109 terms

TOPIC 7: Sediment Transport

Science Olympiad "Dynamic Planet" Carly J. Chambers & Eva W. Lambert
STUDY
PLAY
What are sediments? (Give examples)
Loose Earth materials such as sand that accumulate on the land surface, in river and lake beds, and on the ocean floor.
How do sediments form?
By the weathering of rock.
How are sediments transported? What are these all influenced by primarily?
Wind, water, ice, and mass wasting. All influenced by gravity.
Describe the process of deposition.
When eventually the sediment settles out and accumulates after transport.
What is sedimentology?
The study of sediments and sedimentation.
What are the three basic types of sediment?
Rock fragments/clastic sediments, mineral deposits/chemical sediments, and rock fragments/organic matter/organic sediments.
How do dissolved minerals form?
By weathering rocks exposed at earth's surface.
Where is organic matter derived from?
From the decaying remains of plants and animals. Also produced by a combination of physical and chemical weathering.
When and how do clastic and chemical sediments form?
They form during weathering of bedrock or pre-existing sediment by both physical and chemical processes.
What does physical weathering produce?
Clastic and organic sediment.
What is chemical weathering? What is it caused by?
The decay and dissolution of Earth materials. It is caused by a variety of processes- but primarily from various interactions between water and rock material.
What can chemical weathering do? How does it do this?
It may alter the mineral content of a rock by either adding or removing certain chemical components.
How are some mineral by-products of chemical weathering transported?
Dissolved by water and transported below ground or to an ocean or lake in solution.
Where do some mineral by-products of chemical weathering go after this process?
Later, these materials may precipitate out of their oceans/lakes and form deposits on the roof of a cave or the ocean floor.
What is it called when sediments from on the roof of a cave?
Stalactites.
What does chemical weathering produce?
Clastic, chemical, and organic sediments.
What are erosion and transport of sediments from the site of weathering caused by?
One or more of the following; gravity, wind, water, or ice.
What is mass wasting?
When gravity acts alone to move a body of sediment or rock.
What is it called when gravity acts alone to move a body of sediment or rock?
Mass wasting.
Give six examples of mass wasting!
Rock falls, landslides, mudflows, slump, creep, and subsidence.
When do rock falls occur?
When rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope.
What are landslides? What do they require?
Landslides consist of rapid downslope movement of a mass of rock or soil. They require that little to no water be present.
When do mudflows occur?
Mudflows occur when a hillside composed of fine-grained material becomes nearly saturated by heavy rainfall.
Describe the importance of water in a mudflow?
The water helps lubricate the sediment, and a lobe of mud quickly moves downslope.
What is the most effective agent of transport (even in the desert).
Water.
What are the three forms in which water erodes?
As droplets, in sheets, or as channelized flow.
The less present vegetation...
The more water erodes.
Where is wind an important agents of erosion?
Only where there is little to no vegetation.
Ice in glaciers is very effective at...
eroding and transporting material of all sizes
How big are the boulders that glaciers can move? How far can they move them?
As large as a house for hundreds of miles.
What do erosive agents do?
They remove sediments from the site of weathering.
What are the three ways that erosive agents remove sediments from the site of weathering?
Impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion).
What is corrosion?
Chemical erosion.
How are sediment sizes classified?
By separating them into a number of groups, based on metric measurements.
How big is a boulder?
>256mm
How big is cobble?
256-64mm
Which sediments are non-cohesive?
Cobble and very coarse sand
How big is gravel?
64-2mm
Which sediment is cohesive?
Silt
How big is very coarse sand?
2-1mm
How big is coarse sand?
1-0.5mm
How big is medium sand?
0.5-0.25mm
How big is fine sand?
0.125-0.063mm
How big is silt?
0.062-0.004mm
How big is clay?
0.004-0.00024mm
What happens when particles are eroded and transported by wind, water, or ice?
They become part of the transport medium's sediment load.
What are the three categories of load that may be transported by an erosion agent?
Dissolved load, suspended load, and bedload.
What is wind not capable of doing?
Dissolving minerals- therefore, wind cannot transport any dissolved load.
The dissolved load in water and ice is...
not visible.
How must the dissolved load in water and ice be deposited?
Because it is not visible, it must be chemically precipitated.
How may sediment be suspended?
In wind, water, or ice.
What makes stream water look dirty after a rainstorm?
Suspended sediment.
What is suspended sediment?
Sediment that is not continuously in contact with the underlying surface and so is suspended within the medium of transport.
Which sediment is likely to be suspended?
The sediment with the smallest particles.
What force may sand be suspended by?
Powerful winds.
What force may pebbles be suspended by?
Floodwaters.
Why can any size of sediment be part of the suspended sediment load of a glacier?
Because ice is a solid.
What does bedload consist of?
The larger sediment that is only sporadically transported.
What does bedload always do? How does it move?
Remains in (almost) continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom.
Bedload can be transported by...
All wind, water, and ice!
Why does wind only rarely move beloads coarser than fine sand?
Because of the low density of air.
What greatly increases the power of streams?
Floodwater.
What does flooding cause?
Large sections of a riverbank to be washed into the water and become part of its load
What is often a significant source of a stream's load?
Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting.
Because ice is a solid it can...
Transport virtually any size material if the ice is sufficiently thick and the slope is steep.
The higher the velocity the...
coarser the load.
What causes sediments to become rounder? How?
Transport of sediments... Their irregular edges are removes by both abrasion and corrosion.
Why is beach sand highly rounded?
Because of its endless rolling and bouncing in the surf.
Of the agents of transport, which is the most effective at mechanically rounding (abrading) clastic sediments (coasts)?
Wind!
Why is wind the most effective mechanical rounder?
Because its low density does not provide much of a "cushion" between the grains as they strike one another.
When does sorting happen?
During sediment transport.
Why is sorting limited by?
The medium's velocity and density.
What is the poorest sorter of sediment?
Ice.
What happens when ice flow slows down or stops? Why?
The sediments is not deposited due to the density of the ice.
Sediments deposited directly by ice are usually very...
poorly sorted.
Where does significant sorting only occur?
In glacial sediments that are subsequently transported by meltwater from the glacier.
What is the best sorter of sediment and why?
Win because it can usually only transport sediments ranging in size from sand to clay.
What does occasional variation in wind speed during transport do?
It serves to further sort out sediment sizes.
What happens when velocity decreases in wind or water?
Larger sediments are deposited first. Sediments that were part of the suspended load will drop out and become part of the bedload.
What happens if velocity continues to drop?
Nearly all bedload movement will cease, and only clay and the finest silt will be left suspended.
Describe sediment transport in still water.
In still water, even the clay will be deposited, over the next day or so (based on size) - largest clay particles to the smallest.
What happens to a typical sediment grain during its trip from outcrop to ocean?
It may be deposited, temporarily, thousands of times.
What happens to a typical sediment grain when the transport medium's velocity increases again?
These deposits will again be eroded and transported.
What happens when compacted fine-grained clay deposits are subjected to stream erosion? Why?
They are nearly as difficult to erode as pebbles and boulders because the tiny clay particles are electrostatically attracted to one another and they resist erosion as well as much coarser grains.
Where is the final destination of the sediment? What is this destination known as?
Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.
How is chemical sediment unlike clastic and organic sediment?
Chemical sediment cannot simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution; it must be precipitated.
What is a common way for precipitation to occur?
Evaporation.
What happens if as water evaporates from the surface, it is not replaced by water from another source? Where does this often occur?
Any dissolved materials in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the water. This often occurs in the desert in saltpans or lakes. It may also occur along the sea coast in a salt marsh.
What is another mechanism that triggers mineral precipitation?
A change in water temperature.
What happens when ocean waters with different temperatures mix?
The end result may be seawater in which the concentration of dissolves minerals will precipitate.
What happens (with most minerals) what does the tendency to precipitate correlate with? Which minerals are the reverse exception to this rule?
Their tendency to precipitate increases with decreasing water temperature. Calcite is the reverse exception to this rule.
By what other method may minerals be forced to precipitate? Example?
By the biological activity of certain organisms. For example, when algae remove carbon dioxide from water- the acidity of the water decreases, promoting the precipitation of calcite.
Why do some marine organisms use this water chemistry manipulation? Name some of these organisms.
To promote mineral precipitation and use the minerals to form their skeletons (clams, snails, hard corals, sea urchins).
Why do landscapes form and constantly change?
Weathering and sedimentation.
Rivers are an example of...
Regional depositional environment.
Give five examples of depositional landforms.
Channels, backswamps, floodplains, abandoned channels, and sand bars.
What is a depositional landform?
A landform produced by deposition rather than erosion.
What three elements work together to reshape the earth's surface?
Erosion, weathering, and sedimentation.
What do erosion, weathering, and sedimentation require of humans?
They require us to adapt and adjust to changes in our environment.
What can an abundance of people and disturbance of the land do to these processes?
This can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters.
How much sediment do the world's rivers carry to the ocean every year? How much of this amount can be directly related to human activity?
24 million tons. 2/3 can be directly related to human activity.
Excessive human involvement results in what?
It greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.
What can increased sedimentation case?
Increased size and frequency of flooding.
What happens as stream channels are filled in? What is a result of this?
The capacity of the channel decreases. As a result of this, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding.
Explain how sedimentation can become a major problem on dammed rivers?
Sediment accumulates in the lake created by the damn rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam.
How does sedimentation affect forested areas?
Lakes formed by dams are not as susceptible to the problems displayed in dammed rivers. Sedimentation is not as great due to interception of rainfall by the trees and the underbrush.
How does vegetative cover relate to sedimentation?
Vegetative covers prevents soil from washing into streams by holding the soil in place.
What would sedimentation be like without vegetative cover?
Erosion rates could increase significantly.
What are many state and local governments doing for sedimentation issues?
They are now developing regulation concerning erosion and sedimentation resulting from private and commercial development.
YOU MIGHT ALSO LIKE...