Dynamic Earth Lecture Notes:

Coastal Processes

  • Wave Dynamics

  • Near shore currents

  • Types of coastlines

  • Features at coastlines

  • Engineering to reduce coastal erosion

Shorelines

Shorelines are forever changing in response tides, nearshore currents, sea level changes and the supply of sediment from rivers.

The end result is that existing shorelines will be modified overtime.

Q: Why are these areas so important?

 A. 2/3 of the world’s populations live in narrow bands along coastal shorelines.

Wave Generation

Waves can be generated by several processes: landslides, sea floor faulting, volcanic eruptions. Most commonly waves are produced in response to wind (or wind’s frictional drag).

Wave size is dependent on wind speeds, duration and the distance the wind blows over a continuous water surface or fetch. Lakes and rivers have less surface are so less fetch and smaller waves than the oceans

A particle of water moves in a circular orbit. As water depth increases, the circular orbit size decreases until the surface wave no effects the sea bottom. This level is referred to as wave base. It can be expressed as a distance = ˝ the wave length.

 

Breakers in the Surf Zone

When waves travel into shallow water, they become oversteepened, the wave crest advances faster than the wave form itself until it eventually plunges forward forming as breaker.

Nearshore currents

Most waves impact at an angle to the shoreline. Waves then swash up the beach and back down producing a longshore current parallel to the shoreline. This longshore current transports sediment parallel the shoreline.

In this picture, which direction is the longshore current moving beach sands?

Ask me for the correct answer

Wave Energy & Erosion

Waves moving towards a shore become bent or refracted toward to the coast. This is wave refraction. This helps to concentrate the impact of wave energy parallel to the shore.

Structures such as jetties, groins or piers along the shore can slow the overall transport of sand.
But locally, we see erosion of sand on the down current side of the structure

Coasts Storm & Erosion

Wave energy during storms can have dramatic effects along coastlines. Strong winds, high waves (storm surge) can easily wash away large amounts of sand, ultimately reconstructing the coastline.

Shoreline Deposition

Sediment is transported by rivers to coastal areas where it becomes reworked by waves and longshore currents. If the supply sediment is sufficient, such a coastline will have extensive depositional landforms: beaches, spits, barrier island, or bay mouth bars.

Types of coasts: Depositional Coasts and Uplifted Coasts

Depositional or Submergent coasts
Example is the East Coast of the U.S.
Dominated by deposition of sediments
Uplifted or Emergent coasts
West Coast of U.S.
Dominated by uplift & erosion

Depositional Shoreline features:

  • beach

  • spit

  • baymouth bar

  • barrier island

 

Spit forming north of the Lake Erie shoreline.  Erie, PA.

Bay Mouth Bar, East Coast, USA

 

Coastal Erosion

Erosion along all coasts is produced by the abrasive action of sediment-laden waves.

The type of shoreline material (solid bedrock versus unconsolidated sediment) will also affect the amount of erosion along a shoreline.

Erosion is enhanced if sediment supply is reduced by damming rivers or diversion of longshore drift

Shoreline Erosional Features

Storms, rising lake or sea level & wave refraction tend to concentrate wave energy on irregular coastlines. Features produce are:

  • uplifted (wave-cut) terrace

  • Headlands

  • Sea caves

  • Sea arches

  • Sea stacks

--> Click this link to see examples of these features.

Lake Erie Wave Erosion

Changing Sea Level & Erosion

Global mean rise in sea level (SLR)(1-2 mm/yr) (average for last 100 yrs from tidal gauge data). Melting ice caps and global warming and can increase sea level by as much as 1 m by 2100. Remember some regions are more susceptible if low coastal land surface slope or if land surface is subsiding (sinking).

Engineering to Reduce Erosion

Drain pipes reduce surface water saturation on beachcliffs.

Sea walls are used to reduce wave action are placed parallel to shoreline but, can increase erosion of beaches with less sand.

Breakwaters, jetties and groins. Try to reduce wave action. Jetties usually placed next to a river mouth or lagoon to prevent infilling of the channel.