How can ponded water enlarge the unstable area of a rotational slide?

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landslide This 1995 landslide at La Conchita, a coastal town in California, swept away a hillside road and destroyed a number of houses.

Ponded water can enlarge the unstable area of a rotational slide, or slump, by increasing the weight of the debris and exerting pressure on soil particles, which impairs the overall stability of the slope.1

When water accumulates in depressions or at the head of a slide, it saturates the weathered material and debris. This saturation increases the mass of the material, which, under the influence of gravity, enhances the force driving the downslope movement. Furthermore, the presence of flowing groundwater within the slope exerts pore-water pressure that reduces the internal friction between soil particles, further destabilizing the mass and potentially causing the slip surface to expand or migrate. These processes are critical factors in the development of landslides, as they weaken the structural integrity of the slope and facilitate the separation of material along a plane of least resistance.1