Environmental Engineering Reference
In-Depth Information
expensive alternative.
STREAM FLOW IN CHANNELS
Gravity-induced overland flow is both unstable and inefficient, since land surfaces are
neither homogeneous nor frictionless. Differences in surface micro-relief and material
properties soon concentrate water in parts of the sheet flow at the expense of others.
Erosion commences where the
erodibility
of the surface and
erosivity
of the flow
combine to exceed shear strength. Conversion of sheet-to-channel flow is attributed to a
stochastic
process of random events in a time-dependent sequence and can be seen in the
smallest
rills
, 10
1-3
mm deep and wide. Incision is controlled by specific material-energy
conditions at every turn but the preferred points of concentration vary randomly from one
event to the next. Channels are also initiated where subsurface throughflow converges at
the surface - in slope concavities, for example. Incision creates subsidiary, steeper slopes
which then draw adjacent flow into the rill. This increased proportion of
channelled
water
reduces overall surface-water friction losses. Rills are transient features and may be
infilled by sediment and vegetation, or continue to focus episodic flow and develop at the
expense of neighbours into more enduring
gullies
, an order of magnitude larger (Plate
14.5). The ever-increasing downstream focus of discharge and kinetic energy on the trunk
stream is both inevitable and essential if stream flow and sediment discharge are to
overcome the parallel decline in potential energy. The search for
hydraulic efficiency
begins at the watershed and remains with flowing water throughout the catchment to the
sea via a
drainage network
of successively larger rills, gullies and river channels.
RIVER FLOW
We return to the network later but first need to understand the movement of water in river
channels. It is driven by gravitational energy, acquired through the elevation of
