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<br />1 <br />i <br />1 <br />t <br />1.3.3 Existing Control Types <br />1.3.3.1 General <br />Downstream passage facilities are generally described by two groups. Common groups are physical <br />(primarily oriented to fish dimensions or mass, usually cross-sectional size); and behavioral <br />(primarily oriented around how fish act in response to stimuli, although some physical element may <br />also be involved); and other (management, etc.). Physical elements usually consist of trash racks <br />which are followed by fish screens which are further subdivided by screen velocity into categories of <br />low velocities (<0.5 ft/sec), and high velocities (>0.5 ft/sec) which are normally associated with high <br />rates of impingement (i.e., fish are killed or injured) or an integral fish bypass. Flow and velocity <br />directly controls screen size and cost. Since discharge is calculated by area multiplied by velocity, a <br />large screen area is required to pass high discharges at low approach velocities. This results in <br />higher screen costs. Screens are a common primary element of exclusion. They come in a variety of <br />materials and configurations. Generally, screens are metal and consist of a bar rack, holes in metal <br />plate, woven mesh or wedge-wire. A woven net is yet an additional screen type; distinctive by its <br />material and its means of field mounting. All screens are designed with a 40-60± open area <br />percentage. Features of these categories of physical screens are summarized in Table IA and are <br />described in more detail in immediately following sections of this narrative. <br />The screen/net cost varies considerably by its area and materials of construction. Both variables are' <br />highly correlated with the approach and passage flow velocity and related head loss. There are a full <br />range of screen/net sizes, opening shapes, materials, % open area, configurations or combinations <br />which are potentially applicable. As such, it is not possible to generalize head loss/velocity <br />relationships in a meaningful way without detailed engineering evaluation and possibly model <br />testing of the specific physical situation for which the screening is intended. Some head <br />loss/velocity relationships for standard manufactured units can be made available separately. <br />1.3.3.2 Low Velocity <br />The vast majority of screens are low velocity screens; low velocity meaning less than 0.5 ft/sec and <br />most commonly 0.1 to 0.3 ft/sec. These low velocities are designed to prevent impingement of fish <br />on screens or injury by descaling, starvation, excessive stress, etc. The primary focus of most such <br />screens is the safe exclusion of small anadromous fish about the age of 1 year and approximately 3 <br />inches long. As such, common screen open spaces vary from 3/32- to 1/2-inch (2.38-12.7 mm). <br />Recently, more installations with 3/32 inch openings to pass young of the year salmonids have been <br />constructed. Low velocity screens require specific attention to be directed at both upstream and <br />downstream sediment deposition. <br />1.3.3.3 High Velocity <br />All other screens with velocities greater than 0.5 ft/sec up to a practical upper limit of about 10 ft/sec <br /> are high velocity screens. These high velocities originate from the criteria associated with required <br /> screen strength when clogged with debris and related operations and maintenance considerations. <br /> Either increasing mortality is expected and accepted with increasing velocities associated with high <br /> velocity screens or a fish bypass system is incorporated to carry fish past or away from the screen. <br /> High velocity screens can be much smaller in size and lower in capital cost than low velocity <br /> screens. As flow velocities exceed 3 ft/sec, associated components which become necessary <br /> Control Structure Feasibility Evaluation 1-3 <br /> Miller Ecological Consultants, Inc., February 18, 1997 <br />