1. FISH PASSAGE THROUGH CULVERTS

1.1 Problems posed by migration of fish through culverts In terms of both lower construction and maintenance costs, culverts of many types (circular, elliptical, ovoid, boxed, pipe, arch and bottomless-arch) are an attractive alternative to traditional bridges for the provision of road and motorway crossings of watercourses. Substituting such structures for the natural river channel modifies the local flow characteristics, substrate composition and lighting conditions. They may constitute serious obstructions to fish migration with excessive water velocity (up to 3-4 m/s), insufficient water depth, elevated outlets (i.e. the downstream invert becoming perched above the existing stream-bed), and accumulation of debris being the most frequent causes of impediment for migratory fish. Where culvert gradients are significant, the acceleration of flow at the inlet of the structure can be intense. As a result of the low rugosity values typical of culverts the water velocity accelerates rapidly, often significantly (the flow may even become torrential), and then remains almost constant throughout the structure. This uniformity of the velocit