Gatun Dam

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The Gatun Dam spillway in full discharge during the rainy season (April through December).

The Gatun Dam is a large earthen dam across the Chagres River in Panama, near the town of Gatun. The dam, constructed between 1907 and 1913, is a crucial element of the Panama Canal; it impounds the artificial Gatun Lake, which in turn carries ships for 33 km (20 statute miles) of their transit across the Isthmus of Panama. In addition, a hydro-electric generating station at the dam generates electricity which is used to operate the locks and other equipment in the canal.

Construction of the dam was a great engineering achievement, eclipsed only by the parallel excavation of the Gaillard Cut; at the time of completion, the dam was the largest earth dam in the world, and Lake Gatun was the largest artificial lake in the world.

[edit] Description

The dam is situated in the valley of the Chagres River, about 10 km (6 miles) from its mouth in the Caribbean Sea. The hills bordering the valley of the Chagres form a gap just over 2 km (1.4 miles) wide at this point, with a natural rocky hill in the centre of the gap. The gap is filled by an earth dam, 640 metres (2,100 ft) thick at the base, 2,300 metres (7,500 ft) long along the top, 121 metres (398 ft) thick at the water level, and 30 metres (100 ft) thick at the top, which is 9 metres (30 ft) above the normal lake level.

The spillway for the dam is constructed on the central hill; it consists of a semi-circular concrete dam, which regulates the flow of water down a concrete channel built into the back slope of the hill. The spillway dam itself measures 225 metres (740 ft) along the top; its crest is at 16 feet below the normal lake level. The spillway is designed so that water pouring over the semi-circular dam converges at the bottom from opposite directions and neutralises its own force, thus minimising erosion below.

The spillway dam is topped by 14 gates, supported by concrete piers and each 14 metres (45 ft) wide by 6 metres (20 ft) high. These gates, which are electrically operated, are raised or lowered to control the flow of water; with the lake level at 26.5 metres (87 ft), its planned maximum level, the capacity of the spillway is 4,100 m³ (145,000 ft³) per second, more than the maximum flow of the Chagres River. In addition to this, the culverts in the locks can dispose of 1,400 m³ (50,000 ft³) per second.

Gatun Lake has an area of 425 km² (164 square miles) at its normal level; it stores 5.2 cubic kilometres (4,200,000 acre-feet) of water, which is about as much as the Chagres River brings down in an average year.

[edit] Power Generation

The dam incorporates a hydro-electric generating station, which is situated on the east side of the spillway discharge channel. This uses water from the lake to drive a number of turbine-generators; as first commissioned, three generators were installed, producing a total of 6 megawatts of electricity. The power generated is used for the operation of the lock and spillway machinery, and for the lighting of the locks and the canal villages.

[edit] Construction

This map from the construction era illustrates the area around the dam and locks, which are shown superposed over the previous French excavations.

As described in History of the Panama Canal, the canal effort was began by a French team, who planned to construct a sea-level canal linking the two coasts; this would not have required the dam to be built. When the United States took over this effort on 4 May 1904, some time was spent in preparation and planning before the work got up to full speed, and it was not until 1906 that a lock-based canal was decided upon.

Even before this decision was made, Major George Washington Goethals, the chief engineer for the bulk of the construction effort, had already carried out an investigation into the suitability of the land at Gatun for the building of a large dam. Extensive test borings were made to determine the suitability of the land, and pressure tests were carried out on the material to be used in construction to determine its durability.

The Gatun location was in most ways ideal for a dam; the hills enclosing the Chagres open very wide around the area that is now the lake, then close in to a relatively narrow gap with a natural rock-based hill in the centre. This allows a relatively small dam to enclose a huge body of water, which both provides passage for ships across much of the isthmus, and provides a reservoir of water with which to operate the locks. The central hill was the ideal solid base for the construction of the concrete spillway and its dam, the main part of the dam being earth. The only problem was the huge scale of the dam required.

The dam was constructed by creating two parallel walls of stone, 366 metres (1200 ft) apart, using material excavated from Culebra and the lock site. Between these walls an impervious core was created, using a hydraulic fill technique; this was facilitated by the soft clay present in the valley below. Dredges were used to excavate this clay and pump it up into a pond between the outer walls of the dam; the material was allowed to settle out, and the water was drawn off and pumped back downstream. Thus, a solid core of natural cement was constructed within the dam.

After the dam was built to its desired height, the entire up-stream side was armoured by placing large boulders on the face, particularly where there is strong wave action, to break the force of the waves.

The dam contains some 17,000,000 m³ (22,000,000 cubic yards) of material, and weighs some 27,000,000 tonnes (30,000,000 short tons). It covers 1.17 km² (288 acres) of ground, and contains enough earth and rock to build a wall 1½ metres high and 29 cm thick (or four foot eight inches high and a foot thick) around the earth at the equator.

[edit] References

• History Of The Panama Canal, by Ira E. Bennett

• The Panama Canal, by Colonel George W. Goethals