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Watershed Basics

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What is FGM?

FGM, or "fluvial geomorphology" is the study of how natural stream channels form.  This is then used for natural stream channel restoration after construction or urban expansion.  Its purpose is to return streams to their natural character, reestablish floodplains and riparian buffer zones, limit livestock access to streams and use bioengineering to stabilize soils and decrease stream bank erosion.

Why is it important?

Previously, hydraulic engineering techniques were used to design new stream routes.  This was done when a stream had to be relocated as a consequence of construction, usually that of a road, bridge, or some other population-mandated need. The new streams were designed to handle almost any storm possible, even the cataclysmic 100-year storm.  Unfortunately, this resulted in smooth, straight, high velocity channels that required extensive bank and bottom stabilization.  Concrete, rip-rap and other expensive materials were needed to reinforce the channels to prevent deterioration from the high velocity flow. 

When you stop to consider this technique, it is extremely costly, not only economically, but environmentally as well.  There is a reason in nature for why streams are designed as they are.  These man-made "channels" were very inhospitable to any aquatic organisms that tried to make a home in them.  Algae and macro invertebrates depend on the uneven surface provided by rocks, gravel and sand to provide areas for clinging and shelter.  The high velocity was also detrimental and washed away the any organisms that may have managed to find a tenuous foothold on the bank or bottom.

These channels also developed an unseen problem: sediment collection.  It is ironic that there must be constant maintenance work done to dreg out the sediment that collects in these channels considering the high flow rate of the water.

Changes made through FGM

Years of detailed studies of naturally-formed, stable streams resulted in the concept of FGM.  Through FGM, if s stream must be relocated, it is emulated in its new location as identically as possible.  It was found that stable streams are designed to handle the numerous 1-year and 2-year recurring storms (basically the typical thunderstorm).  There is much more sediment produced by these storms than a rare major-flood event.  natural streams are able to balance the sedimentation resulting from these frequent storms with natural bottom scouring (the removal of sediment) to prevent stream overflow and keep the stream running at a constant level and flow.  If a major-flood event occurred, the flood plains surrounding the stream were put into play to help reduce water velocity and catch sediment.

Using small "work-horse" stream designs, it is possible to prevent the sedimentation problems faced by the smooth channel designs.  Major flood overflow is handled by preserved flood plain vegetation.  If the stream runs under a bridge, culverts or flood flow pipes are placed in the fill at the height of the full-running stream.  Overflow is channeled through these openings into the flood plain on the other side.  The surrounding vegetation decreases the flow through these channels and prevents excess sedimentation.

Benefits of FGM

Employing FGM results in many benefits.  Economically, FGM is much more cost-effective.  Bridge construction is much easier, the number of flood flow pipes can be reduced, span area is minimized.  Less money needs to be spent on channel protection efforts such as concrete and rip-rap.  FGM can also preserve and even enhance the aquatic habitat as well as improving the aesthetics of the construction project.

For in depth information on FGM stream classification and FGM design, see Applied River Morphology by David Rosgen.

Pennsylvania's Keystone Stream Team has developed and published Natural Stream Channel Design Guidelines

A general overview of FGM can be obtained by viewing or downloading: Progress and the Environment from Skelly and Loy, Inc. Note: You must have Adobe Acrobat to view this file.