360 LOGGING 



and other obstructions, and in some cases to block an old 

 channel and divert the water into another course. 



They are built in a manner similar to the piers of crib-dams 

 with cribs from 6 to 8 feet square, and mud-sills fastened to 

 bedrock or lirmly anchored in the stream bed. The cribs are 

 loaded with rock to give them stability. 



Abutments are used to protect the banks of streams during 

 flood time, and prevent them from being worn away. The 

 usual form consists of a cribwork of timber built into the bank. 

 The space between the shore and the timbers is filled with rock 

 to prevent the bank earth from washing out. Where streams 

 pass through wide bottoms and the banks are too low to con- 

 fine the flood water, an artificial channel is sometimes created 

 by constructing false banks of lumber. Cribwork supports a 

 strong frame of timbers on which heavy planking is nailed. 



Fig. 105. — An Artificial Channel used to confine Flood Water in a 

 Narrow Bed. 



Booms. — Backwaters, pockets, low banks, obstructions and 

 shallow places where logs are apt to be lost or stranded occur 

 on most streams. Booms, consisting of long sticks of timber 

 fastened together end for end and moored to objects on shore or 

 to piling or cribs in the stream, are used to confine the logs to the 

 channel. Booms are also used to aid drivers in sluicing logs 

 through dams, for confining logs at sorting gaps and storage 

 points, and for towing. They are built in many forms and are 

 called sheer booms when used to confine logs for storage purposes 

 in given channels and towing booms when used to impound logs 

 for towing purposes. They are again designated as limber and 

 stiff booms according to their manner of construction. Both 

 sheer booms and towing booms are often of the same pattern and 

 are known as the ''plug" boom, "sheep-shank" boom, "chain" 



