Sediment Transport of an EstuaryEstuarine Ecology

Sediment Transport of an Estuary
Estuarine Ecology
WFSC 611
Complied by Loretta Solliday
Fall 97

Sources of Sediment within an Estuary

Freshwater sources:

Freshwater sources of sediment within an estuary (what is an estuary and why are they important) http://www.gem.co.za/enviro/43.htm include the rivers of the world, in fact the rivers of the world discharge 35 trillion tons of water to the oceans every year. (Hunt). The continents are being slowly eaten away by watersheds. (Find out more about watersheds) http://darkwing.uoregon.edu/~gries/watershd.htm The amounts of material carried dissolved in streams, or swept along as suspended sand or mud, down sound impressive when compared to the volume of water. The average river carries only about 140 parts per million dissolved material and perhaps 500 ppm solid particles. Each gallon of water carries only one-fiftieth of an ounce of dissolved matter and four-fiftieths of an ounce of sand and mud, but these add up to around twenty-three billion tons of continental material carried to the seas each year-more than four hundred tons of each square mile of land surface.

Delta Formation (figure 1)

(Hunt)

Delta Sedimentary Structures (figure 2)

(Hunt)

At the mouth of the Mississippi River, lies a huge delta with sediment transported by the river. From the barrier inland along the coast, and reaching 10 to 60 miles inland, lies 6.5 million acre wetland complex estimated to account for 40 percent of the marsh ecosystem of the United States. (Finlayson) In the name of flood control dikes have been built along the banks of the Mississippi, and out on the delta to stabilize the shifting river mouth. In order to maintain constant open shipping channels sediments are dredged out. The river sediments that once settled on the plains are now carried far out into the Gulf of Mexico where they drop into deep water and no longer settle out on the delta wetland communities. Natural compression and organic decay are causing old sediments to subside, and sea levels to rise. Unfortunately, this is causing a loss of as much as 39 sq. miles of wetlands each year. (Finlayson)

NASA JSC Digital Image Collection (Texas Gulf Coast)
http://ston.jsc.nasa.gov/cgi-bin/SFgate?database=ston.jsc.nasa.gov%3A210%2FP

Mississippi Delta
http://www.jpl.nas.gov/radar/sircxsar/delta.html

NASA JSC Digital Image Collection (Mississippi River Delta)
http://ston.jsc.nasa.gov/cgi-bin/SFgate?database=ston.jsc.nasa.gov%3A210%2FP

Ocean Sources:

Deltas only form where rivers bring more sediment into the sea than can be redistributed by marine current. Therefore, where marine currents are strong enough to move the sediment from the land, linear shorelines are formed with bars and beaches running parallel to the coast. Types of linear shorelines consist of two high-energy zones which alternate seawards with two low-energy zones. They are from the land, seaward in this order: Only the first three will be discussed for this project.

1) River deposit called (fluviatile coastal plain)

2) Lagoonal and tidal flat complex

3) Barrier island

Types of Coastal Barriers (figure 3)

(Hunt)

4) Offshore marine shelf.

The coastal plain deposits consist of sand, and mud left by the flowing water of a river emptying into the sea. Fine-grained flood-plain sediments will be the main type of sediments in the coastal plain areas. Along with the mud and sand there will also be bones, wood, and other plant debris and freshwater invertebrates. Lagoon deposits consist of generally fine-grained too, but depending on there size and depth , may deposit sediment ranging from sand to mud. Sedimentary structures of lagoons are similar to those of tidal flats with laminated mud¹s and rippled sand silt and mud. A lagoon is separated form the open sea by a barrier island. The main deposits of the barrier island consist of well-sorted sand with a fragmented from marine fauna.

Cross - Section of a Well Developed Barrier Island (Figure 4)

(Hunt)

(Morning Star)

Sediments found along the shoreline are in constant motion, and waves rearrange the shore sediments. Waves that approach the beach at a slight angle create a water current that flows parallel to the shore, called a longshore current. The longshore current carries loose sediment almost considered a river of sand. Longshore current are often interrupted by rip current.

(The City of Miami Beach Online Production)

A rip current is a narrow seaward stream. This type of current is caused by an excessive amount of water that has come inside a sand bar over which waves have broken shoreward. This accumulation of water becomes higher than sea level and flows seaward. Rip currents return water moved by longshore currents to deeper water. To see an animation of a high-energy shoreline.

To see an animation of a high-energy shoreline.
http://www.vcrlter.virginia.edu/presentations/shaomodel95/hog_decade.gif

Shore Deposits:

Shore zone material that has been eroded from one shore may be transported and deposited to other shore zone areas. Deposits of loose material that are parallel to the shore are called beaches. The sediments found on this type of area are a combination of pieces of rock, broken shell fragments. Size of the fragments range from large cobbles to fine sands, although most beaches are made of fragments that are sand size.

Sand Composition
Composition Characteristics:

quartz: clear and glassy; can be milky; often rounded
feldspar: tan, gray, pink, or salmon: dull luster
hornblende: dark green or black
mica: shiny, soft: breaks into thin sheets
magnetite: dark colored; attracted to magnet
garnet: pink or red; glassy luster
olivine: olive green; glassy luster
calcite: white; pearly luster; breaks into small cubelike pieces
rock fragments and color; made of more than one substance
shell fragments can include corals and other marine organisms
(Hesser)

Understanding beach dynamics:
http://www.con.wesleyan.edu/~eric/sparrow/earth.science/Beach.structure.html

Rocks and shells are constantly being broke down to sand size by wave action, and most shores are composed of grains of resistant material like quart. Sediments are classified according to their size, sizes are as follows:

Diameter Sediment

2.0 to 20.00 mm gravel
0.06 to 0.6 mm sand
0.002 to 0.06 mm silt
less than 0.002 mm clay

See a sediment transport model
http://www.camme.ac.be/~wwwusr/docs_en/models/sedim/model.html

See sediment distribution in an ocean environment: (figure 5)

(Selley)

To learn how do an Sediment analysis
http://www.jasonproject.org/JASON/HTML/CURRICULUM_JASON_7_invest4_methods8.h

Sediment Movement

Natural Interactions:

The movement of sediments in an estuary environment caused by natural interaction which have been discussed previously in this document. Included in the causes of movement of sediments are freshwater run off from watersheds, which carries land sediments to the sea, and cause delta formation. Ocean influences include longshore current, and rip tides, which move the sediments like rivers of sand, eroding in one place and re-depositing in another place.

Mans Activities:

In the name of flood control man has build a labyrinth of levies, dikes, and canals which has altered the natural flow of rivers. It has cause the sediments that would naturally be deposited in the delta regions to be deliver further into the ocean. This has caused depleted the natural build up of wetland areas, and has allowed our shores to be eroded away.

Two excellent articles that focus on mans interaction with the oceans environment are:

Our Slowly Fading Beaches
http://tgl.geology.muohio.edu/Focus/OurSlowlyFadingBeaches.html

Troubled Water II
http://www.tpwd.state.tx.us/news/magazine/water.htm

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