Forest Road Engineering Guidebook - Ministry of Forests

In planning the layout of the structure:
• Choose an appropriate location, along a stream reach with uniform or
uniformly varying flow close to the proposed crossing, to measure a
cross-section. Sketch the cross-section of the stream gully, showing evidence
of the high water level, present water level, and the depth of the
stream across the bottom. The cross-section should extend back from the
stream an appropriate distance to show the terrain that affects the proposed
crossing and road alignment.
• Note any visual evidence of high water.
• Measure and record the average gradient of the stream at the crossing and
at the cross-section if the two are taken at different locations.
• Record the soil type, soil profile, parent material, and substrate material
at the crossing and describe the stream bottom.
• Describe the stream channel (debris loading, bank stability, crossing location
on a fan, bedload problem, etc.).
If the site is a fish stream or a potential fish stream, the Fish-stream Crossing
Guidebook should be consulted for site and design requirements.
Estimating design discharge for streams
The following guidelines apply to determining the design discharge for
streams for a particular recurrence interval. Establishing a return period provides
a benchmark of the relative risk to be attached to any particular design.
These guidelines should not preclude use of other reasonable and accepted
methods for determining the design discharge. Professional engineers, who in
the course of carrying out their professional functions as designers of a
bridge or a major culvert (2000 mm or greater in diameter or with discharges
of 6 m 3 /sec or greater), are ultimately responsible for establishing the design
discharge for that structure. Others determining stream discharges should be
familiar with methods and their limitations, or consult those with training and
experience in stream discharge determination.
Factors affecting runoff
Forest Road Engineering Guidebook
The runoff and behaviour of a stream depends on many factors, most of
which are not readily available or calculable, such as:
• rainfall (cloudbursts; hourly and daily maxima)
• snowpack depth and distribution, and snowmelt
• contributory watershed area, shape, and slope
• topography and aspect
• ground cover
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