The Washington State Department of Fish and Wildlife (WDFW) published a stream-crossing guideline which enumerates nine expected outcomes of a successful stream-simulation culvert design. Hyporheic connectivity differentiates the ecological bottom culvert from traditional culvert options. Of these ten outcomes, nine are expected to be fulfilled by the ecological invert technology, as supported by the results of the testing program (available upon request). The tenth outcome, bed gradation continuity, can be achieved by manual filling of the culvert at installation with natural bed sediments.

Successful Culvert Design Outcomes

• Flood conveyance is the ability for the culvert to pass the required design flood.
• Fish passage is the ability for fish to have appropriate resting areas to allow migration through the culvert.
• Profile continuity is the preservation of the reachwise stream slope through the culvert. Maintaining slope is achieved by maintaining a natural bed through the culvert which reflects changes in the upstream and downstream reaches, and preventing outlet perching and degradation of the bed inside the culvert.
• Hydraulic diversity is the presence of zones of low, zero, or negative (upstream) flow velocities within the culvert along with regions of higher velocities. These zones provide resting habitat for aquatic organisms inside the culvert.
• Sediment transport continuity means that bed material transported through the upstream reach is continuously supplied to the culvert bed and supplied by the culvert to the downstream reach at an equivalent rate, allowing the culvert to reflect the bed structure of the stream reach.
• Low flow continuity is the maintenance of a channel for fish passage during low-flow events.
• Margin habitat is the existence of areas of low velocity and turbulence along and near the banks of a stream where fish, especially juveniles, can use for migration. In the ecological bottom culvert, it is a consequence of developing a low-flow channel in the culvert.
• Debris transport is the ability for the culvert to pass large woody debris or miscellaneous detritus.
• The hyporheic zone is the region beneath and alongside the streambed where there is interaction between the groundwater and the surface water. The hyporheic zone provides ideal habitat for microbes and invertebrates which are critical to the overall health of the stream.
• Bed gradation continuity is the maintenance of a bed material grain size distribution through the culvert which is equivalent to the grain size distribution of bed material upstream and downstream of the culvert. This criteria can be addressed by manual filling of the culvert at installation with natural bed sediments.