In this lesson of Erosion we are going to learn the following:
- What is Abrasion
- Potential energy Vrs Kinetic Energy
- How to measure the discharge of water to determine the force of erosion against limestone
- Limestone reactions to hydraulic action
Stream Erosion
Streams possess two kinds of energy, potential and kinetic. Potential energy is the energy of position, such as that possessed by water behind a dam or at a high elevation. In stream flow, potential energy is converted to kinetic energy, the energy of motion. Moth of this kinetic energy is dissipated as heat within streams by fluid turbulence, but a small amount, perhaps 5%, is available to erode and transport sediment. Erosion involves the physical removal of dissolved substances and loose particles of soil and rock from a source area. Thus, the sediment transported in a stream consists of both dissolved materials and solid particles.
Because the dissolved load of a stream is invisible, it is commonly overlooked, but it is an important part of the total sediment load. Some of it is acquired from the stream bed and banks where soluble rocks such as limestone and dolostone are exposed, but much of it is carried into streams by sheet flow and by groundwater.
The solid sediment carried in streams ranges from clay-sized particles to large boulders. Much of this sediment is supplied to streams by mass wasting, but some is derived directly from the stream bed and banks. The power of running water, called hydraulic action, is sufficient to set particles in motion. Everyone has seen the results of hydraulic action, although perhaps not in streams. For example, if the flow from a garden hose is directed onto loose soil, a hole is soon gouged out by hydraulic action.
Another process of erosion in streams is abrasion, in which exposed rock is worn and scraped by the impact of solid particles. If running water contains no sediment, little or no erosion of solid rock surfaces will result, but if it is transporting sand and gravel, the impact of these particles abrades exposed rock surfaces. Potholes in the bed of streams are one obvious manifestation of abrasion. These circular to oval holes occur where eddying currents containing sand and gravel swirl around and erode depressions into solid rock.
Limestone
A sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Most limestone is composed of skeletal fragments of marine organisms such as coral, foramsand mollusks.
Limestone makes up about 10% of the total volume of all sedimentary rocks. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years.
Velocity and Discharge
Stream velocity and discharge are closely related variables. Velocity is simply a measure of the downstream distance traveled per unit of time. Velocity is usually expressed in feet per second (ft/sec) or meters per second (m/sec) and varies considerably among streams and even within the same stream.
Variations in flow velocity occur not only with distance along a stream channel but also across a channel's width. For example, because friction, flow velocity is slower and more turbulent near a stream's bed and banks than it is farther from these boundaries. The bed and banks cause frictional resistance to flow, whereas the water some distance away is unaffected by friction and has a higher velocity.
Other controls on velocity include channel shape and roughness. Broad, shallow channels and narrow, deep channels have proportionally more water in contact with their perimeters than do channels with semicircular cross sections. Consequently, water in semicircular channels flows more rapidly because it encounters less frictional resistance. In many streams the maximum flow velocity occurs near the surface at the center of the channel; it occurs slightly below the surface because of frictional resistance from the air above. In sinuous (meandering) channels, however, the line of maximum flow velocity switches form one side of the channel to the other and corresponds to the channel center only along straight reaches.
Channel roughness is a measure of the frictional resistance within a channel. Frictional resistance to flow is greater in a channel containing large boulders than in one with banks and a bed composed of sand or clay. In channels with abundant vegetation, flow is slower than in barren channels of comparable size.
The most obvious control on velocity is gradient, and one might think that the steeper the gradient, the greater the flow velocity. In fact, the average velocity generally increases in a downstream direction, even though the gradient decreases in the same direction. Three factors contribute to this: First, velocity increases continuously, even as gradient decreases, in response to the acceleration of gravity unless other factors retard flow. Secondly, in their upstream reaches, streams commonly have boulder-strewn, broad, shallow channels, so flow resistance is high and velocity is correspondingly slower. Downstream, however, channels generally become more semicircular, and the bed and banks are usually composed of finer-grained materials, thus reducing the effects of friction. Thirdly, the number of tributary streams joining a larger stream increases in a downstream direction. Thus, the total volume of water (discharge) increases, and increasing discharge results in increased velocity.
Discharge is the total volume of water in a stream moving past a particular point in a given period of time. To determine discharge, one must know the dimensions of a channel, that is, it's cross sectional area (A), and its flow velocity (V). Discharge (Q) can then be calculated by the formula Q=VA, and it is generally expressed in cubic feet per second (ft^3/sec) or cubic meters per second (m^3/sec). The Mississippi River has an average discharge of 18,000 m^3/sec, but, as for all streams, the discharge varies, being greatest during floods and lowest during long, dry spells.
Legal Disclaimer....kinda
1 As dramatic as the process of weathering sounds, it does not happen overnight. In fact, some instances of mechanical and chemical weathering may take hundreds of years. An example would be the dissolving of limestone through carbonation. Limestone dissolves at an average rate of about one-twentieth of a centimeter every 100 years. If you want to see a layer of limestone (about 150 meters thick) dissolve, plan on watching that layer for about 30 million years.
2 Where we see the effects of weathering often is on our stone monuments and buildings and large rock structures. However, before you can analyze the rate at which these structures are weathering, you need to understand the factors that affect weathering rates. The weathering rate for rocks depends on the composition of the rock; the climate of the area; the topography of the land; and the activities of humans, animals, and plants.
3 A rock's composition has a huge effect on its weathering rate. Rock that is softer and less weather-resistant tends to wear away quickly. What is left behind is harder, more weather-resistant rock. This process is called differential weathering. Quartz is one type of rock whose composition, especially its crystalline structure, makes it resistant to mechanical and chemical weathering. This is why quartz remains unchanged on the Earth's surface after surrounding sedimentary rock has been eroded. There are some rocks, like limestone, that weather more rapidly. Limestone has the compound calcite. It is the carbonization of calcite that causes the increased rate of weathering of limestone. The material found in sediment grains also affects the rate of weathering. The mechanical weathering of rocks like shale and sandstone causes their grains to break up over time and become sand and clay particles. Why? Well, the grains in these two types of rocks are not cemented together firmly. Rocks like conglomerates and sandstones have grains that are cemented strongly with silicates. These rocks and other similar types tend to resist weathering. Geologists have also found that they may resist weathering longer than some types of igneous rocks.
4 A rock's exposure to the weathering elements and its surface area can affect its rate of weathering. Rocks that are constantly bombarded by running water, wind, and other erosion agents, will weather more quickly. Rocks that have a large surface area exposed to these agents will also weather more quickly. As a rock goes through chemical and mechanical weathering, it is broken into smaller rocks. As you can imagine, every time the rock breaks into smaller pieces its surface area or part exposed to weathering is increased. Think about a cube, which has both volume and surface area. To find the surface area of a cube, you need to calculate the sum of the areas for all six sides. Let this cube represent our rock that is exposed to weathering. Already our cube has six sides that are exposed to the elements. If we split our cube into eight smaller cubes, then the total surface area would be doubled. Although the surface area increases, the volume remains constant. Splitting the eight smaller cubes in the same way would have the same effect; the surface area would again be doubled. Increased surface area causes rocks to weather more rapidly.
Did you Know?
Mass wasting (also called mass movement) is defined as the downslope movement of material under the direct influence of gravity. Most types of mass wasting are aided by weathering and usually involve surficial (surface) materials. The material moves at rates ranging from almost imperceptible, as in the case of creep, to extremely fast as in a rockfall or slide. Though water can play an important role, the relentless pull of gravity is the major force behind mass wasting.
Requirements: (please do not put your answers in the log)
Now that we learned a little bit about erosion and the effects it has on limestone, take a moment to study ground zero. With the information above, and information at GZ you should be able to answer the following questions. You have 24 hours to send me an email (found in my geocaching profile) with the correct answers. Failure to do so will result in the removal of your "Found it log". Please add the GC code and title in the subject line. If you are sending answers for multiple people in your group, please add all the names in your email to ensure they don't have their log deleted :) Photos with you/group of the area would be greatly appreciated.
1 A confluence has formed up stream, due to this fact water has used what form of erosion to the Limestone on the other side of the stream?
2 Where is the Potential Energy stored for this water source?
3 Using the formula, determine the discharge at ground zero? (Hint you will need a partner to stand in the middle of a bridge to help figure the velocity of the stream, using a stick or other flotation device release at ground zero and count the time in seconds it takes to reach the bridge, count of distance by using your hand held by standing on the bridge and targeting distance to this earth cache, estimate the berth of the steam to make your calculations. )
4 What is the Kinetic Energy against the limestone?(Hint take the answer of number 4 and multiply it by 5%)
5 Look at the limestone wall, based on your findings, how many years of erosion have already taken place?