56 BULLETIN 71, U. S. DEPAETMENT OF AGEICULTUEE. 



in many cases it lias been found necessary to redredge tlie canal about three 

 years after construction. Tbe surrounding land is tbeu solid enough approxi- 

 mately to hold its place and to make the canal permanent. Canals constructed 

 Avith a hydraulic dredge are better cleared of soft mud at the time of con- 

 struction, and. due to absence of heavy spoil banks close to the edge of the 

 canal, appear to hold their original size more satisfactorily than those con- 

 structed by other forms of di-edge. 



On the newer districts the area in canals is about 1 per cent of the area 

 included within the levees. The average cost per cubic yard for cutting internal 

 canals is between G and 8 cents, the higher figure being applicable to cases 

 where the ground is covered with a growth of timber, when the work is done 

 with a dipper dredge. If the land be free from stumps and sunken timber the 

 work can be done either by dipper dredge or hydraulic dredge for about 6 cents 

 per cubic yard. 



PUMPING PLANTS, 



The theoretical normal capacity of the plants examined averages 1.22 inches 

 in depth of water over the whole area removed in 24 hours of continuous 

 operation. On most of the districts the normal capacity can be increased 

 .somewhat by speeding up the machinery. The reservoir capacity and pump- 

 ing-plant capacity are not proportioned to each other at all uniformly. There 

 is such a wide variation in this regard that either some pumping plants are 

 too large to be economical, or a number of the others are far too small to give 

 drainage. As little flooding of any of these districts has occurred, it would ap- 

 pear that the former is true. There is as much variation in capacity in the 

 newer plants as in the older ones. No settled policy as to required capacity 

 has been established. Some plants have been built of ample size, so that if it 

 became apparent that more water could safely be handled an addition could be 

 made to the area drained. In the summary (facing p. 53) it will be noted that 

 the smallest pumping-plant capacity appears on the largest district, which district 

 has the largest reservoir capacity. Even the smaller plants have their pump- 

 ing capacity divided between at least two units. This allows the use of part 

 of the plant during low-water flow, with as small capacity as is desired. In 

 some of the plants which consist of two duplicate units each unit is nearly 

 large enough to take care of all the water ; this provides a reserve capacity in 

 case of breakage of one of the units. 



The static lift in these pumping plants varies from 3 to 10 feet. The bulk 

 of the water usually is lifted only about 3 feet, the lift increasing as the water 

 in the canal is lowered, so that it is only during the last few hours of pumping 

 that the lift approaches the larger figure given. 



The cast-iron centrifugal pumps are largely used and have been found very 

 ranch more satisfactory than the older types of vertical wooden-case centrifugal 

 pumps. While the first cost of the latter type is less than that of the cast-iron 

 pump the efficiency also is less, and the maintenance much greater. As pumps 

 of the vertical wooden-case type can not be dii'ect connected to engines they 

 are not so reliable in operation, and as they do not discharge into a closed pipe 

 no siphon effect can be arranged, and their actual lift usually is from 2 to 3 

 feet greater than the effective lift. Also they do not lend themselves well to 

 installation on a concrete foundation. 



Only one rotary-pump installation was examined, and while the pump oper- 

 ates verj' satisfactorily it was lifting the water fully twice the necessary 

 height. Rotary pumps of the desired capacity are adapted to a minimum lift 

 of about 10 feet, and as the usual lift is about half this amount the loss of 

 energy is far too large. All the modern plants are equipped with horizontal 



