i6a THE IMPROVEMENT OF RIVERS. 



If a solid crib is used, an opening about 1 5 feet wide should be left in it, 20 or 36 

 feet above the lock, for the disposal of stray drift, and to permit a current through 

 the upper entrance during rises, which would otherwise deposit sediment there. 



The lower entrance is sometimes provided both with a land crib and with a river 

 crib. The former acts partly as a protection to the bank, and has also to support the 

 sediment which floods deposit behind it. The river crib, however, is of doubtful utility, 

 although in some cases it has been built with the object of checking the effect of reac- 

 tions from the dam on the lock gates. We are acquainted with locks, with and with- 

 out river cribs, subject to similar conditions of flow, and in most cases the cribs could 

 have been dispensed with. It is usually best to omit them until their advisability 

 has become apparent. 



On locks having movable dams there is rarely any necessity for a crib or wall either 

 above or below the river wall, but guide walls next the shore should always be pro- 

 vided for convenience in locking tows. 



Where cribs have to be sunk through water to their foundations, as is usually the 

 case where piles are not used, the same methods are employed as described for sinking 

 crib dams, except that the base cribs should not be over 20 or 30 feet long, unless the 

 water is shallow. It is more important to secure a thorough bedding than with a dam, 

 since any settlement will cause the crib to lean. Where the foundation has been 

 dredged out it is a good plan to raise the end of each base crib a foot or two, with the 

 dredge or other power, and then let it fall, continuing the "shaking" until the timbers 

 appear to have found a solid bed. The top can then be leveled up with shims or blocks 

 at the water-line. 



If concrete is to be used for a foundation, it is usually necessary to build a 

 coffer-dam. 



The cost of timber guide-cribs with stone filling, including all labor and material 

 needed to complete them ready for use, varies from two to three dollars per cubic yard 

 of total contents, and from forty to fifty-five dollars per thousand feet B. M., on the 

 basis of the timber. 



Accessories. For holding craft while locking, and to prevent the currents from 

 the valves from bumping them against the walls, check- or snubbing-posts on the coping, 

 or line-hooks built in recesses .in the faces of the chamber walls, are required. The 

 former may be made of wood, or, better still, of cast iron, and should be round and not 

 less than 8 inches in diameter, and about 14 inches high. The latter are usually made 

 of i}-inch round iron, forged to suit. Where the lifts are small line-hooks are very 

 satisfactory, but for high lifts snubbing-posts are preferable, as the mooring-lines can 

 be more easily tended. 



Posts should also be provided on the guide cribs or walls, so that craft can be 

 tied to them while waiting their turn to lock. 



On some locks cast-iron chocks are provided at the ends of the chamber, set close 

 to the edge of the land wall. The mooring-lines are led through these to snubbing- 



