1056 



THE IRKIGATION AGE. 



The Cienega Sub-Surface Dam Near Tuc- 

 son, Arizona 



By G. P. E. Smith* 



In the irrigated sections of the west many streams 

 are popularly believed to carry considerable quantities of 

 underflow and in many places subsurface dams have been 

 projected or discussed for the purpose of intercepting 

 the underflow and raising it to the surface. Few such 

 dams, however, have been built, notable exceptions being 

 those of Pacoima Wash in California and Agua Fria River 

 in Arizona. 



An account of such a structure is of interest in two 

 ways: First, the design is required to meet unique con- 

 ditions; and second, the amount of water recovered con- 

 tributes to our knowledge of the underflow of stream. 



At the head of the Cienega Ranch irrigating ditch, 

 near Tucson, the conditions appeared to be favorable to 

 the construction of a subsurface dam. The river here 

 narrows between hard sandstone walls to a width of 67 

 feet, and the traditions of the locality were that bedrock 

 was reached by sounding rods at a maximum depth of 12 

 feet. The gradient of the river is 41 feet per mile, the 

 summer floods are sudden and torrential and the great 

 difficulty of maintaining an ordinary ditch head against 

 the severe floods occurring every summer made it desira- 

 ble to take the water out through a pipe conduit laid at 

 a safe depth below the river bed. It was expected also 

 that a considerable volume of underflow would be recov- 

 ered. 



The construction of the dam, con- 

 duit and collecting basin has been car- 

 ried out during the spring of 1911. 

 The surface flow was very small and 

 the danger of floods at that season of 

 the year was negligible. The excava- 

 tion of the river, gravel, was carried 

 down to a depth of 25 feet with slip 

 scrapers. At that depth the width of 

 the river bed between rock walls was 

 22 feet and a trench with lagging was 

 started, its cross width being 12 feet, 

 narrowing to 9 feet when bedrock was 

 finally uncovered for the full width of 

 the river bed. 



In order to keep down the seepage 

 water during the excavation and con- 

 struction centrifugal pumps were used, 

 the combined discharge for several 

 weeks approximating 4 second-feet, due 

 in part to the draining out of the sand 

 reservoir above the site. 



The dam is of the arch type as 

 shown by Fig. 1, and is built of plain 

 concrete. The upstream face is verti- 

 cal and the radius of curvature is equal 

 to the span length, 67 feet. The top 

 width is 14 inches, and the bottom 

 width at the extreme depth of 45 feet 

 6 inches is 5 feet. A view of the com- 

 pleted structure is seen in Fig. 2. 



The concrete was proportioned 1 

 cement to 5 of river gravel and boul- 

 ders were driven into the mushy mix- 

 ture after it was placed in the forms. 

 The concrete was bonded to the rock 

 surface with neat cement. On the 

 sides of the canon the rock face was blasted off so as to 

 form a square skewback to take the thrust of the arch. 

 The elevation of the top is that of the normal river bed. 

 Six days sufficed for the concreting, part of the form 

 work being done at night. 



Whether or not the structure as built will withstand 

 the floods has not been proven as yet. If the top portion 

 should fail, the upper 6 or 8 feet can be rebuilt of rein- 



forced concrete and with sufficiently heavy section so that 

 it will be permanent. 



Upstream from the dam, as seen in the plan, Fig. 1, 

 is a concrete cylinder of 8 feet inside diameter and 25 feet 

 depth, to serve as a collecting head. Its thickness is 8 

 inches. The inflow is upward from the bottom and lat- 

 erally through 200 holes each 2 inches in diameter and 

 protected by coarse J4-i ncn mesh screen. The conduit of 

 16-inch cement pipe is started at the bottom of the cylin- 

 der and is laid on a level for 50 feet, thence on an upward 

 incline for 250 feet to the Cienega ditch, the bottom of 

 which is 4 feet lower than the river bed. The cement pipe 

 was \ l /2 inches thick and under the pressure head of 22 feet 

 leaked badly at first, but has slowly improved in that 

 respect. 



The surface flow of water one-fourth mile above the 

 site on June 8 was 0.6 second-foot and the ditch was car- 

 rying 1.5 second-feet, the underflow thus added being ap- 

 proximately 1 second-foot. On previous years the flow at 

 the ranch at this season has been about one-half of a sec- 

 ond-foot in the morning and the ditch has been dry at 

 evening, due to the extremely high rate of evaporation 

 during the day. 



The improvements were designed and successfully 

 constructed by Mr. R. R. Schweitzer, manager of La 

 Cienega Land & Cattle Co. The total cost, including la- 

 bor, supervision, supplies, rent oi" pumping machinery, 

 and cement at $3.15 per barrel, was $7,350. A second-foot 

 of water used for fruit and alfalfa ranching at Tucson 

 can be capitalized at $18,000. Engineering .& Contracting. 



Collecting Head or Wall 



Crest of Dam is at Level of River Bed 



r 5 Gravel Concrete 

 with Boulders included 



[ngXontg 



Fig. 1. Plan Profile and Section of Sub-Surface Dam, Pantano, Wash., near Aucson, Ariz. 



George Austin, John Henry Smith, M. H. Walker, 

 Seymour B. Young and Fred J. Kiesel constitute the 

 committee appointed by Governor William Spry to repre- 

 sent Utah at the National Irrigation Congress to be held 

 at Chicago December 5-9. The committee will work in 

 conjunction with the executive committee of the congress. 



*lrrigatoin Engineer, Agricultural 

 of Arizona, Tucson, Arizona. 



Experiment Station, University 



It was stated recently by Samuel Fortier of the Gov- 

 ernment Agricultural Department that the Weber and 

 Davis canal of Ogden is far ahead of the majority of big 

 irrigation projects in the west, both in point of efficiency 

 and accomplishments in engineering. 



