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Holding Back Floodwaters
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Aerial view of the ARS Hydraulic Engineering Research Laboratory facility with
Lake Carl Blackwell in the backgraound. An earthen dam running along the
shoreline separates and protects the facility from the lake,
which is at a higher elevation.
(K8739-2) |
From a distance, the 50-acre
ARS Hydraulic Engineering Research
Laboratory in Stillwater, Oklahoma, looks more like an archeological excavation
than a world-class research facility. The laboratory and its many outbuildings
and grounds are covered with all types of hydraulic structures. Hydraulics is
the science that deals with the motion of water and other liquids.
Recognized worldwide for modeling, designing, and engineering hydraulic
structures for agriculture, the Stillwater facility today specializes in
studying how water erodes spillways and forms gaps in earthen dam walls or
embankments. The lab is set to play one of its greatest roles—assisting in
the rehabilitation and revitalization of thousands of earthen dams in the
United States.
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This rock chute safely transfers
runoff to a lower elevation.
Near the top of the loose-rock
structure, agricultural engineer
Kem Kadavy (left) and hydraulic
engineer Kerry Robinson monitor
its performance.
(K8735-3) |
"The 3,000-acre Lake Carl
Blackwell located next to the laboratory provides the water used for simulating
and testing hydraulic structures," says the ARS laboratory's research
leader Darrel M. Temple. "The facilities are ample for conducting
full-size prototype tests of many hydraulic structures, like full-scale
vegetated channels."
Such a readily available and abundant water supply allows the laboratory's
scientists to generate the high waterflow rates—up to 130 cubic feet per
second—needed to simulate the forces that cause holes to form in earthen
spillways and embankments.
"The laboratory's unique topography and proximity to the lake make it
possible to deliver these high rates of waterflow," he says.
Water used for experiments can be diverted to the outdoor laboratory or to one
of three buildings where the ARS scientists model and test hydraulic
structures.
"Designs for hydraulic structures are typically tested in one of the
buildings that have various-sized channels," he says. "After we
fine-tune the scale model indoors, we often build and test it outdoors under
conditions that more closely match field conditions."
Of current concern to the Stillwater lab are the nation's earthen dams.
America's countryside is dotted with these small dams. These earthen dams
protect the watershed. Many supply water to municipalities. They also prevent
floods; provide water for irrigation, recreation, fish and wildlife habitats,
and groundwater recharge; and improve water quality.
"Each year," says Temple, "these reservoirs provide Americans
with more than $800 million in benefits." |
Research leader Darrel Temple examines graphic output from SITES, a software
program used for predicting the performance of spillways and designing new
structures.
(K8740-1) |
Dams store and trap sediment and
prevent excess runoff from damaging land downstream. As dams age, they can fill
up with sediment and become unsound structurally, says Temple, who has spent 23
years as an ARS hydraulic engineer working at the Stillwater laboratory.
Temple says, "Many of the more than 10,000 flood-control structures in the
United States were designed to have a 50-year service life. About two-thirds of
them were designed before 1962 to protect communities and rural lands
downstream."
Today, these upstream flood-control dams are in urgent need of revitalization
and rehabilitation. Over the next 10 years, more than a thousand will need
significant repairs and modification. Temple says that "many no longer
work as efficiently as they should. That's mainly because of aging, sediment
filling, and changes in what the land has been used for. These conditions could
not have been anticipated during design and construction."SITES
Software Program
The expertise and database amassed by the ARS laboratory over its 60 years of
research are needed to rehabilitate these dams. And USDA's Natural Resources
Conservation Service (NRCS) will work cooperatively with the Stillwater lab to
develop technologies for rehabilitating and revitalizing the dams.
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A low-drop grade-control structure is used to stabilize the stream or channel
bed where an abrupt drop is needed. Here, agricultural engineer Kem Kadavy
(left) and hydraulic engineer Kerry Robinson measure bed profiles in riprap
(loose rock) upstream in a sealed-down model of the structure.
(K8736-2) |
The software program documenting
these technologies is called SITES (not an acronym). "SITES combines the
principles of geology, hydrology, soil science, and physics to predict the
performance of spillways—both principal and auxiliary," says Temple.
"The current software program predicts how an earthen spillway will
perform and evaluates its potential for failure. Future versions will
incorporate current research that will predict the damage that results from
embankment overtopping. The likelihood of an earthen dam failing will be
determined from the erosion that occurs on the downstream face of the
dam." Adding to the urgency of this problem is the increased risk to life
and property from changes in upstream and downstream land use since the dams
were constructed.
"When these dams were built, most of the surrounding areas were
rural," he says. "As communities expanded and homes were built in
known flood plains, the system of reservoir dams—originally intended to
protect crops and farmland—now safeguards houses, property, and
lives."
A High Price To Pay
When dams fail, land is often devastated, homes and roads can be destroyed, and
people can die.
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Hydraulic engineer Greg Hanson
conducts a jet test to evaluate
soil erodibility.
(K8738-1) |
Temple remembers one
example of dam failure that occurred in May 1983 at Black Creek in Mississippi.
The dam failed because of erosion in the emergency spillway. The flood that
caused the failure was generated by heavy rainstorms that dumped 14 inches of
water on the watershed.
"The spillway was originally 5 feet deep and 100 feet wide. The reservoir
was full of water before the dam failed. After the spillway breached, it was
about 190 feet wide and 40 feet deep. The reservoir drained, releasing 4,500
acre-feet of water at a peak rate of 9,000 to 12,000 cubic feet per second to
the downstream flood plain," says Temple.
He adds, "Luckily, no homes were immediately downstream of the reservoir.
No loss of life occurred. However, the water and sediment released caused
substantial damage downstream."
A county bridge about 1-1/2 miles downstream washed out, and the roadway was
severely damaged. Two downstream ditch bank levees were breached. The water
caused massive erosion and sediment deposits in agricultural fields.
"If a breaching like this occurred today in a reservoir where housing had
encroached on the downstream flood plain, loss of life would be likely,"
he says.
"The Mississippi dam failure drives home the point that dams are a vital
part of our nation's infrastructure, like roads, bridges, and sewage-treatment
plants," says ARS hydraulic engineer Gregory J. Hanson, who has worked for
15 years at the Stillwater laboratory. "An estimated $8.5 billion
infrastructure in 1997 dollars has been developed in the form of these earthen
dam structures," he says.
"As ARS scientists, we work closely with and respond to the needs of NRCS
nationally," Temple says. "The SITES software project is the most
recent example of this cooperation and response."
Developed from both ARS laboratory studies and field data, the SITES software
now provides engineers with a tool for analyzing and predicting the performance
of vegetated earth spillways like those used on many watershed flood-control
reservoirs.
"SITES is a model used for designing new structures or for analyzing how
existing structures perform," he says. "The model considers how
runoff from rainstorms affects the structures and the performance and failure
potential for vegetated spillways. Future refinements will allow us to evaluate
performance and failure potential for dams overtopped by extreme flood
flows."
How SITES Works
The SITES software evaluates spillway surface and subsurface conditions and
determines the location and nature of the erosion posing the greatest risk of
failure.
"New technology is incorporated into the model, such as the use of an
index—called a headcut erodibility index—to describe the resistance
of the exposed geologic materials to erosive attack during the latter stage of
the erosion process," says Temple.
The SITES software also uses mathematical curves for calculating waterflow
rates of vegetated auxiliary spillways. These curves take into consideration
the flow-retarding effects of vegetation in the spillway.
"These capabilities place the SITES software on the leading edge of
technology of earth spillway design and analysis," he says. "The
spillway erosion prediction portion of SITES is based, in part, on data
cooperatively obtained by the long-term joint effort of ARS and NRCS,"
says ARS hydraulic engineer Kerry M. Robinson. He has worked at the Stillwater
laboratory for 17 years.
Started in 1983, the SITES project was aimed at gathering data from field
spillways that had significant waterflow or damage or both. At the same time,
ARS scientists at the Stillwater laboratory began large-scale lab and field
studies to examine the processes associated with the erosion and overtopping of
earthen spillways.
"The SITES water resources site analysis software, released by NRCS in
early 1997, significantly changed that agency's approach to analyzing
spillways," says Temple.
"We're presently refining the hydrologic criteria—the quantity of
rainfall used for dam and spillway design—for use with this
software."
Since SITE's 1997 release, the two federal agencies have been cooperating with
Kansas State University, Manhattan, to simplify how the model can be applied.
An advanced graphic program assists users in applying the model by guiding them
through the needed model inputs and graphically displaying the results of
computations on the monitor of the computer.
"For example," Temple says, "if a user changes the width of the
spillway, the impact of that change will automatically show up in other aspects
of design and performance—sort of a ripple effect."
He says, "This will allow the user to conveniently compare and contrast
the technical implications of various design alternatives. It includes display
screens for data entry and for both text and graphical output."
At the present time, this enhancement—or interface—applies only to a
reservoir site or existing structure with relatively simple upstream watershed
conditions. Work is currently under way to expand the interface to include more
complex watersheds with other reservoirs farther upstream.
"We expect to have this expanded version available by late 2000,"
Temple says. "The first test version of the model will be completed by
February 2000.
"Research efforts are in progress to expand the SITES technology to allow
users to model earthen embankments to determine what conditions will cause them
to fail. Aging of these dams, combined with increasing population densities and
increased environmental concerns, will generate many challenges for each of us
involved in this effort."—By Hank Becker, Agricultural Research
Service Information Staff.
This research is part of Water Quality and Management, an ARS National
Program (#201) described on the World Wide Web at
http://www.nps.ars.usda.gov/programs/nrsas.htm.
Darrel M. Temple,
Gregory J. Hanson, and
Kerry M. Robinson are in the
USDA-ARS Hydraulic
Engineering Research Laboratory, 1301 N. Western St., Stillwater, OK
74075-2714; phone (405) 624-4135, ext. 226/223/225, fax (405) 624-4136.
More information about SITES can be obtained on the World Wide Web at
http://www.wcc.nrcs.usda.gov/water/quality/common/sites/sites.html.
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Kem Kadavy observes the hydraulic
performance of a model of a principal spillway inlet tower. The tower is a
scaled-down version of what will be built under a dam in Mississippi. Water
from above moves through the tower down to a pipe outlet. Air is added to
create bubbles
to study water movement.
(K8733-2) |
This step baffle trash rack on the spillway inlet of Boomer Lake prevents
floating debris from plugging the inlet by forcing the water to enter upwards
through baffles. Water entering the trash rack flows through the principal
spillway conduit to the stream
channel downstream of the dam.
(K8735-2) |
So Many Structures, So Many Needs
Scientists at the ARS Hydraulic Engineering Research Laboratory, Stillwater,
Oklahoma, look at water in terms of physics and math. The concepts and
principles they have developed over the last 60 years have become the standards
for the best design for safe, economical, and lasting hydraulic structures and
channels for agriculture that are in harmony with nature.
"In the United States, USDA's Natural Resources Conservation Service has
assisted in constructing tens of thousands of these hydraulic structures, based
on the procedures developed at the ARS lab," says ARS hydraulic engineer
Gregory J. Hanson.
One area of specialization is grass-lined waterways—natural alternatives
to concrete and rock and other materials considered by some to be ugly and
costly—for draining terraced agricultural lands. Hanson says the
laboratory's worldwide leadership in designing vegetation-lined waterways is a
lasting legacy.
"These vegetation-lined waterways are used today to safely convey runoff
from more than 3 million acres through 500,000 miles of waterways," he
says. "Many were built by NRCS. The grassed-waterway design procedures
developed by ARS are virtually the only reliable ones available, originating
from a solid experimental background."
The grassed-waterway design procedures are the result of a series of tests on
vegetated channels. "Among other things, these tests accounted for
resistance to waterflow caused by vegetation and the geometry of the channel.
Engineers worldwide rely on these studies when they design vegetated
channels," he says.
Hundreds of tests on the hydraulic performance of structures and channels have
been performed at the Stillwater lab.
Some better known products resulting from these tests and the team effort of
USDA's agencies include design criteria for energy-dissipating structures, such
as
- stilling basins, which slow down the flow velocity and reduce the
erosive action of water
- plunge pools, which dissipate the erosive energy of falling water
- hood inlets, which allow a pipe to convey water more efficiently
- step baffle trash racks, which allow water to drain from a reservoir
and which don't get plugged with floating debris
- flow-measurement flumes, which accurately determine the rate of
waterflow
- rock chutes, which are loose rock structures that safely transfer
water to lower elevations (photo on page 6).
"This is just a partial list of the many hydraulic structures studied and
developed here," Hanson says.
Hanson's colleague, ARS hydraulic engineer Kerry M. Robinson recently tested a
hydraulic model of a new water supply dam. It uses roller-compacted concrete
for a structure being built near Randleman, North Carolina.
"With this design, the auxiliary spillway can be placed over the dam, with
a stepped surface on the downstream side of the dam. The stepped surface
decreases the velocity of water and reduces the energy of the flowing water as
it goes over the spillway," he says.
Design modifications resulting from Robinson's study saved about $2 million on
this project.
Besides structures, Hanson and colleagues have developed a circular jet test
that uses a patented device to directly measure soil erodibility by water in
the field or laboratory (photo on page 7).
"The device uses a jet of water to form a cavity in the soil," he
says. "The soil's resistance to erosion is calculated from the difference
between the original and the eroded soil profile. This difference is expressed
as a jet index."
The jet device can measure the erosion potential of a soil in vegetated
channels, road embankments, dams, spillways, and construction sites.
The device has been field and lab tested on several soils at various rates of
waterflow. It was developed in conjunction with the SITES software for
analyzing earth spillways.—By Hank Becker, ARS. |
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"Holding Back Floodwaters" was published in
the February 2000 issue of
Agricultural Research magazine.
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