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►Coastal Classification

►Coastal Dynamics

►Biological Processes and Human Activity

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Two Classification Systems

Without coastal dynamics, the processes that create and shape the oceans’ coastlines, history would be significantly different.

This first classification system is based on geologyand processes that take place over very longperiods of time. Active Coasts: These are close to plate collisions

that result in volcanic activity and earthquakes (the Ring of Fire).

Passive Coasts: These lie far away from activeplate boundaries with little volcanic activity andfew earthquakes.

Oceanographers believe that many other coastalprocesses occur. This second classification systemis based on short-term coastal dynamics. Primary Coasts: Formed by geologic processes not

directly related to the ocean over an extended period of time. Secondary Coasts: Formed by marine action, process takes much less time. A combination coast is one that can be both primary and secondary.

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Primary Coasts

Scientists attribute primary coast development to nonmarine forces. These include land-based erosion (from running water, wind or land ice), sedimentation, volcanic activity and tectonic activity.

Land-based Erosion Coasts – include fjord coastsand drowned river valleys. Scientists believe theseoccurred from erosion cutting into the land duringperiods of low sea level, then the sea level rises,flooding the eroded area. Both fjord coasts anddrowned river valleys can form estuaries – salt wedge estuaries, vertically mixed estuaries,slightly stratified estuaries and highlystratified estuaries.

1. Fjord Coasts – formed by glaciers moving toward thecoastline. The glaciers cut large, deep grooves in the land,which then flooded when the sea level rose (Southern Alaska).

2. Drowned River Valleys – formed by rivers cutting avalley through, which then flooded when the sea levelrose (Chesapeake Bay).

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Primary Coasts (continued)

Sedimentation Coasts – form when materials carried by rivers flow into the ocean, deposit and accumulate. Often sediments accumulated form a wide triangular shape – a delta (Nile deltain Egypt).

Volcanic Coasts – formed by volcanic activity (Hawaiian Islands).

Tectonic Activity Coasts – formed by tectonic activity. Primarily includes fault coasts which form as plates collide. Results when the collision uplifts the seafloor above the water surface, allowing the sea to flood the new area (Tomales Bay, California).

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Secondary Coasts

Secondary coasts result from marine processes that include wave erosion, material deposited by seawater motion, and marine life.

Wave-erosion Coasts – constant pounding by waves erodes and changes a coastline, wearing away land protrusions. Can result in spectacular formations like sea caves, arches and sea stacks.

Marine-deposition Coasts – form when sea action causes ocean sediment to accumulate in one place. Involves ocean sediments moved by water motion in the sea. Barrier islands, beaches, salt marshes, and mud flats are all types of deposition coasts.

Marine Organism Coasts – marine organisms build coasts by providing a structure that reduces the effects of waves and currents. Best known of these is the Great Barrier Reef of Australia built by coral.C

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Longshore Drift

One of the most significant forces shaping the coast is longshore drift. The tendency for materials to move along the coastline due to a longshore current.

Waves arrive on shore at somewhat of an angle, but the water recedes at nearly a 90º angle. Sand and sediment flow back with the water at this angle. The net motion of this backwash combines with the net motion imparted by the waves to cause a longshore current. Longshore drift occurs as the current moves material down the coast.

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Beach Dynamics

Sand comes from erosion. It may be from the effect of waves pounding the shoreline or inland erosion.

In tropical regions sand also comes from biological sources, like the erosion of coral reefs.

A beach is composed of three sections: 1. Foreshore – region from the high- to

low-tide mark. 2. Backshore – region rarely touched by

seawater.Includes dunes and grasses.

3. Offshore – beyond the low-tide terrace.

Factors that shape a beach: They are grainsize of beach sediments, wave energy andthe degree of beach slope.

Coastal Cell: a local region of materialtransport mechanisms that, when combined,form an area with no net sand gain or loss.C

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Large Scale Sand Features

Spits form when a longshore current turns a beach corner into the relatively calm water of a bay. The current slows and can’t carry as much sediment. Sand settles out of the water, forming the spit.

Tombolos are spits that extend between two islands or from an island to the mainland. Tombolos form when the longshore current slows. In this case, current slows around two sides of land, accumulating sand on both sides until the two spits grow together.

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Large Scale Sand Features (continued)

Barrier Islands: there are currently two theories on how they form. 1. Sediment accumulates offshore, eventually building up into an island. 2. They were giant sand dunes that became islands with the last major sea-level rise. Typical barrier islands share five features:

1. Ocean beach 2. Ocean dune 3. Barrier flat 4. Salt marsh 5. Lagoon

Deltas: there are three types. 1. River-dominated deltas have strong rivers and mild wave and tidal action

(Mississippi River delta). 2. Tide-dominated deltas occur in areas with strong tidal changes (Essex River

in Massachusetts). 3. Wave-dominated deltas have significant wave energy that redistributes river

sediments (Senegal delta in west Africa).Co

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Coral Reefs

Coral is perhaps the most significant of all biologicalprocesses that affect the coast. Coral reefs can bemassive, but only the outside layer, the coral polyps,is alive. Individual polyps create a calcium carbonate external

skeleton as they grow. It is this part of its structurethat creates coral reefs.

Fringing reefs lie along an island or mainland coast.They have a fore reef – the outer ocean side with mostbiological activity. A reef crest is the top of the reef thattakes most of the wave energy. A back reef is on theland side and has less biological activity.

Barrier reefs have a similar structure as fringing reefs,but lie further from shore. A barrier reef has a lagoonbetween it and the main coast.

Atolls are ring-shaped coral reefs that encircle a shallow lagoon.

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Plant Communities

In areas where the waves have generally low energy, plant communities can dominate the coast. Seagrasses for example live entirely underwater, but most marine plants live partly out of the water.

Among the most important of the plant-dominated shorelines are the mangrove swamps. Mangroves affect the coast directly by holding sediment in place and absorbing wave energy.

Human Activities There are two primary motivations for humans to modify the coastline:

1. To create new coastal structures. 2. To protect building and structures already on the coast from natural

coastal changes.

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Human Activities (continued)

Human-built coastal structures include: Groins are artificial protrusions jutting out perpendicular to

the shore. These may be built to create an area relatively protected from longshore current, or for recreation.

Jetties are the same as a groin, except they are built to reinforce a harbor entrance.

Breakwaters run parallel to shore or start on shore and curve into the sea. They are used to create an artificial lagoon for use as a harbor or beach.

Seawalls stand either at the water along the shoreor at the top of a beach. They act as a barrier toblock the waves from eroding the land.

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Human Activities (continued)

All of these structures can create problems: Jetties and groins block longshore drift. This tends to cause sand to accumulate on

the upside drift and to become depleted on the downside drift. Spits may form at the top of jetties.

Seawalls effectively absorb energy but create problems at their ends. The unprotected land next to the seawall tends to suffer from increased erosion.

Beach renourishment is the process of bringing in sand or sediment from somewhere else to replenish eroded sand caused by human projects. It is expensive and is only a temporary fix as the sand beach will again erode. Taking

sand from somewhere else effects that environment also and may damage biological communities.

The likely solution to problems created by human structures is a change in coastal attitudes and management. In the long run it is more effective and less costly to work with nature instead of against it.

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