more/less erosive?

steep; more erosive

coasts located near _____________ or across ocean basins from __________ are most at risk

subduction zones (x2)

tsunamis are produced by

sudden displacement of water

there is more wave energy near the _________, and decreases as we go down into the water

surface

E-Line

the expected position of the shoreline after a specified number of years

wave period

time for successive wave crests to pass a reference point

longshore drift

transport of sediment by currents that flow parallel to the shoreline

wave height

vertical difference between crest and trough

rip currents develop when

waves pile up water between the longshore bar and the swash zone; a breakage develops in the longshore bar and then water rushes back through the break (and can pull you in)

waves produced by

wind

how can vegetation impact damage to land from tsunami?

- in areas impacted by smaller waves, trees & dense vegetation protect areas farther inland (slow down the wave)

events capable of triggering tsunamis?

- earthquakes that uplift seafloor - landslides - volcano flank collapse - submarine volcanic eruptions - meteorities

a warning system by itself is not enough. why?

- emergency officials must have an organized plan for evacuating residents - earthquake and tsunami education is necessary for everyone who lives along/visits coastlines

jetties (hard stabilization)

- extend perpendicular to shores at the mouth of a river; they are commonly built in pairs - prevent sediment from accumulating at a river mouth & shelter river channel from large waves

primary effects of tsunamis

- flooding & erosion these destroy beaches, coastal vegetation and infrastructure

number of solutions to control coastal erosion & minimize damage:

- hard stabilization - soft stabilization - land use

rip currents

- horizontal currents that move away from a shoreline - aka undertow - pull people horizontally outwards from the shore

where have large tsunamis taken place over history?

- Lisbon - Portugal (1755), - Krakatoa Volcano - Indonesia (1883) - Sumatra Earthquake - Indonesia (2004) - Tohoku Earthquake - Japan (2011)

Japan Tsunami 2011: - M = ? - Located ______km off the ______ coast of Japan - when was the warning issued? - approx how many people died?

- M = 9.0 - 70km off the east coast of Japan - warning issued about an hour before arrival - over 15 000 died

Indian Ocean Tsunami - 2004 - M = ? - Located off the west coast of ________ - How did it rank in strength over history? - Occurred in a subduction zone between ___________ and ____________ plates

- M = 9.1 - Sumatra - 3rd strongest earthquake in world history - Burman and Indian-Australian plates

passive margins

- a coastline margin that is distant from a plate boundary - wide continental shelves, sandy/pebbly beaches, barrier islands - e.g. east coast of NA, Canadian Arctic

active margins

- a coastline margin that's relatively close to plate boundaries - more rocky shorelines & sea cliffs - e.g. BC coast

sea level rise & erosion the highest erosion rates are ____ annually

- as sea level rises, erosion is affecting areas farther inland; the highest erosion rates are 10 m annually - sea level is projected to rise over the next century as global climate changes & ice sheets continue to melt

earthquake-triggered tsunami development: stage 4

- as the tsunami impacts land, waves can reach heights of dozens of metres. - the wave speed at this time can be up to 50 km/h making them impossible to outrun

soft stabilization

- beach nourishment: involves adding sand to replace sand that has been eroded

breakwaters (hard stabilization)

- built parallel to shores and designed to protect boats in a harbour / marina

earthquake-triggered tsunami development: stage 1

- displacement of seafloor sets waves in motion --> transmits energy outward and upward (when waves reach surface of water, they spread outward)