Culvert and riprap drainage installation on a forested slope showing water management infrastructure

Culvert and riprap drainage on a slope. Source: USDA Forest Service / Wikimedia Commons (Public Domain)

The role of water in slope failure

Water is involved in the majority of coastal landslides in Italy. When rainfall or wave run-up raises the groundwater table inside a slope, the pore-water pressure in the soil increases. This reduces the effective stress — the component of stress that contributes to frictional resistance — and lowers the factor of safety against sliding. On slopes that are already marginal, even a modest seasonal rise in the water table can trigger movement.

In the Cinque Terre and northern Apennine coastal areas, intense short-duration rainfall events are documented as the primary trigger for the translational slides that periodically affect terraced hillsides. Drainage measures aim to intercept infiltrating water before it reaches the failure zone or to remove water already present within the slope body.

ISPRA's national landslide inventory (IdroGEO) records thousands of active and dormant landslide bodies along Italy's coasts. Elevated pore-water pressure is cited as a contributing factor in a substantial proportion of shallow slide events in the database.

Surface drainage

Surface drainage prevents rainfall from infiltrating the slope by collecting and redirecting runoff before it can percolate. The main elements are:

Interception ditches

Lined channels constructed at the crest of a slope intercept runoff from the catchment above. On coastal cliffs they are usually cut into rock or stabilised earth, sloped to a point of safe discharge away from the slope face. The lining material — typically concrete or compacted clay — prevents seepage losses back into the slope.

Benching and terrace drains

On longer slopes, horizontal benches cut into the face reduce the effective slope angle and carry cross-slope drainage. Each bench has a longitudinal channel that directs water to a downslope chute at the side. On the Ligurian terraced coastline, this approach also serves agricultural purposes, making it historically common despite the maintenance burden that abandoned terraces create when the drain system is neglected.

Riprap and surface protection

On slopes where surface erosion by rain splash and sheet flow removes fine material, riprap (loose angular stone) or planted vegetation cover reduces erosion rate and slows the concentration of surface runoff. Geosynthetic erosion-control mats are sometimes used on recently stabilised slopes to support vegetation establishment.

Subsurface drainage

Horizontal drain boreholes

Horizontal drain boreholes are drilled into the slope body at a shallow upward angle (typically 3–8° from horizontal) to intersect the saturated zone and allow water to flow out under gravity. Diameters are commonly 75–100 mm; the borehole is lined with a perforated plastic pipe socketed into a gravel-wrapped collector.

The effectiveness of a horizontal drain depends on its length, orientation relative to the groundwater flow direction, and the permeability of the formation it intersects. On Italian coastal slopes with complex stratigraphy, drilling several boreholes at different depths and azimuths is often necessary to achieve a useful drawdown of the phreatic surface.

Drainage galleries

Where the slope is large and the risk is significant, drainage galleries — small-diameter tunnels driven horizontally into the hillside — provide a framework from which fan-drilled radial boreholes can drain a wide volume. The gallery itself acts as a collector. This approach is used on major landslide bodies in the Apennines where the affected volume is too large for surface measures alone.

Rock armour protection at a coastal site showing stone placement at the water's edge

Coastal rock armour at tide line. Source: Geograph / Wikimedia Commons (CC BY-SA 2.0)

Geosynthetic drainage layers

In reinforced slope construction or behind retaining walls, geocomposite drainage layers — a core of polymeric net or cuspated sheet laminated to a geotextile filter — are placed against the retained face to intercept seepage before it can build up hydrostatic head. This is standard practice behind gabion and sheet-pile walls on Italian coastal projects.

Combined drainage and retention systems

The most effective approaches integrate drainage with structural retention. A typical sequence for a coastal slope in moderately permeable marl might be: install a crest interception ditch, drill a first row of horizontal drains to lower the groundwater, then install ground anchors and a shotcrete or concrete face to carry the residual driving forces. The sequence matters — structural loading should follow drainage wherever the schedule allows, since the drainage lowers the load the structure must carry.

Maintenance requirements

Drainage measures require regular inspection. Horizontal drain pipes can become blocked by calcareous encrustation, root intrusion, or fine-material migration. Surface ditches can fill with silt or become overgrown. On active landslides, drain pipes that have been sheared by slope movement must be replaced. Italian regional administrations responsible for coastal road networks typically specify annual inspection for drainage infrastructure on identified high-risk slopes.

Further reading