Robinson, Ph.D., P.E.; Robert Houghtalen, Ph.D., P.E.
Removal or retrofitting improves public safety
at low-head dams.
Arriving for a day of fishing on the White River in Morgan County, Ind.,
Kenneth Yant saw an 11-year-old boy trapped in the current below the
low-head dam. In an attempt to rescue the young boy, Yant jumped into the
river—a heroic deed with a deadly ending. Yant, whose body was recovered
from the river two hours later, was a victim of what the boating and water
safety community call "drowning machines."
These drowning machines are
low-head dams that can, under certain flow conditions, create dangerous flow
patterns on the downstream side of the dam. Despite educational efforts by
many dam safety and recreational boating organizations, these low-head dams
continue to be the site of numerous drowning deaths every year. Although no
nationwide statistics are available, anecdotal evidence suggests the number
of fatalities at low-head dams is not insignificant. For example, at least
18 people died at the Glen Palmer Dam on the Fox River in Yorkville,
Ill., during the last 25 years.
deaths occur as water enthusiasts seek the recreational opportunities
created by low-head dams, unaware of the danger associated with these
seemingly placid structures. Dam safety organizations, such as the
Association of State Dam Safety Officials, are well aware of these dangers.
However, the same can not be said for the general civil engineering
community. Properly designing new low-head dams and retrofitting existing
structures can eliminate the risks to the public and liability to the dam
Unfortunately, removing or retrofitting low-head dams to eliminate the
dangerous flow patterns is often a low priority for many states. Many older
low-head dams no longer serve their original purpose, and ownership of the
dams is lost to history. However, some states are starting to manage
proactively the hazards created by these structures. The Illinois Department
of Natural Resources (IDNR) recently funded a safety assessment of the
states low-head dams. In addition, several states have developed inventories
of low-head dams and, in some cases, begun to remove or retrofit them.
Defining the problem
low-head dams are classified as run-of-the-river dams with a hydraulic
height (change in elevation from head water to tail water) of less than 10
feet. The Pennsylvania Run-of-the-River Dam Act defines a run-of-river dam
as a structure "constructed across the width of a river or stream to impound
water where at normal flow levels the storage is completely within the banks
and all flow passes directly over the entire dam structure within the banks,
excluding abutments, to a natural channel downstream."
Although the American Association of State Highway and Transportation
Officials (AASHTO) defines a low-head dam as having a hydraulic height of
less than 25 feet, the structures discussed in this article are
significantly smaller and would be classified by AASHTO as run-of-river
dams. It is their low hydraulic height that misleads many to dismiss the
dangers associated with the seemingly placid structures.
Although located throughout the country, most states lack data on the number
and location of low-head dams within their boundaries. While
Pennsylvania has documented approximately 250 low-head dams that meet its
legal definition of a run-of-the-river low-head dam, officials estimate that
more than 2,000 dams in the state could exhibit hazardous flow patterns. The
U.S. Army Corps of Engineers (USACE) notes similar numbers in Ohio, with
1,700 dams, the majority being low-head dams. Private or unknown ownership
of many of the dams and the lack of regulations governing these structures
makes compiling an inventory difficult.
Low-head dams serve many purposes. In the 19th century, many mill dams were
constructed to harness the small amount of hydraulic head needed to turn a
water wheel. The water wheel supplied the necessary power to grind corn and
other grains. However, there are modern applications too. Low-head dams are
used as river diversions for open-channel irrigation and power plant cooling
water. These small dams are constructed to obtain the small amount of head
and storage necessary to divert water from the stream to the "customers."
Some low-head dams are built for recreational purposes, resulting in a small
reservoir, while others are built for water quality purposes, aerating the
water to enhance water quality. Low-head dams are often incorporated as
hydraulic control structures in stream drainage and flood control channels.
Dangerous flow patterns
civil engineers are unaware that low-head dams can present a danger to the
general public. A review of the civil engineering literature on the topic
finds very little written about the safety concerns of low-head dams.
Conversely, canoeing and kayaking literature is replete with articles about
the hazards of these small hydraulic structures. Fortunately, the dangers
are easily evidenced to civil engineers once the flow patterns are examined
in light of some fundamental hydraulic principles.
Figure 1 depicts the most dangerous flow pattern that can arise at a
low-head dam. (This case is known as a drowned hydraulic jump and will be
described in more detail later.) The dam represents an obstruction to flow,
causing the water to rise upstream and flow over the dam, which then acts
like a weir. Air is entrained in the water once it impacts the downstream
water surface. The direction of the current is downward and downstream, and
the flow generally continues to the bottom of the relatively shallow
downstream pool. The bottom of the downstream pool deflects the current, and
the water rises to the surface a short distance downstream accompanied by
the entrained air. A scour hole often results from the current impacting the
stream bottom, creating a curvature that enhances this flow pattern. Once
the water and air mixture surfaces, it produces what canoeists refer to as a
"boil," since it simulates water vapor escaping from a boiling pot of water.
The water surface rise at the boil combines with the water depression at the
point of overflow impact to produce a hydraulic gradient, and thus flow,
toward the low-head dam. This backflow is what canoeists refer to as the
"hydraulic," or within the technical literature as a "roller."
are at least three dangers associated with the hydraulic conditions
described above when a person is ensnared in the water downstream of a
low-head dam. The first danger is the overflowing water, which generally
produces a large dunking force on the person. The second danger is from
entrained air, which inhibits the dunked person from resurfacing. Not only
is buoyancy reduced, but swimming thrust also loses much of its
effectiveness. Finally, when the person does manage to surface, the
hydraulic pushes the person into the overflowing water to begin the process
again. This results in a perpetual dunking machine that wears out even a
strong swimmer in a short span of time.
Floating debris and water temperature represent additional non-hydraulic
dangers associated with being trapped in the hydraulic. Floating debris
represents a danger if it passes over the dam at the wrong time because it
can strike a swimmer, producing blunt force trauma. Once debris passes over
the dam, it too can become trapped in the hydraulic and represents an
obstacle to a person trying to escape. If the water temperature is low
enough, hypothermia can reduce a persons strength and even cause death if
trapped in the hydraulic long enough.
of the most instructive articles in the literature describing the dangers of
low-head dams were written by Hans J. Leutheusser. In the first paper, "Dam
safety, yes. But what about safety at dams?," published in 1988, Leutheusser
describes various states of flow downstream of low-head dams as a function
of tail water depth. Of the four conditions described (swept out jump,
optimum jump, drowned jump, and surface nappe), the drowned jump (Figure 2)
is the most dangerous and produces the hydraulic described previously. In a
subsequent paper in 1991 in the Journal of Hydraulic Engineering, "Drownproofing
of low overflow structures," Leutheusser and Warren M. Birk use dimensional
analysis and similitude to produce a graph of hydraulic velocities based on
weir head and tail water depth as a function of weir height.
dangers described when the drowned jump condition exists are very real.
Using basic hydraulic principles—energy and momentum conservation—along with
Leutheussers paper to estimate hydraulic conditions in a drowning study, a
dunking force in excess of 200 pounds can be calculated. The entrained air
was estimated to be as much as 30 percent, reducing the buoyant force by the
same percentage. The reverse current in the hydraulic was calculated to be
nearly 4 feet per second. Good swimmers can attain velocities of 6 feet per
second, but only for short distances when they are not tired.
the danger of low-head dams is apparent, civil engineers should act
ethically and responsibly to remove the danger when it exists. One
appropriate way to remove the danger is to remove the structure. In some
cases, remnants of old mill dams or diversions have long since served their
purpose. If there is no longer a function to be fulfilled, removal of an old
low-head dam may be an appropriate corrective measure. Often, dam removal
can have beneficial impacts on stream ecology.
Retrofit efforts short of removal may be appropriate in situations where the
economics of removal is prohibitive or the low-head dam in question still
has a purpose. A reliable way of addressing the safety concern is to alter
the tail water conditions to eliminate the hydraulic. This can be
accomplished by adding riprap to the downstream pool or creating a stepped
spillway on the back side of the low-head dam. However, a stepped spillway,
usually made of reinforced concrete, may be expensive. Riprap is likely to
be cheaper, but it needs to be of sufficient size to avoid washing out
during flood flows.
dangerous hydraulic created at the Midtown Dam on the Red River in Fargo,
N.D., was eliminated by creating rock rapids at the downstream face of the
dam. Although at least 19 people had drowned at the dam since its
construction in 1960, the city needed to maintain a water intake in the pool
above the dam. Therefore, complete removal was not an option. To create the
rapids, 3,370 tons of crushed rocks to 5-foot boulders were strategically
placed in the river. The retrofit was completed in 1999 at a cost of
Another retrofit option is creating a notched weir in the low-head dam. In
lieu of allowing the water to overflow the dam along its entire length, flow
should be through a controlled opening near the middle of the dam. This
creates safe areas to swim to on either side of the overflow. In addition,
recirculating horizontal currents are likely to be created that tend to push
a person toward the shore and out of danger. This type of remediation
requires increasing the height of a section of the existing dam or notching
of a center section, which may require spending significant funds.
Removing a low-head dam, in addition to eliminating a public safety hazard,
can impact a rivers ecology. Although positive impacts are frequently
touted, negative impacts also are possible, if only for a limited time
period after dam removal. According to AASHTO research, removal of St. Johns
Dam (a 7-foot-high, 150-foot-long concrete dam located on the
Sandusky River in Seneca County, Ohio) eliminated a public safety hazard and
improved fish and wildlife habitat and water quality. In addition to the
drowning hazard of low-head dams, USACE cited numerous ecological benefits
to the Olentangy River as evidence to support removal of the 5th Avenue
low-head dam in Columbus, Ohio. Built in 1935, the 470-foot-wide,
8-foot-high dam provided a source of cooling water for a power plant.
Low-head dams are known as drowning machines because of the numerous
drowning deaths caused by the dangerous flow patterns created at the base of
the dam. As prevalent features of our nations infrastructure, civil
engineers need to be aware of the risks to the public these structures
present. By either removing the structure or retrofitting the spillway, the
threat to public safety can be eliminated.
Michael Robinson, Ph.D., P.E.,
is an assistant professor of civil and environmental engineering at Rose-Hulman
Institute of Technology. He can be contacted at
or at 812-877-8286. Robert Houghtalen, Ph.D., P.E., is a professor of civil
engineering and chairman of the civil engineering department at Rose-Hulman
Institute of Technology. He can be contacted at
or at 812-877-8449. Cole Marr, Anita Rogacs, and Anizka Garcia participated
in a summer Research Experience for Undergraduates program sponsored by the
Engineering Forensics Research Institute at the Rose-Hulman Institute of
Technology and the National Science Foundation.
drowned hydraulic jump at a low-head dam creates hazardous flow conditions
that can trap even the strongest swimmers.
hydraulic conditions can occur at low-head dams. The drowned hydraulic jump
creates the hazardous "hydraulic" responsible for many drowning deaths.
A fatal history
Glen Palmer Dam, located in Yorkville, Ill., on the Fox River
is a low-head dam with a long history of fatalities. The original mill dam
was built in the 19th century; the current dam was constructed in 1960. The
dam has a modified ogee crest with a spillway length of 530 feet and a
height of approximately 5 feet. Constructed with no riverbed protection, a
scour hole formed at the base of the dam, creating a condition where the
hydraulic jump was submerged for all tail water depths.
1976, concern over the number of deaths prompted local officials to request
a state evaluation of the dam. In response to the discovery of the scour
hole, riprap with an equivalent diameter of 2 feet was placed in the scour
hole in an attempt to eliminate the hydraulic. However, a hydrographic
survey in 1991 showed that the riprap immediately at the base of the dam had
been scoured out, forming a new hole. Although smaller than the original,
the new scour hole was sufficient to again lead to the formation of a
1996, University of Illinois researchers conducted a study to
determine the best method to retrofit the dam. A physical model of the Glen
Palmer Dam was constructed to better understand the processes that led to
formation of the original scour hole and scouring of the riprap placed in
1976, and to determine the best method to retrofit the dam to prevent
formation of the hydraulic. It was determined that a four-stepped structure
would eliminate hydraulics that could entrap a person at the base of the
Illinois Department of Natural Resources (IDNR) evaluated multiple
alternatives to decrease the hazards associated with the dam, including
various retrofits and complete removal of the dam. Full dam removal was the
alternative preferred by IDNR, but local officials preferred a retrofit
alternative. A low-head dam may be a signature structure for a town and can
provide many benefits to an area. The alternative selected was to retrofit
the dam by adding four reinforced concrete steps on the downstream face of
the dam. The estimated cost of the project is $6.5 million, including
construction of several bypasses. A canoe bypass will be constructed to
allow safe passage through the dam by boaters. A fish bypass will also be
constructed to remove a migration barrier for fish and improve stream