2011/09/15

1

EWS132FLUVIAL PROCESSES AND

LANDFORMS

Fluid agents

• Running water is one of the four fluid agents that carry out denudation

• The others are glacial ice, waves and currents, and wind

• Running water (fluvial action) is by far the dominant agent shaping the land surface

Some basic concepts

• Before we go on to fluvial processes and landforms, lets look at some basic concepts!

Surface water flow

• Two types of surface flow– Overland flow occurs when rain falls too

rapidly to infiltrate and runoff occurs on a smooth surface as a continuous thin film called sheet flow

– Overland flow eventually contributes to stream flow which is a much deeper, more concentrated form of runoff

2011/09/15

2

Streams and Channels

• Streams can be defined as long narrow bodies of flowing water occupying trenchlike depressions or channels and moving to lower levels under the influence of gravity

• The channel of a stream is a narrow trough, shaped by the forces of flowing water to its most effective form for moving water and sediment

Flow velocity

• Friction near the channel bed and sides slow down the movement

• In straight and symmetrical channels the highest velocity is located in midstream

• In a curved stream maximum velocity shifts toward the bank on the outside of the curve

Turbulent flow

• Moving water is usually affected by turbulence

• In turbulent flow, water molecules follow irregular paths and mixing between adjacent layers involves transfer of momentum by large scale eddies

• A water molecule will therefore travel a highly irregular corkscrew path moving in a upward, downward and sideway direction

Stream discharge

• Stream flow at a given point is measured by its discharge

• Discharge is defined as the volume of water per unit time passing through a cross section of the stream at that location and is measured as cubic meters per second (m³/s)

2011/09/15

3

Effect of gradient

• Cross-sectional area and average velocity of a stream can change over a short distance even though the stream discharge does not change

• These changes occur because of changes in the gradient

Q = AV = Constant

Landforms

• Erosional landforms are forms resulting from the progressive removal of bedrock mass

• Depositional landformsare forms resulting from transported material deposited elsewhere away from their original position

Denudation

• Denudation is the total action of all processes by which rock surfaces are worn down and the resulting sediments transported to the sea, i.e. an overall lowering of the land surface

• We refer to landforms shaped by running water as fluvial landforms

Soil erosion

• Fluvial action starts with soil erosion from overland flow

• Soil erosion is a natural process that always occurs where flowing water is active and where precipitation occurs

• Under stable conditions, soil erosion is slow and in balance with the soil forming processes, allowing plant communities to maintain themselves

• Soil scientists refer to this state as the geologic norm

2011/09/15

4

Erosion (Cont.)

• Accelerated erosion is when more soil is removed than is formed, usually due to human activities that bare the ground from a protective cover

• Splash erosion is the mobilization of soil particles through the direct force of falling raindrops

• Up to 225metric tons of soil per hectare can be disturbed by a single rainstorm

Erosion (Cont.)

• Splash erosion can seal the natural soil openings and so reduce infiltration

• Reduced infiltration permits a greater depth of overland flow which in turn enhances the rate of soil erosion

• The term sediment yield (metric tonnes per hectare) is used for the amount of sediment removed by overland flow from a unit area in a given unit of time

Erosion (Cont.)

• Sheet erosion is the removal of soil in thin uniform layers by overland flow

• Rill erosion is erosion in which many closely spaced channels are scored into the soil on steeper slopes

Erosion (Cont.)• If rills are not destroyed

by soil tillage they can soon join together into gullies which are steep-walled canyonlike trenches

2011/09/15

5

Erosion (Cont.)

• In arid environments erosion rates are normally high and where clay formations dominate, badlands may develop

Negev, Israel

Wyoming

Badlands, Bryce Canyon,Utah

The geologic work of streams

• Erosion• Transportation

• Deposition

Stream Erosion

• Occur through four processes– Hydraulic action– Abrasion– Bank caving – corrosion

2011/09/15

6

Hydraulic action

• Hydraulic action is the force of flowing water that sets up a dragging action on the bed and bank material

• Two forces play a role– Lift force– Surface drag force

Forces involved in hydraulic action

• Lift force is operating at right angles to the streambed and is related to the Bernoulli effect

• Surface drag force isrelated to the difference in hydraulic pressure on opposite sides of the particle

Abrasion

• Abrasion is the grinding action of particles against each other and the wearing away of material. Potholes of Bourke’s Luck in Mpumalanga are good examples of abrasion

Hawaii

Pothole, Ontario, Canada

2011/09/15

7

Bank Caving• Bank caving is the

process of undermining of river banks that causes banks to slump into the river

Goodwin Creek, Massachusetts, USA

Cole River, UK

Corrosion

• Corrosion is the chemical processes of rock weathering that chemically remove material from the stream channel

Camooweal caves, Queensland

Cave Entrance, Belize, Mexico

Copyright © Jelena Calic-Ljubojevic 2002

Montenegro, Yugoslavia

2011/09/15

8

Stream Transportation

• Stream load can be carried in three ways– As dissolved matter and transported invisibly

as chemical ions– As bed load close to the bed of the channel

by rolling or sliding– As suspension load when particles float in the

water often giving the water a yellow muddy colour

Example of a bed load channel

Example of a bed load channel

Chamonix Valley, France

Bed load channel, Costa Rico

2011/09/15

9

Suspended load in ephemeral river, Namibia Test site, Negev Dessert, Israel

Suspension load 80000 ppm

Yellow River, ChinaConfluence of Colorado and Green

River, Utah

2011/09/15

10

Stream capacity

• Stream capacity is the maximum load a stream can transport at a particular discharge

• A stream’s capacity to carry suspended material or to move bed load increases according to the third or fourth power of its velocity

• This means that when the stream velocity doubles, its capacity to transport bed load is increased from eight to sixteen times

• When water flow increases during the flood stage, rivers will widen and deepen their channels and deposit material again when the water slackens

Graded streams

• Graded streams are streams in equilibrium, meaning they can carry the sediment load supplied to them by their drainage basin

• Graded streams have smooth longitudinal profiles that will flatten with time

Graded streams (cont.)

• Geomorphologically young streams have steep gradients and show deep gorges, waterfalls and rapids and are still in the process of reaching an equilibrium or a graded profile

• The first indication that streams are becoming graded, is when gradients become flat enough for floodplains to develop i.e. when lateral erosion starts

• When this happens rivers will start to meander and over time they will form meanders, cutoffs and ox-bow lakes

Young river without a floodplain

2011/09/15

11

Relatively narrow floodplain Example of well-developed floodplain

Floodplain

Relatively wide floodplain with meandering

Well-developed meandering, cutoffs and oxbow lakes

2011/09/15

12

Deposition

• Graded streams are very sensitive to upstream changes such as vegetation or climatic change

• When bed load increases, additional sediments will be deposited in the channel

• This will raise the elevation of the channel bed, a process called aggradation

Braided channels

• Aggradation disrupts the channel by depositing material in the channel to form braided channels

• Braided channels are generally shallow and characterized by a network of interconnected converging and diverging channels

Braided channel Braided ephemeral channel

2011/09/15

13

Anastomozing channels

• Anastomozing stream patterns are patterns where channels bifurcate, branch and rejoin irregularly to create a net-like formation

Levees

• Deposition during overbank flooding can result in building up of natural leveesalongside the river i.e. sand and silt deposited on both sides of the channel in the form of ridges

• Yazoo streams are streams that are prevented from joining the main stream after a flood has passed because natural levees prevent them from doing so

Degradation

• The opposite of aggradation is degradation, a process whereby the channel is deepened, very often resulting from a decrease in bedload

Degradation on alluvial fan

2011/09/15

14

Alluvial fan as depositional form Terraces

• Lateral cutting into an alluvial deposit will lead to formation of terraces

Alluvial terraces Entrenched meanders

• Tectonic uplift can cause a meandering river to incise its channel to form entrenched meanders

San Juan River, Utah

2011/09/15

15

Entrenched meander, Fish River, Namibia Stream rejuvenation

• Stream rejuvenation is the renewed incision of a stream

• Three types of rejuvenation– Dynamic rejuvenation resulting from epeirogenic uplift– Eustatic rejuvenation resulting from worldwide sea-

level changes. Two types:• Diastrophic eustatism• Glacial eustatism

– Static rejuvenation resulting from• Decreasing bedload• Increase in discharge due to climatic change• Increase in discharge due to stream piracy