SPILLWAYS FOR RESERVOIRS AND CANALS. 11 



addition of such devices invariably serves the primary purpose of 

 the spillway in facilitating the escape of the increased inflow, but 

 lacks the automatic character which must be the distinctive feature 

 of the true spillway. These agencies may be (1) flashboards in 

 their various forms; (2) sliding gates or sluiceways; (3) Taintor or 

 radial gates; (4) tilting or counterweighted gates; and (5) rolling 

 dams or barriers. These are discussed in the order listed. 



FLASHBOARDS. 



Flashboards are the simplest but most unsatisfactory device. They 

 permit of fairly close regulation of head due to the possibility of 

 setting them to any desired width. The boards may be placed be- 

 tween stationary piers or may be held in place by tilting arms or 

 braces set in line on the crest of the spillway so that they may not 

 retard flow during flood, but will afford a certain pondage to pro- 

 vide surplus water from the time of minimum to that of maximum 

 demand. This is shown in figure 1, Plate VII. 



Another method is to have holes in the crest of the dam or spill- 

 way into which iron pins of a certain size and predetermined fiber 

 stress per unit area are set and the boards fastened to these pins by 

 staples. (Fig. 2, Plate VII.) The pins are calculated to resist 

 pressure of the flashboards set against them up to a certain point, 

 and when they bend the boards are released and go down the stream. 



Such practice is not economical and is uncertain, due to the great 

 variation in the strength of "the iron pins. Where the storage of 

 flood water is essential or desirable and the stored water is very 

 valuable, this use of flashboards is not recommended, because they 

 are generally leaky, and when they have gone out they can not be 

 replaced until the freshet or flood has passed, making it impossible 

 to store any of the flood water. 



Flashboards are also installed as a permanent part of the spill- 

 way and are lowered automatically at different stages of water level, 

 as are other devices hereafter discussed. The use of flashboards 

 under any plan in which they must be removed one at a time is 

 dangerous, because of the length of time it takes to remove them, 

 since sudden floods might result in damage to the works before the 

 boards can be removed. 



SLIDING GATES. 



Sliding gates are not adapted to use as emergency escapes for flood 

 water, since they are not automatic in action, although ordinarily they 

 are provided with mechanical means for quick operation. Ordinary 

 slide gates are the slowest of all the types of gates to operate and 

 are not suitable because of the destructive effects of vibration due to 

 high velocity, and are extremely costly when placed in a structure on 



