Forest Road Engineering Guidebook - Ministry of Forests

Forest Road Engineering Guidebook
Cross-drain culvert location
104
How far water should be carried in a ditch before being dissipated away from
the road prism depends on: water volume and velocity, soil types, hillslope
aspect, elevation, vegetation, rainfall intensity, the incidence of rain-on-snow
events, and downslope conditions. With so many factors influencing placement
of cross-drain culverts, it is not recommended that spacing tables be
used unless the designer has experience and augments the tables with consideration
of site-specific conditions.
Typical locations for cross-drain culvert placement are:
• at the top of a steep gradient—the intent is to prevent accelerated ditch,
subgrade, or cut bank erosion by dispersing ditchwater before its volume
and velocity increase downgrade
• at seepage zones
• at zones that have localized overland flow with no defined channels—it
is critical to ensure that ditchwater is dissipated at the downgrade side of
these zones—otherwise water flow will carry on to the next segment of
the ditch, thereby increasing the flow at the start of the next section of
ditchline and increasing the potential for erosion and natural drainage
pattern disruption
• at any location where accelerated ditch erosion could potentially begin—
again ditchwater volume and velocity should be dissipated to prevent
build-up and the risk of adverse impacts on improvements and other
resources
• at low points in the road profile
• where ditchline bedrock approaches the elevation of the finished grade
• immediately prior to sections of cut slope instability or ravelling
• prior to large through cuts that may be drainage divides
• at any other location found necessary during construction, or evident during
maintenance inspections.
Cross-drain culverts and ditches at switchbacks often need site-specific consideration.
For forest road construction, unless soil and runoff conditions require
increased sizes, recommended minimum pipe culvert diameters are 400 mm
for dry sites and 600 mm for wet sites. The culverts addressed here have a
design discharge of less than 6 m 3 /sec and a diameter of less than 2000 mm.
Culverts larger than this must be designed by a professional engineer. Refer
to Chapter 2, “Design and Construction of Bridges and Stream Culverts” for
more information.