Local regrading to facilitate runoff

Category: MODIFYING THE SURFACE WATER REGIME – Surface drainage

Description

Smoothing the topography of the slope surface can prevent surface water from ponding in local depressions (Figure 1), thus reducing the opportunity for infiltration. Any depressions on the slope that might retain standing water should be removed by regrading, infilling and exacavation works (Figure 2), combined with surface and/or shallow drainage (fact sheets 3.1 and 4.1), measures to promote rapid runoff (fact sheet 3.4) and measures to reduce net rainfall thanks to canopy storage in vegetation (fact sheet 3.5).

Figure 1: Ponding in topography disturbed by landsliding, Lillaz, nr. Cogne (AO), Italy (photo: G. Vaciago SGI-MI)
Figure 1: Ponding in topography disturbed by landsliding, Lillaz, nr. Cogne (AO), Italy (photo: G. Vaciago SGI-MI)
Figure 2: Stabilization of Tablachaca Dam Landslide, Peru, crossection (source: Millet et al., 1992)
Figure 2: Stabilization of Tablachaca Dam Landslide, Peru, crossection (source: Millet et al., 1992)

Regrading inevitably damages the residual vegetation cover, which should be reinstated without delay to minimize erosion. Reference may be made to fact sheet 1 for details.

Local regrading as described here should not be confused with general modification of the slope geometry, described in Section 2.



Design methods

The design should balance cut and fill, in order to minimize the cost and environmental impact of the works.

The design should minimize major changes to the mass distribution of the slope, unless this is carried out deliberately as part of the stabilization works in accordance with the principles and methods described in fact sheets 2.



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 1 Most suited to all types of slides and, subject to circumstances in flows. In spreads, only useful as remediation, not as a preventive measure.
Topple 1
Slide 7
Spread 5
Flow 7

Material type

Descriptor Rating Notes
Earth 8 Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode. Potential difficulties in excavation and impermeabilization of ditches in coarse debris.
Debris 7
Rock 2

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 8 Typically applicable to landslides of any depth, but relative effectiveness decreases with increasing depth of movement.
Shallow (0.5 to 3 m) 8
Medium (3 to 8 m) 6
Deep (8 to 15 m) 4
Very deep (> 15 m) 1

Rate of movement

Descriptor Rating Notes
Moderate to fast 3 Can be carried out without special difficulty when the rate of movement is slow (5 cm/day) or less, but may be disrupted and will require additional maintenance or reconstruction as a result of continued movement. May be applicable, with special precautions and limited effectiveness due to continuous disruption, to moderately fast movements.
Slow 6
Very slow 7
Extremely slow 7

Ground water conditions

Descriptor Rating Notes
Artesian 6 Applicable irrespective of groundwater conditions. It does not drain groundwater. Effects on groundwater levels only indirect through reduced infiltration. Potential difficulties in carrying out regarding in areas of high or artesian groundwater levels, depending on the depth of local excavation required.
High 5
Low 7
Absent 7

Surface water

Descriptor Rating Notes
Rain 8 See fact sheet 3.7 for diversion channels for main water courses.
Snowmelt 8
Localized 8
Stream 5
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Effects on stability only indirect. The reliability in the long term may be impaired by further movement or poor maintenance.
Feasibility and Manageability 10 Simple technique. Potential benefits and limits of applicability are well established.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 10 Easily implemented with widely available equipment.
Environmental suitability 4 will be updated
Economic suitability (cost) 10 Low, where applicable.

References

  • Millet et al., 1992

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