Further, that taking the loss of head in passing through the core wall, and the slope assumed by the plane of saturation, the maximum safe height of an earth dam with its top 20 ft. above water level in the reservoir and its outside slope 2 on 1, is 63 to 102.5 ft. This is a remarkable finding in view of the fact that the Titicus Dam, one of the Croton Valley dams examined, has a maximum height above bed rock of 110 ft. and has been in use seven years. This dam is not a fair example to cite in proof of their conclusion, because its _effective head_ is only about 46 ft.[2] [Illustration: BOG BROOK DAM.] [Illustration: MIDDLE BRANCH DAM. FIG. 18.–CROSS-SECTION OF TWO CROTON VALLEY DAMS, SHOWING SATURATION.] Mr. Fteley gave as a reason for the elevation of the water slope found in the outer bank of the Croton dams the fact of their being constructed of fine materials and stated that with comparatively porous materials they would have shown steeper slopes of saturation. Mr. Craven argued that all dams will absorb more or less water, and that porosity is merely a degree of compactness; that slope implies motion in water, and that there is no absolute retention of water in the outer bank of a dam having its base below the plane indicated by the loss of head in passing through the inner bank and then through a further obstruction of either masonry or puddle; that there is simply a partial retention, with motion through the bank governed by the degree of porosity of the material. Fig. 19 is a graphical interpretation of the conclusion reached by the board of experts, as already given on page 41. “A” is an ideal profile of a homogeneous dam with the inner slope 3 on 1 and the outer slope 2 on 1. The top width is made 25 ft. for a dam having 90 ft. effective head, the high-water surface in the reservoir being 10 ft. below the crest of the dam. This ideal profile is a fair average of all the earth dams of the world. Not having a core wall to augment the loss of head, it fairly represents what might be expected of such a dam built of Croton Valley material, compacted in the usual way. It should be noted that the intersection of the plane of saturation with the rear slope of the dam at such high elevation as shown indicates an excessive seepage and a dangerously unstable condition. Preliminary Study of Profile for Dam. The preliminary calculations for designing a profile for an earth dam are simple and will here be illustrated by an example. Let us assume the following values: a. Central height of dam, 100 ft. b. Maximum depth of water, 90 ft., with surface 10 ft. below crest of dam. c. Effective head, 90 ft. d. Weight of water, 62.5 lbs. per cu. ft. e. Weight of material, 125 lbs. per cu. ft. f. Coefficient of friction, 1.00, or equal to the weight. g. Factor of safety against sliding, 10. The width corresponding to the vertical pressure of 1 ft. is, (62.5 × 10)/125 = 5 ft. [Illustration]