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<br />CHAPTER 6. SPILLWAY OPERATION <br /> <br />Section 6.01. Considerations for Spillway Operation <br /> <br />The primary purpose of a spillway is to prevent overtopping of <br />the dam by flood flows in excess of those which the project is designed <br />to regulate. up to the spillway design flood. There are other purposes <br /> <br />for which a spillway may be used. hovlever. For example. gates may be <br />added to an existing spillway to permit storage of water above the spill- <br />way crest level during periods when it is safe to do so. It is sometimes <br />desirable to close gates on the outlet works to take advantage of the <br />limited capacity of the spillway under low heads and thus prevent down- <br />stream damage. This is only feasible. of course. if the flood does not <br />greatly exceed project design magnitudes. Whenever the spillway is used <br /> <br />for such secondary purposes. however. every care must be exercised to <br /> <br />assure that the gates can and would be operated so as to make the full <br />capaci.ty of the spillway and outlets available when needed for protection <br />of the structure. <br /> <br />The size and characteristics of a spillway are based on economic <br />and operation studies of a spillway design flood. In the case of pro- <br />jects where exceeding the spillway capacity would result in a major <br />disaster. it is important to provide a large enough spillway to pass the <br />probable maximum flood (described in Volume 5) without major structural <br />failure. In other cases. a smaller spillway design flood might be satis- <br />factory. The final design of the spillway should be such that it will <br /> <br />l <br />l <br /> <br />6-01 <br />