Ch 9: Waves. Wind waves. Formation of a wind wave

Transcription

1 Ch 9: Waves 1. Features of Waves 2. Deep-water, shallow water and transitional waves 3. Breaking Waves 4. Wind Waves 5. Tsunamis Cf. Fig. 9-2 Waves are created by a disturbance. * wind (wind waves, L= m), where most of ocean s wave energy is located. * earthquakes (seismic waves, L = 200 km) * sun/moon (tidal waves, planetary scale). Restoring force is *gravity (gravity waves). *surface tension (capillary waves) Formation of a wind wave 1. A wave starts out as a ripple = capillary wave, named after the restoring force (surface tension, or capillary force) with very short L < 1.74cm. 2. As wind continues to blow, larger waves will build up that are restored by gravity =gravity wave. 3. H, L, S begin to build up until H/L > 1/7, the wave breaks, white caps form. 9.8 Wind waves Factors that increase wave height: Increasing wind speed Increasing duration (time) of wind Increasing fetch (distance) A fully developed sea is the maximum height of waves produced by conditions of wind speed, duration, and fetch (white caps form) See Table 9-2 for fetch and duration required to produce a fully developed sea

2 9-10. Global wave height acquired with radar altimeter (TOPEX/ Poseidon) Highest waves occur in the Southern Ocean (up to 6 m). Lowest waves were found in the tropics and subtropics. Area where wind generated waves originate is called the sea Swell describes waves that: Have traveled out of their area of origination Exhibit a uniform and symmetrical shape Are sustained not by wind but by energy obtained in the sea Waves with longer L leave sea area first, followed by slower ones = wave dispersion (= sorting of waves by wave length) The sea and swell Figure 9-9

3 Merchant ship laboring in heavy seas as huge wave looms astern. Huge waves are common near the 100-fathom curve on the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log Wave trains Swells often propagate as wave trains (groups of waves) Leading wave keeps disappearing, and is replaced by additional wave in the back. Wave train speed is ½ the speed of a single wave. Constructive Increases wave height Destructive Decreases wave height Mixed Variable pattern Interference patterns In phaseconstructive Out of phase, destructive Mixed, most common Fig. 9-14

4 Interference patterns Rogue waves Constructive Increases wave height Destructive Decreases wave height Mixed Variable pattern Figure 9-15 Fig Example of a rogue wave that is created where storm-driven waves meet strong ocean currents at a current boundary. Rogue waves: Largest wind-generated waves authentically recorded In 1935, the vessel USS Ramapo experienced a large wave while crossing the Pacific Ocean Wave height was measured at 34 meters (112 feet) Rogue wave

5 Ch 9: Waves 1. Features of Waves 2. Deep-water, shallow water and transitional waves 3. Breaking Waves 4. Wind Waves 5. Tsunamis Tsunami Tsunami terminology Often called tidal waves but have nothing to do with the tides Japanese term meaning harbor wave Also called seismic sea waves Created by movement of the ocean floor by: Underwater fault movement Underwater volcanic eruptions Land slides Fig A tsunami is created by an abrupt vertical movement along a fault in the earth s crust which pushes up the ocean water column above the fault. Massive long, low waves are created (L>200km, H=0.5m) which travel very fast (up to 700 km/h). Tsunamis cause strong flood surges up to 40 m above normal sea level.

6 Tsunami characteristics Affect entire water column, so carry more energy than surface waves (shallow-water waves everywhere Can travel at speeds over 700 kilometers (435 miles) per hour Small wave height in the open ocean, so pass beneath ships unnoticed Build up to extreme heights in shallow coastal areas Coastal effects of tsunami If trough arrives first, appear as a strong withdrawal of water (similar to an extreme and suddenly-occurring low tide) If crest arrives first, appear as a strong surge of water that can raise sea level many meters and flood inland areas Tsunami often occur as a series of surges and withdrawals Most tsunami are created near the margins of the Pacific Ocean along the Pacific Ring of Fire 4) Earthquake destruction Magnitude Chile earthquake, Tsunami from Chile earthquake 1960 Magnitude 9.5

7 Tsunami warning system Tsunami damage in Hawaii : From 1960 Chile earthquake, 15 hours later Pacific Tsunami warning system center (PTWC) stationed in Hawaii seismic listening stations track underwater earthquakes that could produce tsunami once a large earthquake occurs, the tsunami must be verified at a nearby tide-measuring station If verified, a tsunami warning is issued Successful in preventing loss of life (if people heed warnings) But damage to property has been increasing Indian Ocean Earthquake and Tsunami (Dec 26, 2004) Indian Ocean Earthquake and Tsunami (Dec 26, 2004) The earthquake took place in a region with previous earthquake activity as shown in the map prepared by the The Global Seismic Hazard Assessment Program (GSHAP) in the framework of the United Nations International Decade for Natural Disaster Reduction (UN/IDNDR). The present earthquake took place in a seismically active region at the plate boundary separating the Indian-Australian and East-Asian Plates. There are 12 plates in the world and earthquakes occur when these collide. A 13th plate was created by the breakup of the Indo-Australian plate was documented in This breakup has set up compression zone near Northern Sumatra.

8 Indian Ocean Earthquake and Tsunami (Dec 26, 2004) The 9.0 Earthquake at 6.58 hours at the epicenter (and in Sri Lanka) led to a sequence of 15 quakes across the Andaman region. The initial eruption happened near the location of the meeting point of the Australian, Indian and Burmese plates. Scientists have shown that this is a region of compression as the Australian plate is rotating counterclockwise into the Indian plate. Tsunamis are rarer in the Indian Ocean as the seismic activity is less than in the Pacific. There have been 7 records of Tsunamis set off by Earthquakes near Indonesia, Pakistan and one at Bay of Bengal in the last century. While earthquakes could not be predicted in advance, once the earthquake was detected it was possible to give about 3 hours of notice of a potential Tsunami. Such a system of warnings is in place across the Pacific Ocean but not in the Indian Ocean. In addition, coastal dwellers are educated in the Pacific littoral to get to high ground quickly following tremors and waves. Ch. 9 and 11: The beach and shoreline processes 1. Landforms and terminology in coastal regions 2. Interaction of waves with shores 3. Longshore current and longshore drift 4. Beach modification by protective structures 5. Wave refraction along an irregular shoreline and features of erosional shores 6. Features of depositional shores 7. Changes in sea-level Ch. 9 and 11: The beach and shoreline processes Landforms and terminology in coastal regions Interaction of waves with shore Figure 11-1 Summertime beach Wintertime beach Fig

9 Perpendicular movement of sand on the beach Summertime and wintertime beach conditions Movement perpendicular ( ) to shoreline Caused by breaking waves Light wave activity moves sand up the beach face toward the berm Heavy wave activity moves sand down the beach face to the longshore bars Produces seasonal changes in the beach Summertime beach Wintertime beach Wave refraction along a straight shoreline Movement parallel to shoreline Figure 9-19 Most waves approach shore at an angle The part of the wave in shallow water slows down The part of the wave in deep water continues at its original speed Causes wave crests to refract (bend) These waves cause a longshore current that moves water (and particles) along a zigzag line downstream.

10 Longshore current and longshore drift (Ch. 11) Longshore currents and rip currents Longshore current = zigzag movement of water in the surf zone Swash-Backwash Longshore drift = movement of sediment caused by longshore current 11-3 Movement of sand on the beach Movement parallel ( ) to shoreline Caused by wave refraction (bending) Each wave transports sand either upcoast or downcoast Huge volumes of sand are moved within the surf zone The beach resembles a river of sand Modification of beaches by hard stabilization: Jetties and Groins Jetties are always in pairs Groins can be singular or many (groin field) Both trap sand upstream and cause erosion downstream Figure 11-22

11 Types of hard stabilization Hard stabilization perpendicular to the coast within the surf zone: Jetties protect harbor entrances Groins designed to trap sand Hard stabilization parallel to the coast: Breakwaters built beyond the surf zone Seawalls built to armor the coast Example for hard stabilization interference of longshore drift: Breakwater at Santa Barbara Harbor, California Breakwater causes deposition in front and in harbor and erosion downstream Sand must be dredged regularly to keep Santa Barbara Harbor free Figure Seawalls and beaches Seawall damage in Leucadia, California Seawalls are built to reduce erosion on beaches Seawalls can destroy recreational beaches Seawalls are costly and eventually fail Figure Figure 10-25

12 Beach compartment s include: Rivers Beaches Submarine canyons Fig Beach compartments in southern California Wave refraction along an irregular shoreline and features of erosional shores Wave energy is concentrated at headlands and dispersed in bays Causes erosion of headlands and creation of erosional features Figure 10-14b Orthogonal lines run perpendicular to wave crests and indicate equal wave energy Features of erosional shores Uplifted, ancient wave-cut benches exposed in southern California Headland Wave-cut cliff Sea cave Sea arch Sea stack

13 Sea stack and sea arch, Oregon Features of depositional shores Spit Bay barrier Tombolo Barrier island Delta Figure 11-7 Barrier coast along coast of Martha s Vineyard Tombolo in the Gulf of California Barrier Islands along North Carolinas Outer Banks Barrier Islands along south Texas coast Fig. 11-9

14 Barrier island, New Jersey Formation of barrier islands Sea level has been rising since the last Ice Age Figure Caused barrier islands to roll toward shore like a tractor s tread Relocation of the Cape Hatteras lighthouse, North Carolina Evidence of emerging and submerging shorelines Figure 11B Emergent features: Marine terraces Stranded beach deposits Submergent features: Drowned beaches Submerged dune topography Drowned river valleys (Chesapeake Bay!)