- Design all stream crossings to handle 50-year peak flows. For culverts up to 6 feet in diameter, use culvert sizing tables I and II in FPA rule 040.02.i., and vi and vii, FPA.
- FPA requires permanent culverts to be 18 inches or larger, except in the upper Snake River Basin, where a 15-inch minimum is allowed.
- Culverts larger than 6 feet in diameter must be designed by a person trained in stream hydrology. Consider alternative structures, such as bridges or ford.
- Relief culverts and those used for seeps, springs, wet areas, and draws must not be less than 12 inches in diameter for permanent use.
- All culverts planned for Class I streams must allow for fish passage.
Railroad flatcars can provide a low cost alternative to conventional bridge construction.
When short-term access to forest land is cut off by a stream, portable bridges are one solution. They offer the flexibility of convenience and low cost. A timber harvest or other forest activity can be carried out over a short period of time and the crossing easily restored to its original condition. This railroad car portable bridge provided access to an eight-acre sale.
An appropriate crossing, approximately 10 feet wide, with firm soil banks, level grade, and requiring minimal vegetation clearing, was selected. The 20-foot-long bridge was hauled into place with a flatbed truck, stretched across the stream, and set into place in one day. Cribbing for the bridge consisted of 10-foot-long timbers laid on the ground approximately 4 feet away from the bank. A small crawler tractor finished the installation by
building the road approaches to the bridge. This durable bridge crossing was used over a 3-week period.
Approximately 25 80,000-pound log truckloads were hauled across it. When the harvest was completed, all logging and skid trail roads were restored and the temporary bridge stabilized. Regardless of whether temporary or permanent, any stream crossing requires a stream channel alteration permit.
Never allow ditch culverts to drain into stream. Culvert drainage should be directed through a vegetation filter before reaching the stream.
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