aoo THE IMPROVEMENT OF RIVERS. 



fixed type should restrict the waterway^ little as possible, and therefore it ought to be 

 as long as the conditions will permit. The greater the length over which the action 

 of the water is extended, the less will be its tendency toward undermining the works 

 and the banks, and the greater will be the facilities for discharging floods. 



No rules of any practical value can be formulated for determining the length of a 

 fixed dam, as can be done for movable dams. In theory it should be such that just 

 before the lock is drowned, boats can pass over the dam in either direction; in other 

 words navigation must never be interrupted. Practically, however, this condition 

 is affected by a number of external elements whose combined effects cannot be fore- 

 told, such as the rapidity of the rises, the length of the pool below, the width or narrow- 

 ness of the river in the first mile below the dam, and similar conditions. The only safe 

 rule is to follow that of experience, which shows that the spillway should be as long as 

 practicable. 



General Design. In this country the greater number of stationary dams are built 

 of timber cribs filled with stone, and decked or floored over. This deck is sometimes 

 made a continuous slope from the crest to near the lower pool level, and sometimes is 

 broken into steps of varying heights and widths. The former are known as slope 

 dams, the latter as step dams. Where practicable these cribs should be founded, at 

 least along the face or down-stream side, upon solid rock. 



The slope and the step dam have each their advantages and drawbacks. The deck- 

 ing of the latter is more easily injured by the passage of ice, drift, saw-logs, etc., but 

 in moderate stages it retards the progress of the water so that it arrives at the lower 

 pool with less velocity than had it passed over a slope, and hence its effect is not felt 

 so far below it. It is also a little cheaper to construct than a slope dam. 



The bottom width of the dam must of course be sufficient to prevent overturning, 

 and its foundation strong enough to resist the pressure that is to come upon it. The 

 ordinary timber and broken-stone dam is usually much wider than its height, one reason 

 for this excess of width being that it gives a more gradual descent to the water from 

 pool to pool, and thus reduces its undermining effect. In one example of old construc- 

 tion, with a lift of 17 feet and a rock foundation, the base width is 43 feet. Another, 

 recently built, with a similar foundation, has a lift of 18 feet and a base of 50 feet, and 

 is 30 feet high. A third has a lift of 14 feet and a base of 33 feet. 



The natural foundation will largely govern the design of the dam. If of rock, 

 the case will not present any special difficulty, but if of gravel or other light material, 

 great care must be used to prevent washing and undermining. In cases where the 

 river is deep the cribs usually rest directly on the bottom, but where it is shallow a 

 trench is dredged out, or piles are driven and cut off just below water and the crib- 

 work built upon them. If the last method is used, an apron crib should always be 

 sunk against the down-stream side of the dam as deep as possible, or the reaction 

 may undermine it. Experience has shown that aprons formed simply of piles framed 

 and decked over, or of cribs resting on piles, are unsuited to rivers of high floods, and 



