[SI] 
The Water Power of Texas. 
35 
dam somewhat as shown in Plate 1 (Fig. 1) [Plate 1 refers to Fig. 13]. 
When logs pass over a sloping dam with the flood they first appear at the 
surface of the ebulition at some distance below the dam, as at m in Plate 
1, and then return along the surface toward the dam. The greater the 
flood depth on the crest the further from the toe of the dam do the logs 
appear and the more swiftly the logs return with the surface current 
toward the dam. 
“ Breakwater . — In a case such as is shown in Plate 1 (Fig. 1) the 
breakwater comes in contact with only the toe of the dam. With eleven 
feet depth on the crest, sixty feet fall and twenty-five feet of backwater, 
the discharge velocity past b, c. f. is great. The water at d then flows 
back over the swift undercurrent with a velocity due to the free head of 
backwater next the dam, but at its surface level cannot reach 'the dam. 
These effects of flow, which may he observed at many dams, seem to have 
been overlooked by most writers on the subject. It is illustrated in part 
by Fig. 14, in which the valley between the down flowing stream and 
the returning current is filled with spray. 
“Fig. 1, on the left of Plate 1, is a reproduction of a sketch relating 
to these matters explained by the writer to members of the Board of Pub- 
lic Works at Austin, in June, 1892, when he first visited the works. The 
foundations of the dam were then in place and the superstructure in 
progress. 
“In computing the stability of a masonry dam, the weight of water 
resultants from a to b, Fig. 1, Plate 1, have usually been neglected. So, 
also, have the reactions of the tail water against the dam througout the 
flowing jet and also the weight reactions at b, c and the weight of the 
water at d, which in this case are sufficient to materially enhance the 
factor of safety. 
“Fall at the Toe . — Referring again to the undercutting at the toe of 
the dam, which occurred at a point about three hundred feet from the 
easterly abutment, we call attention to the appearance of the low and 
moderate flows at the fall over the toe of the dam, as shown in Figs. 11 
and 15 (a view taken of dam from east instead of west as printed under 
illustration). This fall should but slowly cut hard limestone, but might 
cut such soft stone as was said to have been found at the point mentioned. 
At the right of Plate 1 (in Fig. 2) is a sketch suggesting the possible 
effect of such fall on a soft rock or adobe stratum. A fall of one foot to 
surface of backwater gives a velocity of about 8 feet per second, and of 
two feet a, velocity of about 11.34 feet per second, and of two and one- 
half feet, as observed, a velocity of about 12.68 feet per second, each 
independent of the velocity acquired down the slope. 
“The failure of the dam was attributable to a local weakness in the 
rock on which it rested.- It is probable that the friable or soft stratum 
under the part of the dam which first moved, and which was not removed 
