A low cost, flood‐resistant weir to monitor fish migration in small‐ and medium‐sized rivers

A reliable method to count migrating fish at different sites was required within a Danube-tributary system in Austria to assess the efficiency of connectivity measures (Zitek, Schmutz & Jungwirth 2008). Fish needed to be monitored over a maximum time frame of about 4 months per site during their spring/summer spawning migrations, when flood events are expected in Austrian rivers. Due to a high number of different monitoring sites distributed over the whole project area, and expected debris and wood-loaded high floods, a low cost and flood-resistant highly transportable, modular weir for counting migrating fish was developed (Mühlbauer, Traxler, Zitek & Schmutz 2003). The construction was based on the concept of the resistance board weir (Tobin 1994; Stewart 2002), which is an alternative to other weir types, e.g. portable rigid weirs, and has been typically used in Alaskan rivers (Holmes 1992). Portable rigid weirs are usually made of wood or metal, but are vulnerable to washout and downstream losses during high water periods (Anderson & McDonald 1978; Clay 1995). In contrast, resistance board weirs temporarily submerge when pressure created by water volume and debris loading reaches a point that might wash a rigid weir downstream, so demolition and washout are generally avoided. Unfortunately, existing resistance board weir constructions are complex and expensive (Tobin 1994; Stewart 2002). This paper describes a simple design and construction that overcomes these limitations. The system developed consists of panels and traps built in a modular design providing flexibility. The weir consists of a linked array of picket-fence-like panels made of PVC tubes closed at both ends by commercially available end caps. The upstream end of each panel is fixed to concrete lattice girders at the river bottom by shackles mounted on the lowest transverse PVC tube by synthetic ropes; the downstream end is lifted above the water surface by Styrofoam floats (Fig. 1a, b). Lattice girders are anchored in the substratum across the whole river cross section using iron pins (Ø 2 cm) at approximately 1-m spacings. Two sizes of weir panels were constructed, one with 1-cm and one with 2-cm spacing, 6 and 3 m long, respectively. The weir panels are constructed from rectangular PVC tubes with regularly spaced drill holes through which tubes are pushed and glued to form a picket-type fence. Each panel consists of 29 pickets (Ø 2.5 cm, 2-cm spacing) or 43 pickets (Ø 2 cm, 1-cm spacing) and has a width of about 130 cm. Transverse PVC pipes/tubes have diameters of 40 and 50 mm. Styrofoam floats (120 · 20 · 10 cm) are fixed at the