March 1, 2003
Most of the time that restoration practitioners talk about roads and their impacts on water quality and salmon habitat in our North Coast creeks and rivers, they are talking about how roads handle the heavy rainfalls that we often see at this time of year. How well water can drain off of a road surface is as important a consideration as the culverts and bridges that carry creeks under our roads. For salmon habitat, the main issue is how much dirt (sediment) is carried off of the road surface and fill along with that water.
When you drive down a road in the winter, do you ever notice that some roads seem to stay clear of water no matter how much it is raining, and that some always seem to have deep ruts, puddles and mud holes on them? While the geology underlying the road plays a big role, drainage of water off the road surface can determine how much sediment is generated from the road.
For water to drain off of the road, the road needs to be beveled in some direction. This ?tilt? is described in relation to the in-board and out-board sides of the road. If the road is going along a hillside, the uphill side is called the ?in-board? side, and the downhill side is called the ?out-board? side. Correspondingly, a road that slants towards the in-board is called an ?in-sloped? road, while a road that slants toward the out-board is called an ?out-sloped? road.
Most roads constructed since 1950 around here are in-sloped. Water drains off of the road in to an in-board ditch. The water travels downhill in the ditch, and either is deposited on the outboard side by a ditch-relief culvert, or the ditch simply extends into a stream, which then travels under the road in a culvert.
More recently, road designers, particularly those concerned with sedimentation, have seen the advantage to constructing out-sloped roads. Instead of having the in-board ditch/ditch-relief culvert combination, these roads simply slope downhill, and water drains off of them all along the road.
While out-sloped roads are recommended for fisheries reasons, each type of road has advantages and disadvantages depending on the situation. Some people think that out-sloped roads are dangerous in areas that are often frosty or snowy. Out-sloped roads generally are not recommended for areas where roads go across landslides, for the simple reason that you don?t want to have any concentration of water whatsoever going on to a slide.
In most other situations, though, out-sloped roads are the way to go. They require much less maintenance than in-sloped roads. There are no ditch relief culverts that can clog, and there is no ditch that needs to be maintained. Keeping the water draining into the ditch requires vigilance, and ditch relief culverts require replacement on a regular basis.
As a road is being constructed, the engineer or, more likely, the heavy equipment operator, determines if a road is going to be in-sloped or out-sloped. If you are constructing a new road, or re-opening an older one, consider designing it to be out-sloped ? you might find that it saves maintenance, time, and money in the long run, and you?ll know that you are helping to keep your local creek running clear year-round.
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TOC for Forest & River News, Spring 2003