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1.3.6.3 Barrier Nets Characteristics. A barrier net is composed of rope or synthetic twine that is woven into a mesh. The smallest mesh opening is 0.25 inches. The net must be installed far enough away from the intake to insure that velocities through the mesh are maintained well below 0.4 ft/sec. It is important that the percent open area be above 50 percent of the total area of the screen. This will improve the velocity and head loss through the screen significantly. The mesh size and material must be adapted to the fish species to be excluded, and the conditions of the lake or river such as temperature and current. Barrier nets can be mounted on existing trash racks or log booms at a water intake or can ' have their own structure. They usually must have floatation buoys at the top and heavy chains and anchors at the bottom (Figure 1-8). Fish may pass underneath the net since obtaining a positive seal between the lake bottom and net is difficult. Some applications have reduced fish passage by adding ' skirting to the bottom and/or top of the net. High wind, wave action, and submergence are a typical problem with barrier nets. In climates where ice develops the net must be removed or dropped to the bottom during the winter. Typical Existing Applications. Barrier nets have been successful in excluding adult Chinook and Alewives from pump intakes at the Ludington Pumped Storage Plant located on Lake Michigan. They have been applied to hydroelectric intakes and on the approach to diversion canals in Wisconsin. They have also been used to exclude other creatures (turtles) from drowning at intake structures. Typical Advantages i • Flexible to a variety of physical configurations • Inexpensive to purchase and install • No mechanical or electrical parts to var • Flexibilit in water surface level y g y Typical Disadvantages ' • Difficult to maintain seal with lake bottom • Cannot be used during winter due to icing ' • High winds can damage the net • Submergence can be a problem • Required mechanical brushing and special coatings to prevent biofouling • Smallest opening mesh typically available is 1/4-inch Application to Limiting Downstream Passage. Flexibility and cost for the benefit gained are the primary attractive characteristics applied to downstream passage. It is the only barrier which is effective for depths greater than 20 feet. Nets are nicely applied to spillways as they can be out of service in the winter or whenever water is not going through a service spillway. Control Structure Feasibility Evaluation 1-21 Miller Ecological Consultants, Inc., February 18, 1997