| the_work_of_rivers_5th_year.pptx |
Introduction
The river is often divided into three parts or stages. The table below will remind you of some of the main features of each stage.
Upper course (steep gradient)
Vertical (downward) erosion Source; Tributaries; V-Shaped valley; Interlocking spurs; Waterfalls; Rapids; Gorges
Middle course (gentle gradient) Lateral (sideways) erosion starts; Transportation, River beaches (slip off slopes); Meanders; River cliffs
Lower course (very low gradient) Deposition Flood plain; Ox-bow lakes; Levées; Delta; Estuary
Erosion
The main ways in which erosion happens are:
Corrasion - Wearing away of the river bed and banks by the load hitting against them.
Attrition - Wearing down of the load as the rocks and pebbles hit the river bed and each other.
Hydraulic Action - Breaking away of river bed and banks by the sheer force of the water getting into small cracks.
Chemical Action (Corrosion) - Water dissolves minerals from the rocks and washes them away.
Cavitation - air bubbles rise to the top of the river, burst and send ripples outwards causing erosion of the river banks.
Feature of river erosion: Waterfalls
Feature of river erosion and depostion: Meanders and Oxbow Lakes
Feature of river deposition: Deltas
River deposition:
Levees
A levee is a feature of river
deposition. It is a wide, low ridge of sediment deposited on the river banks.
Levees are generally found in the mature and old age stages of a
river.
As rivers enter the middle and lower
course they have a lower velocity due to the gentle slope of these stages. The
slower movement of the river reduces the ability to carry its load
(competence). After heavy rain the river may overflow its banks and flood the
surrounding land. The flooded land either side of the river is known as the
flood plain. The floodwaters deposit a fertile layer of fine sands and silts
called alluvium. The heaviest material is dropped first (closest to the river
side) due to weight and friction (between the water and land) and the lighter
material is carried further by the floodwaters. Eventually ridges of material
may build up on both banks. These ridges are known as levees.
Humans often build artificial levees or
strengthen existing ones to prevent further flooding of towns and
farmlands.
Examples of levees include:
Human activities and
Rivers
Human activity affects river channels
through engineering works including channelization, dam construction, diversion
and culverting (building a sewer or pipe under a road or railway line for river
water to pass through). The less obvious indirect effects of point and reach
changes occurring downstream and throughout the basin, however, are much more
recently appreciated. These are complemented by effects of alterations of land
use, such as deforestation, intensive agriculture and incidence of fire, with
the most extreme effects produced by building activity and
urbanisation.
Changing river channels
are most evident in the channel cross-section where changes of size, shape and
composition are now well-established, with up to tenfold increases or decreases
illustrated by results from more than 200 world studies. In addition the
overall channel planform, the network and the ecology have changed. Specific
terms have become associated with changing river channels including
enlargement, shrinkage and metamorphosis. Although the scope of adjustment has
been established, it has not always been possible to predict what will happen
in a particular location, because of complex response and contingency. The ways
in which changes in cross-section relate to reach and network changes are less
clear, despite investigations showing the distribution of changes along
segmented channels.
When considering the human role in
relation to changing river channels, at least five challenges persist.
First, because prediction of the nature and amount of likely change at a
particular location is not certain, and because the contrasting responses of
humid and arid systems needs to be considered, modelling is required to reduce
uncertainty. Second, feedback effects incorporated within the relationship
between changes at channel, reach and network scales can have considerable
implications, especially because changes now evident may have occurred, or have
been initiated, under different environmental conditions. Third, consideration
of global climate change is imperative when considering channel sensitivity and
responses to threshold conditions. Fourth, channel design involving
geomorphology should now be an integral part of restoration procedures. This
requires, fifthly, greater awareness of different cultures as a basis for
understanding constraints imposed by legislative frameworks. Better
understanding of the ways in which the perception of the human role in changing
river channels varies with culture as well as varying over time should enhance
application of design for river channel landscapes.
The river is often divided into three parts or stages. The table below will remind you of some of the main features of each stage.
Upper course (steep gradient)
Vertical (downward) erosion Source; Tributaries; V-Shaped valley; Interlocking spurs; Waterfalls; Rapids; Gorges
Middle course (gentle gradient) Lateral (sideways) erosion starts; Transportation, River beaches (slip off slopes); Meanders; River cliffs
Lower course (very low gradient) Deposition Flood plain; Ox-bow lakes; Levées; Delta; Estuary
Erosion
The main ways in which erosion happens are:
Corrasion - Wearing away of the river bed and banks by the load hitting against them.
Attrition - Wearing down of the load as the rocks and pebbles hit the river bed and each other.
Hydraulic Action - Breaking away of river bed and banks by the sheer force of the water getting into small cracks.
Chemical Action (Corrosion) - Water dissolves minerals from the rocks and washes them away.
Cavitation - air bubbles rise to the top of the river, burst and send ripples outwards causing erosion of the river banks.
Feature of river erosion: Waterfalls
Feature of river erosion and depostion: Meanders and Oxbow Lakes
Feature of river deposition: Deltas
River deposition:
Levees
A levee is a feature of river
deposition. It is a wide, low ridge of sediment deposited on the river banks.
Levees are generally found in the mature and old age stages of a
river.
As rivers enter the middle and lower
course they have a lower velocity due to the gentle slope of these stages. The
slower movement of the river reduces the ability to carry its load
(competence). After heavy rain the river may overflow its banks and flood the
surrounding land. The flooded land either side of the river is known as the
flood plain. The floodwaters deposit a fertile layer of fine sands and silts
called alluvium. The heaviest material is dropped first (closest to the river
side) due to weight and friction (between the water and land) and the lighter
material is carried further by the floodwaters. Eventually ridges of material
may build up on both banks. These ridges are known as levees.
Humans often build artificial levees or
strengthen existing ones to prevent further flooding of towns and
farmlands.
Examples of levees include:
- The Mississippi River,
(Louisiana,)
USA
- River Po, Italy
- River Nore, Kilkenny.
Human activities and
Rivers
Human activity affects river channels
through engineering works including channelization, dam construction, diversion
and culverting (building a sewer or pipe under a road or railway line for river
water to pass through). The less obvious indirect effects of point and reach
changes occurring downstream and throughout the basin, however, are much more
recently appreciated. These are complemented by effects of alterations of land
use, such as deforestation, intensive agriculture and incidence of fire, with
the most extreme effects produced by building activity and
urbanisation.
Changing river channels
are most evident in the channel cross-section where changes of size, shape and
composition are now well-established, with up to tenfold increases or decreases
illustrated by results from more than 200 world studies. In addition the
overall channel planform, the network and the ecology have changed. Specific
terms have become associated with changing river channels including
enlargement, shrinkage and metamorphosis. Although the scope of adjustment has
been established, it has not always been possible to predict what will happen
in a particular location, because of complex response and contingency. The ways
in which changes in cross-section relate to reach and network changes are less
clear, despite investigations showing the distribution of changes along
segmented channels.
When considering the human role in
relation to changing river channels, at least five challenges persist.
First, because prediction of the nature and amount of likely change at a
particular location is not certain, and because the contrasting responses of
humid and arid systems needs to be considered, modelling is required to reduce
uncertainty. Second, feedback effects incorporated within the relationship
between changes at channel, reach and network scales can have considerable
implications, especially because changes now evident may have occurred, or have
been initiated, under different environmental conditions. Third, consideration
of global climate change is imperative when considering channel sensitivity and
responses to threshold conditions. Fourth, channel design involving
geomorphology should now be an integral part of restoration procedures. This
requires, fifthly, greater awareness of different cultures as a basis for
understanding constraints imposed by legislative frameworks. Better
understanding of the ways in which the perception of the human role in changing
river channels varies with culture as well as varying over time should enhance
application of design for river channel landscapes.
