A road that was closed after a landslide caused by Storm Darragh in mid-December 2024 has now reopened. The A487 in North Wales had been shut due to concerns over unstable ground above the road.
Temporary measures have been put in place to make the road usable again, including debris-catching system. Despite this, authorities are concerned repairs works will continue later into 2025.
North Wales is characterised by a diverse range of ground conditions influenced by its geology and topography. These conditions make the region particularly susceptible to landslides, especially during severe weather events like Storm Darragh.
Ground Conditions in North Wales
Geology:
The region features a mix of hard rock, such as slates, sandstones, and volcanic rocks, alongside softer, more erodible materials like glacial tills and clays.
The presence of fractured and weathered rock layers increases the risk of slope instability, particularly on steep valley sides.
Areas of sedimentary rock can act as aquifers, allowing water to accumulate and increase pore pressure in overlying soils.
Topography:
North Wales is renowned for its rugged landscapes and steep hillsides.
These steep slopes are prone to erosion, especially when saturated with water, creating conditions conducive to landslides.
Soils:
Peaty soils and shallow soils overlying bedrock are common in upland areas.
These soils retain water, becoming heavy and unstable after prolonged rainfall.
How Storms Like Storm Darragh Lead to Landslides
Storm Darragh, characterised by intense rainfall and strong winds, exacerbates these ground conditions through several mechanisms:
Heavy Rainfall and Saturated Ground:
Intense and prolonged rainfall during storms causes water to infiltrate the soil, increasing its weight and reducing its shear strength. This saturation is a critical trigger for landslides.
North Wales often experiences antecedent wet conditions, meaning the ground may already be waterlogged before a storm hits, leaving little capacity to absorb additional rainfall.
Increased Pore Water Pressure:
As water accumulates in the soil, it increases pore water pressure, reducing friction between soil particles and making slopes more prone to failure.
Surface Erosion:
Strong winds and heavy rain strip away vegetation and topsoil, reducing slope stability and exposing underlying materials to further erosion.
Hydrological Stress:
Sudden, high-intensity rainfall can cause rapid runoff and localised flooding, undermining the base of slopes and triggering debris flows or shallow landslides.
Rapid Weathering of Rocks:
Water penetrates cracks and fissures in rocks, promoting freeze-thaw cycles in colder weather, which weaken rock masses and contribute to rockfalls.
Storm Darragh's combination of intense rainfall, wind, and already saturated ground conditions creates the “perfect storm” for landslides in North Wales. Understanding and monitoring these conditions using technologies like InSAR can help predict and mitigate the impacts of such events. SAR-derived insights can inform the implementation of strategies such as improving drainage systems and maintaining vegetation cover can reduce landslide risks in this geologically complex and weather-prone region.
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