Environmental Engineering Reference
In-Depth Information
reached, the form or shape of the landform, vegetation community or soil is constant as
long as the basic controls remain constant. If inputs and outputs remain constant the
system will reach a steady state .
Equilibrium is often regarded as an ideal theoretical state, rarely found in the real
world, as the controlling variables are in a constant state of flux. Climate will fluctuate on
an annual basis, as well as over several years, and also in trends over decades or longer-
term periods. Human activity brings about land-use changes, and, again, this can be
through annual changes or longer-term trends. This dynamism does not make the
equilibrium concept wrong, but it means that equilibrium is critically dependent upon the
time interval being considered.
It is convenient to envisage three different types of time interval, namely cyclic time,
graded time and steady time . Cyclic time changes over long time periods, possibly
millions of years. Changes are progressive in the average state of the system to give a
condition of dynamic equilibrium . Changes in landforms over possibly 100 to 1000 years
occur over graded time . Because of negative feedback, the system is maintained in a
constant condition. In the short steady-time intervals of days and months, most elements
in the landscape are unchanging. Steady-state equilibrium exists over steady time, whilst
dynamic equilibrium occurs over cyclic time (Figure 1.7).
Environmental systems are complex systems in which the relations between variables
are characterized by multiple feedback. Some elements of the system respond rapidly to
changes in external controls, whilst other elements will change only slowly. For example,
in the alluvial channel system of the river Wharfe the channel shape, channel bed forms
and flow velocity adjust quickly to hydrological changes in the catchment, whilst other
characteristics of the catchment, for example valley slopes and channel pattern, take
much longer to adjust. Hydrological changes are often the result of changing land use in
the catchment.
The value of 'time' in studying the environmental system lies also in the causative
factors which are sought to explain the landscape. Factors which govern the landscape
have a different relative importance depending upon the time scale being used. This can
be illustrated by reference to fluvial geomorphology. Rivers usually have long profiles
which are concave in shape and show a fining of sediment size downstream. The median
grain size normally decreases as a result of the attrition and abrasion of particles in bed
load and suspended load. It is also usual for stream discharge to increase downstream as
the catchment area of the stream increases with distance from the source.
The explanation of these downstream relationships in rivers is that the river channel is
evolving so that its discharge and sediment load remain in equilibrium. Over short time
scales of hundreds of years, i.e. over graded time , a 'hydraulic geometry' approach can
be used to explain how a river maintains its equilibrium. Particular discharges and
channel gradients have the potential to mobilize particular sediment sizes. As a result of
this, downstream fining of bed load and suspended load occurs,
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