17.6.1. Flashboards and Stoplogs Flashboards and stoplogs raise the reservoir storage level above a fixed spillway crest level when the spillway is not required to release flood. The flashboards usually consist of individual boards or panels of 1.0 to 1.25 m height. These are hinged at the bottom and are supported against water pressure by struts. Stoplogs are individual beams or girders set one upon the other to form a bulkhead supported in grooves at each end of the span. To increase the spillway capacity, the flashboards or stoplogs are removed prior to the flood. Alternatively, they are designed and arranged so that they can be removed while being overtopped. Flashboards and stoplogs are simple and economical type of regulating devices which provide an unobstructed crest when removed. However, they have the following disadvantages (1):
(i) They present a hazard if not removed in time to pass floods, especially where the reservoir area is small and the stream is subject to flash floods,
(ii) They require attendance of an operator or crew to remove them, unless designed to fall automatically,
(iii) Ordinarily they cannot be restored to position while water flows over the crest,
(iv) If they are designed to fail when the water reaches certain stage, their operation is uncertain, and when they fail they release sudden and undesirably large outflows, and
(v) If the spillway functions frequently, the repeated replacement of flashboards may be costly.
