culvert drainage on dirt roadWelcome to the fourth post in our six-part “Planning Your Road Network” series where we draw on information from one of our favourite resources to answer the question “What’s the Secret to a Good Roadbed?”

We won’t keep you in suspense. The secret to a good roadbed is to keep it well drained. Proper ditching is one of the keys to road stability. The size and frequency of culverts is also very important.

On flat land, ditches should be placed on both sides of the road. Roadways on slopes only require ditching on the topside, sloped to drain water away from the road.

It’s important to avoid long grades because they require extra culverts and are susceptible to water buildup.

A light crown in the centre of the road will assist drainage and minimize potholes. If you have a well-drained, vegetated and stable upslope, you might consider grading a shallow outslope to the road so water drains off rather than constructing a ditch.

What’s so important about culverts?

The purpose of culverts is to drain excess water from roads and ditches and support natural drainage patterns. Culverts should he placed wherever water drains naturally. As a rule of thumb, plan on approximately 5 culverts per kilometre of roadway on average ground. Vary the number according to your specific site conditions.

Culverts can be log culverts constructed on-site or circular metal culverts. In any case, they need to be large enough to handle peak water flows and it is better to err on the side of caution. Wooden culverts should be constructed from cedar to prolong their life.

Untreated Douglas-fir or spruce may be an alternative, though these will require replacement more frequently. Use a layer of geotextile over any wooden structure to minimize siltation. Though metal or plastic culverts are more expensive items, they may be a cost-effective alternative due to their ease of placement and durability.

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How to locate culverts?

  • Place at right angles to roadbed to minimize culvert length
  • Make sure culverts are firmly footed to settle evenly when covered
  • Where road grades are greater than 6%, culverts should be angled or skewed toward 
the grade
  • Provide adequate culvert slope (2–3%) so that water flows freely through the culvert 
and sediment does not build up at lower end
  • Culvert must be large enough to anticipate snowmelt and storm water flows
  • Culverts must be placed at a level to ‘capture’ and channel water flows, but not so low 
that they capture and accumulate debris; a settling pond needs to be placed adjacent 
to the intake to protect the culvert from being blocked by moved material
  • Culvert should be covered with fill to a depth equal to or greater than the culvert 
diameter (or as recommended by the manufacturer)
  • A ditch block should be installed below the culvert, on the intake side, to prevent water 
from flowing further down the ditch; construct the plug or block lower than the shoulder of the road so that any overflow will continue down the ditch and not down the road
  • Armour culvert intakes, outflows, and discharge areas to prevent scouring and erosion.

Take particular care when installing culverts at stream crossings. Use broken rock, vegetative cover or other means to reduce soil movement into the stream and minimize stream bed disturbance.

Federal Fisheries officers and/or officers from the local Ministry of Environment should be consulted regarding specific procedures and timing of construction in fish-bearing streams. Under most circumstances you will be required to use open bottom culverts in order to protect existing fish habitat.

As always, thanks to A Non-forester’s Guide to Small-scale forestry in British Columbia for the information excerpted above. Please stay tuned for the fifth post in the series where we discuss some other structures to consider in your road network. If you can’t wait, you can find the complete resource online here.