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    1. Roadslope types
      • Roadfills or road embankments (1)
      • Roadcuts (2)
    2. Roadslope vegetation - spontaneous colonization
      • Vegetation of roadfills (3)
      • Vegetation of roadfills (4)
      • Vegetation of roadcuts (5)
      • Differences in vegetation cover between roadfills and roadcuts (6)
    3. Physical, chemical and hydrological soil properties of roadslopes
      • Soil sampling (7)
      • Soil water holding capacity (Richard's chamber) (8)
      • Soil water holding capacity (Richard's chamber) (9)
    4. Erosion processes on roadslopes
      • Rill erosion (10)
      • Rill erosion (11)
      • Gully erosion (12)
      • Mass movement (13)
There are 2 types of roadslopes according to the material of origin and the slope angle. Roadfills are built by accumulating and compacting unconsolidated materials from an adjacent area. Slope angles of roadfills are approximately 27º (or 3:1).
There are 2 types of roadslopes according to the material of origin and the slope angle. Roadcuts result from the excavation of high areas. Resulting slopes are steep with angles as high as 35º (2:1) or 45º (3:2). In some cases, roadcuts can be almost vertical.
Once roadfill is built, the addition of topsoil on the slope surface improves the physical and chemical soil properties as the same time as it provides a seed bank which enhances spontaneous revegetation. As a consequence, roadfills show higher levels of fertility, lower levels of soil compaction and erosion, and higher densities of vegetation (>50%) than roadcuts.
Several processes control vegetation establishment and development on roadfills: (1) dispersal and further germination of seeds from surrounding areas to the slopes (2) germination of seeds from the topsoil seed bank (3)germination of hydroseeded species.
In semiarid areas, roadcuts are characterized by a low soil fertility, a high soil compaction, a high degree of erosion and a low vegetal cover.
Two extreme situations: right, a north-facing roadfill with favourable conditions for plant establishment and persistence; left, a south-facing roadcut with unfavourable conditions for plant establishment and persistence.
The important differences in soil properties (physical, chemical and hydrological) between roadfills and roadcuts mainly explain differences in vegetation cover on these two types of slopes.
Water holding capacity is a key soil property for seed germination and seedling establishment on roadslopes. Species able to germinate in few days under water stress are favoured as regard roadslope colonization, and more specifically as regard roadcut colonization.
After roadbuilding and before plants have been able to establish on roadslopes, intense rainfalls enhance water erosion on slopes (here typical rills in slope direction).
The concentrated flow in rills is able to remove soil particles and seeds downslope and to occasionally uproot some plants.
Gullies can be large-sized on roadslopes.
Mass movement can cause severe problems for road safety.