Popular Science Monthly/Volume 61/July 1902/The Panama Route for a Ship Canal I
|THE PANAMA ROUTE FOR A SHIP CANAL.|
THE Panama route as a line of transit across the isthmus was established, as near as can be determined, between 1517 and 1520. The first settlement, at the site of the town of old Panama, six or seven miles easterly of the present city of that name, was begun in August, 1517. This was the Pacific end of the line. The Atlantic end was finally established in 1519 at Nombre de Dios, the more easterly port of Acla, where Balboa was tried and executed, having first been selected but subsequently rejected.
The old town of Panama was made a city by royal decree from the throne of Spain in September, 1521. At the same time it was given a coat of arms and special privileges were conferred upon it. The course of travel then established ran by a road well known at the present time through a small place called Cruces on the River Chagres, about seventeen miles distant from Panama. It must have been an excellent road for those days. Bridges were even laid across streams and the surface was paved, although probably rather crudely. According to some accounts it was only wide enough for use by beasts of burden, but some have stated that it was wide enough to enable two carts to pass each other.
The harbor of the Atlantic terminus at Nombre de Dios did not prove entirely satisfactory and Porto Bello, westerly of the former point, was made the Atlantic port in 1597 for this Isthmian line of transit. The harbor of Porto Bello is excellent and the location was more healthful, yet Porto Bello itself was subsequently abandoned, largely on account of its unhealthiness.
As early as 1534 or soon after that date boats began to pass up and down the Chagres river between Cruces and its mouth on the Caribbean shore, and thence along the coast to Nombre de Dios and subsequently to Porto Bello. The importance of the commerce, which sprang up across the isthmus and in connection with this Isthmian route, is well set forth in the last paragraph on page 28 of the report of the Isthmian Canal Commission:
This commercial prosperity suggested to those interested in it, and soon after its beginning, the possibility of a ship canal to connect the waters of the two oceans. It is stated even that Charles V. directed that survey should be made for the purpose of determining the feasibility of such a work as early as 1520.
From that time on the city of Panama increased in wealth and population in consequence of its commercial importance. Trade was established with the west coast of South America and with the ports on the Pacific coast of Central America. In spite of the fact that it was made by the Spaniards a fortress second in strength in America only to old Cartagena it was sacked and burned by Morgan's buccaneers in February, 1671. The new city, that is the present city, was founded in 1673, it not being considered advisable to rebuild on the old site.
The project of a canal on this route was kept alive for more than three centuries by agitation, sometimes active, and sometimes apparently dying out for long periods until there was organized in Paris in 1876 a company entitled 'Société Civile Internationale du Canal Interocéanique,' with Gen. Etienne Turr as president, for the purpose of making surveys and explorations for a ship canal between the two oceans on this route.
The work on the isthmus for this company was prosecuted under the direction of Lieut. L. N. B. Wyse, a French Naval Officer, and he obtained for his company in 1878 a concession from the Columbian Government conferring the requisite rights and privileges for the construction of a ship canal on the Panama route and the authority to do such other things as might be necessary or advisable in connection with that project. This concession is ordinarily known as the Wyse concession.
A general plan for this transisthmian canal was the subject of consideration at an international scientific congress convened in Paris in May, 1879, and composed of 135 delegates from France, Germany, Great Britain, the United States and other countries with a majority from France. This Congress was convened under the auspices of Ferdinand de Lesseps, and after remaining in session for two weeks a decision, not unanimous, was reached that an interoceanic canal ought to be located on the Panama route, and that it should be a sea level canal without locks. The fact was apparently overlooked that the range between high and low tides in the Bay of Panama, about twenty feet, was so great as probably to require a tidal lock at that terminus.
A company entitled 'Compagnie Universelle du Canal Interocéanique' was organized with Ferdinand de Lesseps as president, immediately after the adjournment of the international congress. The purpose of this company was the construction and operation of the canal, and it purchased the Wyse concession from the original company for the sum of 10,000,000 francs. An immediate but unsuccessful attempt was made to finance the company in August, 1879. This necessitated a second attempt, which was made in December, 1880, with success, as the entire issue of 600,000 shares of 500 francs each was sold. Two years were then devoted to examinations and surveys and preliminary work upon the canal, but it was 1883 before operations upon a large scale were begun. The plan adopted and followed by this company was that of a sea level canal affording a depth of 29.5 feet and a bottom width of 72 feet. It was estimated that the necessary excavation would amount to 157,000,000 cu. yds.
The Atlantic terminus of the canal route was located at Colon and at Panama on the Pacific side. The line passed through the low ground just north of Monkey Hill to Gatun, six miles from the Atlantic terminus, and where it first met the Chagres River. For a distance of twenty-one miles it followed the general course of the Chagres to Obispo, but left it at the latter point and passing up the valley of a small tributary cut through the continental divide at Culebra and descended thence by the valley of the Rio Grande to the mouth of that river where it enters Panama Bay. The total length of this line from 30 ft. depth in the Atlantic to the same depth in the Pacific was about 47 miles. The maximum height of the continental divide on the center line of the canal in the Culebra cut was about 333 ft. above the sea, which is a little higher than the lowest point of the divide in that vicinity. Important considerations in connection with the adjacent alignment made it advisable to cut the divide at a point not its lowest.
Various schemes were proposed for the purpose of controlling the floods of the Chagres River, the suddenness and magnitude of which were at once recognized as among the greatest difficulties to be encountered in the construction of the work. Although it was seriously proposed at one time to control this difficulty by building a dam across the Chagres at Gamboa that plan was never adopted, and the problem of control of the Chagres floods remained unsolved for a long period.
It was estimated by de Lesseps in 1880 that eight years would be required for the completion of the canal, and that its cost would be $127,600,000. The company prosecuted its work with activity until the latter part of 1887 when it became evident that the sea level plan of canal was not feasible with the resources at command. Changes were soon made in the plans, and it was concluded to expedite the completion of the canal by the introduction of locks, deferring the change to a sea-level canal until some period when conditions would be sufficiently favorable to enable the company to attain that end. Work was prosecuted under this modified plan until 1889, when the company became bankrupt and was dissolved by judgment of the French Court, called the Tribunal Civil de la Seine, on February 4, 1889. An officer called the liquidator, corresponding quite closely to a receiver in this country, was appointed by the court to take charge of the company's affairs. At no time was the project of completing the canal abandoned, but the liquidator gradually curtailed operations and finally suspended the work on Mav 15, 1889.
He determined to take into careful consideration the feasibility of the project, and to that end appointed a 'commission d'études,' composed of eleven French and foreign engineers, headed by Inspector-General Guillemain, director of the École Nationale des Ponts et Chaussées. This commission visited the isthmus and made a careful study of the entire enterprise, and subsequently submitted a plan for the canal involving locks. The cost of completing the entire work was estimated to be $112,500,000, but the sum of $62,100,000 more was added to cover administration and financing, making a total of $174,600,000. This commission also gave an approximate estimate of the value of the work done and of the plant at $87,300,000, to which some have attached much more importance than did the commission itself. The latter appears simply to have made the 'estimate' one half of the total cost of completing the work added to that of financing and administration, as a loose, calling it an 'intuitive estimate'; in other words, it was simply a guess, based upon such information as had been gained in connection with the work done on the isthmus.
By this time, the period specified for completion under the original Wyse Concession had nearly expired. The liquidator then sought from the Colombian Government an extension of ten years, which was granted under the Colombian law dated December 26, 1890. This extension was based upon the provision that a new company should be formed and work on the canal resumed not later than February 28, 1893. The latter condition was not fulfilled, and a second extension was obtained on April 4, 1893, which provided that the ten-year extension of time granted in 1890 might begin to run at any time prior to October 31, 1894, but not later than that date. When it became apparent that the provisions of this last extension would not be carried out an agreement between the Colombian Government and the new Panama Company was entered into on April 26, 1900, which extended the time of completion to October 31, 1910. The validity of this last extension of time has been questioned.
A new company, commonly known as the new Panama Canal Company, was organized on October 20, 1894, with a capital stock of 650,000 shares of 100 francs each. Under the provisions of the agreement of December 26, 1890, authorizing an extension of time for the construction of the canal, 50,000 shares passed as full paid stock to the Colombian Government, leaving the actual working capital of the new Panama Company at 60,000,000 francs, that amount having been subscribed in cash. The most of this capital stock was subscribed for by certain loan associations, administrators, contractors and others against whom suits had been brought in consequence of the financial difficulties of the old company, it having been charged, in the scandals attending bankruptcy proceedings, that they had profited illegally. Those suits were discontinued under agreements to subscribe by the parties interested to the capital stock of the new company. The sums thus obtained constituted more than two thirds of the 60,000,000 francs remaining of the share capital of the new company after the Colombian Government received its 50,000 shares. The old company had raised by the sale of stock and bonds not far from $246,000,000, and the number of persons holding the securities thus sold has been estimated at over 200,000.
The Panama Railroad Company holds a concession from the Colombian Government, giving it rights prior to those of the Wyse Concession, so that the latter could not become effective without the concurrence of the Panama Railroad Company. This is shown by the language of Article III. of the Wyse Concession, which reads as follows:
It became necessary therefore in order to control this feature of the situation for the old Panama Company to secure at least a majority of the stock of the Panama Railroad Company. As a matter of fact the old Panama Canal Company purchased nearly 69,000 out of the 70,000 shares of the Panama Railroad Company, each such share having a par value of $100. These shares of Panama railroad stock are now held in trust for the benefit of the new Panama Canal Company. A part of the expenditures of the old company therefore covered the cost of the Panama Railroad Company's shares, now held in trust for the benefit of the new company.
Immediately after its organization the new Panama Canal Company resumed the work of excavation in the Emperador and Culebra cuts with a force of men which has been reported as varying between 1,900 and 3,600. It also gave thorough consideration to the subject of the best plan for the completion of the canal. The company's charter provided for the appointment of a special engineering commission of five members by the company and the liquidator to report upon the work done and the conclusions to be drawn from its study. This report
was to be rendered when the amount expended by the new company should reach about one half of its capital. At the same time the company also appointed a 'Comité Technique' constituted of fourteen eminent European and American engineers, to make a study of the entire project, which was to avail itself of existing data and the results of such other additional surveys and examinations as it might consider necessary. The report rendered by this committee was elaborate, and it was made November 16, 1898. It was referred to the statutory commission of five, to which reference has already been made, which commission reported in 1899 that the canal could be constructed within the limits of time and money estimated. On December 30, 1899, a special meeting of the stockholders of the new company was called, but the liquidator, who was one of the largest stockholders, declined to take part in it, and the report consequently has not received the required statutory consideration.
The plan adopted by the company placed the minimum elevation of the summit level of the canal at 97.5 ft. above the sea and the maximum at 102.5 ft. above the same datum. It provided for a depth of 29.5 ft. of water and a bottom width of canal prism of about 98 ft., except at special places where this width was increased. A dam was to be placed near Bohio, which would thus form an artificial lake with its surface varying from 52.5 to 65.6 ft. above the sea. Under this plan there would be a flight of two locks at Bohio, about sixteen miles from the Atlantic end of the canal, and another flight of two locks at Obispo
about fourteen miles from Bohio, thus reaching the summit level; a single lock at Paraiso, between six and seven miles from Obispo; a flight of two locks at Pedro Miguel, about 1.25 miles from Paraiso, and finally a single lock at Mira Flores, a mile and a quarter from Pedro Miguel, bringing the canal down to the ocean elevation. The location of this line was practically the same as that of the old company. The available length of each lock chamber was 738 ft., while the available width was 82 ft., the depth in the clear being 32 ft. 10 in. The lifts were to vary from 26 to 33 ft. It was estimated that the cost of finishing the canal on this plan would be $101,850,000, exclusive of administration and financing.
In order to control the floods of the Chagres River, and to furnish a supply of water for the summit level of the canal, a dam was planned to be built at a point called Alhajuela, about twelve miles from Obispo, from which a feeder about ten miles long, partly an open canal and partly in tunnels or pipe, would conduct the water from the reservoir thus formed to the summit level.
Although the plan, as described, was adopted, the 'Comité Technique' apparently favored a modification by which a much deeper excavation through Culebra Hill would be made, thus omitting the locks at both Obispo and Paraiso, and making the level of the artificial lake Bohio the summit level of the canal. In this modified plan the bottom of the summit level would be about 32 feet above the sea, and the minimum elevation of the summit level 61.5 feet above the sea. This modification of plan had the material advantage of eliminating both the Obispo and Paraiso locks. The total estimated cost of completing the canal under this plan was about $105,500,000. Although the Alhajuela feeder would be omitted, the Alhajuela reservoir would be retained as an agent for controlling the Chagres floods and to form a reserve water supply. The difference in costs of these two plans was comparatively small, but the additional time required to complete that with the lower summit level was probably one of the main considerations in its rejection by the committee having it under consideration.
This brings the project up to the time when the Isthmian Canal Commission was created in 1899 and when the forces of the new Panama Canal Company were employed either in taking care of the enormous amount of plant bequeathed to it by the old company or in the great excavation at Emperador and Culebra. The total excavation of all classes, made up to the time when that commission rendered its report, amounted to about 77,000,000 cu. yds.
The work of the commission consisted of a comprehensive and detailed examination of the entire project and all its accessories, as contemplated by the new Panama Canal Company, and any modifications of its plans either as to alignment, elevations or subsidiary works, which it might determine advisable to recommend. In the execution of this work it was necessary among other things to send engineering parties on the line of the Panama route for the purpose of making such surveys and examinations as might be necessary to confirm estimates of the new Panama Canal Company as to quantities, elevations or other physical features of the line selected, or required in modifications of alignment or plans. In order to accomplish this portion of its work the commission placed five working parties on the Panama route with twenty engineers and other assistants and forty-one laborers.
The commission adopted for the purposes of its plans and estimates the route selected by the new Panama Canal Company, which is essentially that of the old company. Starting from the six-fathom contour in the harbor of Colon the line follows the low marshy ground adjoining the Bay of Limon to its intersection with the Mindi River; thence through the low ground continuing to Gatun, about six miles from Colon, where it first meets the Chagres River. From this point to Obispo the canal line follows practically the general course of the Chagres River, although at one point in the marshes below Bohio it is nearly two miles from the farthest bend in the river at a small place called Ahorca Lagarto. Bohio is about seventeen miles from the Atlantic terminus and Obispo about thirty miles. At the latter point the course of the Chagres River, passing up stream, lies to the northeast, while the general direction of the canal line is southeast toward Panama, the latter leaving the former at this location. The canal route follows up the general course of a small stream, called the Camacho, for a distance of nearly five miles where the continental divide is found, and in which the great Culebra cut is located about thirty-six miles from Colon and thirteen miles from the Panama terminus. After passing through the Culebra cut the canal route follows the course of the Rio Grande River to its mouth at Panama Bay. The mouth of the Rio Grande where the canal line is located, is about a mile and a half westerly of the city of Panama. The Rio Grande is a small, sluggish stream throughout the last six miles of its course, and for that distance the canal excavation would be made mostly in soft silt or mud.
Although the line selected by the French company is that adopted by the Isthmian Canal Company for its purposes a number of most important features of the general plan have been materially modified by the commission, as will be easily understood from what has already been stated in connection with the French plans.
The feasibility of a sea-level canal but with a tidal lock at the Panama end was carefully considered by the commission, and an approximate estimate of the cost of completing the work on that plan was made. In round numbers this estimated cost was about $250,000,000, and the time required to complete the work would probably be nearly or quite twice that needed for the construction of a canal with locks. The commission therefore adopted a project for the canal with locks. Both plans and estimates were carefully developed in accordance therewith.
The harbor of Colon has been fairly satisfactory for the commerce of that port, but it is open to the north and there are probably two or three days in every year during which northers blow into the harbor with such intensity that ships anchored there must put to sea in order to escape damage. The western limit of this harbor is an artificial point of land formed by material deposited by the old Panama Canal Company; it is called Christoph Colomb and near its extreme end are two large frame residences built for do Lesseps. The entrance to the canal is immediately south of this artificial point. The commission projected a canal entrance from the six fathom contour in the Bay of Limon, in which the harbor of Colon is found, swinging on a gentle curve 6,560 feet radius to the left around behind the artificial point just mentioned and then across the shore line to the right into the. low land southerly of Colon. This channel has a width of 500 ft. at the bottom, with side slopes of one on three, except on the second curve, which is somewhat sharper than the first, where the bottom width is made 800 ft. for a length of 800 ft. for the purpose of a turning basin. This brings the line into the canal proper, forming a well-protected harbor for nearly a mile inside of the shore line. The distance from
the six fathom line to this interior harbor is about two miles. The total cost of constructing the channel into the harbor and the construction of the harbor itself is $8,057,707, and the annual cost of maintenance is placed at $30,000. The harbor would be perfectly protected from the northers, which occasionally blow with such intensity in the Bay of Limon, and it could be made in all weathers by steam vessels seeking it.
The harbor at the Pacific end of the channel where it joins Panama Bay is of an entirely different character in some respects. The Bay of Panama is a place of light winds. Indeed it has been objected that the difficulties sometimes experienced by sailing vessels in finding wind enough to take them out of Panama Bay are so serious as to constitute a material objection to the location for a ship canal on the Panama route. This difficulty undoubtedly exists at times, but the simple fact is to be remembered that Panama was a port for sailing ships for more than two hundred years before a steamship was known. The harbor of Panama, as it now exists, is a large area of water at the extreme northern limit of the bay, immediately adjacent to the City of Panama, protected from the south by the three islands of Perico, Naos and Culebra. It has been called a roadstead. There is good anchorage for heavy draft ships, but for the most part the water is shallow. With the commission's requirement of a minimum depth of water of 35 feet, a channel about four miles long from the mouth of the Rio Grande to the six-fathom line in Panama Bay must be excavated. This channel would have a bottom width of 200 feet with side slopes of one on three where the material is soft. Considerable rock would have to be excavated in this channel. At 4.41 miles from the six fathom line is located a wharf at the point called La Boca. A branch of the Panama Railroad Company runs to this wharf, and at the present time deep draft ships lie up alongside of it and take on and discharge cargo, as do the trains of the Panama Railroad Company. This wharf is a steel framed structure, founded upon steel cylinders, carried down to bedrock by the pneumatic process. Its cost was about $1,284,000. The total cost of this excavated channel, leading from Panama Harbor to the pier at La Boca, is estimated by the commission at $1,464,513. As the harbor at Panama is considered an open roadstead, it requires no estimate for annual cost of maintenance.
Starting from the harbor of Colon the prism of the canal is excavated through the low and for most part marshy ground to the little village called Bohio. The prism would cut the Chagres River at a number of points, and would require a diversion channel from that river for a distance of about five miles on the westerly side of the canal. Levees or protective embankments would also be required on the same side of the canal between Bohio and Gatun, the Chagres River leaving the canal line at the latter point on its way to the sea.
The principal engineering feature of the entire route is found at Bohio; it is the great dam across the Chagres River at that point, forming Lake Bohio, the summit level of the canal. The new Panama Canal Company located this dam at a point about seventeen miles from Colon, and designed to make it an earth structure suitably paved on its faces, but without any other masonry feature. Some borings had been made along the site and test pits were also dug by the French engineers. It was the conviction of the Isthmian Canal Commission, however, that the character of the proposed dam might be affected by a further examination of the sub-surface material at the site. Consequently the boring parties of the commission sunk a large number of bore holes at six different sections or possible sites along the river in the vicinity of the French location. These borings revealed great irregularity in the character and disposition of the material below the bed and banks of the river. In some places the upper stratum of material was almost clear clay, and in other places clear sand, while all degrees of admixture of clay and sand were also found. At the French site the bedrock at the deepest point was found 143 ft. below sea level, with large masses of pervious and semi-pervious sand, gravel, and mixtures of those materials with clay. Apparently there is a geological valley in the rock along the general course of the Chagres River in this vicinity filled with sand, gravel and clay, irregularly distributed and with all degrees of admixture, large masses in all cases being of open texture and pervious to water. The site adopted by the commission for the purpose of its plans and estimates is located nearly half a mile down the course of the river from that selected by the new Panama Canal Company. The geological valley is nearly 2,000 ft. wide at this location, but the deepest rock disclosed by the borings of the commission is but 128 ft. below sea-level. The actual channel of the river is not more than 150 ft. wide and lies on the extreme easterly side of the valley. The easterly or right bank of the river at this place is clean rock and rises abruptly to an elevation of about 40 ft. above the river surface at ordinary stages. The left or westerly bank of the river is compact clay and sand and rises equally as abruptly as the rocky bank of the other side, and to about the same elevation. From the top of the abrupt sandy clay bank a plateau of rather remarkable uniformity of elevation extends for about 1,200 ft. in a southwesterly direction to the rocky hill in which the Bohio locks would be located. The rock slope on the easterly or northerly bank of the river runs down under the sandy river bed, but at such an inclination that within the limits of the channel the deepest rock is less than 100 ft. below sea-level.
After the completion of all its examinations and after a careful study of the data disclosed by them, the commission deemed it advisable to plan such a dam as would cut off absolutely all possible sub-surface flow or seepage through the sand and gravel below the river surface. It is to be observed that such a sub-surface flow might either disturb the stability of an earth dam or endanger the water supply of the summit level of the canal or both. The plan of dam finally adopted by the commission for the purposes of its estimates is shown by the accompanying plans and sections. A heavy core wall of concrete masonry extends from bed rock across the entire geological valley to the top of the structure, or to an elevation of 100 ft. above sea-level, thus absolutely closing the entire valley against any possible flow. The thickness of this wall at the bottom is 30 ft., but at an elevation of 30 ft. below sea-level its sides begin to batter at such a rate as to make the thickness of the wall 8 ft. at its top. On either side of this wall are heavy masses of earth embankment of selected material properly deposited in layers with surface slopes of one on three. As shown by the plans the lower portions of the core wall of this dam would be sunk to bed rock by the pneumatic process, the joints between the caissons being closed and sealed by concrete and by cylinders sunk in recesses or wells, also as shown by the plans.
The profile of this route shows that the summit-level would have an ordinary elevation of 85 ft. above the sea, but it may be drawn down for uses of the canal to a minimum elevation of 82 ft. above the same datum. On the other hand, under circumstances to be discussed later, it may rise during the floods of the Chagres to an elevation of 90 or possibly 91 or 92 ft. above the level of the sea. The top of the dam therefore would be from 8 to 10 ft. above the highest possible water surface in the lake, which is sufficient to guard against wash or overtopping of the dam by waves. The total width of the dam at its top would be 20 ft, and the entire inner slope would be paved with heavy rip-rap suitably placed and bedded.
This dam would create an artificial lake having a superficial area during high water of about 40 sq. miles. The water would be backed up to a point called Alhajuela, about twenty-five miles up the river from Bohio. For a distance of nearly fourteen miles, i. e., from Bohio to Obispo, the route of the canal would lie in this lake. Although the water would be from 80 to 90 ft. deep at the dam, for several miles below Obispo it would be necessary to make some excavation along the general course of the Chagres in order to secure the minimum depth of 35 ft. for the navigable channel.
The feature of Lake Bohio of the greatest importance to the safe and convenient operation of the canal is that by which the floods of the River Chagres are controlled or regulated. That river is but little less than 150 miles ]ong, and its drainage area, as nearly as can be estimated, contains about 875 sq. miles. Above Bohio its current moves some sand and a little silt in times of flood, but usually it is a clear water stream. In low water its discharge may fall to 350 cubic feet per second.
As is well known, the floods of the Chagres have at times been regarded as almost if not quite insurmountable obstacles to the construction of a canal on this line. The greatest flood of which there is any semblance of a reliable record is one which occurred in 1879. No direct measurements were made, but it is stated with apparent authority that the flood elevation at Bohio was 39.3 ft. above low water. If the total channel through which the flood flowed at that time had been as large as at present, actual gaugings or measurements of subsequent floods show that the maximum discharge in 1879 might have been at the rate of 136,000 cu. ft. per second. As a matter of fact the total channel section in that year was less than it is at the present time. Hence if it be assumed that a flood of 140,000 cu. ft. per second must be controlled an error on the safe side will be committed. Other great floods of which there are reliable records are as follows:
|1885—Height at Bohio 33.8 ft. above low water.|
|1888—Height at Bohio 34.7 ft. above low water.|
|1890—Height at Bohio 32.1 ft. above low water.|
|1893—Height at Bohio 2S.5 ft. above low water.|
The maximum measured rate of the 1890 flood was 74,998 cu. ft. per second, and that of 1893, 48,975 cu. ft. per second. It is clear therefore that a flood flow of 75,000 cu. ft. per second is very rare, and that a flood of 140,000 cu. ft. per second exceeds that of which we have any record for practically forty years.
It is obvious that the dam, as designed by the commission, is of such a character that no water must be permitted to flow over its crest, or even in immediate proximity to the downstream embankment. Indeed it is not intended by the Commission that there shall be any waste way or discharge anywhere near the dam. At a point about three miles southwest of the site of the dam at Bohio is a low saddle or notch in the hills near the headwaters of a small stream called the Gigante River. The elevation of this saddle or notch is such that a solid masonry weir with a crest 2,000 ft. long may readily be constructed with its foundation on bed rock without deep excavation. This structure is called the Gigante Spillway, and all surplus flood waters from the Chagres would flow over it. The waters discharged would flow down to and through some large marshes, one called Pena Blanca and another Agua Clara before rejoining the Chagres. Inasmuch as the canal line runs just easterly of those marshes it would be necessary to protect it with the levees or embankments to which allusion has already been made. These embankments are neither much extended nor very costly for such a project. The protection of the canal would be further aided by a short artificial channel between the two marshes, Pena Blanca and Agua Clara, for which provision is made in the estimates of the commission. After the surplus waters from the Gigante Spillway pass these marshes they again enter the Chagres River or flow over the low, half-submerged country along its borders, and thence through its mouth to the sea near the town of Chagres about six miles northwest of Gatun.
The masonry crest of the Gigante Spillway would be placed at an elevation of 85 ft. above the sea, identically the same as that which may be called the normal summit level of the canal. It is estimated that the total uses of water in the canal added to the loss by evaporation taken at six inches in depth per month from the surface of the lake will amount to about 1,070 cu. ft. per second if the traffic through the canal should amount to 10,000,000 tons per annum in ships of ordinary size. This draft per second is the sum of 406 cu. ft. per second for lockage, 207 for evaporation, 250 for leakage at the lock gates and 200 for power and other purposes, making a total of 1,063, which has been taken as 1,070 cu. ft. per second. The amount of storage in Lake Bohio between the elevations of 85 and 82 ft. above sea-level, as designed, is sufficient to supply the needs of that traffic in excess of the smallest recorded low-water flow of the Chagres River during the dry season of a low rainfall year. The lowest monthly average flow of the Chagres on record at Bohio is 600 cu. ft. per second for March, 1891, and for the purposes of this computation that minimum flow has been supposed to continue for three months. This includes a sensible margin of safety. In not even the driest year therefore can it be reasonably expected that the summit level of the canal would fall below the elevation of 82 ft. until the total traffic of the canal carried in ships of the present ordinary size shall exceed 10,000,000 tons. If the average size of ships continues to increase, as will probably be the case, less water in proportion to tonnage will be required for the purposes of lockage. This follows from the fact that with a given tonnage the greater the capacity of the ships the less the number required, and consequently the less will be the number of lockages made.
On the other hand it can be shown that with a depth of five feet of water on the crest of the Gigante Spillway the discharge of that weir 2,000 ft. long will be at the rate of 78,260 cu. ft. per second. If the flood water of the Chagres should flow into Lake Bohio until the head of water on the crest of the Gigante weir rises to 7.5 ft. the rate of discharge over that weir would be 140,000 cu. ft. per second, which, as already shown, exceeds at least by a little the highest flood rate on record. The operation of Lake Bohio as a flood controller or regulator is therefore exceedingly simple. The flood waters of the Chagres would pour into the lake and immediately begin to flow over the Gigante weir, and continue to do so at an increasing rate as the flood continues. The discharge of the weir is augmented by the increasing flood and decreases only after the passage of the crest of the flood wave. No flood even as great as the greatest supposable flood on record can increase the elevation of the lake more than 92 to 92.5 ft. above sea-level, and it will only be at long intervals of time that floods will raise that elevation more than about 90 ft. above sea level. The control is automatic and unfailingly certain. It prevents absolutely any damage from the highest supposable floods of the Chagres, and reserves in Lake Bohio all that is required for the purposes of the canal and for wastage by evaporation through the lowest rainfall season. The floods of the Chagres, therefore, instead of constituting the obstacle to construction and convenient maintenance of the canal heretofore supposed are deprived of all their prejudicial effects and transformed into beneficial agents for the operation of the waterway.
The highest floods are of short duration, and it can be stated as a general law that the higher the flood the shorter its duration. The great floods which it is necessary to consider in connection with the maintenance and operation of this canal would last but a comparatively few hours only. The great flood flow of 140,000 cu. ft. per second would increase the current in the narrowest part of the canal below Obispo to possibly 5 ft. per second for a few hours only, but that is the only inconvenience which would result from such a flood discharge. That velocity could be reduced by additional excavation.
Inasmuch as this system of control, devised and adopted by the Isthmian Canal Commission, is completely effective in regulating the Chagres floods, the reservoir proposed to be constructed, by the new Panama Canal Company at Alhajuela on the Chagres about twelve miles above Obispo, is not required, and the cost of its construction would be avoided. It could, however, as a project be held in reserve. If the traffic of the canal should increase to such an extent that more water would be needed for feeding the summit level the dam could be built at Alhajuela so as to impound enough additional water to accommodate, with that stored in Lake Bohio, at least five times the 10,000,000 annual traffic already considered. Its existence would at the same time act with substantial effect in controlling the Chagres floods and relieve the Gigante Spillway of a corresponding amount of duty.
The locks on the Panama route are designed to have the same dimensions as those in Nicaragua, as was stated in the lecture on that route. The usable length is 710 ft. and the clear width 81 ft. They would be built chiefly of concrete masonry while the gates would be of steel, and of the mitre type.
The great dam at Bohio raises the water surface in the canal from sea-level in the Atlantic maritime section to an ordinary maximum of 90 ft. above sea-level; in other words, the maximum ordinary total lift would be 90 ft. This total lift is divided into parts of 15 ft. each. There is therefore a flight of two locks at Bohio, indeed there are two flights side by side as the twin arrangement is designed to be used at all lock sites on both routes. The general dimensions and the arrangements of these locks with the requisite culverts and other features are shown in theplans and sections. They are not essentially different from other great modern ship canal locks. The excavation for the Bohio locks is made in a rocky hill against which the southwesterly end of the proposed Bohio dam rests and they are less than one thousand feet from it.
After leaving Bohio-Lake, at Obispo, a flight of two locks is found at Pedro Miguel, about 7.9 miles from the former or 21.5 miles from Bohio. These locks have a total ordinary maximum lift of 60 ft., divided into two lifts of 30 ft. each. The fifth and last lock on the route is at Miraflores. The average elevation of water between Pedro Miguel and Miraflores is 30 ft. above mean sea-level. Inasmuch as the range of tide between high and low in Panama Bay is about 20 ft. the maximum lift at Miraflores is 40 ft. and the minimum about 20. The twin locks at Miraflores bring the canal surface down to the Pacific Ocean level, the distance from those locks to the six-fathom curve in Panama Bay being 8.45 miles. There are therefore five locks on the Panama route, all arranged on the twin plan, and, as on the Nicaragua route, all are founded on rock.
Near Obispo a pair of guard gates are arranged 'so that if it should become necessary to draw off the water from the summit cut the level of Lake Bohio would not be affected.'
An unprecedented concentration of heavy cutting is found between Obispo and Pedro Miguel. This is practically one cut although the northwesterly end toward Obispo is called the Emperador while the deepest part at the other end, about three miles from Pedro Miguel, is the great Culebra cut with a maximum depth on the center line of the canal of 286 ft. On page 93 of the Isthmian Canal Commission's report is the following reference to the material in this cut:
When the old Panama Canal Company began its excavation in this cut considerable difficulty was experienced by the slipping of the material outside of the limits of the cut into the excavation, and the marks of that action can be seen plainly at the present time. This experience has given an impression that much of the material in this cut is unstable, but that impression is erroneous. The clay which slipped in the early days of the work was not drained, and like wet clay in numerous places in this country it slipped clown into the excavation. This material is now drained and is perfectly stable. There is no reason to anticipate any future difficulty if reasonable conditions of drainage are maintained. The high faces of the cut will probably weather to some extent, although experience with such clay faces on the isthmus, indicates that the amount of such action will be small. As a matter of fact the material in which the Culebra cut is made is stable and will give no sensible difficulty in maintenance.
(To be continued.)
- The substance of this paper has been delivered as a lecture by the author and will soon be published in a series of lectures on civil engineering.