BellworkComplete your mini quizzes

(to test your OWN knowledge, not the knowledge of the person sitting next to you)

Answers!1. What is evapotranspiration? Evaporation +

Transpiration2. What is the equation for working out

precipitation? (Clue- it includes the following: S=Storage, E=Evapotranspiration, O=Total Runoff (Streamflow))

P= O+E+/-S3. Is groundwater storage found above or under

the ground? Under the ground4. What is river discharge measured in? cumecs5. What is lag time? The amount of time in

between peak rainfall and peak discharge

River ProcessesLearning Objectives:

To understand river processes and the Hjulström Curve

Learning outcomes• Name the 4 erosional

processes• Name the 4

transportational process• Interpret the Hjulström

curve

River Processes

Erosion

Hydraulic ActionCorrasion/Abrasion

AttritionSolution/Corrosion

Transportation

TractionSaltation

SuspensionSolution

Deposition

Low river energy

RIVER PROCESSES:

Three river processes:1. Transportation2. Deposition3. Erosion

Information Hunt• Walk around the room to find the

information about the 3 main river processes in order to complete your sheet

• *Extension Task- Draw a diagram to go with each of the definitions*

River Competence & River Capacity

• River Competence is the maximum size of load a river is capable of transporting.

• River Capacity refers to the total volume of sediment a river can transport.

• It is important to note that the velocity has an influence:– At low velocity only fine partials may be transported

(Clays, silts, and sands).– At a higher velocity larger material can be moved.

• Maximum particle mass that can be moved increases with the power of velocity, so when discharge levels are high (during a flood for example), much larger boulders can be moved.

Hjulstrom Curve

River Processes

Hjulstrom curve

• The Hjulstrøm curve is a graph used by hydrologists to determine whether a river will erode, transport or deposit sediment. The graph takes sediment size and channel velocity into account.

• The curve shows several key ideas about the relationships between erosion, transportation and deposition.

Hjulstrom curve

• The Hjulstrøm Curve shows that particles of a size around 0.5 mm require the least energy to erode, as they are sands that do not coagulate (such as clay).

• Particles smaller than these fine sands are often clays which require a higher velocity to produce the energy required to split the small clay particles which have coagulated.

Hjulstrom curve

• Larger particles such as pebbles are eroded at higher velocities and very large objects such as boulders require the highest velocities to erode.

• When the velocity drops below this velocity, particles will be deposited or transported, instead of being eroded, depending on the river's energy.

THE HJULSTRÖM CURVE• The hjulström curve illustrates the relationship between

velocity and competence. It shows the velocities at which sediment will normally be eroded, transported or deposited.

Very fine particles need higher velocity to erode them than larger particles as materials like clay and sand are cohesive.

Less energy is needed here to erode a particle as less energy is needed to erode sands than clays.

Some of the smallest particles can stay in suspension when the water is still

When the particles are boulders, even the smallest drop in velocity can mean they are deposited.

Drawing the Hjulström Curve

• In order to aid your understanding of what the Hjulström curve is showing, copy out your own version for your notes, noting carefully the logarithmic scale on the x axis

Words Fill!eroded     0.1     silt     120     transported     150

cobbles     200 

At 10 cm per second has eroded whilst   would be deposited. The smallest clay particles require velocities of  cm per second to be eroded. For the smallest silts it is approximately  cm per second. Cobbles are   upwards of 170cm per second.  Deposition starts to occur at   cm per second for particles of approximately 0.01mm in size. Boulders require the smallest velocity for deposition, at only  cm per second for the smallest boulders 250mm in size. Course sand of 0.5 mm in size is   between 3cm per second and 16 cm per second.

silt cobbles

200120

eroded0.1

150

transported

Plenary Critique• How could we critique the Hjulstrom

curve?