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 Panama Canal 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 Panama Canal 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 Gatun Lake 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 Gatun dam and 36 MW at Madden dam. 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 Miraflores 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 Panama Canal Authority (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 Chagres River, 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). N 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.