Chapter IV. Positive Barrier Screens

Chapter IV. Positive Barrier Screens “The significant problems we face cannot be solved at the same level of thinking we were at when we created them.” Albert Einstein This chapter presents an overview of positive barrier screens with detailed planning and design criteria. Positive barrier screens compose a wide range of fish screen concepts that include: < Flat Plate Screens < Drum Screens < Traveling Screens < Submerged Screens − Cylindrical Screens − Inclined Screens − Horizontal Flat Plate Screens < Coanda Screens < Closed Conduit Eicher and MIS Screens Although these screens vary widely in concept and configuration, they have many common characteristics. In all cases, the screen systems generate a “positive barrier” to passage of fish of the selected design size and larger. This requires that openings in screen fabric at seals and between structural members be small enough to prevent passage of the selected fish. The screens are typically designed to effectively screen both debris and fish from the diverted flow and to quickly and safely guide fish back to the natural water body from which they were drawn. In all cases, cleaning and maintenance requirements are important considerations because debris fouling of the screens will reduce both the screens ability to safely exclude fish and reduce the flow capacities of the screens. The following chapter explores initial design requirements and issues that are common to all positive barrier screen concepts. In cases where requirements are generally common but allow exceptions, discussion of the exceptions follows the generalized presentation. This is followed by detailed discussions of unique design requirements and issues associated with each specific screen concept, chapter IV.B. IV-1 Fish Protection at Water Diversions A. Facility Design A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering Freeman Dyson, 1923-, British-born American Physicist, Author 1. Site Selection This chapter presents more detailed discussions of siting considerations with examples of existing fish exclusion structures. The general preference of fishery resource agencies is to maintain fish in the natural water body and not draw them into the diversion. Keeping the fish in the natural water body reduces fish guidance and fish concentrations and eliminates the need for bypasses. As a consequence, in-river and in-diversion pool screens may be preferred over in- canal or closed-conduit fish screens. However, issues such as shallow depths, high river gradients, heavy sedimentation, potential for damage by large debris and ice, and construction difficulties (cofferdams, site dewatering, and construction windows) often force placement of exclusion screens in the diversion canal. The overall hydraulic features of the location, including flow patterns, velocity magnitudes, and fish guidance at and past the screen and bypass, are of paramount importance in the design. These features of the site and design are critical to ensuring effective fish and debris movement and to reducing predation. Objectives typically are to sustain uniformly directed, eddy-free flows that efficiently guide fish past the screen and that do not provide locations for predator and debris accumulation. Placement of the structure in the flow field and configuration of transition structures will strongly influence generated flow patterns. For larger structures or unique designs, fishery resource agencies may require documentation of flow fields and will likely require computational or physical modeling. Site selection considerations will need to address: < Hydraulic requirements < Minimization of predation from all fish, two and four legged animals, and birds < Operation and maintenance costs IV-2 Chapter IV. Positive Barrier Screens < Injury to fish < The need to keep fish in the river or return fish to the river as soon as possible a. In-canal siting The water enters the canal through a headworks. Stream gradients are usually steeper than the diversion canal which tracks away from the diversion with a gentle invert gradient. There is usually sufficient drop to generate gravity flow through the fish bypass conduit. Water levels in the canal are often maintained fairly constant by “checking up” the canal with gate structures along the canal length. If the fish screens are located in the canal, the following considerations must be included in the design: < Fish screens should be located as close to the upstream end of the canal as possible, based on canal hydraulics and site constraints. This placement allows fish to return to the river as soon as possible and reduces potential predation. < Sediment deposition must be addressed. (See chapter IV.A.14 for sediment considerations.) < The fish exclusion facility should be well aligned with the canal and preferably located in a straight reach of canal where uniform flow velocity distributions are provided and good sweeping flow can be achieved (figure 4). < Bypass hydraulics, available head (between the canal and bypass outfall location in the river), and the river location for the fish bypass outfall will need to be evaluated. < Sufficient flow depth must be maintained at the fish screens to ensure that adequate active screen area is provided and that the maximum screen approach velocity is not exceeded. < Scheduling for construction of the fish exclusion structure and bypass will need to be carefully considered, especially if water deliveries will need to be continued during construction. < Debris should be captured at the canal headworks. IV-3 Fish Protection at Water Diversions An example of an in-canal positive barrier fish exclusion facility: The following example presents an in-canal fish exclusion facility concept and includes plan and section drawings and photographs with brief descriptions. The Chandler Canal Fish Facility (Prosser Diversion Dam), Yakima River, Washington, is an example of a moderate to large capacity facility where the fish exclusion screen (drum screen facility) is located well downstream from the headworks at a site that provides both suitable space for facility installation and a well aligned straight canal reach. Immediately downstream from the headworks, the canal passes through a highly developed area (homes and roads). Space for the fish exclusion facility was limited and the canal alignment within the upstream reach included numerous bends. The fish screen structure was located in a straight reach of canal 4,300 ft downstream from the headworks. The site had sufficient space for the screens and for auxiliary fish evaluation and holding facilities. An approximately 500-ft-long straight canal reach leading to the screens was available, establishing a uniform channel approach flow distribution to the fish screens. (A hydraulic model study was used to develop and refine hydraulic features of the screen design, including approach and exit channel configurations). At the screen location, the checked water level in the canal is approximately 10 to 15 ft above typical river water surface elevations. Figures 26 and 27 show plan and section drawings of the constructed fish screen. The maximum diversion discharge capacity at the site is 1,500 cubic feet per second (ft3/s). Figure 26.—Elevation view of Chandler Canal Fish Screen, Washington. IV-4 Chapter IV. Positive Barrier Screens Figure 27.—Plan view of Chandler Canal Fish Screen Structure. Flow is diverted into the existing canal by a diversion dam. Sediment deposition has occurred within the diversion pool to the point that significant quantities of sediment are diverted into the canal. Sediment sluicing capabilities were not included in the original design for the diversion dam. As a result, sediment IV-5 Fish Protection at Water Diversions accumulates in the low velocity reaches of the canal. The existing canal headworks include submerged vertical slide gates that provide flow control and exclude most floating debris. Trashracks were not included as part of the existing headworks. The screen facility includes a trashrack and a fish screen structure where the drum screens are angled to the channel flow in such a way that fish are guided along the screen to intermediate and terminal fish bypasses that lead to a secondary screen/dewatering facility on the combined bypass. From this secondary facility, a portion of the bypass flow is pumped back to the canal, and the remaining flow and diverted fish pass through a buried bypass conduit to a juvenile fish evaluation facility and then back to the river at the bypass outfall (figure 27). Other Examples of In-Canal Positive Barrier Screens include drum screens at Kittitas and Three Mile Falls (left bank), and flat-plate screens at Naches-Selah, Yakima-Tietan, Bachelor Hatton, Snipes Allen, Cascade, and New Cascade. b. In-river siting From a fishery perspective, it is best to locate the fish screen in the river before the flow enters the canal or pumping plant. However, the in-river fish exclusion facility may be exposed to large variations in flow depth, flow velocity, bed sediment transport, debris load, and ice flows that occur because of seasonal and storm events. The facility may be placed in the river channel or at the bank. Since fish remain in the river, a bypass structure is normally not required. The exception is for very long flat plate screens such as at Glenn-Colusa Irrigation District (GCID) (which was placed in a secondary oxbow channel of the main river) in figures 5 and 6, where, because of the potentially long fish exposure time, intermediate bypasses were provided. If the fish screens are located in the river, the following considerations must be included in the design: < The screen structure should be positioned and oriented with careful consideration of the in-river velocity field for a range of river stage and diversion conditions. This positioning will require evaluation of river flow patterns that will occur at the site at various river stages. The facility must then be oriented to yield a sweeping flow capable of moving fish and debris along and past the facility for all flow conditions.

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