Forest Roads and Engineering
What this domain tests
Road location, geometric design, drainage, stream crossings, and the erosion control that keeps a road usable and a watershed intact.
Core rules
- Grade: main haul roads commonly top out at 8 to 10 percent, with short pitches to 12 percent; steeper grades demand more surfacing, drainage, and maintenance.
- Drainage drives design. Outslope or crown the running surface, use ditches and cross drains, and space water bars and relief culverts so water leaves the road before it gains volume and velocity.
- Culverts are sized for a design storm, not average flow; inlet and outlet armor prevents scour, and the pipe must pass debris and fish where required.
- Stream crossings should be perpendicular, minimized in number, and built in low-flow windows. A poorly sited ford can undo an entire BMP plan.
- Curve radius, stopping sight distance, and superelevation follow design speed; a tight curve on a steep grade is a logging-truck accident waiting to happen.
Worked example
Rational method: Q = C times i times A. For a 5-acre road drainage area, runoff coefficient 0.9, and 2 inches per hour intensity, Q = 0.9 times 2 times 5 = 9 cubic feet per second. The culvert must pass 9 cfs at the design headwater depth, so select pipe size from an inlet-control chart and then check outlet velocity for scour.
Traps
Sizing culverts for average rather than peak flow; omitting outlet armor; steep grades with no cross drains; locating roads on unstable fill; assuming a ford is acceptable because it is small.