flashboard
noun
flash·board ˈflash-ˌbȯrd 
: one or more boards projecting above the top of a dam to increase the depth of the water
Example Sentences
Recent Examples on the Web During that week, engineers would also increase the height of the flashboard structure to 8 feet from 4 feet, replacing the plywood with metal. 
John D'anna, azcentral, 18 July 2019 On July 15, the lake level peaked, just inches from the top of the new 8-foot metal flashboards, and less than 7 feet from the top of the dam. John D'anna, azcentral, 18 July 2019
John D'anna, azcentral, 18 July 2019 On July 15, the lake level peaked, just inches from the top of the new 8-foot metal flashboards, and less than 7 feet from the top of the dam. John D'anna, azcentral, 18 July 2019 Word History
First Known Use
circa 1774, in the meaning defined above