Development of Hydraulic Structures
By Thomas J. Rhone, M. ASCE
Research Hydraulic Engineer
Bureau of Reclamation
Presented at the August 8-12, 1988, ASCE Conference on Hydraulic Engineering, Colorado Springs, CO.
Abstract: The Bureau of Reclamation was established in 1902. Since that time, Reclamation has constructed more than 220 dams. Each dam, depending on its function, has two or more principal hydraulic structures. The main hydraulic structures are the spillway and the outlet works. The change in concept of these structures and their energy dissipators as developed by Reclamation from 1902 to the present (1988) is described. Included are stepped spillways, labyrinth spillways, traditional chute and tunnel spillways, and many types of energy dissipators.
April 19, 1938 - the birthdate of the Hydraulics Division and the focus of the 50th Anniversary sessions of this conference. The program says that I will talk about the Development of Hydraulic Structures. That is an overwhelming assignment and a subject that seems to have appeared on many hydraulic-related programs in the past few years. For the sake of brevity, I will confine my reminiscing to the role of the Bureau of Reclamation in the development of hydraulic structures since 1938. Actually, Reclamation had been in the "hydraulic structures" business for 35 years by that time, and Reclamation's Hydraulic Laboratory was about 8 years old and had become a very important part of the Reclamation program.
A low-key review of the status of Reclamation's hydraulic structures in 1938 seems appropriate to establish a baseline. Just how big was the Reclamation program? Consider these statistics: In its first 10 years, Reclamation built 18 dams, including some biggies such as Theodore Roosevelt, Buffalo Bill, and Pathfinder (fig. 1). By the time the Hydraulics Division was formed, the number had grown to 64; and now there are more than 220 dams. Each dam has a minimum of two hydraulic structures: a spillway and an outlet works; a powerplant will add to this number, and an irrigation or M&I (municipal and industrial) conveyance system will add many more. Spillways, outlet works, and energy dissipators are probably the structures that are of most general interest.
Figure 2, an interesting histogram, shows the number of dams built in each of the seventeen 5-year intervals since 1902. It highlights some interesting facts such as the highly productive quarter-century from 1948 to 1972, during which 128 dams were completed - over half of the total inventory. Also shown are the low-productivity periods related to the initial startup, the Depression years, World War II, and the redirection of the past few years.
In the middle of the second low-productivity period, the Reclamation Hydraulic Laboratory was formed. This does not mean that hydraulic investigations were not performed prior to 1930; actually, some of the design units made studies of control valves and some hydraulic structures on a one-time-only basis using very basic facilities.
The Hydraulic Laboratory was formed expressly to fill a need during the design of Boulder Canyon (Hoover) Dam, Nevada. This was to be the highest dam in the world, and new technology had to be developed for the structural, mechanical, and hydraulic designs.
Reclamation's first hydraulic laboratory was at Colorado A&M College (now Colorado State University) at Fort Collins. As the workload grew, Reclamation expanded this facility, added a small annex in the basement of the Old Customs House in Denver, and built a huge outdoor laboratory near Montrose, Colorado. Eventually, these facilities were consolidated in the New Customs House in Denver and in 1945 moved to the present location at the Denver Federal Center. The design units were similarly scattered before being consolidated at the Federal Center.
This brings up the question of what governed the design procedures that Reclamation used for the 39 dams and ancillary hydraulic structures built during this period. Historically, their staff was recruited from the parent agency, the United States Geological Survey, a very knowledgeable engineering organization. Other sources were other Government agencies, construction engineers, private practice engineers, and graduates from highly qualified universities. The supervisory staff has always maintained extremely high engineering standards for their personnel.
Apparently, each design leader assembled a design manual based on his/her training and experience; these were passed on to subordinates who, in turn, added to the standards and eventually became even better qualified designers. In the early documentation concerned with hydraulic structures, the names Horace W. King, William P. Creager, Julian Hinds, Theodore Rehbock, and many other renowned hydraulicians appeared.
A typical page from a 1933 "design manual" is entitled "Types of Scour Protection Below Dams" (fig. 3). Actually, it is a good overview of various types of spillway structures. Eleven structures are shown on figure 3, including a stepped spillway, several ski jump spillways, a couple with forced hydraulic jump energy dissipators, and some with very long paved aprons, presumably for a natural hydraulic jump.
A review of the early Reclamation dams shows a majority with no energy dissipators. Most of them had controlled or uncontrolled spillways, outlet works, or diversion structures, but energy dissipators were unique. The principle was to control the flow for storage and future use. Control systems were present; radial gates, drum gates, slide gates, cylinder gates, ensign valves, and needle valves were there. But if more water was coming in than could be stored, it was turned loose and the problem transferred downstream. You can imagine the disasters that could result!
Before I follow the in-house development of Reclamation energy dissipators, I would like to share some then-versus now hydraulic structure concepts. A few minutes ago, I showed you a stepped spillway that was built in New York State in 1926. This spillway had some features that we reinvented for the Upper Stillwater Dam (Utah) spillway such as varying the height and width of the steps near the crest. Another early version of a stepped spillway was at Lahontan Dam in Nevada, built in 1915 (fig. 4). This is a beautiful structure, but lacking a little in hydraulic performance as shown by the guide walls. The 1987 version of a stepped spillway is at Upper Stillwater Dam (fig. 5), another imposing structure which, built with modern technology, probably has better hydraulic efficiency. Another comparison is a 1910 version of a labyrinth spillway at East Park Dam in northern California (fig. 6). The spillway is a separate structure from the dam. The crest of the spillway is 0.15 m lower than the dam crest but 1.07 m below the dam parapet. The dam parapet was slightly overtopped during floods in 1940 and 1958. Both times the spillway discharge was about 255 m3/s. The 1985 version of a labyrinth spillway is at Ute Dam, New Mexico, designed by Reclamation for the New Mexico Interstate Stream Commission as an inexpensive substitute for a planned, gate-controlled spillway (fig. 7). In 1987 a flood stored over 9 m of water behind this structure, which was overtopped only a couple of centimeters, probably preventing severe downstream damage.
Figure 7. Labyrinth spillway at Ute Dam, New Mexico
Let's return to energy dissipators:
The most prevalent energy dissipater is some form of a stilling basin using the hydraulic jump. The jump has been recognized in one form or another for centuries. Leonardo da Vinci sketched it in one of his notebooks in the 15th century; Venturi wrote about it in the 18th century; and Georgio Bidone of the University of Turin (Italy) "discovered" the jump at about the same time. None of them were interested in it as an energy dissipater. Late in the 19th century and early in the 20th century, research studies were made in the United States at Lehigh University, Worcester Polytechnic University, Cornell University, the University of California, and probably many others. Advanced research was also being accomplished at many European universities.
The lack of energy dissipators and the dangers involved were also noted by Reclamation designers. They began to draw on the experience of European designers, the Tennessee Valley Authority, the Panama Canal designers, and many others. The natural evolution was that some of the more exotic structures featured parabolic humps in the floor, gigantic impact blocks, trapezoidal shapes, and practically anything else that would "slow the flow."
These were gradually eliminated in favor of more standardized designs. The Reclamation stilling basin at that time featured a rectangular shape that used as design parameters the inflow-outflow depths derived from the momentum equation; that is, the lengths were a function of the downstream depth and the heights a function of the incoming flow depth. The width and spacing of the appurtenances were left to the discretion of the individual designer, but usually occupied about half of the basin width. The major development in the 1940's to standardize energy dissipators was the work of Fred Blaisdell and his coworkers at the Saint Anthony Falls (SAF) hydraulic laboratory at the University of Minnesota. SAF was the major hydraulic research station of the Soil Conservation Service. The SAF stilling basin is extensively used throughout the world and is featured on one side of the ASCE Hydraulic Structures Medal.
From 1950 to 1960, Reclamation initiated an extensive research program with the objective of developing standard designs for energy dissipators. The product of this program was Engineering Monograph No. 25, "Hydraulic Design of Stilling Basins and Energy Dissipators."
The first modified hydraulic jump stilling basin in the monograph is referred to as Basin II (fig. 8). The basin contains chute blocks at the upstream end and a dentated end sill, but no intermediate or floor blocks. The end sill seems to have been patterned after the Rehbock sill, an appurtenance that was developed by Theodore Rehbock many years earlier.
Figure 8. Reclamation Basin II energy dissipater
The next most popular basin is Basin III (fig. 9). Basically, it is the same as Basin II except for a solid triangular end sill and a set of floor blocks placed at about the one-third point of the basin. Note the similarity between this basin and the SAF basin. Basin IV (fig. 10) was developed for low Froude number inflow, principally for small canal structures. Usually, a hydraulic jump in this range is not fully developed, is very unstable, and is accompanied by many surface waves. This basin has been revised and appears in the new edition of Design of Small Dams.
Figure 9. Reclamation Basin III energy dissipater
Figure 10. Reclamation Basin IV energy dissipater
Basin V (fig. 11), the so-called sloping apron basin, has been extensively used in the past but seems to have fallen into disfavor with contemporary designers. This basin was developed for Madden Dam in the Panama Canal Zone and is also used at Canyon Ferry Dam, Montana, and Folsom Dam, California, among others.
Figure 11. Reclamation Basin V sloping apron energy dissipater
Basin VII, the solid bucket, was developed for Grand Coulee Dam, Washington, in 1933, and a modified version known as the slotted bucket was developed for Angostura Dam, South Dakota, in 1945 (fig. 12). The slotted bucket is also used for Brantley Dam in New Mexico, the newest Reclamation dam.
Figure 12. Reclamation Basin VII bucket energy dissipater
The foregoing has been a summary of energy dissipators that feature the hydraulic jump, but there are many effective special-purpose energy dissipators. The workhorse of this group is Basin VI, the impact basin (fig. 13). This structure is used mostly at canal turnouts, at wasteways, or at the end of pipelines. It is a very effective energy dissipater.
Figure 13. Reclamation Basin N7I impact energy dissipater
Another special-purpose basin is the hollow-jet valve basin (fig. 14). Outlet works controlled by slide gates can use Basin II, Basin III, or a plunge pool; but due to its unusual jet shape, this valve seemed to require a unique basin. Unfortunately, this basin has proven to have some faults under certain operating conditions. It has worked exceptionally well at Boysen Dam in Wyoming, Falcon Dam in Texas, and Yellowtail Dam in Montana, but was far from satisfactory at Trinity Dam in California and Navajo Dam in New Mexico. It should be noted that the Trinity and Navajo structures operate at heads several times greater than those at Boysen and Falcon; and although Yellowtail is a high-head facility, it has some built-in structural features that apparently helped overcome some of the troubles found at Navajo and Trinity.
Figure 14. Reclamation hollow-jet valve energy dissipater
The baffled apron is a structure that was developed for use on canals as a drop or wasteway (fig. 15). The hydraulic design is related to the unit discharge (discharge per unit width). The objective of the structure is to dissipate the energy as the flow passes down the chute so that the residual energy at the bottom of the chute is equal to or less than the energy at the top of the chute. This proved to be such an effective canal structure that one of the Reclamation design engineers suggested that laboratory studies be made to develop a structure that could be used for larger spillway-type flows. The studies showed that the unit discharge could be increased to any quantity if it is practical to build the structure needed to contain the flow. Recently, many structures have been built that exceed the original unit discharge limit. These include conconully Dam (Washington) spillway at 78 m3/s/m, Marble Bluff Dam (Nevada) spillway at 112 m3/s/m, Soil Conservation Service Dam T or C (New Mexico) at 120 m3/s/m, and Utah Department of Water Resources Dam DMAD at, 100 M3/s/m.
Figure 15. Baffle apron drop energy dissipater
Recently, the trend for terminal structures has turned to the flip bucket, the principle being to direct the flow away from the structure and downstream a sufficient distance where the water can erode its own plunge pool or flow into a pre-excavated plunge pool. Following are some examples of flip buckets (fig. 16). Yellowtail Dam has a combined hydraulic jump/flip bucket; that is, it acts as a hydraulic jump energy dissipater up to a predetermined discharge where the jump flips out and the structure acts as a flip bucket for higher discharges. Glen Canyon Dam, Arizona, has a tunnel spillway through both abutments of the dam, both terminating in a flip bucket. Crystal Dam, Colorado, has an uncontrolled spillway near the top of the dam. A flip bucket directs the jet away from the dam where it impinges nearly vertically into a pre-excavated pool. Flaming Gorge Dam in Utah has a low-angle flip from a tunnel spillway in the left abutment. Trinity Dam tunnel spillway terminates in a flip bucket specifically shaped to direct the jet to the right in a dispersed jet. An example of a pre-excavated lined plunge pool is at Morrow Point Dam, Colorado.
Figure 16. Flip bucket at Glen Canyon Dam, Arizona
As previously mentioned, hydraulic jump basins and plunge pools have some inherent problems. These can include abrasion damage due to circulating debris in the basin, cavitation damage to the appurtenances due to high-velocity flow, or damage to the walls due to vibration. Generally, but not always, these factors can be predicted and corrected by model studies. Cavitation damage by high-velocity flow in tunnels and chutes has also been a problem. Aeration slots have been added to most of the Reclamation tunnel spillways to prevent cavitation and the severe erosion damage that occurred at Hoover, Yellowtail, and Glen Canyon Dams examples of near catastrophes leading to a major change in design concepts.
If you follow the trend in this discussion, you might detect a general theme: Don' t be opposed to trying new design concepts. They can be successful and will often lead to more efficient and economical structures.
The material for this paper represents fragments of information I have retained from reading many Bureau of Reclamation Hydraulic Laboratory reports during my 40-plus years of tenure. I am especially indebted to A. J. Peterka, editor of Engineering Monograph No. 25, "Hydraulic Design of Stilling Basins and Energy Dissipators," who was my mentor during most of that period.
Much of the general historical information came from the fine report of the ASCE Hydraulics Division Task Force on Energy Dissipators for Spillways and Outlet Works of the Committee on Hydraulic Structures, presented in the Journal of the Hydraulics Division, Vol. 90, No. HY 1, January 1964.
The Reclamation historical facts came from the aforementioned laboratory reports supplemented by articles in other Reclamation publications, including Reclamation Era, particularly Volume 63, Nos. 1 and 2, published in 1977 commemorating the 75th Anniversary of the Bureau of Reclamation.
Last reviewed: 10/26/04