surface, with an approximate Hazen-Williams Friction Factor, C, value of between 140–150, the decreased flow area can be offset and hydraulic flow rates potentially increased by way of reduced surface flow resistance. Examples of pipe materials with high C-factors are high-density polyethylene (150) and polyvinyl chloride (140). Environmental impacts Safe and stable stream crossings can accommodate wildlife and protect stream health, while reducing expensive erosion and structural damage. Undersized and poorly placed culverts can cause problems for water quality and aquatic organisms. Poorly designed culverts can degrade water quality via scour and erosion, as well as restrict the movement of aquatic organisms between upstream and downstream habitat. Fish are a common victim in the loss of habitat due to poorly designed crossing structures. Culverts that offer adequate aquatic organism passage reduce impediments to movement of fish, wildlife, and other aquatic life that require instream passage. Poorly designed culverts are also more apt to become jammed with sediment and debris during medium to large scale rain events. If the culvert cannot pass the water volume in the stream, then the water may overflow the road embankment. This may cause significant erosion, ultimately washing out the culvert. The embankment material that is washed away can clog other structures downstream, causing them to fail as well. It can also damage crops and property. A properly sized structure and hard bank armoring can help to alleviate this pressure. Culvert style replacement is a widespread practice in stream restoration. Long-term benefits of this practice include reduced risk of catastrophic failure and improved fish passage. If best management practices are followed, short-term impacts on the aquatic biology are minimal. Fish passage While the culvert discharge capacity derives from hydrological and hydraulic engineering considerations, this results often in large velocities in the barrel, creating a possible fish passage barrier. Critical culvert parameters in terms of fish passage are the dimensions of the barrel, particularly its length, cross-sectional shape, and invert slope. The behavioural response by fish species to culvert dimensions, light conditions, and flow turbulence may play a role in their swimming ability and culvert passage rate. There is no simple technical means to ascertain the turbulence characteristics most relevant to fish passage in culverts, but it is understood that the flow turbulence plays a key role in fish behaviour. The interactions between swimming fish and vortical structures involve a broad range of relevant length and time