Notes

FIGURE 2.6

Road ford with longitudinal slope to allow the crossing of a natural stream with a permanent rolling dip added to divert water from the road surface

Source: © Masila Benson Muteti, Regional Manager at Kenya Rural Roads Authority. Used with permission. Further permission required for reuse. Note: The rolling dip is constructed with concrete, masonry, or rocks (Bender 2009).

Controlling Erosive Areas with Embankment Roads Roads that cross unstable and highly erosive landscapes can be developed to reduce the land-degradation process. Runoff should be concentrated in the more stable streams; cross drainage should not be placed on the steepest or most erodible sections in a watershed.

Erosion can sometimes be controlled in some highly erosive drainage lines by simply not installing culverts or by installing them with high sills. The runoff will disperse and fill the stream along the uphill side of the road, which can help control the specific erosion hot spot.

Capturing Eroded Soils with Roads in Combination with Warping Dams Warping dams are relatively high retaining structures constructed to capture soil in highly erodible landscapes. Warping dams slow water flows, allowing sediment to settle on the upstream side of the dam; over time the accumulating sediment creates stable and fertile terrace land from which runoff water can be better managed. once the land area is filled up, the watercourses can be channeled. Roads can be combined with such warping dams. Warping dams were used extensively to restore land in China’s Loess Plateau (Li, du, and Liu 2016), where the landscape was heavily degraded. Hillsides were treated with a range of measures, and valleys were blocked off with warping dams, creating fertile new land where there had been heavy erosion.

NOTES

1. When a catchment is heavily eroded, the road’s additional impact on sedimentation is proportionally less; in fact, roads can help mitigate some of the erosion. Therefore, in relatively pristine watersheds a road’s contribution to the sedimentation process is proportionally higher but may be lower in absolute numbers. 2. Because there is an extensive and rich literature on preventing landslides during road development, this subject is not addressed in these guidelines.