Chapter 14 The Dynamic Ocean. Agenda 1. Stabilizing the ShoreStabilizing the Shore 1. Stabilizing the ShoreStabilizing the Shore 2. Coastal ClassificationCoastal.

Chapter 14The Dynamic Ocean

AgendaAgenda

1. Stabilizing the Shore1. Stabilizing the Shore

2. Coastal Classification 2. Coastal Classification

3. Tides 3. Tides

Stabilizing the ShoreStabilizing the Shore

• Hard Stabilization Hard Stabilization

• Alternatives to Hard StabilizationAlternatives to Hard Stabilization

• Erosion Problems Along U.S. CoastErosion Problems Along U.S. Coast

Hard StabilizationHard Stabilization

Hard Stabilization is any form of artificial Hard Stabilization is any form of artificial structure built to protect a coast or to structure built to protect a coast or to prevent the movement of sand along a prevent the movement of sand along a beach. beach.

Examples:Examples:1. 1. GroinsGroins2. 2. JettiesJetties3. 3. BreakwatersBreakwaters4. 4. SeawallsSeawalls

GroinsGroins are structures away from a harbor entrance built straight out from the beach. Both jetties and groins serve to stop the flow of the are structures away from a harbor entrance built straight out from the beach. Both jetties and groins serve to stop the flow of the long-shore current. On the up-current side of the groin sand is dropped and the beach builds out. On the down current side the current long-shore current. On the up-current side of the groin sand is dropped and the beach builds out. On the down current side the current is renewed, but now is not carrying any sand, so it erodes the beach on the down-current side. Property owners down-current from is renewed, but now is not carrying any sand, so it erodes the beach on the down-current side. Property owners down-current from such a structure commonly build another groin to protect their beach, and soon a whole set of them, called a groin field, is the result. such a structure commonly build another groin to protect their beach, and soon a whole set of them, called a groin field, is the result.

This figure is from Dean, C, 1999, Against the Tide: the battle for America's This figure is from Dean, C, 1999, Against the Tide: the battle for America's beaches, Columbia Univ Press, NY, NYbeaches, Columbia Univ Press, NY, NY

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Seawalls are continuous barricades built at the shore line in an attempt to prevent any erosion along the whole area. Instead, the wave energy erodes the sand stored in front of the sea wall. This will eventually undermine the seawall and cause it to collapse and fail.

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RelocationRelocation

Relocation deals with relocation of “storm-damaged” or “at-risk” buildings and letting nature reclaim the beach.

- This is a very controversial proposal. People/businesses/corporations spend billions each year trying to attain a view of any coastal area. The problem with this is that many times, they are much too close for comfort, and nature can not exploit itself in all its naturalistic ways. Nature knows no other boundary, and we end up with billions and billions of dollars in damage, not to mention many times, death!

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Alternatives to Hard Alternatives to Hard StabilizationStabilization

Armoring the coast with hard stabilization has several potential drawbacks, including the cost of the structure and the loss of the sand on the beach.

Alternatives to hard stabilization include:• Beach Nourishment• Relocation

This here is This here is a picture of a picture of Miami Miami Beach.Beach.

Many Many scientists scientists want most want most of these of these huge huge buildings to buildings to relocate, so relocate, so nature can nature can take its toll take its toll and regain and regain one of its one of its beaches. beaches.

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Beach NourishmentBeach Nourishment

Beach Nourishment is the process by which large quantities of sand are added to the beach systems to offset losses caused by wave erosion.

* DOWNFALLS *- Beach nourishment, however, is not a permanent solution to the

problem of shrinking beaches because the same process that removed the sand in the first place will eventually remove the replacement sand as well.

- It can also be very expensive too… imagine having to transport huge amounts of sand to a beach from off shore areas nearby.

- Unwanted environmental effects (Hawaii’s sand for instance)

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On East Coast barrier islands, ocean tidal inlets migrate naturally with the longshore current. A On East Coast barrier islands, ocean tidal inlets migrate naturally with the longshore current. A jetty system will permanently disrupt the equilibrium of the beach. This may seriously affect the jetty system will permanently disrupt the equilibrium of the beach. This may seriously affect the tidal circulation and the health of the wetlands between the barrier islands and the mainland. tidal circulation and the health of the wetlands between the barrier islands and the mainland.

Inlets with short Inlets with short jettiesjetties that don't quite reach the surf will clog up with sand. The sand must be that don't quite reach the surf will clog up with sand. The sand must be dredged on a regular basis. A "sand by-passing" system may be built to pump sand around the dredged on a regular basis. A "sand by-passing" system may be built to pump sand around the jetties. The sand pumping may come from within the inlet or from the updrift beach. These jetties. The sand pumping may come from within the inlet or from the updrift beach. These methods are expensive and must be maintained indefinitely. methods are expensive and must be maintained indefinitely.

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Erosion Problems Along U.S. Coast

Atlantic and Gulf CoastsMuch of the coastal development along the Atlantic and Gulf coasts has occurred on barrier islands. Barrier islands consist of wide beaches that are backed by dunes and separated from mainland’s by marshy lagoons. When a storm occurs, the barriers absorb the energy of the waves primarily through the movement of sand.

Pacific CoastThe Pacific Coast is characterized by relatively narrow beaches that are backed by steep cliffs and mountain ranges. A major problem facing the pacific shorelines is the significant narrowing of many of many beaches. This is caused by the man made damns built for irrigation and flood control.

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Picture of a Man Made DamPicture of a Man Made Dam

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What causes tides?What causes tides?Tides are daily changes in the elevation of the ocean

surface. Other than waves, they are the easiest ocean movements to observe.

Tides are created by the imbalance between two forces:1. Gravitational force of the Moon and Sun on Earth

• If mass increases (), then gravitational force increases ()

• If distance increases (), then gravitational force greatly decreases ()

2. Centripetal (center-seeking) force required to keep bodies in nearly circular orbits

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Coastal Classification

• To understand any coastal area, many factors must be considered, including rock types, size and direction waves, frequency of storms, tidal range, and offshore topography.

• Also Tectonics that “uplift” or “downdrop” the land.

• All coastal areas were affected by the melting of the ice age glaciers.

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EMERGENT COASTS

There are 2 basic coastal classifications, and they are based upon changes that have occurred with respect to sea level.

Emergent Coasts: These develop either because an area experiences uplift or as a result of drop in sea levels.

Submergent Coasts: These are created when sea levels rise, or the land adjacent to the sea subsides

Emergent Coast

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Submergent Coast

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The monthly tidal cycle(29½ days)

About every 7 days, Earth alternates between:– Spring tide

• Alignment of Earth-Moon-Sun system (syzygy)

• Lunar and solar bulges constructively interfere

• Large tidal range

– Neap tide• Earth-Moon-Sun system at right angles (quadrature)

• Lunar and solar bulges destructively interfere

• Small tidal range

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The lunar day

• Tidal bulges follow Moon as it rotates around Earth• Lunar day is 50 minutes longer than a solar day because the

Moon is moving in its orbit around Earth

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Relative sizes and distances on Earth, Moon, and Sun

• The Sun is much more massive than the Moon but much further away

• Solar bulges are 46% the size of lunar bulges

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Earth-Moon-Sun positions and the monthly tidal cycle

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Tidal patterns

• Diurnal– One high and one low tide each (lunar) day

• Semidiurnal– Two high and two low tides of about the same

height daily

• Mixed– Characteristics of both diurnal and semidiurnal

with successive high and/or low tides having significantly different heights

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Tidal bore = a true tidal wave

• Wall of water that moves upriver

• Caused by an incoming high tide

• Occurs in some low-lying rivers

• Can be large enough to surf or raft

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Summary of tides on Earth

• Most locations have two high tides and two low tides per lunar day

• Neither the two high tides nor the two low tides are of the same height because of the declination of the Moon and the Sun

• Yearly and monthly cycles of tidal range are related to the changing distances of the Moon and Sun from Earth

• Each week, there would be alternating spring and neap tides

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Gravitational forces on Gravitational forces on Earth due to the MoonEarth due to the Moon• Force decreases Force decreases

with increasing with increasing distancedistance

• Force is Force is directed toward directed toward the Moon’s the Moon’s center of masscenter of mass

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Tidal CurrentsTidal Currents

The term The term Tidal CurrentTidal Current is used to describe the is used to describe the horizontalhorizontal flow of water accompanying the flow of water accompanying the rise and fall of the tides.rise and fall of the tides.

Tidal Flats: Tidal Flats:

This is the area affected by the advancing and This is the area affected by the advancing and retreating of tidal currentsretreating of tidal currents

Tidal Deltas:Tidal Deltas:

An accumulation of sediment on the An accumulation of sediment on the shoreward or ocean-ward side of an shoreward or ocean-ward side of an inlet inlet

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Types of Tidal DeltasTypes of Tidal Deltas

Ebb tidal deltas:

An ebb tidal delta is an accumulation of sand on the seaward side of an inlet formed by the ebb tidal current.

Weak tidal currents relative to wave activity inhibits the formation of ebb tidal deltas along microtidal coasts. Macrotidal coasts typically lack shore-parallel barriers, hence inlets and their deltas.

 Ebb tidal deltas most commonly occur in front of mesotidal (or mixed energy) inlets where they interact with waves and longshore drift. Barriers are often drumstick-shaped with down drift offsets.

The length (degree of protrusion) of an ebb tidal delta reflects the relative strengths of tidal flow and longshore current. The asymmetry, or lean, of the delta is determined by the direction longshore transport.

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Types of Tidal Deltas Types of Tidal Deltas (Cont.)(Cont.)

Flood tidal delta:

A flood tidal delta is an accumulation of sand on the shoreward sided of an inlet. These deltas are initially formed during storm surges and maintained by flood currents.

Flood tidal deltas become stabilized when salt marshes establish on them.

Salt-marsh grasses cause the land to be built up to high-tide level and new land is added to the island

In mesotidal systems, flood tidal deltas transform into marshland causing the inlet to migrate to a new position. New tidal deltas are created along the entire area of inlet migration

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Flood and ebb tidal deltas are deposited on the shoreward and oceanward side of an inlet (mesotidal/mixed energy). Channels occupied by the ebb and flood tidal flows are shown by light and dark blue arrows respectively. The flood ramp throat and main ebb channel form the inlet throat, the deepest portion of the channel. Sand moved into the inlet by waves and flood currents are either deposited on the flood tidal delta or circulated back out to the ebb tidal delta. Waves reorganize sediment in a series of swash bars which eventually migrate and attach to the ends of the barrier. Typically the down drift end receives the most sediment. NextNext

Types of Tidal CurrentsTypes of Tidal Currents

Significance/ImportanceSignificance/Importance Inlets offer access to sheltered bays and lagoons. Inlets offer access to sheltered bays and lagoons. Inlets enable the exchange of water and nutrients between Inlets enable the exchange of water and nutrients between

the ocean and back barrier region required for finfish and the ocean and back barrier region required for finfish and shellfish reproduction. shellfish reproduction.

Tidal deltas are major sediment sinks, capturing a large Tidal deltas are major sediment sinks, capturing a large amount of sediment in the littoral system. amount of sediment in the littoral system.

Ebb tidal deltas refract ocean waves, thereby altering the Ebb tidal deltas refract ocean waves, thereby altering the local littoral dynamics. local littoral dynamics.

Flood tidal deltas aid in widening the back barrier region Flood tidal deltas aid in widening the back barrier region allowing the barrier system to migrate landward. allowing the barrier system to migrate landward.

Stabilization of an inlet require an understanding the Stabilization of an inlet require an understanding the circulation of sediment in and around an inlet as well as circulation of sediment in and around an inlet as well as the relationship between inlet throat dimensions and tidal the relationship between inlet throat dimensions and tidal prism and back barrier area.prism and back barrier area.

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THE DYNAMIC OCEAN

THE END

Centripetal forces on Earth Centripetal forces on Earth due to the Moondue to the Moon• Force is the Force is the

same same everywhere on everywhere on EarthEarth

• Force is directed Force is directed perpendicular to perpendicular to Earth’s center Earth’s center everywhere on everywhere on EarthEarth

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Resultant forcesResultant forces

• Resultant forces are:Resultant forces are:– The difference The difference

between between gravitational (G) and gravitational (G) and centripetal (C) forcescentripetal (C) forces

– Directed away from Directed away from Moon on the side of Moon on the side of Earth opposite MoonEarth opposite Moon

– Directed toward Directed toward Moon on the side of Moon on the side of Earth facing MoonEarth facing Moon

Figure 9-4

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