Running Water & River Systems

CVE 3205Engineering Geology

Wong Jee Khai

Introduction• A stream is a body of water that:– Flows downslope along a clearly defined natural

passageway.– Transports detrital particles and dissolved substances.

• The passageway is called the stream’s channel.• The load is the sediment and dissolved matter

the stream transports. • The quantity of water passing by a point on the

stream bank in a given interval of time is the stream’s discharge.

Human Use of Streams• Large streams are important avenues of

transportation.• People choose to live near streams because:– Valley floors are flat and easy to build on. – Soils tend to be deep and fertile.– Water is available.

• Stream valleys have drawbacks.– They are threatened by floods.– As cities grow, human and industrial wastes pollute

the water.

Streams as Geological Agents

• Streams are vital geologic agents.– Streams carry most of the water that goes from

land to sea (essential part of the hydrologic cycle).– Streams transport billions of tons of sediment to

the oceans each year.

Stream Channels• A stream’s discharge may vary because of

changes in precipitation or the melting of winter snow cover.

• In response to varying discharge and load, the channel continuously adjusts its shape and orientation.

• Two ways to characterize a channel are:i. By measuring its cross-sectional shape.ii. By determining its long profile.

Cross-Sectional Shape• Because the volume of water moving through

a channel generally increases downstream, the ratio of channel width to channel depth is likely to change down stream.

Figure 14.2

Long Profile• The stream’s gradient is the difference in

elevation over a known distance along its course.• The gradient of a stream decreases downstream.• The stream’s long profile is a line drawn along the

surface of a stream from its source to its mouth. – It is a curve that decreases in gradient downstream.

Long Profile• A long profile is not a perfectly smooth curve

because irregularities in the gradient can be produced by:– Beds of resistant rock.– Landslides.– Lava flows.– Hydroelectric dams.

Dynamics Of Streamflow• The average annual rainfall on the area of the

United States is equivalent to a layer of water 76 cm thick. – 45 cm returns to the atmosphere by evaporation

and transpiration.– 1 cm infiltrates the ground,– The remaining 30 cm forms runoff (the portion of

precipitation that flows over the land surface).

Dynamics Of Streamflow• Initially water tends to move down slopes in

broad, thin sheets.– This process is called overland flow.

• After traveling a short distance overland flow begins to concentrate into well-defined channels called streamflow.

Factors in Streamflow• Several basic factors control the way a stream

behaves:– Gradient (expressed in meters per kilometers).– Stream-cross-sectional area (width х average depth,

expressed in square meters [A]).– Average velocity of waterflow (expressed in meters

per second [V]).– Discharge (expressed in cubic meters per second [Q]). – Load (expressed in kilograms per cubic meter).

• Dissolved matter generally does not affect stream behavior.

Discharge, Velocity, and Channel Shape

• The relationship of discharge, velocity, and channel shape for a stream can be expressed by the equation:Q = A x VDischarge Cross-sectional Average (m3/s) area of stream velocity (width x average (m/s) depth) (m2)

Changes Downstream• Traveling down a typical stream from its head

to its mouth:– Discharge increases.– Stream cross-sectional area increases.– Velocity increases slightly.– Gradient decreases.

Changes Downstream• On steep mountain slopes, discharge is low so

the flowing water is shallow.– The stream bed causes much more resistance to the

flow of shallow water.• Discharge increases downstream as each

tributary (a stream joining a larger stream) and inflow of groundwater introduce more water.

• To accommodate the greater volume of water, velocity increases together with the cross-sectional area of the stream.

Floods• A flood occurs when a stream’s discharge

becomes so great that it exceeds the capacity of the channel, therefore causing the stream to overflow its banks.

• Geologists view floods as normal and expected events.

• As discharge increases the water rises in the channel and erosion scours the bed.

Floods• With an increase in velocity, the enlarged

channel is now able to accommodate the increased flood discharge and carry a greater load.

• As discharge falls, the stream is unable to transport as much sediment.– At the end of the flood it returns to its preflood

dimensions.

Floods• Unusually large discharges associated with floods

appear as major peaks on a hydrograph.– A hydrograph is a graph that plots stream discharge

against time.• Regardless of the size of the stream basin, as

discharge increases during a flood, so does velocity.

• This velocity increase has the double effect ofenabling a stream to carry:– Greater load.– Larger particles.

Flood prediction • A flood-frequency curve is produced by

plotting the occurrence of past floods of different sizes on a probability graph.

• The measure of how often a flood of a given magnitude is likely to occur is called the recurrence interval.

• A flood having a recurrence interval of 10 years is called a “10-year flood.”

Flood Prediction• If Earth’s climate changes during the next

several decades, present flood-frequency curves may be a little value in predicting future floods.

Base Level• As a stream flows downslope, its potential

energy decreases and finally falls to zero as it reaches the sea.

• The limiting level below which a stream cannot erode the land is called the base level of the stream.

• The base level for most streams is global sea level.

Base Level• Exceptions are streams that drain into closed

interior basins having no outlet to the sea.• Where the floor of a tectonically formed basin

lies below sea level (for example, Death Valley, California), the base level coincides with the basin floor.

• When a stream flows into a lake, the surface of the lake acts as a local base level.

Natural and Artificial Dams• The courses of many streams are interrupted by

lakes that have formed behind natural dams consisting of:– Landslide sediments.– Glacial deposits. – Glacier ice.– Lava flows.

• Such a dam acts as a local base level and creates an irregularity in a stream’s long profile.

Natural and Artificial Dams• Large artificial dams also disrupt the normal

flow of water in a stream.– They are being constructed in ever-increasing

numbers for:– Water storage. – Flood control.– Hydroelectric power.

Natural and Artificial Dams• Hydroelectric power is recovered from the

potential energy of water in streams as they flow downslope to the sea. – Hydroelectric power is a renewable resource.

• An artificial dam built across a stream creates a reservoir that traps nearly all the sediment that the stream formerly carried to the ocean.– Reservoir siltation limits the useful life of power

dams.

Channel Patterns• Straight channels are rare. – The highest velocity along a straight channel segment

usually is found near the surface in mid-channel.• If a stream channel has many curves the channel

pattern is sinuous.• A deposit of sediment (a bar) tends to

accumulate where velocity is lower.• In many stream, the channel forms a series of

meanders.

Erosion by Running Water• Erosion by water begins before a distinct

stream has formed. It occurs in two ways:– By impact as raindrops hit the ground.– By overland flow during heavy rains, a process

known as sheet erosion.• The effectiveness of raindrops and overland

flows in eroding the land is greatly diminished by a protective cover of vegetation.

Erosion by Running Water• The ability of streams to erode is influenced by the

way water moves through a stream channel.• If the velocity is very slow the water particles travel in

parallel layers, a motion called laminar flow.• With increasing velocity, the movement becomes

turbulent flow.• The ability of a stream to pick up particles of sediment

from its channel and move them along depends largely on:– Turbulence.– Velocity of the water.

The Stream’s Load• The solid portion of a stream’s load consists of

two parts:– The bed load: the coarse particles that move along

the stream bed.– The suspended load: fine particles that are suspended

in the water. • Wherever they are dropped, these solid particles

constitute alluvium. • Streams also carry dissolved substances called

the dissolved load.– These are chiefly a product of chemical weathering.

Bed Load• The bed load generally constitutes between 5

and 50 percent of the total load of a stream.• Particles move discontinuously by rolling or

sliding at a slower velocity than the stream water.

• The bed load may move short distances by saltation (series of short intermittent jumps).

• Coarse-grained sediment is concentrated where the velocity is high.

• Finer-grained sediment is found in zones of progressively lower velocity.

Suspended Load • The muddy character of many streams is due to the

presence of fine particles of silt and clay moving in suspension.

• Because upward-moving currents within a turbulent stream exceed the velocity at which particles of silt and clay can settle toward the bed under the pull of gravity, such particles tend to remain in suspension longer than they would in non turbulent waters.

• They settle and are deposited only where velocity decreases and turbulence ceases, in a lake or in the sea.

Dissolved Load• All stream water contains dissolved chemical

substances that constitute part of its load.• The bulk of the dissolved content of most rivers

consists of seven ionic species:– Bicarbonate (HCO31-). – Calcium (Ca2+).– Sulfate (SO42-).– Chloride (CI1-). – Sodium (Na1+).– Magnesium (Mg2+).– Potassium (K1+).– Dissolved silica as Si(OH)4.

Downstream Changes in Particle Size

• Sediment normally decreases in coarseness downstream.– In mountainous headwaters, rivers may transport

large boulders.• Through times, the coarse bed load is

gradually reduced in size by abrasion.• When the stream eventually reaches the sea,

its bed load may consist mainly of sediment no coarser than sand.

Stream Deposits• When a stream’s turbulence decreases, its transporting

power drops and it deposits part of its load.• When a stream rises during a major flood, the water

overflows the banks and inundates the adjacent floodplain.– The boundary between channel and floodplain may be the

site of a natural levee (a broad, low ridge of alluvium built along the side of a channel by debris-laden floodwater).

• Many stream valleys contain one or more relatively flat alluvial terraces that lie above the floodplain.– A terrace is a remnant of an abandoned floodplain.