FIXED DAMS. 203 



The apron is occasionally made with a considerable upward inclination in the 

 down-stream direction, this construction having been found advantageous in causing 

 deposit along the down-stream face of the dam. 



The width of the apron crib, which in a step dam is formed by the lowest step, is 

 dependent on the depth of water below it and on the lift, since the higher the lift the 

 greater will be the width required. It may be determined on the proportion of 18 inches 

 of width for each foot of lift, with a minimum of 8 or 10 feet. Thus for a dam of 4 

 feet lift the apron would be 8 or 10 feet wide, while for a dam of 12 feet lift it should 

 be 1 8 feet or more. After a flood has reached a depth of 4 or 5 feet on the crest, the 

 profile of overflow becomes the same for slope as for step dams, and a violent reaction 

 occurs where it strikes the lower pool, and not infrequently pieces of the apron deck- 

 ing are loosened by it and sometimes a large number torn up at once. Especial care 

 should therefore be taken to fasten the top stringers to the stringers below, and it is 

 best to make the former of oak or hardwood, which will hold fast the drift-bolts 

 of the decking. If these precautions are taken, pieces will rarely come loose. The 

 decking of the steps or slope above seems to remain practically unaffected by the 

 movement of the water. 



The deck timbers on the down-stream side of a slope dam and on the steps of a step 

 dam are sometimes spaced one-half inch to one inch apart, in order to permit a free 

 circulation of air and the quicker escape of the overflowing water. This, however, is 

 of little practical value, since it has no appreciable effect on the discharge, and it has 

 been found that it is liable to wear away rapidly the filling underneath. With the 

 apron decking, however, it is considered best to space the timbers a little apart, as it is 

 supposed to check the reaction in ordinary stages, and to reduce the tendency to loosen 

 the timbers. Where the reaction has been violent, however, open-spaced decking 

 has been torn off, notwithstanding the precaution of placing the pieces an inch or 

 more apart. 



Methods of Putting in Without Coffer. Crib dams are usually constructed with- 

 out the use of a coffer-dam, during the low-water season. This feature constitutes 

 one of their chief recommendations, as it permits of cheap and rapid construction. 

 The site is first cleared, either by dredging or otherwise, of all things which might 

 interfere with the proper seating of the cribs, and then carefully sounded throughout in 

 order to determine the location of the various irregularities. Its length is then divided 

 into crib sections of such dimensions as it is considered can be handled in the current 

 with the appliances available. These may be up to 100 feet in length and of the 

 whole width of dam, and in practice they are generally put together immediately 

 over the place where it is proposed to sink them ; but this may be done, or partly done, 

 elsewhere, and they may be towed into position when ready. Having determined 

 by the soundings the profile of the bed, the under side of the crib is shaped approximately 

 to fit it by means of additional timbers or blocks, drift-bolted on where required, so 

 that when the crib reaches the bottom its top will be nearly level. The cribs are 



