« PreviousContinue »
in the construction of that plan will enable the estimated time of completion to be shortened rather than lengthened, whereas in the lock plan the complicated character of the construction of the six locks and other masonry works would be likely to extend the time required for construction beyond that estimated.
In reading testimony given subsequent to mine I find some of my observations relating to the transformation of a lock canal to a sealevel canal misunderstood. I stated that it was the opinion of the board, both majority and minority, that if the recommended lock plan should be adopted the cost and time required for the transformation of that particular lock canal to a sea-level canal would be so great as practically to preclude the procedure for an indefinite future period. In other words, the difficulties and cost of that particular transformation would be so great as to make it practically not feasible.
I stated specifically that the best lock plan, in view of future transformation to a sea-level plan, would be found by carrying the sea-level portion on the Caribbean end nearly or quite to Obispo, and on the Pacific end to Miraflores, with the lock portion between these two points. That plan would be far more easily transformable to a sea-level plan than that recommended, and the operation of transformation would involve a minimum of waste. For the reasons extensively given in the majority report and in the testimony, I believe, however, that it would be far better in every way to construct the sea-level plan at
My testimony regarding the silting of the channel of the lock plan in the upper reaches of Gatun Lake between Tavernilla and Obispo has been questioned. The concentration of the current in such a submerged channel as that between Alhajuela and Tavernilla, with such depths of water over the original banks as would exist in Gatun Lake, is precisely what is observed with any submerged river bottom similarly flooded in this country. It is a common hydraulic observation.
The assumption that the water moves uniformly across such a submerged valley and its use in computations of velocity of water under such conditions indicates a gross lack of familiarity with this particular class of river hydraulics, as it is directly at variance with the results of natural observation. The first Isthmian Canal Commission of 1899-1901 recognized the inevitable silting of this submerged channel with precisely the same elevation of water surface, and stated on page 63 of its report that a velocity of 5 feet per second might be reached during floods in the narrowest part of the lake.
The streams located between Gamboa and Bohio, draining a territory from 100 to 150 square miles, and for which provision has not been made for taking their waters entirely away from the canal prism, are all so small that it is difficult to make any definite estimate of even their flood discharges. This amount has been estimated by the minority as high as 29,000 cubic feet per second by taking it at one-fifth of the supposed discharge of the great flood of 1879. It should be borne in mind at the outset that there is not only no record of any such flood volume as 140,000 cubic feet per second at Bohio, but there is no reliable estimate of such a flood.
The most reasonable estimate from indirect evidence is that of General Abbott of only 112,000 cubic feet per second, which would make the maximum estimated flood flow of the small streams in question, if all were in flood concurrently, 22,000 cubic feet instead of 29,000.
This freshet flow of a large number of very small streams is, however, a diffused flow from a drainage area a large portion of which is of such a character as to prevent a quick discharge. Furthermore, the diffusion of a flood flow into a great number of small streams, most of them so small as to be dry in the dry season, will practically destroy the eroding power of any floods that may occur in them, thus reducing to almost insignificant volume the silt brought down by them in comparison with the erosion which would be produced by concentrating the total discharge into two or three streams of greater magnitude. The sedimentation basins, therefore, provided in the sea-level plan between Gamboa and Bohio, are abundantly ample to receive and retain practically all the silt brought down by the rivulets in question.
In my testimony I cited the case of the flow of water from a small underground channel at a depth of about 125 feet below the river surface above it through a boring made in the Hudson River near New Hamburg, N. Y., in the course of investigations for the additional water supply of New York City. As it was intimated in subsequent testimony that this flow was found in a boring through rock, it is well to state the fact that this water was found beneath a thick bed of clear clay a few feet above the bed rock. The water as it flowed up through the boring pipe made a fountain or jet with the water rising 15 or 20 feet above the river surface. It was a clear case of a small underground flow under pressure between two strata of different quality or texture.
In case of more or less permeable strata of varying quality overlying each other, a comparatively large amount of seepage or flow is liable to take place along surfaces of separation where the texture changes from one quality to another. This is so well recognized in the making of artificial filter beds for public water supplies that the greatest care is taken to avoid any sudden change of texture at any point in the filtering mass, because it has been observed that the water is almost certain to find its way in small thin channels wherever such a break or sudden change of texture occurs. The formula for the computation of flow through clear selected sand neglects absolutely any such feature as this change of texture where the freest flow takes place, and they are consequently entirely inapplicable to such conditions. As such conditions are freely found in the subsurface material at the Gatun dam site, as disclosed by the borings, it is obvious that any computations of the volume of seepage based upon neglect of the conditions found there aiding the flow must be wholly erroneous.
Much has been made in the minority report and elsewhere of the alleged necessity for tying up ships in order to pass each other when meeting in a sea-level canal of the dimensions recommended by the majority. It is to be remembered, as stated in my answer to one of Senator Morgan's questions, that the entire sea-level prism, as recommended, is a continuous passing place for ships. That part of the prism which has a bottom width of 150 feet is everywhere wider than the passing places in the Suez Canal, which have a bottom width of 147 feet 6 inches. In the 8 miles through the Culebra Cut the bottom width is 200 feet, with sides practically vertical, affording a much increased width of channel for passing purposes. Under these conditions few, if any, ships short of 550 feet in length and 55 to 60 feet beam would have to be tied up to pass cach other anywhere in the canal. Ships nearly or quite up to those dimensions would pass each
other by slowing their speed and still smaller ships at their regular speed.
As the great bulk of canal traffic will be carried in vessels of less tonnage than those cited above for probably a considerable number of years, it will readily be seen that there can exist but little delay to any part of the canal traffic due to the tying up of the largest ships. Any time of passage of a ship through the sea-level canal, therefore, computed on the assumption that ships must always or generally be tied up whenever they meet and pass, as is done in the minority report, will give
results entirely erroneous. As a matter of fact, as is demonstrated by traffic upon existing maritime canals in Europe, such as the Manchester and the Suez, the majority of vessels seeking the canal, i. e., except those of the largest dimensions, may pass each other, either at speed or by slowing up, but without tying up at meeting places or anywhere else.
It has been suggested in the minority report and in the testimony before the Senate committee, that in the lock plan its capacity might be increased or that facility of passage through it might be enhanced by fleet lockages, i. e., by taking two or more vessels of suitable size into one lock. While it is true that the matter of fleet lockage may be advantageously resorted to when a single lock only exists, as at the Soo, the time of lockage even there is greatly extended by the time required to properly place the two or more vessels brought into the lock at the same time with sufficient closeness without injuring each other, either when brought in or during the emptying or filling of locks.
When, on the other hand, there is a series of locks of either two or three in flight, the operation of moving a number of ships from one lock into the next one below or above must be made with extreme caution and very slowly, in consequence of the increased danger of injuring both ships and gates. No reliable computations can be made as to the greatly extended time required to pass the triple locks in flight at Gatun or the double locks in flight at Sosa Hill in case it should be considered prudent to attempt fleet lockages, as it would depend upon the number and size of ships admitted into a lock. The requisite time, however, would be extended much more than in proportion to the number of ships being locked, as would also the danger with which both vessels and structure would be threatened. Indeed, it would become a grave question whether this system of lockage could be permitted for any vessels whatever in such a flight of locks, except possibly for those of the smallest tonnage.
In connection with my testimony concerning the advisability of shutting off underground seepage through porous strata under an earth dam, I would like to mention the fact that the Charles River dam now being built at Boston is an earth dam. The maximum unbalanced head of water which it will have to retain is that due to the tidal range only in Boston Harbor. The plans for that structure, which I have before me as I write, show that a line of close sheet piling, called a shut-off dam, is driven along that part of it resting on substrata of sand and gravel, so as to preclude any possibility of seepage under the dam itself. Mr. F. P. Stearns, a member of the minority of the Consulting Board, is the consulting engineer for this construction.
The general principle recognized in placing this sheet piling subsurface cut-off in the Charles River dam is that which almost or quite
universally governs the construction of great earth dams at the present time, by either placing them upon absolutely impervious material or carrying a curtain construction of some kind, such as a masonry core, a clay core, a line of close sheet piling, or some other device of that character, down through the permeable strata to bed rock or its equivalent, so that all subsurface seepage or underflow can be effectively prevented. I trust this letter can be admitted as a part of my testimony. Very respectfully,
WM. H. BURR. Hon. J. H. MILLARD, Chairman Committee on Interoceanic Canals,
United States Senate.
Written statement of B. M. Harrod, esq.
The lake idea is logically connected with a lock canal. A lock canal is not built for the sake of the locks. They, per se, are objectionable, unless they provide compensating advantages in the lakes they form. The lock-canal advocates had in this case the opportunity of developing the lake idea, or an unobstructed navigation to the greatest extent, so as to present for 29 out of the 41 miles between the shore lines of the Isthmus a channel, ideal in width and depth, practically equal to navigation in the open sea. On the other hand, the sea-level advocates have been forced by circumstances connected with time and money to reduce their channel throughout to the least possible dimensions in fact, to dimensions which do not comply with the strict injunctions of the act of Congress.
The sea-level canal, as designed, will not “afford convenient passage for vessels of the largest tonnage and greatest draft now in use and such as may be reasonably anticipated.” The dimensions of the project will not allow vessels even now building to pass each other. Ships of average size can not pass each other without one stopping and tying up, but for this no spaces or “gares” are provided. Neither is there any widening of channel proposed at curves. These conveniences, or rather pecessities, can be readily provided, but only with considerable increase in the estimates for both construction and operation. The demand for enlargement will be heard as soon as the inadequacy of the proposed dimensions is developed by operation.
It is proposed, during the construction of a sea-level canal, to conduct the Chagres and its eastern tributaries to the Caribbean Sea through diversion channels. These have been partially excavated by the French, but many million cubic yards of excavation and embankment are required for their completion. Until the Gamboa dam is completed they must have capacity for the flood discharge of the Chagres and its tributaries. After the canal is completed the Chagres will be regulated by the Gamboa dam, and all its tributaries, except the Cano and Gigante, will be turned into its prism. These two will be dammed and diverted in another direction. There will therefore be in the canal a current toward the Caribbean varying, according to the discharge stage of the rivers, estimated at from 1 to 23 miles per hour.
Of the tributaries to be received into the prism of the canal there are 22 of considerable size. Two are known to have a flood discharge of over 3,000 cubic feet per second; eight more have discharges of over 1,000 cubic feet per second. Their flood discharges between Gamboa and Bohio may aggregate 30,000 second-feet. They descend into the canal from heights varying from 13 to 160 feet above sea level. The sea-level plan proposes to overcome this difference of level by masonry-stepped aprons, metallic pipes, or by sloping and lowering the beds of the influent streams, although no designs are presented. Professor Burr in his testimony describes basins at the mouths of these streams, to strain out the sediment and débris, allowing only the water to enter the canal, but that is a personal suggestion and does not appear in the plan. This would certainly add materially to the estimate, and it is doubtful whether it would not be more costly to clean out the several basins, which would rapidly fill up, than to dredge the deposit from the canal itself.
I believe that the discharge of 3,000 cubic feet per second into the canal prism of 8,000 feet cross section would cause cross currents which would prove an absolute obstruction to navigation as long as they prevailed. No ship could hold a direct course under such conditions. She would be driven against the opposite bank. Even lesser discharges would prove proportionately obstructive to navigation.
I believe that the injection of 3,000 cubic feet per second into a canal prism of only 8,000 or 10,000 square feet of sectional area would cause deposit on one side and would abrade the opposite bank unless it were in rock, and that these effects, in combination with a current varying from 1 to 2 miles per hour would give to those parts of the projected sea-level canal through earthen banks the characteristics of an alluvial stream which would ultimately establish meanders or sinuosities that would seriously impair the navigability of the canal for all larger ships, unless these banks were artificially protected, and the bars constantly dredged.
It is proposed in the sea-level plan to divert the Cano, Gigante, and Gigantíto from the canal route by four dams and a spillway. These are all in a region of which little is known by survey. The largest of these dams holds a head of water about 70 feet above sea level, only a few feet less than the Gatun dam, and is about 3,000 feet long. No intimation is given of the method of construction, whether of earth, masonry, or a combination of the two.
The estimate for completing 21 miles of temporary diversion and of several miles of permanent diversion, aggregating many million yards of excavation; for controlling the descent of twenty or more tributaries, by masonry structures, into the canal, and for the building of four dams and a spillway, for which no plan is proposed, in a region where no investigation of foundations has been made, is three and one-half millions plus 20 per cent, which I believe will prove entirely inadequate.
I prefer the Gatun to the Bohio location for a dam because the foundation is more impervious; because the site at Gatun admits of three locks of the required size, while that at Bohio will not hold two of the same length; because it is doubtful if the supply of water at Bohio is sufficient with the larger locks for a traffic of more than 10,000,000 tons, while at Gatun it is sufficient for 40,000,000 tons,