Chapter 6 Flashcards
River
A river is water on the surface of the earth moving downhill in a channel due to gravity.
(4) A river will always try to:
- flow downhill
- find the easiest way downhill
- reduce its gradient
- carry the maximum amount of sediment
(4) Features of processes used to describe a river:
- number, size and shape of the channel(s)
- discharge (Q)
- sediment load
- gradient
How is the shape of a river described?
Shape is described using the ratio of width to depth. (w/d)
Discharge equation
V x W x D
Sediment Load
This is the sediment (solid particles and elements in solution) that are moving or may move.
Gradient
The slope of the surface of the water or channel bottom
(4) RIVER/FLUVIAL PROCESSES
- Weathering
- Erosion
- Transport
- Deposition
Weathering as a fluvial process
Solution weathering is important in many river environments.
Physical weathering is less important.
Erosion in a river
THE IMPACT OF
- moving water
- sediment that is already moving
(Together these two processes are ABRASION)
- Hydraulic lift
Hydraulic lift
The result of an increase in velocity and a decrease in pressure above a grain as water flows around it.
Fluvial Transport
How sediment already in motion is moved by a river.
Moves of Fluvial Transport
- Solution
- Suspension
- Saltation
- Traction
SOLUTION
The way in which minerals dissolve in water.
SUSPENSION
The finest/smallest solid particles that are carried in the water. (This sediment rarely makes contact with the channel bottom.)
SALTATION
Larger sediment grains bounce along the bottom making frequent but only short term contact with the channel bottom.
TRACTION
The largest sediment in motion. These grains roll or slide along the bottom. (too heavy!)
TRACTION + SALTATION
BEDLOAD
Discharge
Volume of flow in a river that passes by a given point in a given unit of time. (cubic metres per second)
Critical/Threshold Velocity (see diagram)
The water velocity necessary to erode (or set a grain in motion).
Why are very small grain sizes (<0.002mm) harder to erode?
- Because they often contain clay minerals (sheet silicates).
- Clay minerals are flat, sheet-like grains that have an electric charge on their surface.
- Because they have a large surface area compared to volume.
- This means that clay minerals stick together (are cohesive) and are hard to erode.
When does deposition of solid particles occur?
when water velocity drops
Clay minerals
Clay minerals are sheet silicates. They are flat, sheet-like grains that have an electric charge on their surface.
Base Level
l?evel below which the stream cannot erode its valley. The lowest point that the river tries to reach.
Deposition
In a river the sediment stops moving when the velocity drops. (However, sediment in solution is deposited when precipitation occurs)
Precipitation usually occurs when…
Water is removed by evaporation and concentration increases.
Sorting
Range of Sizes
Mafic grains are more prone to..
weathering
How does velocity vary in a river and why?
?
What is the effective or dominant discharge?
The volume of water moving in the channel that does the most work. Work may be measured by how much sediment is moving.
Bankfull discharge
the volume of water in the channel when the channel is full.
?
Deposits rather than erodes because velocity is so low
Threshold velocity
necessary velocity needed to set a particular grain in motion
Precipitation in a river
?? Will deposit that sediment load in solution. This happens when you get a chemical reaction, or when elements in solution combine to form a solid mineral. Increased concentration in that river usually happens when evaporation occurs.
How is the river different from the top to the bottom?? ???
- the gradient is steeper : higher velocity at top
- size: smaller at top:
- shape/ratio: higher at top
- sediment load: coarser sediment at the top
- more water at the bottom
Grain size:
gets smaller. big stuff gets left behind because velocity drops because of gradient. breaking apart
Sorting:
Gets better sorter
Roundness:
They get more round
Sphericity
Get more spherical
Composition
more quartz because mafic minerals are heavier and get left behind. more easily eroded and more reactive, so they make them weaker.
Most common mineral at B: quartz: because common and it is resistant
Bed roughness
The irregular shape of the channel, bottom and sides.
smooth bottom: little in the way of roughness
bumpy bottom: significant bed roughness
Bed roughness is important because..
it changes water movement It changes water velocity and direction, in space and time.
Bed roughness creates turbulent flow
It will also “hide” some grain sizes preventing them from being eroded.
Turbulent flow
Water in the channel flows in irregular pathway with much vertical and lateral mixing. The most common type of flow and is caused by: friction.
Capacity
A measure of the volume of sediment a river can carry.
Competency
A measure of the maximum grain size a river can carry.
Laminar Flow
The water in the channel moves in parallel layers with no vertical or lateral mixing.
It is very rare. When it does exist it is only for a short period of time in a limited space.
Laminar Sublayers
The laminar sublayer is a layer of water, usually a very thin layer, adhering to the bottom of the channel.
This is a layer of water that has no velocity - it does not move because of friction with the channel bottom
Relationship between laminar sublayer and sediment erosion.
Depending on the thickness of this layer and the size of sediment, sediment on the bottom may or may not move.
Why is turbulent flow the most important and common type of flow?
Because of friction. Friction with the bottom that is affected by bed roughness and friction within the water itself.
Different types of channel
Bedrock channel (upper channel) Alluvial channel (lowland channel)
Bedrock channel
- Defined by the local bedrock
- Little to no classic sediment in the channel
- Found: higher in the drainage basin
- No sediment load
- Ability of the discharge to remove sediment is greater than the supply of sediment.
- No flood plain (lacks the space to store sediment)
Alluvial Channel
- Defined by the clast sediment in the river
- In this channel there is abundant sediment
- Found: lower in the drainage basin
- Sediment load exceeds the ability of the discharge to remove it.
- Flood plain (room to store sediment)
Briefly explain the relationship between bedrock erosion and sediment load?
Sediment load in a bedrock channel is often absent. However, it is the presence f moving sediment in the channel that is most effect at eroding the channel. If there is too little sediment the channel is not eroded. If there is too much sediment then some of it will remain in the channel forming a deposit that shields the bedrock preventing erosion - what is needed is the right volume of sediment so that it can all move.
