THE LOUISIANA CIVIL ENGINEER - lasce

Case for natural channel design in Louisiana
By Lee W. Forbes, PE
The concept of natural channel design
(NCD) only recently has been introduced to the
water resources engineering community in
Louisiana. Understandably, our water resource
infrastructure improvement efforts have been
focused on immediate needs such as coastal wetland
restoration, drainage, and flood control.
There has been little opportunity to examine
more closely the effectiveness and the long-term
sustainability of Louisiana’s extensive and intricate
network of natural and improved conveyance
channels. They are a seemingly boundless
collection of
• rivers
• bayous
• passes
• creeks
• cuts
• canals
• coulees
• swales
• ditches and
• stormsewers
that handle and convey the vast amounts of surface
waters with which Louisiana is both blessed
and cursed.
Traditional flood control and drainage
improvement design processes in Louisiana have
focused on maximized conveyance to relatively
unconfined outfalls such as the lakes and larger
rivers, or to constructed regional detention areas
and basins. These designs are rooted in attempts
to employ channel configurations of the simplest
geometric forms. This reasonably ensures that
the flows through the resulting structures during
larger storm events can be predicted accurately
and thereby most effectively minimize the potential
for flooding. However, such channel designs
fail to recognize and manage the intrinsic variability
associated with storm flows that — as a
result — cannot be accommodated adequately
by the resulting engineered structures. It will be
attempted herein to demonstrate that it is incumbent
on the engineer responsible for water
resource design to understand and embrace a
new paradigm — natural channel design. This
is especially true here in Louisiana with our
extensive and intricate network of conveyance
channels.
Natural channel design
Natural channel design is an attempt to provide
water resource improvements in — and
restore impaired streams as shown in Figure 1 to
— a state of natural channel stability or dynamic
equilibrium. This is done through the understanding
of fluvial morphology and its application
via hydraulic design methodologies. Fluvial
geomorphology is the science of the formation of
riverbed, flood plain, and stream forms by the
action of water. Though it has only become recognized
in the United States in the last 10 years,
this science is a vital component of water
resources management and it is long established,
well developed and documented through the
extensive studies and writings of Luna B.
Leopold, M. Gordon Wolman, E.W. Lane, David
L. Rosgen, Richard Hey, and other hydrologists
and fluvial geomorphologists.
The schematic balance shown in Figure 2,
developed by E.W. Lane (after Lane, 1955), presents
an analogous, qualitative model of the predominant
factors affecting stream equilibrium
and the natural response of a channel to alterations
in these factors in the form of degradation
and aggradation. River systems express an
inherent and entropic tendency toward minimum
total work and uniform work rate in response to
the natural variations of water and sediment
loads that enter the system.
Natural channel stability is characterized by
the ability of a stream — over time and in its
present climate — to transport the natural variation
of water and sediment loads produced by
its watershed in a manner that it maintains its
Figure 1. Post-construction scene of a typical stream restoration project.
(Image courtesy of Fuller, Mossbarger, Scott, May Engineers.)
Figure 2. Lanes relationship of stream stability and the factors that
affect degradation and aggradation (After Rosgen, 1996).
Lee W. Forbes, PE, earned his BS degree in petroleum engineering from LSU in 1985 and his MS degree in civil engineering from LSU in 1988 with
emphasis in environmental engineering. Forbes is a licensed engineer in Louisiana, Texas, Arkansas, North Carolina, and Michigan. Currently employed
as a Senior Engineer with Geomatrix Consultants in Houston, Texas, he has over 15 years’ experience in private practice with emphasis on environmental
assessment and remedial design. In the mid-1990s, Forbes shifted his focus to natural channel design and sustainable watershed management with
his work on several landmark projects in Michigan, full-delivery stream restoration projects in North Carolina and the development of natural channel
design methodologies for the Gulf Coast region in Louisiana and Texas. He currently serves on the national ASCE Technical Committee for Urban Stream
Restoration where he endeavors to integrate the disciplines of natural channel design into conventional engineering practice to improve and sustain water
resources.
THE LOUISIANA CIVIL ENGINEER / MAY 2006 5