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Owing to Gatun Lake conditions not being comparable with conditions previous to 1914, in matters of evaporation, seepage, retention in swamps, and rainfall direct into the lake, it has become necessary to start a new era of average net and total yields, beginning January, 1914. For this reason no reliable Gatun Lake average long-term yields will be available for some time.

The run-off at Alhajuela for the year 1914 was 10 per cent below the 25-year average, and for the dry season 1915, 30 per cent above the 26-year dry season average. See Tables Nos. 13 and 14 and plate No. 82.

Plate No. 81 gives the total yield, net yield, land-area yield, useful outflow, storage, rainfall on the lake yield, and evaporation, massed, for the Gatun Lake watershed for the year 1914 and the dry season of 1915. The total yield for 1914 was 102 per cent of the capacity of the lake at elevation 85. The water losses in the lake due to lockages, leakage, power, and pumping have considerably increased during the period January to April, inclusive, 1915, over that of any similar length period of 1914. This is due to the fact that one turbine ran from June 1, 1914, whereas three have been taking water since December 29, 1914. The individual high item in water losses due to useful outflow is for hydroelectric power. The ratio between lockage and power water losses varies as 1 to 2.23 during the dry season of 1915. During the total period of lockages, September 13, 1913, to July 1, 1915, there have been lockages as follows:

Gatun locks.....

Pedro Miguel locks.
Miraflores locks.

1,325 1, 330 1, 317

Tables Nos. 9 and 10 give the hydrology of Gatun Lake watershed for the year 1914 and the dry season 1915, respectively. In like manner Tables Nos. 11 and 12 cover the Miraflores Lake hydrology.

Plates Nos. 85, 86, 87, and 88 cover January, February, March, and April, 1915, studies of Gatun Lake heights, respectively. Table No. 17 gives the monthly Gatun and Miraflores Lakes and Chagres River heights for the year 1914. Table 15 gives 1914 monthly discharges at Alhajuela and various data on Gatun Lake.

Plate No. 91 gives the 25-year average (1890-1914, inclusive) discharge duration by months for Chagres River at Alhajuela.

There were no large freshets during the year, the largest two being those of October 7, 1914, and April 3, 1915, at 31,650 and 27,085 c. f. s., respectively, at Alhajuela. Table No. 16 gives the data on the principal freshets. While there were no large freshets in 1914, there were an unusual number of good runs of water well distributed over the wet season. Plate No. 90 gives slope data for past seven years as it has changed, due to lake elevations. The northers of February 9, 10, 11, and April 3. 4, 1915, which did so much damage in Limon Bay, resulted in heavy rainfall over the northern end of Gatun Lake watershed, and gave record yields for those months. All freshets have been handled by the operation of not over five spillway gates, except on October 7, 1914, when six were run for 4 hours 22 minutes, with a maximum discharge of 70,850 c. f. s.

GATUN.

The current meter gaugings at the Gatun spillway gauging station were continued to January 15, 1915. This series of 43 gaugings was begun on December 31, 1913, and taken during spillway discharge at various heights of the Gatun Lake. Average

Q

values of the constant in Francis's formula C=; LH and the rectangular opening

Q
LH√2gH'

formula C1have been calculated from the measured discharges between elevations 84 and 87; a rating curve and table have been prepared. C in Francis's formula at elevation 87 is 3.77; the designing engineer used C=3 in providing a spillway area for passing 144,000 c. f. s. with 14 gates, elevation 87. The true total

3.77

discharge is 3.00 144,000, or very nearly 181,000 c. f. s., with Gatun Lake at 87. At elevation 85 the ordinary wet season lake height C=3.60, and the discharge of 14 gates is 146,500 c. f. s.

ALHAJUELA.

Ninety-eight current-meter measurements were made on the main river at the Calle Larga gauging station during 1914 and 34 were made in 1915 up to July 1, covering elevations 103.10 to 90.91, inclusive. Seven check gaugings were made in 1914 and three in 1915 of the upper tributaries at Dos Bocas (the forks of the Chagres), covering

the discharge from the La Puente, the Pequeni, and the Chagres branches. The results have been satisfactory. The Calle Larga cross section taken January 21, 1915, showed minor changes, and a new discharge curve and table were prepared. Alhajuela and Vigia have been continued as flood-warning stations.

CANAL CURRENT VELOCITIES.

Miraflores Locks.-Surface current velocities at 1 and 2 foot depths, at high and low tide, were measured in the middle of the channel at the lower west operating gate at Miraflores Locks on May 31, 1915. These velocities are due to the intermixing of the salt and fresh water at the opening of the gates. Plate No. 84 gives the results of gaugings. This test was supplementary to those of April, 1914, mentioned in the 1914 Annual Report of The Panama Canal. The maxima velocities of 6.1 and 4.4 feet per second at high and low tides of +8 and -7, respectively, are considerably higher than the velocity of 2.75 feet per second obtained at mean tide in the April, 1914, tests, for the reason that the water in the lower level of the lock was freshened by a second filling from the upper level. Conditions were thus nearer the theoretical ratio of 1.02 to 1 of the specific gravities of salt and fresh water. In the more recent test the current meter was located 55 feet nearer the pintle line of the gate leaves, thus catching the first rush of the water on the opening of the leaves.

Gamboa Bridge. The following data on maximum current velocities, due to Chagres River freshets, have been obtained by current meter measurements taken at 1-foot depths at Gamboa Bridge:

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The currents are at right angles to the Panama Canal channel, the bridge being just north of the east bank of the canal. The back water of this arm of the lake extends about 8 miles above Gamboa Bridge.

Currents in Gatun Lake due to spillway operations.-Current-meter measurements were made on May 7, 8, and 9, 1915, in order to determine the amount of currents set up in the canal when four or five spillway gates are in simultaneous operation. Results obtained are as follows:

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The arrangement of gates open on the 7th was expected to give minimum current in the canal near buoy No. 2, owing to the arc shape of the spillway and the position of Guarapo Island. The arrangement of gates open on the 9th was intended to give the maximum current at buoy No. 2, possible with four gates open.

Local topography at Bohio would seem to be the explanation of the comparative considerable current there, the width between the two islands being 1,200 feet.

Currents about upper approach wall, Gatun Locks, during spillway operations.-Current meter gaugings made at points mentioned below about upper approach wall, Gatun Locks, on May 26, 1915, gave results as below. These gaugings were made at the request of the acting marine superintendent, Commander Butler. It is believed that the spillway gates open, Nos. 6, 7, 8, 9, and 10, are so located as to give the greatest current effect, at Gatun Locks, possible for five gates.

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It may be noted that the wind direction was toward the spillway, and that the current velocity slowed down with the wind. It is probable that the highest current velocity was helped by the gale, which reached its maximum of SE. 25 miles an hour, or 214 knots, at 8.15 p. m.

During the gauging period the lake fell from elevation 85.54 to 85.45. This was partially due to the decrease in wind velocity, the lake elevation having jumped up 0.15 foot in a few minutes during the maximum wind velocity.

Currents in Gaillard Cut during Pedro Miguel lockages.-On December 4, 1914, current meter measurement at 1-foot depth was made in narrowest part of Gaillard Cut at the slide at station 1791+25 to determine the velocity and behavior of the current due to the opening of the valves at Pedro Miguel Lock. The pontoon bridge at Paraiso was open, and dredges and barges were the only vessels in the Cut. wind was blowing northwest 8 miles per hour.

The

Plate No. 92 shows the results obtained. The maximum velocity was 1.36 feet per second or 0.9 mile per hour toward Pedro Miguel.

Time duration of Pedro Miguel lockage effects in Gatun Lake.-On November 7, 1914, a test was made to find the time interval and height from the trough to crest of waves at Pedro Miguel, Gamboa, and Juan Mina. These waves are caused by the opening of the upper valves at Pedro Miguel Lock.

The water stage registers at Pedro Miguel, Gamboa, and Juan Mina were set to the minute at 7.15 a. m. to insure correct timing. The scale on the automatic records was increased to 1 to 5, so as to give accurate heights.

Below is the record made during the lockage of the steamship American Transport, northbound. The Pedro Miguel lock valves at the west chamber were wide open, the Paraiso pontoon bridge was open and no vessels except the regular dredging fleet were in Gaillard Cut. This record is typical under normal working conditions:

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The leakages of valves in the locks have been found to vary considerably at various times, due to obstructions catching under or at the sides. This division made a number of gaugings and weir measurements at Gatun Locks during July and August, 1914, in order to check the leakage found by the testing engineers at Gatun and the Pacific Locks. In September, 1914, further work was done by the lock superintendents in observing the fall of the water in a chamber during the night. This leakage was from both valves and lock gates. These tests, together with those on Gatun spillway, described in last year's annual report, have resulted in the use of the following coefficients in the leakage calculations of the lakes' hydrologies:

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These coefficients are increased when usual leakage conditions obtain, such as when spillway or lock caissons are in use.

MISCELLANEOUS.

Various estimates have been made during the year on time taken for raising or lowering the lake to stated heights.

A current meter car rating station was established in July, 1914, at the Pedro Miguel Pond. This station is well located for satisfactory results. Fourteen ratings have been made at this location and all meters are now in good condition. A HaskellRichie current velocity and direction meter is now under order for special current investigations.

The flow through the north Venturi meter in the Miraflores filtration plant was checked by the measurement of the flow over the municipal division's three-panel rectangular weir, and report made to the municipal engineer.

The usual maintenance work on the various stations has been done. No change in station locations has been made over last year.

The following tables and diagrams accompany the hydrographical section of this report:

Plate No. 81. Yields, storage, and losses mass curves, Gatun Lake watershed. Plate No. 82. Alhajuela average monthly discharges.

Plate No. 83. Total yield of Gatun Lake, 1914, and dry season, 1915.

Plate No. 84. Current-velocity observations, Miraflores Locks.

Plate No. 85.

Plate No. 86.

Plate No. 87.

Study of Gatun Lake Heights, January, 1915.
Study of Gatun Lake Heights, February, 1915.
Study of Gatun Lake Heights, March, 1915.
Study of Gatun Lake Heights, April, 1915.
Alhajuela mass curves.

Ratio curves, Vigia to Gamboa rise.

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Surge in Gaillard Cut due to Pedro Miguel lockage.

Alhajuela discharge duration curve.

Table No. 9. Hydrology of Gatun Lake watershed, 1914.

Table No. 10. Hydrology of Gatun Lake watershed, dry season, 1915.
Table No. 11. Hydrology of Miraflores Lake watershed, 1914.

Table No. 12. Hydrology of Miraflores Lake watershed, dry season, 1915.
Table No. 13. Hydrology of Chagres, 1914.

Table No. 14. Hydrology of Chagres, dry season, 1915.

Table No. 15. Monthly 1914 Alhajuela discharge, Gatun Lake hydrology, 1914. Table No. 16. Principal freshets of 1914 and dry season 1915.

Table No. 17. Maximum, minimum, and mean elevations, by months, at all stations from January to December, inclusive, 1914.

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TABLE 10.-Hydrology of Gatun Lake watershed, dry season, 1915.1
[Drainage area, 1,320 square miles. 491 Pedro Miguel lockages. 492 Gatun lockages.]

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