LO: To understand the river processes and the Hjulstrom Curve What are the processes of a river? RIVER PROCESSES: Three river processes: 1. Transportation 2. Deposition 3. Erosion RIVER TRANSPORTATION The load is transported by 4 ways: (i)
Saltation: when pebbles, sand and gravel (bedload) are lifted up by current and bounced along the bed in a hopping motion. (ii) Traction: when largest boulders and cobbles (bedload) roll or slide along the bed. (iii) Suspension: very fine particles such as clay and silt (suspended load) are dislodged and carried by turbulence in a fast flowing river. (iv)
Solution: water flowing within a river channel contains acids (e.g. carbonic acid from precipitation) dissolve the load such as limestone in running water and removed in solution. Types of Load Bed load Suspended load Dissolved load 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. River erosion. Erosion: wearing away of river bed and bank. There are four main process of erosion: (i)
Corrasion (Abrasion): The rubbing or scouring of the bed and banks by sediment carried along by the river. This can vary from fine particles kept in suspension by turbulent flow to heavier boulders rolled along at times of bank-full flow. Major method by which river erodes both vertically and horizontally. Landforms: potholes (turbulent eddies in the current can swirl pebbles around to form potholes that are hollows in river bed and pebbles are likely to become trapped) potholes
River erosion (continued) (ii) Attrition: Refers to reduction in the size of the sediment particles as they collide with each other, the bed and banks. Pieces of sediment become smaller and more rounded as they move downstream, so it is more common to find rounder, smaller fragments of rock downstream and coarser, more angular fragments upstream (iii) Hydraulic action: The sheer force of water eroding the bank and bed, in a number of ways: The turbulent water current hits river banks and pushes water into
cracks. The air in cracks compressed, pressure increased and in time bank will collapse. Cavitation- form of hydraulic action caused by bubbles of air collapsing. The water simply picking up loose sediment by frictional drag of moving water. River erosion (continued) (iv) Solution/corrosion: This process in independent of river discharge and velocity. Occurs when rocks dissolve in the water and are carried away. This is most common when the rocks in the
channel are carbonate (such as limestone and chalk). PROCESSES OF EROSION Rivers may erode horizontally and vertically, and often it happens at the same time. Vertical Erosion This is a characteristic of faster flowing rivers where there is enough sediment to down-cut, also as there is larger and more angular boulders being moved by the high velocity and there is a more angular bed-load, it creates a relatively quick lowering of the channel floor, generating steepsided valleys. Horizontal Erosion (Lateral Erosion) When a river has a sizeable flood-plain it may meander across the valley, meaning lateral erosion will dominate. This happens particularly when the flood-plain is composed of alluvial (clay, silt or gravel) sediments, as hydraulic action can attack the outside of the meander bend, leading to
undercutting and eventual collapse of river banks. DEPOSITION Deposition happens when a river is no longer competent or has lost the capacity to carry all of its load. Any reduction in river velocity will mean in material being deposited, starting with the coarsest sediment as this requires a lot of energy to remain in suspension. When does it occur? 1. 2. 3.
4. 5. 6. There is a sudden reduction in gradient (e.g. foot of a mountain range) The river enters a lake or the sea Discharge has been reduced following a period of little rainfall Where there is shallower water (e.g. The inside of a meander) There is a sudden increase in the volume of sediment available, such as at a confluence / where a landslide has occurred River overflows its bank so velocity outside channel is reduced. (resulting in floodplain) Hjulstrom Curve
River Processes Hjulstrom curve The Hjulstrm 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 Hjulstrm 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 HJULSTRM CURVE The hjulstrm 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. Some of the smallest particles can stay in suspension when the water is still Less energy is
needed here to erode a particle as less energy is needed to erode sands than clays. When the particles are boulders, even the smallest drop in velocity can mean they are deposited. 1. Consider the statements below and place them on your Hjulstrom curve where you think they should go. There are more than 1 possible location for each statement.
1. 2. 3. 4. 5. 6. 7. Cobbles can only be picked up at this velocity The water still may not be fast enough to lift clays from the bank Sand in suspension will probably fall out here It would take this kind of velocity to lift a boulder As soon as the water slowed, the boulders would fall out here Fine silt might still be in suspension at this low velocity Sand particles would start to be lifted around this velocity
2. For each statement, explain why you located it where you did on the graph, using velocity and particle size information to support your answers. 3. Why might velocities be required to erode both the finest and the coarsest calibre of materials? Hjulstrom Curve Questions a) Identify the relationship between river energy and particle size. b) What speed must the river be traveling at to erode a particle of size 10mm.
Identify the river process at for a particle size of 1mm with velocity 10cm/sec. Explain your answer. Explain why it is more difficult to erode clay and silt. c) d) Hjustrom curve Qs 1. Name the type of sediment that requires the lowest velocity to be eroded.  2.
Name the type of sediment that is likely to be transported at all velocities.  3. Describe and explain the relationship between water velocity and the erosion of clay and sand particles.  4. Explain the variation in water velocity that is required to transport and to deposit sediments of different particle diameter.  Answers
a) Identify the relationship between river energy and particle size. b) As the particle size increases, a river will require more energy in order to erode or to transport its load. However, the above rule does not apply to clay and silt particles which requires high river velocity to erode, as it is extremely cohesive. What speed must the river be traveling at to erode a particle of size
10mm. 100cm/sec Hjulstrom Curve c) Identify the river process at for a particle size of 1mm with velocity 10cm/sec. Explain your answer. c)
Traction. At velocity 10cm/sec, gravel is on the verge of being deposited or transported. Hence the mode of transportation is traction as the river drags the particle from its stationary mode. Explain why it is more difficult to erode clay and silt. This is because clay and silt sticky in texture and tend to coagulate (amass and stick to one another). These particles also tend to stick to the riverbed. SINK OR SWIM exercise. Provide a particle size and a velocity, and the answer they need to shout out within
10 seconds is "SINK!" or "SWIM!" Problems with Hjulstrm: 1.Velocity WHERE? Bed? Banks? Mean? (varies enormously within channel, so hard to apply the graph to real river) 2.Is SIZE (calibre) of the load the important factor? What about different densities? 3.SHAPE of load is important too (why?) 4.SHEAR STRESS is key, not VELOCITY (a function of water depth and gradient) Actually a wide band 150-400 cm/s
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