Fluvial Geomorphology Flashcards
Whats a meandering river?
Clear cut off points and ox bow lakes
Whats a braided river?
Lots of fine sediment, multithread
Whats a anabranching river?
Multithread systems that fall between braided systeams and meandering
Whats a anastomosing river?
Composed of two or more interconnected channels that enclose floodbasins
Sediment flux depends on two things:
The energy or force exerted by the flow
The amount of sediment available for transport
Controls on bedload transport
Sediment size and arrangement on the bed (Resistance to movement).
Force exerted by water flowing over the bed.
Upstream supply of sediment.
Connectivity to upstream sediment supply.
Flume experiments (e.g. Schumm and Khan, 1973) indicate that…
The straight meandering braided platform transition is associated with increased slope (energy) and increased sediment supply.
Bed load
Describes particles in a flowing fluid (usually water) that are transported along the bed.
Suspended load
The amount of sediment that is suspended within the water column.
Wash load
The overland flow thats flowed across the earth surface, collecting fine sediment.
Bed material load
The portion of the sediment that is transported by a stream that contains material derived from the bed.
There are four main processes of transportation. These are:
suspension
solution
saltation
traction.
Nine Degrees of Freedom” for alluvial rivers
- Cross section categories:
- Width (W)
- Depth (d)
- Maximumdepth (dm)
- Velocity (V)
Nine Degrees of Freedom” for alluvial rivers
- Plan form:
- Slope (S)
- Sinuosity (p)
- Meander Arc length (z)
Nine Degrees of Freedom” for alluvial rivers
- Long profile:
- Bedform amplitude(a)
9. Bedform wavelength
Channel adjustment to climate change
Warm Phases:
Rainfall dominated events
Vegetation cover
Reduced weathering rates
Incision into valley fill
Styles of channel adjustment I: Meandering Channels
Rivers flow through the least energetic points in a landscape as quickly as they can.
Enhanced erosion on outside of bend and deposition on inside- leads to meanders.
Styles of channel adjustment II: Braided channels
Braided rivers are built up of confluence: difluence channels and braid bars.
These influence adjustment.
Types of change
Deliberate interventions
Have the specific intention of altering hydrological characteristics
Effects can often be specified exactly
Types of change
Inadvertent interventions
An action can have unintended or “unimportant” effects on streamflow
Effects can be difficult to specify
Effect depends on method of clearance
“Conventional” approach
Clear felling
Extraction of logs by tractor
Clearance by burning
Affect water balance and runoff generation processes
Effect depends on method of clearance
“Minimum impact” approach
Clear felling
Manual extraction
No burning
Affect just water balance
Effect 1: increase in runoff
Reduced interception, hence reduced
evaporation
Effect varies with vegetation type and climate regime
Effect 2: peak flows
Deforestation tends to affect the average/small floods rather than the large ones
Effect 3: change in timing of floods
Largely determined by the deforestation process
‐ particularly the construction of roads
Agriculture
• The most extensive form of land surface manipulation.
• Among the oldest form of land surface manipulation.
• In most cases started with clearance of existing
vegetation.
Agriculture Impacts
• Depends on some form of modification to the
soil (e.g. drainage, nutrient content)
• Increased Soil Erosion
• Increased drainage
What is channelisation?
Direct modification to the physical form
of the river channel
Often part of an arterial drainage scheme.
What is river maintenance?
Action taken to preserve the design performance of a
river channel or to prevent erosion.
Restoration Practice
Habitats
• Conservation‐led (restoration of
habitat)
• Species – led (restoration of
specific habitat)
Restoration Practice
- Intuitive / Expert‐based design
- Feature‐based design and implementation
- Small Scale (Reach‐based)
- Biased to low‐cost options. (Cost:Benefit approaches)
How do certain human activities affect sediment yield?
Construction and urbanization increase sediment yield
Sediment budgets describe what
The input, transport, storage and export of sediment from a system
Describe an alluvial fans.
A fan-shaped mass of silt deposited by a river when its flow is suddenly slowed.
How do braided streams form?
Braided streams form where the sediment load is so heavy that some of the sediments are deposited as shifting islands or bars between the channels.
What is an oxbow lake?
A curved lake formed from a sharp bend in a river where the main stream has cut across the narrow end and sediment buildup blocks the lake from the stream,
Where do braided streams and oxbow lakes occur?
The lower course, where the graident is gradual and deposition takes place.
What is saltation
Saltation is when small material such as pebbles and gravel that is too heavy to be carried in suspension is bounced along the river by the force of the water.
What is traction?
Traction is when large materials such as boulders are rolled and pushed along the river bed by the force of the river.
What are interlocking spurs?
An interlocking spur occur in a river’s upper course, hills tend to jut out into the river valley in a interlocked formation, in a formation like the teeth of a zip.
What is the rejuvenation of a river?
A river is said to be rejuvenated when the base level that it is flowing down to, is lowered.
Four possible causes of rejuvenation?
- Construction of a dam
- Dredging
- Uplift of land
- River capture
What is the effect of drainage basins by people?
The increased development and farming activity, and increasing populations, people have had a negative effect on the land surfaces of catchment areas, underground water, the quality and volume of water in rivers, and the flow characteristics of rivers
Dendritic drainage
Looks like the branching pattern of tree roots
Trellis drainage
Looks like a trellis in a garden
Radial drainage
Develops around a central elevated point, like a volcano
Sediment flux in a river basin is a product of:
- Rates of sediment production (Supply)
- Rates of sediment movement (transport)
- Connectivity between supplies and transport components of the landscape.
Channel Adjustment Case Study: Brazos River, USA.
- Adjustments to restoration schemes
- Series of dam removals
- Incision caused by upstream knick point propagation
- Lower reaches of the Brazos are still yet to adjust
- Adjustments are spatially and temporally variable
Cuneo Creek: Case Study of Channel Response to sediment supply (Smith Pryor et all. 2011) 1
- Steep gravel-bedded stream in California (-3% slope)
- Series of human and natural disturbances:
- cleared for grazing and farming between 1946 and 1966
- large floods in 1955, 1964 and 1997
- Transitions between straight and braided planforms
- Combination of floods and land clearances caused widespread mass failures and sediment delivery to the channel
Cuneo Creek: Case Study of channel response to sediment supply (Smith Pryor et all. 2011) 2
- Major changes in sediment storage and transport rates during aggregational and degradational phases
- Using flume experiments Smith Pryor et al. modelled the effects of aggradation (high sediment flux) and degradation (low sediment flux) on sediment storage.
- Aggradation increased channel gradient
- Degradation decreased channel gradient
- HOWEVER, the response is non- linear (i.e. storage does not respond in a linear fashion to increased sediment flux).
- Variations in sediment transport were associated with changes in channel morphology
History of humans and rivers
Civilisations have always tried to tame the environment, building infrastructure to help build resilience against climatic variability.
Egyptian civilisation used the Nile for irrigation
Ancient Angkor built and intricate network of irrigation channels and reservoirs
Babylonians were renowned for their management of landscapes
