I 4 a THE IMPROVEMENT OF RIVERS. 



their undermining. It has the advantage, however, of permitting excavation to be 

 carried close up to the cribs. These are usually made with a width of base not less 

 than 12 or 14 feet. 



The box coffer consists of horizontal waling-pieces, supported by temporary 

 uprights, and placed 3 to 4 feet apart, inside which planks of ij" to 2" in thick- 

 ness are placed vertically. Heavy tie-rods pass through the wales to prevent spread- 

 ing. The box thus made is filled with gravel, clay, etc. The planks should be driven 

 slightly into the river-bed. 



It will be found sufficient in most cases to place the top of the coffer-dam from 

 6 to 8 feet above low water. When the river has risen to a height of 6 or 7 feet, 

 the leakage usually becomes equal to the capacity of the pumps, and there is risk, 

 moreover, of part of the filling being washed out, but the additional height above this 

 elevation is a safeguard against the overflow of the coffer before it has become filled 

 through the sluices. These should always be provided at the down-stream end, so 

 that the pit can be flooded when required They can be closed with needles or 

 stop- planks. 



It is desirable in many cases to design the coffer so that it can be opened and 

 entered by a dredge early in the season, in order to facilitate excavation of deposit from 

 floods, or of the bank. This can be done without difficulty in a pile coffer, and in a 

 crib coffer the opening can be arranged for with sheet-piling, or with a bank of good 

 material well riprapped. 



Where repairs to a lock or dam are to be made and completed in one season a ser- 

 viceable coffer-dam may be obtained by throwing up a bank of earth around the space 

 to be inclosed, and protecting it with riprap where exposed to scour. This is a very 

 useful method where repairs are to be made to a fixed dam, as it obviates any necessity 

 of drawing off the pool and stopping navigation. We have used it where an entire dam 

 had to be torn out to the bed-rock, and rebuilt, a section at a time, during a season of 

 many and unexpected rises. The crest of the coffer had to be raised several times to 

 prevent the river from overflowing it, and after the work was finished it was finally 

 eaten into at its ends by a flood which rose to a height of 4 feet above the crest of 

 the completed dam. 



Materials of Construction. Modern locks and the foundation of movable dams 

 are almost always built of masonry. Where placed in a derivation, on a river or canal 

 of no great importance, the lock has sometimes been formed by simply placing masonry 

 ends to the chamber, and grading and paving the banks between to form the lock-pit. 

 Wooden locks of modern construction are to be found on the Fox River in Wisconsin, 

 where the extreme flood- range is only about 3 feet, and serve their purpose excel- 

 lently. They have masonry ends, and the chamber is formed by upright posts cov- 

 ered with two thicknesses of 2" dressed plank, jointed but not calked, behind which 

 a dry rubble wall is built. The cost of these locks is about one-third that of masonry 

 locks, but. they have to be renewed about every ten years. 



