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Life's A Beach, Then You Erode

A cache by Three Bottles Send Message to Owner Message this owner
Hidden : 11/20/2008
Difficulty:
1 out of 5
Terrain:
1.5 out of 5

Size: Size:   other (other)

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Geocache Description:

This Earthcache takes you to Long Beach, WA. It is accessible only during daylight hours and is accessible year-round.

TO CLAIM THIS EARTHCACHE:

ÜPart One: Complete the field work listed below and email your answers to Three Bottles

ÜPart Two: Email (please do not post) the answers to the questions listed below.  The answers can be found by reading the text on this Earthcache's page.


FIELD WORK

  1. From where you are standing estimate how many miles the beach extends north and south of you.
  2. Describe the weather (the wind speed, summer/winter, on the day you visited this Earthcache site.  How do you feel the weather was impacting the accretion or erosion of the beach on the day of your visit?
  3. Please take 2 photos of yourself with your GPS at both the parking area with the beach in the background and on the beach. Please post them on this cache page.


QUESTIONS

  1. Describe how sand is carried along Washington's southwest coast.
  2. Washington's southwest coast erosion is affected by six different factors.  What are these factors?
  3. Which one do you feel makes the largest impact on the erosion of Long Beach?

 

columbia6Along Washington's southwest coast, wide sand beaches stretch from the mouth of the Columbia River to Point Grenville.
 

Columbia River sand built beaches and barriers
For the past four to five thousand years, the Columbia River has carried sand to the coast. Currents, waves and wind have swept sand north and south -- from the Columbia River mouth to build up dunes and beaches. Millions of cubic yards of Columbia River sand are stored in these linear dunes and beaches, also called barriers. The Columbia River supplies sand to 100 miles of beaches, from Oregon's Tillamook Head to Washington's Point Grenville. As sand moves north up Washington's coast, the amount available to nourish beaches decreases. At Point Grenville, the trailing end of Columbia River sand is deposited. North of Point Grenville, the contribution of Columbia River sand to beaches is small.
 

Sand for miles
The Long Beach peninsula, a huge spit, was formed by sediments delivered to sea by the Columbia River. Waves and currents slowly reworked the sediments to build the 19 mile long peninsula which is only about a mile wide. Broad sand beaches front an upland sand plain, laced with parallel dune ridges, bogs, lakes, and woodlands.

 

HOW BEACHES ARE BUILT
 
map_drift4

Littoral Drift

Waves come ashore diagonally, as their backwash flows perpendicular to the beach. Sediment particles travel in a zigzag path as they are moved along the beach. This motion creates a current that runs parallel to the beach. Storms can generate high waves that approach the shore at a larger angle, causing a faster current.

 
How sand is carried along Washington's southwest coast

Summer: During the summer, winds come from the northwest; sand moves south.

Winter: During the winter, winds come from the southwest; sand moves north. Because winter winds blow harder, the dominant drift direction is north.
 
For several thousand years, floods washed sand to the mouth of the Columbia River, where it collected in shoals. Longshore currents shuttle sand from these shoals, carry it along shore, and deliver it to beaches. Within the last century, the movement of sand from the Columbia River has been altered by dams, jetties, and other development.

 

HOW THE COAST WORKS
Erosion along Washington's southwest coast is affected by: jetties, dams, sediment supply, geologic history, wave action, and weather.

Jetties caused beaches to grow and possibly erode
Jetties have influenced accretion and possibly erosion patterns on the beaches over distances of 12 miles (20 kilometers) or more.

jetty_diag5

 

Erosion and accretion near jetties
 
Where there is net littoral drift, jetties can block the flow of sand, causing erosion and accretion.

 

  • In the early 1900s, jetties were built at the entrances to the Columbia River and Grays Harbor. These jetties were designed to scour out sandbars and keep navigation channels open. Beaches, inlet entrances, and the nearby sea floor are still changing as a result of jetty construction over a century ago.
     
  • River deltas scoured
    The jetties narrowed inlets and increased tidal currents. These currents flushed sand out of tidal deltas.
     
  • Beach growth and erosion
    Over decades, sand from the scoured deltas accumulated, causing beaches near the jetties to grow. Campgrounds and condominiums were built on this newly accreted land. The delta sand is now gone and beaches near the jetties are experiencing erosion.
     
    ero_can_97_1 Erosion damage to park beach access road at Fort Canby, following winter storms, 1997.

 

Dams on the Columbia River have reduced the sand supply
Dams on the Columbia River have reduced the sand supply to coastal beaches by two thirds.

  • Before dams were constructed, sand moved freely
    Before dams were built on the Columbia River, floods carried sand to the delta, where it gathered in shoals. Longshore currents picked up the sand from the shoals, moved it along shore, and deposited it on beaches. The Columbia River carried about 12 million cubic yards of sediment to the coast per year.
     
  • Dams have restricted the flow of sand by two-thirds
    Beginning in the 1930's, dams were constructed over most of the U.S. part of the Columbia River. Dams restricted sediment supply from 90 percent of the watershed, a quarter of a million square miles. As a result, the Columbia River delta has been starved of critical beach building sand.
     
  • Delayed effects
    Peacock Spit, a huge shoal north of the mouth of the Columbia, supplied sand to coastal beaches for decades. Peacock Spit is now gone. The spit may have been scoured away by jetty-influenced currents. With sand in short supply, Washington's southwest coast may be entering a long-term period of erosion.
     
    Dams have reduced the sand supply
    to coastal beaches

     
    Over 219 dams in the Columbia River basin have reduced the amount of sand and sediment reaching beaches on the coast.

     

El Niño impacts the shoreline
El Niño, a recurring atmospheric phenomenon, can bring higher sea levels, intense storms, and extreme high waves from the southwest.

The El Niño winter of 1997/98, the strongest El Niño of the century, brought frequent storms and high waves. On January 17th, 1997, the Grays Harbor wave gauge reported a wave as tall as a four story building, 48 foot high (14.52 meters). A storm system or squall line surge produced this extreme wave. This storm caused massive overwash and erosion at Ft. Canby State Park.
 

wave_base7
 

Earthquakes hit Washington's coast
Large earthquakes in the past caused the coast to sink 3 to 6 feet suddenly (1 to 2 meters).

plates2



s_sand3 by you.

FUN FACTS:

What's in the sand?
Beach sand from the Columbia River is made of light minerals, lithic fragments, and heavy minerals such as: hornblende, augite, hypersthene, magnetite. Some minerals come from mountains many miles away -- as far as the Rockies. These mountain minerals brought to sea nourish beaches and intertidal life such as razor clams.

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