Stream Simulation

An alternative to hydraulic design is to size and install culverts to avoid constricting the stream or river channel. Where they can be installed with the same slope as the natural streambed, non-constricting culverts will normally provide water depths, velocities, bottom substrates, and channel characteristics that are comparable to the natural stream. Well designed culverts can maintain the continuity of stream bottom and hydraulic conditions, thereby facilitating passage for aquatic organisms utilizing the stream.

Stream Simulation is an approach to culvert design that was developed in Washington State and is now being adapted for use elsewhere. It is a design process that both avoids flow constriction during normal conditions and creates a stream channel within culverts that resists scouring during flood events. Stream Simulation culverts are wider than the natural channel in order to simulate channel forming processes and the entire channel including margins and can be installed at the same slope or at slightly steeper slopes than the natural stream. Construction of a channel within the culvert insures adequate water depth during low-flow conditions. Particular attention is paid to construction of the streambed within the culvert, using bed material that interacts with the stream as a natural bed.

Designing culverts to avoid channel constriction and maintain appropriate channel conditions within the structure, is an effective approach for accommodating the normal movements of aquatic organisms and preserving (or restoring) ecosystem processes that maintain habitats and aquatic animal populations. Where passage for riparian and terrestrial wildlife is desired, stream simulation structures should be adapted to meet minimum height and openness requirements.