Photo by W. W. Little

Shorelines

Wave Motion

Waves are formed as energy moves through water. Water particles travel in a circular pattern, more or less returning their point of origin once the wave has passed.

Breaking Waves

As waves approach shallow depths, they hit bottom and water begins to “pile up.” Due to friction, the upper part of the wave moves more rapidly than the lower part. Eventually, the waves become unstable and “break,” rushing onto the shore.

Types of Shorelines

A variety of shoreline types can develop depending upon the balance between and nature of tectonic and hydrologic factors. Where tectonic processes dominate, coastlines are characterized by erosion. Hydrologically-dominated shorelines exhibit depositional features.

Erosional Coasts

Depositional Coasts

Raised Terraces Drowned River Valleys Fjords

Deltas Beaches and Barrier Islands Reefs

Raised Terraces

In tectonically-active regions, coastlines often become uplifted, forming terraces. These shorelines are dominated by erosional features, such as wave-cut notches, cliffs, and platforms.

Wave-cut Notches

Waves impact the base of a cliff and undercut it by forming an erosional notch. As a notch deepens, the overlying cliff collapses and retreats. A flat surface, called a platform, grades seaward of the notch.

Wave-cut Platforms

Debris from collapsing cliffs is worked back and forth by waves, producing a flat surface that slopes gently seaward.

Drowned River Valleys

When sea level is low, valleys are cut into the continental shelf. If sea level subsequently rises, these valleys become drowned, leaving ridges protruding away from the coast. Ultimately, headlands are eroded and bays are filled, producing a relatively straight shoreline.

Wave Refraction

A wave approaching the shore contacts the seafloor first at headlands and last in bays. A lowering of velocity results in refraction of the wave toward the headland. Therefore, wave energy is concentrated at and erodes headlands and is dispersed within bays. Sediment eroded from the headlands is deposited within the bays.

Sea Stack

Sea Arch

Sea Cave

Sea Caves, Arches, & Stacks

Headland erosion results in a variety of landforms, including caves, arches, and stacks, as the headland retreats through erosion.

Straightening of Coastlines

Ultimately, as headlands are eroded and bays filled, the coastline becomes straightened.

Tombolos

The word tombolo means “bow tie” and refers to a narrow strip of sediment connecting an island to the mainland in a shadow zone protected from wave energy.

Fjords

Fjords are drowned glacial valleys.

Depositional Coasts

Sediment eroded from continents is deposited along the coast as deltas. This sediment is then reworked by coastal processes associated with waves, tides, and marine currents.

Deltas

Deltas are accumulations of sediment located where a stream enters the sea.

Photo by W. W. Little

Waves

Waves are formed by friction between air and water as wind blows across the sea surface.

Photo by W. W. Little

As waves break along the shoreline, they transport large quantities of sediment.

Photo by W. W. Little

Once wave energy has been expended, water is returned to the sea by gravity as sheet wash.

Long-shore Transport

Waves typically approach the shore at an angle and, after breaking, the surf returns to the sea perpendicular to the shoreline. As a result, sediment is transported along the coast in a zig-zag fashion.

Beach Ridges

Long-shore transport adds sediment to beach areas in the form of narrow, flat ridges that build seaward.

Rip Currents

Water that is piled up along the coast by waves must find its way back to sea. This is done by rip currents, which are small “rivers” of water that cut back through the surf and waves.

Barrier Islands

Barrier islands are beaches that are detached from the mainland. Like actual beaches, barrier islands are formed by the long-shore transport of sediment. They are referred to as barrier islands because they provide a barrier between the mainland and the open sea.

Spits

Spits are a type of barrier island that extends away from the mainland and partially across the mouth of an embayment.

Barrier islands are another way in which irregular shorelines are straightened by marine processes.

Baymouth Bars

Barrier islands that completely extend across the mouth of a bay are known as baymouth bars.

Lagoons

The protected area behind a barrier island is a lagoon. Because of low energy conditions, lagoons eventually fill with mud.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Tidal Inlets & Deltas

Long barrier islands are often breached during storms, allowing tides to move in and out of the lagoon. This can lead to the development of tidal deltas on one or both sides of the barrier.

Tides

Tides are caused by the gravitational attraction of the Moon as it passes overhead and “underfoot.” Therefore, tides rise and fall twice a day.

Tidal Flats

Tidal flats are areas along the coast that are covered by water during high tide and exposed to the atmosphere at low tide.

Photo by W. W. Little

Tidal Channels

A complex network of channels is often produced to facilitate the movement of water across a tidal flat, particularly where small streams cross the flat.

Reefs

Reefs are formed by organic processes and require warm, shallow, clear water. These conditions are best met along the margins of continents and islands in a narrow band north and south of the equator.

AtollsAn atoll is a circular reef produced as the island around which it formed slowly sinks into the sea.

Photo by W. W. Little

Photo by W. W. Little

Bio-erosion

In addition to waves, limestone coasts can be eroded by organisms, such as snails, that digest the rock.

Photo by W. W. Little

Modern Reef Deposits

Photo by C. Shuman

Photo by W. W. Little

Reef deposits are often composed of broken framework organisms.

Ancient Reef Deposits

Photo by W. W. Little

Coastal Hazards

Coastal areas are beautiful, but they are also geologically dynamic, leading to many hazards with respect to human activity.

Wave Barriers

Wave energy can be absorbed by barricades placed along the shoreline. These prevent further erosion but need frequent maintenance or repair and deprive areas further down the coast of sediment required to maintain that part of the shoreline.

Jetties

Jetties are designed to keep an inlet open and to prevent it from migrating by blocking sediment transport on the up-coast side and preventing erosion on the down-coast side.

Before 1992 After 1999

Groins

Groins are barriers constructed perpendicular to the shoreline and are designed to block sediment transport. This prevents local beach erosion and leads to widening of the coast.

Breakwaters

Breakwaters are used to create artificial bays. They have two components, one blocks sediment transport along the shore and the other protects the bay from wave energy.