Natural fish passage structures in urban streams (Part 1: Hydrologic and resource issues)
Fish passage is now an integral part of compensatory mitigation under the new Nationwide Permit regulations. Engineered structures and stream restoration designs are common solutions to fish passage; however, in urban systems such solutions may not be feasible. Natural structures such as riffle grade controls and flow constrictor/step pools can provide low-maintenance stable solutions to fish passage in urban systems. Steps to designing such structures include an evaluation of the target fish species characteristics, site-specific baseflow hydrology, and hydraulics of the structure. Analyzing baseflow is essential because urban flood flows exhibit relatively high velocities and short durations precluding upstream migration. Fish characteristics, hydrology, and hydraulics are all used to generate fish passage design criteria.