OCEANOGRAPHY II

Waves

Characteristics (Figure)


Velocity in Deep water = 1.56T (m/sec) and wavelength = 1.56T2 (m). In deep water only the wave form moves (Figure) while in shallow water both the wave form and the water moves. In shallow water the waves will touch bottom when the water depth = L. Friction slows the wave causing H to increase and L to decrease. Also, front of wave is slowed more than rear, thus it breaks. Waves break when H = 1/7 L or about a depth = 1 H. Since waves rarely exceed 6 meters in height, this limits wave erosion to depths of less than 10 meters. There are two types of breaking waves: (Figure)



Swells - large, long period waves formed by the recombination of two or more waves during storms. Can travel thousands of miles. Not unusual for swells that hit California beaches to be generated near Australia.

Average wave height = 2 meters; 10-15% exceed 15 meters; largest ever witnessed 35 meters in height.

Waves are caused by wind. Wave height is dependent on:

The shoreline is often divided into distinct zones by the nature of the wave erosional forces present:

  1. Offshore - that portion of the shoreline beyond the breaker zone where water depths exceed 6 meters.
  2. Inshore - includes the breaker zone and surf zone. In the former, waves crest and break. The surf zone is characterized by foam and turbulence from the breaking waves.
  3. Foreshore - the swash zone where breaking waves surge up onto the beach. Top of the swash zone is marked by a berm or ridge of sand created by wave erosion and surge.
  4. Backshore - that portion of the beach not effected by present day wave activity.

Refraction and Longshore Currents

Refraction - change in a waves direction as it approaches a coastline. Due to drag on waves approaching a coast obliquely. Effect is to cause waves to approach nearly parallel to the shore (Figure).


Although refraction bends the waves until they are nearly parallel to the coast, they nonetheless still approach at a slight angle. This results in a longshore current. The current is caused by the slight oblique angle of the wave, but runoff that is perpendicular to the slope of the beach. This causes grains of sand to be moved along the shore (Figure).


Currents

Currents are driven by the wind. The general effect of currents is to move warm water toward the poles and cold water toward the equator. A major factor in the movement of ocean water is the Coriolis effect. Can use the analogy of a slow moving artillery shell. The shell travels a straight path but the Earth rotates beneath it (Coriolis effect) causing theshell to be deflected relative to the observer. The same process effects currents.

Gyres - Closed loops formed by currents. Major gyres are centered about 30N and S of the equator. Continents have a major effect. At 60 S, where no continents are present currents circle the Earth. (Figure)


Typical major gyre consists of:

Geostrophic Currents

Wind movement and the Coriolis effect combine to cause currents to move at 45 to the actual wind direction. The surface layer drags the next layer of seawater below it which is deflected even farther than the surface layer. This continues downward with depth all the way to the ocean bottom. This is called the Eckman Spiral (Figure).


However, at an average wind speed of 30 mph at a depth of 300 feet the water is moving slowly in the opposite direction from that at the surface. This is considered to be the bottom limit of wind driven currents. The net result of this entire process is actually to cause the water to move at a right angle to the wind direction or toward the center of a gyre. This causes hills up to 6 feet high in the center of a gyre.

The mounded water flows downward and outward from the "hill" under the influence of gravity, but the Coriolis effect deflects it to the right continuously until it is flowing parallel to the hill at which point gravity and the Coriolis effect are balanced. This is termed a geostrophic current (Figure).


Vertical Currents

Because prevailing winds along most coastlines blow more or less parallel to the shoreline (reason to be discussed later in the course) they push the water either toward the shore or offshore causing sinking currents or upwelling currents (Figure). The later are especially important to commercial fishing since upwelling currents provide nutrients for marine life.


Deep Ocean Currents

As noted previously the ocean is layered with differences in salinity and temperature. Since dense, cold water sinks and warm water rises there is a net effect of cold Polar water sinking and moving both northward and southward toward the equator. The cold current flowing along the ocean floor displaces the warmer water upward.

Longshore Current

As waves approach a coastline they undergo a change in direction. They touch bottom and the dragr causes the waves to approach nearly parallel to the shore (refraction).

Although refraction bends the waves until they are nearly parallel to the coast, they nonetheless still approach at a slight angle. This results in a longshore current. The current is caused by the slight oblique angle of the wave, but runoff that is perpendicular to the slope of the beach. This causes grains of sand to be moved along the shore (Figure).


Coastal Features (slides)

Passive Coastline (East coast)

Active Coastline (West coast)