Removal or retrofitting improves
public safety at low-head dams.
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.
Many 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 owners.
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
Many 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.
Most 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."
There 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.
Two 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.
The 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.
Since 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.
The 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 approximately $230,000.
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
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.
hydraulic conditions can occur at low-head dams. The drowned
hydraulic jump creates the hazardous "hydraulic" responsible for
many drowning deaths.
The 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.