Low-head dam on the Little River - Rockford, TN.
Site of multiple drownings.
LOW HEAD DAMS: WHAT ARE THEY? Low head or run-of-river dams, usually spanning the entire river or stream and spilling over all or nearly all the dam crest length, present a safety hazard to the public because of their capability of producing dangerous recirculating currents, large hydraulic forces, and other hazardous conditions sufficient to trap and drown victims immediately downstream from the overflowing water. Low head dams act like uncontrolled spillways and provide little storage function. Thousands of these mostly concrete or masonry weir-type structures, normally producing vertical water surface drops of one to fifteen feet, have been constructed across rivers and streams to raise the water level for improving municipal and industrial water supplies, producing hydropower, feeding navigation canals, and diverting irrigation water. Tens of thousands were built in the 1800s to power gristmills and small industries (Walter and Merritts 2008). While most no longer exist, many have fallen into disrepair or have been abandoned by long-forgotten owners, thus posing dangerous conditions to the public. Increasing numbers of kayakers, canoers, rafters, boaters, anglers, and swimmers are often unaware of, or underestimate, the dangerous forces and currents that these dams or similar hydraulic structures can produce if there are no warning signs, restricted area postings, boat barriers or bypass portages. Waterway users should be aware of all types of drop structures such as bridge or culvert apron drop-offs, grade control structures (GCS), pipe crossings, and large dam spillways that also can produce dangerous submerged hydraulic jump conditions characteristic of low-head dams. Some waterways may warn of the danger, have boat barrier buoys strung across the water, post "exclusion" zones upstream and downstream from dams, or provide portages around low head dams as shown below, but many do not. Fluctuating river flows from upstream hydropower releases or natural runoff – even small flow changes - can significantly increase the water hazard at a low head dam from one hour or day to the next. Experienced swimmers have difficulty overcoming the velocities around these structures. Life vests become less effective in the turbulent foam below low head dams because of greatly reduced buoyancy. Often, boaters or paddlers do not become aware of a dam on the downstream horizon until being pulled over the dam crest from the accelerating draw-down current. The combination of reversed currents, large hydraulic forces, low buoyancy, dangerous rotating submerged objects, hard surfaces, potential hypothermia and disorientation create what has been described as a perfect "drowning machine." The best advice to water users is to avoid being near these dams altogether – both upstream and downstream.
Waterway users also should use caution around large dam intake and outlet structures, including those owned, operated or regulated by federal agencies, such as the U.S. Bureau of Reclamation, TVA, U.S. Corps of Engineers, and the Federal Energy Regulatory Commission (FERC), where sudden spillway and power generation releases can be very hazardous.
TVA provides warning systems at several of its dams with warning signs, horns, strobe lights, and electronic spillway signs activated to alert the public of impending hazardous waters below dams resulting from rapidly rising water and turbulence from flood gate and turbine releases. The Little Rock District of the U.S. Corps of Engineers has published a brochure on hazardous waters and downstream warnings for water users around their dams and spillways.
Photo above right: TVA warning system in heavily-used fishing, swimming and recreation area downstream from Upper East Tennessee South Holston Dam and labyrinth weir (signs, strobe lights and siren)
TVA provides warning systems at several of its dams with warning signs, horns, strobe lights, and electronic spillway signs activated to alert the public of impending hazardous waters below dams resulting from rapidly rising water and turbulence from flood gate and turbine releases. The Little Rock District of the U.S. Corps of Engineers has published a brochure on hazardous waters and downstream warnings for water users around their dams and spillways.
Photo above right: TVA warning system in heavily-used fishing, swimming and recreation area downstream from Upper East Tennessee South Holston Dam and labyrinth weir (signs, strobe lights and siren)
Upstream take-out portage & boat barrier boom.
Boaters should be very cautious upstream from all dams, especially during flooding conditions, where accelerating surface currents are capable of pulling watercraft over spillways and overflow sections. Avoid anchoring a boat immediately upstream from a dam as rising water my cause the boat to drift toward the dam. Several boaters have drowned after their motors have stalled and have been pulled over the dam.
PUBLIC SAFETY REGULATIONS. Most states do not maintain inventories of, or regulate public safety at, these structures because of their small sizes, or because their structural integrity is not considered an issue. Pennsylvania and Illinois are the only states that have comprehensive laws that require owners of run-of-river dams to mark so-called "exclusion zones" above and below their dams, and on the banks immediately adjacent to the dams, with signs and buoys to warn the swimming, fishing, and boating public of the hazards posed by the dam. Virginia has established code and standards for advising the owners of low head dams to mark hazardous areas with warning signs and buoys. Owners who do so are deemed to have met the duty of care for warning the public of the hydraulic hazards. Those who do not mark their dams are presumed not to have met the duty of care for warning the public. Four or five states provide dam owners with general information about the hazards of low head dams and provide recommended warning sign and buoy installation guidance and templates. One federal agency has developed public safety procedures for hazards created by hydraulic structures and operations. The Federal Energy Regulatory Commission (FERC) requires owners of hydropower dams, including low head dams, operating under its jurisdiction to conform to specified public safety guidelines and regulations for protecting the public around these structures.
Warning signs posted upstream from L-H dam
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Turbulent and highly aerated/low bouyancy zone
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LOW HEAD DAM INVENTORY & FATALITY DATA. Data from an early 2014 ASDSO survey of state dam safety officials by Dr. Tschantz shows that of 42 responding states, nine states that maintain some type of inventories reported a total of 916 low head dams. However, 28 states that do not maintain specific inventories of low head dams, but provided approximations in the form of a range with varying degrees of stated confidence, estimated that there may be as many as 1814 to 3510 additional low head dams. Five additional states indicated that they had several dams but did not submit an estimate. This means that there may be 2730 to 4426 low head dams in 42 reporting states.
Hundreds of deaths have occurred at these structures across the U.S. since the 1960s, with drownings and injuries increasing annually as more people participate in water sports. According to the 2013 Outdoor Participation Report, participation in U.S. recreational and whitewater kayaking has increased by over 100%, from 2006 to 2012, to slightly over 10 million kayakers.
Dr. Tschantz has documented for the period 1960 - August 2016, 377 fatalities and/or 103 injuries in 304 incidents at low head dams, with fatalities occurring in 285 of the incidents. Other undocumented low-head dam incidents bring the total to well over 400 deaths. The map on the right shows the distribution of low head dam fatalities by state. As of August 2016, 39 states have had at least one death. Forty-six percent of the documented fatalities have occurred in Iowa, Illinois, Minnesota, Ohio and Pennsylvania (magenta-shaded states). An additional 24 percent of the fatalities have occurred in the six states (red-shaded) of Indiana, Maryland, Missouri, Tennessee, Texas, and Virginia. Some low head dams have produced multiple drownings. Additional drownings, where details remain unavailable, are estimated through newspaper and anecdotal accounts and personal interviews to number about 100, and are not included in the documented 377 deaths. Scores of other drownings have occurred below large dams where gated spillways and turbine releases can produce dangerous turbulence and public hazard conditions. Also, water pressure at grated inlets upstream from dams can force or "suck" unsuspecting swimmers against the grillwork causing them to be helplessly trapped. Dr. Rollin Hotchkiss, Brigham Young University Professor of Civil & Environmental Engineering, and graduate students Edward Kern and John Guyman, have currently (March 2017) described and mapped 531 fatalities at 245 different dam and spillway sites in 39 states on their BYU website.
Current (through 2016) data from Tschantz's research shows that 70 percent, or 266, of the 377 fatalities, have occurred over the 17 years, 2000 through 2016. The same data shows that, in the 3-1/2+ decades since 1980, while "only" 40 people have died as a result of dam failures, over 347, or almost nine times as many, have died from drowning at low head dams. The annual drowning rate at low head dams is accelerating - in the forty years prior to 2002, the number of drownings averaged about 4 deaths per year, but since then the average number has been 17 per year.
For 350 drownings through 2015, forty percent occurred over 2-day weekends during April through August. Further, of 129 fatalities in which life vests, or PFDs, were reported to have been worn or not used, 53 wore PFDs and 76 did not. The relatively close (41% vs 59%) split is not surprising given the low buoyancy environment and other injurious factors that submerged victims are subject to. Data shows that over half of all low-head dam related fatalities have been paddle sport recreationalists such as canoers, kayakers, rafters, or tubers. The same research also shows how risky it is to intentionally or unintentionally go over low-head dams. Of 175 incidents where 390 people were known to jump from or go over a low-head dam, 68 percent either drowned (202) or were seriously injured (63) enough to require medical attention, including resuscitation and hospitalization.
Hundreds of deaths have occurred at these structures across the U.S. since the 1960s, with drownings and injuries increasing annually as more people participate in water sports. According to the 2013 Outdoor Participation Report, participation in U.S. recreational and whitewater kayaking has increased by over 100%, from 2006 to 2012, to slightly over 10 million kayakers.
Dr. Tschantz has documented for the period 1960 - August 2016, 377 fatalities and/or 103 injuries in 304 incidents at low head dams, with fatalities occurring in 285 of the incidents. Other undocumented low-head dam incidents bring the total to well over 400 deaths. The map on the right shows the distribution of low head dam fatalities by state. As of August 2016, 39 states have had at least one death. Forty-six percent of the documented fatalities have occurred in Iowa, Illinois, Minnesota, Ohio and Pennsylvania (magenta-shaded states). An additional 24 percent of the fatalities have occurred in the six states (red-shaded) of Indiana, Maryland, Missouri, Tennessee, Texas, and Virginia. Some low head dams have produced multiple drownings. Additional drownings, where details remain unavailable, are estimated through newspaper and anecdotal accounts and personal interviews to number about 100, and are not included in the documented 377 deaths. Scores of other drownings have occurred below large dams where gated spillways and turbine releases can produce dangerous turbulence and public hazard conditions. Also, water pressure at grated inlets upstream from dams can force or "suck" unsuspecting swimmers against the grillwork causing them to be helplessly trapped. Dr. Rollin Hotchkiss, Brigham Young University Professor of Civil & Environmental Engineering, and graduate students Edward Kern and John Guyman, have currently (March 2017) described and mapped 531 fatalities at 245 different dam and spillway sites in 39 states on their BYU website.
Current (through 2016) data from Tschantz's research shows that 70 percent, or 266, of the 377 fatalities, have occurred over the 17 years, 2000 through 2016. The same data shows that, in the 3-1/2+ decades since 1980, while "only" 40 people have died as a result of dam failures, over 347, or almost nine times as many, have died from drowning at low head dams. The annual drowning rate at low head dams is accelerating - in the forty years prior to 2002, the number of drownings averaged about 4 deaths per year, but since then the average number has been 17 per year.
For 350 drownings through 2015, forty percent occurred over 2-day weekends during April through August. Further, of 129 fatalities in which life vests, or PFDs, were reported to have been worn or not used, 53 wore PFDs and 76 did not. The relatively close (41% vs 59%) split is not surprising given the low buoyancy environment and other injurious factors that submerged victims are subject to. Data shows that over half of all low-head dam related fatalities have been paddle sport recreationalists such as canoers, kayakers, rafters, or tubers. The same research also shows how risky it is to intentionally or unintentionally go over low-head dams. Of 175 incidents where 390 people were known to jump from or go over a low-head dam, 68 percent either drowned (202) or were seriously injured (63) enough to require medical attention, including resuscitation and hospitalization.
Rescue operation below a drop structure.
UNDERSTANDING THE RISK. The lure of a smooth overflow above a low head dam and the attraction of whitewater below are dangerously deceptive traps. In one recent Illinois drowning a 12-yr old youngster wanted to "touch" the smooth overflow wall of water and immediately became trapped by the current. Tschantz’s research shows that some boaters were unaware of the dam before it was too late and were accidentally pulled over the crest by the accelerating current; at least three accidents occurred when boat motors failed or stalled, leaving the boats to drift over the dam crest; and many kayakers, canoers, and rafters deliberately paddled over the dam crest and capsized, apparently underestimating the tremendous power of fast-moving water while overestimating their ability to overcome these forces and currents. Many rescuers have become victims themselves after being pulled upstream toward the dam and capsizing. A classic example of this, described in a Minnesota DNR pamphlet, "The Drowning Machine," occurred on the Susquehanna River, near Binghamton, New York, in late September 1975, with a chain of events that taught rescue teams valuable lessons about the nature of low head dam hazards and rescue operations. In the end, three people were dead and four had been injured. One evening, two rafters were swept over the Rock-bottom Dam and trapped in the current below the structure. Witnesses called for help and a rescue boat was launched with three firefighters. Their boat capsized and all three were thrown into the river. One drowned and the other two, along with the two rafters, were pulled from the water. The next day, on a body recovery operation for the lost firefighter, the fire chief and two firefighters approached the dam from downstream. As their powerboat reached the base of the dam, the turbulence capsized their boat. Attempts to rescue the trio failed. A few minutes later, a rescue boat with two sheriff’s deputies arrived. By this time, two of the firefighters had disappeared and the third was bobbing in the recirculating turbulence. The horror of the two-day event continued as the third rescue boat overturned in the churning water. Luckily, the two deputies and the remaining firefighter were swept clear of the dam and eventually rescued. Rescue teams across the country have learned from tragic incidents like this and have developed special fast water rescue and recovery techniques and protocols (e.g., Elgin, Illinois and Farmington, N.M.). Unfortunately, and despite this event and lessons learned, rescuers continue to die attempting to recover or save people caught in the deadly currents below low head dams.
Waterway users - boaters, kayakers, canoers, rafters, swimmers and anglers - have a responsibility to educate and familiarize themselves about the dangers around these types of structures, observe all warnings, and to understand their own physical limitations against the overwhelming currents and forces that these dams can produce - especially during high water and hidden hazard conditions when life jackets may not offer enough buoyancy or protection. Dam owners, together with local officials and state dam safety and boating safety programs, also have a responsibility to work toward either reducing or eliminating the public hazard around these structures - and, at the very least, warning the public of upstream and downstream dangers. The Canadian Dam Association's (CDA) 2011 Guidelines for Public Safety Around Dams offers a comprehensive and systematic approach for both low and high dam owners to assess public safety hazards associated with their structures and to protect public safety using various risk treatment and control measures. Other public safety guidelines and references are discussed below.
Waterway users - boaters, kayakers, canoers, rafters, swimmers and anglers - have a responsibility to educate and familiarize themselves about the dangers around these types of structures, observe all warnings, and to understand their own physical limitations against the overwhelming currents and forces that these dams can produce - especially during high water and hidden hazard conditions when life jackets may not offer enough buoyancy or protection. Dam owners, together with local officials and state dam safety and boating safety programs, also have a responsibility to work toward either reducing or eliminating the public hazard around these structures - and, at the very least, warning the public of upstream and downstream dangers. The Canadian Dam Association's (CDA) 2011 Guidelines for Public Safety Around Dams offers a comprehensive and systematic approach for both low and high dam owners to assess public safety hazards associated with their structures and to protect public safety using various risk treatment and control measures. Other public safety guidelines and references are discussed below.
Minn. low head dam on Red Lake River replaced with rock ramp (Aadland, Minn DNR, 2010).
REDUCING OR ELIMINATING THE HAZARD.
Eliminating the public hazard by removing low head dams can greatly improve recreational opportunities, such as boating, fishing, swimming, or paddle sports; restore aquatic ecosytems; and increase public safety. However, dam removal also may have adverse physical, chemical, ecological, social, and/or economic impacts. For example, contaminated sediments that have been trapped behind some dams over decades would require study and a plan for preventing them from being transported downstream. In addition, existing or potential benefits from hydropower production, water supply enhancement, or improved navigation would be eliminated. While dam removal may enhance native fish passage and migration, the opportunity for invasive and undesirable species to migrate also increases. Furthermore, some low head dams may have significant historical and cultural value to the local community. Legal questions and disputes over dam ownership and riparian rights of riverfront property owners may have to be resolved. Local, state, and federal regulatory and permitting requirements must be met. Three notable information sources that discuss the impacts of dam removal are (1) DAM REMOVAL – Science and Decision Making (The Heinz Center 2002), (2) A Summary of Existing Research on Low-Head Dam Removal Projects (American Assoc. of State Highway and Transportation Officials 2005) and (3) Reconnecting Rivers: Natural Channel Design in Dam Removal and Fish Passage (Aadland, Minn. DNR 2010). A fourth reference, Small Dam Removal in Pennsylvania – Free-Flowing Watershed Restoration (Penn. Org. for Watersheds and Rivers – POWR 2009) is an excellent guideline and resource on public benefit and cost concerns of dam removal. The Association of State Dam Safety Officials’ (ASDSO) website provides a listing and links to several other resources on the topic of dam removal. Hundreds of dams have been removed across the country for safety and ecological reasons. For example, more than 50 dams have been removed from Ohio streams and rivers during the past 30 years. Approximately 350 dams, including small low head dams have been removed in Pennsylvania (Pittsburg Post-Gazette 2015). Other states such as Minnesota, Wisconsin and Virginia have taken similar steps to remove dams. American Rivers reports that 72 dams were removed in twenty-one states in 2016, making a total of 1384 removals in forty-seven states since 1912. See state removals map and ranking distribution below.
Eliminating the public hazard by removing low head dams can greatly improve recreational opportunities, such as boating, fishing, swimming, or paddle sports; restore aquatic ecosytems; and increase public safety. However, dam removal also may have adverse physical, chemical, ecological, social, and/or economic impacts. For example, contaminated sediments that have been trapped behind some dams over decades would require study and a plan for preventing them from being transported downstream. In addition, existing or potential benefits from hydropower production, water supply enhancement, or improved navigation would be eliminated. While dam removal may enhance native fish passage and migration, the opportunity for invasive and undesirable species to migrate also increases. Furthermore, some low head dams may have significant historical and cultural value to the local community. Legal questions and disputes over dam ownership and riparian rights of riverfront property owners may have to be resolved. Local, state, and federal regulatory and permitting requirements must be met. Three notable information sources that discuss the impacts of dam removal are (1) DAM REMOVAL – Science and Decision Making (The Heinz Center 2002), (2) A Summary of Existing Research on Low-Head Dam Removal Projects (American Assoc. of State Highway and Transportation Officials 2005) and (3) Reconnecting Rivers: Natural Channel Design in Dam Removal and Fish Passage (Aadland, Minn. DNR 2010). A fourth reference, Small Dam Removal in Pennsylvania – Free-Flowing Watershed Restoration (Penn. Org. for Watersheds and Rivers – POWR 2009) is an excellent guideline and resource on public benefit and cost concerns of dam removal. The Association of State Dam Safety Officials’ (ASDSO) website provides a listing and links to several other resources on the topic of dam removal. Hundreds of dams have been removed across the country for safety and ecological reasons. For example, more than 50 dams have been removed from Ohio streams and rivers during the past 30 years. Approximately 350 dams, including small low head dams have been removed in Pennsylvania (Pittsburg Post-Gazette 2015). Other states such as Minnesota, Wisconsin and Virginia have taken similar steps to remove dams. American Rivers reports that 72 dams were removed in twenty-one states in 2016, making a total of 1384 removals in forty-seven states since 1912. See state removals map and ranking distribution below.
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Public safety at and around low head dams can be improved to reduce the hazard and risk exposure by erecting physical boat barriers, portages, and safety devices; installing warning systems; establishing "exclusion" zones; educating the public about the risk; training emergency response and rescue personnel, and modifying a structure to eliminate the dangerous submerged hydraulic jump. These public hazard- and risk-reducing measures have been discussed in several papers, manuals, guidelines, and webinars (see Low-head Reference Sources and Web Links below). One notable ASDSO Journal article, Saving Lives While Improving Fish Passage at “Killer Dams” (Schweiger 2011), discusses and gives examples of the legal responsibility and liability of dam owners and engineered modifications to improve public safety. Presenters in the ASDSO Webinar: Identifying Hazards and Improving Public Safety at Low Head Dams (Schweiger & Tschantz 2013) detail and give examples of several measures for improving public safety. These include Legislation and Regulation, Improving Public Awareness, Non-Structural Measures, and Structural Modifications.
Recent 2017 Low-head Dam Drowning Incidents
3/3/17 Columbus, Ind.: Woman (33), (reported as likely) drowned trying to rescue a dog near a low-head dam on the Flatrock River.
3/31/17 Genoa, Wisc.: Man (52) drowned when his fishing boat capsized at Lock and Dam No. 8 roller gates on the Mississippi River.
4/15/17 Temple, Tex.; Woman (46) drowned searching for a missing man at low head am on House Creek.
6/1/17 Morristown, Tn.; Woman (29) drowned trying to save her children and others at Enka Dam on the Nolichucky River.
5/20/17 Sewickley/Crescent, Pa.; Two women (both 25) kayakers drowned at Dashields low head dam on the Ohio River (27 previous deaths).
Low head dam on the Perkiomen R. horizon line (arrow) from 75 yds upstream.
NOTE TO WATER USERS: Most drowning incidents occur from April through August. If you plan to boat or paddle, please be careful and don't take chances. Know the river and watch for warning signs and take-out portages ....stay away from low-head dams and large dam spillways - both upstream and downstream - especially during turbine or floodwater releases and highwater conditions. Be aware that the danger can vary as the flow changes from one hour or day to the next. Always wear a personal flotation device (PFD). Not all dams are marked! When boating, paddling, or rafting downstream, learn to recognize and be on the lookout ahead for a horizon line across the water surface where a low head dam is likely located - when trees or houses or other objects appear to be cut off or grow straight out of the water or if the water surface ripples suddenly change, you may be looking at a "horizon line" and the smooth overflow at the dam. Be aware that the current increases as you drift closer to the dam overflow.
General currents upstream and downstream from a low-head dam.
DANGER ZONES As you drift downstream toward the crest of a low-head dam the flow tends to squeeze into a smaller space, thus speeding up the water current, which may exceed one's ability to escape from being pulled over the dam by swimming, paddling, or even motorboat. The extent of the upstream danger zone is site dependent and varies with the flow and depth of water behind the dam. Once a boat or person goes over the dam, the force of the falling water may exceed hundreds of pounds which tends to push a person downward and into a recirculating current. A boater, paddler, or rafter who goes over the dam will usually be forced or pulled back toward the face of the dam where the watercraft will capsize into highly turbulent, super-aerated, and recirculating water. The aerated water below the overfall significantly reduces buoyancy, making it very difficult to stay afloat in this environment with or without life vests. This highly aerated zone also reduces ones ability to escape by swimming, paddling, or propelling against the current. Motorboat propellers may "cavitate" and not be functional in this zone. Note that there is a reversed current zone for some distance downstream to the so-called "boil" point where aerated water is upheaving in a very turbulent manner. No one should venture beyond the boil into this dangerous countercurrent zone, where backflow surface currents are capable of pulling the most experienced swimmers back to the dam and into an endless recirculation cycle. Only specially-trained and equipped rescue personnel should attempt to enter this zone. The strength of the currents and forces depend on the river flow, which can fluctuate hourly or daily.
Always remember: Be dam smart - Hidden submerged forces and currents can trap and kill!
click image for source link
REFERENCES. There are several good references and information sources on the hydraulics, hazards, public safety, dam removal, and dam modification associated with low-head dams. Links to a list of references & information sources, along with other applicable low-head dam websites, are provided below. In November 2013, The Association of State Dam Safety Officials (ASDSO) sponsored and hosted a 2-hour webinar Identifying Hazards and Improving Public Safety at Low Head Dams (Tschantz and Schweiger). This webinar presentation, with reference materials and quiz for PDH credit, is available on-demand in digitally archived format from ASDSO. Dam owners, state regulators, public safety officials, policy makers, paddlers and other watersports enthusiasts, design engineers, teachers, rescuers, researchers, and other stakeholders with an interest in public safety around low-head dams will find the material in these resources useful.
Low-head dam reference sources and web links
Bruce A. Tschantz, P.E., 9041 Executive Park Drive, Suite 118, Knoxville, TN 37931 Tel. 865.531.5624 Email: btschant@utk.edu