Design and construction of aquatic organism passage at road-stream crossings: site assessment and geomorphic considerations in stream simulation culvert design
Jackson (2003, current volume) describes the types of damage to aquatic populations and metapopulations caused by barriers to aquatic species movement along stream corridors. Road-stream crossing culverts designed in the traditional way---sized for some rare flood flow---also have predictable detrimental effects on stream channels themselves. These occur not only during floods, when culverts may plug or be overtopped, but also over time if the culvert impedes downstream movement of woody debris and sediment. This paper describes common stream responses to culverts, such as chronic aggradation and degradation; long-term changes in stream stability due to interruption of woody debris transport; and sedimentation sustained when culverts plug and fail, etc. It also describes the range of approaches to crossing design, from a culvert sized only to pass a certain flood to valley-spanning bridges and viaducts. Stream simulation is placed in the context of other design approaches that provide more or less biological and geomorphic connectivity. Biological and geomorphic priorities and risks must be weighed against site constraints and costs to select the appropriate level of continuity for each site. Site assessment procedures for stream simulation design are then described. These include surveying and describing the longitudinal profile and valley cross-sections, bed material assessment, and reference reach selection. Channel stability interpretations needed for design are also discussed.