In hydrology stream competency, also known as stream competence, is a measure of the maximum size of particles a stream can transport. The particles are made up of grain sizes ranging from large to small and include boulders, rocks, pebbles, sand, silt, and clay. These particles make up the bed load of the stream. 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. This refers to the stream bed velocity which is difficult to measure or estimate due to the many factors that cause slight variances in stream velocities. Stream capacity, while linked to stream competency through velocity, is the total quantity of sediment a stream can carry. Total quantity includes dissolved, suspended, saltation and bed loads. The movement of sediment is called sediment transport. 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. Gravity acts upon water to move it down slope. Friction exerted on the water by the bed and banks of the channel works to slow the movement of the water. When the force of gravity is equal and opposite to the force of friction the water flows through the channel at a constant velocity. When the force of gravity is greater than the force of friction the water accelerates. This sediment transport sorts grain sizes based on the velocity. As stream competence increases, the D50 (median grain size) of the stream also increases and can be used to estimate the magnitude of flow which would begin particle transport. Stream competence tends to decrease in the downstream direction, meaning the D50 will increase from mouth to head of the stream.
Importance of Velocity
Stream Power Stream power is the rate of potential energy loss per unit of channel length. This potential energy is lost moving particles along the stream bed.
Ω = ρ w g Q S {\displaystyle \Omega =\rho _{w}gQS}
where Ω {\displaystyle \Omega } is the stream power, ρ w {\displaystyle \rho _{w}} is the density of water, g {\displaystyle g} is the gravitational acceleration, S {\displaystyle S} is the channel slope, and Q {\displaystyle Q} is the discharge of the stream. The discharge of a stream, Q {\displaystyle Q} , is the velocity of the stream, U {\displaystyle U} , multiplied by the cross-sectional area, A C S {\displaystyle A_{\mathrm {CS} }} , of the stream channel at that point:
Q = U A C S {\displaystyle Q=UA_{\mathrm {CS} }}
in which Q {\displaystyle Q} is the discharge of the stream, U {\displaystyle U} is the average stream velocity, and A C S {\displaystyle A_{\mathrm {CS} }} is the cross-sectional area of the stream. As velocity increases, so does stream power, and a larger stream power corresponds to an increased ability to move bed load particles.
Shear Stress and Critical Shear Stress In order for sediment transport to occur in gravel bed channels, flow strength must exceed a critical threshold, called the critical threshold of entrainment, or threshold of mobility. Flow over the surface of a channel and floodplain creates a boundary shear stress field. As discharge increases, shear stress increases above a threshold and starts the process of sediment transport. A comparison of the flow strength available during a given discharge to the critical shear strength needed to mobilize the sediment on the bed of the channel helps us predict whether or not sediment transport is likely to occur, and to some degree, the sediment size likely to move. Although sediment transport in natural rivers varies wildly, relatively simple approximations based on simple flume experiments are commonly used to predict transport. Another way to estimate stream competency is to use the following equation for critical shear stress, τ c {\displaystyle \tau _{c}} which is the amount of shear stress required to move a particle of a certain diameter.
τ c = τ c ∗ ( ρ s − ρ w ) g d 50 {\displaystyle \tau _{c}=\tau _{c}^{\ast }(\rho _{s}-\rho _{w})gd_{50}}
where:
τ c ∗ = {\displaystyle \tau _{c}^{\ast }=} Shields parameter, a dimensionless value which describes the resistance of the stream bed to gravitational acceleration, also described as roughness or friction,
ρ s = {\displaystyle \rho _{s}=} Particle density, and ρ s − ρ w {\displaystyle \rho _{s}-\rho _{w}} is the effective density of the particle when submerged in water (Archimedes principle).
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