By Francois M. Lempérià¨re and Jean-Pierre H. Vigny
Dam engineers can choose from a number of strategies for increasing discharge capacity at existing spillways. Several non-conventional alternatives are available that provide additional protection at relatively low cost.
Many dam owners encounter a need to increase the safety of existing structures against floods that exceed the original design event. There are numerous approaches for achieving this, some of which tend to be used in certain regions of the world more than in others. A dam owner’s choice of a solution is affected by physical and economic conditions, but also by the limitations of local tradition and experience.
As part of its mission to promote the sharing of knowledge and experience concerning dam projects, the non-profit international organization HydroCoop assembled information on the design, suitability, and cost of solutions used worldwide for increasing dam safety against extreme floods. This information is based on laboratory test reports, technical conferences, and investigations by committees of the International Commission on Large Dams (ICOLD). As part of this investigation, HydroCoop evaluated several non-conventional alternatives for adding spillway capacity that provide additional discharge capacity at relatively low cost.
Non-conventional methods foradding discharge capacity
Adding spillway capacity within the constraints of the original design flood typically involves the loss of storage or construction of gates, with a direct or indirect cost in the thousands of U.S. dollars per cubic meter per second (cms). To reduce the cost of adding discharge capacity per cms, a dam owner can increase reservoir levels and accept limited damages in the case of very rare floods.
Ways to increase reservoir levels include:
- Improve the embankment crest;
- Add a device, such as a labyrinth weir, to increase free-surface discharge;
- Install a fuse device; or
- Design an embankment to safely overtop.
Any of these solutions may prove to be practical, depending on project configuration, local material and labor costs, and the amount of additional capacity needed. The following sections provide details on each alternative.
Improving the embankment crest
Raising the reservoir level above the original design flood level can be a low-cost way to increase the discharge capacity of an existing spillway. Embankment improvements associated with this approach may include:
- Raising the crest by steepening the upper slopes of an earthfill dam (usually practical only for new construction);
- Adding a crest parapet; and
- Improving the crest’s imperviousness and resistance to wave erosion.
The cost to gain one unit of discharge capacity may be readily calculated from the weir flow formula and the geometry of the dam. The capacity of a spillway may be computed by:
-
Equation 1:
- Q is the discharge in cms;
- e is the length of the spillway, in meters; and
- H is the permissible height of the water above the sill of the spillway, in meters.
-
Q = 2eH
3/2
where:
The coefficient of 2 is approximate and ranges from 1.7 to 2.2, in metric units.
An approximate formula for the discharge capacity gained by raising the dam crest by 1 meter may be derived from Equation 1. This formula is:
-
Equation 2:
- DQ is the increase in discharge, in cms;
- e is the length of the spillway, in meters; and
- H is the permissible height of the water above the sill of the spillway, in meters.
-
DQ = 3eH
1/2
where:
The cost per cubic meter of discharge can then be estimated by:
-
Equation 3:
- C is the cost to achieve a 1-cms increase in capacity;
- L is the dam length, in meters; and
- a is the cost of raising a 1-meter length of dam crest by 1 meter.
- e is the length of the spillway, in meters; and
- H is the permissible height of the water above the sill of the spillway, in meters.
- Hydraulic performance of the crest shape, if the blocks are designed for overtopping;
- Achieving the proper seal at the base to ensure the design uplift;
- Providing for nappe aeration;
- Buttressing the blocks on the downstream side so they overturn, rather than slide; and
- Separating the blocks (for example by a narrow vertical wall) so they do not interfere with one another mechanically or hydraulically.
-
C = La/(3eH
1/2)
where:
For values of L/e between 5 and 10, and values of H between 2 and 4 meters, C is in the range from 1a to 2a (usually a few hundred U.S dollars per additional cms).
Using labyrinth weirs
A labyrinth weir is made of thin vertical reinforced concrete, and appears in plan view as a series of trapezoids (see Figure 1 on page 36). In a typical installation, the total crest length is about four times the structure length, the nappe depth is half the wall height, and the discharge capacity is roughly double that of a traditional weir. For labyrinth walls that are 3 or 4 meters high, the increase in capacity is about 5 cms per meter of length.
In the United States, the U.S. Department of the Interior’s Bureau of Reclamation designed a labyrinth spillway for Ute Dam in the state of New Mexico. The spillway is 9 meters high and contains 60 cubic meters of concrete per meter of structure length. According to a 1985 report to the ICOLD Congress, the Ute Dam labyrinth weir increased the discharge to 60 cms per meter from 30 cms per meter.
A labyrinth weir, with its vertical walls, is relatively easy to build. The quantity of reinforced concrete needed is roughly 1 cubic meter per cms of flow for structures in the 3- to 4-meter height range. (For Ute Dam, the quantity was about 2 cubic meters per additional cms.) In areas where labor and material costs are relatively low, the cost per cms may be a few hundred U.S. dollars.
The main drawback of a traditional labyrinth weir is that it requires a large amount of space. A labyrinth weir cannot be built on top of a gravity dam or on most spillway structures.
Model studies of labyrinth weirs have been made in various laboratories. Researchers at Biskra University in Algeria are conducting an extensive testing program to identify ways to improve hydraulic efficiency, facilitate construction, and reduce the costs of labyrinth spillways. These objectives could be achieved by optimizing the plan layout or by reducing the wall height by partially filling the cells.
Updating the labyrinth:Piano Key Weirs
Over the past five years, designers have experimented with the Piano Key Weir, a new version of the labyrinth weir that can be placed on an existing gravity dam or spillway. The Piano Key, or P.K., Weir may be used to increase reservoir storage (by providing an equivalent spillway discharge with a higher spillway crest) or to increase the capacity of an existing spillway. The design includes either a pair of overhangs on the upstream and downstream faces of the dam (see Figure 2 on page 38), or a single overhang on the upstream face. The crest layout is rectangular. The first implementation of the Piano Key Weir was constructed in 2006 at Electricité de France’s Goulours Dam in southwestern France.
 In the Piano Key Weir design, water flows up the crest, and then flows down the other side. |
Hydraulic tests performed in various countries have shown that a Piano Key Weir can provide a discharge capacity about three times that of a typical rounded-crest weir. The “N ratio” – the length of the crest as a ratio of the structure length – is optimized between the values of 4 and 6, and the optimal slope of the lower face of the overhang is between 2:1 and 3:2.
A Piano Key Weir can be made of reinforced concrete or of steel, although the practical height limit for a steel weir is 2 to 3 meters. The weir can be built on site, or it can be partially or completely prefabricated. For a reinforced concrete application, the volume of reinforced concrete per meter ranges from 0.4H2 to 0.6H2, where H is the maximum height of the walls. One cubic meter of reinforced concrete can increase the discharge capacity by more than 2 cms. Therefore, the cost per unit discharge is quite low, especially in regions where labor and reinforced concrete costs are low.
 The Piano Key Weir at Electricité de France’s Goulours Dam increases the spillway discharge capacity by a factor of four over a conventional spillway. |
For a steel weir, with H less than 3 meters and using steel plates around 12 millimeters thick, 0.7 ton of steel are required per meter of spillway. One ton of steel increases the discharge capacity by about 4 cms.
Using fuse devices
Fuse devices are spillway structures that open only once and then are lost or permanently displaced. Such devices may be very cost-effective, delivering the benefits of a gated spillway with lower material and construction costs. Their primary drawback is the cost of replacement and lost reservoir storage if the fuse device does deploy. Typically, these types of spillways are used for floods having an annual probability of 1 in 100 or less.
Concrete fuse plugs
Concrete fuse plugs are massive free-standing concrete blocks placed on the sill of a spillway and designed to tilt when the reservoir reaches a certain elevation. Multiple blocks having different thicknesses and weights may be used, so as to tilt for different reservoir levels. These reservoir levels can be predicted quite accurately if the uplift forces on the block can be quantified. The blocks can be engineered for either zero uplift (by placing the block on a void with an upstream seal) or total uplift (using a void with a downstream seal). Experience has shown that the zero-uplift design works well when the block is designed to overturn before it is overtopped. For blocks designed to overturn under an overtopping flow, the full-uplift solution is more easily achieved.
In designing a concrete fuse spillway, the following considerations are important:
In many cases, less than 1 cubic meter of concrete is needed to increase the discharge capacity of an existing spillway by 1 cms. Modifying an existing spillway would typically involve the removal of about 2 cubic meters of existing concrete for each cubic meter of fuse block concrete added (see Figure 3). For a new spillway, fuse blocks involve about half the concrete of a conventional rounded crest.
Fuse gates
The fuse gate is a more highly engineered concept than fuse block. Fuse gates are designed to overturn at a precise reservoir level, by the mechanism of an uplift chamber connected by a well to the reservoir. Fuse gates have been used for more than ten years in countries such as Australia, France, India, South Africa, Switzerland, and the United States, where they have been used to discharge as much as 30,000 cms. Because of the relatively high level of effort involved in the design of fuse gates, they are most attractive for large spillways and discharges. They can be used for discharging up to 100 cms per meter and can be designed in a labyrinth shape, combining the benefits of a labyrinth and fuse device.
Earth and other fuse devices
Earthfill fuse plugs have been used in China and in the United States for the discharge of as much as several thousand cms. Although relatively inexpensive to build, they require a large amount of space. Furthermore, questions persist about their long-term reliability (in particular, changes over time in the cohesion and compaction of the fill) and the downstream consequences of their deployment.
Flashboards are another device used by owners of small dams, especially in the United States. Most flashboards are wooden boards supported by vertical steel pipes set in the sill of the dam. They may either be removed by hand before the flood season, or allowed to fail through bending of the supports. Although imprecise, they can be very practical for increasing the storage of small dams by a meter or less.
Designing embankments to overtop
Embankment dams that feature a long, 5- to 10-meter-high section may be converted to emergency spillways by placing roller-compacted concrete (RCC) on the downstream face. RCC has been used at about 100 low dams in the United States. For an overtopping depth of 3 meters, each 1-cms increase in flow requires 2 to 3 cubic meters of RCC at a cost of several hundred U.S. dollars.
Some consideration also has been given to using sections of concrete-faced rockfill dams as spillways. This concept has recently been implemented in China and Australia.
Enhancing capacity, controlling costs
Most of the solutions for increasing reservoir levels described in this article are intended for rare events, such that possible minor damages are acceptable given the probability or frequency of occurrence. Under these circumstances, dam owners may benefit from considering these non-conventional alternatives that provide additional protection at a relatively low cost.
Messr. Lempérià¨re and Vigny may be reached at Hydrocoop, 4 Cité Duplan, Paris 75116 France; (33) 1-45017206; E-mail: [email protected].
References
“Cost-Effective Increase in Storage and Safety of Most Dams Using Fusegates or P.K. Weirs,” answer 72 to Question 84, International Commission on Large Dams, 22nd Congress, Barcelona, Spain, 2006.
“New spillway design offers versatility, cost savings,” HRW, Volume 13, No. 3, July 2005, page 40.
Francois Lempérià¨re is chairman of the International Commission on Large Dams’ ad hoc committee on cost savings in dam construction. He is also chairman of Hydrocoop, a non-profit organization whose mission is to promote international technical cooperation in dam engineering. Jean-Pierre Vigny is general manager of Hydrocoop.
