Fluvial Processes and Landforms

Unit 1 Module 2

Hydrological cycle• Continuous movement of water through the atmosphere,

over land and beneath the surface

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Introduction• Rivers and streams are dynamic systems

that continually adjust to natural and human-caused changes

• Running water is the most important geologic agent modifying Earth’s land surface and is a source of fresh water for industry, agriculture, and domestic use

• Management of erosion and flooding requires considerable effort and cost

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Importance of RiversRivers:

Provide water and nutrients for agricultureProvide habitat to diverse flora and faunaProvide routes for commerceProvide recreationProvide electricity

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The Action of RiversRivers are the most widespread agents of

denudation and deposition. The amount of erosion, transportation and

deposition of the river is dependent on the energy of the river which in turn is dependent on: The gradient of the slope The volume of water The shape of the channel

Generally, the volume of the river increases from the source to the mouth. Exceptions may be such as when the river passes through a hot desert.

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Fluvial ProcessesConcepts associated with fluvial processes and landforms

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Velocity Stream velocity is the speed of the water in the stream. Units are distance

per time (e.g., meters per second or feet per second). Stream velocity is greatest in midstream near the surface and is slowest along the stream bed and banks due to friction.

Factors influencing river velocity1. Shape of the river channel

• The shape of a channel’s cross-section affects the amount of friction that the river has to overcome and hence the river velocity

• Deep narrow channels and wide shallow ones are less efficient in transporting both water and load than deep wide channels of semi-circular shape

• Channel efficiency can be measured by the hydraulic radius:Hydraulic Radius = Cross-sectional area (CA) Wetted Perimeter

• The larger the hydraulic radius, the more efficient the channel shape as less energy is needed to overcome friction

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Example 1:CA = 10mWP = 12m

10mHR = –––– = 0.8

12m

Example 2:CA = 10mWP = 9m

10mHR = –––– = 1.1

9m

Conclusion:The channel in Example 2 is more efficient than the one in Example 1

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2. Gradient of the river channel• A river flowing down a steep slope has greater

velocity than one that flows down a gentle slope

3. Roughness of the river channel• The presence of resistance in the form of boulders or

unevenness in the river bed interferes with the river flow, thus reducing the velocity re: Manning’s equation

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Manning’s EquationQ = (AR2/3 S1/2)/n

Q = dischargeA = cross sectional

areaR = hydraulic radiusS = channel slopeN = coefficient of bed

roughness

Some e.g. of Manning’s n

Mountain stream, rocky bed0.04 – 0.05

Alluvial channel (large dunes) 0.02 – 0.035

Alluvial channel (small ripples) 0.014 – 0.024

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Stream DischargeStream discharge is the quantity (volume) of

water passing by a given point in a certain amount of time. It is calculated as Q = V * A, where V is the stream velocity and A is the stream's cross-sectional area. Units of discharge are volume per time (e.g., m3/sec or million gallons per day, mgpd).

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FlowsAt low velocity, especially if the stream bed is

smooth, streams may exhibit laminar flow in which all of the water molecules flow in parallel paths. At higher velocities turbulence is introduced into the flow (turbulent flow). The water molecules don't follow parallel paths.

Streams carry dissolved ions as dissolved load, fine clay and silt particles as suspended load, and coarse sands and gravels as bed load. Fine particles will only remain suspended if flow is turbulent. In laminar flow, suspended particles will slowly settle to the bed.

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Laminar vs. Turbulent Flow

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Sediment TransportHjulstrom's Diagram plots two curves representing 1)

the minimum stream velocity required to erode sediments of varying sizes from the stream bed, and 2) the minimum velocity required to continue to transport sediments of varying sizes. Notice that for coarser sediments (sand and gravel) it takes just a little higher velocity to initially erode particles than it takes to continue to transport them. For small particles (clay and silt) considerably higer velocities are required for erosion than for transportation because these finer particles have cohesion resulting from electrostatic attractions. Think of how sticky wet mud is.

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The Hjulstrom Curve

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Sediment Transport (continued)You should note the following:Sand is easily transported at lower velocities. More velocity is needed to pick up material than to

carry it in suspension. In times of highest discharge, velocity increases, as

does erosion. The division between Transportation and deposition is

small. This means that only a small decrease in velocity leads to sedimentation.

Competence is the maximum size of material a river can transport.

Capacity is the total load actually transported.Desmond Mohammed

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Competence and CapacityStream competence refers to the heaviest particles a stream

can erode and therefore transport. Stream competence depends on stream velocity (as shown on the Hjulstrom diagram above). The faster the current, the heavier the particle that will be transported.

Stream capacity is the maximum amount of solid load (bed and suspended) a stream can carry. It depends on both the discharge and the velocity (since velocity affects the competence and therefore the range of particle sizes that may be transported).

As stream velocity and discharge increase so do competence and capacity. But it is not a linear relationship (e.g., doubling velocity and discharge do not simply double competence and capacity). Competence varies as approximately the sixth power of velocity. For example, doubling the velocity results in a 64 times increase in the competence.

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Competence and Capacity (continued)Capacity varies as the discharge squared or

cubed. So tripling the discharge results in a 9 to 27 times increase in the capacity.

Therefore, most of the work of streams is accomplished during floods when stream velocity and discharge (and therefore competence and capacity) are many times their level during low flow regimes. This work is in the form of bed scouring (erosion), sediment transport (bed and suspended loads), and sediment deposition.

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Variations in a river flow (regimes)

The regime of a river is expected to have a seasonal pattern of discharge during the year. This is due to factors such as climate, local geology and human interaction. Equatorial rivers have regular regimes but in the UK where seasons exist one or two peaks may be recognisable.

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Variations in a river flow (regimes)

Simple regimes

These show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.

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Variations in a river flow (regimes)

Complex regimes

If a river has more than one period of high water levels and/or low water levels, a more complex regime results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, The Rhine.

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The Significance of Base Level

Base level is the lowest level to which any stream can erode

Sea level is taken to be the ultimate base level, but the rising of the sea or subsidence of land over geologic time make this concept a relative one

Local base levels may control erosion and deposition

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