Lesson 3 - Tips For Building or Fixing Roads or Bridges, Information Sheet

Acknowledgement: Taken from "Living on the Land 2001"

1. Outslope roads so that they distribute any subsurface water from the upslope area that the road forces to the soil surface.
2. Outslope roads to distribute any water falling on the road surface.
3. Surface roads with gravel that does not erode with tire traffic under the impact of falling rain. Surfacing material should allow water to run off or infiltrate without puddling to avoid erosion.
4. If a road concentrates water above the road and moves it through a culvert, armor the drainage way to make sure the concentrated water does not cause erosion on soil that cannot resist the force of this much flowing water.
5. Keep raised roads out of floodplains. Most roads in floodplains have been elevated to keep them dry. This reduces the floodplain area, deepening and accelerating flood flows.
6. Keep simple "two-track" roads out of floodplains. During floods, the stream may capture roads that are not elevated, because their smooth, straight and steeper surface allows accelerated flow and roadbed erosion.
7. Where roads must cross floodplains, construct them to allow floodwater passage over a wide area. Do this by lowering the road to floodplain level except at the bridge where the water is deep, or by providing many culverts with armored collection areas and downstream dissipation aprons.
8. Where roads must be elevated, consider making a causeway by elevating the road on stilts or by making a very long bridge that spans the floodway.
9. Where a bridge or culvert will accelerate water and cause incision, armor the upstream bed to prevent the incision from becoming a headcut. Dissipate the energy below the bridge or culvert with a plunge pool or engineered structures.
10. Get help from experts to consider all upstream and downstream effects of your project. Make sure they obtain needed permits before beginning projects.
11. Build culverts and bridges big enough for the rare events. The 100-year flood has a 1 percent chance of occurring in any given year, even after a recent flood.

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