8th IAHR ISHS 2020 Santiago, Chile, May 12th to 15th 2020

DOI: 10.14264/uql.2020.609

Some historical aspects on the hydraulic design of the Gatun Spillway in the

A.V. Bal1, F. Re2, M.R. Lapetina2 & N. Badano2 1Panama Canal Authority Balboa, Panama 2Stantec Buenos Aires, Argentina E-mail: [email protected]

ABSTRACT

The Gatun spillway in the is built on top of the sea-level canal project, which was excavated between 1881 and 1887 by the Universal Company of the Interoceanic Canal, of France. The project was changed in October 1887 to a lock canal project. The design of the Gatun Spillway was developed between 1909 and 1911 by the Isthmian Canal Commission (ICC), an organization which reported directly to the United States Secretary of War, and which had the support of some of the best engineering minds working at the best universities, engineering companies and government institutions of the United States and Europe. A 1:32 scale physical model was used to aid in the spillway design. The spillway was completed in 1913 and the Panama Canal began operating on August 15th, 1914. This paper presents some engineering and historical aspects of the hydraulic design of the Gatun Spillway. The spillway design hydrograph and the methodology used to estimate the number of spillway gates required is contrasted to the current engineering practice. A detailed hydraulic engineering study was performed for the spillway between 2011 and 2013, in order to evaluate its hydraulic performance and to determine its discharge rating curve, using the OpenFoam CFD model and a physical model at a scale of 1:40. Pool level routing simulations were performed using a Probable Maximum Flood hydrograph developed for the 3338 Km2 Panama Canal watershed, resulting in a complete assessment of spilling capacity for present and future operation policies.

Keywords: Spillway, hydraulic design, hydrology, history.

1. INTRODUCTION

The Panama Canal consists of an inland lake with a set of locks to transit vessels in a system that works by gravity, as is shown in Figure 1. Water supply is provided by the rainfall which falls in a 3338 Km2 watershed. Storage is provided by Gatun and Alhajuela lakes. There are dams and navigation locks at both ends of Gatun Lake. A secondary benefit of the dams is the production of hydroelectric power, for which there is an installed capacity of 24 MW at and 36 MW at .

Gatun Lake has a total volume of roughly 5500 Hm3. By the time of its construction, it was the largest artificial lake in the world. Its elevation is regulated by 14 gates in Gatun Spillway, each 13.7 meters wide. This lake guarantees the water supply to the nearby cities helps to maintain the proper levels in the Panama Canal navigation channel. Gatun Dam is 32 meters high. In Figure 2, two aerial views of Gatun dam and spillway are shown. One bridge crosses the discharge channel and another one was recently built just downstream of it. There is a drop of 18 meters measured from the ogee crest to the discharge channel level. The energy dissipation system consists of a curved spillway in plan view, with 21 impact baffles located at the beginning of the 293-m long and 87 meter wide discharge channel. As is shown in Figure 3 (a), baffles No. 15 and 16 were removed in 1927, in order to protect the powerhouse, which is located in the right side of the discharge channel, from the flow coming from the gates in the West side of the spillway. Figure 3 (b) shows how those baffles dissipate the energy during the spills. Figure 4 shows a section on the centerline of Gatun dam and spillway.

Gatun Spillway is a centenary structure which has operated even beyond the conditions foreseen by its designers. This article presents a brief description of the history of the spillway, and related hydrologic and hydraulic studies.

Pedro Gatun Miguel Atlantic Gatun Pacific Lake Locks Locks Ocean Locks (52 Km) Ocean Miraflores Lake (1.7 Km) (a) (b) Figure 1. (a) The Panama Canal Watershed. (b) Profile of the Panama Canal.

(a) (b) Figure 2. Aerial views of Gatun Spillway. (a) July 31st, 2019. (b) Spill of October 20th, 2005. Photos provided by the (ACP).

Two baffles removed N

(a) (b) Figure 3. (a) General Plan of Gatun Dam (Drawing 4050, ICC, October 27th, 1910). (b) Effect of the baffles on the East side of the spillway, during the spill exercise of November 26th, 2009. Photo provided by ACP.

100 feet (30.5 m) Figure 4. Section on Center Line of Gatun Spillway (partial view of Drawing 4060, Isthmian Canal Commission, February 25th, 1911). The elevations shown are in feet.

2. RIVER DIVERSION DURING THE CONSTRUCTION

The scheme used for the river diversion during Gatun spillway construction is shown in Figure 5. The drawing shows a superposition of the Gatun locks, Gatun dam, Gatun spillway and the French the sea-level canal project. The excavation of a sea-level canal project was attempted between 1881 and 1887 by the Universal Company of the Interoceanic Canal, of France. The project was changed in October 1887 to a lock canal project, due to the difficulties found during construction. However, a good portion of the project was built. Figure 5 shows in light green the navigation channel of the French sea-level canal. Two river diversions associated to that project are shown in red. Their function was to prevent the flood waters from the rivers in the Panama Canal basin to reach the navigation channel. The figure also shows in brown the borrow areas for the Gatun dam fill. Only the borrow areas downstream of the dam are shown. It can be observed that the spillway discharge channel ends in what today seems to be a section of the , but it was not part of that river. It was excavated during the dam construction. The original path of the Chagres River is shown in blue. The river was diverted during the construction of the spillway to the diversions shown in red. After the spillway was built to a certain elevation, the river flow went through the main body of the spillway and was controlled with cylindrical valves during the dry season, while allowing the overtopping through a partially constructed spillway during floods (see Figure 6).

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Figure 5. Pre-American conditions with superposition of Gatun Dam, Gatun Locks, Gatun Spillway, Mindi Dike, Chagres River, French Sea-Level Navigation Canal and French Canal Project River Diversions (partial view of Drawing 6104-27, The Panama Canal Section of Surveys, October 17th, 1936).

(a) (b) Figure 6. River diversion during construction. (a) Flow through the spillway body, which was regulated by cylindrical valves. Photo from ACP archives. (b) Flow over the partially constructed spillway. Detroit Publishing Company Collection (Library of Congress).

3. HYDROLOGIC STUDIES

The Gatun reservoir with a summit lake level at elevation 25.9 m (85 feet) was first proposed in the Minority Report of the Board of Consulting Engineers for the Panama Canal in 1906, which reported to the United States Government. In previous projects, the summit lake dam of the Panama Canal had been located approximately 15 Km upstream from Gatun, at a location called Bohio, where the Chagres river valley was narrower. For example, the American Isthmian Canal Commission of 1899-1901, adopted a plan with a dam at Bohio, which would form a lake with a normal elevation at 25.9 m (85.0 feet) above sea level, but fluctuating between 25.0 and 27.4 m (82.0 and 90.0 feet) (Goethals, 1915:18).

The main reason to change the location of the dam from Bohio, downstream to Gatun, was to increase the summit lake volume and the navigation distance in the lake. But it was also considered that “there would be less seepage beneath a dam built at Gatun than at Bohio”. Gatun Lake would be regulated between elevations 25.0 and 26.2 m (82.0 and 86.0 feet) (Board of Consulting Engineers: 68, 74). In order to increase the lake volume and therefore the water supply, this range of operation was increased in January 22nd, 1908, by chief engineer George Goethals to be between 24.4 and 26.5 m (80.0 and 87.0 feet) (The Canal Record: 195). The lake is currently operated between 24.0 and 26.8 m (78.8 and 88.0 feet).

The crest of Gatun Dam was placed at elevation 35.0 m (115 feet). The destructive effect of a flood would be felt first at the navigation lock structures, where the floor elevation was set at 28.0 m (92 feet), that is, 1.5 m (5.0 feet) above the maximum lake operation level of 26.5 m (87.0 feet).

The spillway discharge capacity was computed using a maximum river discharge of 4960 m3/s (175000 feet3/s), after adjusting the flow measured at Bohio by a factor of 1.62, in order to account for the larger drainage area at the dam site location. Keeping this inflow constant and with a spillway capacity of 4360 m3/s (154000 feet3/s), it was estimated that Gatun Lake would raise one foot from 26.5 to 26.8 m (87 to 88 feet) in 50 hours (Hodges, 1915: 4, 46). Note that this means that a rectangular hydrograph was used to route the inflow through the spillway, which is a conservative assumption that gives a certain factor of safety. Later it was found that the discharge capacity of the spillway was actually 4880 m3/s (172200 feet3/s), that is, 12% greater than what it was originally assumed, increasing even more the factor of safety of the design (Hodges, 1915: 49). This discharge value is close to the value of 4949 m3/s (174743 ft3/s), as is shown in the Panama Canal Flood Control Manual and to 4634 m3/s (163648 ft3/s), as was measured with a hydraulic physical model at a scale 1:40 in Argentina in 2011-2013.

Another way of checking the discharge capacity of the spillway was to compute the time that it would take for the lake to reach the coping of the navigation lock walls at elevation 28.0 m (92.0 feet), with all the spillway gates closed. Sibert wrote that starting with the lake at 26.5 m (87.0 feet) and taking the maximum known discharge of Chagres River for a period of 33 hours, which at the time had been estimated to be 3880 m3/s (137000 ft3/s), it would take 47 hours before the lake reached the top of the lock walls (Sibert, 1915: 185). Sibert also argued that the six lock culverts could also be used to spill, with a combined discharge capacity of 1130 m3/s (40000 ft3/s), which would give an additional margin of safety. Haskin presented a very similar lock culverts discharge value of 1390 m3/s (49000 ft3/s) (Haskin, 1915:37). The lock culverts discharge capacity presented in the Panama Canal Flood Manual is 1360 m3/s (48000 ft3/s), a value which is close to those presented a century ago. All the arguments and computations mentioned were recently checked for this paper and were found to be right.

There was little river discharge data available at the dam site when the spillway was being designed, between 1909 and 1911. The inflows considered in the analyses presented were based on only 30 years of recorded river gage data at Bohio. It was known at the time that the return period of the largest flood data available would occur only once in a century (Sibert, 1915:184). This can be understood as meaning that its return period was roughly about 100 years. This estimate was not very bad, considering that after a century of operations of the Panama Canal, the flood of December 2-4, 1906 is considered to be the third largest by volume.

After the opening of the Panama Canal in 1915, as more hydrological data became available, several hydrologic studies were performed on the Panama Canal Watershed, which allowed for a better understanding of the characteristics of the watershed. Some of these studies are the following: Kirkpatrick (1929), Riter (1931), Brod (1941), U.S. Weather Service – USACE, WES (1943), Randolph (1945), USACE (1979), HRC (2011). The studies led to the conclusion that it was necessary to augment the spilling capacity in Gatun Lake. An additional spillway for Gatun Lake was studied since the early 1930s and possibly before. In 1945, E. S. Randolph, principal engineer of the Panama Canal, presented a report with 12 possible locations for a new spillway for Gatun Lake. Randolph recommended the construction of a new spillway near the west end of the Gatun dam, with a discharge capacity similar to that of the existing Gatun spillway.

The Corps of Engineers of the United States, conducted between 1977 and 1980 a series of studies of flood control facilities in the Panama Canal. They included meteorological studies using the methodology of probable maximum precipitation (PMP) and hydrological studies of probable maximum flood (PMF). In 1979, the United States Corps of Engineers (USACE), made an estimation of the Probable Maximum Flood (PMF). The hydrograph of the PMF is compared in Figure 7 (a) to the storms of 2010 and 1906. It is seen that the storm of 1906, which was used or the design of the Gatun Spillway to justify its spilling capacity, is small in peak flow and volume when compared to the PMF. Figure 7 (b) shows that the PMF computed in 1979 lies on the Francou-Rodier envelope, which relates the maximum flow to the basin area, for basins with an area greater than 300 Km2 (Berga, 2004).

The Panama Canal Authority (ACP) used the PMF hydrograph to perform pool-level routing analyses of Gatun Lake, in order to evaluate the existing spilling capacity. It was decided to limit the flood level to 27.9 m (91.5 feet) during the PMF, because the floor elevation at the navigation locks was at elevation 28.0 m (92.0 feet), allowing for a 0.15 m (0.5 feet freeboard). It was found that an additional spilling capacity was needed if the maximum operational level is kept at its current level. Therefore, in order to guarantee the flood safety, the Panama Canal Authority needs to decide on one of the following options: provide additional spilling facilities or lower the maximum operational lake level. This last option would attain the desired safety level by increasing the available flood storage in the reservoir, but at the cost of decreasing the water conservation storage and increasing the risk of having navigation draft restrictions.

25,000 1,000,000

A < 300 Km2 A > 300 Km2 20,000 100,000 /s)

3 10,000 15,000 1,000 / s) 3 10,000 100 Peak Flow (m 10 Flow (m 5,000 1 0 2 1234 Catchment Area (Km ) Day ICOLD 2003 IAHS 1984 PMF ‐ Gatun (USACE 1979) PMF ‐ Madden (USACE 1979) 1906 2010 ½ PMF PMF 3.820 m3/s threshold Envelope (a) (b) Figure 7. (a) Hydrographs of major storms in the Panama Canal Basin and their comparison to the PMF and ½ PMF hydrographs. The blue line marks the applied threshold level which defines the estimated volume of the storms. U.S. army Corps of Engineers (1979) and ACP hydrological records. (b) Envelope curve of the extreme floods registered in the world (Berga, 2004).

More recently, the ACP decided to update the meteorological and hydrological 1979 study of the USACE, using additional historical information, as well as the latest techniques and technological tools available. The ACP and the Hydrologic Research Center in San Diego, California, conducted in 2007 a study of extreme rainfall in the Panama Canal basin, in which the MM5 mesoscale weather model was used. Although there are daily data since 1908 it was decided to use only data after 1950 in the statistical development of the models. With the participation of the ACP, 20 intense storms observed in the Panama Canal basin were selected after 1979, which were simulated with the model. With the same model, 40 additional synthetic storms were generated. In December 2010 occurred the worst storm in the history of the Panama Canal. As can be seen in Figure 7 (a), the hydrograph of that storm can be compared in volume and peak flow with ½ PMF hydrograph. Using the information gathered during that storm, the series was extended to ten thousand years. The rainfall data was used in a hydrological model and the results were an estimation of the risk of damage associated with the high levels of Gatun Lake and a relationship between storm volume and return period (Georgakakos, 2011).

4. HYDRAULIC STUDIES

4.1. Initial design

The logic behind the hydraulic design of the spillway is described by Sherman, Design Engineer of the Isthmian Canal Commission (ICC). He explains that in order to be able to route the design flood, a very long spillway would be necessary, had an uncontrolled crest at lake level been chosen. So the first criteria was to adopt a crest elevation below the operational lake level and to control the discharge with steel sluice gates of the Stoney type. It was found that about midway in along the Gatun Dam axis, there was a rocky hill outcropping which provided an excellent site for the foundation of the masonry structure, so it was selected to locate the spillway. The extent of the available foundation determined in part the length of the spillway, and consequently, the elevation of the crest was set at 21.03 m (69 ft), 4.88 m (16 ft) the below normal lake elevation of 25.91 m (85 feet). A fan shape for the spillway was chosen because it was found to be an excellent solution, which provided more crest length in the limited available space.

In order to determine the spilling capacity, the designer studied the matter with great care. Sherman explains that the 13.72 m (45 feet) gate width of the 14 gates was determined for a desired discharge capacity of 3965 m3/s (140000 feet3/s) at the normal lake elevation of 25.91 m (85 feet), assuming a discharge coefficient C of 3.47 (1.92 in metric units). A 0.61 m (2 feet) rise of the lake was expected under these conditions. The value of C was estimated to be somewhere between 3.0 and 3.5 in English units (between 1.66 and 1.96 in metric units), if the formula used to compute the discharge was q = C (L – 0.2 h) h1.5, where q is the flow, C is the discharge coefficient, L is the length and h is the head. The problem was that at that time, the head over the sill of the spillway gates was greater than which had been provided in any structure which had been built before. Since the normal lake elevation was 25.91 m (85 feet) and the sill was placed at elevation 21.03 m (69 feet), there was a head of 4.88 m (16 feet) over the sill. The greatest head for which there was definite information at the time was 2.13 m (7 feet) at La Grange dam in California. After the spillway had been built, a value of 3.94 (2.18 in metric units) was measured during a flood, for a head of 5.18 m (17 feet), which gave a sense of security. The spillway was thought to be able to discharge a flow larger than it was likely ever to be required.

The current crest profiles did not exist at that time, so the designer chose a shape that could follow the lower nappe of the jet, a criteria which is similar criteria to the modern design guidelines. He selected an intermediate gate opening, estimated the velocity of the jet, and then selected a parabola for the upper part of the downstream face, with a shape given by the equation h2 = 42 v, where h is the abscissa and v is the ordinate. The lower part of the profile was a circular, concave up curve of long radius. Sherman also explains that “the design of the downstream face was controlled by the principle that the nappe should adhere to the masonry to prevent air from entering under it to cause chattering and the consequent lifting action, which, with such an enormous quantity of water, might be dangerous to the structure”.

The elevation of the discharge channel was set at 3.05 m (10 ft), with no pool to serve as a cushion. The designer estimated that the water velocity would be about 18 m/s (60 ft/s), which would be dangerous for the discharge channel floor. The stream depth could not be determined at that time. The uncertainty of the conditions at the toe of the spillway led the designer to choose a method which could reduce the velocity at the toe of the spillway and at the same time to obtain a more uniform flow, at a reasonable velocity in the discharge channel. Therefore, a system of baffle blocks was introduced, somewhat similar to the baffles used at Wachusett Dam. The discharge channel was made 292 m (960 ft) long, so the water would be discharged at a point were erosion would do no harm to the toe of the dam. The stream below the baffle piers, would then have a depth of 6.1 m (20 ft) and a velocity of 6.1 m/s (20 feet/s). (Sherman, 1916: 135-142).

4.2. Numerical models

A complete hydraulic study of the lower Chagres River and the Gatun Spillway was performed in Argentina between 2011 and 2012, by the engineering company Montgomery Watson Harza (MWH). The studies included a verification of the hydraulic operation of the spillway structure, considering new operating policies of Gatun Lake, which include a 0.46 m (1.5 feet) increase in the maximum operating level. Possible modifications to the spillway to increase its discharge capacity were also investigated. The study included the development of three numerical models. A HEC-RAS 1-D model was developed for the 12-Km section of the Chagres River located downstream of the spillway. This model helped to provide information on the boundary condition downstream of

the spillway discharge channel. In the roughly 2.5-Km long segment which is located just downstream of the spillway, the MOHID 2-D Modeling System was used. This model had been applied by MWH during the studies for the Third Set of Locks of Panama Canal. The model uses a finite volume approach and was developed by the Marine, Environment and Technology (MARETEC) center of the Technical University of Lisbon.

The OpenFoam 3-D model was also used in this project, as is shown in Figure 8. Its code can simulate anything from complex fluid flows involving chemical reactions, turbulence and heat transfer, to solid dynamics, electromagnetic and the pricing of financial options. The core technology of OpenFOAM is a flexible set of efficient C++ modules. They are used to build solvers, which simulate specific problems in engineering mechanics. Utility modules are used to perform pre- and post-processing tasks ranging from simple data manipulations to visualization. The model has mesh processing capabilities and libraries, which are accessible to the solvers and utilities, such as libraries of physical models. Turbulence was modeled according to a LES (Large Eddy Simulation) approach. In order to speed up computations, in lieu of the LES approach, the RANS (Reynolds Averaged Navier-Stokes) approach was used. As sub-grid scale (SGS) model for the LES approach, a sub-grid kinetic energy equation eddy viscosity model was used. The Deardorff’s method was selected to define the filter cutoff length. A wall model was considered to treat the boundary conditions at solid borders. Spalding law-of- the-wall was selected for the velocity. A zero normal gradient condition was taken for the remaining variables. The methodology was validated by previously simulating a standard ogee crest and then comparing the results with the available data and literature. The results of the Gatun Spillway numerical CFD model were later compared with the physical model, but no calibration of the numerical model was made.

(a) (b) Figure 8. (a) Water level and velocity distributions from spillway and chute 3D models. MWH, June 2012. (b) Streamlines for La Purisima Storm (December 8th, 2010). Detail of circulation. MWH, June 2012.

4.3. Physical models

A 1:32 scale physical model was used during the design and construction of the Gatun Spillway, as is shown in Figure 9. Roughly a century later, in 2012, another physical model at a scale of 1:40 according to Froude similitude, was built for Gatun Spillway at the Hydraulic Laboratory of the Argentinian National Water Institute (Instituto Nacional del Agua, or INA) in Ezeiza, Argentina. This model is shown in Figure 10. It was used in conjunction with the already presented 3-D OpenFoam model, in order to obtain the discharge capacity curve of the spillway, which is shown in Figure 11. The purpose of the model was to analyze the hydrodynamic performance of the existing spillway due to the potential increase of its maximum flow and the possible increase in the total flow discharged into the Chagres River. Possible modifications to the spillway were also studied, in order to allow for a greater discharge of the spillway at the maximum reservoir level expected in the future and also to evaluate the spillway operating policies to improve the operating conditions in the discharge channel. There is limited information available on how the previous theoretical discharge curve shown in Figure 11 was developed. It is believed that the physical model presented in Figure 9 was used to compute the values presented in the historical literature of 1915, which are shown in the fourth paragraph of Section 4 (page 4) and in the second paragraph of Section 5.1 (page 6). However, none of those values coincide perfectly with the curve shown in Figure 11, even though they are close. It is therefore believed that later computations were performed on the spillway discharge capacity. The most likely source of the previous theoretical curve shown in Figure 11, is a computer code written in the Basic language in 1983 and seems to have been revised in 1987 and 1993. The discharge computation includes a correction in order to consider the effect on the friction losses of the protecting rock fill shown in Figures 12 and 13, which is located in front of the spillway.

Figure 9. Physical model of Gatun Spillway used for hydraulic experiments at a scale of 1:32 during the construction of the spillway. February 1910. Isthmian Canal Commission, 1910 Annual Report.

Figure 10. Physical model of Gatun Spillway at a scale of 1:40. Ezeiza, Argentina, August 6th, 2012.

29.0

28.0

27.0

26.0

Elevation Gatun Lake (m PLD) 25.0 Previous Theoretical discharge curve

24.0 Numerical discharge curve

Physical Model

23.0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Discharge (m3/s)

Figure 11. Discharge rating curve estimation for the Gatun Spillway, 2012.

5. WORKS TO PROTECT THE SPILLWAY DURING WORLD WAR II

During World War II, several measures were taken to protect the Panama Canal. One of them was the construction of an underwater berm in front of the spillway. It had protection nets on top of it, in order to protect the spillway gates from bombs which could be dropped from an airplane and impact the spillway after jumping over the lake surface. Railroad tracks were also placed on top of the berm. This provided a second crossing over the spillway, in addition to the bridge over the discharge channel. Figure 12 shows the underwater berm and the railroad trestle. Figure 13 shows an aerial picture during a spill with the protection installed. Nowadays, the trestle over the berm has been removed, but the berm is still in place.

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(a) (b) Figure 12. Protecting rock fill in front of Gatun Spillway (partial view of Drawing 5186-60, The Panama Canal Department of Operation and Maintenance, September 25th, 1940). (a) Plan View. (b) Sections A–A and B–B. The elevations shown are in feet.

Figure 13. View of a spill before the trestle over the berm was removed. Photo from ACP archives.

6. CONCLUSIONS

The following conclusions are derived from a careful review of the historical, hydrological and hydraulic documentation related to Gatun Lake Spillway:

• The Gatun Spillway was designed between 1909 and 1911, with only 22 years of hydrological information, to operate Gatun Lake up to a maximum of 26.5 m (87 feet) over sea level. The volume of the 1906 flood, which was used to design the Gatun Spillway during the construction of the Panama Canal, is only 18% of the volume of the Probable Maximum Flood (PMF) computed by USACE in 1979. That flood has a return period of less than 100 years, which is below the 10.000-year return period that is usually associated with the PMF.

• In 1980, the Gatun Lake operating level was increased from 26.5 to 26.7 m (87.0 to 87.5 feet). The current operation level is 26.8 m (88.0 feet). It is desired to increase it further to 27.1 m (89.0 feet), in order to gain additional storage volume. Raising the maximum operational level of Gatun Lake from 26.7 to 27.1 m (87.5 to 89.0 feet), resulted in a reduction of 37.1% of the storage volume of Gatun Lake for flood control, considering an

allowed maximum flood elevation of 27.9 m (91.5 feet). These changes made the structure operate regularly with more demanding conditions that it was designed for.

• During the December 2010 “La Purisima” extreme flood event, the spillway operated with all 14 gates open during 35 hours, with lake levels over 26.5 m (87.0 feet), reaching a maximum elevation of 27.0 m (88.5 feet). According the physical model results, a maximum discharge of 5200 m3/s (184000 feet3/s) was measured at the maximum lake elevation, for a head of almost 6.1 m (20 feet). This discharge condition is higher than the design scenario made 100 years before, and the spillway performed very well.

• Gatun Spillway is a centenary structure with a very thoughtful design. Advantage was taken of the space available to allow the discharge of a large flood. It was a hydraulic structure which was advanced for its time. The operating conditions changed during its useful life, exceeding the original design conditions. In 2010, the spillway routed a flood which was much larger than the design hydrograph and suffered little damage.

7. REFERENCES

Bal, A.V, Leis, G. (2013). Chagres River level measurements made during the spill of November 16th, 2012. Panama Canal Authority.

Berga (2004). Dams and Floods. New Trends in Hydrological Safety of Dams. Proceedings of the Workshop on Dam Safety Problems and Solutions – Sharing Experience. ICOLD 72nd Annual Meeting. Seoul, Korea.

Board of Consulting Engineers for the Panama Canal (1906). Government Printing Office, Washington.

Francou, J. & Rodier, J. A. (1967). Essai de classification des crues maximales observees dans le monde. In: Cah. ORSTOM. ser. Hydrol, vol. IV(3).

Georgakakos, Holly, Shamir, Spencer (2011). Risk-Based Analysis of Gatun Spillway Capacity After the Extreme Rainfall of Year 2010. Hydrologic Research Center. San Diego, California, U.S.A.

Goethals (1915). Introduction. Transactions of the International Engineering Congress on the Panama Canal. San Francisco, California.

Haskin (1915). The Panama Canal. Doubleday, Page & Company. New York.

Hodges (1915). General Design of the Locks, Dams and Regulating Works of the Panama Canal. Transactions of the International Engineering Congress on the Panama Canal. San Francisco, California.

National Weather Service (1978). Probable Maximum Precipitation Estimates for Drainages Above Gatun and Madden Dams, .

Panama Canal Authority (2019). Manual de Control de Inundaciones (Flood Control Manual). Panama.

Randolph, E. S. (1945). Report on Preliminary Office Study on Additional Spillway Capacity from Gatun Lake. Principal Engineer of the Panama Canal.

Sherman, E.C. (1916). The spillways of the Panama Canal. Journal of the Boston Society of Civil Engineers. Vol. III, No. 4.

Sibert, W.L. and Stevens, J. F. (1915). The construction of the Panama Canal. D. Appleton Company. New York and London.

Canal Record (a periodical publication, published weekly under the authority and supervision of the Isthmian Canal Commission, U.S.A).

U.S. Army Corps of Engineers (1979). Development of Probable Maximum Flood and Review of Flood Routing Procedures. Phase III and Phase IV Studies. Addendum Report. Engineering Services for the Panama Canal. Mobile District. U.S.A.