Monday December 13, 2010
One of the advantages of a resistance board weir over a rigid weir is the ability to withstand higher flows. Rigid weirs have a tendency to get blown-out when increased velocity and debris put excessive force against the weir, whereas resistance board weirs have flexibility and can shed debris over the top. This type of weir is made up of an array of resistance panels that span the majority of the river channel. Each panel consists of evenly spaced PVC conduit pickets held together by UHMW polyethylene (high density plastic) stringers. The upstream end of the panel is attached to a cable and rail system (i.e. substrate rail) that is anchored to the channel bottom, and the downstream end is lifted above the water surface by a 2 ft. x 3 ft. resistance board that planes upward in flowing water.
The resistance board weir is comprised of more than 35 unique pieces of hardware, some of which are readily available, but others are individually fabricated prior to assembly. There are 5 different types of stringers that are constructed for each panel. Three inch wide stringers are fabricated out of ½-in. thick UHMW polyethylene sheets and the edges are rounded with a router to reduce water resistance. Hole locations are marked with a template and drilled (see photo above); there are 130 holes in 10 stringers (3-21 holes per stringer) for each panel.
Not surprisingly, resistance boards are the key feature in the function of a resistance board weir (aka fish counting weir, floating weir, Alaskan weir). A resistance board is fabricated out of ½ in. pieces of marine grade plywood, which are primed and painted with marine grade paint. A polystyrene board is laminated between two sheets of the painted plywood, to improve buoyancy, and is secured with stainless steel hardware. The resistance board it is attached to the panel with hinges to allow the user to adjust the angle. This is one of the many ways that the resistance board weirs can be customized to fit each unique location and situation.
Photo source: FISHBIO