Road Drainage

By Thembi Borras

Chances are you have seen the result of poor road drainage manifested in water that has been allowed to concentrate and reach a velocity that moves soil causing accelerated erosion. Addressing road drainage is central to meeting two of the road management goals, reducing chronic delivery of sediment and reducing maintenance.

There are three ways to drain a road. Insloping is where the roadbed is tipped toward the cutbank, water flows to the inside ditch where it mixes with flow intercepted from the hillslope. The water is then carried to a ditch relief culvert and underneath the road to the outside edge of the road. Outsloping is where the roadbed is tipped out; water is not concentrated and flows to the outside edge of the road. Rolling dips supplement outsloping by insuring water gets across the road. The third way to drain a road is crowning, 1/2 the roadbed is tipped out and 1/2 the roadbed is tipped in.

Any of these methods, installed well, will minimize chronic erosion. The key is to drain roads well and frequently onto stable surfaces.

After road improvement, chronic erosion will continue. However, improvements are intended to minimize and redirect the sediment generated to stable locations and filter strips, such that the sediment has a chance to drop out before reaching the waterway, thus disconnecting roads from streams.

For more information on road drainage reference the Handbook for Forest and Ranch Roads by Pacific Watershed Associates or the "Roads" video, adapted from the Handbook. Both are available through the Mendocino County Resource Conservation District (707-468-9223) and the Navarro River Resource Center (707-895-3230).