FLASHBOARDS 91 



and nearer for others, with the necessary cross stiffening girders. 

 The upstream face shall be at an angle of not over 45 with the 

 horizontal and the downstream face not over 60. No part of 

 the dam shall be less than 12 inches thick. 



" If the dam is on rock foundations, the front face must have a 

 heavy cut-off wall built into the rock. If on gravel and clay foun- 

 dations both faces must have deep cut-off walls and a heavy rein- 

 forced flooring with weep holes to relieve the water pressure under 

 the flooring. Drainage must be provided in interior pockets for 

 seepage waters, and, if practical, the interior must be made acces- 

 sible to allow for inspection. 



!< The crest of the spillway, and for 3 feet below, must be thick- 

 ened and heavily reinforced, and the entire dam and bulkheads 

 protected from ice and floating bodies the same as masonry dams. 

 The dam must be well anchored to the bulkheads." 



2. FLASHBOARDS 



The maintaining of a constant water level above the dam is 

 naturally very desirable. This water surface fluctuates con- 

 siderably during the different seasons of the year, depending on 

 the flow, and it was previously shown that the spillway must be of 

 ample capacity to discharge the flood waters and prevent the 

 water above the dam from flooding such land as has not been 

 included in the flowage area. It is furthermore desirable to keep 

 the surface at approximately the same level during the low- 

 water periods and thus maintain a constant head. This is accom- 

 plished by providing flashboards, which are placed on the top of 

 the dam, and arranged to be raised or lowered with the variation 

 in the water level. It has also been found that for installations 

 with steam reserve plants the operating arrangement that will 

 insure the most efficient use of the river flow is to maintain the 

 level in the storage reservoir at nearly the crest of the flashboards, 

 carrying by the auxiliary plant any excess load until such time as 

 reports from the watershed above indicate a freshet. Then the 

 stream plant is shut down and the water drawn down in the reser- 

 voir to such an extent as to allow it to be filled by the anticipated 

 freshet. 



There are numerous designs of such flashboards, the most 

 common being as follows: 



1. Stationary flashboards. 



