64 BULLETIN 71, U. S. DEPARTMENT OF AGRICULTURE. 



it is cleaned out, the drainage secured is comparatively poor. The land would 

 not be so badly cut up if tile were used, and the necessity for a great number 

 of small bridges would thus be removed. 



Reservoir Canals. 



The primary requisite of reservoir canals is that they give sufficient outlet 

 to the lateral ditches. To do this they must be spaced not more than one-half 

 mile apart and usually they should be located in parallel lines. In building 

 the levee around a district the resulting canal can sometimes safely be made 

 on the inside of the levee and consequently used as a reservoir canal. On 

 small districts this canal may be extensive enough to make up the entire reser- 

 voir-canal system. The practice of placing the levee canal inside the district 

 has proved either a success or a failure, according to local conditions. If the 

 berm between the levee and canal is wide, if the base is above ordinary stages 

 of outside water, if there is no canal immediately outside the levee, and if the 

 storm tide is low, such construction should not result in any great amount of 

 seepage or subject the district to any danger from storm tide. On the other 

 hand, if the canal is placed outside the district the seepage will be less, the 

 levee will be safer in time of storm tide, and the canal can be used for navi- 

 gation purposes. In all but the most favorable locations present practice tends 

 toward placing the levee canal outside the district and cutting an interior sys- 

 tem of reservoir canals. 



The reservoir canal should be of such depth and cross section that the water 

 will ordinarily be held at least 4 feet below the surface, although immediately 

 after heavy precipitation the water may safely stand at the level of the lowest 

 land for several hours. The reservoir canal serves a twofold purpose: (1) To 

 take the water from the lateral ditches and carry it to the pumping plant, and 

 (2) to store up the dry-weather flow of the ditches, so that the pumping 

 plant will not need to be operated so frequently. Canals designed with only the 

 first consideration in mind are smaller than when any considerable reservoir 

 capacity is desired. If, however, the canals are correctly designed for storage 

 capacity the question of flow will be taken care of. When heavy rains occur 

 the storage capacity in the canals will take part of the run-off and temporarily 

 relieve the pumping plant. Thus by an increase of reservoir capacity a less 

 capacity of pumping plant will be required, and the plant can be operated more 

 economically. The relative capacities of reservoir canal and pumping plant 

 should be such that the interest and depreciation on the two investments, plus 

 the cost of operation, would be a minimum. A complete set of records main- 

 tained on a number of typical reclamation districts for several years will be 

 necessary before a relation can be established that would be capable of general 

 application. 



In some of the districts already constructed the reservoir capacity is about 

 0.6 inch in depth of water over the whole area. In these the average slope of 

 the water surface in the canals is theoretically less than 0.2 foot per mile, with 

 an average depth of flow of 6 feet and a run-off of 1 inch per day. If the reser- 

 voir capacity is so proportioned that the velocity of flow is nearly uniform in 

 all parts of the system, the slopes can be held very close to the above figure. 

 These slopes will of course increase when the water is lowered to near the bot- 

 tom of the canals, but as the bulk of the water will be pumped at the time when 

 the canals are full this increase of slope is not especially objectionable. 



The local conditions will decide to some extent the question of using broad, 

 shallow canals or deep, narrow ones. A broad, shallow canal has more of its 



