Environmental Engineering Reference
In-Depth Information
Plate 14.5 Active gully erosion during a rainstorm on a
footpath subjected to compaction and removal of vegetation
by trampling on the North Yorkshire Moors.
Photo: Nick Musgrove.
water vapour into the atmosphere and tectonic uplift of the land surface, and subject to
downstream exponential energy decay (Chapter 13). It is resisted by friction, primarily at
the water-channel interface but also between water and solid sediment, individual
ribbons of flow within the river and with the atmosphere. Dynamic viscosity also
influences flow resistance, increasing directly with dissolved and suspended sediment
load and inversely with temperature. Water moves in one of two ways. Laminar flow
occurs at low velocities in shallow rivers with smooth channels, when the lowest water
lamina (thin layer) is retarded by channel boundary friction. Overlying laminae move
successively faster past each other with a velocity maximum ( v max ) at the surface (Figure
14.10). This rarely survives for long beyond the immediate boundary layer and breaks
down into turbulent flow at higher velocities, or at greater water depth and in irregular
channels. This eddy viscosity consumes energy as ribbons of water shear past each other,
creating more uniform velocities as faster and slower ribbons mingle (Plate 14.6).
Turbulence occurs at random at all scales but definite patterns occur within meandering
channels (see below), with a spiral or helical flow pattern and accompanying transverse
currents.
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