Laserfiche WebLink
<br />To the uninitiated. a dam might seem little more sophisticated than a plug stuffed into a <br />pipe. a few shovelfuls of dirt thrown across a ditch. But a deeper look brings an entire <br />world of technological expertise into view. Engineers must identify appropriate sites for a <br />dam. locate materials for its construction. conceive the basic shape of the dam. decide <br />whether or not to build an associated hydroelectric power plant and calculate the neces. <br />sary size of emergency spillways. The design of a dam is directly tied to its fundamental <br />purpose. Run.of.the.river designs are usually low in elevation. have small upstream <br />reservoirs. and modify the natural flood and sediment.transport cycle only slightly. Alter. <br />natively. high dams with large upstream reservoirs can store many months. if not years. of <br />natural streamflow and can generate prodigious amounts of hydroelectric energy due to <br />the fall of the river at the dam. <br />Dams and reservoirs <br />differ not only in their <br />sizes. but also in opera. <br />tional strategies. Dams <br />ofthe same size may <br />hold varying amounts of <br />water depending on <br />their ultimate purpose. <br />A flood control dam <br />keeps its reservoir low at <br />the onset of each flood <br />season, while a water. <br />supply reservoir tries to <br />remain full as long as <br />possible. Because the <br />actual inflow into a <br />reservoir can never be <br />precisely anticipated, <br />each of these operating <br />strategies carries <br />inherent risks that the <br />reservoir will overflow or <br />that the reservoir will go <br />dry. The task of the <br />water.resource engineer is to develop techniques that will increase the chances of achiev. <br />ing the desired objectives of the dam, while accepting some risk that other situations. less <br />relevant to the dam's ultimate purpose, will occur. In other words. a flood control dam has <br />some chance of going dry and a water.supply dam has some chance of spilling. <br />But we can't have it all. all of the time. The challenge for today's manager is that yet <br />other factors now exist in the dam.management equation - those centered on manage. <br />ment of the downstream river corridor and its ecology. Scientifically-based management <br />ofregulated rivers adds a new layer of objectives to the already complex task ofthe water. <br />resource engineer who designs the strategies of multi. purpose reservoir management <br /> <br />7 <br /> <br /> <br />6 <br /> <br />5 <br /> <br />~ <br />o <br />'0 <br />1: <br />E <br />" <br />z <br /> <br />4 <br /> <br />3 <br /> <br />2 <br /> <br />o <br /> <br />1910 t920 1930 t940 1950 1960 1970 t980 <br />Year <br /> <br />Dates of closure for dams in the continental United States with reservoir <br />capacity of a million acre-feet or more (comPiled by Gmf, 1993). <br /> <br />t <br /> <br />4 <br />