This approach can also ameliorate any perching problem. The shape and orientation of baffles depends on the culvert barrel shape and its gradient. The recommended solutions, based on recent research are shown in Figures 7.2 and 7.3. This involves installation of a pre-fabricated steel frame rather like a ladder which lies along the bed of the culvert. The rocks thus provide the turbulence and depth to allow fish to migrate.
Bridges generally represent much more benign conditions for fish passage than culverts.
The main potential for problems is with bridge aprons or sills, installed either when the bridge was constructed or retro-fitted to correct erosion that potentially threatens the bridge structure. These can be perched in the same manner as culverts, making it difficult or impossible for fish to ascend at certain flows. Flat bridge aprons can also present problems for fish passage at low flows as the available water is spread very thinly over the width of the channel. The most straightforward solution to perching at bridge aprons is the installation of low stone weirs as described in Sections 7.8 and 7.9. This approach may also be appropriate to overcome inadequate water depth problems on the apron itself.