THE IRRIGATION AGE. 



145 



TYPES OF SMALL DAMS AND WHAT THEY COST 



By EVERETT B. MURRAY 



Assoc. M. Am. Soc. C. E.; M. Am. Soc. T. M.. of Kanws City, Mo. 



homa City, Okla. 



THE first and simplest 

 type of dam is what is 

 known as the earth dam. 

 This is generally used where 

 the soil is composed of a 

 good grade of clay that will 

 insure a satisfactory bond at 

 the base of the dam with 

 the natural ground and also 

 insure a satisfactory core 

 wall, which is one of the 

 most important factors in 

 this type of structure. 



Under ordinary condi- 

 tions this dam is more sat- 

 isfactory where the length 



of it is restricted and it will not be subject to sud- 

 den extreme rises in the stream in which it is im-. 

 pounded. The crown width of the dam should 

 not be less than 6 feet, and where it is to be used 

 as a roadway it should be at least 12 feet. The side 

 slope on the water side should be at least 2J/2 to 1 

 foot, while the opposite slope may be made 2 to 1. 

 Under no conditions should the crown of dam be 

 less than 2 l / 2 feet above what is known as extreme 

 high-water, and it is always safer to make this dis- 

 tance more if possible. 



The materials used in the construction of a dirt 

 or earth dam should be carefully considered, only 

 good clean earth, either of clay or sandy loam, 

 should be used. Any earth which has large num- 

 bers of stones or gravel or vegetable material, sub- 

 ject to decay, should be rejected. It might be well 

 to mention here that in locating a dam one of the 

 most essential features is to select the site where 

 there is a good solid earth foundation, and that the 

 axis of the embankment or dam should be placed 

 at right angles to the general direction of the flow 

 of the water which is to be impounded. The basr 

 of the dam should be carefully gone over and all 

 rock, saturated earth, loose stone and decayed vege- 

 table matter should be grubbed out. The dam site 

 should be well broken up by plowing longitudinal 

 furrows in order to insure a good bond with the 

 natural surface of the ground. After this has been 

 done the core wall should be dug. 



This core wall consists of a trench at least 3 

 feet wide and as deep as may be considered neces- 

 sary to intercept any seepage that is likely to occur 

 at the base of the dam. As a general thing this is 

 hard to determine unless the local conditions can 

 be observed. This wall should in no case be less 

 than 5 feet. After this excavation has been made, 

 located between the inner slope of the dam and the 

 center line and nearer to the center line, it should 

 be filled with good clay carefully puddled in order 

 to make it impervious to what seepage occurs. 



The construction of the dam proper should be 

 in even' horizontal layers, not more than 1 foot in 

 thickness, and should extend out to the slope stakes 

 to start with, and, as the embankment is brought 



Irrigated sweet potatoes raised by J. E. Lucas of Okla 

 Courtesy of the Rock Island Railway. 



up, gradually narrowed so 

 as to conform to the pre- 

 scribed slopes. It is a gen- 

 erally conceded fact that 

 team work makes the best 

 embankment, inasmuch as it 

 packs these layers hard as 

 they increase in height. 



If for any reason after 

 the dam is one-half com- 

 pleted in height the work is 

 abandoned for a time and 

 then resumed, care should 

 be taken to scar, either by 

 means of furrows or other 

 means the top surface be 



fore resuming the work. If this is not done there 

 is likely to be a joint where there will be no bond 

 between the new and the old work. An extra pre- 

 caution which is highly advisable is that where the 

 material is loose and dry it should be thoroughly 

 wet and rolled between the succeeding layers. 



It is well to take into consideration some pre- 

 ventative for the crawfish nuisance. This is fre- 

 quently handled very satisfactorily by placing just 

 below the surface of the ground in the slope of the 

 dam on the upstream side a fine wire mesh which 

 prevents the animal from penetrating and causing 

 failure by reason of the water's percolating in 

 after it. 



This type of dam will vary in cost with the lo- 

 cality, the size, difficulties of getting labor and 

 maintaining a camp. However, the cost should 

 vary from 15 to 25 cents a yard. 



The second class of structures to be consid- 

 ered is what is known as timber or crib dams. 

 These are usually built where the base of the dam 

 rests on a solid rock foundation or in large coarse 

 gravel. One of the most common types is to frame 

 the dam with a very flat slope extending upstream 

 and with a vertical face downstream or a much 

 steeper slope than the upstream side. Heavy tim- 

 ber on the upstream side should be held in place by 

 correspondingly heavy structures, the size of all 

 the members depending entirely on the height of 

 the dam and other general conditions governing the 

 work. These structures or supports should be car- 

 ried down to a sill which is placed on the rock bed, 

 this sill in turn resting on cross pieces of heavy 

 timber which should be imbedded in to the rock or 

 grave. This is to prevent slipping. 



Between the supports, which act in a way simi- 

 lar to buttresses on a retaining wall, there should 

 be placed quantities of heavy stone and clay, this 

 to rest on the sill before mentioned and is for the 

 purpose of giving the dam extra weight. On top 

 of the up and downstream slopes there should be 

 placed on these buttresses heavy planking spiked 

 as close together as possible, said planking not to 

 be less than three inches in thickness. On the up- 

 stream side there should be placed a considerable 



