Safety first at low head dams11 August 2011
Although small in size, low head dams can be a great danger for recreational river users. A recent drowning at a US dam has highlighted why both dam owners and the general public must put safety at the forefront of their minds. Suzanne Pritchard reports
The decision to remove Brantley Dam on the Dan River in Virginia was unanimous. On 18 January 2011 Danville City Council in the US voted for the 60-year old low head structure to be taken down. Originally built to divert water to an intake facility at the Brantley steam plant, the 1m high, 153m long dam is no longer in use.
Environmental concerns were not top of the council’s agenda when they voted for the dam removal option. It was the death of a five-year-old boy in April 2010 which prompted a review of safety measures, specifically at low head dams. Four deaths have occurred at Brantley dam over the past 45 years but these are not isolated events. Similar incidences have occurred across the US at low head dams, causing some people to label them as ‘drowning machines’.
Experts agree that low head dams can lull river users into a false sense of security. They do not appear to be dangerous due to their smaller size, which is particularly true if viewed from a boat or canoe upstream. They are also pleasant places to be in the summer when water drops gently over them. But low head dams are deceptive, agrees consulting firm Dewberry and Davis in its report on the Danville City dams. They do not appear to be dangerous but it is their tailwater hydraulics which cause the problem.
According to the report: “Hydraulic jumps (boils) form below low head dams when supercritical (high velocity) water flows over the steep slope of the downstream face, and re-enters the subcritical (low velocity) flow of the channel tail water. The hydraulic jump is typically seen as a step, standing wave or ‘roller’ where energy is released. The water flow is then directed back upstream towards the dam entraining air, debris and potentially people in a re-circulating flow pattern. The shape, size and location of the hydraulic jumps are variable and highly dependent on the upstream flow conditions.”
Low head dams become more dangerous in periods of high runoff and there other safety hazards for potential victims too. These include:
• The faces of the dam usually consist of a vertical concrete abutment which means victims will struggle to climb out.
• Branches and other debris trapped in the hydraulic pose an additional hazard for victims.
• The temperature of the water also decreases survival time.
• Air bubbles mixing in the water decrease buoyancy by one-third. Victims have a hard time staying afloat even if they have a personal floatation device.
The above factors, combined with the hydraulic current, create what has been termed as a nearly perfect drowning machine.
According to Charlie Walbridge from American Whitewater, low head dams are responsible for almost 10% of moving water deaths reported to the organisation. “Almost all victims are inexperienced paddlers,” he says. “A few did not know the dam was there; others knew but ran it anyway.”
Kevin Colburn is the National Stewardship Director at American Whitewater. “Low head dams have resulted in many deaths in the US,” he claims. “Some low head dams are difficult to see or hear from upstream, and many are designed in a manner that forms a virtually inescapable hydraulic that holds paddlers and boats. River users that either accidently or intentionally go over such a dam often are held by the hydraulic and drown. Victims either are not aware of the dam’s existence or location, or are not aware of its inherent danger.”
Since 1974, 25 people have died at low head dams in Minnesota. Twenty-two others have been injured or rescued. Victims have been boaters, canoeists, swimmers and fishermen. “Boaters and canoeists get into trouble when they don’t gather enough information about the river on which they are travelling before they start off on their journey,” explains Tim Smalley, boat and water safety specialist at the Minnesota Department of Natural Resources. “We have found that people entering the water from below the dam to swim and play in the water are especially vulnerable. They don’t understand the true hazardous nature of dams, especially small low heads, and the danger associated with the power of the current.”
Creating greater awareness of the potential danger at low head dams seems to be the key to avoiding tragedy. American Whitewater’s advice for paddlers is clear. “The advice is simple,” Colburn says. “Take out well above dams and portage around them. This is one of the first lessons learned by paddlers - one taken very seriously. It is not always known or understood though by very inexperienced paddlers or those who do not receive any instruction or mentoring. Those uninitiated paddlers are more at risk.”
So what else can be done for recreational river users who throw caution to the wind? Adequate warning signs at the river access and upstream of the dam itself are necessary to warn of the dam’s location and risk. A well marked and maintained portage trail is also necessary.
In 1999 the Pennsylvania Department of Environment and Protection passed legislation that required owners of low head dams to post signs and buoys. Any owner failing to install or maintain these can be fined US$250-5000. While an unauthorised person who enters a restricted dam area can also be fined. Maryland, Virginia and Ohio have similar rules.
It is equally important to ensure that there is effective maintenance of any signage and warning programme, Dewberry and Davis add. The following should be carried out:
• Conduct frequent inspections to ensure that signs are maintained as needed.
• Take precautions to help reduce vandalism to signs.
• Repair or replace damaged signs and buoys quickly (maintain a stock of them).
• Check the reflectivity of signs.
• Setup reporting protocols for employees or visitors to report signs in need of repair.
However, Walbridge cautions that this approach is not foolproof. “As a practical matter it’s not easy to place and maintain signs and buoys,” he says. “They are expensive, often costing tens of thousands of dollars. The must also be placed close enough to the water to be seen and invariably this means that they’ll also get washed away by floods.”
Unfortunately many incidents occur during high water periods or during the immediate aftermath before signs and buoys can be replaced. One exception though is to post signs on upstream bridges which are seldom washed away.
“In Minnesota we have found graphic signage that actually shows the mechanism of the recirculating current at dams are more likely to be obeyed than a sign with words alone,” Smalley adds. “Since we started installing signs that graphically show the danger, visible to people shoreward on either side of the dam, non-boat related deaths at low heads dams have decreased.”
Minnesota’s experience of buoying off dams from upstream has also helped to decrease the number of boats and canoes caught in the current. Over the past five years there have been no fatalities at their low head dams.
A similar experience has occurred at Bow River Weir in Alberta, Canada. After completion in 1975 eight people drowned over the next seven years. Dam managers took a number of steps which included installing a multi-layered warning system using different kinds of graphic signs. Two buoy lines also remove the temptation to paddle to the edge of the dam in order to shorten portage around it. A weir safety committee was also formed in 1982.
Charles Walbridge from American Whitewater states that the programme at the Bow River Weir serves as a good model for dam owners and managers everywhere. He says that an ideal safety programme links commonsense regulation, enforcement and a far-reaching community education programme. Elements it should include are that:
• All boaters should be required to wear personal floatation devices. Unfortunately this alone will not be sufficient and an enforcement programme to ensure widespread compliance will be necessary.
• The dangers of drinking/taking drugs on the water should be explained.
• Advice is given about caution in extreme conditions of very high or cold water. One possible approach could be to use colour coded flags to indicate progressively greater risks, as is used successfully on many US beaches.
• Safety information should include warnings of foot entrapment and the correct whitewater swimming position. Plus the importance of sound judgement when making the decision to get on the river, as well the need to dress properly for the appropriate conditions.
Ultimately, educating the public is seen as the most productive strategy. Information should be put out to the public through the media and visits to schools can pass on the river safety message.
“Dam owners can put out articles in the local papers and partner with local water rescue personnel in doing training and public outreach and awareness programmes,” says Tim Smalley from Minnesota Department of Natural Resources. “Safety brochures can be distributed at local fishing supply stores and marinas. One press release and one sign placed isn’t enough though,’ he cautions. ‘Dam operators need to have an ongoing dam safety public awareness programme to keep reminding folks about the dangers of low head dams.”
Even though safety programmes can very good there are still no guarantees that they will prevent tragedy. To date a total of 14 people have drowned at the Bow River Weir in Canada. “The existing weir, no matter how well marked with signs and buoys, remains a potentially deadly hazard,” Walbridge claims. Unlike Brantley dam in Virginia which is no longer in operation, the Bow River Weir is part of the Western Headworks System. It plays a vital role, being constructed in 1975 to supply water to the Western Irrigation District. The dam removal approach favoured at Brantley dam could not even be considered here.
Designed to reduce danger
“Dams do not have to be dangerous,” Walbridge says. “The hydraulic behind each dam is specifically designed to dissipate force and protect the structure. Engineers can create new dams to eliminate its retentive characteristics. Current dams can also be modified.”
One such firm utilising its expertise in this area is consultant klohn-crippen Berger. Unfortunately, it acknowledges, the Bow River Weir has created an extreme drowning hazard and claimed several lives over the years, but now the company is project manager for a scheme to reduce the danger that the weir presents. Construction started in 2008 and is scheduled for completion in October 2011. With construction costs estimated at C$16.4M, the project is on schedule and on budget.
The primary objects of the project are to enable river passage for non-motorised boats and enhance fish passage while maintaining water supply functions. The main features consist of construction of a low water channel for novice boaters and a high water channel for more experienced boaters. A dividing island will separate the two channels and within each one a series of boulder drop structures and pools will be incorporated to create the desired flow conditions. Modifications to the weir will include incorporating slots at specific locations for boat and fish passage, and adding a concrete infill to change the ogee profile to create the desired flow conditions. Currently the difference between upstream and downstream water levels at the weir is around 1.5m. This will reduce to less than 300mm when the new structure is in place.
As Kevin Colburn from American Whitewater adds: “A better design can significantly reduce their [low head dams] risk to the public.”
The hydraulic characteristics of low head dams can be potentially hazardous to river users. In certain circumstances project modifications or even dam removal are being considered as solutions to avoiding deaths at such structures. Nonetheless, effective public information systems still need to be maintained to ensure that the correct safety measures are continually reinforced. However, these are not infallible.
“Boaters and water recreationists need to learn about the dangers of dams,” says Tim Smalley from Minnesota Department of Natural Resources. “They have to obey signs and buoys to stay away from dangerous areas. But I’d say that they are taking it more seriously,’ he adds. ‘Deaths at dams have decreased in Minnesota over the last 23 years.”
The onus is on both dam owners and the general public. If safety is at the forefront of everyone’s mind, recreational activities are just one of the many functions that dams can continue to provide.
|Removing Brantley Dam|
Brantley Dam is one of four low head dams in the City Of Danville in Virginia. Dams on the Dan River have been described as being iconic and an integral part of the local culture. The majority of deaths on the Dan River since 1965 have been as a result of the public"™s use of the river as a recreational amenity. Each of the four documented deaths occurred at the base of the Brantley Dam. Furthermore, due to its popularity with local fisherman, the hydraulic jump at Brantley Dam was also a concern.