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Physics (also Earth and Environmental Sciences)

Sediment Entrainment and Channel Lifetimes in Braided Streams part of Vignettes:Vignette Collection

Braided streams are typically found on steep slopes (high energy environments) with an abundance of sediment. Their channels are constantly choking up with sediments and shifting. This gives rise to the concept of the lifetime of a channel, i.e., how long water continues flowing in it. The channel lifetime is known to be proportional to the square root of its length1. This vignette summarizes research that suggests that the origin of this relationship is in the mechanics of particle transport. We show that transport distances relate to particle sizes. Then, if the channel's length is proportional to how far sediment is transported in it, this argument succeeds. The process of sediment entrainment (lifting into the flow) initiates fluvial (water) and aeolian (wind) transport. The wide range of beautiful patterns of ripples and dunes, the dimensions and slopes of channels, the development of Sahara Desert dust storms, the sorting of sediments, as well as the dynamics of braided streams, derive from sediment transport processes. Understanding of entrainment began with Bagnold, an English engineer interested in the saltation (bouncing transport, Figure 1) of sand grains in Northern Africa. Bagnold's description of sediment entrainment derived from basic fluid dynamics and Newtonian mechanics of particles on the surface2. Consider Bagnold's argument. Turbulent flow near the earth's surface (in a stream, or the atmosphere) is described by a vertical velocity profile (Fig. 2), in which the mean velocity, <v> increases logarithmically with height, <v> = v*ln(y/y0) Here, v* is called the friction velocity, while y0 is a parameter understood as the bed roughness. The bed stress, a force per unit area which can be exerted on a particle to lift it into the flow, is given by (1/2)ρv*2, where ρ is the density of the fluid. The area presented by a spherical particle of diameter d is proportional to d2, and the lifting force, Flift, is then proportional to (1/2)d2ρv*2. If Flift is greater than or equal to the effective weight, Fg, of the particle, itself proportional to (ρs-ρ)gd3, the particle is entrained (Fig. 3). Here g is the acceleration due to gravity. An important result obtained from equating these two forces is that the volume d3 of the largest particle entrained in a given flow is proportional to v*6. Physics usually offers more than one way to understand a phenomenon and an alternate method based on energy3,4 rather than forces allows new interpretations. Consider this energy perspective. If the energy a particle absorbs from the fluid exceeds the gravitational potential necessary to lift it over its nearest neighbor, the particle is entrained into the flow. The advantage here is that one can easily address the issue of what happens to the entrained particle if the force is greater than necessary for entrainment. The excess kinetic energy may be converted to gravitational potential energy, and knowledge of that potential energy (how high the particle can be pushed) and an understanding of the process of settling in turbulent flow allows predictions of the total distance downstream a particle is carried before it falls back to the bed or ground. The following considers, for simplicity, a streambed with equal sized particles. A particle of volume d3 can absorb an energy (1/2)ρv*2d3 from turbulent flow3. If this energy exceeds (ρs-ρ)gd3(d), the particle is lifted off the bed (or ground). Equating the two energies leads to the identical result for the maximum particle size entrained found above. But now the height, h, a given particle can reach is found by setting its initial excess kinetic energy equal to its final potential energy4: (1/2)ρv*2d3 = (ρs-ρ)gd3(d+h) and solving for h. The distance that the particle is transported before it settles is approximately the product of v*t, where t is the time the particle takes to settle back to the bed. In turbulent flow t has been given as Using this result and the frequency, v*/d, such a particle is likely to be entrained, it is possible to calculate an effective rate of transport of particles as a function of their diameter4. The combination of variables (v*2/gd) can be put into the following form, (v*2/gH) (H/d), where H is the depth of the stream and v*2/gH is the square of the (unitless) Froude number, Fr. Fr2 is the ratio of stream kinetic to potential energy. When Fr=1, these two energies are equal and the flow is termed critical. The Froude number is also relevant to wave propagation in a stream. When Fr < 1 (Fr > 1), surface wave propagation is downstream (upstream), but at critical flow standing wave patterns (Fig. 4) develop5,6. Constant Fr with constant g implies the existence of a constant ratio of length to time squared in the remaining factors, e.g., v* and H. It seems no coincidence that the relationship of channel length to channel lifetime squared1 is the same ratio of units required by constant Fr. In fact, if the length of a channel tends to be proportional to the typical transport distance of the particles found in it, the above formula for the distance of transport of particles in turbulent flow can be used to show that the lifetime squared of a channel is proportional to its length7.