Cliff instability at Dingli: armour or retreat?
A field class surveyed a 600 m stretch of cliff near Dingli, Malta. The cliff is capped by 8 m of Upper Coralline Limestone, below which lies 2 m of Greensand and 15 m of Blue Clay. At the base, the clay is undercut by 1.2 m, and wave-cut debris has been removed by recent storms. Two tension cracks run parallel to the edge, 12 m and 18 m inland; the nearer crack widened 3 mm between January and December, with most movement after 48 hours of rainfall totalling 95 mm. A small chapel stands 25 m from the edge, and a road serves three farmhouses. The local council has asked whether to fund rock armour at the base or to relocate the road and chapel. Annual visitors number about 40,000, mostly in spring. The survey also recorded a 0.4 m drop in the clay surface at a seepage point.
1. Using the geological and monitoring evidence, explain why the cliff is unstable and why the movement is episodic rather than continuous.
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Mechanism. The cliff is a layered system, not a single rock mass. The Blue Clay is impermeable, so rainfall and seepage raise pore-water pressure within it; the recorded 0.4 m drop at a seepage point shows water is moving through and weakening the clay. The Greensand between the clay and the limestone is relatively weak and can be eroded by water, removing support from the limestone cap. Marine undercutting of 1.2 m at the base removes the toe, so the upper limestone is effectively a cantilever. Tension cracks 12–18 m inland define a block whose base is clay. When the clay is saturated, its shear strength falls; the block can slip or topple.
Why episodic. Movement is not continuous because the clay needs a threshold pore pressure. The 3 mm widening occurred mainly after 95 mm of rain in 48 hours, so most years produce little movement, while rare storms trigger jumps. Storm-wave removal of toe debris is also episodic. The 3 mm/year average therefore understates risk: the cliff may be stable for years and then fail suddenly. The chapel at 25 m and the crack at 12 m mean the chapel is within the likely block if the nearer crack propagates. The correct judgement is that average rates are a poor guide; the council should use rainfall thresholds and crack monitoring, not a single annual figure.
2. Evaluate the two options: rock armour at the base versus relocation of the road and chapel. Recommend one, with conditions.
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Criteria: safety, cost, reversibility, ecological impact, heritage, and maintenance.
Rock armour. It addresses marine undercutting, but the cliff is 600 m long and the failure mechanism is largely clay seepage and tension-crack development. Armour at the base would not drain the clay or stop the block sliding. It is expensive, requires heavy access on a cliff, may shift erosion downdrift, and creates a false sense of safety. It could buy time for the chapel, but only if coupled with drainage and crack sealing; otherwise it treats a symptom.
Relocation. It removes people and heritage from the hazard zone, is reversible in principle, and avoids downdrift impacts. But moving a chapel is heritage-sensitive and costly; moving the road may isolate farmhouses. It does not stop cliff retreat, so a setback zone is still needed.
Recommendation: a dual approach-relocate the road and chapel to a setback beyond the 18 m crack plus a safety buffer, and monitor and drain the clay; do not fund full rock armour. Conditions: a statutory setback, rainfall-triggered closure, and a study of downdrift effects if any armour is used. If the chapel is of exceptional heritage value, a small, reversible toe protection could be justified only with drainage.
3. Identify two further data sets the council should collect before deciding, and explain how each would change the decision.
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High-resolution rainfall and pore-pressure records. The decision hinges on threshold behaviour. A tipping-bucket gauge and piezometers in the clay would show whether the 95 mm event was exceptional and how quickly pore pressure decays. If pressure remains high, relocation is urgent; if it drains quickly, monitoring may suffice.
A geological survey of crack depth and clay continuity. The surface crack may be shallow. Seismic refraction or boreholes would show whether a continuous slip surface exists. If the crack terminates in limestone, the block is smaller and drainage may be viable; if it reaches clay, relocation is stronger.
Visitor and traffic data. About 40,000 annual visitors create revenue and evacuation risk. A count of peak-day vehicles would show whether the road can be closed seasonally. This changes the cost of relocation and the acceptable timing.