The framing is shown in the figure, consisting of alternate longitudinal and transverse square timbers; the former are staggered one above another excepting in the upstream face, the latter are placed 8 feet centres; the longitudinals are doubled at the apron end and are laid close in the top streak under the crown. The entire structure is covered with 3-inch planking, to make it as water-tight as possible, prevent the washing out of any of the filling material, and protect the crib timbers. The crown is given a slight slope to prevent the lodgement of floatage; its upstream edge is rounded and sheeted with iron plates. The substructure of a timber spillway is of the same type as for concrete spillways, consisting of the cut-off, the bearing piles, if the location is in soft material, and of the apron; for the structural type of the latter heretofore described, a rock-filled trench may be substituted, that is if heavy rock is available, and the trench should be not less than three feet deep, and its width should be equal to half of the spillway height in order to insure that all of the overfall strikes this rock fill. A gravel and earth fill may be placed against the upstream side of the spillway, but it should not be expected to add to the resistance weight of the structure, as the upstream side will always be exposed to the hydrostatic head of its full height. As a matter of fact, this upstream side will sooner or later fill in from the sediment and silt carried by the stream.

TABLE 15.-APPROXIMATE QUANTITIES OF THE MATERIAL REQUIRED FOR TIMBER SPILLWAYS OF THE DESIGN HERE DESCRIBED IN LENGTHS OF EIGHT FEET AND FOR VARYING HEIGHTS.

Sluices of any type may be arranged in a timber spillway. To decide between timber and concrete spillways is practically wholly a question of comparative cost and of the maintenance and repair charges.

Diagram 30 gives the approximate first cost of the timber spillway superstructure of the types here described, per linear foot of the spillway, and comparatively with the cost of concrete spillways for different heights and varying market values of timber and of concrete, both placed in the structure.

Fig. 57, section II, shows a modification of the timber spillway, the upstream side being inclined on half horizontal in one vertical and the apron fully sheeted to the shape of the overfall curve; its characteristics are: S = 16 ft., B 0.357 B.

=

1.75 S, C

=

ARTICLE 71.-Occasionally it may be desired to construct the spillway at first to only a part of the total height to be ultimately utilized; this is practicable both with the solid and the gravity types. For the first the downstream slope should then be left in steps with dowels, in order to secure the best practicable connection for the future addition to the original section; the foundation and the apron must be sufficient for the final height. For gravity spillways the partitions are likewise stepped on the apron side and are covered with a timber instead of the concrete-steel apron, the latter being constructed when the spillway is completed to its final height.

Or it may be desired to raise an existing spillway, which may also be practicable but cannot be treated in a general manner; each such case must be considered from its own conditions.

ARTICLE 72. Spillway Abutments.-The spillway terminates in abutments which may be of natural rock walls when the location is in a palisaded gorge; but in the majority of cases the spillway does not complete the empounding of the watercourse, and the additional structures, which are required to dam the river, are joined on the spillway, and the abutments then form the connecting links.

Abutments should be of the same height as the adjoining reservoir structures, not less than three feet above the maximum overflow level, and in outline they must be of sufficient dimensions to cover completely the ends of the banks which they are to protect against the overflowing water; they have, however, little in common with bridge abutments, partaking rather of the character of retaining walls. For solid spillways the section proper needs no abutment, which is formed around and

overlapping it on all sides as much as the section of the adjoining reservoir structures require; for gravity spillways the abutment becomes a complete wall, forming, in part, the end partition of the spillway; while for timber spillways the abutment is a separate structure throughout. The abutments may be of timber, masonry, concrete, or of concrete-steel construction, their design being based upon the theory of earth-retaining structures.

F

J

C

Fig. 58

B

the bank. It is assumed that if the wall is suddenly removed a portion of the fill, represented by CDE, would follow after it, and the slope of rupture, CE, is accepted as being the bisect of an angle = 90° — p. This theory is not well proved,—that is, as to the locus and form

of the line of rupture; it may be accepted as representing the conditions of a sand bank, provided it is dry.

The author has examined a number of slides in canal banks of from 20 to 40 feet high, chiefly of clay, gravel, and sand formation, with a view of determining the causes and effects of such subsidences, and has found the lines of rupture to be always vertical for one-third to one-half