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Forum—Hydraulic Lifelines for
Soil, Water, and People
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In the United States, the drought of the 1930s
was followed by periodic flooding in the 1940s and early 1950s. These events,
plus the postwar baby boom, its accompanying housing boom, and other
developments, made America's agricultural community keenly aware of its
responsibility for stewardship of the soil and water resources needed to meet
increasing demands for food and fiber.
USDA responded in a number of ways to allow farmers and others to maintain
future resources while increasing food and fiber production throughout the last
half of the 20th century.
Federal research on hydraulic structures and engineered stream channels has
played a major role in America's food success story and our farmland's
sustainability. In particular, the ARS
Hydraulic Engineering Research Laboratory at Stillwater, Oklahoma, is
recognized worldwide for modeling, designing, and evaluating hydraulic
structures.
This year, the lab celebrates its 60th anniversary. Throughout its history, ARS
hydraulic research has played a major role in developing the knowledge and
procedures for successfully and economically designing and constructing
different structures for alleviating flooding and controlling erosion.
The 1930s-era droughts across the central United States made clear the need to
keep water and soil in place. To this end, many thousands of acres of cropland
have been terraced. Most terraces are drained by grassed waterways designed to
convey excess water to stream channels without erosion. An estimated
half-million miles of these channels have been constructed in the United States
and elsewhere. Most, designed over the past 50 years, use engineering criteria
developed and tested at Stillwater.
Natural processes, changes in land use (especially from development), and
dredging combined to make some stream channels unstable. Eroding streambeds and
banks destroy land and move excess sediment downstream. Stream-stabilizing
structures are often required to control bed erosion and let trees, grasses,
and other vegetation anchor the banks. These measures, properly applied, allow
people to productively share the floodplain with the rest of the ecosystem.
Floods can cause extreme destruction to cropland and river environments. In
many areas, especially the central United States, seasonal thunderstorms can
dump very large volumes of water on small watersheds.
To protect lives, property, and transportation and communication systems, the
nation has invested about $14 billion in an infrastructure of upland
flood-control reservoirs built with assistance from USDA and similar watershed
programs. The purpose of these reservoirs is to temporarily contain floodwater
and release it at a rate the downstream channel can hold.
Each year, the return on this investment—in preventing property and crop
losses—is estimated to exceed $800 million. Besides reducing flooding and
sedimentation, the reservoirs provide recreational fisheries, wildlife habitat,
and wetlands above the containment.
Each upland flood-control reservoir consists of multiple components. Each
component is a hydraulic structure that must operate properly if floodwaters
are to be controlled.
Flow to the downstream channel is conveyed by a principal spillway—usually
a pipe—through the reservoir. Often, trash racks are needed to prevent
floating flood debris from clogging the pipe's inlet. ARS scientists have
designed, developed, and tested many inlets and trash racks used worldwide.
To prevent damage to a floodplain below the reservoir, the tremendous energy of
the large volume of water released from a reservoir must be dissipated before
it enters the downstream channel. This is done with a stilling basin at the
spillway's outlet. The most common type of stilling basin is the riprap-lined
plunge pool into which water drops as it exits the pipe. Both the pool and its
protective lining of stone riprap must be properly sized.
It is generally impractical to provide reservoir storage for extremely large
and infrequent floods. On most watershed flood-control structures, an auxiliary
spillway safely diverts these large flows around the dam. These spillways are
usually wide, steep, grass-lined channels. The channels must be properly sized
to convey the maximum amount of water without eroding the spillway and causing
it to fail. The Stillwater laboratory has developed exacting design criteria
for these vegetated channels.
Still, in some instances a vegetated spillway is impractical. Instead,
structural spillways—usually concrete-lined channels—are used. These
spillways must be properly designed to dissipate the water's energy before the
flow returns to the downstream channel.
Many stilling basin designs are available. But the Saint Anthony Falls stilling
basin, developed more than 50 years ago by USDA hydraulic engineer Fred W.
Blaisdell, is still recognized worldwide as one of the smallest, most
efficient, and most economical designs for dropping water level.
Thousands of flood-control structures were built from the late 1940s through
the 1990s. Many were designed for 50 years of service and are near the end of
their planned life. Rehabilitating and upgrading them poses new technical
challenges. ARS scientists will meet these challenges while protecting the
environment and agriculture.
Darrel M. Temple
Research Leader, Hydraulic
Engineering Research Laboratory
Stillwater, Oklahoma |
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"Forum" was published in the
February 2000
issue of Agricultural Research magazine.
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