River Channel Processes & LandformsPresentation Transcript
RIVER CHANNEL PROCESSES & LANDFORMS, FLOOD & MANAGEMENT (1) RIVER PROCESSES. (2) DEPOSITION & SEDIMENTATION (HJULSTROM CURVE) (3) VELOCITY & DISCHARGE (4) PATTERNS OF FLOW (5) CHANNEL TYPES (6) CHANNEL LANDFORMS (7) FLOOD (CAUSES, IMPACT AND MANAGEMENT)
Three river processes:
The load is transported by 4 ways:
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 particle such as clay and silt (suspended load) are dislodged and carried by turbulence in a fast flowing river.
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.
Deposition : when velocity begins to fall, it has less energy and no
longer had competence and capacity to carry all its load so largest
particles, materials begins to be deposited.
Low discharge during period of low precipitation
Less velocity when river enter sea or lake.
Shallow water occurs on inside of a meander.
The load suddenly increase (debris from landslide)
River overflow its bank so velocity outside channel is reduced. (resulting in floodplain)
Erosion: wearing away of river bed and bank.
There are four main process of erosion:
Corrasion : occurs when the river picks up materials and rubs it along its bed and banks, wearing them away by abrasion, effective during flood. 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)
Attrition : As bedload moved downstream, boulders collide with
other material and the impact may break the rock into smaller pieces. In time angular rocks become increasingly rounded.
(iii) Hydraulic action : The sheer force of the water as the turbulent current hits banks (outside of meander) pushes water into cracks. The air in cracks compressed, pressure increased and in time bank will collapse.
Cavitation: is a form of hydraulic action caused by bubbles of air collapsing.
(iv) Solution/corrosion : This process in independent of river discharge and velocity. It is related to chemical composition of water e.g. concentration of carbonic acid and humid acid.
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.
The Hjulstrøm Curve shows that particles of a size around 1mm require the least energy to erode, as they are sands that do not coagulate. 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.
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 called the line of critical velocity , particles will be deposited or transported, instead of being eroded, depending on the river's energy.
Velocity and Discharge:
Discharge : is the amount of water originating
as precipitation which reaches the channel
by surface runoff, throughflow and baseflow.
Q = A X V
(Q: discharge, A: cross-sectional area and
1. Velocity: speed of a river (m/s)
2. Velocity of a river is influence by 3 factors:
Channel shape in cross-section.
Roughness of the channel’s bed and banks.
Patterns of flow
As water flows downhills under gravity, velocity decreases.
This is not only due to friction found along river bed and
banks, but also internal friction of water and air resistance
on the surface. There are two patterns of flow:
1. Laminar flow : horizontal movement of water
(rarely found), common in lava flow.
2. Turbulent: a series of erratic eddies, both vertical and
horizontal, in a downstream direction.
3. Helicoidal flow : a corkscrew movement, in a meander.
It is responsible for moving material from the
outside of one meander bend and depositing
on the inside of the next bend.
(a) Straight channel
(b) Braided channel
(c) Meander channel
Braided channel Braided reach of Lillooet River, southwestern British Columbia. What? A braided stream has islands/eyots of deposited material within the channel. Description: Overall channel is straight with eyots and smaller channels. It rapidly and frequently change position. When? It occurs when the load contain high proportion of coarser sands and gravel. Area: Semi-arid environment.
WHAT? Bends in course of a river channel.
Begin when a river approaches its middle course & gradient channel
is less steep.
2. It results from helicoidal flow with faster current spirals downstream in
corkscrew fashion. Movement result in erosion on outside bend of meander to
form river cliff and deposit on inside bend called slip off slope.
River cliff on outside of bend and gentle
sloping slip- off slope called point bar on
inside of bend of meander.
Read textbook (Geography An integrated approach, Waugh, D., p:79)
write briefly formation of Meander and ox-bow lake.
Riffle and pools
Riffles : deposition of a coarse material that create
areas of shallow water.
Pools: areas of deeper water between riffles.
Pools and riffles developed in section along river channel
which create different gradient of channel.
Coarse pebbles create steeper gradient than eroded pools.
Effects of fluvial erosion:
(a) V-shaped valleys
A river erode vertically by traction or saltation which resulted in a steep-
sided valley called a V-shaped valley.
Steepness of valley sides depend of factors such as:
Climate: valley are steeper where there is sufficient rainfall. (for mass movement and allow river to transport bedload and erode vertically)
Rock structure: resistant, permeable rocks such as limestone produce vertical sides.
Vegetation: it helps to bind soil together and keep the hillslope more stable.
(b) Interlocking spurs
It forms because the river is forced to follow a winding course around
the protrusions of the surrounding highland, resulting in spurs interlock.
2. Water fall:
A waterfall form when a river, after flowing over relatively hard rock
meets a band of less resistant rock flow over the edge of a plateau.
Over a period of years, the edges of this shelf will gradually break away
and the waterfall will steadily retreat upstream, creating a gorge of
Havasu fall, Arizona.
Rapids develop where the gradient of the river bed increases without a
sudden break of slope (as in a waterfall) or where stream flows over a
series of gently dipping bands of harder rock. Rapid increase the
turbulence of a river and hence its erosive power.
Effect of fluvial deposition.
Deposition of sediment takes place where there is a decrease in energy
or an increase in capacity which makes the river less competent to
transport its load.
It can occur anywhere from upper course, where boulders may be left,
to the mouth where fine clays may be deposited.
A floodplain is a mostly flat area of land bordering a river subjected to
periodic flooding. It is made of silts and sands which have been
deposited over many years by the river.
When river overflows its banks, the increase in friction produced by the
contact with the floodplain causes material to be deposited. The
coarsest material is dropped first to form a small, natural embankment
(levee) alongside the channel. During subsequent periods of low
discharge, further deposition will occur within main channel causing
bed of the river to rise and the risk of flooding to occur.
Floodplains and levees.
(3) Braiding channels:
For short periods of year, some rivers carry a very high load in relation
to their velocity e.g. during snow melt periods in Alpine or Arctic areas.
When a river’s level falls rapidly, competence and capacity are
reduced, and channel become chocked with material, causing the river
to braid ( divide into a series of diverging and converging segments)
It is composed of fine sediment which is deposited when a river losses
energy and competence as it flows into an area of slow moving water
such as a lake or sea. The shape resembled that of delta, the fourth
letter of the Greek alphabet ( )
Delta provide world’s fertile land, while shallow and frequently changing
river channels hinder navigation.
There are three types:
Arcuate (fan-shaped delta) : having rounded, convex outer margin e.g. Nile.
(b) Cuspate (tooth’s delta) : where material brought down by a river is spread out evenly on either side of its channel. E.g. Tiber
(c ) Bird’s foot : where the river has many distributaries bounded by sediment and which extent out to sea like the claws of a bird’s foot.e.g. the Mississippi.
Arcuate delta e.g. River Nile. Bird’s foot e.g. Mississippi river. Cuspate delta e.g. Tiber .
CAUSES (Human and physical factors)
(I) PHYSICAL FACTORS:
When does flooding occur?
Water overflows river banks onto surrounding area.
Occur when water available is more than infiltration capacity.
When does water overflow?
Prolong rainfall in saturated soil.
i.e. clay prone to overlandflow (smaller pores)
Soil already saturated thus reduce infitration capacity. (prone to flood)
3. Sudden increase in temperature (rapid snow melt)
(II) HUMAN FACTORS :
Land use (drainage system, digging ditch, ploughing up and down slope)
Urbanisation (land made impermeable in road building)
Impact of flooding (case study: uk and brazil) Reference: AS Geography Pallister J. & Bowen, Ann, p: 34-38) Flood management (Colorado & Bangladesh) Ref: AS Geography, Pallister J. & Bowen, Ann, p:39-43) IMPACTS AND FLOOD MANAGEMENT Read from textbook below, draw a mind map/concept map