Description of Waves

The simplest way of looking at waves is the concept of a wave as a harmonic oscillation. It can then be described by its:

• period – the time it takes 2 successive wave crests to pass a fixed point

• frequency = 1 / – the number of waves passing a fixed point per second

• wavelength – horizontal distance between two successive wave crests

• wave speed c = /

• wave height H = 2A (A = amplitude) – vertical distance between wave crest and adjacent trough

• wave steepness = H /

Calm water

Ocean waves can be classified in various ways:

Disturbing Force- the forces which generate the waves. 1. Meteorological forcing (wind, air pressure); sea and

swell belong to this category. 2. Earthquakes; they generate tsunamis, which are

shallow water or long waves. 3. Tides (astronomical forcing); they are always shallow

water or long waves.

Wave Classification

Hurricane Andrew- 1992

"Tsunami" - a Japanese word meaning "great wave in harbor". It is a series of ocean waves commonly caused by violent movement of the sea floor by submarine faulting, landslides, or volcanic activity. A tsunami travels at the speed of nearly 500 miles per hour outward from the site of the violent movement.

Fault displacement under water displaces water, water moves to fill vacuum, generating large waves.

Tsunami

Tsunamis struck Kahului in 1946, 1957, 1960, and 1964. The earliest historically recorded tsunami in Kahului occurred on November 7, 1837, when a large tsunami traveled 800 yards inland and destroyed a Hawaiian village.

The 1960 tsunami was caused by a violent earthquake in Chile on May 22, 1960. It took approximately 15 hours for the tsunami to travel from Chile to the Hawaiian Islands. The tsunami killed 61 people in Hilo on the Big Island, but there were no other human casualties on any of the other islands. The tsunami caused moderate damage in Kahului.

MauiPuunene Avenue

Aftermath of a Tsunami in Kahului, 1960

A giant wave engulfs the Hilo pier during the 1946 tsunami. The red arrow points to a man who was swept away seconds later.

Passage of a tsunami as seen in a sea level record from Hilo, Hawaii. The observed sea level shows high frequency variations with a period of approximately 20 minutes and an initial amplitude of nearly two meters (total tsunami wave height 3.7 m)

Earthquake originated in Anchorage, AK

tides w/out tsunami

This mathematical simulation (above) shows the tsunami created by the Cascadia Subduction Zone earthquake on January 26, 1700, as it reaches Hawaii on its way across the Pacific Ocean (5 hrs).

Free Waves, Forced Waves

• Free waves- a wave that is formed by a disturbing force such as a storm. Waves continue to move without additional wind energy

• Forced wave- a wave that is maintained by its disturbing force, e.g., tides

Restoring Force

Force necessary to restore the water surface to flatness after a wave has formed in it

• Capillary waves- wavelength < 1.73 cm

• Gravity waves- wavelength > 1.73 cm

Deep-water, Transitional, & Shallow-water waves

Wavelength- determines the size of the orbits of water molecules within a wave

Water depth- determines the shape of the orbits

Deep-water waves- more circular orbits

Water Depth 1/2 of wavelength

Transitional waves- intermediate-shaped orbits

1/20 wavelength depth ½ wavelength

Shallow-water waves- orbits are more flattened

Water Depth 1/20 of wavelength

Wind Waves- gravity waves formed by the transfer of wind energy into water

• Wave ht- usually <3m

• Wave length- 60-150m

Factors that affect wind wave development:

• Wind strength

• Wind duration

• Fetch- the uninterrupted distance the wind blows

http://www.newportsurf.com/tides.html

Interference and Rogue WavesInterference waves: when waves from different

storm systems overtake one another. They add (constructive interference) or subtract (destructive interference) from the other.

Constructive Destructive Mixed

Rogue waves:

• freak waves that come out of nowhere

• created by constructive interference

• formed by the interaction of a wind wave and a swift surface current

• common in southeastern tip of Africa

Types of Breaking Waves:• Plunging breaker• Spilling breaker• Surging breaker

Factors that determine the position and nature of the breaking wave:• Slope• Contour• Composition

Waves approaching shoreWaves approaching shore

a gradual sloping bottom generates a milder wave

• doesn't break, because it never reaches critical wave steepness

• breaker diminishes in size and looses momentum• Found on beach with a very steep or near vertical

slope

Surging BreakerSurging Breaker

Sunset Beach

What type wave are these?

Waikiki

Wave Refraction- when a wave approaches an inclined surface (shore) from an angle, the wave slows and bends, paralleling the shoreline, creating odd surf patterns

Wave Diffraction- Propagation of a wave around an obstacle

Wave Reflection- a progressive wave striking a vertical barrier and being reflected in the direction from where they came

The Wedge, Newport Harbor, Ca

waves

Rip Current

Internal Waves- at thermocline/pycnocline layer

• constant coral species turnover associated with mortality and recruitment• rarely thicker than a single coral colony

Wave exposed environment:

• Breakage• Scour• Abrasion

Mortality on wave exposed environment due to:

Depth- lack of coral accretion in shallow open ocean coastline due to wave energy

Absence of mature barrier reef in Hawaiian Islands

5 types of open ocean swells that cause disturbance to coral:

Destructive waves-causes high mortality on reef building corals:

1. North Pacific winter waves on north and western coastline

2. hurricane generated swells on south or southwest coastline (40 yr cycle)

Low moderate nondestructive waves- optimizes mixing and nutrient uptake or exchange, usually beneficial due to increased circulation and nutrients between water and organisms:

3. Tradewinds generated from northeast or east; ht. of 1-3 m, occurs 90% of summertime and 55-65% of wintertime

4. Long period southerly swell from southern ocean during the Austral winter; common between April and September (1-2 m in ht)

5. Kona storm generated waves (~4m); occasionally may be destructive and cause beach and shoreline erosion 

Reef Front in a Low Energy Environment

Reef Front in a High Energy Environment

Algal RidgeAlgal Ridge

Upper Reef Slope of a High Energy Environment

Upper Reef Slope

Upper Reef Slope of a Lower Energy Environment

Site Depth (m)

Coral cover

%

Coral Diversity (H’)

Algal Cover %

Bare Limeston

e %

Sand %

Dominant coral,

algae

Coral growth (mm/y)

Kaneohe Bay

1 2±5 0.16 5 1 95 P.c. Negligible

2-5 69±20

0.35 9 3 19 P.c.M.v

7.66

Hanauma Bay

1 <1 <0.01 90 10 0 P.o. Negligible

12 73±14

0.87 0 5 10 P.c.P.l.

8.13

Mamala Bay

1 6±3 0.15 90 5 5 P.m. Negligible

12 10±5 0.35 2 40 40 P.l 10.1

Sunset Beach

1 9±8 0.53 60 20 20 P.l. Negligible

12 15±13

0.68 20 65 65 P.m. 8.08

P.l.- Porites lobata; P.C.- Porites compressa; M.v.- Motipora verrucosa, P.m.- Pocillopora meandrina; P.o.- Porolithon oncodes (coralline algae)

Table 1. Community structure and growth of coral reef at sites selected for study. Attributes of community structure are based on one 50 m transect at each station. Annual coral growth rates are averages of 10 colonies.