aoa THE IMPROVEMENT OF RIVERS. 



top or near the top of the cribwork, and there connect with the decking of the upper 

 slope. Some engineers cut it off about 2 feet below the top, and continue it with short 

 plank, for easier repairing. Immediately up stream of this sheeting should be placed 

 an embankment of gravel and clay, riprapped on its upper surface for a distance of 

 10 or 15 feet away from the dam. The down-stream face may be left open between 

 the timbers unless the spaces exceed 7 or 8 inches, when they should be reduced by 

 fillers, or the filling stone will be washed out. The up-stream face may be carried to 

 within 3 or 4 feet of crest height ; the lower face should stop at or near lower pool level. 



The line of the crest is ordinarily placed from 8 to 12 feet from the upper face, 

 and the sloping decking connecting these points should be practically water-tight. It 

 is placed on a slope to permit the passage of drift, etc. 



If the dam is of the step type, the height of each step should not much exceed 

 one-third the width of the next below, or in certain stages the falling water may miss 

 the step and strike the one below, and if drift or ice is running, the decking will suffer 

 accordingly. Thus, where the steps are 10 feet wide, the limiting height should be 

 about 3 feet 4 inches. The step descending to the apron, however, if the latter is wide, 

 may be as high as 5 feet. The width of these successive steps may vary between 8 

 and 12 feet, 10 feet being usual. The decking of the up-stream slope should be of two 

 thicknesses of 2-inch or 3-inch white oak or hardwood plank, lap-jointed so that when 

 the top layer is worn it can be renewed without destroying the lower one. The 

 decking of the other steps may be from 4 to 10 inches in thickness, depending largely 

 upon the amount of ice, drift, etc., passing over the dam, which soon wear away the 

 surfaces. 



In certain cases the top of the cribwork at the crest is stopped from 6 to 12 inches 

 below the upper pool, and the remaining height obtained by spiking a "comb-stick " on 

 the decking. This stick should be of oak, and be well secured with drift-bolts and 

 straps, as it has to stand heavy blows from drift. It is frequently necessary to use this 

 method in order to level up or raise the crest of a dam which has settled, which always 

 happens as the timbers become old, and in point of fact the settlement usually dates 

 from the completion of the work. 



In rivers with small flood heights, especially where it is desired to keep the pool 

 at about the same level, flash-boards are often used; these are plank laid horizontally 

 along the crest and supported against pins. As the river rises one or more planks are 

 removed, and replaced as it falls, thus securing a regulation of the pool. 



Where slope dams are built the up-stream side is made as for a step dam, and 

 the down-stream side is made with a slope varying from 2' 6" to 4 feet base to i foot rise. 

 As generally built these dams provide for but little protection below, but it has been 

 found necessary in almost every case to place apron cribs below them later in order to 

 prevent undermining. Where this method is adopted in the first case, as it always 

 should be, the cross section of the dam itself may be reduced, as the apron can be 

 made part of the structure. 



