Rivers Landforms and Processes Geographical Association 22.09.11
Lecturer Chris Parker (Fluvial Geomorphologist, UWE, Chris2.Parker@uwe.ac.uk)
Lecture Notes
Key point: Rivers move. They are dynamic, not static landforms. The dynamic nature of river
systems has direct affects on flood risk.
A. Why are we concerned that rivers move? Affects on:
1. Flood risk. Flooding is a direct result of both increased discharge and increased sedimentation
(the build up of sediment by deposition).
A diagram to illustrate flood risk:
1. Flood risk is increased by an increase in discharge (cumecs)
above bankfull level
Key terms
An aggraded bed is a river bed that
becomes raised due to increased levels of
deposition (process is called aggradation).
This reduces channel capacity.
A degraded bed is when a river bed is
eroded or scoured, increasing channel
capacity (process is called degradation).
2. Flood risk can also be increased by reduced channel capacity
(an aggraded bed – see key terms) caused by increased
sedimentation.
Dredging is the artificial scouring
(degradation) of river beds to increase
channel capacity and reduced flood risk.
Lane et al (2007) observed that increased rainfall as a result of climate change will increased
likelihood of flooding by 12.2% between 2002 and 2012. Whilst increased sedimentation and
reduced channel capacity, as an indirect result of climate change, will increase the likelihood of
flooding by 5.7% between 2002 and 2004!
Key point: Sedimentation is a major contributing factor to flood risk. Cumbria floods of 2009 the
Environment Agency were accused by the local press and residents of compounding the impact of
the flooding at Cockermouth because they had reduced dredging. Press and residents claimed this
had reduced channel capacity, and the aggraded bed had resulted in extra severe flooding.
2. Navigation (trade and economy) E.g. The Mississippi is an important arterial route way. Economy
is heavily dependent on the river for trade and shipping. Intensive agriculture on floodplain
increased sedimentation. Depositional bars hampered shipping lanes and continue to pose problems
to planners and navigators.
3. Damage to infrastructure – flashy rivers need room to breathe. Culverts introduced in urbanised
areas can be damaged at times of intense rainfall as river shifts and operates at bankfull level.
4. Damage to ecology – continual realignment, flooding and sedimentation disturbs ecosystems.
Habitats swamped if sedimentation is too extreme, yet if river velocity is too great this can also lead
to damage. Need for equilibrium.
B. Why do rivers move?
Key Point: River movement is a product of erosive energy and sediment capacity. Put simply a river
channel will expand and become more sinuous if there is excess energy.
Question posed in ‘Job of the River’ academic paper – what is the role of the river – to transport
water or sediment or both?
Fluvial system is at battle with tectonic system – tectonic processes (isostatic uplift/orogeny) are
raising our landscapes whilst rivers are trying to level the playing field through erosive power and
degrading.
Some simple river rules:
If…
Capacity to transport sediment > sediment available to be transported = excess energy to erode/degrade
river, therefore…
- channel size will expand
- meanders will become more sinuous
If…
Sediment available to be transported > capacity to transport sediment = no energy for erosion, so
deposition occurs, bed is aggraded, therefore…
- channel size will shrink
- meanders will become less sinuous
Key point = less energy a river has, the less sinuous it will be and the more aggraded (raised) the bed
will be. Remember rivers vary over space (i.e. erosive power often more dominant in upper course,
whilst deposition more dominant in lower course), but also vary over time. So erosion will occur in
the lower course at certain times/seasons.
C. How will rivers move in the future?
Cellular catchment models (Coulthard et al) divide catchment in 10 x 10 m2 cells, based on laws of
gravity/human and physical factors, models can predict what water will do in the catchment. Charts
rainfall, river flow, channel and changes in sinuosity.
Coulthard et al noted that an increase in rainfall, results in an increase in sedimentation. On the
River Eden, Cumbria they charted marked increases in rainfall intensity and duration resulted in
increased levels of sedimentation and increased river sinuosity.
Conservative climate change predictions tells us there will be an increase in rainfall, resulting in a
42% increase in sedimentation. And it is sedimentation that increases flood likelihood.
Sedimentation Case Study – Toutle
River, Mt St Helens, Cascade
Range, Washington State, USA
Since May 18, 1980, sediment
transport rates for the rivers
flanking Mount St. Helens,
especially the Toutle River, have
been among the highest in the
world. More than 20 million tons of
suspended sediment was
transported from the Toutle River
basin in the first 7 months after the
May 18, eruption, or 15 million tons in only 13 days. About 39 million tons of suspended sediment
was transported from October 1981 to September 1982, enough to cover an average city block to a
depth of 8 kilometers.
This sediment has aggraded channel beds with excess sand and gravel for tens to hundreds of
kilometers downstream. Such aggradation promotes lateral migration of channels and causes
serious flooding during rainstorms, due to loss of channel capacity necessary to convey floodwaters.
SUMMARY
-
Rivers are not static, they are dynamic moving bodies of water
-
Migration across the floodplain impacts on flood risk, navigation, infrastructure and ecology
-
Increase in rainfall due to climate change will result in an increase in sediment transport.
-
Increased sediment transport will increase sedimentation rates – and this is the real threat.
-
Sedimentation is the greatest flood risk as we enter the future
Questions to consider:
Assimilation
1.
How does sedimentation lead to an increase in flood risk? What is an aggraded bed?
2.
Why should we be concerned by river movement?
3.
What conditions lead to…
a) increased channel size and sinuosity
b) reduced channel size and straightening?
4.
Why are insurance companies choosing now to work closely with fluvial geomorphologists?
5.
Chris Parker said that ‘arrogance’ has been at the heart of our desire to use hard engineering
to manage flooding. How then should we approach the problem of increased flood risk from
sedimentation in the future? Is dredging our only option?
6.
Link the eruption of Mt St Helens in May 1980 to increased flood risk in the state of
Washington.
Extension