The record rainfall that drenched South Carolina over the weekend has given us a lesson in the destructive power of flooding.
The soil can handle a certain amount of rain, but once the soil is saturated, the water runs off instantaneously, making its way over land into smaller streams or storm-water drainage pipes and then to larger streams and rivers.
When the carrying capacity of the storm-water drainage system is exceeded, or when branches and other types of storm debris clog the normal drainage routes, the water backs up in the roadways.
The fast-moving overland flow can cause soil erosion, particularly as it flows down hills that are not covered by vegetation.
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Soil erosion is particularly dangerous in earthen dams, as we saw in the Midlands on Sunday. The rising water behind the dam and the rainfall on the dam increase the pressure of the water in the pores of the dam, weakening it to the point that it gives way instantly, releasing water that adds to an already-heavy water flow and exacerbating flooding.
Even though the rainfall is largely over, the worst of the floods may be yet to come. It can take hours or even days for floodwater from outlying areas to reach an outlet, and most outlets are choke points that cannot handle the increased flows.
It could take days or even weeks, but eventually the flows will recede as the water drains into the ocean, beginning with the coastal areas and then progressively the inland areas. Some of the water will stay around for a long time, eventually draining into the soil or evaporating into the atmosphere.
And that is where we find the silver lining of this rainfall: Many parts of South Carolina were under drought conditions just a few days ago. With the rainfall this has changed. Even better, some of the water that does not drain into the ocean or evaporate — water in the ponds, lakes, streams and rivers — will continue to infiltrate over time and replenish our groundwater aquifers, making our state better able to survive the next long dry spell.
USC Professor of Earth and Ocean Sciences