Surface drainage works (Ditches, Channels, Pipeworks)



Surface drainage works are used to collect and direct surface runoff in a controlled manner, to minimize the quantity of surface water flowing into actually or potentially unstable slopes.

Surface drainage works are especially important at the head of the slope to intercept the run-off and reduce the surface water flowing downstream across the face of the slope. This may be achieved by open ditches at the head of the slope.

Ditches on the main landslide body are used to dispose of local surface runoff and any water arising from deep drainage works.

Different types of ditches are used to drain surface runoff. The cross-section of ditches is usually trapezoidal, although small ones may be V or U-shaped or semicircular; their dimensions vary according to the expected runoff, the need for open water storage, the risk of bank erosion, the need to accommodate the transit of construction or maintenance equipment and the available means for maintenance (Figures 1 and 2 and Table 1).

Table 1: Typical dimensions of open ditches (

Type of ditch



Bed width (m)

Side slope


Maximum side slope (v: h)


0.3 to 0.6


1: 6








0.3 to 1

As required



Figure 1: Typical arrangement of open ditches   (source:
Figure 1: Typical arrangement of open ditches
Figure 1: Typical arrangement of open ditches   (source:
Figure 2: Typical arrangement of open ditches

Ditch gradient should be at least 2% to ensure rapid flow away from the potentially unstable areas and to promote self cleaning from any windblown or other debris that would tend to accumulate, causing local blockage and spillage.

Ditches should be lined to minimize erosion and uncontrolled infiltration. The lining may consist of cast-in-place or prefabricated concrete, pitched stone (Figure 3), rip rap, gabion mattresses or baskets, speciality geotextiles or geocomposites, zinc coated steel or PVC half-pipes. Flexible, self-healing lining or pipes should be used in areas susceptible to cracking and movements.

Figure 3: Open ditch lined with pitched stone, Gimillan nr. Cogne (AO), Italy (photo: G. Vaciago, SGI-MI)
Figure 3: Open ditch lined with pitched stone, Gimillan nr. Cogne (AO), Italy (photo: G. Vaciago, SGI-MI)

Where permeable linings are used, this should be in association with an impermeable geomembrane to minimize infiltration. Geomembranes may also be used by themselves for temporary or emergency applications, but they are easily damaged by wind and direct sunlight and should not normally be used by themselves for permnnent applications.

Techniques must be adapted to ground conditions and local technology; an example is provided by Anderson and Holcombe (2004; 2008) who describe the development and application at community level of good drainage practices with locally available, affordable technologies in St Lucia, West Indies, consisting of ditches lined with a specialised plastic, held in place by a wire mesh (Figure 4).

Figure 4: STARTM  drainage system installed by residents in St Lucia, West Indies  (source: Anderson and Holcombe, 2008)
Figure 4: STARTM drainage system installed by residents in St Lucia, West Indies
(source: Anderson and Holcombe, 2008)


Design methods

Ditches must have enough capacity to transport the drainage water in wet period; however they are sometimes made wider than needed in order to create more storage in the open water system. Such temporary storage is a good way of diminishing the peak outflow from the area, as occurs after heavy rains. Thus it reduces the required capacity of downstream constructions, such as the larger watercourses, culverts and pumpung stations.

Ditches are often relatively unaccessible and may receive less maintenance than would be appropriate. Accordingly, it is advisable to design them with a generous freebord to minimize the risk of blockage and spilling.

Steps or other energy dissipation systems should be used on and at the toe of steep sections, to prevent excessive flow speeds and the resulting erosion.

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 0
Slide 8
Spread 4
Flow 7

Material type

Descriptor Rating Notes
Earth 9 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 1

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) 3
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 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.
Slow 6
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 5 Applicable irrespective of groundwater conditions. It does not drain groundwater. Effects on groundwater levels only indirect through reduced infiltration.
High 7
Low 6
Absent 4

Surface water

Descriptor Rating Notes
Rain 9 See fact sheet 3.7 for diversion channels for main water courses.
Snowmelt 8
Localized 9
Stream 4
Torrent 1
River 1

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 6 will be updated
Economic suitability (cost) 10 Low, where applicable.


  • Anderson M.G., Holcombe E.A. (2004). “Management of slope stability in communities” Insight, Vol 6, 15-17.

  • Anderson M.G., Holcombe E.A. (2008). “A new sustainable landslide risk reduction methodology for communities in lower income countries”. In: Proc. of the First World landslide Forum, Tokyo, Parallel Session Volume, 61-64.

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