MOVABLE DAMS. 215 



"As the stage that would just cover the pier and abutment is 16.5 feet, it is thought 

 best to fix the length of the weir at 140 feet, particularly as the rises during the summer 

 are generally quite sudden. 



"With the sill of the pass at low- water level, and 130 feet long; the sill of the 

 weir 6 feet above that of the pass, and 140 feet long; for the 8-foot stage Z becomes 

 equal to 0.53 foot, for the g-ioot stage it is less than 0.5 foot, and when the pier and 

 abutment are submerged Z will not exceed 0.5 foot appreciably." 



Foundations, etc. The foundation for a movable dam should be of masonry, 

 and wherever practicable should be begun on bed-rock; where this is not feasible 

 substantial aprons or other protection must be provided for the down-stream face. 

 This type of dam, owing to the regulation of the pool, is more subject to the dangers 

 of currents and reactions than a fixed dam, and should any settlement or undermin- 

 ing occur it may throw the whole superstructure out of order. For this reason the 

 substructure should be made secure and of permanent material, as, if repairs have to 

 be made to it later, it will necessitate expensive coffer-dams and probably a long inter- 

 ruption of navigation. A fixed dam will safely stand a settlement which would be 

 dangerous to a movable dam, and can, moreover, be much more easily repaired. The 

 long experience of European engineers in this field has shown the wisdom of using 

 nothing but permanent foundations for movable dams, and they discarded long since 

 the use of timber for such purposes. 



Where the coping of the masonry is of cut stone the two upper courses should be 

 doweled together, and in many cases long bolts are run horizontally from the up-stream 

 to the down-stream face. Especial care must be taken to make the former proof against 

 undermining, since there is no mass of backing to prevent leakage, as with a fixed dam. 



The sills supporting the bottom of the curtain of the dam should be made prefer- 

 ably of cast iron, although where always under water wood has been generally em- 

 ployed in this country. Cast iron is, however, much preferable, since the wood 

 becomes gradually worn away by the currents and can only be replaced with difficulty 

 and expense. The depth behind the sill, or the recess, should be sufficient to allow 

 all movable parts to lie down, with an inch or two of clearance above them, so that any 

 submerged object will strike the sill rather than the parts below. The depth of these 

 recesses varies from 15 inches to 20 inches or more, one recent example of a Boule 

 dam having a depth of 3^ feet. It should, however, always be made as small as pos- 

 sible, since it acts as a catch-basin for gravel, sunken drift, etc. 



The height of the sill above the masonry on the upper side is from 4 to 6 inches, 

 as may be required to support the ends of the needles or wickets. It is made small, 

 to prevent gravel and loose stone catching against the sill when the dam is down, 

 which would interfere with the raising. 



Recesses should be provided in the coping of the masonry into which uprights can be 

 placed for use in coffering off any portion of the dam for repairs to the superstructure.' 



Drift-chute and Regulating-weir. It is very desirable, particularly in high dams, 



