Insider Blog: Why Tropical Storm Hilary caused sinkholes in the Coachella Valley

In many places, like Florida, sinkholes aren’t an uncommon occurrence. In Florida, much of the bedrock is made of limestone, a sedimentary rock. The problem with sedimentary rocks like limestone is that they can easily dissolve in acidic water.

When rain falls, small amounts of gasses can dissolve into the water, turning the water ever so slightly acidic. Over time, this slightly acidic water can erode away the limestone, leading to a void under the surface.

This can cause the surface to collapse, forming a sinkhole. This process is known as chemical weathering since the rock is being broken down chemically.

However, in places like the Coachella Valley, the geology makes limestone or cave-like sinkholes significantly rarer. I spoke with Dr. Kerry Cato, a professor of geology at California State University at San Bernardino who explained why.

According to Dr. Cato, "the floor of the Coachella Valley can have from 0 to up to 100 feet of alluvium - loose deposits of small particles like sand, silt, clay and gravel. This sits atop a strong bedrock – like granite, but also dense sedimentary and metamorphic rock. The bedrock is resistant to the formation of natural sinkholes, but in some cases the alluvium can be affected by subsurface erosion".

The reason we saw a few sinkholes open up on our roads comes down to how they are built. When roads are being built, utility lines, pipes and more are laid down first. A fill material, usually sand and gravel, is then densely packed around these utilities before asphalt or concrete is laid on top. When water, especially flowing water, makes its way into cracks in the road, it can wash away the fill material, leading to structural weakening of a section of the road and eventually a sinkhole.

Dr. Cato emphasized that "what are commonly called sinkholes in the southwestern US are caused by alluvium being eroded underneath roadways. This is a normal phenomenon and not a flaw in the design of our roads. It is almost impossible to monitor the structural status of a whole road system beneath the surface. This is why we cannot really predict where or when sinkholes will open up".

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