Stream competency
[1] The particles are made up of grain sizes ranging from large to small and include boulders, rocks, pebbles, sand, silt, and clay.
Stream competence was originally simplified by the “sixth-power-law,” which states the mass of a particle that can be moved is proportional to the velocity of the river raised to the sixth power.
Total quantity includes dissolved, suspended, saltation and bed loads.
Initiation of motion involves mass, force, friction and stress.
Gravity and friction are the two primary forces in play as water flows through a channel.
[4] This sediment transport sorts grain sizes based on the velocity.
Stream power is the rate of potential energy loss per unit of channel length.
[7] This potential energy is lost moving particles along the stream bed.
As velocity increases, so does stream power, and a larger stream power corresponds to an increased ability to move bed load particles.
Flow over the surface of a channel and floodplain creates a boundary shear stress field.
Although sediment transport in natural rivers varies wildly, relatively simple approximations based on simple flume experiments are commonly used to predict transport.
[8] Another way to estimate stream competency is to use the following equation for critical shear stress,
which is the amount of shear stress required to move a particle of a certain diameter.
, is directly proportional to both the depth of water and slope of stream bed (flow and velocity), and inversely proportional to Shield's parameter and the effective density of the particle.
Velocity differences between the bottom and tops of particles can lead to lift.
The difference in velocities results in a pressure gradient that imparts a lifting force on the particle.
Turbulent flows result velocities that vary in both magnitude and direction.
These erratic flows help keep particles suspended for longer periods of time.
[7] Another important property comes into play when discussing stream competency, and that is the intrinsic quality of the material.
In 1935 Filip Hjulström published his curve, which takes into account the cohesiveness of clay and some silt.
[12] By observing the size of boulders, rocks, pebbles, sand, silt, and clay in and around streams, one can understand the forces at work shaping the landscape.
Ultimately these forces are determined by the amount of precipitation, the drainage density, relief ratio and sediment parent material.
[7] They shape depth and slope of the stream, velocity and discharge, channel and floodplain, and determine the amount and kind of sediment observed.
This is how the power of water moves and shapes the landscape through erosion, transport, and deposition, and it can be understood by observing stream competency.
Differences in bedrock will affect the general slope and particle sizes in the channel.
[6] Slight variations in underlying material will affect erosion rates, cohesion, and soil composition.
Vegetation has a known impact on a stream's flow, but its influence is hard to isolate.
A high root density will result in a reinforced stream channel.
Vegetation that gets caught in the middle of a stream will disrupt flow and lead to sedimentation in the resulting low velocity eddies.
When these channels are closed, or opened in the case of a breach, the flow characteristics of the stream are disrupted.
