Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 282

Gradient
current

Wind shear; 12 h at gale force 7-8

t o

Wind shear setup

P re-storm
thermocline
at 9-10º

10º 11º 11.5º 11.9º

A static equilibrium is possible if
applied wind stress is balanced
by water surface elevation gradient
of magnitude u * /gh, where h is
depth and u * is wind shear velocity

9º

8º

Very high
thermal
gradients

7.5º

Storm gradients lie in range 10 -6 to
10 -7

1 km

10 m

Lake Windemere, Cumbria, UK

Fig. 6.62 Extreme wind shear causes isothermal displacement, density inversion, windward wave setup, and gradient currents.

There is a general anticlockwise circulaion.
This can be used in conjunction with the
more important winter circulation to
obtain mean annual vorticity, of great
practical use in applied environmental
engineering and pollution control

Fig. 6.63 Mean summer surface circulation in the Great Lakes of North America.

mass transport of surface water by wind shear, most effective
in very large and deep lakes (Fig. 6.63). The application or
disappearance of wind stress causes lake-surface and inter-
nal oscillations known as seiches , which may further mix
surface waters or subsurface stratified layers and cause
erosion and entrainment along shorelines.

Ganges. The magnitude of any channel may be described
in terms of its width, w , and depth, h , for bankfull flow.
The bigger the channel, the more water it can carry, so we
must also characterize channels according to the magni-
tude of the mean annual discharge, Q . Since the mean flow
velocity, u , in any channel is Q/wh , we have Q whu .

Expressing width, depth, and mean velocity of flow as
functions of the mean discharge, we can derive the basic
expressions of hydraulic geometry as w 6.7.3 Rivers and the hydraulic boundary layer aQ d , h bQ e ,

cQ f , where a u 1. The

magnitudes of the exponents and constants vary according
to different stream types and climatic conditions.
The remarkably smooth, concave longitudinal profiles
of most channels once they emerge from their bedrock val-
leys reflect a long-term ability of rivers to overcome initial
or imposed gradient irregularities. This can be readily under-
stood by noting that the downstream increase in discharge
associated with the stream network must be accompanied by
a downstream decrease in slope if equilibrium, that is,

b c 1 and d e f

The hydrological cycle ensures that some part of the pre-
cipitation that falls on the Earth's surface eventually finds
itself flowing as channelized runoff: river channels are con-
duits for the dispersal of weathering products derived from
their catchments and are highly sensitive indicators of tec-
tonic slope changes, sourceland geology, and climate.
River channels vary greatly in size, over more than four
orders of magnitude, from mere ditches to greater than
20 km wide lower reaches of the Brahmaputra and the