Lecture 12 Running Water and Streams

1. Hydrologic cycle

2. Stream hydraulics

3. Stream erosion

4. Transportation of sediment by streams

5. Deposition of sediment by streams

6. Flooding

1. Hydrologic cycle:

• Powered by solar energy, Earth's Water is constantly moving among the hydrosphere, the atmosphere, the solid Earth, and the biosphere. This unending circulation of Earth's water supply is called the hydrologic cycle.

• Balance of water in the hydrologic cycle.

• What happens to the water after falling on the land?

• Much of the water falling on the land returns to the oceans by seepage into the ground (called infiltration) and by runoff over the surface (when rainfall is beyond the land's ability to absorb it). Some of the water that infiltrates the ground is absorbed by plants, which later release it into the atmosphere (a process called transpiration).

2. Stream hydraulics• flow types

Water may flow in one of two ways, laminar flow or turbulent flow. The stream's velocity is a primary controlling factor.

• Laminar flow: When velocity is low, water particles flow steadily parallel to each other and to the channel, without mixing.

• Turbulent flow: When velocity is high, streamlines will mix, cross each other, and form swirls and eddies.

• (Left) Most stream flows are turbulent. (Right) This stream is closer to a laminar flow. Continuous records of discharge are collected by the USGS at more than 7000 gauging stations like this in the U.S. (Tarbuck and Lutgens)

• velocity

• When the channel is straight, the highest velocities occur in the center of the channel just below the surface. It is here the friction is least. Minimum velocities occur along the sides and bottom (bed) of the channel where friction is the greatest.

• When the channel is bent, the zone of highest velocities shifts away from the center towards the outside of the bend. Thus active erosion occurs on the outside of the bend and deposition occurs on the inside.

• Stream velocity is controlled by (1) the gradient, (2) the shape, size, and roughness of the channel, and (3) the discharge.

• The Manning equation: v proportional to d2/3s1/2/n, d: water depth s: gradient n: bed roughness

• Gradient:

Gradient is the vertical drop of a stream over a fixed distance. For example, portions of the lower Mississippi River have gradients of only 10 cm per km or less.

• The shape of a channel: The shape of the cross-section of a channel

determines the amount of water in contact with the channel and hence affects the frictional drag and stream velocity. The most efficient channel is one with the least perimeter for its cross-sectional area.

• Influence of channel shape on velocity. The cross-sectional areas of (A) and (B) are the same, but water flows more rapidly in (B) because it has less water in contact with the channel and hence less friction.

• discharge• The discharge is the amount of water flowing past a certain p

oint in a given unit time, thus

discharge = (cross-sectional area) x (flow velocity)

• Discharge is measured in cubic feet per second (cfs) for streams but for water supply and sewage treatment in millions of gallons per day (MGD) (1 ft3/sec=0.646 MGD).

• The discharge of the World's largest river, Amazon, is 7.5 million cfs. Just one day's discharge of Amazon could supply the water needs of New York City of for 9 years! The Mississippi River ranks 7th in the world by discharge.

3. Stream erosion• Streams erode their channels by lifting loose particles

and by abrasion.

• Erosive power is proportional to the square of the velocity. Thus when discharge increases, the depth increases and the velocity increases, resulting in dramatic increase of erosive power.

• The scour and removal of bed materials during flooding may undermine foundations for engineering structures located in stream channels (such as bridges, loading facilities).

• Sediment-filled floodwaters. The greatest erosion and sediment transport occur during these high-water periods. (Davis/Stone Images)

• Sands and gravels are great tools of erosion. Transported by a river, they act as powerful abrasives, cutting through the bedrock as they are moved by the stream.

• Potholes in a river bed. The rotational motion of swirling pebbles acts like a drill to create potholes. (T. Till)

4. Transportation of sediment by streams

• Streams transport their loads in solution, in suspension, and along the bottom of the channels (bedload).

• Ions in solution may include calcium, magnesium, chloride, nitrate, sulfate, and silica.

• Usually only fine sand-, silt-, and clay- particles can be carried in suspension, except during flood stage where larger particles are carried as well.

• Sediment-filled floodwaters. The greatest erosion and sediment transport occur during these high-water periods. (Davis/Stone Images)

• Particles too large to be carried in suspension may be moved by streams along the stream bottom as bedload. The maximum-size particle a stream can move is determined by its velocity. (Tarbuck and Lutgens)

5. Deposition of sediment by streams

• When stream velocity decreases due to reduced depth or gradient, the particles of sediment are deposited. The coarsest particles of bedload are deposited first, followed by finer and finer particles in suspension.

• base level:• defined as the lowest elevation to which a stream can cut

its channel. The ocean is the ultimate base level of all streams. Local base levels include lakes, a dam, and resistant layers of rock.

• The ability of a stream to do work is closely related to its base level. When a base level is raised, e.g. by a dam, the reduced gradient lowers its velocity, causing sediment to deposit until the stream again has a gradient sufficient to carry its load.

• The base level of a stream is raised when a dam is built and a reservoir forms. This reduces the gradient and leads to the reduction of velocity and deposition of sediment upstream from the reservoir.

• Landform features of streams

• Both through erosion and deposition, streams alter the appearance of the land surface.

Drainage patterns. (a) Dendritic, forming on gentle slope and uniform substrate. (b) Radial, forming on a cone-shaped mountain flow. (c) Rectangular, forming on a rectangular grid of vertical joints. (d) trellis, forming on parallel valleys and ridges. (W.W. Norton)

The continental divide separate drainage basins that flow into different oceans. The Mississippi drainage basin is one of the several in North America. (W.W. Norton)

Niagara Falls. The resistant Lockport dolostone serves as a local base level for Lake Erie. (W.W. Norton)

Niagara Falls. The undercutting of the soft shale layers causes the resistant dolostone to break off. (W.W. Norton)

• More common than straight channels, meandering streams tend to form in gently sloping areas of unconsolidated sediments. Meanders in the Sevier River west of Yuba Reservoir, Utah.

Erosion occurs faster on the outer bank while deposition takes place on the inner curve (to form point bars). An oxbow lake is formed when the stream eventually cuts through the meander neck. (W.W. Norton)

People building communities along a riverbank mistakenly assume that the shape of a meandering stream will remain fixed for a long time. In fact, in a natural meandering river system, the river channel migrates back and forth across the floodplain. View 1 illustrates the processes of erosion and deposition, and View 2 shows the evolution, in map view, of a meandering stream. [by Stephen Marshak]

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Evolution of a Meandering Stream

• The outside of a meander is a zone of active erosion (often referred to as the cut bank). The Neaukum River, Washington in January 1965 (A) and March, 1965 (B). (P.A. Glancy, USGS)

• Alluvial fans develop on land where the gradient of a stream changes abruptly from steep to flat (e.g. from mountain terrain to flat valley floor). Death valley has many large alluvial fans as shown (Hamblin and Christiansen).

• Deltas form where a stream flows into a standing body of water (e.g., oceans, lakes). The transported sediment is deposited because of decreased velocity. Today New Orleans located in the Mississipi Delta is built where there was ocean less than 5000 years ago. The river flows over the delta to form the tributaries (Modified from Hamblin and Christiansen, 1988 Tarbuck and Lutgens)

6. Flooding

• When the discharge of a stream becomes so great that it exceeds the capacity of its channel, it overflows its banks as a flood.

• The 1931 great flood of Yellow River in China killed 4 million people. The 1993 Midwest flood in the upper Mississippi River Basin caused direct property damage exceeding $10 billion.

• Satellite views of the Missouri River flowing into the Mississippi River before (top) and during (bottom) the 1993 flood.

• Water rushes through a break in an artificial levee in Monroe County, Illinois during the record-breaking 1993 Midwest floods.

• The cause of flood is weather. But human interference can make it worse. One example is urbanization, which shortens the lag time between rainfall and flood peak and increases the flood peak because of less infiltration and more rapid runoff.

• When an area changes from rural to urban, the lag time between rainfall and flood peak is shorted and the flood peak is higher because of less infiltration and more rapid runoff. (Hamblin and Christiansen)

A channel was constructed to take additional run-off from the new parking lot.