198 



THE IRRIGATION AGE. 



site be of such a loose and porous charac- 

 ter as to not be capable of retaining water, 

 but such as would allow it to sink and 

 -escape too readily from the reservoir, that 

 would be a fatal objection. In all cases 

 under consideration, thorough examinations 

 should be made to determine the condi- 

 tion in this particular. 



Another matter of importance is the 

 liability of the reservoir to till up with 

 materials having a specific gravity greater 

 than water. Such materials are usually 

 ibroughtdown from above by heavy floods, 

 and the checking of the current when the 

 water charged with them reaches the 

 reservoir causes them to settle. This ob- 

 jection can sometimes be overcome by 

 intercepting the materials before they 

 reach the reservoir site or by sluicing them 

 out after they are in. The former method 

 is practicable when the materials are 

 coarse, provided that a suitable place can 

 be found in which to intercept them, and 

 the latter when they consist of fine sand 

 or soil and an opportunity exists for con- 

 structing scour or sluice gates for removing 

 them. Materials having a less specific 

 gravity than water cause no difficulty as 

 they will float on the surface and escape 

 over the waste weir. 



REQUIREMENTS FOR A DAM SITE. 



The conditions which make a site desir- 

 able for the construction of a dam are a 

 narrow passage to be closed, a reasonable 

 depth to material suitable for a founda- 

 tion, good opportunity for draining the 

 foundation, and proximity of suitable mate- 

 rials for constructing the dam. 



All of these things materially influence 

 the cost of a dam and in this way affect 

 the feasibility of a project. Before any 

 recommendation of a storage project is 

 made, surveys and borings to determine 

 the exact length of dam required, the 

 depth to a foundation and the amount of 

 water to be handled in draining the foun- 

 dation should be completed. Then the 

 class of materials required for a dam 

 can be settled, the data already obtained 

 enabling us to decide what type of dam to 

 build. Next the point from which the 

 materials for construction are to come 

 ought to be ascertained, after which their 

 cost can be accurately estimated. 



Finally comes the estimate of the total 

 cost of a dam based on all of the above by 

 taking into account the contents of the foun- 



dation and section of the dam, the amount 

 and cost of handling the material to be 

 excavated for the foundation and the cost 

 of cuts, tunnels or pumping water for 

 drainage, together with the cost of finish- 

 ings, gate tower, other equipments, super- 

 intendence and other incidental expenses. 



SUFFICIENCY OF WATER -SHED. 



In order to derive the greatest profit 

 from a storage reservoir its tributary water- 

 shed should be capable of filling it at least 

 once every year. 



If the water-shed is limited so that the 

 reservoir cannot with certainty be de- 

 pended on to fill every year, a portion of 

 the water has to be carried over each year, 

 so as to ensure an ample supply for the 

 ensuing year. In a case of this kind a 

 large reservoir may lose a very consider- 

 able portion of its value, as the duty 

 which it can perform wiil depend entirely 

 upon how much of the reservoir will fill in 

 years of minimum precipitation. 



Thus it will appear that a large reser- 

 voir site and an easy dam site are not the 

 only things to be sought, but that an ade- 

 quate water-shed for supplying it is of 

 quite as much importance. 



An examination of the water-shed tribu- 

 tary to a reservoir site is therefore a mat- 

 ter of great moment and in the following 

 articles we will briefly discuss the points 

 to be investigated and the lines upon which 

 these investigations ought to be carried 

 out. 



All of the territory draining into a 

 reservoir site above the location of the 

 dam is known as its tributary Avater-shed. 

 To determine the area of this water-shed is 

 the first matter of importance in investi- 

 gating it. 



Surveys from which the number of 

 square miles or acres can be calculated 

 should be made. These surveys should 

 also be topographical in a measure, as the 

 differences in elevation are important, so if 

 the water- shed differs very much in ele- 

 vation it should be classified accordingly. 

 The exactness with which it is necessary 

 to determine the area of a water-shed must 

 be left entirely to the judgment of the en- 

 gineer. Sometimes it may be so much 

 larger or smaller than necessary that only 

 a fair approximation of its size is required 

 for making a report. In such cases it is 

 usually sufficient to locate the principal 

 points along the boundary lines of the 



