Environmental Engineering Reference
In-Depth Information
where ρ
g
is the
specific weight
of water (density × gravitational acceleration) and
Q
,
R
and
S
are standard parameters used above. Boundary shear stress multiplied by mean
velocity gives the
specific
stream power:
and is high (over 1 kW m
−2
) in steep, high-discharge rivers and low (under 100 W m
−2
) in
gentle, low-discharge rivers. This confirms the importance of channel efficiency in
establishing a
power threshold
, at which available power is just sufficient to overcome
friction resistance to mean water and sediment discharge. Below this threshold, the
stream deposits a proportion of its sediment load to restore efficiency. Above the
threshold, the stream actively erodes its channel.
CHANNEL NETWORKS
Principles of channel process and form extend from individual segments to the entire
river and, indeed, the drainage basin. Downstream changes discussed so far are applied
later to the geomorphological development of fluvial landsystems, after first reviewing
their significance for the basin-wide
network
. Drainage networks are organized systems
of channels which transfer water and sediment, in incremental amounts, through the
catchment via a definite sequence (e.g. rills → gullies → tributary river channels → trunk
rivers). Networks possess measurable order (hierarchy), density and pattern, and reflect
the principal catchment attributes, including stage of development and catchment shape.
Stream order
recognizes the nature and development of channel hierarchy as more
aggressive channels 'capture' neighbours. Having looked earlier at how rills focus flow
in a downslope manner, upslope and lateral development extend this simple model. Local
convergence of water in the channel, from (ideally) isotropic or
equipotential
throughflow or overland flow, causes
spring sapping
and
headward retreat
of the point
of initiation. This may continue via branching until the remaining catchment area is too
small to feed any additional channels (Figure 14.12). At its simplest, stream ordering
identifies all streams lacking tributaries as
first-order
streams and each sequential stream
by either arithmetic progression (2, 3, etc.) or as the sum of feeder streams. Shrève's
scheme is preferred to Strahler's, which fails to recognize the scale of increasing
discharge, although it is thought to generate a number of useful correlations between river
connectivity and the catchment (Figure 14.13).
Drainage density
,
D
d
, refers to the channel length (
L
) draining a unit area (
A
):
