|
|
||
 Fundamentals of Fluvial Geomorphology and Channel Processes
If we change a river we usually do some good somewhere and
"good" in quotation marks. That means we achieve some kind of a result
that we are aiming at but sometimes forget that the same change which we
are introducing may have widespread influences somewhere else. I think
if, out of today's emphasis of the environment, anything results for us it is
that it emphasizes the fact that we must look at a river or a drainage basin
or whatever we are talking about as a big unit with many facets. We
should not concentrate only on a little piece of that river unless we have
some good reason to decide that we can do that.
2.1.1.2 The System is Dynamic
In each of the idealized zones described above, a primary function is listed. Zone 1
is the sediment source that implies that erosion of sediment occurs. Zone 2 is the transfer
zone that implies that as rainfall increases soil erosion from the watershed, some change must
result in the stream to enable transfer of the increased sediment supply. Zone 3 is the zone
of deposition and change must occur as sediment builds in this zone, perhaps the emergence
of wetland habitat in a lake then a change to a floodplain as a drier habitat evolves. The
function of each zone implies that change is occurring in the system, and that the system is
dynamic.
From an engineering viewpoint some of these changes may be very significant. For
example, loss of 100 feet of stream bank may endanger a home or take valuable agricultural
land. From a geomorphic viewpoint, these changes are expected in a dynamic system and
change does not necessarily represent a departure from a natural equilibrium system. In
planning stabilization measures, we must realize that we are forced to work in a dynamic
system and we must be try to avoid disrupting the system while we are accomplishing our
task.
2.1.1.3 Complexity
Landscape changes are usually complex (Schumm and Parker, 1973). We are
working in a system and we have defined a system as an arrangement of things to form a
whole. Change to one portion of the system may result in complex changes throughout the
system.
When the fluvial system is subjected to an external influence such as channelization
of part of a stream, we can expect change to occur throughout the system. Channelization
usually increases stream velocity and this would allow the stream to transfer more sediment,
resulting in erosion upstream and deposition downstream of the portion of the stream
channelized. For example, some Yazoo Basin streams in north Mississippi that were
channelized in the 1960s responded initially, but an equilibrium has not yet been reestablished
as repeated waves of degradation, erosion, and aggradation have occurred.
5
|
||
|
||
