A DESCRIPTION OF THE SWEETWATER DAM.

359

The CHAIRMAN. Leaving out what you find on the sloping hills and taking into consideration the alluvial land which is in a body, how much land is there here?

Mr. GUNN. I can not answer that question definitely. There are gentlemen who are capable of giving you definite figures in regard to that, and their statement would be better than a loose one from me.

STATEMENT OF THOMAS S. SEDGWICK, OF SAN DIEGO.

The CHAIRMAN. How long have you resided in San Diego?
Mr. SEDGWICK. I have been making my home here for the last year.
The CHAIRMAN. You have been familiar with it how many years?
Mr. SEDGWICK. Twenty years.

The CHAIRMAN. Have you paid any attention to irrigation and the water supply for that purpose?

Mr. SEDGWICK. To some extent, yes.

The CHAIRMAN. Have you any suggestions to make with regard to the Sweetwater Reservoir?

Mr. SEDGWICK. No; I can not say that I have.

The CHAIRMAN. Have you any suggestions to make as to the improvements that might be made in it?

Mr. SEDGWICK. No; Mr. Hinton was asking me some questions about it this morning. He asked me in regard to the principles of construction, aud I made some explanations in that direction.

The CHAIRMAN. The Sweetwater Dam is that which we have visited to-day. Will you give us a general description of it in the absence of the engineer, Mr. Schuyler?

Mr. SEDGWICK. As you observed to-day, the dam is placed across the upper end of the ravine of the Sweetwater Creek, some 7 miles from its débouché into San Diego Bay.

The dam is 76 feet long at the bottom, and 396 feet long at the top, and 90 feet high. Its thickness at the bottom is 46 teet, and at the top 12 feet, including parapet and cornice, with a parapet wall on top on the up-stream side 34 feet high and 2 feet thick, and a hand-rail on the down-stream side. The profile of the dam batters 15 feet on the up-stream face and 19 feet in three slopes on the down-stream face. (See profile, fig. 2.)

The horizontal plan of the dam is a curve of 213 feet radius.

The waste-weir is 45 feet long by 5 feet depth, divided into eight sections by masonry piers, inclined on the up stream face to receive loose flash boards, which, when in place, increase the depth of the reservoir by 5 feet. A training-wall carries the waste water well away from the foot of the dam. It is estimated that the wasteway will allow a continuous flow from the full reservoir of 1,500 cubic feet of water a second, which is believed to be about the flood regimen of the stream.

The masonry of the dam is from the rock forming the walls of the ravine, laid in Portland cement mortar, with large, rough blocks, uncoursed or otherwise ranged, the down-stream face presenting an even face.

The reservoir formed by this dam is about 3 miles long and about three fourths of a mile wide at its widest place, covering 720 acres, and has a storage capacity of 786,390,000 cubic feet, equal to 5,882,248,000 gallons.

The catchment basin feeding the reservoir is estimated to cover 186 square miles, flowing by the Sweetwater River from the southern slopes

of the Cuyamaca Mountain, lying nearly east from the Bay of San Diego, and about 60 miles distant.

The distributing system comprises 57 miles of iron pipes varying from 36 inches to 6 inches diameter, and supplies water for irrigation to some 10,000 to 15,000 acres of land, including the town of National City and other outlying settlements.

Q. Do you comprehend the principles of designing and constructing dams for the storage of water?-A. I believe I do.

Q. Will you kindly prepare a short paper on the subject, to be embodied in the report of the committee?-A. I will be pleased to do so to the best of my ability.

EARTH DAMS.

Embankments or dams for obstructing the flow of streams to form reservoirs of water may be successfully made of earth, for moderate heights, especially if suitable clays can be had by excavating for enlargment of the area, or depth of the reservoir, or in the near vicinity.

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361

In such cases the embankments are wide on the base and have very flat slopes, generally from 2 to 3 feet horizontally on 1 of height. If the clay be of good character, such a dam or embankment will generally be impervious to the water; but if the clay be of uncertain imperviousness, a core of puddled clay or a thin wall of masonry may be placed in the middle of the embankment to render it water-tight.

The slopes are usually faced with hand-laid protection of stone or brick, clay tiles, or even boards, to protect the slopes from wear by overflowing water, and from sliding, which is prevented by the weight of the paving. If the embankment is to be constructed with a view to allow it to be submerged when the stream is in flood, the top of the embankment and its down-stream slope should be covered with a good quality of pavement to prevent injury from drift likely to be carried over the dam in flood stages of the obstructed stream. Often the wasteway extends along a part only of the dam, or is placed at either end thereof, as may be indicated by some favorable condition of the local

FIG. 1-Nethertrees reservoir embankment, Paisley, England.

ity. Figure 1 shows an earth-embankment dam of this description, which is a good example of English practice.

The original surface of the ground on which such an embankment is to be built should be removed to a stratum of consistent material of clay or earth, or to an underlying rock.

There is no mathematical principle of mechanics necessary to be applied to the construction of an earthen dam, with reference to its weight and the pressure of the impounded water, for the reason that material for the dam is usually plentifully at hand, and can be so economically obtained as to render the cost of putting it in the dam an almost inconsiderable item, with reference to the importance of the benefits to be derived from the success of the projected dam and reservoir. Such a dam will be of ample stability if it be solidly made, but it may fail because of infiltration of water through it when of faulty construction, so that it may crack from settlement, which may also break the puddle wall.

There have been a great many such earth-embankment dams made in India for irrigation purposes, some of which are believed to be two thousand years old. The material is said to have been carried in baskets on the heads of the laborers, and deposited so as to have been compacted by the continual tramping of the workmen, which would account for their solid condition. There are many such dams in England and in our own Eastern States.

When the cause of the failure of this type of dams has been inve gated it has generally been found that the embankment had been constructed or had been carelessly taken care of. Such a dam a cho de Rientes, Spain, gave way in the year 1802, causing the 600 people and destroying some $7,000,000 value of property. the Dale Dike Dam, near Sheffield, England, 95 feet high,

causing the death of 300 persons and destroying much property. In 1874 a dam near the town of Williamsburgh, Mass., failed, and 150 persons lost their lives and $1,000,000 of property was destroyed. The most disastrous failure was that of the Conemaugh Dam, near Johnstown, Pa., in June this year (1889), completely washing away three or four villages, with a loss of some 3,500 lives and millions of dollars value of property. Unless the top and slopes of earth dams are well protected, the overflow water will wear away the embankment and destroy the dam.

MASONRY DAMS.

When the valley of drainage desired to be closed by a dam to form a reservoir is of less width, and requires a higher dam than is usually ventured to be made in an earth embankment, say, of 70 or 80 feet in height, stone masonry is resorted to as the proper material for the dam, and the mathematical rules of applied mechanics are taken into consideration and applied to the problem of proportioning the dimensions of the dam and disposing the forces acting on it in accordance with the science of civil engineering, so as to make the structure stable; that is to say, that it must be of such a weight that it shall not be pushed down stream nor overturned by the pressure of the water in the reservoir.

The forces to be considered in the construction of a masonry dam are the weight of the masonry and the pressure of the water resting against it. The weight of the material of the dam resting on its foundations is readily found.

The pressure of water against the vertical face of the dam at the top is equal to the weight of a cubic foot of water (62 pounds) for the first foot of depth, and increases as the depth, so that at a depth of h feet it is 162 times the depth (623 × h). Now, as the pressure at the top is nothing and at the bottom 624 h, the average pressure is one-half of 621 h, or ; and this multiplied by the depth h gives the whole

623h
2

pressure against the face of h feet height, and 624h

2

the algebraic expression for the pressure of water on a vertical face of the height h.

It is demonstrated in the text-books on engineering that this pressure acts by concentration at a point two-thirds of the depth from the surface of the water. If we consider the case of a dam of a given height, say, 100 feet, and 100 feet long, the pressure for each foot of length will 623h2 62x100x100 be or 312,500 pounds, and for the whole length of the dam 312,500 × 100=31,250,000 pounds. For the purposes of discussion of the sectional proportions of the dam it is usual to consider 1 foot only of length, which section is called its "profile."

2

or

2

Considering the dam to have a profile of uniform thickness, A B C D, Fig. 2, supporting 100 feet depth of water, the pressure is 312,500 pounds, as has just been computed. That the dam shall not slide on its base, it must have a weight equal to this pressure. For the purposes of this discussion stone masonry will be considered as having a weight of 150 pounds to the cubic foot; consequently our dam must have a sectional or profile area, to be found by dividing the weight of the pressure F (312,500 pounds) by the weight per cubic foot of the masonry, which is

312500

150

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), 2,0834 cubic feet of masonry for each foot in length of full

height. As our dam is 100 feet high its thickness must be

20831 100

20.83-+ feet wide on C D, the area of the profile expressing also the cubic feet of one foot in thickness of this profile.

It is customary to increase the computed dimension of width proportion, as one-half, or by doubling it, to insure stability by ing the weight of the dam proportionately.