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1 <br />1 <br />1.3.5 Low Velocity Screens - Traveling <br />1 1.3.5.1 Rotating Drum <br />Characteristics. This screen design is very popular and in common use on diversions up to 3,000 <br />cfs, particularly in the Pacific Northwest. The screen consists of a horizontal drum with a screen <br />mounted around its circumference that rotates within a metal frame (Figure 1-4). The frame is <br />lowered into slots constructed in support piers at each end. Larger installations are usually equipped <br />with an electric motor mounted on the frame to rotate the drum. Power to rotate the drum has been <br />successfully produced from a paddle wheel located immediately downstream for installations where <br />electric power is unavailable or unreliable. Smaller screens can be driven by internal turbines or <br />paddle wheels that rely on the differential head between the upstream and downstream water surface <br />and/or the flow velocity of the canal. <br />Drum screens work well at sites with high debris loads and small water surface fluctuations. The <br />screen must operate continuously for debris cleaning. Debris is either carried over the screen and <br />passed downstream or, for installations where the screen in oriented at an angle to flow, is diverted <br />into the fish bypass where handling is required. Some applications require mechanical brushing or <br />water jetting to clean algae from the screen surface. <br /> Drum sizes range from 18 inches in diameter by 3 feet long, up to 19 feet in diameter by 12 feet <br /> long, and 15 feet in diameter by 24 feet long. Seals require good design, installation and ongoing <br />maintenance to assure reliability. Neoprene seals are most common and may last 1 to 5 years <br /> depending on sediment loading. They are required between the drum and frame, and between the <br /> frame and the concrete support piers and floor. Large screens are difficult to manufacture and meet <br />opening tolerances on the perimeter. Maintenance on large screens can be a significant dollar and <br /> personnel commitment. <br /> Typical Existing Applications. Horizontal drum screens are mainly applicable to canal intakes, <br /> rather than deep water inlets, due to the limited depth of water screened. They have proven to be <br /> reliable in California and the Pacific Northwest. Drum screens have been proven to demonstrate <br /> nearly 100 percent overall efficiency and survival from comprehensive testing conducted at various <br /> large installations. Recent installations in the Yakima River basin in Washington and the Umatilla <br /> River basin in Oregon have been biologically evaluated and shown to pass juvenile salmonids with 0 <br /> to 2 percent of the bypassed fish killed or descaled. <br /> Typical Advantages <br /> • High fish exclusion efficiency for target species and life stages when facility is in optimal <br /> condition <br /> • Fish friendly <br /> Typical Disadvantages <br /> • Out of roundness or structural sag <br /> • Large drum screens require considerable expenditure of time and money for maintenance <br /> • May require costly and problematic maintenance of seals <br /> Control Structure Feasibility Evaluation 1-12 <br /> Miller Ecological Consultants, Inc., February 18, 1997